Red Mud Minimisation and Management for the …...Bauxite residue (red mud), a waste from the Bayer process for refining bauxite to alumina, is highly alkaline (pH~13) and its treatment

Red Mud Minimisation and Management for the

Alumina Industry by the Carbonation Method

A thesis submitted in fulfilment of

the requirement for the degree of

Doctor of Philosophy Environmental Engineering

By CUONG PHUOC TRAN

February 2016

School of Chemical Engineering

Faculty of Engineering, Computer and Mathematical Sciences

The University of Adelaide

i

DECLARATION

I certify that this work contains no material which has been accepted for the

award of any other degree or diploma in my name, in any university or other tertiary

institution and, to the best of my knowledge and belief, contains no material

previously published or written by another person, except where due reference has

been made in the text. In addition, I certify that no part of this work will, in the

future, be used in a submission in my name, for any other degree or diploma in any

university or other tertiary institution without the prior approval of the University of

Adelaide and where applicable, any partner institution responsible for the joint-award

of this degree.

I give consent to this copy of my thesis, when deposited in the University

Library, being made available for loan and photocopying, subject to the provisions of

the Copyright Act 1968.

I also give permission for the digital version of my thesis to be made

available on the web, via the University’s digital research repository, the Library

Search and also through web search engines, unless permission has been granted by

the University to restrict access for a period of time.

CUONG PHUOC TRAN

Adelaide, 2016

ii

ACKNOWLEDGEMENTS

This thesis has been a truly exciting and extremely enriching experience for

me, both academically and personally. The outcomes of the research had the

assistance and cooperation of many individuals and organisations. I would like to

offer my grateful thanks to all of them. Particularly, I am thankful to the University

of Adelaide and the Vietnamese Government for offering me a prestigious

scholarship with which to conduct this study.

My special thanks must first go to my supervisor Associate Professor Dzuy

Nguyen, for his continuous assistance, guidance, active supervision, and kindness to

support me during my research candidature. He guided me initially on

comprehensive research of materials and encouraged me since the early stages of

research. I am greatly indebted to Associate Professor Dzuy Nguyen.

I would also like to express my appreciation to Associate Professor Brian

O’Neill, who was my previous co-supervisor and initially helped me in critical

thinking, supported and corrected my English at the beginning of this project. I am

also thankful to Associate Professor Yung Ngothai, who replaced my previous co-

supervisor in 2014. She always encouraged and shared with me difficulties occurring

during the research. She provided me with the best facilities from her lab for my

experiments.

My sincere thanks also go to Professor Allan Pring from Museum of South

Australia, who provided me with permission to use facilities in SA Museum. My

heartfelt thanks go to all academic members, office staff and other PhD students in

the School of Chemical Engineering, School of Physical Sciences, and Adelaide

Microscopy for their helps, friendship, encouragement, and understandings on many

occasions. My grateful thanks go to my colleagues, who shared with me their

research experience and made me feel confident through their talks and

iii

companionship. It has been a memorable time of my life and one that I will never

forget. I also thank Rio Tinto Alcan for kindly donating the red mud sample used in

this study.

To my family members, who gave support from the beginning, I have greatly

appreciated it and will try to do all those things that I promised. I would like to thank

my lovely wife, Loan Thi Thuy Nguyen, for her love, inspiration, and endless

encouragement. She has gracious and much patience in looking after our two lovely

and gentle children, Vy Thuy Tran and Trong Phuoc Tran (Ken Tran). Their love and

affection kept me sane during my research in Adelaide. Finally, I am thankful to my

parents and my siblings for their unconditional love and constant prayers, as well as

my parents-in-law, for their great understanding and support.

Adelaide, 2016

iv

ABSTRACT

Bauxite residue (red mud), a waste from the Bayer process for refining bauxite

to alumina, is highly alkaline (pH~13) and its treatment and management have posed

environmental challenges to the alumina industry. Carbonation of red mud using

carbon dioxide (CO2) has previously been demonstrated to be feasible in both

permanently capturing the CO2 and neutralising this solid waste. A systematic study

of the neutralisation of red mud by CO2 over a range of different operating

conditions is essential in order to optimise the carbonation process and maximise the

volume of CO2 captured by red mud.

The objectives of this study were to determine the acid neutralisation capacity

of red mud and its solid and aqueous phase contribution to the acid neutralisation

capacity via the analyses of red mud compositions. A red mud sample, provided by

Rio Tinto Alcan, was carbonated in a range of different operating conditions with the

intent of establishing the optimal condition for the carbonation process. The

carbonation was carried out at room temperature and atmospheric pressure using a

stirred tank reactor operating at different conditions such as total gas flow rate, CO2

concentrations, stirring speeds, and solids concentrations in red mud. A range of

analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy

(SEM) coupled with Energy Dispersive X-ray (EDX), Inductively Coupled Plasma

Mass Spectrometry (ICP-MS) and Carbon-Hydrogen-Nitrogen Elemental Analyser

(CHN) were used to ascertain the different mineral phases, change of chemical

composition before and after carbonation, and carbonation capacity of the mud.

Finally, based on the information of red mud composition, an equilibrium chemical

model using MINEQL+ version 5.0 was developed for the carbonation process.

v

The acid neutralisation capacity of red mud was measured by both rapid and

long-term titration of red mud slurries to pH endpoint of 4.5, 6, 8, and 10. At the

endpoint of pH 4.5 corresponding to the bicarbonate endpoint, the acid neutralisation

capacity of the red mud was found to be 0.79 and 1.91meq/g red mud for rapid and

long-term titration, respectively. Furthermore, it is estimated that the solid phase

contributed approximately 81% to the acid neutralisation capacity, while contribution

from the liquid phase was only 19% in the final long-term acid neutralisation

capacity determination.

The carbonation process was observed to be significantly dependent on

concentration of CO2, total gas flow rate and stirring speeds, whereas the

concentration of solids in red mud seemed to have a little effect based on only three

concentrations studied. For the carbonation of red mud slurry, it took from 30-75

minutes to establish the equilibrium pH of 7.5-6.6 in the range of CO2 concentrations

of 10%-100%. In contrast, when the carbonation of red mud liquor only was

performed at the same range of CO2 values, the stable pH of 7.0-6.3 (0.3-0.5 pH unit

lower) was reached within 15-30 minutes. After carbonation, the pH from carbonated

red mud slurries, exposed to atmosphere CO2, rebound quickly and took about 20-25

days to reach pH of 9.7. The carbonated liquor, however, showed a lower rate of pH

recovery, and took a month to equilibrate to pH of 9.7.

The XRD patterns of carbonated red mud revealed the appearance of calcite

and the increase of gibbsite due to the dissolution of sodalite and the breakdown of

cancrinite minerals in the carbonation of red mud. The quantifications confirmed the

precipitation of calcite from 0% to 1.51%, and the increase of gibbsite from 1.04% to

5.15% in raw red mud and carbonated red mud, respectively. XRD patterns and the

quantifications associated with other results such as EDX and CHN analyses

vi

indicated that the most optimal conditions for carbonation process were 30% CO2

concentration and total gas flow rate of 200mL/min. At this condition, the amount of

CO2 captured for the whole red mud (both solid and liquid phases) was highest at

65g CO2/kg of red mud, and the alkalinity decreased from 11,610mg/L to 2,104mg/L

as CaCO3. Stirring speeds were found to be effective in boosting the extent of red

mud carbonation and the amount of CO2 sequestration. The results showed that when

stirring speeds rose from 250rpm to 700rpm, the amount of CO2 sequestration

increased by 3.4g/kg of red mud, from 65 to 68.4g CO2/kg of red mud.

The simulation for heavy metals dissolved in long-term titration of red mud at

different pH levels of 4.5, 6, 8, 10, and 12.5 was performed using chemical

equilibrium modelling system MINEQL+ 5.0. The modelling suggested that four key

dominant metals Al, Na, Ca, and Fe were found to govern the aqueous chemistry of

the red mud carbonation process due to their presence in both soluble and solid forms

in red mud. Measured metal concentrations from long-term titration at various pH

values indicated that boehmite (AlO(OH)) and hematite (Fe2O3) did not dissolve in

the system, therefore, both Al and Fe were not responsible for the control of

carbonation process as their concentrations remained unchanged. However, Na and

Ca were considered the major solids controlling the process. The dissolution of

sodalite (Na8(AlSiO4)6(OH)2.4H2O) and cancrinite (Na6(AlSiO4)6(CaCO3)(H2O)2)

were attributable to Na and Ca concentrations in the system. The key reactions are as

below:

Na8(AlSiO4)6(OH)2.4H2O + 18H+ = 8Na+ + 6Al3+ + 6Si(OH)4 + 2H2O

Na6(Al6Si6O24)(CaCO3)(H2O)2+24H+ = 6Na++6Al3+ + 6Si(OH)4 +Ca2++CO32-+2H2O

vii

For carbonation process, a chemical model was formulated in MINEQL+ 5.0

to calculate the final equilibrium pH values for both carbonation of RM slurry and

RM liquor at different concentration of CO2. The results revealed that the simulated

pH values for the carbonation process at different PCO2 were 0.3-0.45 pH units higher

than the experimental pH values. In other words, the difference in final pH

equilibrium values between experimental and simulated carbonation of red mud

varies from 4.0-6.0%. This difference is about 2 times lower than that of previous

work done by Khaitan (2009b). The key reactions of carbonation of red mud are as

follows:

Liquid phase reactions:

2OH-(aq) + CO2(aq) ↔ CO3

2- + H2O

H2O + CO32- + CO2(aq) ↔ HCO3

-(aq) + H+

(aq)

[Al(OH4)-](aq) + CO2(aq) + Na+

(aq) ↔ Al(OH)3(s) + Na+(aq) + HCO3

-(aq)

Solid phase reactions:

Na8(AlSiO4)6(OH)2.4H2O + 18H+ ↔ 8Na+ + 6Al3+ + 6Si(OH)4 + 2H2O

Na6(AlSiO4)6CaCO3(H2O)2 ↔ 6NaAlSiO4 + CaCO3(s) + H2O

viii

TABLE OF CONTENTS

DECLARATION .......................................................................................................... i

ACKNOWLEDGEMENTS ......................................................................................... ii

ABSTRACT ................................................................................................................ iv

TABLE OF CONTENTS .......................................................................................... viii

LIST OF FIGURES .................................................................................................... xi

LIST OF TABLES ................................................................................................... xvii

CHAPTER 1 INTRODUCTION .............................................................................. 1

1.1. Background ....................................................................................................... 1

1.2. Objectives .......................................................................................................... 6

1.3. Organisation of Thesis ....................................................................................... 7

CHAPTER 2 LITERATURE REVIEW .................................................................. 8

2.1. Overview of Bauxite Residue Management ...................................................... 8

2.1.1. Bauxite Residue Generation Process .......................................................... 8

2.1.2. Physicochemical and Mineralogical Properties of Red Mud .................... 11

2.1.3. Methods Utilised for Disposal of Red Mud .............................................. 14

2.2. The Red Mud Utilisation Options ................................................................... 20

2.2.1. The Utilisations of Red Mud in Construction ........................................... 21

2.2.2. The Utilisations of Red Mud in Chemical Applications........................... 22

2.2.3. The Utilisations of Red Mud in Metallurgy.............................................. 23

2.2.4. The Utilisations of Red Mud in Agriculture ............................................. 24

2.2.5. The Utilisations of Red Mud in Environmental Treatment ...................... 26

2.3. Red Mud Neutralisation Methods ................................................................... 28

2.3.1. Neutralisation of Red Mud with Seawater ................................................ 29

2.3.2. Neutralisation with Gypsum (CaSO4.2H2O) ............................................ 30

2.3.3. Neutralisation of Red Mud by Acid Mine Wastes .................................... 31

2.3.4. Neutralisation of Red Mud Using Mineral Acid ...................................... 32

2.3.5. Neutralisation of Red Mud by Fly Ash ..................................................... 33

2.3.6. Neutralisation of Red Mud by Carbon Dioxide (CO2) Gas ...................... 33

2.3.7. Perspectives of Red Mud Carbonation ..................................................... 35

2.3.8. Mechanism of the Carbonation of Red Mud ............................................ 42

2.4. Summary ......................................................................................................... 45

ix

CHAPTER 3 MATERIALS AND METHODS .................................................... 49

3.1. Materials .......................................................................................................... 49

3.2. Materials Preparation ...................................................................................... 50

3.3. Methods ........................................................................................................... 50

3.3.1. Acid Titration Procedures ......................................................................... 50

3.3.2. Determination of Total Alkalinity of Raw RM and Carbonated RM ....... 52

3.3.3. X-ray Diffraction (XRD) .......................................................................... 53

3.3.4. Scanning Electronic Microscopy and Energy Dispersive X-ray (SEM-

EDX) ................................................................................................................... 53

3.3.5. Carbon-Hydrogen-Nitrogen Elemental Analyser ..................................... 54

3.3.6. Thermal Analysis (TGA-DSC) ................................................................. 55

3.3.7. Fourier Transform Infrared Spectroscopy (FT-IR) ................................... 55

3.4. Carbonation Experiments ................................................................................ 56

3.4.1. Construction of Reaction Chamber........................................................... 56

3.4.2. Carbonation of RM ................................................................................... 58

3.4.3. pH Rebound of the Carbonated Red Mud ................................................ 59

3.5. Chemical Equilibrium Modelling .................................................................... 59

CHAPTER 4 RESULTS AND DISCUSSIONS ................................................... 62

4.1. Acid Neutralising Capacity (ANC) of the raw RM ......................................... 62

4.1.1. Rapid Titration of RM slurry and RM liquor ........................................... 62

4.1.2. Long Term Titration of Red Mud ............................................................. 64

4.2. Carbonation of Red Mud ................................................................................. 68

4.2.1. Effect of CO2 Concentration on Carbonation of RM ............................... 68

4.2.2. Effect of Total Gas Flow Rate on Carbonation of RM ............................. 72

4.2.3. Effect of Stirring Speed on Carbonation of RM ....................................... 74

4.2.4. Effect of Solids Concentrations in RM on Carbonation of RM ............... 76

4.2.5. pH Rebound in Carbonated RM ............................................................... 78

4.2.6. Longer Carbonation of RM....................................................................... 80

4.3. Mineralogical Characterisation of Red Mud and Carbonated Red Mud ......... 84

4.3.1. X-ray Diffraction Analysis ....................................................................... 84

4.3.2. Micro-morphological Characterisation of raw RM and Carbonated RM by

SEM .................................................................................................................... 96

4.3.3. Chemical Composition Changes by EDX ................................................ 99

4.3.4. Determination of Alkalinity of RM and Carbonated RM ....................... 111

x

4.3.5. Thermal Analysis using TGA-DSC ........................................................ 113

4.3.6. FT-IR Spectroscopy ................................................................................ 115

4.4. Determination of CO2 Sequestration ............................................................. 116

4.4.1. Determination of CO2 sequestered in 2-hour carbonation of RM .......... 116

4.4.2. Determination of CO2 sequestered in 5-day carbonation of RM ............ 118

4.5. Modelling of Carbonation Process ................................................................ 122

4.5.1. Modelling of potentially dissolved metals .............................................. 123

4.5.2. Modelling of RM carbonation ................................................................ 130

4.6. Summary ....................................................................................................... 136

CHAPTER 5 FINDING OUTCOMES AND CONCLUSIONS .......................... 138

5.1. Major Findings of This Research .................................................................. 138

5.1.1. Acid Neutralisation Capacity (ANC) of Red Mud ................................. 138

5.1.2. Carbonation of Bauxite Residue ............................................................. 139

5.1.3. Modelling of the Carbonation Process.................................................... 142

5.2. Conclusions ................................................................................................... 144

CHAPTER 6 RECOMMENDATIONS FOR THE FUTURE WORK ................ 146

REFERENCES ......................................................................................................... 147

APPENDIX .............................................................................................................. 165

xi

LIST OF FIGURES

Figure 1.1. Global production rate and cumulative inventory ..................................... 2

Figure 2.1. Schematic of a general Bayer process ....................................................... 9

Figure 2.2. Lagooning red mud disposal .................................................................... 16

Figure 2.3. Schematic of dry stacking system............................................................ 19

Figure 2.4. A possible flowsheet for recovery of Fe, Al, and Ti from bauxite residue

.................................................................................................................................... 24

Figure 3.1. Carbonation reaction chamber ................................................................. 57

Figure 3.2. The experimental apparatus system for carbonation of RM .................... 58

Figure 4.1. Rapid RM liquor titration compared with that of RM slurry (44%wt).... 63

Figure 4.2. Long-term titration of RM ....................................................................... 64

Figure 4.3. XRD pattern of raw RM overlapped with titrated RM at pH 6 ............... 67

Figure 4.4. XRD pattern of raw RM overlapped with titrated RM at pH 4.5 ............ 67

Figure 4.5. Carbonation of RM slurry at different CO2 concentrations, fixed TF of

200mL/min and stirring speed of 250rpm .................................................................. 69

Figure 4.6. Carbonation of RM liquor at different CO2 concentrations, fixed TF of


Figure 4.7. Comparison of carbonation between RM slurry and RM liquor at some

different CO2 concentrations, fixed TF of 200mL/min and stirring speed of 250rpm

.................................................................................................................................... 70

Figure 4.8. Carbonation rate constant (k) for both RM slurry and RM liquor at

different CO2 concentration, total gas flow rate 200mL/min and speed 250rpm ...... 72

Figure 4.9. Carbonation of red mud by 30% of CO2, 250rpm and different TF of gas

.................................................................................................................................... 73

Figure 4.10. Carbonation rate constant (k) for RM slurry at 30% CO2 concentration,

stirring speed 250rpm, and different total gas flow rate ............................................ 74

xii

Figure 4.11. Carbonation of red mud by 30% of CO2, TF of 200mL/min and different

stirring speeds ............................................................................................................ 75

Figure 4.12. Rate constant (k) for carbonation of RM slurry by 30% CO2

concentration, TF of 200mL/min and different stirring speeds ................................. 76

Figure 4.13. Carbonation of red mud by 30% of CO2, TF of 200mL/min and stirring

speed of 250rpm, and different solids concentrations in RM .................................... 77


concentration, TF of 200mL/min, and different solids concentrations in RM........... 78

Figure 4.15. pH rebound for both RM slurry and liquor at three CO2 concentrations,

TF of 200mL/min, stirring speed of 250rpm ............................................................. 79

Figure 4.16. pH rebound of carbonated RM slurries at different solids concentrations

.................................................................................................................................... 80

Figure 4.17. Longer carbonation of RM slurry at different CO2 concentrations ....... 81

Figure 4.18. Longer carbonation of RM slurry at fixed 30% CO2, stirring speed of

250rpm and at different total gas flow rate ................................................................ 82

Figure 4.19. Longer carbonation of RM slurry at fixed 30% CO2, total gas flow rate

of 200mL/min and different stirring speeds ............................................................... 83


of 200mL/min, stirring speeds of 250rpm and different solids concentrations of RM

.................................................................................................................................... 83

Figure 4.21. Variation of powder XRD pattern of raw RM ....................................... 84

Figure 4.22. Phase composition quantification of raw RM ....................................... 85

Figure 4.23. XRD pattern of carbonated RM compared with raw RM...................... 86

Figure 4.24. Phase composition quantification of carbonated RM at 15% CO2

concentration and total gas flow rate of 200mL/min ................................................. 88





xiii



Figure 4.28. Phase composition quantification of carbonated RM at fixed 30% CO2







concentration and stirring speed of 350rpm ............................................................... 94





Figure 4.34. SEM imaging of raw RM: (a) Sodalite in “cotton ball” form, and (b)

Structure of crystalline sodalite .................................................................................. 96

Figure 4.35. SEM imaging of carbonated RM at different CO2 concentration, TF of


Figure 4.36. The amounts of C and CO2 absorbed by RM after 2-hour carbonation at

different CO2 concentration, TF of 200mL/min and stirring speed of 250rpm ....... 101

Figure 4.37. Amounts of C and CO2 absorbed by RM after 5-day carbonation ...... 103

Figure 4.38. Amounts of C and CO2 absorbed by RM at a given 30% CO2

concentration, 250rpm and different TF of gas ........................................................ 105

Figure 4.39. Amounts of C and CO2 absorbed by RM at given 30% CO2, TF of

200mL/min and different stirring speeds ................................................................. 106

Figure 4.40. Amounts of C and CO2 captured by RM in different solids

concentrations .......................................................................................................... 109

xiv

Figure 4.41. Comparison of amounts of C and CO2 captured between 2-hour and 5-

day carbonation at fixed TF of 200mL/min, 250rpm and different CO2 concentrations

.................................................................................................................................. 111

Figure 4.42. Changes in HCO3-, CO3

2-, and OH- alkalinity in raw RM and carbonated

RM at different concentrations of CO2, TF of 200mL/min, 250rpm ....................... 112

Figure 4.43. Acid titration curves for a) Raw RM and b) Carbonated RM at 30%

CO2, TF of 200mL/min and stirring speed of 250rpm ............................................. 113

Figure 4.44. TGA-DSC plots indicating weight loss of RM ................................... 114

Figure 4.45. TGA-DSC plots indicating weight loss of carbonated RM ................. 115

Figure 4.46. Fourier Transform Infrared (FT-IR) spectra of RM and carbonated RM

.................................................................................................................................. 116

Figure 4.47. Amounts of CO2 sequestered by RM after 2-hour carbonation at

different CO2 concentrations, stirring speed of 250rpm .......................................... 117

Figure 4.48. Amounts of CO2 captured by RM (A): solid, (B): liquor, after 5-day

carbonation at different CO2 concentrations, TF of 200mL/min and speed of 250rpm

.................................................................................................................................. 119

Figure 4.49. Comparison of CO2 amounts captured between 2-hour and 5-day

carbonations ............................................................................................................. 120

Figure 4.50. Amounts of CO2 captured by RM carbonated 30% CO2 concentration,

TF of 200mL/min and at different stirring speeds ................................................... 120

Figure 4.51. Amounts of CO2 captured by RM with different solids concentrations

carbonated at 30% CO2 concentration, TF of 250mL/min and 250rpm .................. 121

Figure 4.52. Metal concentrations in RM liquor as a function of pH ...................... 124

Figure 4.53. Comparison of simulated and experimental carbonation of RM liquor at

different CO2 concentration and TF of (A): 100mL/min, (B): 200mL/min ............. 133


different CO2 concentration and TF of (C): 300mL/min, (D): 400mL/min ............. 133

Figure 4.55. Comparison of simulated and experimental carbonation of RM slurry at

different CO2 concentration and TF of (A): 100mL/min, (B): 200mL/min ............. 134

xv


different CO2 concentration and TF of (C): 300mL/min, (D): 400mL/min ............. 134

Figure B-1. Carbonation of RM slurry at different CO2 concentrations, fixed TF of

100mL/min and stirring speed of 250rpm ................................................................ 172





Figure B-4. Carbonation of RM liquor at different CO2 concentrations, fixed TF of






Figure B-7. Carbonation of RM by 25% CO2 at different TF of gas ....................... 174



Figure B-10. Carbonation of red mud by 30% of CO2, TF of 100mL/min at different

stirring speeds .......................................................................................................... 175


stirring speeds .......................................................................................................... 175


stirring speeds .......................................................................................................... 175

Figure B-13. Carbonation of red mud by 30% of CO2, TF of 100mL/min, stirring

speed 250rpm at different solid concentrations of RM ............................................ 176





xvi









Figure B-20. pH rebound for both RM slurry and liquor at some CO2 concentrations,

TF of 200mL/min, stirring speed of 250rpm ........................................................... 178

Figure C-1. Phase composition quantification of carbonated RM at 20% CO2

concentration, total gas flow rate 200mL/min ......................................................... 200

Figure C-2. Phase composition quantification of carbonated RM at 50% CO2

concentration, total gas flow rate 200mL/min ......................................................... 201

xvii

LIST OF TABLES

Table 2.1. Chemical constituents of red mud in different locations (%wt) ............... 12

Table 2.2. Buffering common reactions in aqueous solution of bauxite residue ....... 13

Table 2.3. Dissolution reactions of common buffering solids present in bauxite

residues ....................................................................................................................... 14

Table 2.4. Summary of CO2 amount captured in previous studies on RM carbonation

.................................................................................................................................... 41

Table 2.5. Reactions taking place in the carbonation of red mud .............................. 43

Table 3.1. Major mineral composition of raw RM .................................................... 49

Table 3.2. Concentration of raw RM and liquor ........................................................ 61

Table 4.1. Comparison between rapid and long term ANC for RM .......................... 65

Table 4.2. Metal concentrations in RM liquor at different pH values ....................... 66

Table 4.3. Effect of CO2 concentrations on the composition of solid phase in

carbonated RM as quantified by XRD ....................................................................... 88

Table 4.4. Effect of total gas flow rate on the composition of solid phase in

carbonated RM as quantified by XRD ....................................................................... 92

Table 4.5. Effect of stirring speed on the composition of solid phase in carbonated

RM as quantified by XRD.......................................................................................... 94

Table 4.6. Major elemental composition (%w/w in average) of RM and carbonated

RM at different concentrations of CO2, TF of gas 200mL/min, stirring speed 250rpm

.................................................................................................................................... 99

Table 4.7. Major compound composition (%w/w in average) of RM and carbonated


.................................................................................................................................. 101


RM at 15%-60% CO2, TF of 200mL/min, 250rpm in 5 days of carbonation .......... 102


RM at 15%-60% CO2, TF of 200mL/min, 250rpm in 5 days of carbonation .......... 103


RM at 30% CO2 concentration, 250rpm and different total gas flow rate ............... 104

xviii


RM at 30% CO2 concentration, 250rpm and different total gas flow rate ............... 105

Table 4.12. Major element composition (%w/w in average) of RM and carbonated

RM at 30% CO2, TF of 200mL/min and different stirring speeds ........................... 107


RM at 30% CO2, TF of 200mL/min and different stirring speeds ........................... 107


RM at 30% CO2, TF of 200mL/min, 250rpm and different solids concentrations .. 108


RM at 30% CO2, TF of 200mL/min, 250rpm and different solids concentrations .. 109

Table 4.16. Concentration of raw RM and liquor .................................................... 123

Table 4.17. Solid precipitation/dissolution reactions in red mud model.................. 125

Table 4.18. Solid dissolution/precipitation and liquid reactions in RM simulation 131

Table A-1. Rapid titration of RM by 0.1N HCl ....................................................... 165

Table A-2. Rapid titration of RM liquor to pH 4.5 by 0.1N HCl ............................ 166

Table A-3. Long-term titration of RM to pH 4.5 by 0.1N HCl................................ 167



Table A-6. Long-term titration of RM to pH 10 by 0.1N HCl................................. 170

Table A-7. Metal concentrations in RM liquor measured at different pH values .... 171

Table A-8. Simulated metal concentrations in RM liquor at different pH values ... 171

Table B-1. Carbonation of RM at different CO2 concentrations and total gas flow rate

of 100mL/min, stirring speed of 250rpm ................................................................. 179









xix







Table B-9. Carbonation of RM by 30% CO2 concentrations, stirring speed of 250rpm

and different total gas flow rate ............................................................................... 187

Table B-10. Carbonation of RM by 30% CO2 concentrations, total gas flow rate of

200mL/min and different stirring speeds ................................................................. 188

Table B-11. Carbonation of RM by 30% CO2 concentrations, TF of 200mL/min,

speeds of 250rpm and different solids concentrations in RM .................................. 189

Table B-12. Longer carbonation of RM at 15% - 30% CO2 concentrations, TF of


Table B-13. Longer carbonation of RM at 40% - 60% CO2 concentrations, TF of


Table B-14. Longer carbonation of RM at by 30% CO2 concentrations, stirring speed

of 250rpm and different total gas flow rate .............................................................. 191

Table B-15. Longer carbonation of RM at by 30% CO2 concentrations, stirring speed

of 250rpm, TF of 200mL/min and different solids concentrations in RM............... 191

Table B-16. Carbonation of RM liquor at different CO2 concentrations, total gas flow

rate of 100mL/min and stirring speed of 250rpm .................................................... 192











xx





Table D-1. Simulated carbonation of RM at different CO2 concentrations and total

gas flow rate of 100mL/min ..................................................................................... 202







1

CHAPTER 1 INTRODUCTION

1.1. Background

Bauxite residue, commonly called red mud (RM), is a principal waste products

created from the Bayer process in the alumina production. It is composed of hydrous

muddy silt combined with very high alkaline solid waste with a pH in the range of

10.5-13 (Borges et al. 2011). This cocktail is generated by physical and chemical

treatments of bauxite in the alumina production (György & Tran 2008; Zhang et al.

2001). The worldwide alumina industry has experienced significant growth over past

decades and many producing countries are now coping with severe problems in

managing bauxite residues. The quantity of RM has been increasing alarmingly due

to the rapid increases in the global demand of aluminium metal for the development

of construction, packaging, and transportation.

Normally, the production of 1 tonne of alumina results in the generation of 1-

1.5 tonnes of RM (Brunori et al. 2005; Kumar et al. 2006). However, depending on

the efficiency of alumina extraction and quality of bauxite source, the quantity of

waste generated may vary from 0.3 to 2.5 tonnes for the high-grade or for very low

grade bauxite, respectively (Borges et al. 2011; Kalkan 2006; Nguyen & Boger

1998; Paramguru et al. 2005; Sushil & Batra 2008). Consequently, the volume of

bauxite residue waste has grown exponentially. In 1940, the inventory of bauxite

waste was 22 million tonnes associated with the annual production rate of 1 million

tonnes of aluminium metal. By 1985, the generation rate of the waste grew to

roughly 48.5 million tonnes per annum, and the global inventory of red mud reached

2

1 billion tonnes (Klauber et al. 2011; Power et al. 2009, 2011). In 2007, it was

predicted that approximately 120 million tonnes of red mud were produced per year

and the inventory of the waste had grown to 2.7 billion tonnes (Fig. 1.1) (Clark et al.

2009; Klauber et al. 2011; Power et al. 2011). Of greater significance was the rapid

increase in the rate of waste generation. It can be seen that the first billion tonnes of

RM were produced within nearly a century, but the second billion tonnes were

accumulated approximately 15 years.

Figure 1.1. Global production rate and cumulative inventory (Power et al. 2011)

Future predictions suggest that approximately 3 billion tonnes of RM will be

generated in the period from 2010 to 2015 (Power et al. 2009, 2011). The bauxite

residue tonnage was often estimated by applying an overall ratio of 1.5 to alumina

production (Power et al. 2011; Sutar et al. 2014). In 2012, the global alumina

production as reported by Dentoni et al. (2014) reached 90.17 million tonnes. It is

projected that the annual amount of bauxite residue will be generated about 135

million tonnes worldwide, thus, the global production of red mud could possibly

reach over 5 billion tonnes in 2030. This mass of bauxite residue will pose a

significant environmental problem as the RM is highly alkaline (pH>13), fine-

3

grained in nature (more than 90% is under 10µm), and contains elevated

concentrations of sodium (over 50g/kg of RM) (Johnston et al. 2010). Clearly,

appropriate management methods and utilisation practices must be devised to

improve this problem. Unfortunately, there are no acceptable solutions for dealing

with the RM in terms of its management and its potential utilisation. Treating the

waste would lead to a significant penalty to production cost. As a result, it seems that

the RM disposal problem may be ignored by the industry, the public and

governments (Power et al. 2009, 2011).

The most important constraints to the management, utilisation, and remediation

of RM are its very high level of sodicity and alkalinity, and the complex physical and

chemical properties. These properties result in the high impedance of reusing red

mud. One of the efforts amongst the several pH-reduction processes to be

incorporated is neutralisation by acid. To determine the amount of acid needed for

neutralisation, the acid neutralisation capacity (ANC) of red mud must be measured.

In addition, the contributions of both solid and liquid phases to the ANC must be

identified. This process is not only challenging due to the complicated chemical

constitutes in red mud, but because of slow reactions taking place between the acid

and the solids. The ANC of red mud was measured by rapid and long-term acid

titrations. Data from the titration of red mud slurries was fitted by using chemical

equilibrium modelling to identify the aqueous and solid reactions in the system.

Furthermore, metal concentrations from long-term titrations at pH endpoints of 4.5,

6, 8 and 10 were determined to confirm the dissolution/precipitation of Na, Al, Ca

and Fe corresponding to their solid phases (Khaitan, et al. 2009a).

4

Another promising effort is the use of industrial wastes such as CO2 emission

for solving the problems of other wastes. The capture and storage of CO2 by RM

could play a significant role in mitigating the release of greenhouse gases to the

environment. It is estimated that one third of all CO2 emissions due to human’s

activities derive from fossil fuels used for power plants, which annually emit several

million tonnes of CO2 (Rai 2013). The aluminium industry is of special interest to the

global warming mitigation strategy as it is one of the largest energy consumers with

approximately 14MWh/ton of aluminium required for the aluminium refining

process.

The use of CO2 from the atmosphere or from industrial emissions is a

potentially significant source of acid for neutralizing red mud. Up to date, a variety

of solutions have been conducted for neutralising RM by aqueous CO2, (Enick et al.

2001; Shi et al. 2000), SO2 from flue gases (Fois et al. 2007), seawater treatment

(Cooling 2007; Hanahan et al. 2004), Mg and Ca rich salts, acid treatment

(Bonenfant et al. 2008; Khaitan et al. 2009b). Also, CO2-containing emission gas or

CO2 gas phase can be bubbled through red mud slurries to form the carbonic acid in

the aqueous phase that would react with basic components of red mud lowering the

pH of slurries (Szirmai et al. 1991). The extent of neutralisation of red mud by CO2

gas as a function of CO2 partial pressure has been measured to identify the

geochemical reactions responsible for carbon sequestration (Khaitan et al. 2009b).

Carbonation of red mud was observed to be affected by operating parameters.

Cardile et al. (1994) characterised the influence of temperature, gas flow rate, stirring

rate, total sodium content and slurry density by using factorial experiment design.

These operating factors can help to optimise the carbonation process.

5

Although the pH of the aqueous phase decreased quickly from a pH of 12-13 to

7-8 when red mud contacting with CO2 gas, it soon rose again to unacceptable levels

because of additional alkaline material leaching from red mud. In a study of red mud

carbonation by CO2 (Rai 2013), the pH was observed to rebound back to 9.5 each

time the red mud was neutralised in the multiple cycle experiments. In another study

of red mud neutralisation using CO2 sequestration cycle (Sahu et al. 2010), the pH of

carbonated slurry rebound slowly for few days after that it remained at 8.45. The pH

recovery back to 9.5 was also observed when red mud was carbonated by pure CO2

for 14 hours (Cardile et al. 1994). Additionally, the phenomenon of pH rebound was

found to be inversely proportional to the partial pressure of CO2 used for carbonation

of red mud (Khaitan et al. 2009b).

This research aims to investigate the RM neutralisation by carbon dioxide gas

over a range of different operating conditions such as CO2 concentrations, total gas

flow rate, stirring speeds and concentrations of RM. Furthermore, it was to identify

the equilibrium pH reached for different concentrations of CO2 and to understand the

potential for carbon sequestration by red mud. Liquid and solid phase reactions that

control the carbonation of red mud slurries were investigated with the assistance of

chemical equilibrium modelling. The final equilibrium pH values affected by

different concentrations of CO2 during carbonation were also simulated and

compared to the experimental values. Finally, the calculation of CO2 volume

permanently captured by RM should also be examined.

6

1.2. Objectives

Nowadays, global warming and climate change have been becoming the most

challenging environmental problems as the atmospheric CO2 concentration would be

reaching from 500-700ppm by the end of the 21st century (IPCC 2014). Global

warming is of special interest to alumina industry because it requires a high demand

of energy and disposes of large amounts of CO2 emissions and waste red mud.

Annually, direct atmospheric emissions of greenhouse gases such as CO2 from

bauxite refinery and aluminium smelting have been estimated worldwide at 110

million tonnes of carbon dioxide (Martcheck 2003). Additionally, approximately 135

million tonnes of red mud have been generated per year from the Bayer process in

over the world by applying the ratio of 1.5 to alumina production (Sutar et al. 2014).

The disposal of this waste is often expensive and may cause negative impacts on the

environment. Therefore, the reuse of red mud as an important material for the capture

of CO2 could potentially make a significant contribution to the global warming

mitigation. This research aims to focus on the investigation of neutralisation of red

mud using CO2 gas with the following objectives.

1. To investigate the acid neutralisation capacity (ANC) of red mud and

evaluate the contribution extent of liquid phase and solid phase to the ANC of

red mud.

2. To characterise the carbonation of red mud over a range of different operating

conditions such as concentrations of CO2, total flow rate of gas, stirring

speeds and concentrations of red mud in order to optimise the carbonation

process.

7

3. To characterise the materials using a range of techniques such as XRD, SEM,

EDX, FT-IR, TG-DSC, ICP-MS, CHN, and Total Organic Carbon (TOC)

Analyser.

4. To determine and evaluate the effect of operating parameters on the

carbonation process, then suggest the optimal condition for the carbonation

process.

5. To simulate the effects of different concentrations of CO2 on the equilibrium

pH values, then compared to the experimental values.

6. To calculate the potential amount of CO2 captured by the red mud over the

range of these conditions.

1.3. Organisation of Thesis

The thesis consists of 6 chapters. Chapter 1 describes background to the

problems studied. Chapter 2 provides a comprehensive literature review of the

process of bauxite residue generation and the properties of RM. In addition, this

chapter addresses current methods of RM management and treatment. Research

methodology and techniques deployed to conduct the research are detailed in Chapter

3. Chapter 4 presents and discusses the results and outcomes of the research. Chapter

5 summarises the findings and conclusion of the project and scope for future work is

detailed in Chapter 6.

8

CHAPTER 2 LITERATURE REVIEW

2.1. Overview of Bauxite Residue Management

2.1.1. Bauxite Residue Generation Process

Bauxite residue or red mud is produced as an undesirable by-product during the

extraction of alumina from bauxite ore in the Bayer process as shown in Figure 2.1.

This process was patented by Karl Josef Bayer in the period from 1887 to 1892

(Power et al. 2011; Sparks 2010). Now, the process becomes dominant for the

production of alumina because of its unexpected success. Though this process

contributes significantly to the alumina processing industry, it results in an

unavoidable consequence. The key issue is the generation of the very large amount

of difficult to treat waste. This waste could provide a source of valuable metals if an

economic treatment could be devised. The exponential increase in the RM produced

was due to the development of the Hall-Heroult process for smelting alumina to

aluminium in the 19th century (Klauber et al. 2009).

As can be seen from Figure 2.1, the Bayer process includes a variety of

processes that treat bauxite feed to produce RM waste and product liquor. Generally,

the Bayer process consists of bauxite milling, pre-desilication, digestion,

clarification, and of counter-current decantation (CCD) washing or thickener series.

However, before discharging the RM into a bauxite residue disposal area, a further

thickening or filtration step was employed. Each of these unit processes has an

impact on the chemical composition of the residue, as well as their physical and

mineralogical properties of red mud (Power et al. 2009).

9

Figure 2.1. Schematic of a general Bayer process (Power et al. 2011)

In the Bayer process, gibbsite (Al(OH)3) and/or boehmite (AlOOH) from

bauxite were extracted by dissolving these minerals in the concentrated NaOH

solution. The elemental composition, expressed in terms of oxide, of Al, Si, Ti, Fe,

Na and Ca, in the red mud varies from places to places as shown in Table 2.1. This

could be due to a difference in the chemical composition of the bauxite used as feed.

To minimise silica contamination of the product, lime (Ca(OH)2) may be added in

the pre-desilication stage, just before the digestion stage, to form insoluble

cancrinite. Sodalite is the most common desilication products formed during the pre-

desilication process, whereas cancrinite might be formed in the presence of Ca

during the digestion of boehmite-rich bauxite with high temperature. The

10

concentrations of sodalite and cancrinite were identified to have value of 16-24%

(Castaldi et al. 2008), and up to 50% (Garau et al. 2007), respectively in bauxite

residues from Eurallumina plant processing Weipa bauxite. However, these sodalite

and cancrinite were often disposed with the bauxite residues. These products

contributed significantly to the acid neutralisation capacity of the red mud (Gräfe &

Klauber 2011). Other additives such as iron oxides (Cook 1970), MgSO4, apatite

solids (Bernard et al. 2007), and other unwanted impurities were either separated

from the NaAl(OH)4 rich solution, to be used for specific purposes or be disposed of

with the bauxite residues.

The management of red mud starts from the separation process of green liquor

after digestion (Power et al. 2011). The residue will, then, be transferred to a series

of washing units called counter-current decantation (CCD), where the residue will be

washed and NaOH and Al(OH)4- will be recovered for recycling back to the process.

After washing in the CCD, the collected waste is often subjected to a further

thickening or filtration process to increase the solids content before being discharged

into the disposal area (Power et al. 2011).

Basically, bauxite residue comprises essentially insoluble compounds of Fe, Ti

and some un-digested soluble alumina minerals and others (György & Tran 2008).

Bauxite residue also consists of the quantity of coarse particles called “sand”. The

amount of sand can range from <1% to about 50% depending on the bauxite source,

but the average is about 5% (György & Tran 2008). The large quantity of RM

generated from the Bayer process is usually discharged directly into the environment

via a tailing dam without further treatment. This direct disposal method, being cheap

and convenient, will pose an adverse impact on the ecosystem because of high

alkalinity level of the RM.

11

2.1.2. Physicochemical and Mineralogical Properties of Red Mud

Red mud is a very fine material with a particle size distribution, typically

varying from 1 to 150µm (over 90% by volume is <10µm) (Clark et al. 2009;

Johnston et al. 2010). It has a very low settling rate (1.4x10-6-3.6x10-3m/s), a range

of density of 2.6x103-3.5x103kg/m3 (György & Tran 2008), and a high surface area

(Sushil & Batra 2008). The pH of RM varies from 10 to 13.5, therefore, it is a highly

caustic waste, which can harm the environment (György & Tran 2008; Johnston et

al. 2010; Sushil & Batra 2008).

Red mud consists of a mixture of metal oxides, which were originally present

in bauxite ore and may be created in the Bayer process (Agrawal et al. 2004; Singh et

al. 1997; Sushil & Batra 2008). Depending on the bauxite source and the parameters

of digestion process, the chemical and mineralogical properties of RM can vary

significantly (Altundogan et al. 2000; Bertocchi et al. 2006; Cengeloglu et al. 2003;

Collazo et al. 2005; Gong & Yang 2000; György & Tran 2008; Koumanova et al.

1997; Park & Jun 2005; Pontikes et al. 2007; Tsakiridis et al. 2004; Yalcin & Sevinc

2000). The main compositions of RM comprise a significant quantities of metal

oxides such as Fe2O3, Al2O3, SiO2, TiO2, Na2O, CaO, MgO and several other minor

elements like K, Cr, V, Ni, Cu, Zn, Mn, etc. (Singh et al. 1997; Sushil & Batra

2008). Table 2.1 shows the chemical compositions of RM produced in various plants.

The mineralogical composition of bauxite determined the preferred reaction

temperature, pressure, and concentration of NaOH to be employed in the extraction

process (Gräfe et al. 2011). These preferred reaction conditions in the digestion often

lead to the consequence of mineralisation in the Bayer process, which is related to

12

the common equilibrium reactions of liquid and solids in bauxite residues listed in

Tables 2.2 & 2.3.

Table 2.1. Chemical constituents of red mud in different locations (%wt) (adapted

from György & Tran (2008))

Weipa

(Australia)

Trombetas

(Brasil)

South

Manchester

(Jamaica)

Darling

Range

(Australia)

Iszka

(Hungary)

Parnasse

(Greece)

Digestion

temperature 2400C 1430C 2450C 1430C 2400C 2600C

Al2O3 17.2 13.0 10.7 14.9 14.4 13.0

SiO2 15.0 12.9 3.0 42.6 12.5 12.0

Fe2O3 36.0 52.1 61.9 28.0 38.0 41.0

TiO2 12.0 4.2 8.1 2.0 5.5 6.2

Na2O 9.0 9.0 2.3 1.2 7.5 7.5

CaO - 1.4 2.8 2.4 7.6 10.9

L.O.I 7.3 6.4 8.4 6.5 9.6 7.1

Others 3.5 1.0 2.8 2.4 4.9 2.3

Buffering reactions shown in Tables 2.2 & 2.3 represent the ability of the

solids in red mud to maintain the concentration of alkaline anions in the solution. The

solids that are likely soluble to some degree, and H+ acceptance by the alkaline anion

in the solution are necessary for the buffering reactions taking place. At pH>10.2,

Ca2+ species is absent as it is lost to the formation of insoluble calcite (CaCO3), and

therefore, Na2CO3 governs the concentration of HCO3-/CO3

2- in the solution. Apart

from that, there are some major alkaline anions existing in red mud such as HCO3-

/CO32-, Al(OH)4

- and OH- that are responsible for the buffering property.

Furthermore, some systems H2SiO42-/H3SiO4

-/H4SiO4 and PO43-/HPO4

2-/H2PO4-,

which are at lower concentrations, may also help to buffer the pH of solution as well.

As shown in Table 2.2, the pH region between two pKa values is the buffering

region, for instance the buffer region of HCO3- is around pH of 8.3 (the pH average

of 10.2 and 6.35), and the precipitation of gibbsite Al(OH)3 occurs rapidly at pH

below 10 (Gräfe et al. 2011).

13

Table 2.2. Buffering common reactions in aqueous solution of bauxite residue

(adapted from (Stumm & Morgan 1981))

Reaction Acidity constants

OH− + H3O + ⇆ 2H2O

Al(OH)4−·2H2O + H3O+ ⇆ Al(OH)3·3H2O(s) + 2H2O

CO32− + H2O ⇆ HCO3

− + OH−

HCO3− + H3O+ ⇆ H2CO3 + OH−

H2SiO42− + H2O ⇆ H3SiO4

–

H3SiO4− + H2O ⇆ H4SiO4

PO43− + H2O ⇆ HPO4

2− + OH−

HPO42− + H2O ⇆ H2PO4 + OH−

H2PO4− + H3O+ ⇆ H3PO4 + OH−

pKw = 14.0

pKa4 ~10.2

pKa2 = 10.2

pKa1 = 6.35

pKa2 = 12.95

pKa1 = 9.85

pKa3 = 12.35

pKa2 = 7.2

pKa1 = 2.25

The introduction of slaked lime (Ca(OH)2) at different stages before, during

and after digestion results in the formation of Bayer process characteristic solids

(BPCSs) (Whittington 1996). These solids impart a significant buffering capacity to

the red mud solutions via a number of reactions as listed in Table 2.3. The abundance

of these solids relies on the conditions of bauxite processing. Therefore, the addition

of Ca(OH)2 helps not only enhance the overall extraction of gibbsite and/or boehmite

from bauxite, but also reduce the formation of sodalite by instead transforming some

to cancrinite. Furthermore, the introduction of Ca(OH)2 also favours the formation of

TCA or hydrogrossular by the exchange of 4OH- for one SiO44-, although this

substitution reaction seems limited (Whittington 1996). As shown in Table 2.3, it is

impossible to evaluate the buffering capacity of most the solids from their reactions

because of the paucity of dissolution constants and/or solubility products.

Nevertheless, the BPCSs must be responsible for the buffering effect because the

oxide minerals of Fe, Al, Ti and Si are not the buffering solids (Gräfe et al. 2011).

14

Table 2.3. Dissolution reactions of common buffering solids present in bauxite

residues (adapted from (Greenberg & Chang 1965; Greenberg et al. 1960; Stumm &

Morgan 1981; Vieillard & Rassineux 1992))

Dissolution reaction Solubility constants

Natron-decahydrate

Na2CO3·10H2O(s) + H2O ⇆ 2Na+ + HCO3− + OH− + 10H2O

Calcite

CaCO3(s) ⇆ Ca2+ + CO32-

Hydrocalumite

Ca4Al2(OH)12·CO3·6H2O + 7H2O ⇆ 4Ca2+ + 2Al(OH)3(aq) +

HCO3− + 7OH− + 6H2O

Tri-calcium aluminate (TCA or hydrogrossular, n = 0)

Ca3Al2[(OH)12−4n](SiO4)n(s) + H2O ⇆ 3Ca2+ + 2Al(OH)3 + 6OH−

Hydroxysodalite

Na6[Al6Si6O24]∙2NaOH + 24H2O ⇆ 8Na+ + 8OH− + 6Al(OH)3 +

6H4SiO4

Cancrinite

Na6[Al6Si6O24]∙2CaCO3 + 26H2O ⇆ 6Na+ +

2Ca2++ 8OH− + 2HCO3− + 6Al(OH)3 + 6H4SiO4

pKsp = 1.31

pKsp = 8.42 (6.2)

pKsp = n/d

pKsp = n/d

pKsp = n/d

pKsp = n/d

2.1.3. Methods Utilised for Disposal of Red Mud

Prior to the 1970s, two major conventional wet disposal methods were utilised

in most alumina refineries, namely marine discharge and lagooning (Klauber et al.

2011). Generally, the marine disposal practice sounds simpler than the lagooning in

terms of economy and technology. However, these practices make a significant

contribution to the management of RM at that time.

2.1.3.1. Marine Disposal Practice

Marine discharge method was employed in France, Greece, the USA, Australia

and Japan for several years given the neutralisation capacity of seawater to the

causticity of RM (György & Tran 2008). During marine disposal, the RM slurry

15

produced from the Bayer process is directly discharged into the deep ocean via the

pipeline system. Although reports on environmental impact assessment revealed no

negative impacts on the coastal ecosystem at the sedimentation areas in Japan, this

disposal method has not been encouraged by the United Nations Industrial

Development Organisation (UNIDO 1985). Consequently, a transition from ocean or

river disposal to lagooning or alternative methods has been adopted, and the

available literature revealed that no refineries established after 1970 employed the

marine disposal method. Currently, 2-3% of alumina production worldwide use

marine discharge for the treatment of red mud (Power et al. 2011).

The Gramercy plant in Louisiana in the USA dumped its red mud into the

Mississippi river until 1974, when it adopted lagooning for waste storage and

subsequent treatment. The transition removing RM from the river and relocating it to

land-based impoundments was voluntarily initiated by the Kaiser Company

(Kirkpatrick 1996). Some Japanese refineries consider marine discharge a main

method due to the limitation of land area available for disposal. Nevertheless, these

refineries made a commitment to International Marine Organisation to stop dumping

their RM waste into the ocean by 2015, and will conduct other alternative methods

for disposal of red mud (International Marine Organisation 2005). Similarly,

refineries at Gardanne in France, which still utilise marine disposal method, are also

scheduled to stop discharging their waste into the sea by 2015 (Martinet-Catalot et

al. 2002).

2.1.3.2. Lagooning Method

The use of lagoon is now normally practised for dumping red mud (György &

Tran 2008). The bauxite residue slurry is directly discharged into the land-based

16

impoundments (Power et al. 2011) called a red mud pond (Fig. 2.2). The practice

requires increased engineering input considering topography, linings and material’s

issues, and construction complexity, etc. when compared to the marine option, but it

is now widely applied in many refineries in the world. These requirements aim to

reduce potential leakage of caustic and alkaline water into the soil and ground water.

Unfortunately, in many plants built before 1960, there was often no special sealing

applied at the bottom of the pond. This resulted in the contamination of the

surrounding soils and ground water, with consequent negative impacts on human’s

health (György & Tran 2008).

Figure 2.2. Lagooning red mud disposal (György & Tran 2008)

It is suggested that, the best lagooning method is to line the pond with a single

compacted clay bed or multiple ones of up to 300-400mm thick in order that the red

mud can be separated from the original soil or rock of the pond (György & Tran

2008; Liu et al. 2009). Moreover, another security solution can be employed by

utilising multiple layers of plastic or geo-membrane materials to create a seal

between the red mud and the supporting clay bed (Cooling 1989).

17

Nevertheless, previous studies have noted that the caustic soda content of the

bauxite residue reacts with the clays over several decades to form amorphous sodium

aluminium hydro silicates and ultimately zeolite through a series of complex

reactions (Gerrise & Thomas 2008). These reactions increase the hydraulic

conductivity of the clay bed, resulting in potential contamination of the aquatic

system after several decades. These problems could be reduced by neutralising the

RM slurry before discharging, for example, by using mineral acids such as sulphuric

acid, or by mixing the slurry with seawater (Power et al. 2009, 2011).

Generally, lagooning method is the simplest land-based disposal that is

globally utilised. However, this practice is still dependent on good engineering

practice for residue storage; hence, it may lead to the high cost of operation. In

addition, this method is often more problematic due to its high risk of leakage,

liquefaction, and instability. The statistics show that around 103 major tailings dam

failures have been recorded worldwide since 1960, leading to at least 1,838 human

deaths and untold environmental degradation (WISE 2015). The newest red mud

problem that tragically happened in Hungary in October 2010 (Enserink 2010), and

the other two tragedies of Hpakant jade plant in Myanmar and Germano iron mine in

Brasil demonstrated in November 2015 (WISE 2015) was the tailings dams failures

killing 140 people and causing long-term environmental damage.

2.1.3.3. Dry Stacking Method

An alternative disposal method for the storage of bauxite residue called dry

stacking has been introduced. This method relates to the progressive deposition of

dewatered red mud onto sloped and under-drained drying bed (Sofrá & Boger 2002)

to facilitate the consolidation process. This practice shows a paradigm shift of

18

engineering. In addition, the tailings should be engineered to meet the requirements

of residue discharge. This means it is very important to understand how the residue

transportation process and the characteristics of deposition can be influenced by

material properties and operational parameters (Sofrá & Boger 2002).

The dry stacking process was employed the first time at the Burntisland

Alumina plant in Scotland in 1941 (György & Tran 2008). Unlike lagooning, dry

mud stacking is termed thickened tailings disposal and implies that the washed

residue slurry is dewatered or consolidated to a paste with an initial solids content of

approximately 48-55% prior to disposal as shown in Figure 2.3 (Alcoa 2011).

Therefore, it was considered to be cost-effective and less environmental problems

(Purnell 1986).

The next pioneer in conducting dry stacking method in the early 1970s was the

Giulini GmbH refinery in Germany (Haerter & Shefer 1975). This practice considers

reducing the area of land needed for disposal and maximising the return of soda and

alumina to the process. This practice was introduced at the Alcan refinery followed

by the achievement of Alcoa in 1985 (Paradis 1992). By comparison with the

lagooning method, two sealing layers are simultaneously needed at the bottom of the

disposal site. The first is an at least 600mm thickness compacted clay liner, which is

placed at the bottom. Then, a plastic membrane, which is often high-density

polyethylene (HDPE) is used as the upper layer. This plastic layer offers a good

resistance to high soda and pH environment (György & Tran 2008).

19

Figure 2.3. Schematic of dry stacking system (Alcoa 2011)

2.1.3.4. Dry Cake Disposal Method

Dry cake disposal is also a dry disposal practice, where the slurry is

mechanically dewatered as much as possible in order to create a dry cake with more

than 65% of solid content before discharging (Power et al. 2011). The most

significant feature that differs from dry stacking practice is that there is no further

dewatering once the red mud waste has been dumped at the storage site. Similar to

the other practices, this method also offers an advantage feature that risk of alkalinity

and caustics can be further diminished by neutralising or washing in the filtration on

vacuum drum filters (Shah & Gararia 1995).

20

However, it is impossible to obtain a dry cake by using only means of

thickening; therefore, it does require a filtration phase (Power et al. 2011). This

combination is essential to overcome the capillary resistance inside the residue’s

pores in order to dewater the cake thoroughly. This practice has been successfully

deployed in a pilot scale, for example at the Hindalco plant at Renukoot (Shah &

Gararia 1995) and the Aluminiumoxid Stade (Germany) plant with the solids content

of over 75% achieved (Bott et al. 2002). However, this practice seems to be unlikely

specified for utilisation in alumina refineries as such in available information because

the dry stacking is often used as its advantages and preferred disposal strategy.

2.2. The Red Mud Utilisation Options

It is clear that the current rate of red mud production and its alkaline and

caustic properties will pose adverse impacts on the environment and human health if

there are no proper minimisation and management solutions. Currently, many

alumina plants in the world are still discharging the red mud into the ocean, or land-

based impoundments, or dry stacking ponds. These practices are just conventional

solutions for storing red mud in different ways. They will not actually help to

minimise and reuse this waste as a valuable material in terms of economic,

environmental, and social considerations. Confronted with the problem, many

different technologies have been developed in the efforts of long-term remediation

and utilisation applications of red mud in order to minimise to the environmentally

acceptable level.

21

2.2.1. The Utilisations of Red Mud in Construction

Building materials seem to be one of the most successful applications that

could consume a significant red mud quantity. The study conducted in early 1936

(Thakur & Sant 1983a) showed that the contents of iron and alumina in red mud can

contribute a crucial part to the setting and strength properties of the cement, but the

soda is detrimental. The use of calcium replacing soda in the study can foster the

ability of red mud as an additive; however, the process of residue calcinations has to

be conducted at a very high temperature of 10000C. Similarly, Liu et al. (2009) also

advocated that the production of cement from the mix of 50% red mud and other

solid wastes and modifiers has reached the standard of Portland 42.5 cement, but the

limited content of sodium in cement according to the newest standard could lead to

the prohibition of using red mud in cement production.

Additionally, research on using a mixture of gypsum, bauxite and red mud for

preparing special cements was performed by Singh et al. (1996, 1997). The authors

concluded that with the red mud content of 20-50% by dry weight added in the

composition of the material mix, it is possible to produce cements with superior

setting strengths when compared to normal Portland cement. A case study on using

red mud for manufacturing bricks and blocks have been conducted in Jamaica by

McCarthy et al. (1992). It indicated that the manufacturing of bricks from residue has

illustrated the technical feasibility, and the alumina producers were willing to support

the development of the process. However, the main brick properties such as long-

term stability, leaching and ionising radiation were not mentioned. Pinnock (1991;

1999) reported that the radon levels from bauxite residue bricks recorded were

22

approximately 2-3 times higher than that of the conventional concrete. This will pose

an impact on human health.

A recent investigation on developing unsintered bricks from red mud generated

by Shandong Aluminium Plant in China has been conducted (Yang & Xiao 2008). In

this study, researchers considered five key experimental plans in order to identify the

optimal ratio of red mud to other materials such as fly ash, the percentage of sand,

the effect of lime, gypsum, and Portland cement. It is noteworthy that the

manufacturing process of unfired bricks was implemented at the ambient

temperature. The results concluded that the optimal percentages (in weight) of red

mud bricks consist of red mud (25-40%), fly ash (18-28%), sand (30-35%), lime (8-

10%), gypsum (1-3%) and Portland cement (just 1%). The products have met the

first class brick standard of China with good durability in severe climatic conditions

and high strength. Furthermore, such products can compete with traditional bricks as

the production process consumed much lower energy and of low cost.

2.2.2. The Utilisations of Red Mud in Chemical Applications

In the context of using bauxite residue as catalysts, many potential applications

have been explored due to the presence of Fe2O3 and TiO2 and its high surface area

and the low cost of the source materials (Klauber et al. 2009, 2011). Recent studies

conducted by Sushil & Batra (2008, 2012) have demonstrated the use of red mud as a

catalyst in hydrogenation, hydro-dechlorination, exhausted gas clean-up and other

areas. These authors concluded that unmodified red mud had a poor performance as a

catalyst compared with pure iron oxide or commercial catalysts.

In the production of ceramics, red mud is considered a beneficial material that

creates a barrier for radiation shielding (Amritphale et al. 2007). This is because of

23

the formation of a dense ceramic matrix produced from liquid phase sintering. This

matrix demonstrated that the shielding thickness and compressive and impact

strength being better compared to normal Portland cement-based shielding materials.

Although the applications in this area are really broad, they consume a low volume

of bauxite residue and require some pre-treatments.

2.2.3. The Utilisations of Red Mud in Metallurgy

There were about 135 patents granted between 1964 and 2008 related to

metallurgical applications, 17% out of which are subject to bauxite residue (Klauber

et al. 2011). The major metals recovered from bauxite residue are iron, aluminium,

titanium, and sodium. The process of iron, aluminium and titanium recovery

separately and in combination has been reviewed in previous studies (Paramguru et

al. 2005; Thakur & Sant 1983b). However, the available literature showed that this

proposed recovery process is technically complex requiring large investment in an

energy intensive plant (Fig. 2.4) (Piga et al. 1993). As a result, there is no large-scale

extraction of metals from bauxite residue to date.

24

Figure 2.4. A possible flowsheet for recovery of Fe, Al, and Ti from bauxite residue

(Piga et al. 1993)

2.2.4. The Utilisations of Red Mud in Agriculture

A number of investigators have studied the potential use of untreated red mud

to improve soil condition for agriculture or safety given its ability for fixation of

heavy metals (Liu et al. 2011). The biological and chemical assessments study of in

situ fixation of metals in soil using red mud conducted by Lombi et al. (2002a;

2002b) is a typical example. This study reported a remarkable decrease in the

concentration of heavy metals in the soil pore water following treatment using 2%

red mud and 5% beringite (an alkaline alumino silicate). The research also confirmed

that red mud treatment could significantly accelerate the production of soil microbial

biomass.

25

Similarly, research has been conducted using batch, pot and field experiments

involving gravel sludge and red mud to deal with contaminated agricultural soils near

a former Pb/Zn smelter in Austria (Friesl et al. 2006). The field experiment results

showed that heavy metals such as Cd, Pb and Zn could be minimised by up to 96%,

99% and 99%, respectively in treated soils. As well, red mud can retain the content

of phosphorus significantly and reduce soil acidity effectively (Liu et al. 2011; Snars

et al. 2004; Summers et al. 2001). Furthermore, in a pot experiment, Friesl et al.

(2003) found that compared to unamended soil, red mud could considerably decrease

Cd, Zn, and Ni uptake in fescue and amaranthus (Amaranthus hybridus L.) by up to

87%, 81% and 87%, respectively.

It is evident that bauxite residue can improve the cycle of phosphorus in

agricultural areas with sandy soils having a low phosphate and other nutrient holding

capacities (Klauber et al. 2011). This occurs because the mud has two useful

properties with respect to phosphate cycling, namely reducing phosphate leaching

into ground and surface water, and creating a phosphate pool which is subsequently

available to plants for improved growth. A number of studies employed by the

Western Australian Department of Agriculture (Summers et al. 1996a; Summers et

al. 1996b; Summers et al. 1993; Summers & Pech 1997; Summers et al. 1999)

reported that bauxite residue is useful for improving P retention, reducing run-off

into the Peel Inlet and the Harvey Estuary by up to 75%. Concurrently, pasture yields

were increased by 25% on average and in well-controlled areas by up to 200%. This

retention of P in the soil reduces the damaging effects of eutrophication caused by

phosphate leaching into the Peel-Harvey ecosystems.

26

2.2.5. The Utilisations of Red Mud in Environmental Treatment

Bauxite residue can be used to mitigate environmental problems given its

capacity to remove metals and metalloids due to its high alkalinity and oxidising

potential. The mud’s alkalinity causes most metal ions to hydrolysed to form

hydrolysis products and hydroxide precipitates, and the presence of high

concentrations of iron, aluminium, and titanium oxides assists sorption reactions of

metals and metalloids to occur quickly. As well, red mud contains a considerable

amount of TiO2, which facilitates oxidisation reactions thereby lowering the toxicity

of metals for example by converting As+3 to As+5, Cr+6 to Cr+3 in their salted

compounds.

Wastewater treatment applications of red mud in the removal of contaminants

both metals and metalloids from water streams, have been widely touted (Genc-

Fuhrman et al. 2005; Genc-Fuhrman et al. 2007; Genc-Fuhrman et al. 2004a, 2004b;

Genç et al. 2003; Gupta et al. 2004; Gupta & Sharma 2002; Li et al. 2006; Liu et al.

2007; Liu et al. 2011; Palmer et al. 2010; Wang et al. 2009; Zhang et al. 2008).

These studies confirmed that red mud demonstrates a promising ability to remove

toxic heavy metals, inorganics and organics, metalloids, phenolic compounds and

bacteria alike in wastewater. Improving wastewater is a consequence of the basic

advantage of red mud, which is a versatile mixture of adsorbents and flocculants that

can sequester or adsorb pollutants from the wastewater (Liu et al. 2011).

The result from the study of removing phosphate from wastewater by using red

mud reported that up to 70% of phosphate in the pH range of 6.5-7.5 can be removed

(Couillard 1982). Similarly, at pH = 7, pH removal reached 99% if red mud was

treated with HCl (concentration = 0.25 mol/L) at 7000C for 2 hours (Li et al. 2006;

27

Liu et al. 2007), and its performance was better than treated fly ash. An experiment

on comparison of nitrate adsorption between activated red mud with 20% HCl and

raw red mud was conducted by Cengeloghu et al. (2006). The authors observed that

the adsorption capacity of the activated mud was fivefold higher than that of the raw

mud, and the rate of nitrate removal reduced at pH values exceeding 7. The effect of

pH on the rate of adsorption was explained by ligand exchange reactions occurring

between metal oxides and the nitrate ion in the red mud and the activated red mud.

Numerous studies using red mud as an adsorbent for removal of heavy metals

in wastewater have been conducted (Altundogan et al. 2002; Altundoğan & Tümen

2003; Genc-Fuhrman et al. 2004a; Gupta et al. 2001; Gupta & Sharma 2002; Li et al.

2010; Soner Altundogan et al. 2000; Zhu et al. 2007). These studies concluded that

red mud has an effective capacity for adsorbing and precipitating heavy metals due to

its high pH. Furthermore, after physical and chemical treatments such as heating, or

the addition of seawater, HCl, or H2O2, a very finely grained red mud with high

surface and charge ratios is produced. This mud strongly binds to heavy metal ions

(Altundogan et al. 2002; Brunori et al. 2005; Lin et al. 2004; Liu et al. 2011). As

well, the results confirmed that pH, contacting time, adsorbent dosage, initial

pollutant concentrations, and presence of other ions are the key factors for successful

adsorption. The efficiency of the adsorption process may increase if the solution pH

is kept above 5 (Gupta & Sharma 2002). Oxide components such as Fe2O3, Al2O3,

and TiO2 play an important role in the removal of heavy metals due to their high

adsorption affinity. Unfortunately, this study (Liu et al. 2011) did not identify which

oxide possessed the highest affinity for a given heavy metal ion.

28

Apart from the inorganic removal, substantial efforts have been also made in

eliminating organics such as phenol and its derivatives, dye and bacteria from

wastewater with red mud (Liu et al. 2011). Many methods of dye removal, such as

sedimentation, filtration, chemical treatment, oxidation, biological treatment, and

adsorption and ion exchange, have used low cost red mud as adsorbents (Gupta &

Suhas 2009; Liu et al. 2011). A study on using red mud as an adsorbent for removing

congo red and acid violet from aqueous solution has been conducted (Namasivayam

& Arasi 1997; Namasivayam et al. 2001). The result showed that the removal

capacity of red mud for the dye was recorded to be 4.05mg/g and the mechanism of

adsorption is mostly ion exchange.

An experiment on using red mud mixed with sand for removing bacteria and

virus has been reported (Wang et al. 2008). The study was implemented by column

technique and using red mud neutralised by 5% gypsum and incorporated to form

30% of amended sand. The result showed that the removal rate of Escherichia coli,

Salmonella adelaide and poliovirus-1 in secondary effluent from wastewater

treatment plant was significantly improved. The order of removal was Poliovirus>E.

coli>S. adelaide (Wang et al. 2008).

2.3. Red Mud Neutralisation Methods

Untreated red mud is alkaline with pH as high as 13. Therefore, this high

alkaline waste posed an environmental hazard, and needs to be neutralised before

discharging into the environment. Several red mud neutralisation methods such as

neutralisation with seawater (Hanahan et al. 2004; Menzies et al. 2004),

neutralisation with the addition of gypsum (CaSO4.2H2O) (Barrow 1982; Courtney &

Timpson 2005; Ho et al. 1989; Kopittke et al. 2004; Wong & Ho 1993),

29

neutralisation of red mud by acid mine wastes (Glenister & Thornber 1985; Ho et al.

1985; Wong & Ho 1994), neutralisation of red mud using mineral acid (Glenister &

Thornber 1985; Khaitan et al. 2009a; Thornber & Hughes 1986), neutralisation of

red mud by fly ash (Khaitan et al. 2009; Santini & Fey 2015), and carbonation of red

mud by CO2 gas (Cardile et al. 1994; Cooling et al. 2002; Shi et al. 2000) have been

reported. These neutralisation methods not only resolve the significant amount of

pollutants, but also enhance the importance of reusing the neutralised wastes as a

valuable resource in an economic aspect.

2.3.1. Neutralisation of Red Mud with Seawater

Neutralisation of red mud with seawater has been explored by several groups

(Glenister & Thornber 1985; Hanahan et al. 2004; Menzies et al. 2004). This

neutralisation method involves the addition of seawater to bauxite residue resulting

in the precipitation of insoluble hydroxides (Mg3(OH)6) and carbonates (CaCO3 and

MgCO3), and hydroxycarbonates (Mg6Al2(CO3)(OH)16.4H2O,

CaAl2(CO3)2(OH)4.3H2O) (Johnston et al. 2008). Seawater does not eliminate

hydroxide alkalinity from the system, but it uses the soluble Ca2+ and Mg2+ content

within the seawater solution to neutralise the red mud slurry. The neutralisation of

red mud by seawater not only helps to reduce the pH level of bauxite residue slurry

to approximately 8.5 (Hanahan et al. 2004; Menzies et al. 2004), but also to increase

the long-term acid neutralisation capacity of red mud without decreasing the short

term neutralisation capacity compared with untreated red mud (Paradis et al. 2007).

Furthermore, this neutralisation method could enhance the nutrient retention capacity

of the soil due to increasing removal of P, and improve physical and chemical

30

properties of red mud that promote plant growth (Akhurst et al. 2006; Hanahan et al.

2004; Johnston et al. 2008). Chemistry of seawater neutralisation shows that in the

neutralisation process Mg reacted effectively with hydroxide, while Ca was found

more effectively with carbonates reaction. Key chemistry reactions of the

neutralisation process are the precipitation of hydrotalcite and para-

aluminohydrocalcite as below, though other minor simple carbonates and hydroxides

reactions are also possible (Hanahan et al. 2004; Johnston et al. 2008):

Hydrotalcite:

6MgCl2+12OH-+2Al(OH)4- +CO3

2-+12Na++4H+↔Mg6Al2CO3(OH)16.4H2O+12NaCl

Para-aluminohydrocalcite:

CaCl2 + 2Al(OH)4- + 2CO3

2- + 2Na+ ↔ CaAl2(CO3)2(OH)4.3H2O + 4OH- + 2NaCl

2.3.2. Neutralisation with Gypsum (CaSO4.2H2O)

The use of gypsum as an ameliorant or soil amendment has been widely

applied at bauxite processing residue disposal sites (Courtney & Timpson 2005;

Renforth et al. 2012). The addition of gypsum as a soluble calcium source to the

bauxite residue provides Ca2+ displacing Na+ from exchange complexes resulting in

the reduction of pH of the solution (Wong & Ho 1993). The previous studies

(Barrow 1982; Courtney & Timpson 2005; Ho et al. 1989; Kopittke et al. 2004;

Wong & Ho 1993) noted that the addition of gypsum brought the pH of red mud to

around 8.3, and the alkalinity of red mud can be precipitated in similar reactions to

those of seawater neutralisation.

31

Ca2+ + CO32- ↔ CaCO3 (calcite/aragonite)

3Ca2+ + 4OH- + 2Al(OH)4- ↔ Ca3Al2(OH)2 (tricalcium aluminate)

4Ca2+ + 4OH- + 2Al(OH)4- + CO3

2- ↔ Ca3Al2(OH)12.CaCO3.5H2O (hydrocalumite)

6Ca2+ +4OH- + 2Al(OH)4- + 3SO4

2- ↔ Ca3Al2(OH)12.3CaSO4.26H2O (ettringite)

When adding gypsum to weathered red mud with pH typically around 11, the

pH of red mud was lower to an acceptable level of approximately 8.5-9.0. In contrast

to weathered red mud, Thornber and Hughes (1986) reported that there are carbonate

(CO32-), aluminate (Al(OH)4

-) and free hydroxide (OH-) ions existing in alkalinity of

fresh red mud, therefore, the pH reduction can be achieved at 10.5 with the addition

of gypsum. Moreover, the addition of gypsum to red mud resulted in the

improvement of chemical characteristics and concentrations of nutrients and

enhancement of plant growth because of enhanced hydraulic quality and drainage

that require for successful revegetation at red mud impoundment sites (Courtney &

Timpson 2005; Woodard et al. 2008).

2.3.3. Neutralisation of Red Mud by Acid Mine Wastes

Acidic industrial wastes such as copperas (FeSO4) or mixture of ferrous

sulphate and sulphuric acid produced from the extraction of rutile from ilmenite and

from metal cleaning using sulphuric acid were found effectiveness in the

neutralisation of bauxite residue (Glenister & Thornber 1985; Ho et al. 1985;

Thornber & Hughes 1986; Wong & Ho 1994). The studies showed that when

copperas was added to the red mud, it acted as a strong acid resulting in the reduction

of pH. The extent of the decrease in the pH level was found positively related to the

32

quantity of acidic ameliorants in the copperas. Because copperas was highly soluble,

it reduced quickly the pH of the mud to about 5.0 to 6.0. This pH level was ideal for

the disposal of red mud at impoundment sites (Glenister & Thornber 1985; Wong &

Ho 1994).

2.3.4. Neutralisation of Red Mud Using Mineral Acid

The use of strong acids (H2SO4, HCl, HNO3) to neutralise bauxite residue as

a rapid way of reducing the pH of the red mud for safer storage has been reported in

previous studies (Glenister & Thornber 1985; McConchie et al. 2002). However, this

method has not been carried out at a plant-scale due to its high cost. There were some

lab-scale experiments reported (Glenister & Thornber 1985; Khaitan et al. 2009a).

These studies found that after acid addition the alkalinity within the liquid phase

reacted relatively quickly compared to the solid phase and leading to the immediately

decreased pH level. As mineral acids such as H2SO4, HCl, HNO3 are often strong

acids, thus if neutralisation of red mud using mineral acids, in theory any equilibrium

pH levels can be achieved depending on the quantity of acid added to the red mud

solution (Kirwan et al. 2013). However, there are drawbacks with this neutralisation

method as it introduces large volumes of sulphate or chloride impurities to the

process water stream (Cooling 2007). Strong acids have also been used to create

activated red mud as cheap adsorbents for removal of toxic metals, fluoride, and

phosphate in water or wastewater treatment and soil reclamations (Liang et al. 2014;

Pradhan et al. 1998; Shannon & Verghese 1976; Shiao & Akashi 1977).

33

2.3.5. Neutralisation of Red Mud by Fly Ash

The neutralisation of red mud can also be achieved with fly ash solid waste

produced from coal plants. Fly ash varies significantly in composition and shows a

range of acid-base characteristics when exposure to water (Theis & Wirth 1977). A

study performed by Khaitan et al. (2009) investigated the potential of fly ash for

neutralisation of highly alkaline red mud slurry and evaluate the kinetics of the

neutralisation process. Different acidic fly ashes mixed with red mud at fly ash/red

mud weight ratios ranging from 0.05 to 1.0 were used in the study to identify the

dose of fly ash required to neutralise a fixed amount of red mud. Experimental results

revealed that a variety of fly ash at fly ash/red mud weight ratios of 0.6-1.0 could

lower the pH from 12.5 to 10.8 in approximately 150 days. In addition to the large

doses of fly ash needed, the kinetics of neutralisation was rather slow (Khaitan et al.

2009). Due to the large amounts of fly ash required, this approach of neutralization is

not economically feasible. Furthermore, the slow rate of neutralisation showed that

fly ash is not a promising agent for bauxite residue neutralisation.

2.3.6. Neutralisation of Red Mud by Carbon Dioxide (CO2) Gas

In the environmental context, the current volume of CO2 in the atmosphere is

rapidly increasing and reaching the level of approximately 398ppm on October, 2015

(Dlugokencky & Tans 2015), and it is projected that atmospheric CO2 concentration

will be at around 700ppm by the end of the 21st century (IPCC 2014). This will lead

to the climate change and severe global warming affecting human health, industries

and agricultural activities. Therefore, using industrial wastes, such as metal oxide

bearing materials, for capturing CO2 may potentially make a significant contribution

to global warming mitigation. Possible materials could be alkaline and alkaline earth

34

oxides (MgO, CaO) naturally present in silicate rocks or alkaline industrial residues

such as slags from aluminium industry (IPCC 2005).

Carbonation of red mud is defined as the process of adding gaseous or liquid

CO2 to the red mud slurry before discharging into the disposal areas (Cooling et al.

2002). The carbon dioxide reacts with alkaline compounds in the slurry to form

carbonate species leading to a reduction in pH of the red mud. The carbonation

process involves the following reactions (Cooling et al. 2002):

NaOH(aq) + CO2(aq) ↔ NaHCO3(aq) (2.1)

Na2CO3(aq) + CO2(aq) + H2O ↔ 2NaHCO3(aq) (2.2)

NaAl(OH)4(aq) + CO2(aq) ↔ NaAlCO3(OH)2(s) + H2O (2.3) (dawsonite)

Na6[AlSiO4]6.2NaOH(aq) + 2CO2(aq) ↔ Na6[AlSiO4]6(s) + 2NaHCO3(aq) (2.4) (Desilication product Na2O) (sodium alumino silicate)

3Ca(OH)2.2Al(OH)3(aq) + 3CO2(aq) ↔ 3CaCO3(s) + Al(OH)3(s) + 3H2O (2.5) (tricalcium aluminate-6) (calcite) (gibbsite)

In the carbonation process, CO2 neutralises the red mud by reacting with free

soda in the form of NaOH, Na2CO3, and Al(OH)4- in the liquid phase. Consequently,

the soluble carbonate and bicarbonate ions were the dominant products of the

carbonation process as shown in equations (2.1) & (2.2) (Johnston et al. 2008).

Nevertheless, in some previous studies (Guilfoyle et al. 2005; McConchie et al.

2002) on the carbonation of red mud by CO2, it is indicated that the CO2 reacted with

ionised sodium aluminate to form solid dawsonite as described by equation (2.3).

The equations (2.4) and (2.5) take place in the solid phase where the reactions of

desilication products (e.g. sodalite) (Glenister & Thornber 1985) and calcium

35

mineral (e.g. tricalcium aluminate) dissolution and the precipitation of calcite

(Khaitan et al. 2009b) occurred that would decrease the pH of red mud slurry in

long-term carbonation.

Carbonation of red mud is considered an inexpensive and safe treatment

process that results in the formation of thermodynamically stable products (Huijgen

et al. 2005). This process could prove to be an effective approach to make red mud

less hazardous prior to disposal. As well, carbonation of red mud may provide other

significant benefits (Cooling et al. 2002). It could decrease the risk of seal material

failure (clay or HDPE) in the storage pond, thereby reducing the risk of underground

water pollution. Further, this treated non-hazardous red mud could be used for useful

purposes such as soil amendment and construction materials. Finally, it removed

greenhouse gases and minimised its adverse impacts on the quality of drainage water.

2.3.7. Perspectives of Red Mud Carbonation

Carbonation of red mud offers opportunities for the reuse of red mud as a

valuable resource which to date has been limited due to its high pH. The carbonated

products could potentially be used in industrial and agricultural activities for the

removal of toxic metals and nutrients and for soil amendment. The capacity of this

waste in capturing CO2 has been reported in a number of studies.

The first pilot study on carbonation of red mud was implemented by Alcan at

their Saramenha Ouro Preto refinery in Brazil in 1983 (Power et al. 2011).

Subsequently, the process was employed by Alcoa with the aim of lowering the pH

of the slurry before disposing of as a dry stacking waste. Many trials were conducted

between 1991 and 1996 and these studies confirmed the potential of the red mud as

an effective CO2 adsorbent, for example it was employed at the Kwinana refinery in

36

2000 (Cooling et al. 2002). The carbonation plant established in the Kwinana has

operated at the range of CO2 dose rate varying from 16 to 25.5 kg of CO2 per m3 of

red mud slurry (48%wt solids). It was observed that the lowest pH was 8.5 at the

CO2 dose rate of 25.3 kg/kL or 17.8g CO2/kg RM (an estimated density of

2,600kg/m3) under the current operating conditions at the Kwinana (Cooling et al.

2002).

In one study (Shi et al. 2000), red mud was mixed with liquid CO2 at 297K and

10MPa for the period of 5-15 minutes. The experiment was performed under high

pressure liquid CO2 rather than vapour phase CO2 in an attempt to neutralise both the

free soda and bound soda in the red mud. The pH value of the water in contact with

liquid CO2 was less than 3.0 so that it can neutralise the basic compounds of the red

mud leading to the rapid reduction of pH. The experiment suggested that the pH of

the slurry reduced immediately to 7.0 following the exposure to liquid CO2. After

treatment, the pH rebound slowly back and levelled off at 9.5 due to the release of

bound soda via desilication reaction. The approximate quantity of CO2 sequestration

was found 23g of CO2/kg of dewatered red mud. The amount of CO2 sequestered

was higher than that reported by Alcoa.

From the literature on red mud carbonation, it can be seen that preliminary

experiments clearly demonstrated the potential of carbonation to achieve the desired

pH. However, the outcome of the carbonation process is also influenced by a range

of parameters, such as partial pressure of CO2 or CO2 concentrations, chemical and

physical properties of red mud, flow rate of gas, temperature of reaction, etc. A study

on carbon dioxide treatment of red mud (Cardile et al. 1994) was performed using

factorial experimental design to characterise the influence of five factors namely

temperature of reaction, CO2 gas flow rate, stirring rate, total sodium content in

37

solution and slurry density. The responses of five factors were investigated in a fully

structured analysis by using the Jass statistical computing package and “relative

significant” Pareto plots. The determination of fitted responses, residuals, and

Normal plots of the residuals were also included in the analysis procedure. The

outcomes of the study stated that the minimum pH of 7.0 can be achieved and it

remains firmly in the solution as long as the addition of CO2 gas continues. If the

CO2 addition is stopped the pH conversion starts increasing to 9.5 after 14 hours of

treatment because the new establishment of equilibrium between the solution and

atmospheric CO2 occurred. The rebound of pH is due to the most alkaline materials

such as tricalcium aluminate or sodalite becomes reactive to CO2.

Furthermore, the study (Cardile et al. 1994) suggested that the efficiency of the

CO2 gas/mud reaction depends largely on the stirring speed and flow rate of gas,

whereas the temperature and sodium content have a little effect. These factors also

have impacts on the final pH rebound of the slurry. Accordingly, total Na content in

the RM and slurry density were found the main factors showing a remarkable impact

on the final pH recovery of the equilibrated system, while other physical behaviours

do not seem to be effective on this aspect. However, this research did not work out

the specific operating parameters that would be necessary for achieving the optimal

conditions for the carbonation process in these experiments.

In a study conducted by Khaitan et al. (2009b), bauxite residue was carbonated

in different partial pressures of carbon dioxide in the air. The partial pressures of CO2

of 10-3.5, 0.01, 0.1, and 1atm were used in the study by mixing CO2 gas with air in

different volume proportions. The results showed that the rate and extent of

carbonation was directly proportional to the partial pressure of CO2. At the pressure

of 1atm CO2, an equilibrium pH level was achieved in one day of carbonation, while

38

it took about 9 days to establish a steady state pH of 9.8 at CO2 pressure of 10-3.5.

The pH conversion phenomenon for all treatments was found to reach the same value

of 9.9 within one day. The pH rebounded was attributable to the dissolution of

tricalcium aluminate in the red mud slurry (Smith et al. 2003). The CO2 sequestration

potential for red mud liquor was estimated to be 5.8mg CO2/kg RM, while the

amount of CO2 for the whole bauxite residue was found 11.9mg CO2/kg RM. It is

estimated that the amount of carbon sequestration potential of annual red mud

production (30 million metric tonnes/year) was estimated in the order of 0.029-0.057

million metric tonnes/year (Khaitan et al. 2009b). This quantity is small in

comparison with annual production of bauxite residues.

Based on the results from carbonation of red mud at different partial pressures

of CO2 of 10-3.5, 0.01, 0.1, and 1atm, the modelling program MINEQL+ was used to

calculate the final equilibrium pH values for the carbonation. The simulation results

were then plotted and compared with experimental data. The simulation showed that

the modelled curve was similar in shape to the experimental data. The modelled

results at PCO2 values of 0.01, 0.1, and 1atm yielded a higher pH value by about 0.5

pH units, a difference not well understood (Khaitan et al. 2009b). Furthermore, the

modelling suggested that the dominant solid-phase pH buffering was caused by the

dissolution of tri-calcium aluminate (Ca3Al2O6(s)), sodium-aluminate silicate

(NaAlSiO4) and calcite (CaCO3) (Khaitan et al. 2009b).

A preliminary study on carbonation of raw red mud to determine the capacity

of waste in capturing CO2 showed that the carbon removal process is rapid and the

added carbon dioxide produced a large increase in bicarbonate alkalinity (Jones et al.

2006). The raw red mud slurry contacted with gaseous CO2 at a constant pressure of

68.9kPa, and flow rate of 200mL/minute in a range of different times. Although this

39

is a preliminary study, the capacity of red mud to capture CO2 has been proven.

Based on the amount of CO2 (4 litres) captured by red mud and its density (1.87

kg/m3), it is estimated that there are about 748g of CO2 taken up by 1kg of wet red

mud (Jones et al. 2006). In other words, the volume of 748kg of CO2 (wet weight)

was captured by per tonne of red mud. However, this amount of CO2 captured was

not feasible because there was a discrepancy in data reporting and a mistake in CO2

sequestered calculation found by Johnston et al. (2010). Accordingly, the CO2

sequestration calculated in the study conducted by Johnston et al. (2010) was 17g

CO2/kg RM.

Yadav et al. (2009) studied the sequestration of carbon dioxide by using red

mud. Their work set out to determine the main mineral phases in red mud responsible

for the carbonation process, and to evaluate the capacity of CO2 uptake in different

size red mud fractions at the ambient temperature and a fixed CO2 pressure of 3.5bar.

The conclusion was that red mud has an effective potential for sequestration of CO2,

and the principal minerals responsible for the carbonation were chantalite

(CaAl2SiO4(OH)4) and cancrinite (Na6Ca2Al6Si6O24(CO3)2). In addition, the red mud

fraction of different sizes also has different effectiveness of CO2 capture, and red

mud fraction with the average size of 30µm, a pH of 7 and its relative density of 1.8

g/cm3 was found more effective for CO2 capture, whilst the carbonation capacity was

53g of CO2/kg RM.

In another study (Sahu et al. 2010), pure CO2 gas with the flow rate of

5mL/min was passed into the red mud slurry for 5, 10, 20, 24, 48 and 72 hours. The

neutralised red mud (NRM) collected after centrifugation for 20 minutes at 3000rpm

was treated as cycle-1 for each carbonation period. Cycle-2 and 3 were repeated by

using only 5h carbonation processes. The results showed that the amount of CO2

40

removed for cycle 1, 2, 3 and NRM were 3.54, 2.28, 0.63, and 0.57g CO2/100g of

RM, respectively. Totally, calculated CO2 removal was 7.02g/100g of RM or 70.2g

of CO2/kg RM. The phenomenon of pH rebound was also observed in this study. The

pH of NRM was observed to increase slowly in the first few days and then reached a

constant value of 8.45 (Sahu et al. 2010).

In another CO2 neutralisation of red mud, Rai (Rai 2013) investigated the pH

rebound phenomenon. Two different carbonation experiments (30%wt solids) were

performed. The first experiment was called multiple cycle carbonation, where red

mud slurry was carbonated at 0.1 atm pressure for 15 minutes. The initial and final

pH were recorded, and the slurry was then stored for a week. After a week, the pH of

the slurry was measured and the slurry was carbonated again. The pH was measured

after carbonation and the slurry was stored again for another week. This procedure

was repeated in the seven-week experimental program. The second one was named

as single cycle carbonation, where the red mud slurry (30%wt) was carbonated only

once for 15 minutes. The pH of the carbonated slurry was recorded and the slurry

was then stored for a week. At the end of each week, the pH of the slurry was

measured for a period of seven weeks.

The results of the study indicated that the pH rebound was observed in both the

multiple cycle and single cycle carbonation experiments. The pH rebound in both

experiments was found to be about 9.5-9.7 within a week suggesting that pH rebound

happened irrespective of the number of cycles the slurry was carbonated. This means

that only the liquid phase was getting carbonated and the pH rebound occurred due to

leaching of the alkaline by-product in the red mud solids driving up the pH (Rai

41

2013). Unfortunately, this study did not work out the amount of CO2 sequestered by

red mud.

Table 2.4 summarises the amount of CO2 sequestered by red mud from

previous carbonation studies.

Table 2.4. Summary of CO2 amount captured in previous studies on RM carbonation

Studies on carbonation of

RM by CO2

The amount of CO2 captured

References gCO2/kg

RM

gCO2/kg

RM solid

gCO2/kg

RM liquor

Carbonation of RM at

Kwinana refinery in 2000. 17.8 - -

(Cooling et al.

2002)

Carbonation of RM by CO2

liquid at 297K, 10MPa. 23 - - (Shi et al. 2000)

Carbonated of RM in

different CO2 partial

pressure.

11.9 - 5.8 (Khaitan et al.

2009b)

Carbonation of RM at CO2

pressure of 68.9kPa, flow

rate of 200mL/min.

17 - - (Johnston et al.

2010)

Carbonation of RM at

15vol% CO2 and 85vol%

N2 at flow rate of 5mL/min

41.5 - - (Bonenfant et al.

2008)

Carbonation of RM at fixed

CO2 pressure of 3.5bar. 53 - -

(Yadav et al.

2009)

Carbonation of RM by CO2

at flow rate of 5mL/min. 70.2 - - (Sahu et al. 2010)

42

2.3.8. Mechanism of the Carbonation of Red Mud

The mechanism of red mud carbonation has been identified in both liquid and

solid phase reactions as summarised in Table 2.5. In the liquid phase, it is the

reaction of CO2 with hydroxide to form bicarbonate and the reversibility of key

alkalinity reactions between hydroxide, carbonate, and bicarbonate (Johnston et al.

2010). When initial injection of CO2 gas into the red mud slurry (pH~13), it absorbs

into the red mud liquor, then diffuses through the liquor and reacts with free

hydroxide (OH-) present in the liquor to form carbonate (CO32-) alkalinity (Eqn.

(2.6)) in Table 2.5, and lower the pH of solution to ~10.3. With further CO2 addition,

the carbonate is converted to bicarbonate illustrated by equation (2.7), therefore

lowering the liquor pH to <8.5 (Cardile et al. 1994; Johnston et al. 2010; Khaitan et

al. 2009b; Kirwan et al. 2013; Sahu et al. 2010).

However, much of the hydroxide for reaction with CO2 also comes from the

solubilisation of residual aluminium such as aluminate (Al(OH)4-). Thus, the

reactions taking place in liquid phase also involves the consumption of free

hydroxide from aluminate anion resulting in the precipitation of alumina as shown in

equation (2.8) (Johnston et al. 2008; Jones et al. 2006; Kirwan et al. 2013).

43

Table 2.5. Reactions taking place in the carbonation of red mud

Reactions

Liquid phase reactions:

2OH-(aq) + CO2(aq) ↔ CO3

2- + H2O

H2O + CO32- + CO2(aq) ↔ HCO3

-(aq) + H+

(aq)


(aq) ↔ Al(OH)3(s) + Na+(aq) + HCO3

-(aq)

(2.6)

(2.7)

(2.8)

Solid phase reactions:

Calcium-bearing mineral dissolution (e.g. tricalcium aluminate):

Ca3Al2(OH)12 + 6H+ ↔ 3Ca2+ + 2Al(OH)3 + 6H2O

DSP dissolution (e.g. sodalite):

Na6Al6Si6O24.Na2CO3.yH2O + 18H2O + 7H+ ↔ 8Na+ + 6Al(OH)3 +

6Si(OH)4 + HCO3- + yH2O

The precipitation of carbonate (e.g. calcite):

Ca2+ + CO32- ↔ CaCO3(s)

(2. 9)

(2.10)

(2.11)

In the solid phase, the reactions involved hydrogen ions of carbonic acid

causing the dissolution of alkaline tricalcium aluminate (TCA) and the dissolution of

desilication products (DSPs) such as sodalite, cancrinite and the precipitation of

carbonates such as calcite (Glenister & Thornber 1985; Johnston et al. 2010; Smith

et al. 2003). Solids like TCA act as an alkaline store and its dissolution (Eqn. (2.9))

can serve to buffer any neutralisation agent to a pH value of about 11 (Gräfe et al.

2009). The DSPs solid such as sodalite, cancrinite can be an alkalinity contributor

and its anion could also offer some buffering capacity for pH in the range of 8.3 to 11

(Kirwan et al. 2013). However, the buffer associated with the dissolution of these

solids (Eqn. (2.10)) can lead to a stable pH of nearly 8.0 (Glenister & Thornber

44

1985). The dissolution of solids was observed to occur slowly as the red mud was

carbonated (Khaitan et al. 2009a). Apart from that, the precipitation of carbonates

such as calcite (CaCO3) also occurs following the dissolution of solids. The

precipitation of calcite (Eqn. (2.11)) was observed taking place after the dissolution

of TCA and lowering the equilibrium pH of carbonated red mud (Khaitan et al.

2009b). The final pH value remains constant at around 7.0 as long as the injection of

CO2 continues. If the CO2 addition is ceased, the equilibrium pH of the slurry

increases because of alkaline solids becoming reactive to CO2 (Eqns. (2.9)&(2.10))

causing the pH to rebound (Cardile et al. 1994; Glenister & Thornber 1985; Khaitan

et al. 2009b).

The rate of conversion of CO2 into H+ and HCO3- ions in an aqueous solution

plays an important role in many geological processes, especially the dissolution and

precipitation of carbonate minerals (Dreybrodt & Buhmann 1991; Dreybrodt et al.

1996). The dissolution of CO2 into water is slow and dependent on pH and

temperature, while the dissociation of carbonic acid into carbonate (CO32-) (Eqn.

(2.6)) and bicarbonate (HCO3-) (Eqn. (2.7)) as shown in Table 2.5 occurs rapidly in

an aqueous solution. The forward rate constant of CO2 dissolution in aqueous

solution is kCO2=3.10-2s-1, whereas the dissociation (Eqn. (2.7)) becomes dominant at

high pH (>9) with a forward rate constant k=8.5.10-3mol-1s-1 at 250C (Pan et al. 2012;

Stumm & Morgan 1981). The dissolution rate of calcium bearing (e.g. TCA) (Eqn.

(2.9)) and DSP (e.g. sodalite) (Eqn. (2.10)) minerals and the precipitation rate of

carbonates (e.g. calcite) (Eqn. (2.11)) in the carbonation process are related to the

dissociation of CO2 in the liquid phase and the pH of solution as well (Dreybrodt &

Buhmann 1991; Pan et al. 2012). As discussed earlier, the dissolution of these

minerals bearing alkaline anions occurs slowly at the mineral surface as the red mud

45

was being carbonated. Both TCA and DSP minerals begin dissolving slowly at pH

below 9, but the reaction of calcium ions combining with carbonate ions is very fast

at pH range of 4.5-6.0 (Khaitan et al. 2009a; Pan et al. 2012). The dissolution

kinetics of solids improved with increasing temperature; however, carbonation

precipitation was retarded at higher temperature because of reduced CO2 solubility

(Costa et al. 2007; Gerdemann et al. 2007; Park et al. 2003). Therefore, it is

important to keep the temperature stable at room condition during the experiments in

this study.

2.4. Summary

The mass of bauxite residue generated from the Bayer process has posed a

significant impact on the environment because of its high alkalinity. Many different

disposal methods have been employed in the efforts of the management of red mud

in the world. The first two conventional wet methods used in alumina refineries prior

to the 1970s were marine discharge and lagooning. Although the marine method

sounds simple, it had adverse impacts on the ocean ecosystem, so it has been

discouraged by the UNIDO. As a result, the lagooning practice has been widely

applied as a main method of disposal of red mud in refineries established after 1970.

This practice is the simplest land-based disposal and requires increased engineering

input for red mud storage. However, this method is also problematic due to high risk

of leakage, instability, and liquefaction that can lead to human deaths and long-term

environmental depletion caused by tailings dam failures.

Other disposal methods that have been introduced to alumina refineries for

red mud storage are dry stacking and dry cake disposal practices. Both methods are

related to the process of dewatering red mud as much as possible before disposing of.

46

While the dry stacking method is considered to be cost-effective and less

environmental problems, the dry cake practice offers an advance feature that the risk

of alkalinity and caustics can be further diminished by neutralising or washing by the

filtration process. However, the dry stacking is often employed in refineries because

of its advantages and preferred disposal strategy compared with the dry cake disposal

practice.

There are many different technologies deployed in the efforts of long-term

remediation and utilisation of red mud in the environmentally friendly applications.

Red mud has been used as a building material for manufacturing bricks and blocks in

construction. In this way, the application contributed a significant part to the

reduction of the volume of red mud discharging into the environment. The red mud

bricks showed a good durability in severe climate conditions and high strength. More

importantly, such products could compete with other building materials since they

consume lower energy and production cost. Nevertheless, it is noticed that the

ionising radiation level from the red mud bricks was approximately 2-3 times higher

than that of the conventional concrete. This is a potential impact on human health.

Using red mud as catalysts in some chemical applications such as

hydrogenation, hydro-dechlorination, exhausted gas clean-up has been widely

published. However, this practice was not a promising solution as it had a poor

performance compared with the commercial ones. Also, red mud has been

successfully employed in agricultural application. Untreated red mud can improve

soil given its ability for fixation of heavy metals. Furthermore, bauxite residue is also

useful in improving P retention in agricultural areas having sandy soil with low

phosphate content. The ability of P retention can help reduce the damaging impacts

of eutrophication in the ecosystem. The use of red mud in environmental treatment is

47

the most successful application. Many studies have confirmed that red mud has an

effective capacity for adsorbing and precipitating heavy metals, inorganics and

organics, metalloids, phenolic compounds and bacteria alike in wastewater.

Additionally, the efficiency of adsorption may increase if the mud can be activated

by HCl or the solution pH is retained above 5 since the red mud contains Fe2O3,

Al2O3 and TiO2 that play an significant role in removal of heavy metals.

Apart from technologies of utilisation of red mud mentioned above,

neutralisation methods have been widely employed to enhance the reuse of

neutralised waste. Red mud can be neutralised with seawater to precipitate the

insoluble hydroxides and carbonates. Although this neutralisation method does not

eliminate hydroxide alkalinity in the mud, it helps to reduce the pH level of the

residue slurry, and to increase the long-term acid neutralisation capacity of RM. The

neutralisation of red mud can be done by the addition of gypsum as a soluble calcium

source to the bauxite residue. The neutralisation of RM by gypsum can improve

chemical properties and concentrations of nutrients, and enhance the growth of plant

at red mud impoundment sites.

The fastest way of reducing pH and alkalinity of bauxite residue for safer

storage is the neutralisation of red mud with strong acids (HCl, H2SO4, HNO3). The

advantage of this method is that any equilibrium pH levels can be achieved

depending the amount of acid added to the RM. Nevertheless, this practice has not

been employed at a plant-scale because of its high cost and producing a huge amount

of sulphate or chloride impurities. Alternatively, the use of fly ash solid waste from

coal plants to neutralise the RM can be possible. Fly ash was considered efficacy of

neutralisation because of its varieties of composition and acid-base characteristics

when exposure to water. However, this method would be unlikely an economically

48

feasible remedy because of the slow rate of neutralisation and the large quantity of

fly ash required for neutralisation of RM.

The neutralisation of RM using CO2 gas is the most promising solution. The

carbonation of RM is considered an inexpensive and safe treatment process since it

produces thermodynamically stable products. This practice could help reduce the risk

of underground water pollution in the storage pond. Moreover, it could lower the risk

of future classification of RM as a hazardous waste, and improve the usefulness of

RM in other purposes. Finally, the carbonation of RM can contribute a significant

part to the greenhouse gas reduction and global warming mitigation strategies.

49

CHAPTER 3 MATERIALS AND METHODS

3.1. Materials

The raw red mud (RM) sample used throughout the experimental program was

supplied by Rio Tinto Alcan Queensland, Australia in the form of slurry with 44%

solids by weight (44%wt). The sample was stored in a 20-litre plastic bucket and sent

from the Rio Tinto Alcan to the university laboratory. Then, the RM was split into

smaller plastic containers for easier storage. The RM slurry samples stored in plastic

containers were covered by a layer of Argon gas on the top to prevent any air or

other gases contacting with the mud. The mineral compositions of raw RM as

determined by Bruker D4 Endeavor Powder X-ray Diffractometer (XRD) and

quantified by TOPAS V4.2 are given in Table 3.1.

Table 3.1. Major mineral composition of raw RM

Component in

RM

Possible formula wt (%)(1)

Sodalite

Cancrinite

Hematite

Boehmite

Gibbsite

Quartz

Anatase

Na8(AlSiO4)6(OH)2.4H2O

Na6(AlSiO4)6(CaCO3)(H2O)2

Fe2O3

AlO(OH)

Al(OH)3

SiO2

TiO2

19.75

2.32

58.03

8.35

1.04

3.74

6.77

(1): Quantified by Diffracplus TOPAS software associated with Bruker D4 XRD instrument

50

Analytical grade hydrochloric acid (HCl) used in this study was available in the

lab. Research grade CO2 gas cylinder was purchased from Coregas Adelaide and

used without further purification.

3.2. Materials Preparation

All RM slurries (44%wt) contained in plastic containers were homogenised

first by stirring with an impeller for all experiments. Particle size of the raw RM was

in the range of 0.25-224.40µm as measured by Master Sizer 2000, version 5.60

Malvern, UK. For each set of batch experiment, 100g of RM slurry was weighed on

a 4-figure balance Ohaus Model AX324-Ohaus Corporation, USA. The mud

suspension for each batch of carbonation study used an initial solid: water ratio of

1:5 by mixing 100g of RM with 500mL of distilled water to create slurry suitable for

dissolution of soluble alkaline components in the carbonation reaction (Johnston et

al. 2010). All experiments were conducted in duplicates to minimise errors and

improve reproducibility. All carbonation experiments and measurement techniques

were performed at ambient conditions.

3.3. Methods

3.3.1. Acid Titration Procedures

Acid titration of RM slurry was done to determine acid neutralisation capacity

(ANC) of available species in the solid and liquid phases of the RM slurry. The ANC

represents the amount of mineral acid required to reach a specific pH endpoint

51

(Carter et al. 2008; Lin et al. 2004; Snars et al. 2004). The acid titration of RM was

performed by using 0.1N hydrochloric acid (HCl). The method of acid titration for

RM slurry was adapted from standard procedure for soil titration (Page et al. 1982).

Accordingly, 10g of RM slurry (44% solid by weight) was transferred to a glass

beaker and continuously stirred by a magnetic bar and titrated by adding aliquots of

0.1N HCl to target pH endpoint 4.5, where all hydroxide and carbonate alkalinity

were converted to bicarbonate. A calibrated pH meter Jenway 3510 was used to

record pH values during the titration process. Duplicate ANC measurements were

obtained by repeating the same procedure as above.

Long-term titration of RM slurry by HCl was performed at pH values of 4.5, 6,

8 and 10 to get an estimate of the total ANC of the RM. 10 grams of RM was titrated

against 0.1N HCl to target the final pH values as above on the daily basis until the

desired pH value was stable. Duplicate long-term titrations were made by using the

same procedure to get the average value. After long-term titration, the slurry was

filtered by using Whatman 40 filter paper to obtain the liquor. Then, this liquid was

diluted by distilled water for dissolved metal analysis using Inductively Coupled

Plasma Mass Spectrometry (ICP-MS).

To distinguish the difference in contributions to ANC between the liquid and

solid phases, only RM liquor was used to perform the rapid titration. In this titration,

10g of RM liquor, obtained by centrifugation from 50g of RM slurry and filtered by

Whatman 40 paper, was titrated by adding aliquots of 0.1N HCl within 2 minutes

allowed for reactions after each acid addition (Khaitan et al. 2009a). Duplicate

measurements were done to get the average value.

52

3.3.2. Determination of Total Alkalinity of Raw RM and Carbonated RM

Total alkalinity (TA) is a measure of the ability of a water sample to neutralize

strong acid. Unlike acid neutralisation capacity, the TA was determined on a filtered

sample (Rounds 2012), which means it is the property of the liquid phase only. The

TA determination of RM slurry was carried out based on the method detailed by

Rounds (2012). Accordingly, a 10g of RM sample (wet weight) was directly titrated

against 0.1N HCl from an initial pH of 12.5 to a final pH of 4.0. The process was

performed in two steps. Firstly, the slurry was titrated to a pH of 8.3 (carbonate

endpoint) at which all the dissolved aluminium hydroxide in the mud precipitated.

Next, the sample mixture was filtered using Whatman 40 filter paper to separate the

precipitated hydroxide. Then, the filtrate was further titrated against 0.1N HCl to the

pH of 4.0 (bicarbonate endpoint).

For carbonated RM, 10g quantities of sample (wet weight) were also titrated

against 0.1N HCl to the bicarbonate endpoint (pH 5), since all carbonated samples

have pH<8.1 and >5.0 (Rounds 2012). Next, the mixture was filtered by using

Whatman 40 filter paper. The filtrate was then further titrated against 0.1N HCl to

pH of 4.0. Finally, web-based alkalinity calculator V2.22 (USGS 2007) was used to

obtain alkalinity values for RM and carbonated RM. In the web-based alkalinity

calculator, two columns of data included titrant volume and pH values were entered

in the titration data area. Other fields in the program such as sample temperature,

sample volume, acid concentration were filled with their availabilities. Filtered

sample area was selected “yes”, acid correction factor for HCl was 1.0, and burette

titration was selected in titration type area. In the fixed endpoint section, for raw RM

samples, carbonate (pH 8.3) and bicarbonate (pH 4.0) endpoints were entered, but for

53

carbonated RM carbonate field was left blank, and bicarbonate field was entered 5.0.

Finally, click “calculate” button for running the program.

3.3.3. X-ray Diffraction (XRD)

X-ray Diffraction analysis was used to characterise the crystalline materials

and provide the information about the structure of minerals in RM. The RM samples

were oven dried at 650C in 24h before their analyses. The identification of the

minerals and quantitative phase analysis were undertaken using the Bruker D4

Endeavor Powder X-ray Diffractometer with a Co-Kα radiation source generated at

35kV and 30mA (λ= 1.788970Å). Diffraction data in the 2θ range from 10 to 80°

were collected and matched with ICSD (Inorganic Crystal Structure Database)

reference patterns (ICSD 2012) using standard software associated with the

instrument. Mineral composition quantitative analysis of different phases was

determined by using TOPAS V4.2 software, Bruker AXS GmbH, Germany.

3.3.4. Scanning Electronic Microscopy and Energy Dispersive X-ray (SEM-EDX)

Scanning Electronic Microscopy (SEM) Quanta 450 was used to investigate

crystalline structures and spatial variations in chemical compositions of red mud

before and after treatment. In addition, element and chemical compositions of major

oxides of the samples were identified by using Energy Dispersive X-ray (EDX)

coupled with the SEM. Powder samples were placed on the mounting adhesive stubs.

As analysed by the EDX, the mounted samples were coated with a very thin carbon

to minimise sample-charging problems (Willis et al. 2002). The samples were then

analysed in a very high vacuum mode chamber. For EDX analysis, a carbon

coefficient of 14 (radioactive carbon) as default in the program was turned on, and

the element and major oxides results were obtained from spot analysis of SEM

54

images and pallet of the samples. All measurements were done in triplicates to get an

average value.

3.3.5. Carbon-Hydrogen-Nitrogen Elemental Analyser

Carbon content of the carbonated RM samples was determined by using

PerkinElmer® 2400 Series II CHNS/O Elemental Analyser (EA), in CHN mode.

Carbonated RM finely powdered samples were accurately weighed to 5mg (± 2mg)

on a 6-figure balance in tin capsules, folded, and were placed in the EA carousel.

Analysis was conducted with a furnace temperature of 9250C in Helium gas used as

carrier and provided inert atmosphere. S2 standards were run after every 10 samples

to check the calibration of the EA and validate the sample readings, i.e. S2: 29.99 ±

0.3% C; and CaCO3: 12.00 ± 0.3% C. If the standards were not within range, the

samples were labelled as invalid and were analysed again. The carbon content in

solid phase was estimated in percentage (%) and converted to gCO2/100g of RM.

Total inorganic carbon (TIC) content in raw red mud slurry was determined by

the difference between total carbon (TC) and total organic carbon (TOC) content

(TIC=TC-TOC) (Pansu & Gautheyrou 2007). To identify the TC, 50g of RM sample

was heated to 9250C for 5-10 minutes to convert all carbon to CO2 and the TC was

obtained. Duplicates were performed to get the average value. To identify the TOC,

two samples 44g & 46g of RM slurry were transferred to the two glass vials. Next,

0.2ml of phosphoric acid (H3PO4) were added to each vial to lower the pH of

samples for expelling the inorganic carbon at 2500C, then the samples were heated at

9250C to get the TOC. The TIC of samples was identified by the difference.

For carbonated RM liquor samples, the carbon content was determined by

using Total Organic Carbon (TOC) Analyser, model TOC-VCSH/CSN + TNM-1,

55

Shimadzu Corporation, Japan. Carbonated RM liquor samples filtered through 0.45µ

before analysis. To determine total carbon, the samples were injected into a catalyst

packed combustion tube, at a furnace temperature of 720°C. Organic carbon is

oxidised to CO2 and inorganic carbon is decomposed to CO2, which is measured by a

nondispersive infrared detector. Inorganic carbon is determined by injection of the

sample into a reaction chamber where it is acidified and the CO2 generated was

carried to the detector and measured. The carbon content in liquid phase was

estimated in moles C/L RM liquor and converted to gCO2/100g of RM liquor.

3.3.6. Thermal Analysis (TGA-DSC)

The Thermogravimetric Analysis and Differential Scanning Calorimetry

(TGA-DSC) was used to determine changes in physical or chemical composition of

substances in both raw RM and carbonated RM. The TGA-DSC analysis of samples

was performed using TGA-DSC 2 STAReSystem, METTLER TOLEDO, USA. To

do this analysis, 5-10 mg of sample was transferred to crucible and alumina was used

as a reference. Measurements were carried out in N2 atmosphere at a heating rate of

100C/min from 250 to 9100C.

3.3.7. Fourier Transform Infrared Spectroscopy (FT-IR)

The FT-IR spectra study of carbonated and noncarbonated red mud samples

helps to understand the changes in chemical bonds or stretching vibrations in a

molecule. The spectra of samples were collected by using Nicolet 6700 FT-IR

Spectrometer (USA). The spectrum was registered from 4000 to 400cm-1 because the

absorption radiation of most organic compounds and inorganic ions is within this

56

region (Thermo Electron Corporation 2004). Firstly, background spectrum was

collected without samples. It is a measurement of the response of the spectrometer

alone without absorption due to samples. Then, a small amount of dried sample was

placed in the sample holder for scanning in the registered spectrum 4000-400cm-1,

and data collection for samples was obtained.

3.4. Carbonation Experiments

3.4.1. Construction of Reaction Chamber

The carbonation reaction chamber as shown in Figure 3.1 was designed to

safely operate over the likely range of relevant parameters for the RM carbonation

process (e.g. materials of construction, temperature, pressure, residence time, etc.).

The absorption reactor used in the project was a 1000mL stainless steel cylinder

vessel with a height of 128mm and a diameter of 100mm. A stainless impeller

controlled by a rotor IKA® RW20 digital was placed at port no.1 in the middle of the

reactor to attain a uniform agitation. The port no.2 was designed up to 3 bar for

pressure vent to the environment if the reactor was used in high pressure. Other

apparatus include pH probe and temperature probe, which were specifically designed

for the reactor. The temperature and pH probes from calibrated pH meter Jenway 350

were inserted in port no. 3 and 4 to monitor the temperature and record pH values of

the solution. Air and CO2 gas controlled by mass flow rate meters Brooks MFCs

were mixed together and injected into the chamber via the inlet-gas port no.5. The

mass flow rate controller Brooks® 4800 series model 4850ABC, Brooks Instrument

USA, has the following characteristics:

57

- Flow ranges in full scale (FS): 25mL/min - 40L/min

- Accuracy: ±1.0% of FS

- Repeatability: ± 0.15% of FS

- Response time: Flow signal <0.3 second, flow control <0.5 second

- Pressure: 0 - 10 bar

- Temperature: 0 - 500C and humidity of 5 - 95% (ambient)

The gas was distributed to the solution by a circle-shape gas diffuser placed at

the bottom inside the reactor. The outlet-gas port no.6 was used for excessive gas in

the chamber escaping to the environment. The whole system of experimental

apparatus is described in Figure 3.2.

Figure 3.1. Carbonation reaction chamber

58

Figure 3.2. The experimental apparatus system for carbonation of RM

3.4.2. Carbonation of RM

Both red mud slurry and liquor samples were carbonated in the stainless steel

chamber as described in section 3.4.1. The samples were contacted with the mixture

of air and CO2 gas in a range of different operating conditions such as CO2

concentrations, total flow rate of gas, stirring speeds and solids concentrations in

RM. The specific different experimental conditions were used in the project as

follows:

- Total flow rate (TF) of gas: 100mL/min, 200mL/min, 300mL/min, and

400mL/min.

- CO2 concentrations ranging from 10% to 100% corresponding to the above TF.

- Stirring speeds: 250rpm, 350rpm, 500rpm, and 700rpm.

- Solids concentrations in RM: 35%, 40%, and 44% solids by weight.

59

All carbonation experiments were performed in the reaction chamber under

ambient temperature and pressure. For each experimental batch, 100g of RM was

mixed with 500mL of distilled water (solid: water ratio 1:5 by mass) to attain a

sufficient volume of slurry for carbonation. Air and CO2 gas from cylinders

controlled by mass flowrate controllers (Brooks MFCs) were mixed together then

passed through the samples via a circle-shape gas diffuser to create the small bubbles

increasing the surface area available for reactions to take place. The pH of the

solution as a function of time was recorded during the carbonation process until it

reached the equilibrium. Then, the neutralised RM was oven-dried at 650C in 24h,

ground to a powder for analysis. The carbonation experiments were repeated in a

change of different conditions.

3.4.3. pH Rebound of the Carbonated Red Mud

The increase in pH after the carbonation of red mud due to caustic soda

leaching after its initial leaching from the mud is commonly known as pH reversion

or rebound (Rai et al. 2013). Carbonated red mud samples obtained at different

operating conditions were used to evaluate pH rebound upon the equilibrium with the

atmospheric CO2 level. The pH values were recorded every 24 hours in the period of

35 days.

3.5. Chemical Equilibrium Modelling

The modelling was performed in this study using the chemical equilibrium

program MINEQL+ 5.0 (Schecher & McAvoy 2015). This software and its

precursor, REDEQL (Morel & Morgan 1972), were developed to solve the

60

expressions of mass balance by using equilibrium constants. Today, the MINEQL+

program incorporates the combination of numerical structure and the up-to-date

thermodynamic database in chemical speciation from WATEQ3 (Ball et al. 1981)

program through the minimisation of Gibbs free energy matrix.

The principles and capabilities of MINEQL+ 5.0 exist on three different levels:

modelling of first principles, secondary and conceptual modelling, and systems

modelling (Schecher & McAvoy 1992). The first level aims to calculate mass

balance and possible electroneutrality conditions. This can be done by using all

thermodynamic data such as stoichiometric coefficients, equilibrium constants, and

enthalpy values to make a specific chemical species of complexes or solids. The

second level considers addressing the model characteristics such as surface

complexation models, corrections for temperature and ionic strength, models for

calculation of pH and conductivity. The final level focuses on the definition of a

chemical system via database modelling. This level allows users to change

thermodynamic data of a system, and insert or delete chemical species, which are not

available in a database or problem set or not essential for chemical equilibrium

model (Schecher & McAvoy 1992).

In this study, the MINEQL+ 5.0 program was used to model the metal

concentrations from long-term titration in order to evaluate dissolved species and

their solid phases controlling the aqueous chemistry system. In addition, the

simulation was carried out to validate the final equilibrium pH values obtained from

the carbonation of RM in different CO2 concentrations.

All information on the chemical components in both RM slurry and liquor as

shown in Table 3.2 was used in the MINEQL+ program. The simulation of heavy

metals in long-term titration was performed at various pH values of 4.5, 6, 8, 10, and

61

12.5 with fixed carbonate content (TOTCO3) value as given in Table 3.2. In contrast,

the modelling for carbonation of RM was conducted at different partial pressure

(PCO2) of CO2. The ionic strength (I) corrections setting was kept on, and the method

of ionic strength calculated by the program was selected as this method is more

likely to give the accurate results (Schecher & McAvoy 2015).

Table 3.2. Concentration of raw RM and liquor

Constituent

Concentration

in dried solids

(g/kg solid)(1)

Concentration

in RM slurry

(g/L liquor)(2)

Concentration

in slurry (M)

Concentration

in liquor (M)

Carbonate

Na

Al

Si

Ca

Ti

Fe

5.1(3)

98.2

125

85.6

8

41.5

200

4.01(3)

77.2

98.2

67.3

6.3

32.6

157.1

0.76

3.35

3.64

2.40

0.16

0.68

2.81

0.011(4)

0.198(5)

0.076(5)

8.58E-05

0.00017(5)

1.42E-05

5.36E-06(5)

(1): Determined by EDX. (2): Converted to RM liquor volume basis (from RM 44% solid by weight as received). (3): Determined in this study (5.1mgTIC/gRM) and converted to liquor volume basis (5.1*1000/0.56). (4): Measured by Total Organic Carbon Analyser, Model TOC-VCSH/CSN + TNM-1, Shimadzu Corporation, Japan. (5): Determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Model Agilent 7500cs, Agilent Technologies,

USA.

62

CHAPTER 4 RESULTS AND DISCUSSIONS

4.1. Acid Neutralising Capacity (ANC) of the raw RM

Acid neutralisation capacity (ANC) reflects the amount of mineral acid

available in the red mud for neutralisation to reach a preselected pH endpoint, and it

was determined by the titration of an aqueous solution against a strong acid (Carter et

al. 2008; Lin et al. 2004; Liu et al. 2006; Stumm & Morgan 1981). In this study, the

ANC of raw RM slurries and liquors was performed in order to assess the

contribution of mineral solids and liquor chemical species to the ANC. The rapid

ANC was performed for RM liquor only, and the long-term ANC was done for RM

slurries because of the complex chemical composition in RM and the slow acid-base

reactions with the solid phases.

4.1.1. Rapid Titration of RM slurry and RM liquor

Figure 4.1 shows rapid titration results of RM samples carried out to a pH level

of 4.5 in order to determine the ANC of the RM. The initial pH of both RM slurry

and its liquor was recorded at around 12.5 and 12.4, respectively. It can be seen that

rapid titration of the RM slurry (0.79g solids and 1g RM liquor) yielded an ANC of

0.79 milliequivalents per gram (meq/g) of RM for titration to pH of 4.5. The ANC of

1 gram of RM liquor was about 0.22meq.

As illustrated in Figure 4.1, there was no difference between the two curves at

the initial phase of titration from pH of 11.5-12.5. However, at the later part of

titration, the gap between the two curves was more visible at pH 11 or lower. This

63

confirmed that the initial phase of titration was controlled by RM liquor, and the later

titration was controlled by the dissolution of solids in the RM. Similar observations

were reported by Khaitan et al. (2009a).

Figure 4.1. Rapid RM liquor titration compared with that of RM slurry (44%wt)

Furthermore, based on the titration curves in Figure 4.1 it can be concluded

that when acid was injected in the samples, H+ reacted with soluble aluminium in the

form of Al(OH)4-, NaOH and NaCO3

- in the liquid phase causing the resistance to pH

changes of RM liquor and buffering OH- (Khaitan et al. 2009a). All the titration data

are presented in Tables A.1-A.2 in the Appendix. The key controlling reactions are

as below.

Al(OH)4-(aq) + H+ = Al(OH)3

0 + H2O

NaOH(aq) + H+ = Na+ + H2O

NaCO3-(aq) + H+ = Na+ + HCO3

-

4

5

6

7

8

9

10

11

12

13

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

pH

meq/g liquor or meq/g liquor in 1.79g RM

1g RM liquor

1g RM liquor in RM

64

4.1.2. Long Term Titration of Red Mud

Figure 4.2 illustrates the results of long-term titration of RM slurry samples to

different pH levels of 4.5, 6, 8, and 10. The measurements were performed daily by

adding acid 0.1M HCl to adjust to the desired pH values until they remain

unchanged. It took over 40 days to complete the titration to specific pH levels of 4.5,

6, and 8. In long-term titration, the ANC recorded for the RM at pH 4.5 was about

1.91 meq/g RM. The long term ANC is double the ANC obtained from rapid titration

at the same pH value. It can be seen that the long neutralising titration time (45 days)

associated with the higher ANC obtained at pH levels of 4.5, 6 and 8 endpoints under

slow titration conditions suggest that solid dissolution occurs at these endpoints,

leading to the relatively high ANC values. Furthermore, there was an over 50%

difference between rapid and long term ANC as indicated in Table 4.1. This was due

to the complex chemical composition in RM and the slow acid-base reactions of

solid phases.

Figure 4.2. Long-term titration of RM

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 4 8 12 16 20 24 28 32 36 40 44

AN

C (

meq

/g R

M)

Time (day)

pH 4.5

pH 6

pH 8

pH 10

65

Table 4.1. Comparison between rapid and long term ANC for RM

pH value Rapid ANC

(meq/g RM)

Long term ANC

(meq/g RM) % Difference

12.5

10.0

8.0

6.0

4.5

0

0.12

0.2

0.43

0.79

0

0.34

0.69

1.03

1.91

0

65

71

58

59

In comparison between rapid liquor titration (Fig. 4.1) and long-term RM

titration (Fig. 4.2), it can be seen that solid phase plays a significant role in long-term

ANC titration process. The titration of 1 gram of RM liquor required 0.22meq or the

liquor (0.56g) contributes about 0.12meq in 1 gram of RM (44% solid by weight).

Therefore, it is estimated that the solid phase contributes approximately to 81% of

ANC, whereas this number was 19% for the liquid phase in the final long-term ANC

determination. The ANC contribution of liquid phase in this study agrees with that

reported by Liu et al. (2006), who stated that the rapid ANC accounted for less than

20% of the total ANC of red mud. Additionally, by comparing the percentage of

solid contribution to the ANC (81%, RM 44%wt) in this study with that of previous

study (76%, RM 40%wt) (Khaitan et al. 2009a), it could be confirmed that the higher

the concentration of RM, the higher percentage of solid phase contribution to the

ANC.

Based on the rapid and long term ANC for the RM (Fig. 4.1 & 4.2, Table 4.1),

it can be seen that most ANC was derived from the solids dissolution, and rapid

titration was not able to capture all the ANC of the RM. Moreover, the ANC is also

conditional upon the final pH of the system. As indicated in Figure 4.2, the ANC

(under 0.3meq/g RM) at pH 10 did not significantly increase after 45 days of

66

titration, and there was a little or no solids dissolution with time confirmed by the

results of metal concentrations by using ICP-MS (Table 4.2). However, when the

endpoint of pH was reduced from 10 to 8, 6, and 4.5, there was a significant increase

in the amounts of solids dissolved, and the greater ANC values obtained. This

collaborates with the observations of earlier researcher (Khaitan et al. 2009a), which

described the higher ANC observed at lower pH endpoints by chemical equilibrium

modelling.

Table 4.2. Metal concentrations in RM liquor at different pH values

Component Concentration (M)

Initial liquor pH 10 pH 8 pH 6 pH 4.5

Al

Na

Ca

Fe

0.076

0.198

0

0

0

0.229

0.0015

0

0

0.262

0.032

0

0

0.28

0.035

0

0

0.487

0.104

0

In long-term titration, key reactions were the dissolution of sodalite

Na8(AlSiO4)6(OH)2.4H2O and cancrinite Na6(Al6Si6O24(CaCO3)(H2O)2 in the lower

pH environment. Weber (2001) and Newson et al. (2006) reported that the solubility

of sodalite and cancrinite was pH dependent. In other words, these minerals are less

soluble at high pH environment, but at low pH their solubility becomes much more

significant. As shown in XRD patterns in Figure 4.3 & 4.4, cancrinite and sodalite

have several common peaks located at 16.10, 270, 390, 480, 580 and 780 2theta

because they have the same framework stoichiometry [AlSiO4]6 as reported by

Gerson & Zheng (1997) and Barnes et al. (1999a). At pH 6 sodalite and cancrinite

peaks decreased but were still present in the RM. However, these peaks virtually

disappeared at the pH 4.5. Hence, at pH 6 sodalite and cancrinite were found to be

partly dissolved, but they become completely dissolved at pH lower than 5.5 (Zhao et

al. 2004). The long-term titration data as a function of time for RM by 0.1M HCl are

67

reported in Tables A.3-A.6. The key reactions in solid phase are thus the dissolution

of sodalite and cancrinite as shown below (Schecher & McAvoy 2015):

Na8(AlSiO4)6(OH)2.4H2O + 18H+ ↔ 8Na+ + 6Al3+ + 6Si(OH)4 + 2H2O (4.1)

Na6(AlSiO4)6(CaCO3)(H2O)2+24H+ ↔ 6Na++6Al3++6Si(OH)4+Ca2++CO32-+2H2O (4.2)

Figure 4.3. XRD pattern of raw RM overlapped with titrated RM at pH 6

Figure 4.4. XRD pattern of raw RM overlapped with titrated RM at pH 4.5

68

4.2. Carbonation of Red Mud

Carbonation of red mud was performed at different operating conditions such

as CO2 concentrations, total gas flow rate (TF), agitation speeds, and concentration

of solid in red mud. The experiments aimed to evaluate the effect of these operating

conditions on the carbonation process. The phenomenon of pH rebound was also

examined under these conditions.

4.2.1. Effect of CO2 Concentration on Carbonation of RM

Carbonation was performed in both RM slurry and RM liquor alone in order to

evaluate the effect of CO2 concentrations on the rate of carbonation in solid and

liquid phases. The carbonation experiments were performed at different CO2

concentrations ranging from 10% to 100% depending on the total flow rate of gas.

The results showed that the equilibrium pH of the neutralised RM decreased with

increasing CO2 concentrations in both RM slurry and RM liquor. Figure 4.5 indicates

that at a fixed total gas flow rate of 200mL/min, the steady state pH was reached

from 7.5 to 6.6 for CO2 concentration values ranging from 15% to 100%.

Additionally, it can be seen that the rate of carbonation increased with the increase in

the concentration of CO2 as observed in Figure 4.5. When using concentration of 10-

15% CO2, it took about 60-75 minutes for establishing a pH of 7.5, whereas a steady

state pH of 6.9 and 6.6 was reached after 30 and 15 minutes of carbonation at 50%

and 100% CO2 concentration, respectively.

69

Figure 4.5. Carbonation of RM slurry at different CO2 concentrations, fixed TF of

200mL/min and stirring speed of 250rpm

In contrast to RM slurry, the carbonation of RM liquor occurred very fast as

there is no solid reaction to slow down the rate. As illustrated in Figure 4.6, it took

more than half an hour to obtain an equilibrium pH of 7.0 when carbonated at 10-

15% of CO2. If the concentration of CO2 is increased to 50% and 100%, the

carbonation process took about 15 minutes to complete and reach the steady state pH

of 6.5 and 6.3, respectively. This indicates that the CO2 concentration had a large

positive influence on the carbonation process.

Figure 4.6. Carbonation of RM liquor at different CO2 concentrations, fixed TF of


70

By comparing the carbonation of RM slurry and RM liquor, Figure 4.7 reveals

that at the same CO2 concentration values ranging from 15% to 100%, it took about 1

hour to reach the equilibrium pH of 7.5-6.6, respectively for RM slurry, while the

steady state pH values attained for the RM liquor were from 7.0 to 6.3 (0.3-0.5 pH

unit lower). In the carbonation of RM liquor, the pH curves in Figure 4.7

demonstrates three distinct inflexions reflecting the concentrations of hydroxide (OH-

), carbonate (CO32-), and bicarbonate (HCO3

-) that were previously observed in the

determination of ANC of RM as illustrated in Figure 4.1. These inflexions appeared

hard to distinguish in the pH curves of RM slurry carbonation. The inflexion of

converting carbonate to bicarbonate in RM slurry at pH<8.3 seemed to disappear in

Figure 4.7. This might be due to some carbonates precipitating and adhering to the

RM slurry causing the absence of carbonate to bicarbonate conversion in RM slurry

carbonation. This suggested that the carbonation process in the short time of the

experiment is mainly due to the liquid phase neutralisation. The solid phase

carbonation reactions are much slower (Khaitan et al. 2009b).

Figure 4.7. Comparison of carbonation between RM slurry and RM liquor at some

different CO2 concentrations, fixed TF of 200mL/min and stirring speed of 250rpm

71

Figure 4.8 describes the carbonation rate constant for both RM slurry and RM

liquor as a function of CO2 concentration at fixed total gas flow rate of 200mL/min

and stirring speed of 250rpm. The data of the pH taken from the carbonation process

was regressed to obtain the reaction model. Based on the R2 analysis, the best fit was

achieved using the first-order regression model given as follows (Frost & Pearson

1961):

ln c = ln co - kt

where c is the concentration of H+, co is the initial concentration of H+, t is the

reaction time in minutes and k is the rate constant. Using the first-order regression

model, the rate constant, k is selected such that the R2 accuracy is above 95%. This

approach for determining the rate constant, k was employed for all the carbonation

reactions in this work (Tables B.1 – B.23). From Figure 4.8 it can be seen that the

rate of carbonation of RM liquor were faster than that of RM slurry. The liquid

carbonation occurred so fast because of the absorption of CO2 to form aqueous

carbon dioxide and convert the hydroxide to bicarbonate and the reversibility of key

alkalinity reactions between hydroxide, carbonate, and bicarbonate (Eqns. 2.1-2.3).

Moreover, with the excesses of CO2 gas in the carbonation process, free hydroxide in

the form of NaOH, NaCO3- and Al(OH)4

- in the liquor reacts with the aqueous

carbon dioxide to precipitate as aluminate hydroxide (Eqns. 2.4 & 2.5). The other

data and plots of pH change as a function of time for the carbonation of both RM

slurry and RM liquor at different CO2 concentrations is presented in Figure B.1-B.6

and Tables B.1-B.8 and B.16-B.23.

72

Figure 4.8. Carbonation rate constant (k) for both RM slurry and RM liquor at

different CO2 concentration, total gas flow rate 200mL/min and speed 250rpm

4.2.2. Effect of Total Gas Flow Rate on Carbonation of RM

The carbonation of RM at different total gas flow rates (TF) was done in a

range of 100mL/min to 400mL/min. The air and CO2 gas from cylinders controlled

by Brooks flow rate meter were mixed together to create desired total flow rates of

gas mentioned above for each experiment. The kinetics of neutralisation of RM

slurry at different TF of gas is illustrated in Figure 4.9, and time-based data for the

residue carbonation is reported in Table B.9.

73

Figure 4.9. Carbonation of red mud by 30% of CO2, 250rpm and different TF of gas

Figure 4.9 indicates the dependence of carbonation process on the total flow

rate of gas. At the fixed 30% concentration of CO2, the equilibrium pH of the

carbonated residue decreased from 12.5 to 7.3 after an hour of carbonation at the

lowest TF of gas in the study (100mL.min-1). When the TF of gas was double the pH

values reached at 7.0 after half an hour and the carbonation time was only 4 times

less than at the highest TF of gas (400mL.min-1) in the study, indicating a strong

correlation between carbonation process and TF of gas. Figure 4.10 indicates that the

rate of carbonation increases with increasing total gas flow rate at a given CO2

concentration. This is because at higher total gas flow rate the absorption of CO2 gas

in the solution is perhaps complete (Çopur et al. 2007) leading to the sufficiency of

CO2 that speeds up the rate of carbonation process. The results of RM carbonation at

other CO2 concentrations at different total gas flow rate are reported in Figures B.7-

B.9.

74

Figure 4.10. Carbonation rate constant (k) for RM slurry at 30% CO2 concentration,

stirring speed 250rpm, and different total gas flow rate

4.2.3. Effect of Stirring Speed on Carbonation of RM

Plots and data of carbonation of RM slurry at different stirring speeds are

shown in Figure 4.11 and Table B.10. The stirring speeds investigated in the

carbonation of bauxite residue were at four different levels ranging from 250rpm to

700rpm. It can be seen from Figure 4.8 that while the impeller speed has no effect on

the final equilibrium pH, it has positive effect on the rate of pH reduction. When the

stirring speed was increased from 250rpm to 700rpm, the rate of reaction between

the gas and the mud increased, suggesting that at 250rpm the pH of the solution was

12.5, which decreased to a pH of 7.3 at 700rpm. The steady state pH level of 7.3 was

achieved after 30 minutes of carbonation at the stirring speed of 250rpm compared to

only 15 minutes at 700rpm.

75

Figure 4.11. Carbonation of red mud by 30% of CO2, TF of 200mL/min and

different stirring speeds

According to Figure 4.12, the rate of carbonation was proportional to the

agitation speeds. This is because the rate of CO2 absorption into the liquid phase is

enhanced by the agitation provided by the rotating impeller. If there was no stirring,

CO2 gas still absorbs into the liquid but at a very slow rate mainly due to molecular

diffusion. However, with the increase in stirring speeds the rate of CO2 absorption

was immediately boosted, indicating a physical process being involved in the

distribution of CO2 to the reaction site (Cardile et al. 1994). The results of

carbonation of RM at other CO2 concentrations and total flow rate of gas at different

agitation speeds were presented in Figures B.10-B.12.

76


concentration, TF of 200mL/min and different stirring speeds

4.2.4. Effect of Solids Concentrations in RM on Carbonation of RM

Figure 4.13 shows the effect of solids concentrations in RM slurry on the

carbonation process. The experiments were performed in three different solids

concentrations of RM slurry, namely 35%, 40%, and 44% solids by weight. It can be

seen that the equilibrium pH was achieved at 7.5 after 30 minutes of carbonation in

all three concentrations of RM slurry. The pH curves suggest that there was no effect

of solids concentrations of slurry in the study on neutralisation reaction efficiency.

This would be due to the CO2 gas just reacts with soluble alkaline components in the

liquid and lowering pH of the solution, while the solid did not contribute to the

reduction of pH. In a previous systematic study, Cardile (1994) used the factorial

experimental approach to evaluate the effect of RM slurry density on the carbonation

process. This experimental approach concluded that RM slurry density was the factor

77

having a little significance for reaction efficiency of carbonation process. This

conclusion has well supported for the above discussions.

Figure 4.13. Carbonation of red mud by 30% of CO2, TF of 200mL/min and stirring

speed of 250rpm, and different solids concentrations in RM

From Figure 4.14, it can be seen that at a given 30% CO2 concentration, total

gas flow rate of 200mL/min and stirring speed of 250rpm, the rate of carbonation of

different solids concentrations in RM was virtually unchanged. Furthermore, all red

mud samples were diluted by adding distilled water in the carbonation experiments.

Hence, the variation in the amount of solids is very small for these red muds. This

evidence confirmed the no effect of solids concentrations in RM on the carbonation

process. The results of RM carbonation at other CO2 concentrations, total flow rate

of gas and rotating speeds at different solids concentrations in RM were illustrated in

Figures B.13-B.19 and carbonation data are given in Table B.11.

78


concentration, TF of 200mL/min, and different solids concentrations in RM

4.2.5. pH Rebound in Carbonated RM

Figure 4.15 shows pH rebound of short term carbonated RM and carbonated

liquor at three CO2 concentrations of 15%, 50%, and 100%. It can be seen that

carbonated samples at different concentrations of CO2 have different rates of pH

rebound when exposing to atmospheric CO2 environment. For carbonated RM, the

pH rebound was very fast after one day of carbonation and gradually increased and

stabilised to a final value of about 9.7 after 20-25 days when equilibrated with the

atmosphere. In contrast, the pH rebound of carbonated liquor increased slowly and

took nearly a month to equilibrate with the atmosphere, which was a week slower

than that obtained with the carbonated RM slurry. The same phenomenon was

observed in previous study conducted by Rai (2013) when RM slurry was carbonated

in a multiple cycles of 7 weeks (pH rebound to 9.5). The basicity of the RM slurry

again increases as much caustic soda adhered to the red mud particles was slowly

leached in the solution. For carbonated liquor, the pH also rebound back with time

once CO2 gas added ceased. This was due to the carbonated liquors still contains the

79

mixture of CO32-/HCO3

- with pH values ranging from 8.3-10.6 (Cardile et al. 1994),

which drives the pH in the system back to 9.7. The plot of pH recovery for both

carbonated RM and carbonated liquor at other CO2 concentrations was illustrated in

Figures B.20.

Figure 4.15. pH rebound for both RM slurry and liquor at three CO2 concentrations,

TF of 200mL/min, stirring speed of 250rpm

Figure 4.16 describes the pH rebound of carbonated RM with different

concentrations of solids by weight (35%wt-44%wt). Accordingly, the experiments

suggested that the rate of pH rebound was influenced by the solids concentration

within two weeks, and then it reached to the same pH level of about 9.5-9.7. It can be

seen that carbonated RM with lower percentage of solid has a slower rate of pH

rebound compared with carbonated RM with a higher solids loading. Additionally,

while solids concentration has no effect on pH reduction by carbonation as discussed

previously, but it has some effects on pH rebound after carbonation. This may be

explained by the fact that pH reduction by carbonation was due to only the free soda

content in RM slurry was getting carbonated, whereas the bound soda concentration

80

adhered to solid phase was leached to the solution after carbonation driving the

phenomena of pH rebound. Previous study (Cardile et al. 1994) stated that total Na

content and RM slurry density were the main factors showing a significant effect on

the final pH value after carbonation. Therefore, it is thought that the pH rebound was

dependent of solids concentrations observed in this study because RM slurry with

higher % of solid (44%wt) may have more bound soda contents adhered to the RM

particles than RM with lower % of solid (35%wt).

Figure 4.16. pH rebound of carbonated RM slurries at different solids concentrations

4.2.6. Longer Carbonation of RM

Due to the time frame of experiment, longer carbonation of bauxite residue was

performed for 5 days. The carbonation aims to investigate the potential impact of

mineral transformation on the steady state pH level in the carbonation process.

Initially, the longer carbonation of RM slurry was done at the concentration of CO2

ranging from 15% - 60%, total gas flow rate of 200mL/min and stirring speed of

250rpm. However, 30% CO2 concentration was found to be effective because of a

81

greater amount of CO2 sequestration. Thus, the concentration of 30% CO2 was

selected for further carbonation experiments in the changes of different total gas flow

rate varying from 100mL/min-400mL/min, stirring speeds (250rpm-700rpm), and

solids concentrations in RM (35%wt-44%wt). The resulting pH values reached were

from 7.4 to 6.6 as indicated in Figure 4.17 and Tables B.12 - B.13.

Figure 4.17. Longer carbonation of RM slurry at different CO2 concentrations

Figure 4.17 shows that the concentration of CO2 still has effect on the

carbonation of RM in longer experiments. Accordingly, the different steady state pH

levels were obtained for different concentrations of injected carbon dioxide. When

using 15% of CO2 concentration, the final pH equilibrium level reached at 7.4,

whereas the pH was observed at 7.1 when the CO2 concentration was double, and 6.6

at the concentration of 60% CO2 after the first day of carbonation. From the first to

the last day of carbonation, there was a slight increase in the final pH equilibrium

level, for instance from 7.4 to 7.6 and 6.6 to 6.8 for the concentrations of 15% and

60% CO2, respectively. This observation was also found in previous study (Sahu et

82

al. 2010), which conducted the carbonation of RM slurry in three days. This may be

due to the dissolution of minerals buffering the pH values in the solution during the

carbonation process. The longer carbonation would suggest the dissolution of

sodalite as indicated by equation (4.1), and/or the break-up of cancrinite in the CO2

environment to release calcite (Sirbescu & Jenkins 1999; Yadav et al. 2009) as

below:

Na6(AlSiO4)6CaCO3(H2O)2 ↔ 6NaAlSiO4 + CaCO3 + H2O (4.3)

Figure 4.18. Longer carbonation of RM slurry at fixed 30% CO2, stirring speed of

250rpm and at different total gas flow rate

The final pH equilibrium results for long carbonation of RM slurry at a given

concentration of 30% CO2 at different total gas flow rate (100-400mL/min), stirring

speeds (250-700rpm) and solids concentrations in RM (35-44%wt) were plotted in

Figures 4.18-4.20. It can be seen that the final pH levels was constant at 7.0 for all

cases of carbonation in the period of 5 days. In other words, total gas flow rate,

stirring speed, and solid concentrations have no effect on the final pH of 5-day

carbonation of RM. This is because when CO2 gas was injected in RM slurry, it took

83

about an hour to reach the pH equilibrium of 7.0 as discussed previously. Further, if

the CO2 gas is consecutively added to the slurry in the period of 5 days, it will lead to

the excess CO2 amount absorbed into the system keeping the pH of the solution

constant.


of 200mL/min and different stirring speeds


of 200mL/min, stirring speeds of 250rpm and different solids concentrations of RM

84

4.3. Mineralogical Characterisation of Red Mud and Carbonated Red Mud

4.3.1. X-ray Diffraction Analysis

4.3.1.1. Solid phase composition in raw RM as quantified by XRD

Figure 4.21 shows the X-ray diffraction patterns of raw RM. The mineral

composition of this RM is comprised of predominantly hematite (Fe2O3), sodalite

(Na8(AlSiO4)6(OH)2.4H2O), and aluminium mineral compounds (Al2O3 &

AlO(OH)). Although the mineral analysis of RM has been reported in numerous

papers, the compositions of each RM sample differ because of the original

compositions of bauxite ore and the operating conditions used to extract alumina.

The elemental abundance in bauxite residue generally follow the order Fe > Si ~ Ti >

Al > Ca > Na (Gräfe et al. 2009; Liang et al. 2014). The mineral phase compositions

of the raw RM used in this study identified by XRD pattern consist of sodalite

(Na8(AlSiO4)6(OH)2.4H2O), cancrinite (Na6(AlSiO4)6(CaCO3)(H2O)2), boehmite

(AlO(OH)), gibbsite (Al(OH)3), anatase (TiO2), quartz (SiO2), and hematite (Fe2O3).

Figure 4.21. Variation of powder XRD pattern of raw RM

85

The quantification results determined by TOPAS V4.2 software in Figure 4.22

suggest that hematite occupied a large proportion (58%) as the major composition of

raw RM, followed by sodalite (~20%). Boehmite and anatase made up a minority of

~8.4% and ~6.8% of the total composition, respectively. Quartz accounted for only

~3.7%, while cancrinite and gibbsite accounted for even smaller amounts of ~2.3%

and ~1%, respectively. The broadness of phase peaks and quantifications in the XRD

patterns align with the following remarks from Grafe et al (2009), that approximately

70% (by weight) of bauxite phases are crystalline whereas the remaining 30% are

amorphous materials.

Figure 4.22. Phase composition quantification of raw RM

Figure 4.23 illustrates variation of carbonated XRD pattern compared to raw

RM. Changes to the XRD intensity associated to some mineral phases can be clearly

seen. This shows that these minerals are unstable in the CO2 environment during the

carbonation process. In particularly, Figure 4.23 indicates the dissolution of sodalite

(Na8(AlSiO4)6(OH)2.4H2O) and cancrinite-(Na6(AlSiO4)6CaCO3(H2O)2) and the

formation of calcite in the carbonation process. Both sodalite and cancrinite

86

structures have the same framework stoichiometry [AlSiO4]6, (Barnes et al. 1999a;

Gerson & Zheng 1997), the presence/absence of sodalite, therefore, is not easily

defined due to the high degree of overlap of the sodalite diffraction pattern by

cancrinite (Gerson & Zheng 1997). In Figure 4.23, there is no calcite peak in raw

RM pattern, but this peak is present in carbonated RM pattern at 330 2theta. Sodalite

and cancrinite peaks at 16.10, 270, 390 and 480 2theta show a decline but still present

after 5 days of carbonation, indicating the dissolution and/or break-up of these phases

caused the formation of calcite. Furthermore, the XRD pattern of treated red mud

revealed that the intensity of gibbsite at peak 210 2theta was significantly increased

while boehmite peaks at 170 and 310 2theta were decreased. The dissolution of

sodalite is responsible for the increase of gibbsite and over the long term carbonation

boehmite is converted to the more stable phase gibbsite (Khaitan et al. 2009b) as

indicated in Figure 4.23.

Figure 4.23. XRD pattern of carbonated RM compared with raw RM

87

4.3.1.2. Effect of CO2 concentrations on solid phase composition in carbonated RM

The mineral content of carbonated red mud at different concentrations of CO2

gas using TOPAS V4.2 was reported in Figures 4.24-4.27, and Table 4.3. It can be

seen that after carbonation at different CO2 concentrations, there were marked

changes in the content of certain minerals in the treated mud. In particularly, at 15%

of CO2, the amounts of sodalite and cancrinite were markedly reduced from 19.75%

and 2.32% to 16.03% and 1.53%, while the amounts of gibbsite and calcite rose from

1.04% and 0% to 3.47% and 0.81%, respectively. Other minerals also showed a

decrease after carbonation such as boehmite (from 8.35% to 8.3%), quartz (3.74% to

2.93%) and anatase (6.8% to 5.7%). However, when the concentration of CO2 was

doubled (30%), the percentages of sodalite and cancrinite decreased more

significantly to 15% and 0.82%, respectively. Similarly, the proportion of gibbsite

and calcite increased to 5.05% and 1.51%, respectively. It can be seen that the

increase in the calcite content (1.51%) corresponded to a similar decreased in

cancrinite (from 2.32% to 0.8%). This confirmed that the amount of calcite formed in

the carbonated RM was attributable to the breakdown of cancrinite in the CO2

environment (Yadav et al. 2009).

88


concentration and total gas flow rate of 200mL/min

Table 4.3. Effect of CO2 concentrations on the composition of solid phase in

carbonated RM as quantified by XRD

Composition Raw

RM

Phase quantification (%) of carbonated RM at fixed total gas flow

rate 200mL/min, stirring speed 250rpm and different CO2

concentrations

15%CO2 20%CO2 30%CO2 40%CO2 50%CO2 60%CO2

Sodalite

Cancrinite

Hematite

Boehmite

Quartz

Anatase

Gibbsite

Calcite

19.75

2.32

58.03

8.36

3.74

6.77

1.04

0.00

16.03

1.53

61.23

8.30

2.93

5.70

3.47

0.81

16.27

1.20

61.15

8.26

2.53

6.07

3.39

1.13

15.01

0.82

61.24

8.21

2.53

5.64

5.05

1.51

15.83

1.10

61.10

8.24

2.24

6.17

4.09

1.22

17.25

1.45

61.00

8.17

2.64

5.97

2.62

0.91

16.89

1.54

61.60

8.27

2.39

6.42

2.07

0.81

89

According to Figure 4.26, when the concentration of CO2 increased from 30%

to 40%, the percentage of gibbsite and calcite formed began to decline from 5.05% to

4.09% and 1.51% to 1.22%, respectively. The quantity of sodalite and cancrinite

correspondingly showed a slight increase from 15% to 15.83%, and 0.8% to 1.1%,

respectively. This means that less amount of sodalite and cancrinite was dissolved. If

the carbonation was done at higher concentration of CO2, for instance at 60%, as

shown in Figure 4.27 and Table 4.3, it can be seen that the dissolution of sodalite and

cancrinite was observed even lower (by ~2.9% and ~0.8% compare with the raw

RM), leading to the decrease in the formation of gibbsite and calcite to 2.07% and

0.81%, respectively. The quantifications of the minerals in the carbonated RM at

other CO2 concentrations are reported in Figures C.1-C.2.



90





91

4.3.1.3. Effect of total gas flow rate on solid phase composition in carbonated RM

The mineral phase composition of carbonated RM at a given 30% CO2

concentration, stirring speed of 250rpm and solids concentration of 44%wt but at

different total gas flow rate (TF) was presented in Figures 4.28-4.30, and Table 4.4.

It can be seen that the proportion of sodalite, cancrinite, and boehmite decreased at

all TF of gas. The dissolution of these minerals contributes to the increase in gibbsite

and calcite as discussed before. However, the amounts of gibbsite and calcite formed

at TF of 200mL/min were higher than that of other TF (100mL/min, 300mL/min, and

400mL/min). This again confirmed the efficiency of carbonation at 30% CO2 and TF

of 200mL/min.



92

Table 4.4. Effect of total gas flow rate on the composition of solid phase in

carbonated RM as quantified by XRD

Composition Raw

RM

Phase quantification (%) of carbonated RM at fixed

30% CO2 concentrations, stirring speed 250rpm, and

different total gas flow rate

100mL/min 200mL/min 300mL/min 400mL/min

Sodalite

Cancrinite

Hematite

Boehmite

Quartz

Anatase

Gibbsite

Calcite

19.75

2.32

58.03

8.36

3.74

6.77

1.04

0.00

17.97

1.58

61.13

8.31

2.78

6.97

1.50

0.77

15.01

0.82

61.24

8.21

2.53

5.64

5.05

1.51

15.71

1.33

61.15

8.28

2.62

5.66

4.24

1.01

15.79

1.60

61.63

8.30

2.38

5.77

3.79

0.84



93



4.3.1.4. Effect of stirring speed on solid phase composition in carbonated RM

Carbonated RM samples at fixed 30% CO2 concentration and TF of

200mL/min but at different stirring speeds were quantified the phase composition

and plotted in Figures 4.31-4.33 and Table 4.5. The results show that when the

stirring speed rose from 350rpm to 700rpm, the formation of calcite and gibbsite also

increased associated with the decrease in amounts of sodalite and cancrinite

suggesting that the impeller can boost the dissolution of sodalite and cancrinite.

94


concentration and stirring speed of 350rpm

Table 4.5. Effect of stirring speed on the composition of solid phase in carbonated

RM as quantified by XRD

Composition Raw

RM

Phase quantification (%) of carbonated RM at fixed

30% CO2 concentrations, total gas flow rate

200mL/min, and different stirring speeds

250rpm 350rpm 500rpm 700rpm

Sodalite

Cancrinite

Hematite

Boehmite

Quartz

Anatase

Gibbsite

Calcite

19.75

2.32

58.03

8.36

3.74

6.77

1.04

0.00

15.01

0.82

61.24

8.21

2.53

5.64

5.05

1.51

17.02

0.80

60.92

8.13

3.27

6.56

1.73

1.55

16.81

0.71

60.65

8.23

3.37

6.52

2.12

1.59

16.71

0.69

60.57

8.22

3.22

6.45

2.49

1.66

95





96

4.3.2. Micro-morphological Characterisation of raw RM and Carbonated RM by

SEM

Figure 4.34 describes the SEM images of raw red mud before treatment with

CO2. The SEM images showed that raw RM composed of the large rounded shape

aggregate particles. Sodalite’s SEM photomicrograph showing “cotton ball” type

morphology reported by others (Barnes et al. 1999a, 1999b) can be seen here in

Figure 4.34a. As previously discussed, sodalite and cancrinite structures have the

same framework stoichiometry [AlSiO4]6, (Barnes et al. 1999a; Gerson & Zheng

1997) and were identified by the same peaks in XRD pattern, but the “cotton ball”

type morphology of sodalite may be used to distinguish the two (Fig. 4.34b) (Deng et

al. 2006). However, due to its small concentration (2.32% as quantified by TOPAS),

cancrinite is morphologically indistinguishable from the surrounding sodalite crystals

in the SEM photomicrograph (Barnes et al. 1999a).

Figure 4.34. SEM imaging of raw RM: (a) Sodalite in “cotton ball” form, and (b)

Structure of crystalline sodalite

97

SEMs of carbonated red mud treated at different CO2 concentrations are shown

in Figure 4.35. In addition to the changes in the mineral content over the

experimental period by contacted with CO2, SEM showed a corresponding change in

crystalline morphology. Raw RM before treatment often has its large rounded shape

particles. However, after carbonation these large rounded shape objects were

decreased as indicated in Figure 4.35a, and more porous materials, which were

absent in raw RM, appeared in carbonated RM (Fig. 4.35a-f). This indicates that

some mineral phases such as sodalite, cancrinite containing in RM are more soluble

or broken-up in acidic environment to form porous materials (Huijgen et al. 2005;

Newson et al. 2006; Sahu et al. 2010; Yadav et al. 2009).

After carbonation, a small concentration of calcite (1.51%) was observed by

XRD. This phase was derived from the break-up of cancrinite during the period of

experiment. SEM image showed the presence of a hexagonal prismatic crystal in

carbonated RM (Fig. 4.35f) as cancrinite morphology (Barnes et al. 1999a), and the

porous coating adhering around was determined as calcite (Yadav et al. 2009).

Therefore, a conclusion that the formation of calcite in carbonated RM might be due

to the break-up of cancrinite in CO2 environment can be made.

98

Figure 4.35. SEM imaging of carbonated RM at different CO2 concentration, TF of

200mL/min and stirring speed of 250rpm (a) 15%CO2 (b) 20%CO2 (c) 30% CO2 (d) 40%CO2 (e) 50%CO2 (f) 60%CO2

99

4.3.3. Chemical Composition Changes by EDX

Major chemical compositions of solids in raw RM and carbonated RM

determined by Energy Dispersive X-ray (EDX) from spot analysis and pallet of the

samples at different concentrations of CO2 are given in Tables 4.6 and 4.7, and

Figure 4.36.



Major

element

Raw

RM

Red mud carbonated at different concentrations of CO2

15% 20% 30% 40% 50% 60% 75% 100%

C

O

Na

Al

Si

Ca

Ti

Fe

1.84 ±0.04

42.34 ±1.50

9.82 ±0.27

12.50±0.50

8.56 ±0.57

0.80 ±0.14

4.15 ±0.30

19.99±1.37

4.37 ±0.75

37.77±1.07

7.14 ±0.69

11.14±0.96

6.39 ±0.29

0.88 ±0.21

4.91 ±0.80

27.40±1.65

4.55 ±0.34

39.97±1.52

9.46 ±0.28

11.65±0.34

8.14 ±0.16

0.95 ±0.04

4.36 ±0.08

20.91±1.85

5.68 ±0.54

39.01±2.40

9.09 ±0.83

12.51±0.72

8.86 ±0.55

1.03 ±0.55

4.21 ±0.41

19.62±1.76

4.68 ±0.08

41.70±2.02

8.65 ±0.89

12.70±0.27

7.56 ±0.23

0.83 ±0.14

3.97 ±0.67

19.92±1.83

4.28 ±0.05

41.83±1.57

9.92 ±0.52

11.91±0.27

8.13 ±0.39

0.75 ±0.12

3.64 ±0.06

19.54±2.83

4.57 ±0.20

38.77±2.38

10.05±0.33

13.89±0.33

10.06±0.59

0.81 ±0.25

3.93 ±0.34

17.93±1.07

4.15 ±0.58

40.20±1.84

8.85 ±0.49

11.56±0.70

8.12 ±0.15

0.96 ±0.16

4.35 ±0.38

21.80±2.41

3.89 ±0.04

41.40±3.56

9.02 ±1.29

11.65±0.53

7.98 ±0.35

0.95 ±0.06

4.48 ±0.39

20.63±1.15

Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

The results of elemental composition and chemical composition of major

oxides determined by EDX were obtained from short-term (2 hours) carbonation

experiments. The results showed that the raw RM used in this research has a

particularly high content of aluminium, indicating that the operating conditions for

aluminium extraction were not optimised. The extent of carbonation was found to be

100

dependent on concentration of CO2 gas. After carbonation, carbonated RM samples

were rich in C at all concentrations of CO2, and reached the highest amount (5.68%

w/w) at 30% CO2 concentration, over 3 times higher than before treatment. The

amount of CO2 (%w/w) absorbed in carbonated RM increased with the increasing of

CO2 concentration, from 14.2%w/w at 15% CO2 to 14.87%w/w at 50% CO2, and

reached the highest amount of nearly 18%w/w at 30% CO2 concentration, 2.5 times

higher than raw RM. After that, the amount of CO2 absorbed by RM decreased

gradually with increasing CO2 concentration, and the lowest extent of carbonation

occurred at 100% CO2, where the amounts of both C and CO2 absorbed by RM was

recorded at 3.89% and 13.57%, respectively. This proved that there was a positive

effect of CO2 concentration on the amount of CO2 captured by RM in the

carbonation process. For the short-term carbonation, the highest extent of

carbonation would take place at 30% CO2 concentration with nearly 18% of CO2

absorbed by RM as illustrated in Figure 4.36. To confirm this result, further

experiments of RM carbonation at CO2 concentrations ranging from 15% to 60%

were carried out in 5 days, and then other carbonation experiments at fixed 30% CO2

concentration were investigated at different total flow rate of gas, stirring speeds, and

solids concentrations in RM.

101



Major

compound

Raw

RM


15% 20% 30% 40% 50% 60% 75% 100%

CO2

Na2O

Al2O3

SiO2

CaO

TiO2

Fe2O3

6.89 ±0.37

13.41±0.58

23.98±1.15

18.62±1.25

1.14 ±0.18

7.00 ±0.38

28.95±1.38

14.21±0.78

8.93 ±1.37

19.44±1.82

12.52±1.86

1.14 ±0.22

7.56 ±0.88

36.19±1.86

15.13±0.43

12.00±0.33

20.57±0.62

16.16±0.23

1.24 ±0.04

6.84 ±0.16

28.06±2.22

17.77±0.17

11.15±0.82

21.24±0.47

16.81±0.61

1.32 ±0.77

6.35 ±0.58

25.36±2.11

16.01±0.85

11.20±0.47

22.84±1.47

15.34±0.70

1.10 ±0.16

6.31 ±0.90

27.21±2.05

14.87±0.04

12.94±0.86

21.68±0.85

16.69±1.08

1.02 ±0.17

5.86 ±0.14

26.95±1.47

14.39±0.48

12.43±0.79

23.75±0.32

19.20±0.32

1.02 ±0.28

5.93 ±0.33

23.27±0.59

13.97±0.50

11.35±0.95

20.67±0.79

16.34±0.84

1.27 ±0.22

6.89 ±0.51

29.52±1.57

13.57±0.84

11.82±0.14

21.27±1.78

16.44±1.48

1.29 ±0.05

7.23 ±0.34

28.38±1.81

Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Figure 4.36. The amounts of C and CO2 absorbed by RM after 2-hour carbonation at

different CO2 concentration, TF of 200mL/min and stirring speed of 250rpm

102

The results of elemental and chemical compositions of carbonated red mud in

5-day carbonation are demonstrated in Figure 4.37 and Tables 4.8 & 4.9. It can be

seen that the amounts of C and CO2 absorbed rose significantly after longer

carbonation (5 days), reaching 5.39%w/w for C, and 16.02%w/w for CO2,

respectively at 15% CO2 concentration. The longer carbonation confirmed the

highest extent of carbonation again occurred at 30% CO2, where the amounts of C

and CO2 absorbed permanently by the solid were 9.07% and 23.93%w/w,

respectively. The extent of carbonation started going down at 40% CO2 with

7.64%w/w for C and 21.77%w/w for CO2. The study showed that during carbonation

process while the CO2 concentration increased, equilibrium pH of the solution

decreased but the amount of CO2 absorbed was found to reduce. This means that the

extent of carbonation would be effective at a particular CO2 concentration, and it was

at 30% CO2 in this research.


RM at 15%-60% CO2, TF of 200mL/min, 250rpm in 5 days of carbonation

Major

element

Raw

RM


15%

CO2

20%

CO2

30%

CO2

40%

CO2

50%

CO2

60%

CO2

C

O

Na

Al

Si

Ca

Ti

Fe

1.84 ±0.04

42.34 ±1.50

9.82 ±0.27

12.50 ±0.50 8.56 ±0.57 0.80 ±0.14 4.15 ±0.30

19.99 ±1.37

5.39 ±0.23

35.99 ±0.69

10.47 ±0.28

12.87 ±0.41 9.57 ±0.36 0.85 ±0.13 4.10 ±0.36

20.75 ±0.39

6.68 ±0.32

40.65 ±0.19 8.46 ±0.73

10.49 ±0.54 6.99 ±0.29 0.89 ±0.09 4.38 ±0.22

21.47 ±1.15

9.07 ±0.58

32.38 ±0.48 8.01 ±0.68

11.49 ±0.12 8.23 ±0.56 0.99 ±0.25 4.63 ±0.30

25.21 ±1.54

7.64 ±1.12

36.10 ±1.46 9.40 ±0.61

12.36 ±0.65 8.57 ±0.90 1.34 ±0.57 4.12 ±0.12

20.41 ±0.73

6.27 ±0.19

40.76 ±0.34

10.54 ±0.31

12.25 ±0.08 8.11 ±0.21 0.40 ±0.01 3.04 ±0.26

18.63 ±0.30

4.83 ±0.35

39.33 ±1.46 7.81 ±0.18

11.57 ±0.77 8.04 ±0.39 1.19 ±0.20 4.74 ±0.10

22.47 ±0.76

Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0

103


RM at 15%-60% CO2, TF of 200mL/min, 250rpm in 5 days of carbonation

Major

compound

Raw

Red

Mud

Red mud carbonated at different CO2 concentrations

15%

CO2

20%

CO2

30%

CO2

40%

CO2

50%

CO2

60%

CO2

CO2

Na2O

Al2O3

SiO2

CaO

TiO2

Fe2O3

6.89 ±0.37

13.41 ±0.58

23.98 ±1.15

18.62 ±1.25 1.14 ±0.18 7.00 ±0.38

28.95 ±1.38

16.02 ±0.69

12.47 ±0.27

21.09 ±0.51

17.46 ±0.46 1.03 ±0.14 5.98 ±0.54

25.96 ±0.73

21.59 ±0.86

10.54 ±0.98

18.16 ±1.05

13.57 ±0.61 1.14 ±0.13 6.73 ±0.32

28.27 ±1.37

23.93 ±0.69 8.76 ±0.36

17.17 ±0.76

13.60 ±1.78 1.10 ±0.23 6.24 ±0.38

29.20 ±1.48

21.77 ±0.66

10.93 ±0.47

19.72 ±0.22

15.17 ±0.69 1.61 ±0.79 5.91 ±0.21

24.88 ±0.69

20.19 ±0.44

13.17 ±0.45

21.20 ±0.03

15.74 ±0.36 0.52 ±0.01 4.66 ±0.37

24.52 ±0.51

15.76 ±1.54 9.77 ±0.38

20.13 ±1.07

15.71 ±0.47 1.54 ±0.24 7.32 ±0.07

29.76 ±0.66

Total 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Figure 4.37. Amounts of C and CO2 absorbed by RM after 5-day carbonation

104

Tables 4.10 & 4.11 and Figure 4.38 describe the major element and

compound composition of carbonated RM at a given 30% CO2 concentration and

different total gas flow rate. As discussed previously, total flow rate (TF) of gas has a

positive effect on the carbonation process, but the most effective extent of

carbonation occurred at total gas flow rate of 200mL/min (Fig. 4.37). At TF of

100mL/min, the amounts of C and CO2 absorbed by RM was about 5.24% and

17.24%w/w, respectively. When the TF of gas was doubled, the extent of

carbonation increased significantly with the absorbed C and CO2 amounts of 9.07%

and 23.93%w/w. The amounts of C and CO2 absorbed by RM started reducing at

higher total gas flow rate (300mL/min), with 7.05%w/w for C and 21%w/w for CO2.

Unfortunately, the higher the total flow rate of gas used the less effective is the

carbonation process and the smaller the amounts of C and CO2 captured by the RM.

The lowest amounts of C and CO2 (4.88% and 16.78%w/w, respectively) was

observed at TF of 400mL/min in the study.


RM at 30% CO2 concentration, 250rpm and different total gas flow rate

Major

element

Raw Red

Mud

Red mud carbonated at different TF of gas


C

O

Na

Al

Si

Ca

Ti

Fe

1.84 ±0.04

42.34 ±1.50

9.82 ±0.27

12.50 ±0.50

8.56 ±0.57

0.80 ±0.14

4.15 ±0.30

19.99 ±1.37

5.24 ±0.04

39.66 ±1.65

12.78 ±0.53

11.36 ±1.31

7.23 ±1.20

0.93 ±0.11

3.88 ±0.65

18.93 ±0.19

9.07 ±0.58

32.38 ±0.48

8.01 ±0.68

11.49 ±0.12

8.23 ±0.56

0.99 ±0.25

4.63 ±0.30

25.21 ±1.54

7.05 ±0.54

37.70 ±0.60

8.25 ±0.29

12.20 ±0.28

8.21 ±0.35

1.21 ±0.20

4.74 ±0.39

20.65 ±0.68

4.88 ±0.16

41.23 ±0.31

14.32 ±1.29

11.29 ±1.58

6.81 ±0.64

1.11 ±0.21

4.20 ±0.32

16.15 ±0.36

Total 100.0 100.0 100.0 100.0 100.0

105


RM at 30% CO2 concentration, 250rpm and different total gas flow rate

Major

compound

Raw Red

Mud

Red mud carbonated at different TF of gas


CO2

Na2O

Al2O3

SiO2

CaO

TiO2

Fe2O3

6.89 ±0.37

13.41 ±0.58

23.98 ±1.15

18.62 ±1.25

1.14 ±0.18

7.00 ±0.38

28.95 ±1.38

17.24 ±0.34

16.15 ±0.92

19.90 ±1.37

14.26 ±1.55

1.22 ±0.14

6.02 ±1.01

25.20 ±1.93

23.93 ±0.69

8.76 ±0.36

17.17 ±0.76

13.60 ±1.78

1.10 ±0.23

6.24 ±0.38

29.20 ±1.48

20.99 ±1.15

9.80 ±0.23

20.01±0.55

15.00 ±0.50

1.47 ±0.23

6.91 ±0.67

25.82 ±1.07

16.78 ±0.71

18.61 ±1.83

20.39 ±1.69

13.86 ±1.24

1.49±0.28

6.72 ±0.45

22.15 ±0.70

Total 100.0 100.0 100.0 100.0 100.0

Figure 4.38. Amounts of C and CO2 absorbed by RM at a given 30% CO2

concentration, 250rpm and different TF of gas

106

The large positive effect of stirring speeds on the extent of carbonation was

verified by results in Figure 4.39 and Tables 4.12 & 4.13. The results showed that the

extent of carbonation presented by the absorbed CO2 content was directly

proportional to the stirring speeds. It can be seen that there was a steady increase in

the CO2 content from nearly 24%w/w at 250rpm to 26%w/w at 350rpm, followed by

27.5%w/w at 500rpm, respectively, then, remarkably rose to 32.2%w/w at 700rpm.

The higher stirring speeds may help increase the solubility of CO2 gas in the RM

solution and boost the surface area of reaction between CO2 and the mud leading to

the greater extent of carbonation (Jones et al. 2006; Sahu et al. 2010).

Figure 4.39. Amounts of C and CO2 absorbed by RM at given 30% CO2, TF of

200mL/min and different stirring speeds

107


RM at 30% CO2, TF of 200mL/min and different stirring speeds

Major

element

Raw Red

Mud

Red mud carbonated at different stirring speeds


C

O

Na

Al

Si

Ca

Ti

Fe

1.84 ±0.04

42.34 ±1.50

9.82 ±0.27

12.50 ±0.50

8.56 ±0.57

0.80 ±0.14

4.15 ±0.30

19.99 ±1.37

9.07 ±0.58

32.38 ±0.48

8.01 ±0.68

11.49 ±0.12

8.23 ±0.56

0.99 ±0.25

4.63 ±0.30

25.21 ±1.54

9.51 ±0.31

35.48 ±1.87

9.21 ±1.18

12.60 ±1.49

8.79 ±1.67

1.03 ±0.07

3.93 ±0.17

19.45 ±1.14

10.34 ±0.54

34.99 ±1.89

8.28 ±0.60

12.29 ±1.13

8.23 ±0.73

0.99 ±0.19

4.50 ±0.07

20.40 ±0.70

10.98 ±0.48

40.47 ±1.65

7.79 ±0.16

10.43 ±0.50

6.72 ±0.38

0.85 ±0.21

3.86 ±0.45

18.91 ±0.89

Total 100.00 100.00 100.00 100.00 100.00


RM at 30% CO2, TF of 200mL/min and different stirring speeds

Major

compound

Raw Red

Mud

Red mud carbonated at different stirring speeds


CO2

Na2O

Al2O3

SiO2

CaO

TiO2

Fe2O3

6.89 ±0.37

13.41 ±0.58

23.98 ±1.15

18.62 ±1.25

1.14 ±0.18

7.00 ±0.38

28.95 ±1.38

23.93 ±0.69

8.76 ±0.36

17.17 ±0.76

13.60 ±1.78

1.10 ±0.23

6.24 ±0.38

29.20 ±1.48

25.88 ±1.46

10.38 ±0.97

19.38 ±1.35

14.88 ±0.95

1.18 ±0.02

5.39 ±0.53

22.91 ±1.29

27.46 ±0.97

9.19 ±0.46

18.65 ±1.33

13.80 ±0.43

1.11 ±0.18

6.08 ±0.29

23.71 ±1.08

32.18 ±0.65

9.12 ±0.41

16.83 ±0.85

12.08 ±0.60

1.02 ±0.27

5.53 ±0.55

23.23 ±0.55

Total 100.00 100.00 100.00 100.00 100.00

108

The solids concentrations in RM show its effect on the amounts of C and CO2

absorbed as given in Tables 4.14 & 4.15 and Figure 4.40. Accordingly, the contents

of C increased steadily but the amount of CO2 demonstrated a remarkable rise in all

treatments. Bauxite residue with 35% of solids by weight was carbonated yielding an

extent of about 12.6% of CO2 absorbed, doubled that before treatment. The extent of

CO2 absorbed was rising to 17.8% when the solids concentration in RM increased by

5%. From Figure 4.40, it is noticeable that if the solids concentration rose by ~10%,

from 35%wt to 44%wt, the extent of CO2 absorbed increased twofold. The

experiments suggest that the more CO2 can be captured if the RM slurry contains

more content of solids in the composition. This was due to RM with higher solids

concentrations may contain more cancrinite mineral that is responsible for reacting

with CO2 to form carbonates (Yadav et al. 2009).


RM at 30% CO2, TF of 200mL/min, 250rpm and different solids concentrations

Major

element

Raw Red

Mud

Red mud carbonated at different solids

concentrations

35%wt 40%wt 44%wt

C

O

Na

Al

Si

Ca

Ti

Fe

1.84 ±0.04

42.34 ±1.50

9.82 ±0.27

12.50 ±0.50

8.56 ±0.57

0.80 ±0.14

4.15 ±0.30

19.99 ±1.37

3.37 ±0.40

44.29 ±0.23

10.69 ±0.60

13.02 ±0.41

8.26 ±0.74

0.82 ±0.22

3.23 ±0.18

16.31 ±1.24

5.15 ±0.37

42.70 ±1.16

9.30 ±0.35

11.49 ±0.26

7.72 ±0.16

0.64 ±0.11

3.86 ±0.89

19.14 ±1.08

9.07 ±0.58

32.38 ±0.48

8.01 ±0.68

11.49 ±0.12

8.23 ±0.56

0.99 ±0.25

4.63 ±0.30

25.21 ±1.54

Total 100.0 100.0 100.0 100.0

109


RM at 30% CO2, TF of 200mL/min, 250rpm and different solids concentrations

Major

element

Raw Red

Mud

Red mud carbonated at different solids

concentrations

35%wt 40%wt 44%wt

CO2

Na2O

Al2O3

SiO2

CaO

TiO2

Fe2O3

6.89 ±0.37

13.41 ±0.58

23.98 ±1.15

18.62 ±1.25

1.14 ±0.18

7.00 ±0.38

28.95 ±1.38

12.56 ±0.40

14.54 ±0.84

24.86 ±0.86

17.87 ±1.63

1.17 ±0.30

5.44 ±0.32

23.55 ±1.76

17.80 ±0.93

12.12 ±0.81

20.88 ±0.67

15.82 ±0.31

0.86 ±0.13

6.17 ±0.21

26.36 ±0.87

23.93 ±0.69

8.76 ±0.36

17.17 ±0.76

13.60 ±1.78

1.10 ±0.23

6.24 ±0.38

29.20 ±1.48

Total 100.0 100.0 100.0 100.0

Figure 4.40. Amounts of C and CO2 captured by RM in different solids

concentrations

110

By comparing the extent of carbonation between 2-hour and 5-day

carbonation processes, it can be seen that in the longer carbonation the degree of

absorption of C and CO2 was significantly increased. In Figure 4.41, at 15% CO2

concentration, there was not much difference in the amounts of C (1%) and CO2

(1.8%) captured by solids in RM between 2-hour and 5-day carbonations. However,

the prominent difference between the short-term and long-term carbonations was

identified at CO2 concentrations varying from 20%-50% in the study. It is estimated

that at 30% CO2 concentration, the amounts of C and CO2 absorbed after 5-day

carbonation were 3.4% and 6.2%, respectively, higher than that of the 2-hour one.

The degree of carbonation at the higher CO2 concentrations also illustrated

that at the 40% CO2 concentration, this difference was dropped to 3% and 5.7% for C

and CO2, respectively, then continued to decrease to 2% and 5% for C and CO2 at the

50% CO2 concentration. Interestingly, the difference in the amounts of C and CO2

between short term and long-term carbonations seemed to be disappeared at the 60%

CO2 concentration. The comparison shows that the most optimal extent of

carbonation occurred at 30% CO2 concentration for both 2-hour and 5-day

carbonations because of the greater amount of CO2 absorbed by the RM. The study

suggests that at higher CO2 concentration, for instance 60% CO2, the extent of CO2

absorbed by the mud remained unchanged regardless of short term or long-term

carbonation.

111

Figure 4.41. Comparison of amounts of C and CO2 captured between 2-hour and 5-

day carbonation at fixed TF of 200mL/min, 250rpm and different CO2 concentrations

4.3.4. Determination of Alkalinity of RM and Carbonated RM

Total alkalinity is defined as concentration of free and combined sodium

hydroxide (NaOH) plus concentration of sodium carbonate (Na2CO3) in the solution,

and expressed as mg/L of CaCO3 equivalent (Rounds 2012). In this study, the

variations of alkalinity of raw RM and carbonated RM are indicated in Figure 4.42.

Raw red mud at an initial pH of 12.5 yielded an estimated alkalinity of 232 meq/L or

11,610 mg/L as total carbonate. The major contributor to the alkalinity as determined

by using the advanced speciation method (USGS 2007) was hydroxide and carbonate

anions with the values of 309mg/L and 6381mg/L, respectively, whereas the

concentration of bicarbonate anions (72.2mg/L) was much lower. After carbonation

with different concentrations of CO2, total gas flow rate of 200mL/min and stirring

speed of 250rpm, total alkalinity dropped rapidly to 3,088 mg/L, having lost over

8,500mg/L of alkalinity (initial pH of 7.5 at 15% CO2), while all hydroxide alkalinity

was consumed (non-detectable), and carbonate alkalinity was almost consumed,

112

reducing to 2mg/L. In contrast to these two fractions, bicarbonate alkalinity increased

to 3,761mg/L after carbonation with CO2. These changes of alkalinity in both raw

RM and carbonated RM are also in accordance to investigations reported by Jones et

al. (2006).

In raw red mud, from Figure 4.43a three distinct contributors to the alkalinity

were observed, reflecting the concentration of hydroxide (OH-), carbonate (CO32-),

and bicarbonate (HCO3-). However, from Figure 4.43b after carbonation at 30% CO2

concentrations (pH~7.1), only peak of bicarbonate was observed, indicating that the

concentration of OH- (non-detectable) and CO32- (0.8-2 mg/L) were virtually absent

in all treatments. It can be seen from Figure 4.42, total alkalinity of carbonated RM

decreased to 2,104 mg/L as total carbonate at 30% CO2 concentration, confirming

that the carbonation process would be optimised at this condition.

Figure 4.42. Changes in HCO3-, CO3

2-, and OH- alkalinity in raw RM and

carbonated RM at different concentrations of CO2, TF of 200mL/min, 250rpm

113

Figure 4.43. Acid titration curves for a) Raw RM and b) Carbonated RM at 30%

CO2, TF of 200mL/min and stirring speed of 250rpm

4.3.5. Thermal Analysis using TGA-DSC

The TGA-DSC analysis aims to not only measure physical or chemical

changes in both raw RM and carbonated RM but also determine the composition of

substances after carbonation. Figure 4.44 shows that the weight loss of RM takes

place in several steps. The first step occurred between 600-2000C where the RM

undergoes a weight reduction of 0.84% and the DSC peak centred at 650C due to the

removal of physically adsorbed moisture (Sushil et al. 2010). Another weight loss of

1.5% taking place in the range of 2000C to 4000C with the broad DSC peak at 2480C

can be attributed to the loss of structural H2O (Liu et al. 2007), and also the removal

of H2O from Al(OH)3 (Sahu 2011). The third endothermic effect between 4000C and

5750C, which corresponds to a weight loss of 0.6%, and the final step occurring in

the range from 5750C to 7000C associated with the weight loss of 0.35%, can be

attributed to the dehydroxylation of Ca(OH)2 to lime and partial dehydration of

silicates that continues up to over 7000C (Navarro et al. 2010).

114

Figure 4.44. TGA-DSC plots indicating weight loss of RM

For the carbonated RM, the thermal decomposition behaviours also occurred in

four steps as shown in Figure 4.45. Between 500C and 1700C, the weight loss was

determined about 2.12% of the total weight. Probably, this loss can be attributed to

the evaporation of physically absorbed water in the carbonated RM. Next step taking

place in the range from 2200C to 3800C was found to be 1.36% of weight loss. This

was proposed to be the loss of loosely and strongly bound water H2O in the minerals

(Sahu 2011), which may be due to the removal of H2O from sodalite and cancrinite

in the carbonated RM. The third step occurred in the range of 4400C-6000C with the

weight loss of 1.13%, possibly corresponding to the release of CO2 during

calcination of CaCO3 to CaO (Liu et al. 2007; Sushil et al. 2010; Zhang & Pan

2005). Finally, in the range from 6000C-7100C may take place the remaining

endothermic evolution of H2O and CO2 in the carbonated RM, the weight loss

associated to this process is about 0.6%.

115

Figure 4.45. TGA-DSC plots indicating weight loss of carbonated RM

4.3.6. FT-IR Spectroscopy

Raw RM contains a complex chemical composition of different minerals. Thus,

after contacting with CO2 gas some minerals may react with CO2 to form chemical

bonds in a molecule or a group of products, such as carbonate products. FR-IR

Spectroscopy helps identify these chemical bonds or stretching vibrations of

products. The FT-IR spectra of pristine raw RM and carbonated RM are presented in

Figure 4.46. The peaks illustrated at ~3300 cm-1 and ~1650 cm-1 region for both fresh

RM and carbonated RM are due to the stretching vibrations of OH group and of

molecular H2O, respectively (Cardell et al. 2009; Gok et al. 2007). This form of

water hydroxyl-stretching vibrations derived from water adsorbed on the outer

surface and free water between layers of RM structures. These vibrations, therefore,

are more intense in an infrared spectrum because of the large change in dipole

moment (Palmer et al. 2009). The bands that appear at 1412 cm-1 and 1410 cm-1 were

ascribed to C=O stretching vibrations, which confirm the existence of carbonate

groups (Cardell et al. 2009; Haberko et al. 2006). This was to prove that CO2 has

been absorbed by the pristine RM during carbonation process. Additionally, the

116

intensity of this peak in carbonated RM was decreased, which to a certain extent

confirmed the break-up of carbonate minerals such as cancrinite, when the RM

contacts with CO2.

The absorptions between 966 cm-1 and 963 cm-1 in both RM and carbonated

RM were characteristics of Si-O, or O-Si-O stretching modes of silica and silicates.

The bands at 527 cm-1 and 440 cm-1 correspond to Si-O-Al stretching vibrations and

Fe-O bonds, respectively (Gok et al. 2007; Navarro et al. 2010).

Figure 4.46. Fourier Transform Infrared (FT-IR) spectra of RM and carbonated RM

4.4. Determination of CO2 Sequestration

The amount of CO2 sequestered by RM can be estimated based on the quantity

of CO2 consumed by both the solid phase and liquid phase in the carbonation.

4.4.1. Determination of CO2 sequestered in 2-hour carbonation of RM

The amounts of CO2 sequestered by RM when carbonated at different

concentrations of carbon dioxide in the period of 2 hours are presented in Figure

4.47. Overall, the quantities of CO2 captured by RM differed when the mud was

117

carbonated at different CO2 concentrations. The experiments indicated that the

significant amount of CO2 captured by RM was recorded in the range of 15%-60%

CO2 concentration. Total gas flow rate (TF) of 200mL/min was found to be the most

effective where the amount of CO2 captured was prominent.

It can be seen that the highest amount of CO2 captured by RM was estimated to

be 4.56g CO2/100g of RM (or 45.6g CO2/kg RM) at 30% CO2 concentration. In the

range of CO2 concentration from 15% to 30%, the amount of CO2 captured by RM

increased significantly then decreased when the concentration of CO2 was higher

than 30%. However, the experiments suggested that the amounts of CO2 sequestered

by RM (3.9g and 3.51g CO2/100g of RM or 39g and 35.1g CO2/kg RM) at the

concentration of CO2 40% and 50% were still higher than that (3.67g and 3.45g

CO2/100g of RM or 36.7g and 34.5g CO2/kg RM) at 20% and 15%. In summary, the

most effective CO2 concentration for the sequestration was 30%, where the CO2

amount captured was highest in this study.

Figure 4.47. Amounts of CO2 sequestered by RM after 2-hour carbonation at

different CO2 concentrations, stirring speed of 250rpm

118

4.4.2. Determination of CO2 sequestered in 5-day carbonation of RM

The amounts of CO2 sequestered by both RM solid and RM liquor in 5-day

carbonation process at CO2 concentrations 15%-60%, total gas flow rate of

200mL/min and stirring speed of 250rpm are presented in Figures 4.48.

The 5-day carbonation experiments show the same trend of CO2 capture as that

observed with the 2-hour carbonation process as indicated in Figure 4.47. It can be

seen from Figure 4.48b that the amounts of CO2 consumed by RM liquor were

insignificant. The maximum quantity of CO2 absorbed by the liquor was found to be

0.139g/100g of RM liquor or ~1.4g CO2/kg RM liquor at 30% CO2 concentration,

whereas this amount in RM solid accounted for 6.36g CO2/100g of RM solid or

63.6g CO2/kg RM solid. This means that the liquor contributed only 2.2% to the CO2

sequestered by RM, while the remainder (~98%) came from the solid.

Totally, the amounts of CO2 sequestered by the whole RM were considered

both the solid phase and the liquid phase. In Figure 4.48, the amount of CO2 captured

by RM increased slightly from 4.0g CO2/100g of RM (or 40g CO2/kg RM) at 15%

CO2 concentration to 4.2g (or 42g CO2/kg RM) at 20% CO2 concentration. Again,

this amount reached its maximum value of 6.5g CO2/100g of RM (or 65g CO2/kg

RM) at 30% CO2 concentration, then decreased to 4.7g (or 47g CO2/kg RM) at 40%

CO2 concentration. The experiments also confirmed that the significant amount of

CO2 captured by RM occurred in the concentration of CO2 from 15%-60% as

discussed above.

119

Figure 4.48. Amounts of CO2 captured by RM (A): solid, (B): liquor, after 5-day

carbonation at different CO2 concentrations, TF of 200mL/min and speed of 250rpm

By comparing the potential of CO2 sequestration between 2-hour and 5-day

carbonations, it can be seen from Figure 4.49 that the longer time carbonation would

have yielded higher amounts of CO2 captured than the shorter time carbonation. The

experiments suggested that the amount of CO2 captured in the longer carbonation

was nearly 40% higher than that in the shorter carbonation at the same concentration

of CO2 (30%). This did confirm that the long-time carbon sequestration was

associated with interaction with bauxite residues, whereas short-time capture was

associated to reaction with the liquor. Carbon sequestration by the RM liquor is

associated with the conversion of hydroxide alkalinity to carbonate and bicarbonate

alkalinity (Cardile et al. 1994; Rai 2013; Shi et al. 2000), while the longer-time

carbon sequestration involved the reaction, dissolution and/or break-up of sodalite

and cancrinite to form calcite (Yadav et al. 2009) as discussed previously.

120

Figure 4.49. Comparison of CO2 amounts captured between 2-hour and 5-day

carbonations

Stirring speed had a positive effect on the carbonation efficiency as discussed

earlier. This also contributed to the carbon sequestration efficiency. The experiments

show that there was an increase in CO2 captured by RM when the stirring speed

increased. The CO2 amount captured rose by 0.34g/100g of RM (or 3.4g CO2/kg

RM) from the stirring speed of 250rpm to 700rpm as illustrated in Figure 4.50.

Figure 4.50. Amounts of CO2 captured by RM carbonated 30% CO2 concentration,

TF of 200mL/min and at different stirring speeds

121

With respect to carbon sequestration, the CO2 capture capacity also varied with

solids concentrations in RM studied in this research. Figure 4.51 demonstrates that

RM with higher concentration of solids by weight (44%wt) sequestered more CO2

(65g CO2/kg RM) than the RM with lower concentration of solids (40%wt and

35%wt, 39.6g and 33.2g CO2/kg of RM, respectively). This provides further

evidence that the carbon sequestration potential involved reaction with the solids of

red mud.

Figure 4.51. Amounts of CO2 captured by RM with different solids concentrations

carbonated at 30% CO2 concentration, TF of 250mL/min and 250rpm

In comparison of CO2 sequestration capacity of RM in this study with previous

researchers’ estimated results reported such as 23g CO2/kg of RM (Shi et al. 2000),

11.9g CO2/kg of RM (Khaitan et al. 2009b), 53g CO2/kg of RM (Yadav et al. 2009),

70.2g CO2/kg of RM (Sahu et al. 2010), 41.5g CO2/kg of RM (Bonenfant et al.

2008) as summarised earlier in Table 2.4, this study produced 65g CO2/kg of RM. In

122

fact, it is quite inappropriate to make the direct comparison of CO2 sequestration

capacity amongst RM from different studies due to vastly different mineralogy of the

residues.

Based on the annual production of 135 million tonnes of RM and future

inventory prediction of 5 billion tonnes by 2030 worldwide (Dentoni et al. 2014), it

is estimated that the amounts of CO2, which could be potentially captured by RM

were about 7.8 million tonnes per year and 325 million tonnes for the cumulation,

respectively. Similarly, the annual production of RM in Australia is about 30 million

tonnes (Sutar et al. 2014), which could potentially be used to capture ~2 million

tonnes of CO2. Presently, a full-scale carbonation facility has been established in

Western Australia by Alcoa. This facility helps to enhance the storage capacity of

RM by neutralising the RM before disposal, and to capture up to 4% of CO2 from the

refinery (Cooling et al. 2002). It is expected that the utilisation of red mud waste can

potentially remove large amounts of CO2 produced from industrial activities.

Moreover, the process of capturing CO2 or other forms of carbon by using the RM

waste would lock up large amounts of greenhouse gas released to the atmosphere and

to either solve environmental problems or mitigate and defer global warming.

4.5. Modelling of Carbonation Process

Modelling was performed on two processes: metal concentrations from long-

term titration and carbonation of RM. The chemical equilibrium modelling system

MINEQL+ 5.0 (Schecher & McAvoy 2015) was used to predict the heavy metal

species concentrations from long-term titration data and final pH equilibrium values

obtained from carbonation of both RM slurry and RM liquor.

123

4.5.1. Modelling of potentially dissolved metals

The general chemical equilibrium constants were used in the program to

calculate the metal ions concentrations during that accounts for the CO2 absorption

behaviour observed in the long-term titration at pH values of 4.5, 6, 8, 10, and 12.5.

The selection of solids governing aqueous chemistry as input data in the model was

based on the measured concentration of dominant constituents as indicated in Table

4.16. Additionally, the rapid and long-term titration data as discussed previously in

Figures 4.1&4.2 illustrated pH region where the dissolution of these solids occur, and

this information was used to choose the solids. The selection of solids was also based

on the measured metal concentrations at different pH values as given in Figure 4.52

(dots only). This information identifies the metals dissolving or precipitating when

red mud was titrated from pH of 12.5 to 4.5 corresponding to bicarbonate endpoint.

Table 4.16. Concentration of raw RM and liquor

Constituent

Concentration

in dried solids

(g/kg solid)(1)

Concentration

in RM slurry

(g/L liquor)(2)

Concentration

in slurry (M)

Concentration

in liquor (M)

Carbonate

Na

Al

Si

Ca

Ti

Fe

5.1(3)

98.2

125

85.6

8

41.5

200

4.01(3)

77.2

98.2

67.3

6.3

32.6

157.1

0.76

3.35

3.64

2.40

0.16

0.68

2.81

0.011(4)

0.198(5)

0.076(5)

8.58E-05

0.00017(5)

1.42E-05

5.36E-06(5)

(1): Determined by EDX. (2): Converted to RM liquor volume basis (from RM 44% solid by weight as received). (3): Determined in this study (5.1mgTIC/gRM) and converted to liquor volume basis (5.1*1000/0.56). (4): Measured by Total Organic Carbon Analyser, Model TOC-VCSH/CSN + TNM-1, Shimadzu Corporation, Japan. (5): Determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Model Agilent 7500cs, Agilent Technologies,

USA.

124

Figure 4.52. Metal concentrations in RM liquor as a function of pH

Chemical components for the RM slurry tabulated in Table 4.16 were used in

the MINEQL+ 5.0 program. As the MINEQL+ 5.0 focuses on concentrations

normalised to the liquid phase volume, the dried solid concentration of each

constituent in Table 4.16 was therefore input on a liquid volume basic. For carbonate

content, the TOTCO3 setting was changed to simulate a “Closed to the atmosphere”

system in equilibrium with a fixed value as given in Table 4.16. Because the system

does not contain sodalite and cancrinite, it is essential to include these minerals in the

system. In the runtime manager, the ionic strength (I) corrections setting was kept on,

and the method of ionic strength calculated by the program from the final

equilibrium composition was selected as this method is more likely to give the

accurate results (Schecher & McAvoy 2015). For each metal considered in the

simulation, it is important to choose metal compound in the solids that may

potentially dissolve. Finally, the metal ions species simulations were conducted at the

125

selected pH values of 4.5, 6, 8, 10 and 12.5 to produce outputs in terms of metal

concentrations and dissolution/precipitation reactions using logK values for the

reactions from the database in the system. A list of solid dissolution/precipitation

reactions associated with their logK values was presented in Table 4.17. The metal

compounds in the RM solids were also inputted in the simulation, as these metal ions

will control cation concentrations in the suspending medium or liquid. The logK

values were obtained from the MINEQL+ 5.0 database except for sodalite and

cancrinite.

Table 4.17. Solid precipitation/dissolution reactions in red mud model

Solid

formed

Reactions LogK(1)

Sodalite

Cancrinite

Boehmite

Hematite

Calcite

2H2O + 6Al3+ + 8Na+ + 6Si(OH)4 =

Al6Na8Si6O26H2(s) + 26H+

6Al3+ + Ca2+ + CO32- + 6Na+ + 6Si(OH)4 =

Al6CaNa6Si6O24CO3(s) + 24H+

2H2O + Al3+ = AlOOH(s) + 3H+

3H2O + 2Fe3+ = Fe2O3(s) + 6H+

Ca2+ + CO32- = CaCO3(s)

-34.2

-36.9

-8.578

1.418

8.48

(1): LogK values were obtained from MINEQL+ 5.0 database, except for sodalite (fitted), cancrinite (fitted)

4.5.1.1. Analysis of solids controlling Al

Al exists in bauxite residue slurry in the forms of soluble Al (predominantly

Al(OH)4- at high pH ~11-13) in RM liquor and Al solid (boehmite-AlO(OH) and

gibbsite-Al(OH)3). Additionally, previous work (Akitt et al. 1972; Mesmer & Baes

1971; Sposito 1989) confirmed that Al at high pH existed in various forms including

Al2(OH)24+, Al3(OH)4

5+, Al(H2O)63+, Al2(OH)2(H2O)8

4+, Al13O4(OH)24(H2O)127+ and

Al8(OH)20(H2O)x4+. However, it is important to determine which form of Al from

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these sources can control Al in bauxite residue and the carbonation process. As

discussed previously, Figure 4.1 indicated that the initial phase of titration was

controlled by RM liquor, and the later titration was controlled by the dissolution of

solids in the RM. This was confirmed by Khaitan et al. (2009a) stating that soluble

Al (predominantly Al(OH)4- at high pH ~11-13) was responsible for the initial buffer

region. In this study, it is not necessary to simulate all various forms of Al because

one of these species Al3(OH)45+ has been modelled by Khaitan et al. (2009a), and

stated that the results did not change significantly. The ANC in long-term titration of

RM (Fig. 4.2, Table 4.1) indicates that only 19% of ANC comes from liquid phase.

Thus, it is concluded that soluble Al in RM liquor would not affect the ANC, and Al-

containing solids in RM such as boehmite and gibbsite may control Al concentration.

Al solid was found to be present in RM in the form boehmite (AlO(OH)(s)) and

gibbsite (Al(OH)3(s)) as reported by numerous papers (Hanahan et al. 2004; Liang et

al. 2014; Rai 2013; Yadav et al. 2009). These solids were also identified in XRD

analysis and they might control Al concentration in the RM. Nevertheless, when

choosing potentially dissolved solids for simulation of Al metal, gibbsite and

boehmite cannot co-exist in a Gibb’s phase matrix for entire system because it leads

to a phase rule violation. This problem was also struggled by previous simulator

(Khaitan et al. 2009a), and concluded that gibbsite was found not to control Al in red

mud as it is the more stable phase and less soluble than boehmite. Therefore, in this

study, boehmite was selected in the Gibb’s phase matrix for entire system and it

worked smoothly with a logK of -8.578 by default matching well with initial Al

concentration of 0.076M.

The simulation of Al metal concentrations measured at different pH values in

Figure 4.52 describes the dissolution behaviour of boehmite in the system. Based on

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the data from Figure 4.2 as discussed earlier and Table A.7 in the appendix, there

was no boehmite dissolution observed in the long-term titration process. It is because

if boehmite could have dissolved during titration process, its dissolution would have

resulted in the exceedance of the ANC values. This was confirmed by Al

concentrations in RM liquor at different pH values (both experiment and simulation

data) from Figure 4.52, where Al remains unchanged at pH level of 4.5. Further, this

was well supported by the XRD analysis and quantitative results of carbonated RM

at different CO2 concentrations (Fig. 4.24-4.27), different total gas flow rate (Fig.

4.28-4.30), and different stirring speeds (Fig. 4.31-4.33), showing no change in the

proportion of boehmite.

4.5.1.2. Analysis of Na controlling solids

XRD results showed the presence of Na in two solid phases called sodalite

Na8(AlSiO4)6(OH)2.4H2O and cancrinite Na6(AlSiO4)6(CaCO3)(H2O)2. Previous

investigators (Barnes et al. 1999b; Gerson & Zheng 1997) reported that in Bayer

process plant, NaAlSiO4 is present mainly in the form of sodalite and to a lesser

extent cancrinite. The quantification results in this study also confirmed the

proportion of sodalite (19.75%) is many times higher than that of cancrinite (2.32%).

Thus, simulation focuses on the Na concentration controlled by sodalite. Simulation

was tried with two equilibrium constant values (logK) of sodalite, the first logK=34.2

reported by Wannenmacher et al. (2005), and the second logK=39.2, which was 5

orders of magnitude higher, reported by Deng et al. (2006). The results indicate that

formation of logK=34.2 was the best fitted modelled corresponding to initial Na

concentration 0.198M in Table A.7. The simulated results in Figure 4.52 show that at

pH 10, Na simulated concentration matched well with Na experimental value, but

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from pH 9 or lower, Na concentration increased quickly (0.32M at pH 8 and 0.36M

at pH 6) compared with experimental data (0.26M at pH 8 and 0.28M at pH 6) as

indicated in Tables A.7&A.8. Therefore, the simulation suggested that NaAlSiO4(s),

which is present mainly in the form of sodalite (Barnes et al. 1999b; Gerson &

Zheng 1997), dissolves at pH 9 rather than pH 8 as reported by Khaitan et al.

(2009a). The increase of Na concentration as a function of pH was attributable to the

dissolution of sodalite as illustrated by equation (4.1). In addition, the dissolution of

sodalite was confirmed by quantitative results in Figures 4.24-4.33 illustrating the

decrease of sodalite in carbonated RM at different CO2 concentrations, total gas flow

rate, and stirring speeds.

4.5.1.3. Analysis of Ca controlling solids

Although XRD analysis in Figure 4.21 did not show the presence of calcite in

raw RM, this pattern was prominently observed at 330 2theta in carbonated RM as

indicated in Figure 4.23. Further, the concentration of Ca in RM slurry as shown in

Table 4.16 was 0.16M, while the initial Ca concentration in RM liquor was virtually

zero (6.7mg/L or ~0.00017M). Therefore, it was concluded that Ca concentration

(0.16M) cannot be controlled by calcite in RM, but it should be controlled by a Ca-

bearing solid in RM. Many studies have noticed that Ca-bearing solid such as

tricalcium aluminate (Ca3Al2O6) existing in raw RM is often the main source to

produce Ca in porewater of RM and control the Ca concentration (Cardile et al.

1994; Khaitan et al. 2009a). However, in this study there was no tricalcium

aluminate peak to be observed in the bauxite residue pattern (Fig. 4.21) instead of

having the peak of another Ca-containing solid, namely cancrinite

Na6(AlSiO4)6(CaCO3)(H2O)2 at 16.10, 270, 390 and 480 2theta. This was used to

129

confirm that no tricalcium aluminate to be present in RM used in this study. Thus, it

was evident that cancrinite is the only source of controlling the Ca concentration.

XRD analysis in Figure 4.21 shows the possibility of the presence of cancrinite

at multi-peaks overlapping with sodalite. Sirbescu (1999) reported that there were

numerous types of cancrinite in the crystal structure database, but the cancrinite with

the structural formula expressed as Na6(AlSiO4)6(CaCO3)(H2O)2 is the only one that

can form or release calcite and nepheline. Although the amount of cancrinite in RM

was quite small (2.3%), it acts as a solid that controls Ca concentration in the system.

The formation logK= -36.9 of cancrinite as reported by Deng et al. (2006) was

applied in the system and worked well at different pH levels. The shape of the

simulated curve as shown in Figure 4.52 matches well with the experimental data.

Accordingly, the concentration of Ca increased as the pH decreased to 4.5, and this

was predicted accurately by the model. This confirmed that cancrinite dissolved to

control Ca concentration. This was further substantiated by the decrease in the

proportion of cancrinite in carbonated red mud at different operating conditions as

indicated by quantitative results in Figures 4.24-4.33.

4.5.1.4. Analysis of Fe controlling solids

The bauxite residue pattern in Figure 4.21 confirmed the presence of hematite

(Fe2O3(s)) in the RM. Therefore, it was included in the Gibb’s phase matrix for entire

system of the model. The dissolution of hematite was studied by modelling the

measured Fe concentration at different pH levels as shown in Figure 4.52. Data from

long-term titration in Figure 4.2 demonstrated that no hematite could dissolve during

the titration process. This was confirmed by the zero concentration of Fe being

measured at different pH levels in Table A.7. The equilibrium constant of Fe

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(logK=1.418) provided by the program did not constrain any simulated results of Fe

at any pH levels. Therefore, simulation result of Fe concentration, which was plotted

in Figure 4.52 and tabulated in Table A.8, matches well with Fe experiment

concentration at all pH levels. The model predicted no dissolution of hematite which

was also observed by previous investigator (Khaitan et al. 2009a).

4.5.2. Modelling of RM carbonation

The chemical equilibrium program MINEQL+ 5.0 (Schecher & McAvoy 2015)

was used to simulate the final equilibrium pH values from RM carbonation. The

calculation of equilibrium pH in the model was done for both RM slurry and RM

liquor. The concentration of all components from RM slurry and RM liquor in Table

4.16 was used throughout the program. The amount of total inorganic carbon (TIC)

of RM was determined by the difference between the total carbon (TC) and total

organic carbon (TOC). The average TC and TOC were 6.5mg/g of RM and 1.4mg/g

of RM, respectively, and the TIC achieved from RM was 5.1mg of C/g of RM. Based

on the solids concentration of RM (44%wt), the amount of TIC was converted to

9107mgC/L or 0.76M, and reported as carbonate in Table 4.16. The carbonate

content obtained from RM liquor was 0.011M. This indicates that carbonate content

in RM liquor occupied a very small proportion (1.5%), while the remainder (98.5%)

was in solid phase. The carbonate content in RM liquor was mainly in the forms of

CO32- and NaCO3

- predicted by the MINEQL+ as shown in Table 4.18.

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Table 4.18. Solid dissolution/precipitation and liquid reactions in RM simulation

Solid

formed

Potential Precipitation/Dissolution Reactions LogK(1)

Sodalite

Cancrinite

Calcite

Boehmite

2H2O + 6Al3+ + 8Na+ + 6Si(OH)4 =

Al6Na8Si6O26H2(s) + 26H+

6Al3+ + Ca2+ + CO32- + 6Na+ + 6Si(OH)4 =

Al6CaNa6Si6O24CO3(s) + 24H+

Ca2+ + CO32- = CaCO3(s)

2H2O + Al3+ = AlOOH(s) + 3H+

-34.2

-36.9

8.48

-8.578

Aqueous Species reactions

H2CO3 = 2H+ + CO32-

H+ + CO32- = HCO3

-

Al(OH)4- + 4H+ = 4H2O + Al3+

CO32- + Na+ = NaCO3

-

16.681

10.329

-22.688

1.270

(1): LogK values were obtained from MINEQL+ 5.0 database, except for sodalite (fitted) and cancrinite (fitted).

The procedures of modelling were similar to that of metal concentrations.

However, some adjustments were made in the program in order to achieve the good

results. For carbonate content, in the field labelled “Total CO3”, the TOTCO3 setting

was switched to an “Open to the atmosphere” system in that the partial pressure of

carbon dioxide (PCO2) was fixed. The concentrations of CO2 used for carbonation of

RM in this study were in percentage, so they were converted to partial pressure (PCO2)

to meet the requirement of the program. Because equilibrium pH values need to be

simulated, therefore the functions of “pH is calculated by MINEQL+” and “Base pH

calculation on Electroneutrality” in the calculation type of the Calculation Wizard

were selected in order to get more accurate results (Schecher & McAvoy 2015).

From the modelling performed, a list of solid and liquid phase reactions established

by MINEQL+ 5.0 was given in Table 4.18.

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4.5.2.1. Simulation of RM liquor

The chemical equilibrium model MINEQL+ 5.0 was used to calculate final pH

equilibrium values at different partial pressures of CO2 (PCO2 ranging 0.1-1atm)

corresponding to different CO2 concentrations (10%-100%). The concentration of

components for the RM liquor in Table 4.16 was used as input data for the

simulation system. The simulated results were plotted in comparison with the

experimental data in Figures 4.53&4.54.

It can be seen from the Figures 4.53 & 4.54 that the simulated results in all

cases were higher than that of experiment. Although the shape of simulated curves

was correlative to the experiments, the difference between experimental and

simulated carbonation of red mud was observed greater in some points of PCO2 values.

Generally, the RM liquor simulation in most PCO2 values yielded 0.1-0.2 pH units

higher than experimental data. Particularly, the RM liquor simulation at other PCO2

values from 0.4 to 1.0 in Figure 4.54 yielded 0.3 pH units higher than experimental

data. This means that the difference between experiment and model prediction is 4%.

The difference was also observed to increase with increasing total gas flow rate. The

carbonation of RM liquor means no solid phase present during the carbonation.

Therefore, the solid phase did not resulted in this difference. The only explanation

provided to the difference between simulated and experimental data was due to the

final pH equilibrium values obtained from RM liquor carbonation was carried out

under stirring speeds of 250rpm and different total gas flow rate. Meanwhile the

chemical equilibrium-modelling program MINEQL+ 5.0 was merely performed with

the information of RM liquor from Table 4.16. By comparison with the difference

(0.5 pH units) for the bauxite residue pore water only in previous study (Khaitan et

133

al. 2009b), the difference in this study was smaller. The data for experimental and

simulated carbonation of RM liquor at different CO2 concentrations and total gas

flow rate was tabulated in Tables D.1-D.4.


different CO2 concentration and TF of (A): 100mL/min, (B): 200mL/min


different CO2 concentration and TF of (C): 300mL/min, (D): 400mL/min

4.5.2.2. Simulation of RM slurry

Simulation of RM slurry was performed similarly to RM liquor, and all

information about the concentration of solids in the RM slurry as shown in Table

4.16 was used in MINEQL+ 5.0. The results for experimental and simulated

134

carbonation of RM slurry at different CO2 concentrations and total gas flow rate are

illustrated in Figures 4.55 & 4.56.


different CO2 concentration and TF of (A): 100mL/min, (B): 200mL/min


different CO2 concentration and TF of (C): 300mL/min, (D): 400mL/min

Like RM liquor simulation results, it can be seen from Figures 4.55 & 4.56 that

the modelled carbonation of RM slurry was also predicted higher pH values than

experiments. The simulated carbonation of RM slurry showed the constraint of

135

relative solids in the slurry resulting in a difference of 6.0% higher than the

experimental pH values as given in Tables D.1-D.4. The slow dissolution of sodalite

and cancrinite as predicted by the model may be responsible for the higher pH values

compared with the experimental data. Furthermore, as discussed in simulation for

RM liquor, the experimental pH values in RM slurry were reduced under the effect

of other physical factors such as stirring speed and total gas flow rate. Such factors

cannot be added or adjusted in the MINEQL+ 5.0 program. Therefore, if the

carbonation of RM slurry were performed in the laboratory without the effect of

stirring speed and total flow rate of gas, the simulated values would match well with

the experimental data. The maximum difference between experimental pH values

and modelled pH values for carbonation of RM slurry in this study was observed

about 6.0%. This difference could be in an acceptable level as it is about 2 times

lower than that of previous work (Khaitan et al. 2009b). The data for experimental

and simulated carbonation of RM slurry at different CO2 concentrations and total gas

flow rate were given in Tables D.1-D.4.

The modelling results in this study provided the insights about the potential of

heavy metal leaching from RM at different pH values, and the final equilibrium pH

values in the carbonation of red mud. The metal leaching results predicted by the

model helps to determine the key metals controlling the aqueous chemistry of the red

mud carbonation process. Although the simulated equilibrium pH difference for both

RM liquor and RM slurry was from 4.0-6.0% higher than experiments, this

difference was still lower than that of previous work (6.5-11.2%) (Khaitan et al.

2009b). However, the simulation results in this study were higher than the

experiment data, while the simulations presented earlier (Khaitan et al. 2009b) were

lower than experiments. This can be explained that the RM carbonation experiments

136

in this study were performed with the effects of physical factors such as stirring

speeds, total gas flow rate, whereas the experiments by Khaitan (2009b) were

conducted without these factors. The chemical model developed in this study can be

used to calculate metal speciation, solubility equilibria for projects working with the

dissolution of heavy metals and pH equilibrium calculation in an aqueous solution.

4.6. Summary

Acid neutralisation of the whole red mud slurries was done by both rapid and

long-term titration to pH endpoints of 4.5, 6, 8, and 10. The acid neutralisation

capacity was 0.79meq/g RM for rapid and 1.91meq/g RM for long-term titration,

respectively. The analysis of the long-term titration data for the whole RM slurry

illustrated that solids in RM slurry contributed about 81% of the ANC to pH 4.5,

whereas, the contribution of the liquid phase was only 19% at the same pH value.

The carbonation of bauxite residue was found to be significant dependent on

the concentrations of CO2, total flow rate of gas, and agitation speeds, while the

solids concentration in RM had a little effect on the carbonation process. The

carbonation of RM liquor involves the conversion of hydroxide and carbonate

alkalinity. The carbonation of solid phase involves the reactions of sodalite and/or

cancrinite dissolution resulting in the increase of gibbsite and the formation of

calcite, respectively. After carbonation, the pH of carbonated RM slurry and RM

liquor rebound back to 9.7 within 20-30 days. The amount of CO2 sequestered by

RM liquor was 1.4g/kg RM, meanwhile this quantity in RM slurry occupied about

63.6g/kg RM. The carbonation process was found to be the most efficient at 30%

CO2 concentration, total gas flow rate of 200mL/min with 65g CO2/kg RM captured

by the RM slurry. The CO2 sequestration capacity implies that there would be about

137

7.8 million tonnes of CO2 captured per year worldwide in general, and approximately

2 million tonnes of CO2 sequestered in Australia in particular.

Four key metals Al, Na, Ca, and Fe were found to control the chemistry of the

carbonation of RM as predicted by the simulation. However, the results of measured

metal concentrations from long-term titration of RM indicated that both Fe and Al

were not responsible for the control of carbonation process because they did not

dissolved in the titration at different pH values. The major solids governing the

carbonation process were considered as Na and Ca, which resulted from the

dissolution of sodalite and cancrinite. For the simulation of carbonation process, the

final equilibrium pH modelled values in all cases of both RM slurry and RM liquor

were higher than that of experimental data. The difference between the experimental

data and simulated results in both RM slurry and RM liquor was from 4.0-6.0%. The

difference was also increased with increasing total flow rate of gas. The slow

dissolution of sodalite and cancrinite may contribute a part to this difference.

138

CHAPTER 5 FINDING OUTCOMES AND CONCLUSIONS

Global warming and climate change have been of special interest to the

alumina industry because of its big demand of energy for the aluminium refining

process resulting in large CO2 emissions to the atmosphere. In addition, an enormous

amount of RM generated annually from this sector has posed major environmental

concerns. To confront these problems, neutralisation of RM by using CO2 gas brings

a promising potential for the industry to deal with both the CO2 emissions and RM

disposal problems. The process of capturing CO2 and mixing it with RM would lock

up large amounts of the greenhouse gases that otherwise would be discharged

directly into the atmosphere. A number of feasibility studies have evaluated treating

bauxite residue with different acidic sources such as SO2 from flue gases, seawater,

hydrochloric acid, acidic fly ash, and even CO2 aqueous and gas, which were

discussed in the literature review. However, none of them suggests any particular

condition, which the carbonation process would be optimised. Thus, a lab-scale

experimental carbonation process has been performed in a range of different

conditions in order to work out the specific condition for the carbonation process.

5.1. Major Findings of This Research

5.1.1. Acid Neutralisation Capacity (ANC) of Red Mud

Red mud was titrated in rapid and long-term titration scheme to pH 4.5 in order

to determine the acid neutralisation capacity of RM. Rapid titration of RM to pH 4.5

yielded an ANC of 0.79meq/g RM, while long-term titration got the value of

1.91meq/g RM at the same pH level. The rapid titration did not give the actual ANC

139

of the RM as it skipped the contribution of the solids to ANC. The liquid phase key

reactions taking place in the rapid titration were attributable to the ANC of the RM

liquor.

Al(OH)4-(aq) + H+ ↔ Al(OH)3

0 + H2O (5.1)

NaOH(aq) + H+ ↔ Na+ + H2O (5.2)

NaCO3-(aq) + H+ ↔ Na+ + HCO3

- (5.3)

Results from long-term titration of RM slurry suggested that long term ANC is

double that of rapid titration at the same endpoint pH. Solid phase plays as a major

contributor to the ANC of the RM with an estimate of 81% compared to 19% from

the liquid phase. The key reactions in the solid phase responsible for the ANC of the

RM slurry were as follows:

The dissolution of sodalite and cancrinite:

Na8(AlSiO4)6(OH)2.4H2O + 18H+ ↔ 8Na+ + 6Al3+ + 6Si(OH)4 + 2H2O (5.4)

Na6(AlSiO4)6(CaCO3)(H2O)2 +24H+↔ 6Na+ +6Al3+ +6Si(OH)4+CaCO3 +2H2O (5.5)

5.1.2. Carbonation of Bauxite Residue

In this project, bauxite residue was carbonated in a range of different

conditions such as concentrations of CO2 (10%-100%), total flow rate of gas (TF)

(100mL/min-400mL/min), stirring speeds (250rpm-700rpm) and solids

concentrations in RM (35%wt-44%wt). The results show that the carbonation

process was significantly dependent on the first three conditions mentioned above,

whereas the concentration of solids in RM was observed having a little effect on the

140

carbonation process. The pH of the RM decreased with increasing concentrations of

CO2, total flow rate of gas and stirring speeds, indicating the physical behaviour of

mixing and distribution of CO2 being important factor in the carbonation process. In

other words, the rate and extent of carbonation process was directly proportional to

the concentrations of CO2, total flow rate of gas and stirring speeds, while the

concentration of solids in RM had little effects on the carbonation process.

In the carbonation process, the reactions occurred in both the liquid and solid

phases. The carbonation reaction in the liquid phase with hydroxide to form

carbonate involved the following species NaOH, NaCO3- and Al(OH)4

- and the

formation or the precipitation of gibbsite.

2OH-(aq) + CO2(aq) → CO3

2- + H2O (5.6)

H2O + CO32- + CO2(aq) → HCO3

-(aq) + H+

(aq) (5.7)


(aq) → Al(OH)3(s) + Na+(aq) + HCO3

-(aq) (5.8)

The longer carbonation experiments resulted in the dissolution of sodalite

and/or cancrinite in the acidic environment. The dissolution of sodalite contributed to

the increase of gibbsite, while the dissolution of cancrinite helped to release calcite,

lowering the pH of the slurry. The longer carbonation reactions are expressed as

equations (5.4) & (5.5) above.

It was found that at the same CO2 concentrations ranging from 10%-100%, it

took about 1 hour to reach the equilibrium pH of 7.5-6.6, respectively for RM slurry,

while the steady state pH for the RM liquor was lower 7.0-6.3 (0.3-0.5 pH unit

lower) in the short term carbonation reaction. This suggested that the liquor

141

chemistry had an influence on the short-term carbonation and that there was no solid

dissolution occurred in this process.

In both carbonated RM slurry and RM liquor, the pH recovery phenomena

were found to rebound back to 9.7, but in different times and rates. The pH from

carbonated RM slurry rebound very fast and took about 20-25 days to stabilise at 9.7,

while the carbonated liquor showed a lower rate of pH recovery, and needed a month

(about one week slower) to equilibrate with the atmosphere. For carbonated RM

slurry, the pH rebound occurs due to bound soda adhered to the RM particles slowly

leached in the solution (Rai 2013), while for the RM liquor, the CO32-/HCO3

- mixture

was found existing in the liquor (Cardile et al. 1994) causing the reverse reactions as

expressed above.

In this work, the maximum quantity of CO2 absorbed by the liquor was found

to be ~1.4g/kg of RM liquor, whereas this amount in RM solid accounted for 63.6g

CO2/kg of RM solid. This means the liquor contributed only 2.2% to the CO2

sequestration, while the remainder (~98%) came from the solid. Furthermore, the

carbonation of RM in different concentrations of CO2 yielded the different amount of

CO2 captured, and the longer carbonation (5 days) gave more CO2 sequestration

potential than the short term (2 hours). The other physical factors such as total gas

flow rate, stirring speeds and solids concentrations in RM also had different effects

on the CO2 sequestration in carbonation process. However, the carbonation process

was found to be most efficient at 30% CO2 concentration, total gas flow rate of

200mL/min, where the captured CO2 amount obtained at the maximum value of 65g

CO2/kg of RM. At this condition, gibbsite and calcite concentration were observed to

142

be highest 5.05% and 1.51%, respectively as the result of dissolution of sodalite and

cancrinite during the carbonation process (Eqns. (5.4) & (5.5)). In addition, the

alkalinity decreased to 2,104mg/L as total carbonate at the CO2 concentration of 30%

and TF of 200mL/min, confirming the carbonation process was optimised at this

condition. The stirring speed was observed to have a very positive effect on the

carbonation efficiency and CO2 capture as well. The CO2 amount sequestered by RM

increased by 3.4g/kg RM when stirring speed rose from 250rpm to 700rpm.

Based on the CO2 sequestration potential values of 65g/kg of RM, it is

estimated about 7.8 million tonnes of CO2 would be captured per year from the

annual RM waste generation, and approximate 325 million tonnes of CO2

sequestered by the cumulative residue. These estimates were made using an annual

production of 135 million tonnes of RM and global inventory prediction of 5 billion

tonnes by 2030. For Australian alumina industry, it is estimated that the annual

production of red mud is about 30 million tonnes, which could be potentially used to

sequestered approximately 2 million tonnes of CO2. Thus, CO2 sequestration using

RM waste would be considered as an environmentally friendly technology for

strategies of mitigating the most challenging problem of global warming and climate

change.

5.1.3. Modelling of the Carbonation Process

The general chemical equilibrium modelling system MINEQL+ 5.0 was used

to simulate the heavy metal concentrations dissolved in long-term titration of red

143

mud at different pH levels of 4.5, 6, 8, 10, and 12.5 and final pH equilibrium values

obtained from carbonation of both RM slurry and RM liquor. The modelling

suggested that four key dominant metals Al, Na, Ca, and Fe were found to govern the

aqueous chemistry of the red mud carbonation process due to their presence in both

soluble and solid forms in red mud. Measured metal concentration from long-term

titration at various pH values indicated that boehmite (AlO(OH)) and hematite

(Fe2O3) did not dissolve in the system, therefore, both Al and Fe were not

responsible for the control of carbonation process as their concentrations remained

unchanged. However, Na and Ca were considered the major solids controlling the

process. The dissolution of sodalite (Na8(AlSiO4)6(OH)2.4H2O) and cancrinite

(Na6(AlSiO4)6(CaCO3)(H2O)2) as discussed earlier were attributable to Na and Ca

concentrations in the system. The key reactions are illustrated in equations (5.4) and

(5.5) above.

The chemical model was formulated in MINEQL+ 5.0 to calculate the final

equilibrium pH values for both carbonation of RM slurry and RM liquor at different

partial pressures of CO2 (PCO2 ranging 0.1-1atm) corresponding to different CO2

concentrations (10%-100%). In both RM solid and RM liquor simulation, the final

equilibrium pH simulated results in all cases were higher than that of experimental

data. Specifically, the modelled data in most PCO2 values yielded from 0.1-0.4 pH

units higher than experimental data. The difference was also observed to increase

with increasing total gas flow rate. The slow dissolution of sodalite and cancrinite as

predicted by the model may be a part of the higher pH values compared with the

experimental data. Additionally, the experimental pH values in RM slurry were

reduced lower because of the effect of physical factors such as stirring speed and

total gas flow rate in the carbonation process. Such these factors cannot be added or

144

adjusted in the MINEQL+ 5.0 program. The simulated values would match well with

the experimental data in the case of the carbonation of RM slurry performed without

the effect of stirring speed and total flow rate of gas. However, the difference would

be in an acceptable threshold as it is about 2 times lower than that of previous work

done by Khaitan (2009b).

5.2. Conclusions

Today, the use of industrial wastes as a waste remediation technology to solve

another environmental problem has become a promising solution worldwide. This

method not only helps to improve the activities of pollution control, but to provide an

economic incentive to reuse waste as a resource. Aluminium industry is one of the

industrial sectors producing large amounts of waste red mud and high levels of CO2

emissions in the environment. Therefore, the disposal of red mud is a major problem

for most alumina plants. One of the viable solutions, which could be developed and

applied for disposing of the waste economically and safely to the environment, is the

carbonation of RM. Neutralisation of red mud using carbon dioxide (CO2) will

convert the highly caustic waste to the less hazardous state, and reducing the release

of greenhouse gas to the environment.

This research has performed the carbonation of RM over a range of different

operating conditions in order to optimise the carbonation process and determine the

potential amount of CO2 captured by the red mud. From the lab-scale results of the

study, it is concluded that this research would provide alumina refineries with

145

promising solutions to the problem involving in red mud storage and utilisation.

Furthermore, the research also provide data and information about optimal conditions

of carbonation process that can be implemented for practical scale up and design of

red mud carbonation plant. Finally, the outcomes of this research could support other

industries that can use wastes from red mud as valuable materials in the efforts of

reducing CO2 emissions, and of contributing to the global warming and climate

change mitigation strategies.

146

CHAPTER 6 RECOMMENDATIONS FOR THE FUTURE

WORK

Annually, the enormous quantities of red mud have been quickly generated

worldwide posing an alarming environmental problem. Future predictions suggest

approximately 5 billion tonnes of red mud will be produced by 2030. It is essential to

develop more appropriate management methods and utilisation practices to

ameliorate this challenging problem. This research reveals only the optimal condition

for the carbonation of red mud in limited considerations. Therefore, future research

needs to take into account the following suggestions:

1. Carbonation process should be carried out under more different operating

conditions such as temperature, content of sodium in red mud, higher

stirring speeds, pressure of gas, and density of red mud in order to

determine the most involved factors controlling the efficiency of the

process.

2. Red mud can be carbonated by using flue gas (emissions) from other

sources instead of pure CO2 gas as done in this research, or the mix of flue

gas with other acid gas sources under different conditions. Then, the results

achieved from the most optimal condition can compare with those when the

red mud is carbonated by pure CO2.

3. A pilot plant of large-scale carbonation of red mud using CO2 gas should

be highly recommended for the sustainable development and cleaner

production of alumina industry.

147

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APPENDIX

Table A-1. Rapid titration of RM by 0.1N HCl

Volume of 0.1N

HCl (mL)

Meq/g

RM(*)

Meq/g

PW(**)

pH

Rep 1

pH

Rep 2

pH

Average

Std Dev

pH

0

4

8

12

15

20

25

28

31

34

37

40

43

46

50

54

57

60

64

68

72

74

76

78

79

0.00

0.04

0.08

0.12

0.15

0.20

0.25

0.28

0.31

0.34

0.37

0.40

0.43

0.46

0.50

0.54

0.57

0.60

0.64

0.68

0.72

0.74

0.76

0.78

0.79

0.00

0.07

0.14

0.21

0.27

0.36

0.45

0.50

0.55

0.61

0.66

0.71

0.77

0.82

0.89

0.96

1.02

1.07

1.14

1.21

1.29

1.32

1.36

1.39

1.41

12.42

11.6

10.44

9.55

8.86

7.83

7.23

7.01

6.74

6.56

6.38

6.08

5.98

5.74

5.49

5.38

5.25

5.1

5

4.8

4.7

4.63

4.6

4.55

4.48

12.49

11.66

10.53

9.71

9.04

8.03

7.4

7.09

6.85

6.62

6.42

6.3

6.1

5.85

5.6

5.36

5.29

5.19

5.05

4.91

4.78

4.72

4.67

4.6

4.54

12.46

11.63

10.49

9.63

8.95

7.93

7.32

7.05

6.80

6.59

6.40

6.19

6.04

5.80

5.55

5.37

5.27

5.15

5.03

4.86

4.74

4.68

4.64

4.58

4.51

0.05

0.04

0.06

0.11

0.13

0.14

0.12

0.06

0.08

0.04

0.03

0.16

0.08

0.08

0.08

0.01

0.03

0.06

0.04

0.08

0.06

0.06

0.05

0.04

0.04

(*) Mass of RM used: 10g (**) Normalised to RM liquor basis, divide by 0.56 as RM contains 44% solids.

166

Table A-2. Rapid titration of RM liquor to pH 4.5 by 0.1N HCl

Volume of 0.1N

HCl (mL)

Meq/g

PW(*)

pH(**)

Rep 1

pH(**)

Rep 2

pH

Average

Std Dev

pH

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

17.5

18

18.5

19

19.5

20.0

20.5

21.0

21.5

21.7

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

0.12

0.13

0.14

0.15

0.16

0.17

0.18

0.18

0.19

0.19

0.20

0.20

0.21

0.21

0.22

0.22

12.42

12.28

12.21

12.14

12.05

11.95

11.81

11.62

11.32

10.73

10.15

9.96

9.83

9.74

9.6

9.35

8.97

7.85

6.85

5.89

5.8

5.57

5.32

5.2

5.12

4.62

4.55

4.38

12.46

12.35

12.27

12.18

12.09

11.97

11.83

11.63

11.32

10.71

10.16

10.02

9.92

9.79

9.68

9.47

9.13

8.31

7.3

6.43

6.2

5.8

5.71

5.56

5.27

4.97

4.72

4.43

12.44

12.32

12.24

12.16

12.07

11.96

11.82

11.63

11.32

10.72

10.16

9.99

9.88

9.77

9.64

9.41

9.05

8.08

7.08

6.16

6.00

5.69

5.52

5.38

5.20

4.80

4.64

4.41

0.03

0.05

0.04

0.03

0.03

0.01

0.01

0.01

0.00

0.01

0.01

0.04

0.06

0.04

0.06

0.08

0.11

0.33

0.32

0.38

0.28

0.16

0.28

0.25

0.11

0.25

0.12

0.04

(*) Mass of RM liquor used: 10g

(**) Titration to pH 4.5

167

Table A-3. Long-term titration of RM to pH 4.5 by 0.1N HCl

Time

(day)

Acid

added(*)

0.1N HCL

mL, Rep 1

Acid added

meq/g

RM(**) Rep

1

Acid

added(*)

0.1N HCL

mL, Rep 2

Acid added

meq/g

RM(**) Rep 2

Acid added

Average

(Rep1,

Rep2)

Acid

added

Std

Dev

0

1

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

44

45

90.5

116.0

137.0

160.5

171.5

179.1

182.8

184.9

186.2

187.0

187.5

188.1

188.7

189.3

189.9

190.6

191.2

191.5

191.7

191.9

192.1

192.3

192.4

192.4

192.4

0.91

1.16

1.37

1.61

1.72

1.79

1.83

1.85

1.86

1.87

1.87

1.88

1.89

1.89

1.90

1.91

1.91

1.92

1.92

1.92

1.92

1.92

1.92

1.92

1.92

87.0

107.5

124.0

143.4

155.5

162.9

168.7

173.2

176.6

179.0

181.1

182.6

183.8

184.7

185.7

186.2

186.9

187.4

187.7

187.9

188.1

188.3

188.5

188.7

188.8

0.87

1.08

1.24

1.43

1.56

1.63

1.69

1.73

1.77

1.79

1.81

1.83

1.84

1.85

1.86

1.86

1.87

1.87

1.88

1.88

1.88

1.88

1.89

1.89

1.89

0.89

1.12

1.31

1.52

1.64

1.71

1.76

1.79

1.81

1.83

1.84

1.85

1.86

1.87

1.88

1.88

1.89

1.89

1.90

1.90

1.90

1.90

1.91

1.91

1.91

0.02

0.06

0.09

0.12

0.11

0.11

0.10

0.08

0.07

0.06

0.05

0.04

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

(*) pH was kept at 4.5 after each titration. (**) Mass of RM used: 10g

168


Time

(day)

Acid

added(*)

0.1N HCL

mL, Rep 1

Acid added

meq/g

RM(**) Rep

1

Acid

added(*)

0.1N HCL

mL, Rep 2

Acid added

meq/g

RM(**) Rep 2

Acid added

Average

(Rep1,

Rep2)

Acid

added

Std

Dev

0

1

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

44

45

49.5

60.2

66.7

72.1

74.7

77.0

79.3

81.9

84.1

86.1

87.4

88.4

89.6

90.8

92.1

93.3

94.4

95.6

96.6

97.7

98.7

99.8

100.9

101.6

101.8

0.50

0.60

0.67

0.72

0.75

0.77

0.79

0.82

0.84

0.86

0.87

0.88

0.90

0.91

0.92

0.93

0.94

0.96

0.97

0.98

0.99

1.00

1.01

1.02

1.02

50.2

61.4

68.2

74.2

77.1

79.7

82.2

84.6

87.0

88.9

90.3

91.4

92.6

93.8

94.9

95.9

97.1

98.2

99.2

100.2

101.2

102.0

103.0

104.0

104.5

0.50

0.61

0.68

0.74

0.77

0.80

0.82

0.85

0.87

0.89

0.90

0.91

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

1.01

1.02

1.03

1.04

1.04

0.50

0.61

0.67

0.73

0.76

0.78

0.81

0.83

0.86

0.87

0.89

0.90

0.91

0.92

0.93

0.95

0.96

0.97

0.98

0.99

1.00

1.01

1.02

1.03

1.03

0.00

0.01

0.01

0.01

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.01

0.02

0.02


169


Time

(day)

Acid

added(*)

0.1N HCL

mL, Rep 1

Acid added

meq/g

RM(**) Rep

1

Acid

added(*)

0.1N HCL

mL, Rep 2

Acid added

meq/g

RM(**) Rep

2

Acid added

Average

(Rep1,

Rep2)

Acid

added

Std

Dev

0

1

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

44

45

24.7

31.2

34.7

39.6

42.4

44.8

47.2

48.7

50.5

52.1

53.6

54.8

56.2

57.4

58.4

59.7

61.0

62.1

63.3

64.7

65.8

66.7

67.3

67.9

68.2

0.25

0.31

0.35

0.40

0.42

0.45

0.47

0.49

0.51

0.52

0.54

0.55

0.56

0.57

0.58

0.60

0.61

0.62

0.63

0.65

0.66

0.67

0.67

0.68

0.68

25.3

32.1

35.8

41.3

44.6

47.4

50.2

51.9

53.6

55.5

57.0

58.5

60.1

61.3

62.4

63.4

64.4

65.2

66.2

67.3

68.3

69.0

69.6

70.2

70.5

0.25

0.32

0.36

0.41

0.45

0.47

0.50

0.52

0.54

0.56

0.57

0.59

0.60

0.61

0.62

0.63

0.64

0.65

0.66

0.67

0.68

0.69

0.70

0.70

0.71

0.25

0.32

0.35

0.40

0.44

0.46

0.49

0.50

0.52

0.54

0.55

0.57

0.58

0.59

0.60

0.62

0.63

0.64

0.65

0.66

0.67

0.68

0.68

0.69

0.69

0.00

0.01

0.01

0.01

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.03

0.03

0.03

0.03

0.03

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02


170

Table A-6. Long-term titration of RM to pH 10 by 0.1N HCl

Time

(day)

Acid

added(*)

0.1N HCL

mL, Rep 1

Acid added

meq/g

RM(**) Rep

1

Acid

added(*)

0.1N HCL

mL, Rep 1

Acid added

meq/g

RM(**) Rep

1

pH

Average

Std

Dev

pH

0

1

2

4

6

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

44

45

11.2

13.9

15.3

17.0

18.4

19.8

21.0

22.3

23.8

25.1

26.1

27.3

28.8

29.8

30.9

32.0

32.9

33.6

34.3

35.0

35.7

36.3

36.7

37.1

37.3

0.11

0.14

0.15

0.17

0.18

0.20

0.21

0.22

0.24

0.25

0.26

0.27

0.29

0.30

0.31

0.32

0.33

0.34

0.34

0.35

0.36

0.36

0.37

0.37

0.37

9.2

11.6

13.2

14.5

15.6

17.0

18.1

19.1

20.3

21.7

22.7

23.5

24.5

25.5

26.6

27.7

28.6

29.2

29.7

30.2

30.6

30.9

31.1

31.3

31.4

0.09

0.12

0.13

0.15

0.16

0.17

0.18

0.19

0.20

0.22

0.23

0.24

0.25

0.26

0.27

0.28

0.29

0.29

0.30

0.30

0.31

0.31

0.31

0.31

0.31

0.10

0.13

0.14

0.16

0.17

0.18

0.20

0.21

0.22

0.23

0.24

0.25

0.27

0.28

0.29

0.30

0.31

0.31

0.32

0.33

0.33

0.34

0.34

0.34

0.34

0.01

0.02

0.01

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.04

0.04

0.04

0.04

0.04

(*) pH was kept at 10 after each titration. (**) Mass of RM used: 10g

171

Table A-7. Metal concentrations in RM liquor measured at different pH values

Component

Concentration (M)

Initial RM

liquor pH 10 pH 8 pH 6 pH 4.5

Al

Na

Ca

Fe

0.076

0.198

0

0

0

0.229

0.0015

0

0

0.262

0.032

0

0

0.280

0.035

0

0

0.487

0.104

0

Table A-8. Simulated metal concentrations in RM liquor at different pH values

Component

Concentration (M)

Initial RM

liquor pH 10 pH 8 pH 6 pH 4.5

Al

Na

Ca

Fe

0.052

0.23

0

0

0

0.24

0.0055

0

0

0.32

0.035

0

0

0.36

0.04

0

0

0.53

0.17

0

172







173







174

Figure B-7. Carbonation of RM by 25% CO2 at different TF of gas



175


stirring speeds


stirring speeds


stirring speeds

176


speed 250rpm at different solid concentrations of RM





177







178



Figure B-20. pH rebound for both RM slurry and liquor at some CO2 concentrations,

TF of 200mL/min, stirring speed of 250rpm

179

Table B-1. Carbonation of RM at different CO2 concentrations and total gas flow rate of 100mL/min, stirring speed of 250rpm

Time

(min)

25% CO2 30% CO2 40% CO2 50% CO2 60% CO2

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

12.4

11.77

10.88

10.3

10.01

9.37

9.42

9.07

8.70

8.44

8.21

8.05

7.97

7.89

7.83

7.77

7.73

7.69

7.65

7.63

7.59

7.57

7.55

7.53

7.51

12.59

12.02

10.81

10.24

9.95

9.52

9.01

8.48

8.15

7.97

7.83

7.77

7.69

7.63

7.61

7.57

7.55

7.51

7.49

7.47

7.45

7.45

7.45

7.45

7.45

12.50

11.90

10.85

10.27

9.98

9.45

9.22

8.78

8.43

8.21

8.02

7.91

7.83

7.76

7.72

7.67

7.64

7.60

7.57

7.55

7.52

7.51

7.50

7.49

7.48

0.13

0.18

0.05

0.04

0.04

0.11

0.29

0.42

0.39

0.33

0.27

0.20

0.20

0.18

0.16

0.14

0.13

0.13

0.11

0.11

0.10

0.08

0.07

0.06

0.04

12.42

11.76

10.65

9.94

9.45

9.04

8.57

8.27

7.91

7.66

7.52

7.43

7.38

7.33

7.27

7.24

7.21

7.21

7.20

7.20

7.20

7.20

7.20

7.20

7.20

12.28

11.62

10.45

10.22

10.07

9.64

8.99

8.25

7.87

7.6

7.46

7.35

7.28

7.25

7.23

7.22

7.21

7.21

7.22

7.22

7.22

7.22

7.22

7.22

7.22

12.35

11.69

10.55

10.08

9.76

9.34

8.78

8.26

7.89

7.63

7.49

7.39

7.33

7.29

7.25

7.23

7.21

7.21

7.21

7.21

7.21

7.21

7.21

7.21

7.21

0.10

0.10

0.14

0.20

0.44

0.42

0.30

0.01

0.03

0.04

0.04

0.06

0.07

0.06

0.03

0.01

0.00

0.00

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.32

10.6

9.68

9.01

8.21

7.51

7.31

7.25

7.15

7.09

7.06

7.04

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

12.42

11.00

10.21

9.61

8.57

8.07

7.63

7.37

7.27

7.21

7.18

7.16

7.10

7.10

7.10

7.10

7.10

7.10

7.10

7.10

7.10

7.10

7.10

7.10

7.10

12.37

10.8

9.95

9.31

8.39

7.79

7.47

7.31

7.21

7.15

7.12

7.1

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

0.07

0.28

0.37

0.42

0.25

0.40

0.23

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

0.08

12.58

10.52

9.38

8.06

7.45

7.17

7.07

6.99

6.95

6.93

6.93

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

12.51

10.76

9.69

8.40

7.64

7.33

7.17

7.09

7.03

6.99

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

12.55

10.64

9.54

8.23

7.55

7.25

7.12

7.04

6.99

6.96

6.95

6.94

6.94

6.94

6.94

6.94

6.94

6.94

6.94

6.94

6.94

6.94

6.94

6.94

6.94

0.05

0.17

0.22

0.24

0.13

0.11

0.07

0.07

0.06

0.04

0.03

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

12.5

10.31

8.74

7.60

7.20

7.14

6.97

6.94

6.92

6.89

6.89

6.89

6.89

6.89

6.89

6.89

6.89

6.89

6.89

6.89

6.89

6.89

6.89

6.89

6.89

12.34

10.17

8.62

7.50

7.10

6.84

6.89

6.86

6.84

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

12.42

10.24

8.68

7.55

7.15

6.99

6.93

6.9

6.88

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

0.11

0.10

0.08

0.07

0.07

0.21

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06 (*) Mass of red mud used: 100g

180

Table B-2. Carbonation of RM at different CO2 concentrations and total gas flow

rate of 100mL/min, stirring speed of 250rpm

Time

(min)

75% CO2 100% CO2

pH(*)

Rep1

pH(*)

Rep2 Avg pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2 Avg pH

Stdev

pH

0

2.5

5

7.5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

12.64

11.27

10.1

9.17

8.06

7.2

6.93

6.85

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

6.79

12.65

11.39

10.2

9.36

8.23

7.3

7.01

6.89

6.85

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

12.65

11.33

10.15

9.27

8.15

7.25

6.97

6.87

6.82

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

0.01

0.08

0.07

0.13

0.12

0.07

0.06

0.03

0.04

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

12.52

10.85

9.28

8.22

7.35

7.05

6.85

6.76

6.69

6.66

6.64

6.62

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

12.40

10.75

9.24

8.10

7.30

7.02

6.79

6.70

6.67

6.65

6.63

6.61

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

12.45

10.80

9.25

8.15

7.32

7.03

6.82

6.72

6.68

6.66

6.64

6.62

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

0.08

0.07

0.03

0.08

0.04

0.02

0.04

0.04

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

(*) Mass of red mud used: 100g

181


Time

(min)

15% CO2 20% CO2 25% CO2 30% CO2 40% CO2

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

12.42

11.69

10.65

10.20

9.82

9.54

9.13

8.64

8.26

8.02

7.85

7.75

7.68

7.62

7.60

7.56

7.54

7.52

7.52

7.52

7.52

7.52

7.52

7.52

7.52

12.30

11.60

10.55

10.11

9.74

9.46

9.04

8.58

8.21

7.94

7.80

7.69

7.62

7.56

7.54

7.50

7.48

7.46

7.46

7.46

7.46

7.46

7.46

7.46

7.46

12.36

11.65

10.60

10.16

9.78

9.50

9.09

8.61

8.24

7.98

7.83

7.72

7.65

7.59

7.57

7.53

7.51

7.49

7.49

7.49

7.49

7.49

7.49

7.49

7.49

0.08

0.06

0.07

0.06

0.06

0.06

0.06

0.04

0.04

0.06

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

12.3

11.41

10.36

9.77

9.35

8.84

8.28

8.02

7.70

7.55

7.47

7.45

7.44

7.38

7.37

7.35

7.33

7.32

7.3

7.3

7.28

7.28

7.28

7.28

7.28

12.50

11.60

10.61

10.14

9.67

9.20

8.60

8.20

8.00

7.80

7.61

7.57

7.50

7.48

7.46

7.44

7.42

7.40

7.38

7.38

7.38

7.38

7.38

7.38

7.38

12.40

11.51

10.49

9.96

9.51

9.02

8.44

8.11

7.85

7.68

7.54

7.51

7.47

7.43

7.42

7.40

7.38

7.36

7.34

7.34

7.33

7.33

7.33

7.33

7.33

0.14

0.13

0.18

0.26

0.23

0.25

0.23

0.13

0.21

0.18

0.10

0.08

0.04

0.07

0.06

0.06

0.06

0.06

0.06

0.06

0.07

0.07

0.07

0.07

0.07

12.57

11.46

10.47

9.98

9.44

8.81

8.32

7.93

7.69

7.55

7.45

7.38

7.33

7.3

7.28

7.26

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

12.46

11.01

10.10

9.47

8.75

8.17

7.75

7.55

7.43

7.35

7.29

7.25

7.23

7.21

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

12.52

11.24

10.29

9.73

9.10

8.49

8.04

7.74

7.56

7.45

7.37

7.32

7.28

7.26

7.24

7.23

7.22

7.22

7.22

7.22

7.22

7.22

7.22

7.22

7.22

0.08

0.32

0.26

0.36

0.49

0.45

0.40

0.27

0.18

0.14

0.11

0.09

0.07

0.06

0.06

0.05

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

12.34

10.81

9.91

9.11

8.20

7.70

7.52

7.40

7.32

7.25

7.22

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

12.43

10.60

9.73

8.90

7.92

7.50

7.39

7.25

7.21

7.15

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

12.39

10.71

9.82

9.01

8.06

7.60

7.46

7.33

7.27

7.20

7.18

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

0.06

0.15

0.13

0.15

0.20

0.14

0.09

0.11

0.08

0.07

0.06

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

12.35

10.43

9.12

7.82

7.30

7.13

7.06

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

12.48

10.26

8.95

7.67

7.15

7.00

6.95

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

12.42

10.35

9.04

7.75

7.23

7.07

7.01

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

6.98

0.09

0.12

0.12

0.11

0.11

0.09

0.08

0.09

0.09

0.09

0.09

0.09

0.09

0.09

0.09

0.09

0.09

0.09

0.09

0.09

0.09

0.09

0.09

0.09


182


Time

(min)

50% CO2 60% CO2 75% CO2 100% CO2 pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

0

2.5

5

7.5

10

12.5

15

20

25

30

35

40

45

50

60

70

80

90

100

110

120

12.48

11.35

10.68

9.7

8.8

8.01

7.5

7.24

7.13

7.11

7.07

7.05

7.03

7.03

7.03

7.03

7.03

7.03

7.03

7.03

7.03

12.4

11.20

10.00

9.54

8.70

7.48

6.86

6.80

6.78

6.76

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

12.44

11.28

10.34

9.62

8.75

7.75

7.18

7.02

6.96

6.94

6.91

6.90

6.89

6.89

6.89

6.89

6.89

6.89

6.89

6.89

6.89

0.06

0.11

0.48

0.11

0.07

0.37

0.45

0.45

0.31

0.25

0.25

0.23

0.22

0.21

0.21

0.21

0.21

0.21

0.21

0.21

0.21

12.45

10.83

9.81

8.85

7.67

7.15

6.95

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

12.40

10.92

9.82

8.90

7.68

7.18

7.00

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

12.43

10.88

9.82

8.88

7.68

7.17

6.98

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

0.04

0.06

0.01

0.04

0.01

0.02

0.04

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.42

10.37

8.98

7.93

6.95

6.62

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

12.43

10.29

8.72

7.87

6.89

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

12.43

10.33

8.85

7.90

6.92

6.59

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

0.01

0.06

0.18

0.04

0.04

0.04

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

12.36

10.38

7.84

7.13

6.8

6.65

6.6

6.58

6.54

6.54

6.54

6.54

6.54

6.54

6.54

6.54

6.54

6.54

6.54

6.54

6.54

12.47

10.33

7.78

7.07

6.81

6.67

6.61

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

12.42

10.36

7.81

7.10

6.81

6.66

6.61

6.58

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

6.56

0.08

0.04

0.04

0.04

0.01

0.01

0.01

0.00

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03


183


Time

(min)

10% CO2 15% CO2 20% CO2 25% CO2 30% CO2

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

12.48

11.02

10.16

9.48

8.70

8.10

7.71

7.63

7.54

7.52

7.50

7.50

7.50

7.50

7.50

7.50

7.50

7.50

7.50

7.50

7.50

7.50

7.50

7.50

7.50

12.36

10.82

10.00

9.30

8.40

7.84

7.65

7.48

7.44

7.42

7.42

7.42

7.42

7.42

7.42

7.42

7.42

7.42

7.42

7.42

7.42

7.42

7.42

7.42

7.42

12.42

10.92

10.08

9.39

8.55

7.97

7.68

7.56

7.49

7.47

7.46

7.46

7.46

7.46

7.46

7.46

7.46

7.46

7.46

7.46

7.46

7.46

7.46

7.46

7.46

0.08

0.14

0.11

0.13

0.21

0.18

0.04

0.11

0.07

0.07

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

12.38

11.14

10.03

9.29

8.54

8.05

7.68

7.55

7.43

7.34

7.32

7.28

7.26

7.26

7.26

7.26

7.26

7.26

7.26

7.26

7.26

7.26

7.26

7.26

7.26

12.32

10.4

9.87

9.51

8.36

7.83

7.6

7.37

7.33

7.30

7.3

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

12.35

10.77

9.95

9.4

8.45

7.94

7.64

7.46

7.38

7.32

7.31

7.26

7.25

7.25

7.25

7.25

7.25

7.25

7.25

7.25

7.25

7.25

7.25

7.25

7.25

0.04

0.52

0.11

0.16

0.13

0.16

0.06

0.13

0.07

0.03

0.01

0.03

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.34

10.75

9.93

9.23

8.36

7.82

7.63

7.52

7.38

7.32

7.27

7.25

7.23

7.20

7.17

7.17

7.17

7.17

7.17

7.17

7.17

7.17

7.17

7.17

7.17

12.34

10.09

9.51

8.97

8.08

7.88

7.63

7.44

7.38

7.28

7.25

7.23

7.21

7.2

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

12.34

10.42

9.72

9.10

8.22

7.85

7.63

7.48

7.38

7.30

7.26

7.24

7.22

7.20

7.18

7.18

7.18

7.18

7.18

7.18

7.18

7.18

7.18

7.18

7.18

0.00

0.47

0.30

0.18

0.20

0.04

0.00

0.06

0.00

0.03

0.01

0.01

0.01

0.00

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.34

10.42

9.91

9.1

8.22

7.85

7.63

7.42

7.21

7.18

7.15

7.13

7.11

7.11

7.11

7.11

7.11

7.11

7.11

7.11

7.11

7.11

7.11

7.11

7.11

12.54

10.58

9.49

8.84

7.78

7.23

7.05

7.06

7.15

7.14

7.13

7.11

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

12.44

10.5

9.7

8.97

8.0

7.54

7.34

7.24

7.18

7.16

7.14

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

0.14

0.11

0.30

0.18

0.31

0.44

0.41

0.25

0.04

0.03

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.40

10.34

9.42

8.13

7.69

7.32

7.19

7.17

7.13

7.09

7.07

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

12.38

10.22

9.10

7.71

7.13

7.10

7.03

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

12.39

10.28

9.26

7.92

7.41

7.21

7.11

7.07

7.05

7.03

7.02

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

0.01

0.08

0.23

0.30

0.40

0.16

0.11

0.14

0.11

0.08

0.07

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05


184


Time

(min)

40% CO2 50% CO2 60% CO2

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

0

2.5

5

7.5

10

12.5

15

20

25

30

35

40

45

50

60

70

80

90

100

110

120

12.57

11.33

10.35

9.67

8.82

7.67

7.28

7.07

6.95

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

12.52

11.05

10.15

9.39

8.52

7.65

7.2

7.02

6.98

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

12.55

11.19

10.25

9.53

8.67

7.66

7.24

7.05

6.97

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

0.04

0.20

0.14

0.20

0.21

0.06

0.04

0.02

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

12.49

10.90

9.96

8.99

8.15

7.39

7.09

6.97

6.91

6.89

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

12.4

10.74

9.69

8.56

7.86

7.2

6.96

6.86

6.82

6.80

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

12.45

10.82

9.83

8.78

8.01

7.30

7.03

6.92

6.87

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

0.06

0.11

0.19

0.30

0.21

0.13

0.09

0.08

0.06

0.06

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.52

10.52

9.26

8.3

7.39

7.11

6.95

6.88

6.84

6.8

6.76

6.76

6.76

6.76

6.76

6.76

6.76

6.76

6.76

6.76

6.76

12.44

10.31

9.19

8.1

7.26

6.85

6.82

6.76

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

12.48

10.42

9.23

8.20

7.33

6.98

6.89

6.82

6.79

6.77

6.75

6.75

6.75

6.75

6.75

6.75

6.75

6.75

6.75

6.75

6.75

0.06

0.15

0.05

0.14

0.09

0.18

0.09

0.08

0.07

0.04

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01


185


Time

(min)

10% CO2 15% CO2 20% CO2 25% CO2

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

12.43

11.05

10.1

9.65

9.05

8.57

8.02

7.8

7.7

7.65

7.6

7.57

7.55

7.52

7.52

7.52

7.52

7.52

7.52

7.52

7.52

7.52

7.52

7.52

7.52

12.33

10.65

10.05

9.55

8.95

8.23

7.89

7.72

7.6

7.5

7.46

7.43

7.43

7.43

7.43

7.43

7.43

7.43

7.43

7.43

7.43

7.43

7.43

7.43

7.43

12.38

10.85

10.08

9.60

9.00

8.40

7.96

7.76

7.65

7.58

7.53

7.50

7.49

7.48

7.48

7.48

7.48

7.48

7.48

7.48

7.48

7.48

7.48

7.48

7.48

0.07

0.28

0.04

0.07

0.07

0.24

0.09

0.06

0.07

0.11

0.10

0.10

0.08

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

12.36

11.12

10.01

9.52

8.78

8.16

7.91

7.79

7.66

7.54

7.48

7.42

7.4

7.39

7.39

7.39

7.39

7.39

7.39

7.39

7.39

7.39

7.39

7.39

7.39

12.42

10.24

9.75

9.18

8.42

7.90

7.55

7.41

7.38

7.38

7.34

7.32

7.29

7.29

7.29

7.29

7.29

7.29

7.29

7.29

7.29

7.29

7.29

7.29

7.29

12.39

10.68

9.88

9.35

8.6

8.03

7.73

7.6

7.52

7.46

7.41

7.37

7.345

7.34

7.34

7.34

7.34

7.34

7.34

7.34

7.34

7.34

7.34

7.34

7.34

0.04

0.62

0.18

0.24

0.25

0.18

0.25

0.27

0.20

0.11

0.10

0.07

0.08

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

12.32

10.42

9.39

7.96

7.4

7.29

7.15

7.13

7.11

7.09

7.09

7.09

7.09

7.09

7.09

7.09

7.09

7.09

7.09

7.09

7.09

7.09

7.09

7.09

7.09

12.42

10.48

9.27

8.66

8.0

7.51

7.47

7.43

7.41

7.37

7.32

7.27

7.23

7.19

7.19

7.15

7.15

7.15

7.15

7.15

7.15

7.15

7.15

7.15

7.15

12.37

10.45

9.33

8.31

7.70

7.40

7.31

7.28

7.26

7.23

7.21

7.18

7.16

7.14

7.14

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

0.07

0.04

0.08

0.49

0.42

0.16

0.23

0.21

0.21

0.20

0.16

0.13

0.10

0.07

0.07

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

12.3

10.1

8.71

7.68

7.21

7.05

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

12.46

9.72

8.41

7.42

7.03

7.00

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

6.96

12.38

9.91

8.56

7.55

7.12

7.03

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

6.99

0.11

0.27

0.21

0.18

0.13

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04


186


Time

(min)

30% CO2 40% CO2 50% CO2

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

0

2.5

5

7.5

10

12.5

15

20

25

30

35

40

45

50

60

70

80

90

100

110

120

12.38

11.30

9.80

9.05

8.10

7.32

6.98

6.92

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

12.32

11.12

9.75

8.89

7.80

7.02

6.95

6.88

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

12.35

11.21

9.78

8.97

7.95

7.17

6.97

6.90

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

0.04

0.13

0.04

0.11

0.21

0.21

0.02

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

12.55

10.9

9.88

8.82

7.94

7.26

6.97

6.93

6.91

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

12.5

10.7

9.65

8.46

7.78

7.10

6.86

6.82

6.80

6.80

6.80

6.80

6.80

6.80

6.80

6.80

6.80

6.80

6.80

6.80

6.80

12.53

10.80

9.77

8.64

7.86

7.18

6.92

6.88

6.86

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

0.04

0.14

0.16

0.25

0.11

0.11

0.08

0.08

0.08

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

0.07

12.36

10.85

9.74

8.86

8.01

7.31

7.03

6.95

6.89

6.87

6.87

6.87

6.87

6.87

6.87

6.87

6.87

6.87

6.87

6.87

6.87

12.49

10.55

9.55

8.31

7.51

7.05

6.89

6.85

6.84

6.84

6.80

6.80

6.80

6.80

6.80

6.80

6.80

6.80

6.80

6.80

6.80

12.43

10.70

9.65

8.59

7.76

7.18

6.96

6.90

6.87

6.86

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

0.09

0.21

0.13

0.39

0.35

0.18

0.10

0.07

0.04

0.02

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05


187

Table B-9. Carbonation of RM by 30% CO2 concentrations, stirring speed of 250rpm and different total gas flow rate

Time

(min)

TF of 100mL/min TF of 200mL/min TF of 300mL/min TF of 400mL/min

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

12.42

11.76

10.65

9.94

9.45

9.04

8.57

8.27

7.91

7.66

7.52

7.43

7.38

7.33

7.27

7.24

7.21

7.21

7.20

7.20

7.20

7.20

7.20

7.20

7.20

12.28

11.62

10.45

10.22

10.07

9.64

8.99

8.25

7.87

7.6

7.46

7.35

7.28

7.25

7.23

7.22

7.21

7.21

7.22

7.22

7.22

7.22

7.22

7.22

7.22

12.35

11.69

10.55

10.08

9.76

9.34

8.78

8.26

7.89

7.63

7.49

7.39

7.33

7.29

7.25

7.23

7.21

7.21

7.21

7.21

7.21

7.21

7.21

7.21

7.21

0.10

0.10

0.14

0.20

0.44

0.42

0.30

0.01

0.03

0.04

0.04

0.06

0.07

0.06

0.03

0.01

0.00

0.00

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.34

10.81

9.91

9.11

8.20

7.70

7.52

7.40

7.32

7.25

7.22

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

12.43

10.60

9.73

8.90

7.92

7.50

7.39

7.25

7.21

7.15

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

12.39

10.71

9.82

9.01

8.06

7.60

7.46

7.33

7.27

7.20

7.18

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

0.06

0.15

0.13

0.15

0.20

0.14

0.09

0.11

0.08

0.07

0.06

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

12.40

10.34

9.42

8.13

7.69

7.32

7.19

7.17

7.13

7.09

7.07

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

7.04

12.38

10.22

9.10

7.71

7.13

7.10

7.03

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

6.97

12.39

10.28

9.26

7.92

7.41

7.21

7.11

7.07

7.05

7.03

7.02

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

7.01

0.01

0.08

0.23

0.30

0.40

0.16

0.11

0.14

0.11

0.08

0.07

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

12.38

11.30

9.80

9.05

8.10

7.32

6.98

6.92

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

6.90

12.32

11.12

9.75

8.89

7.80

7.02

6.95

6.88

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

12.35

11.21

9.78

8.97

7.95

7.17

6.97

6.90

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

6.88

0.04

0.13

0.04

0.11

0.21

0.21

0.02

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03


188

Table B-10. Carbonation of RM by 30% CO2 concentrations, total gas flow rate of 200mL/min and different stirring speeds

250rpm 350rpm Time

(min)

500rpm 750rpm

Time

(min)

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

Time

(min)

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

12.44

10.81

9.91

9.11

8.2

7.7

7.52

7.4

7.32

7.25

7.22

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

12.53

10.60

9.73

8.90

7.92

7.50

7.39

7.25

7.21

7.15

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

12.49

10.71

9.82

9.01

8.06

7.60

7.46

7.33

7.27

7.20

7.18

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

0.06

0.15

0.13

0.15

0.20

0.14

0.09

0.11

0.08

0.07

0.06

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0

2.5

5

7.5

10

12.5

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

100

110

120

12.56

11.28

10.38

9.7

8.92

8.35

7.99

7.62

7.42

7.34

7.28

7.26

7.24

7.24

7.22

7.22

7.22

7.22

7.22

7.22

7.22

7.22

7.22

7.22

7.22

12.64

11.3

10.43

9.8

9.05

8.46

8.06

7.67

7.48

7.36

7.32

7.28

7.26

7.26

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

12.60

11.29

10.41

9.75

8.99

8.41

8.03

7.65

7.45

7.35

7.30

7.27

7.25

7.25

7.23

7.23

7.23

7.23

7.23

7.23

7.23

7.23

7.23

7.23

7.23

0.06

0.01

0.04

0.07

0.09

0.08

0.05

0.04

0.04

0.01

0.03

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0

1

2

3

4

5

6

7

8

9

10

12

14

16

20

24

28

32

40

50

60

70

80

100

120

12.63

12.09

11.12

10.61

10.22

9.8

9.36

8.93

8.59

8.34

8.14

7.87

7.69

7.57

7.42

7.34

7.3

7.28

7.26

7.26

7.26

7.26

7.26

7.26

7.26

12.55

11.9

10.96

10.46

10.04

9.56

9.04

8.64

8.35

8.13

7.96

7.72

7.56

7.46

7.34

7.28

7.24

7.22

7.22

7.22

7.22

7.22

7.22

7.22

7.22

12.59

12.00

11.04

10.54

10.13

9.68

9.20

8.79

8.47

8.24

8.05

7.80

7.63

7.52

7.38

7.31

7.27

7.25

7.24

7.24

7.24

7.24

7.24

7.24

7.24

0.06

0.13

0.11

0.11

0.13

0.17

0.23

0.21

0.17

0.15

0.13

0.11

0.09

0.08

0.06

0.04

0.04

0.04

0.03

0.03

0.03

0.03

0.03

0.03

0.03

12.59

11.8

10.84

10.32

9.76

9.16

8.69

8.37

8.13

7.95

7.82

7.62

7.51

7.43

7.33

7.27

7.25

7.25

7.25

7.25

7.25

7.25

7.25

7.25

7.25

12.5

11.75

10.84

10.31

9.81

9.24

8.76

8.42

8.17

7.99

7.84

7.64

7.52

7.44

7.32

7.28

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

7.24

12.55

11.78

10.84

10.32

9.79

9.20

8.73

8.40

8.15

7.97

7.83

7.63

7.52

7.44

7.33

7.28

7.25

7.25

7.25

7.25

7.25

7.25

7.25

7.25

7.25

0.06

0.04

0.00

0.01

0.04

0.06

0.05

0.04

0.03

0.03

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01


189

Table B-11. Carbonation of RM by 30% CO2 concentrations, TF of 200mL/min, speeds of 250rpm and different solids concentrations in RM

Time

(min)

RM-35%wt by solid RM-40%wt by solid Time

(min)

RM-44%wt by solid

pH(*)

Rep 1

pH(*) Rep

2 Avg pH Stdev pH

pH(*)

Rep 1

pH(*) Rep

2 Avg pH Stdev pH

pH(*)

Rep 1

pH(*) Rep

2 Avg pH Stdev pH

0

2.5

5

7.5

10

12.5

15

17.5

20

25

30

35

40

45

50

55

60

65

70

75

80

90

100

110

120

12.5

11.72

10.92

10.52

10.22

10.01

9.72

9.41

9.02

8.12

7.76

7.43

7.26

7.18

7.12

7.1

7.08

7.08

7.08

7.08

7.08

7.08

7.08

7.08

7.08

12.43

11.6

10.8

10.42

10.2

9.84

9.64

9.28

8.78

8.01

7.42

7.28

7.18

7.12

7.07

7.04

7.02

7.02

7.02

7.02

7.02

7.02

7.02

7.02

7.02

12.47

11.66

10.86

10.47

10.21

9.93

9.68

9.35

8.90

8.07

7.59

7.36

7.22

7.15

7.10

7.07

7.05

7.05

7.05

7.05

7.05

7.05

7.05

7.05

7.05

0.05

0.08

0.08

0.07

0.01

0.12

0.06

0.09

0.17

0.08

0.24

0.11

0.06

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

12.48

11.52

10.86

10.52

10.14

9.86

9.54

9.12

8.68

8.06

7.62

7.44

7.36

7.28

7.22

7.18

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

12.45

11.32

10.62

10.3

10.06

9.8

9.45

9.0

8.52

7.92

7.56

7.38

7.3

7.22

7.18

7.15

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

7.12

12.47

11.42

10.74

10.41

10.10

9.83

9.50

9.06

8.60

7.99

7.59

7.41

7.33

7.25

7.20

7.17

7.14

7.14

7.14

7.14

7.14

7.14

7.14

7.14

7.14

0.02

0.14

0.17

0.16

0.06

0.04

0.06

0.08

0.11

0.10

0.04

0.04

0.04

0.04

0.03

0.02

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

115

120

12.44

10.81

9.91

9.11

8.2

7.7

7.52

7.4

7.32

7.25

7.22

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

7.19

12.53

10.60

9.73

8.90

7.92

7.50

7.39

7.25

7.21

7.15

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

7.13

12.49

10.71

9.82

9.01

8.06

7.60

7.46

7.33

7.27

7.20

7.18

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

7.16

0.06

0.15

0.13

0.15

0.20

0.14

0.09

0.11

0.08

0.07

0.06

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04


190

Table B-12. Longer carbonation of RM at 15% - 30% CO2 concentrations, TF of 200mL/min and stirring speed of 250rpm

Time

(day)

15% CO2 20% CO2 30% CO2

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

0

1

2

3

4

5

12.48

7.45

7.47

7.6

7.61

7.64

12.52

7.36

7.45

7.56

7.56

7.62

12.5

7.41

7.46

7.58

7.59

7.63

0.03

0.06

0.01

0.03

0.04

0.01

12.48

7.2

7.2

7.21

7.21

7.31

12.45

7.18

7.19

7.19

7.2

7.32

12.47

7.19

7.20

7.20

7.21

7.32

0.02

0.01

0.01

0.01

0.01

0.01

12.56

7.23

7.18

7.18

7.1

7.16

12.46

7.12

7.13

7.12

7.09

7.15

12.51

7.18

7.16

7.15

7.10

7.16

0.07

0.08

0.04

0.04

0.01

0.01

Table B-13. Longer carbonation of RM at 40% - 60% CO2 concentrations, TF of 200mL/min and stirring speed of 250rpm

Time

(day)

40% CO2 50% CO2 60% CO2

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

0

1

2

3

4

5

12.48

7.1

6.93

6.9

6.88

6.96

12.5

7

6.95

6.9

6.87

6.95

12.49

7.05

6.94

6.90

6.88

6.96

0.01

0.07

0.01

0.00

0.01

0.01

12.52

6.92

6.81

6.8

6.8

6.9

12.5

6.82

6.78

6.78

6.79

6.83

12.51

6.87

6.80

6.79

6.80

6.87

0.01

0.07

0.02

0.01

0.01

0.05

12.53

6.62

6.6

6.7

6.7

6.78

12.46

6.52

6.58

6.66

6.64

6.75

12.50

6.57

6.59

6.68

6.67

6.77

0.05

0.07

0.01

0.03

0.04

0.02


191

Table B-14. Longer carbonation of RM at by 30% CO2 concentrations, stirring speed of 250rpm and different total gas flow rate

Time

(day)

TF of gas 100mL/min TF of gas 200mL/min TF of gas 300mL/min TF of gas 400mL/min

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

0

1

2

3

4

5

12.44

7.17

7.21

7.36

7.34

7.36

12.45

7.18

7.2

7.35

7.34

7.33

12.45

7.18

7.21

7.36

7.34

7.35

0.01

0.01

0.01

0.01

0.01

0.01

12.56

7.23

7.18

7.18

7.1

7.16

12.46

7.12

7.13

7.12

7.09

7.15

12.51

7.18

7.16

7.15

7.10

7.16

0.07

0.08

0.04

0.04

0.01

0.01

12.48

7.13

7.2

7.27

7.1

7.31

12.47

7.15

7.19

7.26

7.12

7.33

12.48

7.14

7.20

7.27

7.11

7.32

0.01

0.01

0.01

0.01

0.01

0.01

12.48

7.22

7.17

7.25

7.34

7.34

12.5

7.21

7.19

7.26

7.35

7.36

12.49

7.22

7.18

7.26

7.35

7.35

0.01

0.01

0.01

0.01

0.01

0.01

Table B-15. Longer carbonation of RM at by 30% CO2 concentrations, stirring speed of 250rpm, TF of 200mL/min and different solids

concentrations in RM

Time

(day)

RM-35%wt RM-40%wt RM-44%wt

pH(*)

Rep 1

pH(*) Rep

2 Avg pH Stdev pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH Stdev pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH Stdev pH

0

1

2

3

4

5

12.52

7.08

7.12

7.05

7.08

7.12

12.49

7.1

7.1

7.09

7.1

7.08

12.51

7.09

7.11

7.07

7.09

7.10

0.02

0.01

0.01

0.03

0.01

0.03

12.48

7.18

7.12

7.16

7.15

7.12

12.55

7.1

7.11

7.13

7.1

7.09

12.52

7.14

7.12

7.15

7.13

7.11

0.05

0.06

0.01

0.02

0.04

0.02

12.56

7.23

7.18

7.18

7.1

7.16

12.46

7.12

7.13

7.12

7.09

7.15

12.51

7.18

7.16

7.15

7.10

7.16

0.07

0.08

0.04

0.04

0.01

0.01


192

Table B-16. Carbonation of RM liquor at different CO2 concentrations, total gas flow rate of 100mL/min and stirring speed of 250rpm

Time

(min)

25% CO2 30% CO2 40% CO2 50% CO2

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

0

2.5

5

7.5

10

12.5

15

17.5

20

22.5

25

30

35

40

45

50

55

60

65

70

75

80

85

90

12.5

12.17

11.43

10.85

10.48

10.3

10.24

10.12

10.02

9.79

9.5

8.96

8.07

7.57

7.3

7.13

7

6.94

6.88

6.88

6.88

6.88

6.88

6.88

12.47

12.11

11.34

10.78

10.42

10.23

10.14

10.10

10.00

9.78

9.42

8.94

8.00

7.50

7.20

7.05

6.98

6.96

6.90

6.88

6.88

6.88

6.88

6.88

12.49

12.14

11.39

10.82

10.45

10.27

10.19

10.11

10.01

9.79

9.46

8.95

8.04

7.54

7.25

7.09

6.99

6.95

6.89

6.88

6.88

6.88

6.88

6.88

0.02

0.04

0.06

0.05

0.04

0.05

0.07

0.01

0.01

0.01

0.06

0.01

0.05

0.05

0.07

0.06

0.01

0.01

0.01

0.00

0.00

0.00

0.00

0.00

12.48

11.64

10.87

10.41

10.23

10.08

9.96

9.85

9.69

9.3

8.47

7.63

7.23

7.03

6.93

6.87

6.83

6.81

6.79

6.77

6.75

6.75

6.75

6.75

12.46

11.6

10.8

10.33

10.1

10.04

9.92

9.80

9.66

9.27

8.43

7.60

7.21

7.00

6.91

6.85

6.83

6.80

6.78

6.76

6.76

6.76

6.76

6.76

12.47

11.62

10.84

10.37

10.17

10.06

9.94

9.83

9.68

9.29

8.45

7.62

7.22

7.02

6.92

6.86

6.83

6.81

6.79

6.77

6.76

6.76

6.76

6.76

0.01

0.03

0.05

0.06

0.09

0.03

0.03

0.04

0.02

0.02

0.03

0.02

0.01

0.02

0.01

0.01

0.00

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.48

11.02

10.4

10.21

10.04

9.83

9.59

9.22

8.5

7.47

7.05

6.88

6.78

6.72

6.68

6.66

6.66

6.66

6.66

6.66

6.66

6.66

6.66

6.66

12.52

11.1

10.46

10.24

10.09

9.86

9.62

9.25

8.55

7.50

7.12

6.92

6.80

6.74

6.70

6.65

6.65

6.65

6.65

6.65

6.65

6.65

6.65

6.65

12.50

11.06

10.43

10.23

10.07

9.85

9.61

9.24

8.53

7.49

7.09

6.90

6.79

6.73

6.69

6.66

6.66

6.66

6.66

6.66

6.66

6.66

6.66

6.66

0.03

0.06

0.04

0.02

0.04

0.02

0.02

0.02

0.04

0.02

0.05

0.03

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.47

10.6

10.23

10.02

9.78

9.44

8.81

7.92

7.45

6.98

6.78

6.68

6.62

6.58

6.56

6.54

6.52

6.52

6.52

6.52

6.52

6.52

6.52

6.52

12.46

10.55

10.2

10.00

9.75

9.41

8.78

7.90

7.41

6.93

6.72

6.65

6.60

6.57

6.55

6.53

6.53

6.53

6.53

6.53

6.53

6.53

6.53

6.53

12.47

10.58

10.22

10.01

9.77

9.43

8.80

7.91

7.43

6.96

6.75

6.67

6.61

6.58

6.56

6.54

6.53

6.53

6.53

6.53

6.53

6.53

6.53

6.53

0.01

0.04

0.02

0.01

0.02

0.02

0.02

0.01

0.03

0.04

0.04

0.02

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

(*) Mass of red mud liquor used: 100g

193


Time

(min)

60% CO2 75% CO2 100% CO2

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

0

2.5

5

7.5

10

12.5

15

17.5

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

12.51

10.32

10.02

9.65

8.94

7.86

7.35

6.86

6.66

6.56

6.52

6.48

6.46

6.44

6.4

6.4

6.4

6.4

6.4

6.4

6.4

6.4

6.4

12.48

10.27

10.06

9.68

8.96

7.88

7.40

6.88

6.69

6.60

6.56

6.52

6.50

6.48

6.45

6.43

6.43

6.43

6.43

6.43

6.43

6.43

6.43

12.50

10.30

10.04

9.67

8.95

7.87

7.38

6.87

6.68

6.58

6.54

6.50

6.48

6.46

6.43

6.42

6.42

6.42

6.42

6.42

6.42

6.42

6.42

0.02

0.04

0.03

0.02

0.01

0.01

0.04

0.01

0.02

0.03

0.03

0.03

0.03

0.03

0.04

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

12.5

10.04

9.58

8.47

7.5

7.03

6.8

6.56

6.48

6.42

6.36

6.34

6.34

6.34

6.34

6.34

6.34

6.34

6.34

6.34

6.34

6.34

6.4

12.46

10

9.55

8.44

7.42

7.00

6.77

6.57

6.50

6.43

6.39

6.35

6.33

6.33

6.33

6.33

6.33

6.33

6.33

6.33

6.33

6.33

6.43

12.48

10.02

9.57

8.46

7.46

7.02

6.79

6.57

6.49

6.43

6.38

6.35

6.34

6.34

6.34

6.34

6.34

6.34

6.34

6.34

6.34

6.34

6.42

0.03

0.03

0.02

0.02

0.06

0.02

0.02

0.01

0.01

0.01

0.02

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.02

12.48

9.8

9.25

8.1

7.22

6.78

6.51

6.38

6.34

6.3

6.29

6.27

6.27

6.27

6.27

6.27

6.27

6.27

6.27

6.27

6.27

6.27

6.27

12.45

9.76

9.22

8.00

7.14

6.75

6.47

6.35

6.32

6.28

6.25

6.25

6.25

6.25

6.25

6.25

6.25

6.25

6.25

6.25

6.25

6.25

6.25

12.47

9.78

9.24

8.05

7.18

6.77

6.49

6.37

6.33

6.29

6.27

6.26

6.26

6.26

6.26

6.26

6.26

6.26

6.26

6.26

6.26

6.26

6.26

0.02

0.03

0.02

0.07

0.06

0.02

0.03

0.02

0.01

0.01

0.03

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01 (*) Mass of red mud liquor used: 100g

194


Time

(min)

15% CO2 20% CO2 25% CO2 30% CO2 40% CO2

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

pH(*)

Rep1

pH(*)

Rep2

Avg

pH

Stdev

pH

0

2.5

5

7.5

10

12.5

15

17.5

20

22.5

25

30

35

40

45

50

55

60

65

70

75

80

85

90

12.49

11.8

11

10.5

10.24

10.11

10

9.9

9.8

9.48

8.89

7.93

7.48

7.26

7.16

7.12

7.08

7.06

7.06

7.06

7.06

7.06

7.06

7.06

12.47

11.98

11.1

10.66

10.34

10.24

10.12

10.01

9.86

9.53

8.95

8.02

7.56

7.3

7.18

7.14

7.1

7.08

7.08

7.08

7.08

7.08

7.08

7.08

12.48

11.89

11.05

10.58

10.29

10.18

10.06

9.96

9.83

9.51

8.92

7.98

7.52

7.28

7.17

7.13

7.09

7.07

7.07

7.07

7.07

7.07

7.07

7.07

0.01

0.13

0.07

0.11

0.07

0.09

0.08

0.08

0.04

0.04

0.04

0.06

0.06

0.03

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.52

11.63

10.73

10.26

10.14

10

9.85

9.65

9.01

7.93

7.46

7.22

7.08

7

6.98

6.96

6.94

6.94

6.94

6.94

6.94

6.94

6.94

6.94

12.43

11.76

10.75

10.27

10.17

10.02

9.87

9.66

9.02

8.01

7.55

7.24

7.1

7.03

7

7

6.98

6.96

6.96

6.96

6.96

6.96

6.96

6.96

12.48

11.70

10.74

10.27

10.16

10.01

9.86

9.66

9.02

7.97

7.51

7.23

7.09

7.02

6.99

6.98

6.96

6.95

6.95

6.95

6.95

6.95

6.95

6.95

0.06

0.09

0.01

0.01

0.02

0.01

0.01

0.01

0.01

0.06

0.06

0.01

0.01

0.02

0.01

0.03

0.03

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.49

11.48

10.6

10.17

10

9.82

9.61

9.28

8.57

7.56

7.19

6.99

6.91

6.87

6.85

6.83

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

12.46

11.47

10.57

10.13

9.96

9.78

9.66

9.23

8.36

7.29

7.05

6.95

6.91

6.89

6.87

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

6.85

12.48

11.48

10.59

10.15

9.98

9.80

9.64

9.26

8.47

7.43

7.12

6.97

6.91

6.88

6.86

6.84

6.83

6.83

6.83

6.83

6.83

6.83

6.83

6.83

0.02

0.01

0.02

0.03

0.03

0.03

0.04

0.04

0.15

0.19

0.10

0.03

0.00

0.01

0.01

0.01

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.03

12.47

10.84

10.13

9.92

9.67

9.32

8.81

7.91

7.29

6.99

6.85

6.79

6.77

6.77

6.77

6.77

6.77

6.77

6.77

6.77

6.77

6.77

6.77

6.77

12.45

10.76

10.1

9.88

9.77

9.35

8.86

7.94

7.34

7

6.86

6.82

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

6.74

12.46

10.80

10.12

9.90

9.72

9.34

8.84

7.93

7.32

7.00

6.86

6.81

6.76

6.76

6.76

6.76

6.76

6.76

6.76

6.76

6.76

6.76

6.76

6.76

0.01

0.06

0.02

0.03

0.07

0.02

0.04

0.02

0.04

0.01

0.01

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

12.48

10.42

10

9.75

9.41

8.62

7.72

7.27

7.01

6.88

6.74

6.66

6.64

6.62

6.6

6.6

6.6

6.6

6.6

6.6

6.6

6.6

6.6

6.6

12.5

10.46

10.04

9.8

9.44

8.67

7.82

7.34

7.03

6.85

6.75

6.67

6.65

6.65

6.65

6.65

6.65

6.65

6.65

6.65

6.65

6.65

6.65

6.65

12.49

10.44

10.02

9.78

9.43

8.65

7.77

7.31

7.02

6.87

6.75

6.67

6.65

6.64

6.63

6.63

6.63

6.63

6.63

6.63

6.63

6.63

6.63

6.63

0.01

0.03

0.03

0.04

0.02

0.04

0.07

0.05

0.01

0.02

0.01

0.01

0.01

0.02

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

(*) Mass red mud liquor used: 100g

195


Time

(min)

50% CO2 60% CO2 75% CO2 100% CO2

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

0

2.5

5

7.5

10

12.5

15

17.5

20

22.5

25

30

35

40

45

50

55

60

65

70

75

80

85

90

12.5

10.25

9.77

9.3

8.35

7.51

7.07

6.83

6.73

6.67

6.59

6.55

6.53

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

12.51

10.24

9.89

9.48

8.44

7.77

7.13

6.86

6.7

6.62

6.58

6.54

6.52

6.5

6.48

6.48

6.48

6.48

6.48

6.48

6.48

6.48

6.48

6.48

12.51

10.25

9.83

9.39

8.40

7.64

7.10

6.85

6.72

6.65

6.59

6.55

6.53

6.51

6.50

6.50

6.50

6.50

6.50

6.50

6.50

6.50

6.50

6.50

0.01

0.01

0.08

0.13

0.06

0.18

0.04

0.02

0.02

0.04

0.01

0.01

0.01

0.01

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

12.49

9.89

9.58

9.04

7.77

7.18

6.9

6.7

6.63

6.57

6.54

6.52

6.5

6.48

6.48

6.46

6.46

6.46

6.46

6.46

6.46

6.46

6.46

6.46

12.44

9.85

9.57

9.05

7.74

7.1

6.8

6.69

6.6

6.58

6.56

6.48

6.44

6.44

6.44

6.44

6.44

6.44

6.44

6.44

6.44

6.44

6.44

6.44

12.47

9.87

9.58

9.05

7.76

7.14

6.85

6.70

6.62

6.58

6.55

6.50

6.47

6.46

6.46

6.45

6.45

6.45

6.45

6.45

6.45

6.45

6.45

6.45

0.04

0.03

0.01

0.01

0.02

0.06

0.07

0.01

0.02

0.01

0.01

0.03

0.04

0.03

0.03

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.46

9.66

9.38

8.65

7.61

7

6.74

6.59

6.51

6.46

6.44

6.42

6.4

6.4

6.38

6.38

6.38

6.38

6.38

6.38

6.38

6.38

6.38

6.38

12.5

9.71

9.45

8.63

7.58

6.98

6.72

6.56

6.48

6.45

6.43

6.41

6.39

6.39

6.39

6.39

6.39

6.39

6.39

6.39

6.39

6.39

6.39

6.39

12.48

9.69

9.42

8.64

7.60

6.99

6.73

6.58

6.50

6.46

6.44

6.42

6.40

6.40

6.39

6.39

6.39

6.39

6.39

6.39

6.39

6.39

6.39

6.39

0.03

0.04

0.05

0.01

0.02

0.01

0.01

0.02

0.02

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.51

9.5

9.22

7.92

6.98

6.64

6.49

6.41

6.37

6.33

6.31

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

12.47

9.47

9.28

7.9

6.94

6.6

6.40

6.37

6.34

6.30

6.28

6.28

6.28

6.28

6.28

6.28

6.28

6.28

6.28

6.28

6.28

6.28

6.28

6.28

12.49

9.49

9.25

7.91

6.96

6.62

6.45

6.39

6.36

6.32

6.30

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

6.29

0.03

0.02

0.04

0.01

0.03

0.03

0.06

0.03

0.02

0.02

0.02

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01


196


Time

(min)

10% CO2 15% CO2 20% CO2 25% CO2

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

0

2.5

5

7.5

10

12.5

15

17.5

20

22.5

25

30

35

40

45

50

55

60

65

70

75

80

85

90

12.47

12.08

11.64

11.16

10.64

10.4

10.26

10.14

10

-

9.9

9.74

9.44

8.9

8.18

7.7

7.4

7.3

7.26

7.26

7.26

7.26

7.26

7.26

12.5

12.14

11.7

11.25

10.8

10.49

10.4

10.26

10.2

-

10.06

9.8

9.45

8.91

8.2

7.74

7.46

7.34

7.3

7.28

7.25

7.25

7.25

7.25

12.49

12.11

11.67

11.21

10.72

10.45

10.33

10.20

10.10

-

9.98

9.77

9.45

8.91

8.19

7.72

7.43

7.32

7.28

7.27

7.26

7.26

7.26

7.26

0.02

0.04

0.04

0.06

0.11

0.06

0.10

0.08

0.14

-

0.11

0.04

0.01

0.01

0.01

0.03

0.04

0.03

0.03

0.01

0.01

0.01

0.01

0.01

12.5

11.82

11.14

10.54

10.34

10.2

10.01

9.88

9.64

-

9.17

8.13

7.54

7.29

7.17

7.11

7.07

7.05

7.03

7.03

7.03

7.03

7.03

7.03

12.43

11.78

11.17

10.4

10.16

10

9.86

9.7

9.5

-

9.12

8.13

7.4

7.13

7.01

6.97

6.95

6.95

6.93

6.93

6.93

6.93

6.93

6.93

12.47

11.80

11.16

10.47

10.25

10.10

9.94

9.79

9.57

-

9.15

8.13

7.47

7.21

7.09

7.04

7.01

7.00

6.98

6.98

6.98

6.98

6.98

6.98

0.05

0.03

0.02

0.10

0.13

0.14

0.11

0.13

0.10

-

0.04

0.00

0.10

0.11

0.11

0.10

0.08

0.07

0.07

0.07

0.07

0.07

0.07

0.07

12.44

11.52

10.58

10.18

10

9.8

9.61

9.25

8.7

7.94

7.39

7.05

6.97

6.92

6.92

6.92

6.92

6.92

6.92

6.92

6.92

6.92

6.92

6.92

12.48

11.57

10.64

10.25

10.08

9.96

9.78

9.36

8.78

8

7.43

7.1

7.01

6.94

6.9

6.9

6.9

6.9

6.9

6.9

6.9

6.9

6.9

6.9

12.46

11.55

10.61

10.22

10.04

9.88

9.70

9.31

8.74

7.97

7.41

7.08

6.99

6.93

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

6.91

0.03

0.04

0.04

0.05

0.06

0.11

0.12

0.08

0.06

0.04

0.03

0.04

0.03

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.47

11.24

10.4

10.12

9.91

9.7

9.29

8.36

7.71

7.3

7.01

6.89

6.84

6.82

6.8

6.78

6.78

6.78

6.78

6.78

6.78

6.78

6.78

6.78

12.5

11.3

10.54

10.13

9.84

9.5

9.1

8.44

7.62

7.22

7.04

6.91

6.88

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

6.86

12.49

11.27

10.47

10.13

9.88

9.60

9.20

8.40

7.67

7.26

7.03

6.90

6.86

6.84

6.83

6.82

6.82

6.82

6.82

6.82

6.82

6.82

6.82

6.82

0.02

0.04

0.10

0.01

0.05

0.14

0.13

0.06

0.06

0.06

0.02

0.01

0.03

0.03

0.04

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06


197


Time

(min)

30% CO2 40% CO2 50% CO2 60% CO2

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

0

2.5

5

7.5

10

12.5

15

17.5

20

22.5

25

30

35

40

45

50

55

60

65

70

75

80

85

90

12.5

10.66

10.1

9.8

9.42

8.48

7.7

7.29

7.05

6.93

6.81

6.76

6.74

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

12.44

10.62

10.02

9.75

9.31

8.38

7.5

7.1

6.87

6.77

6.75

6.73

6.71

6.71

6.71

6.71

6.71

6.71

6.71

6.71

6.71

6.71

6.71

6.71

12.47

10.64

10.06

9.78

9.37

8.43

7.60

7.20

6.96

6.85

6.78

6.75

6.73

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

0.04

0.03

0.06

0.04

0.08

0.07

0.14

0.13

0.13

0.11

0.04

0.02

0.02

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.48

10.32

9.9

9.28

8.01

7.36

7

6.82

6.73

6.67

6.64

6.62

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

6.59

12.5

10.4

9.98

9.34

8.16

7.48

7.08

6.92

6.81

6.74

6.67

6.63

6.61

6.6

6.6

6.6

6.6

6.6

6.6

6.6

6.6

6.6

6.6

6.6

12.49

10.36

9.94

9.31

8.09

7.42

7.04

6.87

6.77

6.71

6.66

6.63

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

0.01

0.06

0.06

0.04

0.11

0.08

0.06

0.07

0.06

0.05

0.02

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.45

10.12

9.48

7.97

7.2

6.87

6.71

6.63

6.59

6.57

6.53

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

12.48

10.18

9.53

8.11

7.36

6.99

6.82

6.71

6.62

6.6

6.57

6.55

6.53

6.53

6.53

6.53

6.53

6.53

6.53

6.53

6.53

6.53

6.53

6.53

12.47

10.15

9.51

8.04

7.28

6.93

6.77

6.67

6.61

6.59

6.55

6.53

6.52

6.52

6.52

6.52

6.52

6.52

6.52

6.52

6.52

6.52

6.52

6.52

0.02

0.04

0.04

0.10

0.11

0.08

0.08

0.06

0.02

0.02

0.03

0.03

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

12.46

9.97

9.23

7.67

7.01

6.75

6.6

6.53

6.49

6.47

6.45

6.43

6.43

6.43

6.41

6.41

6.41

6.41

6.41

6.41

6.41

6.41

6.41

6.41

12.52

10.07

9.28

7.72

7.09

6.84

6.64

6.59

6.48

6.46

6.44

6.44

6.44

6.42

6.42

6.42

6.42

6.42

6.42

6.42

6.42

6.42

6.42

6.42

12.49

10.02

9.26

7.70

7.05

6.80

6.62

6.56

6.49

6.47

6.45

6.44

6.44

6.43

6.42

6.42

6.42

6.42

6.42

6.42

6.42

6.42

6.42

6.42

0.04

0.07

0.04

0.04

0.06

0.06

0.03

0.04

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01


198


Time

(min)

10% CO2 15% CO2 20% CO2

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2 Avg pH

Stdev

pH

0

2.5

5

7.5

10

12.5

15

17.5

20

22.5

25

30

35

40

45

50

55

60

65

70

75

80

85

90

12.63

12.28

11.67

10.98

10.55

10.27

10.2

10.12

10.04

9.94

9.71

9.42

8.93

8.11

7.65

7.45

7.35

7.31

7.27

7.27

7.27

7.27

7.27

7.27

12.55

12.26

11.36

10.53

10.3

10.18

10.08

10.02

9.92

9.85

9.58

9.29

8.44

7.84

7.52

7.34

7.22

7.2

7.17

7.15

7.15

7.15

7.15

7.15

12.59

12.27

11.52

10.76

10.43

10.23

10.14

10.07

9.98

9.90

9.65

9.36

8.69

7.98

7.59

7.40

7.29

7.26

7.22

7.21

7.21

7.21

7.21

7.21

0.06

0.01

0.22

0.32

0.18

0.06

0.08

0.07

0.08

0.06

0.09

0.09

0.35

0.19

0.09

0.08

0.09

0.08

0.07

0.08

0.08

0.08

0.08

0.08

12.57

11.72

10.74

10.31

10.16

10

9.8

9.56

9.21

8.58

7.61

7.27

7.13

7.07

7.04

7.02

7.02

7.02

7.02

7.02

7.02

7.02

7.02

7.02

12.46

11.84

10.86

10.33

10.01

9.9

9.85

9.58

9.19

8.34

7.48

7.22

7.1

7.06

7.05

7.02

7.02

7.02

7.02

7.02

7.02

7.02

7.02

7.02

12.52

11.78

10.80

10.32

10.09

9.95

9.83

9.57

9.20

8.46

7.55

7.25

7.12

7.07

7.05

7.02

7.02

7.02

7.02

7.02

7.02

7.02

7.02

7.02

0.08

0.08

0.08

0.01

0.11

0.07

0.04

0.01

0.01

0.17

0.09

0.04

0.02

0.01

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

12.42

11.18

10.35

10.12

9.9

9.62

9.19

8.35

7.75

7.41

7.08

6.98

6.92

6.9

6.9

6.9

6.9

6.9

6.9

6.9

6.9

6.9

6.9

6.9

12.54

11.22

10.27

10.17

9.88

9.56

9.03

8.28

7.94

7.37

7.07

6.88

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

12.48

11.20

10.31

10.15

9.89

9.59

9.11

8.32

7.85

7.39

7.08

6.93

6.88

6.87

6.87

6.87

6.87

6.87

6.87

6.87

6.87

6.87

6.87

6.87

0.08

0.03

0.06

0.04

0.01

0.04

0.11

0.05

0.13

0.03

0.01

0.07

0.06

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04


199


Time

(min)

25% CO2 30% CO2 40% CO2 50% CO2

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

pH(*)

Rep 1

pH(*)

Rep 2

Avg

pH

Stdev

pH

0

2.5

5

7.5

10

12.5

15

17.5

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

12.45

10.78

10.25

9.96

9.59

8.91

7.89

7.42

7

6.86

6.82

6.8

6.78

6.78

6.78

6.78

6.78

6.78

6.78

6.78

6.78

6.78

6.78

12.5

11

10.3

10.01

9.68

9.05

8.16

7.65

7.15

6.96

6.92

6.88

6.86

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

6.84

12.48

10.89

10.28

9.99

9.64

8.98

8.03

7.54

7.08

6.91

6.87

6.84

6.82

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

6.81

0.04

0.16

0.04

0.04

0.06

0.10

0.19

0.16

0.11

0.07

0.07

0.06

0.06

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

12.53

10.72

10.15

9.83

9.36

8.26

7.55

7.19

6.9

6.78

6.74

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

6.72

12.42

10.66

10.18

9.82

9.3

8.08

7.38

7.06

6.9

6.74

6.69

6.67

6.65

6.65

6.65

6.65

6.65

6.65

6.65

6.65

6.65

6.65

6.65

12.48

10.69

10.17

9.83

9.33

8.17

7.47

7.13

6.90

6.76

6.72

6.70

6.69

6.69

6.69

6.69

6.69

6.69

6.69

6.69

6.69

6.69

6.69

0.08

0.04

0.02

0.01

0.04

0.13

0.12

0.09

0.00

0.03

0.04

0.04

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

0.05

12.53

10.41

10.01

9.59

8.24

7.37

6.98

6.81

6.72

6.65

6.63

6.61

6.61

6.61

6.61

6.61

6.61

6.61

6.61

6.61

6.61

6.61

6.61

12.49

10.33

9.97

9.45

8.12

7.31

6.92

6.81

6.69

6.62

6.6

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

6.58

12.51

10.37

9.99

9.52

8.18

7.34

6.95

6.81

6.71

6.64

6.62

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

6.60

0.03

0.06

0.03

0.10

0.08

0.04

0.04

0.00

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

0.02

12.51

10.05

9.66

8.9

7.55

6.96

6.68

6.61

6.56

6.53

6.51

6.49

6.48

6.48

6.48

6.48

6.48

6.48

6.48

6.48

6.48

6.48

6.48

12.55

10.1

9.74

9.02

7.86

7.06

6.78

6.7

6.65

6.61

6.58

6.55

6.54

6.54

6.54

6.54

6.54

6.54

6.54

6.54

6.54

6.54

6.54

12.53

10.08

9.70

8.96

7.71

7.01

6.73

6.66

6.61

6.57

6.55

6.52

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

6.51

0.03

0.04

0.06

0.08

0.22

0.07

0.07

0.06

0.06

0.06

0.05

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04

0.04


200

Figure C-1. Phase composition quantification of carbonated RM at 20% CO2 concentration, total gas flow rate 200mL/min

201

Figure C-2. Phase composition quantification of carbonated RM at 50% CO2 concentration, total gas flow rate 200mL/min

202

Table D-1. Simulated carbonation of RM at different CO2 concentrations and total gas flow rate of 100mL/min

CO2 concentration

(%)

PCO2 (atm) -Log(PCO2) Experimental

RM liquor pH

Simulated RM

liquor pH

Experimental

RM slurry pH

Simulated RM

slurry pH

25

30

40

50

60

75

100

0.25

0.3

0.4

0.5

0.6

0.75

1

0.602

0.522

0.397

0.301

0.221

0.124

0

7.0

6.8

6.7

6.5

6.4

6.3

6.3

7.20

7.13

6.93

6.71

6.55

6.52

6.40

7.35

7.20

7.14

6.94

6.85

6.81

6.60

7.45

7.37

7.24

7.15

7.07

6.90

6.87


CO2 concentration

(%)

PCO2 (atm) -Log(PCO2) Experimental

RM liquor pH

Simulated RM

liquor pH

Experimental

RM slurry pH

Simulated RM

slurry pH

15

20

30

40

50

60

75

100

0.15

0.2

0.3

0.4

0.5

0.6

0.75

1

0.823

0.698

0.522

0.397

0.301

0.221

0.124

0

7.3

7.09

6.85

6.7

6.5

6.4

6.4

6.3

7.4

7.22

7.13

6.93

6.71

6.55

6.52

6.4

7.5

7.4

7.2

7.0

6.9

6.8

6.6

6.6

7.67

7.54

7.37

7.24

7.15

7.07

6.90

6.87

203


CO2 concentration

(%) PCO2 (atm) -Log(PCO2)

Experimental

RM liquor pH

Simulated RM

liquor pH

Experimental

RM slurry pH

Simulated RM

slurry pH

10

15

20

25

30

40

50

60

0.1

0.15

0.2

0.25

0.3

0.4

0.5

0.6

1.0

0.823

0.698

0.602

0.522

0.397

0.301

0.221

7.43

7.1

6.99

6.9

6.78

6.7

6.6

6.5

7.56

7.4

7.22

7.2

7.13

6.93

6.71

6.55

7.5

7.3

7.18

7.1

7.0

6.9

6.8

6.7

7.78

7.67

7.54

7.45

7.37

7.24

7.15

7.07


CO2 concentration

(%) PCO2 (atm) -Log(PCO2)

Experimental

RM liquor pH

Simulated RM

liquor pH

Experimental

RM slurry pH

Simulated RM

slurry pH

10

15

20

25

30

40

50

0.1

0.15

0.2

0.25

0.3

0.4

0.5

1.0

0.823

0.698

0.602

0.522

0.397

0.301

7.3

7.1

6.9

6.8

6.7

6.6

6.5

7.56

7.4

7.22

7.2

7.13

6.93

6.71

7.5

7.3

7.14

7.0

6.9

6.8

6.8

7.78

7.67

7.54

7.45

7.37

7.24

7.15

Documents

Red Mud Minimisation and Management for the …...Bauxite residue (red mud), a waste from the Bayer process for refining bauxite to alumina, is highly alkaline (pH~13) and its treatment