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To my mum and dad, my cheerful skters and brother,

and my loving wife, Liziren

This study focuses on several fùndamental and practical aspects of peracetic acid (Pa)

brightening of softwood kraft pulps.

A PaEop bnghtening sequence was developed and optimised for a conventiond

Hemlock kraft pulp bleached by a solvent-assisted ozone treatment and subsequent oxidative

extraction (ZnEo pulp). At a final brightness of about 88% ISO the strength properties and

brightness stability of this TCF fully bleached pulp are similar to or better than those of a

conventionally chiorine bleached pulp.

The stability of an aqueous solution of peracetic acid was studied. It was found that

peracetic acid is consumed by three reactions: i) spontaneous decomposition, ii) hydrolysis

and iii) transition metal catalysed reactions. The spontaneous decomposition rate is maximal

at pH 8.2 while both the hydrolysis and metal ion catalysed reactions increase with increasing

pH. At pH 10.5 or higher, the hydrolysis becomes dominant when the metal ion catalysed

reactions are minimized by metal chelation. The kinetics of the spontaneous decomposition

and aIkaIine hydrolysis of peracetic acid were developed.

The transition metal ion induced peracetic acid decomposition and the effect of the

addition of chehtdadditives on the decomposition were studied under typical peracetic acid

pulp bleaching conditions. The results show that a significant amount of peracetic acid may

be wasted when metal ions are present, resulting in less efficient bleaching. A chelation

pretreatment stage for pulp with high metal ion contents, especially rnanganese, leads to

irnproved subsequent peracetic acid bleaching. However, for pulps with a low rnetal ion

such as DTPA or DTMPA, to the peracetic acid solution minimizes the Cu(I1) and Fe(II1)

indud decomposition of peracetic acid but increases the Mn(I1) induced decornposition. The

addition of sodium pyrophosphate eflectively deactivates the metal ions and consequently

improves the bleaching performance.

The peracetic acid consumption in wasteful reactions was quantified by detemining

the gaseous products evolved. It is found that aithough carbon dioxide is fonned under

certain conditions, oxygen is the dominant product of the wasteful reactions. One mole of

oxygen is forme. per two moles of oxidant (peracetic a d and hydrogen peroxide) consumed.

Therefore, the amount of peracetic acid consumed in oxidation reactions can be determined

as the dxerence between the total arnount of peracetic acid consumed and that consumed in

wastetùl reactions as determined by oxygen formation. It was shown for an oxygen delignified

softwood kraft pulp that there is a unique relationship between pulp brightness gain and

peracetic acid consumption due to pulp oxidations irrespective of the brightening conditions.

This suggests that the oqgen formation can be used for control of an industrial peracetic acid

treatment.

Finally, a kinetic mode1 of peracetic acid briçhtening of a soivent-assisted ozone

delignified pulp was developed. The model consists of three parallel and one consecutive

reaction, and is based on reaction pathways reported for Iignin model compounds with

peracetic acid. An optirnization search algorithm was used to find the kinetic parameters of

the model. The kinetic model provides an adequate description of the disappearance rate of

the chromophores in pulp at constant pH and peracetic acid concentration.

ACKNOWLEDGEMENT

1 would like to express my deepest appreciation to my thesis s u p e ~ s o r s , Drs. Ni and

van Heiningen, for al1 their help, encouragement and invaluable guidance throughout the

course of this study. Without them this work would not have been possible!

1 wish to thank Mr. M. d'Entremont for his help with some of the experiments.

It is a pleasure to extend gratitude toward a11 the members of the Dr. Limerick Pulp

& Paper Research and Education Centre for their assistance and suggestions during the

course of this work. Special thanks to Le Zeng, XiaoZhu Zhang, Russell Girard, Teik Ooi and

Drs. Kang , Li and Ghosh for al1 the valuable discussions.

Financial support fiom NSERC in the fonn of a Strategic Grant is greatly appreciated.

I also wish to thank Dr. RW. Thring, Mr. & Mrs. Hildebrand, Mr. M. Peters for their

moral support dunng my study in Canada.

My greatest thanks must go to my dear wife, Lizhen, whose love, support,

understanding and encouragement have bolstered me through this endeavour.

BFUGHTENING OF A SOFTWOOD KRAFT PULP DELIGNIFIED BY METHANOLASSISTED

..................................... OZONATION 32

..................................................... Abstract 33

Introduction .. ................................................. 34

Experimental .............................................. 36

Results and Discussion .......................................... 37

Peracetic Acid Brightening (Pa) of ZnEo Delignified Pulp .............. 37

Further Brightening of ZnEoPa Treated Pulp in an Eop Stage .........*. 40

Further Brightening of ZnEoPaEop Treated Pulp by a Final Pa Treatment . . 46

................... Brightness Stability of the TCF Fully Bleached Pulp 46

Chernical Cost Evaluation of the ZnEoPaEop Sequence ................ 46

Conclusions ......................................... . 47

References ................................................... 49

KINETICS OF PERACETIC ACID DECOMPOSITION PART 1: SPONTANEOUS DECOMPOSITION AT TYPICAL PULP BLEACHING CONDITIONS .......... 54

Abstract ..................................................... 55

Introduction .................................................. 56

Experimental ................................................. 59

Approcach ................................................. 60

.......................................... Results and Discussion 62

vii

References ................................................ 68

CHAPTER 5 KINETICS OF PERACETIC ACID DECOMPOSITION PART II: ALKALINE HYDROLYSIS ................. 75

Abstract ..................................................... 76

Introduction ........................................... 7 7

Experimental ................................................. 78 .......................................... Results and Discussion 79

Establishment of Conditions for Determination of Hydrolysis Kinetics ..... 79 .................. Kinetics of the Alakline Hydrolysis of Peracetic Acid 84

Predictions Obtained with the Hydrolysis and Decomposition Kinetics ..... 87

................ Effect of MgSO. on the Consumption of Peracetic Acid 88

Condusions .................................................. 90

References ................................................... 92

CHAPTER 6 THE ROLE OF TRANSITION METAL IONS DURING PEFWCETIC ACID BLEACHING OF

.............................. CHEMICAL PULPS 101

Abstracf .................................................... 102

Introduction ................................................. 103

Experimental ................................................ 104

......................................... Results and Discussion 106

Effect of Additives on the Meta1 Catalysed Decomposition of .............................................. Peracetic Acid 108

The Role of Transition Metal Ions during Peracetic Acid ................................... Bleaching of Chernical Pulps 112

Conclusions ................................................. 117

References .................................................. 119

CHAPTER 7 THE FORMATION OF GASEOUS PRODUCTS AND ITS RELATION TO PULP BLEACHING DURING

.............. TEE PERACETIC ACID TFtEATMENT 129

Abstract . ...................1.....o......................... 130

Introduction ..........................e...................... 131

......................................... Results and Discussion 135

The Formation of Gaseous Products during the Decomposition of .................... ............. .... Peracetic Acid ,. .. 135

The Effect of DTPA and DTMPA on Gas Formation and Peracetic .......................................... Acid Decomposition 138

The Formation of Gaseous Products and Its Relation to Peracetic Acid Bleaching .............................................. 140

Conclusions ................................................. 144

References ........................................... 146

BRIGHTENING OF AN OZONE DELIGNIFIED ........................ SOF'TWOOD KRAFT PULP 160

Abstract .................................................... 161

Introduction ................................................. 162

Experimental ................................................ 163

......................................... Results and Discussion 165

............................... Development of the Kinetic Mode1 166

.................................. Estimation of the Initial Values 171

........................................... Modelling Results 174

Conclusions ................................................. 177

References .................................................. 179

.................................. CHAPTER 9 CONCLUSIONS 191

GeneralSummary ............................................ 192

.............................. Recommendations for Future Work 196

THE COMPUTER OPTIMIZATION PROGRAM FOR THE KINETIC STUDY OF PERACETIC

............................ ACID BRIGHTENING 197

Table 3.1 . The Fractional Factorial Design of b(39 for Pa Brightening of ZnEo Delignified Pulp ........................ 38

Table 3.2 .

Table 3.3 . Table 3.4 .

Table 3-5 . Table 3.6 . Table 3-7 .

Table 4 4 .

Table 4.2 .

Table 5.2 .

Table 5.3 .

Table 6.1.

Table 6.2 .

Table 6.3 .

The Effect of Reaction Time on Peracetic Acid Bleaching .................................. of ZnEo Delignified Pulp 40

The Effect of NaOH Charge on the Eop Stage ................. 41

The Addition of DTPA to the Eop Stage ...................... 43

The Addition of MgSO. to the Eop Stage ..................... 44

The Effect of Temperature and Reaction Time of the Eop Stage ... 45

Estimation of Bleaching Chernical Cost of the ZnEoPaEop Sequence .................................... 47

The Observed Second-order Rate Constant as a Function of pH a t 40°C ................................... 63 Rate Constant of the Spontaneous Decomposition as a Function of Temperature .............................. 66

Rate Constant of the Peracetic Acid Hydrolysis .............................. as a Function of Temperature 87

The Required Time to Convert 95% of Residual ........................ Peracetic Acid to Hydrogen Peroxide 88

The Composition of Magnesium Species in Aqueous Solution a t 40 OC with Varying pH and MgSO. Addition ................ 90

Meta1 Ion Contents of the Materials Used in This Study ........ IO5

Conditional Stability Constants of Metal Ion to DTPA ............................. Complexes a t pH 7.0 and 20°C 109

Peracetic Acid Bleaching of Oxygen Delignified Softwood Kraft Pulp (O Pulp) ............................. 113

Table 6-4. Peracetic Acid Bleaching of Ozone Delignified Softwood Kraft Pulp .................................... 117

Table 7-1. The Formation of Gaseous Products and Its Relation to Bleaching during Peracetic Acid Treatment of an Oxygen Delignified Pulp ................................. 141

Table 8-1. The Development of the Light Absorption Coeficient a t pH 6.0, ............ 50°C and Different Peracetic Acid Concentrations 166

Table 8-2. The Kinetic Parameters Obtained by the Book and Jeeves Search Algorithm at 50°C for the

.................................. Brightening of ZE Pulp 176

xii

Figure 4-1. The effect of stabilizers on the decomposition of peracetic acid at 40°C, pH of 7.0 . . . . . . . . . . . . . . . . . . . . . . . . . , , . 69

Figure 4-2.

Figure 4-3.

Figure 4-4.

Figure 4-5.

Figure 4-6.

Figure 5-1.

Figure 5-2.

Figure 5-3.

Figure 5-4.

Figure 5-5.

Figure 5-6.

Figure 5-7.

Plot of k, versus 2M/(l+M)' at 40°C, pH = 5.5 -9.0 . . . . . . . . . . . . 70

Comparison of the predicted and experimental relationship between &, and pH at 40°C . . . . . . . . . . . . . .. .. . .. . 71 Koubek's plot of Logk, versus pH at 25"C, with 103 m o n EDTA [8] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Ranalysis of Koubek's data 181 with the kinetic mode1 developed in this study at 2s0C, with IO3 m o n EDTA . . . . . . . . . . 73

The Arrhenius plot of the spontaneous decomposition of peracetic acid .. . .. . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . .. 74

Addition of DTPA or DTMPA on the peracetic acid decomposition a t pH 9.5 and 40°C.. . . . . . .. . . . . . . . . . . . . . . . . . . 94 Addition of DTPA or DTMPA on the peracetic acid decomposition a t pH 8.2 and 40°C. . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Effect of H'O, concentration on the disappearance rate of peracetic acid at pH 8.2 and 40°C . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

Effect of pH on the consumption of peracetic acid (40°C, 8.2-12.0 pE,0.5 g/L DTMPA). . . . . . . . . . . . . . . .. . . .. . .. . 97

Determination of the rate constants of the alkaline hydrolysis of peracetic acid (40-60°C, 10.5-12.0 pH, 0.5 g/L DTMPA) . . . . . . . 98 Comparison of determined and predicted peracetic acid and hydrogen peroxide concentrations (pH 9.5, 40°C, 0.5 g / L DTMPA) . . . . . . . . . . . . . . .. . . . .. . . . . .. .. 99

Effect of MgSO, on the consumption of peracetic acid at 40°C and pH of 9.5 . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 100

Figure 6-2.

Figure 6-3.

Figure 6-4.

Figure 6-5.

Figure 6-6.

Figure 6-7.

Figure 7-1.

Figure 7-2.

Figure 7-3.

Figure 7-4.

Stability of a peracetic acid soiution ............. (40°C, pH 7,0,0.75 ppm metal ion concentration) 122

Effect of additives on copper catalysed peracetic acid decomposition (40°C, pH 7.0,0,75 pprn Cu2', 0.5 g/L DTPA, DTMPA or MgSO,, 2 g/L Na,P20,) ......................... 123

Effect of additives on iron catalysed peracetic acid decomposition (40°C, pH 7.0, 0.75 ppm FeY, 0.5 g / L DTPA, DTMPA or MgSO,, 2 g/L Na,P20,). ........................ 124

Effect of additives on manganese catalysed peracetic acid decomposition (40°C, pH 7.0,0.75 pprn Mn2', 0.5 g/L DTPA, DTMPA or MgSO,, 2 g/L Na,P20,) .................. 125

Effect of DTPA on the manganese catalysed decomposition of .... peracetic acid(40°C, pH 7,0,0.75 ppm Mn2', 0-5 g/L DTPA) 126

Effect of DTPA or DTMPA on the manganese catalysed decompositioo of peracetic acid (40°C, pH 5.0,0.75 pprn Mn2', 0.5 g/L DTPA or DTMPA) ................................ 127

Effect of DTPA or DTMPA on the manganese catalysed decomposition of peracetic acid (40°C, pH 6.0,0.75 pprn Mn2', 0.5 g/L DTPA or DTMPA) ................................ 128

The experimentai setup for the determination of gaseous products during peracetic acid decomposition ................ 148

The formation of gaseous products and the consurnption of peracetic acid at 25OC, pH 8.2, F(He) of 100 mumin and with 05 g/l DTMPA addition ............................. 149

The stoichiometry of the peracetic acid consumption and the oxygen formation at 2S°C, pH 8.2, F(He) of 100 mL/min and with 0.5 g& DTMPA addition ......................... 150

Gas formation and the consumption of peracetic acid in the presence of 0.75 pprn Mn(II) and a t 25"C, pH 8.2, F(He)of 100mWmin .................................... 151

Figure 7-6.

Figure 7-7.

Figure 7-8.

Figure 7-9.

Figure 7- 10.

Figure 7- 1 1.

Figure 7-12.

Figure 8-1.

Figure û-2.

Figure 8-3.

the presence of 0.75 ppm Mn(1I) and at 2S°C, pH 5.0, ..................................... F(He) of 70 mumin 152

The stoichiometr-y of peracetic acid consumption and carbon dioxide formation in the presence of 0.75 ppm Mn(U) and a t 25"C, pH 5.0, F(Be) of 100 milmin) ............. 153

Plausible mechanism of the carbon dioxide formation during peracetic acid decomposition ........................ 154

The effect of DTPA addition on peracetic acid consumption and the oxygen formation at 25"C, pH 8.2, F(He) of 100 mumin and with 0.02 moVL DTPA added at 30 min, . ................ 155

The effect of DTMPA addition on peracetic acid consumption and oxygen formation in the presence of 0.75 ppm Mn@) at 2S°C, pH 8.2, F(He) of 100 mumin and with 0.02 m o n

................................. DTMPA added at 30 min 156

The relationship between the wasted peracetic acid and hydrogen peroxide (O or OQ pulp, 40-60°C, pH 5-7,1.5-5% peracetic acid charge, 0.5-1.5% pulp consistency, 30-120 min) . . 157

The relationship between the peracetic acid consumption due to the pulp oxidation and the pulp brightness (O or OQ pulp, 40-60°C, pH 5-7, 1.5-5% peracetic acid charge, 0.5-1.5% pulp consistency, 30-120 min) ............... 158

The relationship between the pulp viscosity and the oxidant consumed in wasteful reactions (O or OQ pulp, 40-60°C, pH 5-7, 1.5-5% peraceticacid charge, 0.5-1.5% pulp consistency,

............................................ 30-120 min) 159

The effect of pH on the Iight absorption coefficient development during Pa brightening of ozone delignified softwood kraft pulp (50°C and [CH3C03H] of 0.0493 moVL) ..................... 183

The proposed kinetic mode1 for peracetic acid brightening ..................... of ozone delignified softwood kraft pulp 184

Estimation of C3, and k, (50°C, pH 7.0, [CH3C03H] of 0.0493 m o n ) .................. 185

Figure 8-5. Cornparison of mode1 predictions and experimental results (50°C, p H 7.0, [CH,CO,H] of 0.0493 moi&) .................. 187

Figure 8-6. Predictions of the development of Cl, C,, C, and C .................. (50°C, pH 3.0, [CH,C03H] of 0.0493 moi&) 188

Figure 8-7. Predictions of the development of Cl, C2, C3 and C .................. (50°C, pH 7.0, [CH3C0,HJ of 0.0493 m o n ) 189

Figure 8-8. Predictions of the development of C,, C,, C3 and C (50°C, pH 10.0, [CH3C0,H] of 0,0493 moüL) ................. 190

xvi

NOMENCLATURE

[c l = chromophore concentration

[CH3C03H], = total titratable peracetic acid concentration, (mol/L)

DTPA = diethylenetriaminepentaacetic acid

DTMPA = diethylenetriarninepentamethylenephosphonic acid

EDTA = ethylenediarninetetraacetic acid

E = alkaline extraction stage

Ea = activation energy, (kcaVmol)

ECF = Elemental Chlorine Free

Eo = alkaline oxidative extraction

EOP = hydrogen peroxide reinforced oxidative extraction stage

EP = hydrogen peroxide reinforced extraction stage

% ISO = international standard organization brightness

k = reaction rate constant

Ka = acid ionization constant

ko = pre-exponential constant

ko, = observed rate constant

k = ionization constant of water

M = the ratio of proton concentration to the dissociation constant

P = hydrogen peroxide bleaching stage

xvii

Q

TCF

Zn

= chelating stase

= Totally Chlorine Free

= solvent-assisted ozone bleaching stage

CHAPTER 1

INTRODUCTION

BACKGROUND

In pnnciple, bleaching of chemical pulps consists of two steps: (1) to remove the

residual lignin, and (2) to brighten the pulp. Oxidants, such as chlorine, chlorine dioxide,

hypochlorite, oxygen, hydrogen peroxide, peracids and ozone, are known as good

delignification andfor brightening chemicals. An aikaline stage is usudly included in the

bleaching process to solubilize the oxidized lignin, and to reactivate the residual lignin for

subsequent bleaching treatments.

New regulations are forcing the pulp and paper industry to reduce the environmental

impact of pulp bleaching. Several operational practices are rapidly being introduced into the

pulp manufàcturing process such as: (1) lowering the lignin content of pulp pnor to entering

the bleachery by extended delignification (e.g. MCC or RDH) 11-31 andor oxygen

delignification [4], (2) modifjmg the bleaching process by complete substitution of chlorine

with chlorine dioxide, e.g. the so-called ECF (Elemental ChIorine-Free) process [5,6], and the

adoption of an alkali oxidative extraction stage (Eo) or a peroxide enhanced Eo stage (Eop)

[7,8], (3) (partial) system closure of the bleach plant, such as in the Bleach Filtrate Recycle

process (BFRm) [9].

The ECF bleaching process is now becoming the dominant bleaching technology for

chemical puIps. Totally Chlorine Free (TCF) bleaching processes are also emerging 16, IO].

Ozone, oxygen, hydrogen peroxide and peracids are potential TCF bleaching chemicals.

Among them, hydrogen peroxide and peracids are possible substitutes for chlorine dioxide

in the final brightening stages due to their high brightening capacity [6] .

Peracetic acid, although known as an effective delignifier for nearly fifty years, was

considered too expensive for commercial application. However, recent advances in peracetic

acid production technology have significantly reduced the cost, enabling peracetic acid to

become a promising candidate for pulp bleaching [l 1,131. It has been shown that peracetic

acid can be used as a delignification agent [Il - 181, or as an activation agent for oxygen or

hydrogen peroxide stages [13,19], or as a brightening agent in the final stages of an ECF or

TCF process [l3,19-21 J.

Through earlier studies 122-261, it was demonstrated that a solvent-assisted ozone

process can be used to deligniQ chernical pulps to a low kappa number, while maintaining

good viscosity and strength properties. In order to produce a high strength, high brightness

TCF pulp, one of the objectives of the present project was to develop the subsequent

bnghtening sequence with peracetic acid and hydrogen peroxide.

Sirnilar to hydrogen peroxide, peracetic acid decomposes at pulp bleaching conditions.

Koubek [27] shidied the peracetic acid decomposition in several buffer solutions. However,

since Our preliminary experimental data showed that his kinetics did not accurately predict the

decomposition of peracetic acid at the typical pulp bleaching conditions, the kinetics of

spontaneous decomposition were further investigated in the present work. For a complete

description of the disappearance rate of peracetic acid the kinetics of alkaline hydrolysis of

peracetic acid must be known at a pH higher than 8.2. Also it is necessary to investigate the

role of transition metal ions during peracetic acid bleaching, which is typically performed at

a neutral pH. Others have found that some of the transition metal ions may catalyse the

decomposition of peracetic acid under acidic conditions (27-3 11. It was also reported 127-

29.3 11 that gaseous products, such as O, and CO, , are formed dunng peracetic acid

decomposition. However, the relationship between the gas formation and the efficiency of

peracetic acid bleaching has not been established. Finally, a kinetic model to describe the

peracetic acid brightening is lackinç.

OBJECTIVES

The gened objective of the present project is to study the firndarnental and practical

aspects of peracetic acid brightening of s o ~ o o d kraft pu1 ps. More specificall y the objectives

to develop and optimize a TCF brightening process including peracetic acid treatment

of an ozone delignified hemlock kraft pulp

to study the decomposition of peracetic acid under conditions typical to those of pulp

brightening by peracetic acid

to examine the role of transition metals during peracetic acid decomposition and their

importance in peracetic acid bleaching

to identa the relationship between the efficiency of pulp brightening and the gaseous

products fonned as a result of peracetic acid decomposition

to develop a kinetic model for peracetic acid brightening of an ozone deliçnified

Hernlock kraft pulp

OUTLINE OF THE THESIS

This thesis focuses on the fundamental and practical aspects of peracetic acid

brightening of softwood kraft pulps.

The relevant literature review of peracetic acid decomposition, peracetic acid

bleaching and hydrogen peroxide bleaching is presented in Chapter 2.

In Chapter 3, the inclusion of peracetic acid in a TCF bnghtening sequence for a

Hemlock kraft pulp delignified by a methanol-assisted ozone process is described. The

optimum process conditions for the peracetic acid stage (Pa) and the hydrogen peroxide

reinforced oxidative extraction stage (Eop) are also determined.

The kinetics of spontaneous decomposition of peracetic acid under conditions typical

of pulp brightening by peracetic acid are described in Chapter 4.

In Chapter 5, the effect of pH on the disappearance of peracetic acid in an aqueous

solution is discussed. The alkaline hydrolysis kinetics of peracetic acid are also reported.

The role of transition metal ions during peracetic acid bleaching of chernical pulps is

investigated in Chapter 6. The fundamental phenornena associated with the presence of

additives such as D m DTMPA, Na,P,Q, and MgSO, on the peracetic acid decomposition

and bleaching are examined.

In Chapter 7, the formation of gaseous products and its relation to pulp bleaching

dunng the peracetic acid treatment is studied. The potential use of the quantitative

determination of the amount of gaseous products formed by peracetic acid decomposition

as an indicator of the efficiency of peracetic acid pulp bleaching is discussed.

In Chapter 8, a kinetic mode1 of peracetic acid brightening of an ozone delignified pulp

is developed based on the reaction pathways reported in literature for peracetic acid treatment

of lignin mode1 cornpounds.

Chapter 9 summarizes the contributions to knowledge resulting fiom this study.

The work describeci in Chapters 3,4,5,6,7, and 8 have been written as self-contained

papers suitable for publication with IittIe or no fürther modifications. Thus each chapter has

its own abstract and references. As far as possible, uniform and standard syrnbols are used

throughout the thesis. The major content of these chapters is goinç to be or has been

submitted or presented at respectively refereed journals and technical conferences.

Specifically :

Chapter 3: Submitted to Papen ja Puu, April, 1997

Chapter 4: The Canadian Joumal of Chernical Engineering, Vol. 75, Feb., 1997, p. 37-4 1

Chapter 5: The Canadian Journal of Chernical Engineering, Vol. 75, Feb., 1997, p. 42-47

Chapter 6: Accepted for publication by Puip & Paper Canada, Feb., 1997

Chapter 7: Submitted to Tappi Joumal, March, 1997

Chapter 8: Submitted to Joumal of Wood Chemistry and Technology, March, 1997

REFERENCES

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2. Volk J. 1. and J. J. Young, Successfùl Lo-solidsn' Cooking at Hinton, Preprints the 82* CPPA Annual Meeting, Tech. Sect., CPPA, Montreal, 1996. p. B3 7

3. Andrews, E. K., RDH Kr& Pulping to Extend Deligiification, Decrease Emuent, and Improve Productivity and Puip Properties, Tappi J. 72(11):55 (1 989)

4. van Lierop, B., N. Liebergott, G. Teodorescu, and G . J. Kubes, Oxygen in Bleaching Sequence - an overview, Pulp Paper Can., 87(5): T 193 ( 1986)

Axesard, P., Substituting Chlorine Dioxide for Elemental Chlorine Makes the Bleach Plant Effluent Less Toxic, Tappi J. 69(10):54 (1 986)

Dence, C. W. and D. W. Reeve, Putp Bleaching-Principles and Practice, TAPPl PRESS, Atlanta, Georgia, 1996, p. 4 13

Young, J., G. Start and J. Lazorek, Utilization of an Eop Stage During 100% CIOz Bleaching at Weldwood Hinton, CPPA Pacific and Western Branch AnnuaI Meeting Preprints, Tech. Sect., CPPq Montreat, IW2, Section 4 4 Paper No.3

Anderson, J. R., Hydrogen Peroxide Use in Chemical Pulp Bleaching, 1992 Tappi Bleach Plant Operations Short Course Notes, TAPPI PRESS, Atlanta, p. 123

Ferguson, K. H., Champion Moves Toward Closure with BFR Startup at Canton, N. C., h l p & Paper, 70(4):47 ( 1 996)

Exclusive to Pulp and Paper Canada, No Matter What You Call It, Chlorine-fiee Bleaching 1s Here to Stay, h l p Paper Canada, 90(5):22 (1 992)

Hill, R T., P. B. Walsh, and J. A. HoUie, Peracetic Acid, an Effective Alternative for Chlorine Compound Free Delignification of Kraft Pulp, TAPPI 1992 Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p. 12 19

Basta, J., L. Holtinger, P. Lundgren, H. Fasten, and R. Fredriksson, Alternatives for Achieving High Brightness TCF Pulps, Proceedings, 1994 International Non-chlonne Bleaching Conference, Sponsored by Pulp & Paper and Emerging Technology Transfer Inc., Amelia Island, FL, sec. 1 1-3

Anderson, J. R, B. Amini and W. Wilkinson, On-site Generation and Use of Peroxy Acids in Chemical Pulp Bleaching, Preprints, 81" CPPA Annual Meeting, Tech. Sect., CPPA, Montreal, 1995, p. B59.

Christiansen, C. B., and W. F. Parker, Laboratory Studies of Acetic Acid Bleaching, Preprint, TAPPI Annual Meeting, 1 965, p. 13 2

Rapson, W. H. and C. B. Anderson, Bleaching in Five Stages to Asymptotic Lirnit Using Only One Oxidant and Sodium Hydroxide, 1985 International Pulp Bleaching Conference Proceedings, Quebec, 1985, p. 227

Devenyns, J., F. Desprez and N. Troughton, Peracetic Acid as a Selective Prebleaching Agent: an Effective Option for the Production of Fully Bleached TCF Kraft Pulps, Proceedings, 1 993 Non Chlorine Bleaching Conference, Sponsored by Pulp & Paper and Emerging Technology Transfer Inc., HHI, SC, sec. 8-1

Gellerstedt, G., Peracid Technology Review, Preprint, 1993 Workshop on Emerging Pulping and Chlorine-Free Bleaching Technology, Raleigh, N.C., March, 1 993

Bailey, C. W. and C. W. Dence, Peroxyacetic Acid Bleaching of Chemical Pulp, Tappi J., 49(1):9 (1966)

Liebergott, N., Peracid Delignification and Bleaching, 1 994 TAPPI Pulping Conference Proceedings, TAPPi PRESS, Atlanta, p. 357

Devenyns, J., F. Desprez and N. Troughton, Peroxygen Prebleaching and Bleaching Technologies for Step-Wise Conversion fiom Conventional Chlorine Bleaching via ECF toTCF, TAPPI 1 993 Pulping Conference Proceedings, T APPI PRESS, Atlanta p. 341

Troughton, N., F. Desprez and J. Devenyns, Peracids: The Pathway to High Brightness TCF Kraft Pulps, Proceedings of 1994 International Non-chlorine Bleaching Conference, Sponsored by Pulp & Paper and Emerging Technology Transfer Inc. Arnelia Island, FL, sec. IO- 1

Ni, Y. and A R P. van Heiningen, Ozone Dioxane Bleaching of Chemical Pulps, U.S. Patent 5354423, 1994

Solinas M. and T. H. Murphy, Ozone Selectivity Improved with Use of Organic Solvent, Pulp & Paper, 70(3): 133 (1 996)

Ni Y., A. R P. van Heiningen, J. Lora, L. Magdzinsk and E. K. Pye, A Novel Ozone Bleaching Technology for the Alcellm Process, J. Wood Chem. and Technology, 16(4):367-380 (1 996)

Kan& G. J., Y. Ni and A. R. P. van Heiningen, Mechanism of Cellulose Protection in a Novel and Selective Ozone Pulp Bleaching Process, Prepnnts, 82"' CPPA Annual Meeting Tech. Sect., CPPA, Montreai, 1996, p.B303

Ni, Y. and A. R. P. van Heiningen, Improved Ozone Bleaching by Impreçnation of Chemical Pulps with An Acidified Mixture of 1,4-Dioxane and Water, 1996 International Pulp Bleaching Conference Proceedings, Washington, D.C., April, 1996, p.2 13

Koubek, E., The Kinetics and the Mechanism of the Decomposition of Aliphatic Peroxyacids in Aqueous Solutions, Ph.D thesis, Brown Univ. . 1964

28. Kagan, M. J. and G. D. Lubarsky, The Intermediate Stages of Aldehyde Oxidation, J. Phys. Chem., 39, p.837 (1935)

29. Vrbaski, T. and 1. Brihta,, The Kinetics of Oxidation of Aldehydes to Acids and Acid Anhydrides, Arhiv Za Kemiju, 26, p.267 (1 954)

30. V a s p t p , Ya. M., V. O. Gavenko and 1. A. Garbuzyuk, Influence o f Minera1 Salts on the Decay rate of Peroxyacetic Acid in Aqueous Solution, Ukrainskii Khimicheskic Zhumaf, 55(6):584 (1 989)

3 1. Allen, G. C. and A. Aguilo, Metal Ion Catalysed Oxidation of Acetaldehyde, Advances in Chemistry Senes, 76, p.363 (1 968)

CHAPTER 2

LITERATURE FkEVIEW

1. PERACETIC ACID AS AN ALTERNATIVE CHEMICAL IN THE BLEACH PLANT

New bleaching technologies which do not use chlorine-based chernicals continue to

be developed in the pulp and paper industry worldwide in order to reduce the environmental

impact of the bleach plants. Recent results have shown that peracetic acid is a low capital

intensive, easily retrofitted and highly selective non-chlorine bleaching agent [l-91.

1.1. Production and Properties of Peracetic Acid

Peracetic acid is the mono-acetyi derivative of hydrogen peroxide. It has a single

acidic proton with a pKa of 8.2 at 25°C [IO]. The most important and widely used method

of production of peracetic acid is the direct, acid-catalysed reaction of hydrogen peroxide

with acetic acid, as shown in Equation (1): - CH3CQH + H202 -- CH3C03H + H20

Since the reaction is reversible, an equiiibnum mixture of the reactants is normally available.

However, pure peracetic acid can be obtained by a vacuum distillation technology in which

excess acetic acid, hydrogen peroxide and sulfùric acid catalyst are removed and recycled

[l,9]. Peracetic acid is reported to be insensitive to impact, but explodes violently when

heated to 110°C [I I l . Peracetic acid is classified as a hazardous substance and its

transportation is restricted by Transporiation of Dangerous Goods (TDG) reçulations [ I I l .

Although peracetic acid is used for chernical syntheses, the scale of consumption in a pulp

bleaching application requires d e and economicai generation of peracetic acid at the mil1 site.

The manufacture of distilled peracetic acid requires the installation of a vacuum still. The

process hazards associated with the distillation of the acetic acid-peracetic acid solution are

related to the detonabifity of concentrated mixtures, and must be considered in the design and

operation of a distilled peracetic acid system [9].

Peracetic acid has a sharp, pungent odour. It has a boiling point of 103°C and a

vapour pressure of 20 rnrnHg at 25°C [Il]. It is a weaker acid than acetic acid. The oxidation

potential of peracetic acid is about 1 . 0 6 ~ with the hdf reaction shown in Equation (2):

CH3C03H + 2H + + 2e- CH3C02H + H20 (2)

The principal hazard of peracetic acid is its corrosive nature, which can cause severe

bums to skin, eyes and body tissue. Contacted areas must be washed immediately with large

volumes of water to avoid tissue damage. Exposure to the vapour causes lachrymation and

possibly lung damage. Long-term exposure to peracetic acid can lead to liver damage,

decreased white bIood count, and lung tumors [Il]. Personal protective equipment should

include a respirator in addition to splash proof goggles, rubber acid suit, and rubber gloves.

1.2. Reactions of Peracetic Acid with Lignin

Most of the present knowledge of the reactions of peracetic acid and lignin has been

obtained tiom studies with lignin mode1 compounds. Generdly, three discrete pathways were

identified: sidechain oxidation of a-carbonyl groups by the Baeyer-Villiger mechanism [ 12-

153; aromatic ring oxidation initiated by ortho or para-hydroxylation [ 16- 19 j; and side-chain

epoxidation of alkenes [20].

1.2.1. Baeyer-Villiger reaction

This is a reaction whereby peracids oxidize ketones to esters [21]:

The reaction has been shown [22] to proceed by a mechanism involving nucleophilic

attack of the peracetic acid on the carbonyl carbon, followed by migration of one of the

ketone's alkyl or aryl groups. Products consistent with the Baeyer-Villiger mechanism were

identified dunng peracetic acid treatment of lignin mode1 compounds with a-carbonyl groups

at 60°C and pH 3 [ I 53. Aiso it was found that a ketone reacts somewhat slower than an

aldehyde, while the etherification of a phenolic hydroxyl group or the presence of an extra

methoxyl in the aromatic nucleus appears to have linle or no influence on the reaction rate

1.2.2. Epoxidation of alkenes

Alkenes are converted to oxiranes by direct transfer of oxygen as in Equation (4) [20].

The oxirane ring may be opened to give a glycol ester if strong acids are present.

1.2.3. Hydroxylation

The oxidation of phenols and their ethers by peracetic acid is commonly initiated by

ortho and para-hydroxylation reactions [23]. The resulting catechols fonned by the former

reaction are first oxidized to ortho quinones, and then to muconic acids, as shown in Equation

(5) [16,17]. Hydroxylation at the para site gives hydroquinones which are subsequently

converted to p-quinones and then to maleic and fùmaric acid derivatives [I7].

McDonougb [18] found tbat the oxidation of creosol by peracetic acid in moderately

alkaline solutions can be accounted for by a mechanism involving electrophilic attack of

undissociated peracetic acid on the phenolate anion followed by demethyIation and

subsequent nucleophllic attack by the peracetic anion on the resulting quinone intermediates.

Furthmore, Boeseken and Engieberts [17] showed that a direct ring cleavage could

also ocarr without the formation of a quinone intermediate as show in Equation (6), wbich

was later confirmed by Kawamoto et al. [24].

1.3. Peracetic Acid Bleaching

The use of peracetic acid as a delignifjhng agent was initially investigated by Poijak

in 1948 as a method for preparing holocdulose fiom wood [25]. Other studies confirmed that

peracetic acid is higldy selective in removing lignin fiom wood [26,27]. The subject has

recently been reviewed by Liebergott [ 5 ] . It has been shown that peracetic acid can be used

as a deIignification agent[ l,6,9,28-321, an activation asent for oxygen or hydrogen peroxide

stages [5,9] and as a briçhteninç agent in the final brighteninç stages of an ECF or TCF

bleaching sequence [2,3,5,9].

Bailey and Dence [28] published a research paper describing the use of peracetic acid

for both delignification and brightening of chemical pulps. It was found that the maximum

brightness gain during peracetic acid treatment of spruce kraft pulp occurred at neutrai pH,

while consistency (3 to 6%) and temperature (50 to 85°C) had little effect on the final pulp

properties. At pH of 7 to 9, the reduction in kappa number obtained with peracetic acid was

essentially similar to that of an equimolar quantity of chlorine. Also it appears that a peracetic

acid (or Pa) treatment at slightly acidic conditions of for exarnple, pH of 2.5 to 3.5, leads to

a good delignification [29,30]. More recent work [1,3 1,321 supports the earlier findings that

peracetic acid is a good delignification agent. For example, a charge of 5% distilled peracetic

acid decreased the kappa number by 62% and 41% after an alkaline extraction stage for

hardwood and sofiwood kraft pulps respectively.

Peracetic atid can dm be used as an activation agent before or after an oxygen as well

as hydrogen peroxide stage to increase the delignification degree or pulp brightness gain

[3,5]. For exarnple, peracetic acid pretreatment of an unbleached softwood kraft pulp at 1%

Pa charge expressed as H,O, can increase the delignification degree from 35% to 48.9%

during an Eop stage [5]. A similar peracetic acid treatment of an oxygen delignified pulp

results in an increase in the deligiification degree of a following Eop stage fiom 20% to 40%

[5]. Troughton et al. [3] showed that at a total hydrogen peroxide charge of 3% an oxygen

delignified softwood kraft pulp with an initial kappa number of 12.4 can be bleached to a

brightness of 74.8% ISO following a QPP sequence, while a brightness of 82.1% ISO is

obtained by a QPPaP sequence.

Peracetic acid is aIso a good brightening chernical which can be used in the final stages

of a bIeaching sequence [5,33 1. Rapson 1333 reported that a peracetic a d treatrnent of a well

delignified kraft pulp with a brightness of 69% ISO leads to the production of hlly bleached

pulp of 85% ISO brightness afier 4 hr at a 1.07% peracetic acid charge, pH 7, 70°C and 8%

pulp consistency. The viscosity was unaffected. Liebergott [ 5 ] found that CE delignified

sulphite pulps were bleached to 85 - 90% ISO brightness with a peracetic acid charge of

about 1%. Brightening of a soflwood kraft pulp with CIO2 or peracetic acid produced a

brightness of 83% at 0.6% CIO, and 86.5% at 0.45% Pa charge expressed as H202.

Rapson [34] reported that the optimum pH and temperature for chlorine dioxide and

peracetic acid bleaching are similar and that the reactions between these two chernicals are

negligible. Thus he concluded that chlorine dioxide and peracetic acid bleaching may be

combined in a single stage, which resulted in a higher brightness gain than either chlorine

dioxide or peracetic acid bleaching alone.

Peracetic acid may also replace hydrogen peroxide in TCF bleachinç processes

[4,5,35,36]. Phillips et al. [35] compared the effect of peracetic acid to hydrogen peroxide in

the sequences of OZEZP and OZEZPa. The resulting pulps had a brightness of 85.9 and

89.1% ISO respectively. A surprising fact was that the Pa bleached pulp showed normal

reverted brightness (87.3%), while the peroxide pulp brightness decreased to 76.9% after 2

hr treatment at 1 0S0C. More recent work [5] showed that a Pa treatment of an OZEZ

bleached softwood kraft pulp at a charge of 1 .O% Pa as H,02 increased the pulp bnghtness

from 81.3 to 92.4% ISO, which is superior to a brightness of 90.1% ISO obtained with a

peroxide stage at a charge of 1 %.

Until recently, the drawback of peracetic acid bleaching was its high manufacturing

cost. AIso it is not sufficiently stable to be shipped or stored for extended periods of time

19,111. Dupont, Eka Nobel and Degussa AG are arnong the companies who are marketing

peroxy acids in the pulp and paper industry [1,8,9].

2. PERACETIC ACID DECOMPOSITION AND HYDROLYSIS AT TYPICAL PULP BLEACHING CONDITIONS

Peracids undergo decomposition and hydrolysis by a variety of reactions which

depend on their structures, temperature, pH and the presence of catalysts, notably transition

metd ions. Under typical peracetic acid bleaching conditions, it is expected that in addition

to the lignin oxidation reactions, peracetic acid may aIso be consumed by three wastefiil side

reactions, namely i) spontaneous decornposition, ii) hydrolysis and iii) transition rnetal

catdysed reactions.

2.1 Spontaneous Decomposition

Peracids decompose spontaneousiy in an aqueous solution to give the corresponding

carboxylic acid and oxygen [ZO]. Studies of the spontaneous decomposition of various

peracids have shown that the decomposition rate reaches its maximum when pH is equal to

the pKa of the peracid [lO,37-4 11. The rate of disappearance of the peracid, ROOH, is given

b y:

- d[RooH1 = k [ROOH] [ROO -1 dt

When peracetic acid containinç about 4% CH,COO1xOIXH was used, oxygen appeared

predominately (83%) as OIW-O'Y [IO]. This leads to the conclusion that the electrophilic site

is carbonyl carbon rather peroxidic oxygen, as shown in Equation (8) [ l O]:

--

The decomposition kinetics of peracetic acid were first studied by Koubek [l O] over

the pH range of 5.9-10.2 in carbonate, phosphate and borate buffers with

ethylenediaminetetraacetic acid (EDTA) as stabilizer. He found the kinetics to be second-

order in total peracetic acid concentration. The reaction rate at 25°C reaches a maximum of

2.7 x 105 Wrno1.s at pH 8.2, which is also the pKa of peracetic acid. The rate constant

inmeases proportionally with pH until it reaches 7.5 and then decreases proportionally when

the pH is higher than 9.0 as follows:

The above kinetics do not include the pH range of 7.5 to 9.0. Equations (9) and (1 O)

show that two straight-line portions with a dope of +1 and - 1 should be obtained if logk,

is plotted against pH. However, Koubek's data showed that the slopes of the two straight line

sections deviated considerably fiom unity. He explained this by the hypothesis that the systern

was complicated by the metal ion catalysed decomposition of peracetic acid [10]. Later

McDonough [18] investigated the decomposition of peracetic acid in an aqueous solution in

the presence of DTPA, and found that the metal catalysed reaction between peracetic acid and

hydrogen peroxide is stilI considerable when DTPA is present, especidly at a pH higher than

8.0. Based on the experirnental results, he conduded that acid hydrolysis and the reaction of

peracetic acid with hydrogen peroxide were responsible for the deviation. He also found that

the rate constant is lower in an aqueous solution than that in a phosphate buffer. This he

explained with the hypothesis that phosphate may catdyse the hydrolysis of peracetic acid.

Rucker and Cates [42] reported that the peracetic acid consumption in a borate buffer at a

pH range of 7.0 to 8.9 is mainly due to hydrolysis, and not to spontaneous decomposition.

This is due to the fact that borate reacts rapidly with hydrogen peroxide to form

peroxyborates so that peracetic acid hydrolysis is dominant. Therefore, the above literature

shows that hydrolysis should be considered, and that a suitable chelating agent must be used

to suppress the transition metal ion catalysed reactions when the spontaneous decomposition

kinetics of peracetic acid are studied.

2.2. Hydrotysis

Hydrogen peroxide and acetic acid are formed upon hydrolysis of peracetic acid. Thus

there is no loss of active oxysen in reaction (1 1):

CH,CO,H + H20 + CH,CO,H + H,Oz (1 1)

However, hydrolysis represents a decrease in real bleaching power since hydrogen peroxide

is not an effective bleaching agent under typicai peracetic acid bleaching conditions.

Acid hydrolysis of peracetic acid was studied by Bunton et al. [43] and Koubek [ 1 O].

It was found [IO] that acid hydrolysis proceeds by nucleophilic attack of water on the

protonated peracid molea.de, with its rate beinç first order in peracetic acid concentration as

weil as acid concentration.

Aikaline hydrolysis has been the subject of only cursory investigation [ 10,441. D'Ans

and Mattner [44] reported that peracetic acid hydrolysis increases with increasing pH and

MgSO, addition, and becomes the dominant mechanism in a 0.9 moVL NaOH solution at

12°C when the transition metal catalysed decomposition of peracetic acid and hydrogen

peroxide is suppressed. However, the kinetics of the alkaline hydrolysis were not reported.

Koubek [IO] observed that alkaline hydrolysis proceeds at a rapid rate which increases with

increasing pH, but again the kinetics were not given.

2.3 Transition Metal Catalysed Reactions

2.3.1 Transition metal catalysed decomposition of peracetic acid

The first study conceming the catalytic decomposition of organic peracids in an

aqueous solution was reported by Kagan and Lubarsky [45]. They found that manganese

catalyses the decomposition of peracetic acid under formation of oxygen, carbon dioxide and

c h o n monoxide. Vrbaski and Brihta [46] reported the effect of cobalt on the decomposition

of peracetic acid. Both studies did not address the kinetics and mechanism of the

decomposition of peracetic acid. Koubek [IO] studied the metal ion catalysed decomposition

of peracetic acid at pH 5.4 in an ammonium acetate buffer. He found that the kinetics were

first-order in the peracetic acid concentration, and that the order of decreasing catalytic

activity was Co(II), Mn(II), Fe (II), and Cu(I1). He also showed that the reaction order in

cobaItous ion concentration varied fiom one at 10" mol/L to one-half at moVL. The

activation energy was found to be 21.6 kcal/mol. He proposed that the decornposition

followed a radical mechanism. Vasyutyn et al. [47] reported that in an aqueous solution of

peracetic acid without pH controt, Mn(I1) is the most harmful metal ion, followed by Co(I1)

and Fe(m). Furthemore, they confirrned that the kinetics are first order with respect to the

peracetic acid concentration. On the contrag, Ailen and Aguilo [48] reported second-order

kinetics in peracetic acid concentration when they studied the manganese catalysed

decomposition of peracetic acid in an acetic acid solution. However, the above authors didn't

extend their study to a neutral pH range which is of interest for peracetic acid bnghtening.

The importance of transition metal catalysed decomposition of peracetic acid on the chemical

pulp bleaching efficiency has not been examined. Aiso it is worthwhie to investigate the effect

of additives such as DTPq DTMPA Na,P,û,, and MgSO, on peracetic acid decomposition

and bleaching.

2.3.2 Reaction of peracetic acid with hydrogen peroxide

The reaction between peracetic acid and hydrogen peroxide was first noted in 195 1

by D'Ans and Mattner [44] in their studies of the decomposition of peracetic acid in an

alkaline solution. Later several workers [38,4 1,49,50] have reported that hydrogen peroxide

reacts with peracids at a pH above 10 as:

RCO, + Hz02 + RCO; + H20 + 0, (1 2 )

Akiba and Simamura [49] carried out this reaction with ''O labelled peroxybenzoate,

and concluded that the evolved O2 came almost entirely fiom hydrogen peroxide.

McDonough [18] studied the reaction of peracetic acid and hydrogen peroxide in aqueous

solutions at a pH range of 7 to 9.5. The reaction rate was found to increase with increasing

pH. He proposed that there are two reactions between peracetic acid and hydrogen peroxide,

one of which is catalysed by traces of transition metals. Reaction mechanisms were proposed

for these two reactions.

Evans and Upton [SOI examineci the reaction of peracetic acid and hydrogen peroxide

at a pH of I O to I 1. They concluded that the mechanism of the uncatalysed reaction of

peracetic acid and hydrogen peroxide is analogous to that of the spontaneous decomposition

of peracetic acid given by Equation (8) as:

However, this reaction is much more difficult than the nucleophilic attack of peracetic acid

anion on peracetic acid since hydrogen peroxide is a much poorer electrophile than peracetic

acid. Furthemore, Evans and Upton [SOI found that the reaction between peracetic acid and

hydrogen peroxide is strongly catalysed by transition meta1 ions, and that the cataiytic effect

can be minimized by addition of phosphonc acid chelating agents. Bal1 and Edwards [38]

reported that the addition of EDTA stopped the catdytic reaction between Caro's acid

(monoperoxysulphuric acid) and hydrogen peroxide at a pH range of 6 to 8.37.

A radical mechanism of the catatytic reaction between peracetic acid and hydrogen

peroxide given by the overall reaction Equation ( 12) was proposed by Evans and Upton [50]

as:

RCO, + M"' + H1O - RCO, + (MOH)"' + OH-

R C 0 , - + Hz07 - RCOOH + HO.

HO2' + H' ' 02-- ( 16)

(MOH)"' + O?'' + OH- + Mn+ + Oz ( 17)

2.4. Formation of Gaseous Products during Decomposition of Peracetic Acid

Gaseous products are generated during the decomposition of peracetic acid and the

reaction of peracetic acid with hydrogen peroxide [ 1 O,45,46,48 3. Kagan and Lubarsky [45]

studied the manganese caîalysed decomposition of peracetic acid in both acetic acid and water

solutions. They found that the composition of the gaseous products evolved during the

peracetic acid decomposition was different for the two solutions. Carbon dioxide and oxygen

were the main gaseous products formed in the acetic acid solution and water solution

respectively. Later Allen and Aguilo [48] confirmed that carbon dioxide is the main gaseous

product formed fiom the manganese catalysed decomposition of peracetic acid in an acetic

acid soIution. Vrbaski and Bnhta [46] investigated the cobalt catalysed decomposition of

peracetic acid in an aqueous solution without pH control, and claimed that the only gas

formed is oxygen. Contrary to this finding, Koukek [IO] later found that both oxygen and

carbon dioxide are formed at a molar ratio of about one for cobalt catalysed decomposition

of peracetic acid at pH 5.4 in an ammonium acetate buffer. Furthemore, Koubek [l O] also

established that roughly 80% of the carbon dioxide formed arises from the acetate ion of the

buffer. The above authors did not extend their study to a neutral pH range, the optimum pH

for peracetic acid brightening [28.33].

3, HYDROGEN PEROXIDE BLEACHING OF CHEMICAL PULPS

The use of hydrogen peroxide as a pulp bleaching agent has been well documented

since its introduction for mechanical pulp bleaching in 1940. There has been a growing use

of hydrogen peroxide in chemical pulp bleaching since the late 1980s due to the pressure to

reduce the use of chlorine in the bleach pIant.

A hydrogen peroxide treatment may be included in the final stages of a bleach plant

to achieve higher brightness (292% ISO) and improved brightness stability for specialty puIps

[ l 11. Another typical appIication of peroxide is to reinfiorce an alkaline extraction stage (Ep)

or an oxygen reinforcd extraction stage (Eop), which has been used in some ECF sequences

to reach the final brightness target and decrease the chemical cost [5 1-52]. Hydrogen

peroxide is also one of the p n m q chernicals used in TCF bleaching of chernical pulps. It has

been shown that brightness and brightness stability are optimum when hydrogen peroxide is

applied at the end of sequences such as OZP, OZEopP and O(QZ)Q(PO) [53]. Recently,

Devenyns et al. 1541 reported a modified version of hydrogen peroxide bleaching, which is

operated at high consistency, high temperature (r 100°C) and under pressure, the so called

P(0) process. Its commercial application showed an improved bleaching response at a Iower

hydrogen peroxide consumption [55,56].

Some transition metals catalyse the hydrogen peroxide decomposition and contribute

to the formation of hydroxyl radicals, HO- and superoxide anion radicals, 02.- [ 1 11. Although

it has been reported that the presence of low concentrations of these radicai species may

actualIy improve hydrogen peroxide bleaching 1571, it is generally accepted that a high

concentration of such radicais ieads to a poor selectivity and impaired bleaching Alkaline

earth metal (e.g., magnesium and calcium) compounds and sodium silicate appear to reduce

the hydrogen peroxide decomposition when added to an alkaline peroxide stase 1581. These

compounds may form complexes with transition metais to lower their catalytic activities.

Proper management of metal ions is essential to achieve efficient hydrogen peroxide

bleaching. Two methods have been shown to yield successfiil bleaching results: 1 ) chelation

with DTPA at pH 4-7 followed by a washinç or pressing stage [59] and 2 ) an acid treatment

at pH < 3 followed by washing and addition of magnesium salts to replenish the magnesium

removed in the washing stage [60]. A Mg/Mn ratio of over 30 is needed for effective

hydrogen peroxide bleaching at a temperature range of 60 to 90°C, and the MgMn ratio must

be higher than 100 if the P(0) process is applied [6 11.

Chelants, which are polydentate ligand molecules, are ofien used in the pulp and paper

industry to minirnize the negative effect of metal ions. Some typical chelants include

ethylenediarninetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA).

DiethyIene triamuiepentamethylenephosphonic acid (DTMPA) has also been show to be an

effective chelant [62].

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CHAPTER 3

PERACETIC ACID AND HYDROGEN PEROXIDE

BRIGHTENING OF A SOlVWOOD KRAFT PULP DELIGNIFIED

BY METHANOGASSISTED OZONA'IION

ABSTRACT

In earlier studies at the Limerick Pulp and Paper Centre, it was demonstrated that

conventional chernical pulps cm be delignified by a solvent-assisted ozone process to a low

kappa number while still maintaining good viscosity and strength properties. In this chapter,

a final brightening sequence was developed for this ozonated pulp with a peracetic acid

treatment and an Eop stage.

A fiactional factorial design with four parameters, each perforrned at three levels, was

used to optimize the process conditions of the peracetic acid stage. It was found that a single

peracetic acid stage cm not produce a fùlly bleached pulp with a brightness above 88% ISO.

However, this brightness level is achieved by sequential application of a peracetic acid and

an Eop stage. The effect of operating variables of the Eop stage, such as the caustic charge,

MgSO, charge, temperature and time, were also investigated. Finally it was argued that a

conventional softwood kraft pulp bleached to 88% ISO brightness by this TCF sequence of

ZnEoPaEop has strength and bnghtness stability properties similar to or better than those of

chlorine and chlorine dioxide bleached pulps.

Keywords: peracetic acid brightening, ozone delignified s o b o o d kr& pulp, hydrogen

peroxide, totally chlorine fiee bleaching, brightness, viscosity

INTRODUCTION

Commercial production of TCF fblly bleached softwood kraft and sulfite pulps is now

a reality [l-81. However, these TCF processes still have a number of weaknesses. In most

insiances, the final pulp brightness achieved in these bleaching processes is below 88% ISO,

and generalIy in the range of 80 - 85% ISO [3-5,8]. Furthemore, it appears that the pulp

strength properties, such as the tear index (at the same tensile index), are typically 10 - 15%

lower than those of pulps with equd final bnghtness obtained by conventional chlorine-based

bleaching [3,4,7,8]. Althougli a few papers [9-101 reported that TCF fùlly bleached pulps cm

be produced with strength properties similar to those of conventionally bleached pulps, these

TCF sequences require oxygen delignification ancilor other extended delignification

techniques, which are capital intensive and generally less selective than the chlorine based

processes.

Earlier studies at the Limerick Pulp and Paper Centre and at MacMillan Bloedel[l 1-

151 showed that a solvent-assisted ozone bleaching process can replace chlonnation as the

delignification step while maintaining the same iignh-carbohydrate selectivity as that of

chlorine bleaching. For example, a well delignifed pulp with a kappa number of 5.8 and a

viscosity of 28.4 mPa.s can be obtained when an unbleached Hemlock kraft pulp with a kappa

number of around 30 and a viscosity of about 33 rnPa.s was subjected to a ZnEo sequence,

where Zn represents the solvent-assisted ozone bleaching stage.

Peracetic acid, although known as an effective deligni fication chemical for nearl y fi@

years- was until recently considered too expensive for commercial application [ 161. However,

recent advances in the manufacture of peracetic acid have significantly reduced the cost,

making peraçetic acid a promisinç candidate for pulp bleaching [16,18]. Most of the literature

on peracetic acid in pulp bleaching deals with delignification [ 1 6,17,2 1 -241, and limited results

are available on the use of peracetic acid as a brightening chemical to produce fùlly bleached

chemical pulps [19,25-281. Rapson [28] reported the effect of pH on the briçhtness and

viscosity development during peracetic acid treatrnent of a CEHCE delignified kraft pulp with

a brightness of 69% ISO. The pulp brightness increased to 85% ISO after 4 hours at a 1.07%

charge of peracetk acid, pH 7,70°C and 8% pulp consistency. A minimum viscosity loss was

observed in the pH range of 8 to 9. Liebergott [19] found that CE delignified sulphite pulps

could be bleached to 85 - 90% ISO brightness with a peracetic acid charge of about 1%.

Brightening of a softwood kraft pulp with C10, or peracetic acid produced a brightness of

83% ISO at a charge of 0.6% CIO,. and 86.5% ISO at a peracetic acid charge of 0.45%

expressed as equivalent weight of hydrogen peroxide [i 91.

Hydrogen peroxide is a good brightening chernical when the optimum conditions are

used [8,29-3 11. A hydrogen peroxide reinforced extraction stage (Eop) has been widely

applied to achieve a higher final bnghtness at a reduced chemical cost [30-3 11. It is known

[32,33] that a-carbonyl and quinone structures are effdvely removed, while a, P-unsaturated

double bonds are stable and continuously formed durinç conventional hydrogen peroxide

bleaching. On the other hand, double bonds in residual liçnin are very reactive toward

peracetic acid [34]. Therefore, one might expect that a higher brightness could be achieved

by a sequentiai treatment of chemical pulp with peracetic acid and hydrogen peroxide, as was

confirrned by Liebergott (351 as well as Ni and Ooi [36].

The main objective of the present study was to develop a TCF brightening sequence

usin3 peracetic acid and hydrogen peroxide for the ZnEo sofiwood kraft pulp to produce a

fÙlI brightness, high strength product comparable to conventionally bleached pulp.

EXPERIMENTAL

Peracetic acid was purchased fiom Aldrich (Milwaukee, USA). Its composition is

about 34% w/w peracetic acid, 5% wlw hydrogen peroxide, 40% wlw acetic acid with the

remainder being water. Reagent gade -O2, NaOH, MgSO, and DTPA obtained fiom Fisher

Scientific (Nepean, Ontario) were used without fùrther purification. The quantities of

chernicals are expressed as a weight percentage of pure chernical on oven dried pulp. 2.24%

peracetic acid charge equals to 1% H,O, on an equivalent oxidant basis.

The ZnEo pulp with a brightness of 56.5% ISO, kappa number of 5.8 and viscosity

of 28.4 mPa-s was obtained fiom MacMillan Bloedel Ltd.. The kappa number and viscosity

of the original unbleached Hemlock kraft pulp were about 3 0 and 3 3 mPa.s respectively. The

methanol-assisted ozone bleaching (Zn) was camed out at a consistency of about 35% with

a 70% methanol solution of pH 2 as impregnation liquor. The ozone charge was 2%, of which

over 90% was consumed during the course of the reaction. The ozonated pulp was then

subjected to an Eo stage at the following conditions: 2% NaOH charge, 205 kPa 0, pressure,

10% pulp consistency, 7OoC, 1 hour. The ZnEo treated pulp has a transition metal ion content

of 61.8 ppm Fe, 2.7 ppm Mn and 1.3 ppm Cu.

The peracetic acid brightening was camed out in a poIyethylene bag containing a

suficient quantity of NaOH and water to provide the required initial pH and consistency.

M e r Pa treatment, the pulp was washed thoroughIy with deionized water and the spent

liquor was titrated for its residual peracetic acid and hydrogen peroxide concentration

following the iodometric method developed by Greenspan and Mackellar [37].

The Eop stage was performed under oxygen pressure in a specially designed

stainless steel bomb with tefion liner to avoid direct contract between the puip sample and

the metal surface. An oil bath was used to keep the bomb at the required temperature.

After the Eop stage, the pulp s l u q was washed thoroughly with deionized water and the

spent liquor was analysai for residual H,O,.

Pulp brightness, viscosity and kappa number were determined in accordance with

TAPPI methods T452 om-92, T230 om-89, and T236 cm-85 or Tappi UM246, respectively.

RIESULTS AND DISCUSSION

1. Peracetic Acid Brigbtening (Pa) of ZnEo Delignified Pulp

The ZnEo semi-bleached pulp was subjected to a peracetic acid treatment (Pa). A

hctiond fadorial design ofh(3') was used to optimize the process conditions. Temperature,

pH, peracetic acid charge and pulp consistency were chosen as the four factors, and each was

investigated at three levels. Table 3-1 Iists the bleaching results &er 3 hr. Also included are

the standard data analysis results, where Ij is the average brightness or viscosity when

bleaching is performed at the first level of factorj and various levels of the other three factors.

For example, 1, is the average brightening result obtained at the first level (1%) of the first

factor (peracetic acid charge) at different pHs, temperatures and pulp consistencies; II, is the

average brightening result obtained at the second level (pH 7.0) of the third factor (initial pH)

at different peracetic acid charges, temperatures and pulp consistencies.

Table 3-1. The Fractional Factorial Design of b(3') for Pa Brighiening of ZnEo Delignified Pulp

Bleaching Results I

Run Fa1

Pulp Conc. (%)

IO 1

Pa Charge

(Yo)

1 .O

Initial PH

Temp. (OC)

The maximum differences listed indicates the maximum possible change in pulp properties

which were caused by the variation in the Ievel of a specified factor. For example, a maximum

brightness difference of 5.6% ISO was obtained between the pulps prepared at different

Brightness (% ISO)

Max. Diff.

Viscosity (mPa.s)

Visco.: Ii

II,

IIIi

M U

25.5

24.7

22.8

initial pH levels of 5.0, 7.0 and 9.0.

A cornparison of the maximum differences for both brightness and viscosity shows

that pH is the most important operating parameter affecting the performance of a Pa

treatment. The optimum pH is in the neutral range, which is consistent with the findings of

eariier studies [ I9,24,28]. Table 3- 1 shows that the pulp brightness increases with increasing

temperature, while the pulp viscosity decreases slightly at the highest temperature. An

additional experiment performed at a temperature of 80°C shows that the brightness does not

increase fùrther above that obtained at 60°C while the pulp viscosity is significantly fùrther

reduced to 14.8 mPa.s. The latter can be attributed to the hct that at higher temperature more

peracetic acid is consumeci in wastefùl side reactions which generate harmful reactive species

(see Chapter 6 for details). Therefore, a temperature of about 60°C is optimum for peracetic

acid brightening of the ZnEo delignified pulp. Table 3-1 also indicates that a peracetic acid

charge of 1.5% is appropriate since the fiirther increase in btightness is very small when the

peracetic acid charge is raised fiom 1.5 to 2%. Finally, an increase in puIp consistency from

10% to 20% improves the bleaching results by an additional 3.3% ISO brightness units.

However, a fbrther increase in consistency to 30% lads to a slightly smaller brightness gain.

This may be explained by the difficulty of mixing pulp and chemicals at 30% consistency.

Based on the above analysis, it can be concluded that the optimum condition for

peracetic acid bnghtening of ZnEo delignified pulp is 1.5% peracetic acid charge, 20% pulp

consistency, 60°C and an initial pH of 7.0. The ZnEoPa pulp produced at these optimum

conditions has the following properties: 82.7% brightness, 26.6 mPa.s viscosity and 1.9

kappa number.

For al1 the runs perfomed so far in this study a reaction time of 3 hours was chosen.

We fùrther studied the efFect of bleaching time at the above specified optimum conditions.

The results listed in Table 3-2 show that there is no need to extend the bleachins time beyond

3 hours. Furtherrnore, it shows that the viscosity loss during peracetic acid bleaching is

relatively smail when operated at the optimum conditions, confirming that peracetic acid is

rather selective dunng final brightening [ 1 7,24,28].

Table 3-2, The Effect of Reaction Time on Peracetic Acid Bleaching of ZnEo Delignified Pulp

Other Conditions: 1.5% Pa charge, 20% pulp consistency, 60°C, initial pH of 7.0

2. Further Bnghtening of ZnEoPa Treated Pulp in An Eop Stage

Bleaching Time ( h o

O

1

2

3

4

In the previous section, it wras shown that the optimum peracetic acid charge is about

1 .5%. By fùrther increasing the charge of peracetic acid it can be shown that a brightness

ceiling of about 84% ISO is obtained at a peracetic acid charse of 5%. Therefore, we fùrther

treated the ZnEoPa pulp with either an Eo or an Ep or an Eop stage. It was found that an Eop

stage gave the best brightening result. The pulp brightness afler either the Eo or Ep stage was

about 2-3 units Iower than that after the Eop stage. Therefore, the Eop stage was used for

Brightness (% ISO)

56.5

78.2

80.7

82.7

82.9

Viscosity (m Pas)

28.4

27.2

26.8

26.6

26.0

Residual Pa (% on charge)

-

28.9

21.6

15.7

12.4

further brightening of the ZnEoPa pulp, and the determination of the optimum conditions of

this stage is described in the following sections.

2.1 H202 charge

A fùlly bleached pulp of 88.0% brightness and 1 8.9 mPa.s viscosity can be obtained

when subjecting the ZnEoPa pulp to an Eop stage with charges of 0.5% H202 and 1.5%

NaOH. A fiirther increase in the HzOz charge fi-om 0.5% to 1.0% raises the brightness to

89.0°/0, however the viscosity decreased to 15.1 rnPa.s. Therefore, 0.5% H,O, charge will be

used in the Eop stage.

2.2 NaOH charge

It is known that the perhydroxyl ion, the dominant form of H,O, under alkaline

conditions, is the reactive species during peroxide brightening. However, excessive caustic

addition may cause aikali darkening. Therefore optimization of the alkalinity is important for

a maximum brightness gain. The results in Table 3-3 suggest that a NaOH charge of 1 -5%

is the optimum level for the present conditions.

Table 3-3. The Effect of NaOH Charge on the Eop Stage

Other conditions: 0.5% H202 charge, 0.05% MgSO, , 20% consistency, I05"C, 60 psi O, Dressure, 60 min.)

NaOH Charge (% on 0.d. pulp)

0.5

Residual H,O, (% on charge)

26.1

Brightness (% ISO)

85.9

Viscosity (m Pas)

31.1

2.3 DTPA addition

The effect of DTPA addition to the Eop stage was investigated for the ZnEoPaEop

sequence (Table 3-4). As mentioned earlier, a TCF bleached pulp with a brightness of 88.0%

1SO and viscosity of 18.9 rnPa.s was obtained without DTPA in the Eop stage. With a DTPA

charge of 0.2%. the brightness did not change significantiy but the viscosity decreased to 1 6.7

mPa.s. A fûrther increase in the DTPA charge resulted in both a decrease in viscosity and

brightness. Also less residual hydrogen peroxide was present at the end of bleaching when

more DTPA was charged to the Eop stage. Similar results were obsenred for hydrogen

peroxide bleaching of an oxygen delignified kraft pulp [38]. However, this is contrary to

other references which report that the inclusion of DTPA in the peroxide bleaching liquor

results in a higher brightness gain as weU as a higher peroxide residual [39,40]. The latter was

explained by the well known fact that DTPA in the bleaching liquor chelates and thus

inactivates the metal ions released by the pulp [29]. Colodette et a1.[41] found that DTPA

addition to a hydrogen peroxide solution reduces the copper and manganese catalysed

decomposition of hydrogen peroxide, but enhances the iron catalysed decomposition. It was

also reported [42,43] that Fe3' chelated by EDTA or DTPA is still catalytically active

towards hydrogen peroxide decomposition. Therefore, it is reasonable to assume that the

effect of the addition of DTPA to the Eop stage depends on the metal ion profile in the

original pulp. Thus, it is expected that a beneficial effect will be observed if the pulp sample

contains more manganese and copper. However. a negative effect is expected if the iron

content is relatively high in the pulp. In the experimental section, it was reported that iron is

by far the most dominant transition metal species in the ZnEo delignified pulp. This then

explains why in Our system the addition of DTPA has a detrimental effect on the performance

of the Eop stage.

Table 3-4. The Addition of DTPA to the Eop Stage

Other conditions: 0.5% H,O, charge, 1.5% NaOH, 0.05% MgS0, . 20% consistency, IO5OC 60 Dsi 0- m e . 60 min )

i

2.4 MgSO, addition

Residual H202 (% on charge)

20.5

12.1

O

MgSO, is known as a stabiiizer for hydrogen peroxide bleaching [44,45]. The results

in Table 3-5 show that a brightness gain of about 3 to 4% 1SO and Mscosity increase of 6 to

7 mPa.s are obtained when 0.05 to O. 1% MgSO, is added to the Eop stage. The cornpiete

consumption of residual hydrogen peroxide in the absence of MgSO, is an indicator of

significant peroxide decomposition and thus less efficient use of hydrogen peroxide for

brightening reactions. The addition of too much MgSO,, however, reduces the brightness

development. This is in agreement with the çeneral consensus [46-491 that a suitable ratio of

magnesium to transition metal ions should be maintained in hydrogen peroxide bleaching.

2.5 Temperature and reaction time

The effect of temperature and reaction tirne on the Eop stage is shown in Table 3-6.

Viscosity (mPas)

18.9

16.7

12.1

r

DTPA charge (% on O-d. pulp)

O

0.2

0.4

Brightness (% ISO)

88.0

88.3

84.3

0.6 84.0 10.6 O

A cornparison of pulps with similar brightness (Run 2 and 5) shows that a decrease in

Table 3-5. The Addition of MgSO, to the Eop Stage

II Other conditions: 0.5% H,O, charge, 1.5% NaOH, 20% consistency, 105"C, 60 psi Q 11 pressure, 60 min.)

Residual H,02 (% on charge)

O

20.5

27.3

temperature fiom 105°C to 9S°C improves the Mscosity by about 2 to 3 units. Note also that

more hydrogen peroxide remains at 95OC than at 10S°C, indicating that less hydrogen

peroxide is decomposed during bleaching at 95°C. To compensate for the decrease in reaction

rate at a lower temperature , the reaction time of the Eop stage at 95°C was extended fiom

60 to 90 minutes. A hrther increase in the reaction time to 90 minutes at 105°C or to 120

minutes at 95°C deçreased the final Mscosity. Therefore the optimum reaction time should be

about 90 minutes at 95°C.

2.6 The optimum Eop condition

B a d on the above reported results it is now possible to define the optimum operating

conditions of the Eop stage for the ZnEoPa treated pulp: 0.5% H,0, ,1.5% NaOH and 0.1%

MgSO,, 95OC, 20% consistency, 60 psi oxygen pressure and 90 minutes. No DTPA is needed

for this particular pulp since iron is the dominant transition rnetal ion present in the pulp. The

Viscosity (mPa.s)

12.2

18.9

19.9

MgSO, charge (% on 0.d. pulp)

O

0.05

O. 1

Brightness (% ISO)

84.4

88.0

88.2

0.2

0.4

87.5

87.1

19.5

19.0

17.2

15.3

final brightness and viscosity for these conditions are 88.1% 1SO and 2 1.8 mPa.s.

Table 3-6. The Effect of Tempenture and Reaction Time of the Eop Stage

-

Other conditions: 0.5% H,O, charge, 1.5% NaOH, 0.1% MgSO, . 20% consistency, 60 psi 0, pressure

2.7 The optimized ZnEoPaEop pulp

Forber [50] concluded that a fully bleached TCF pulp will exhibit strength properties

comparable to those of conventionally bleached pulp if its viscosity is about 16 mPa.s or

higher. Therefore, it is reasonable to assume that the fùlly bleached Hemlock softwood krafi

pulp obtained by the optimized ZnEoPaEop sequence will be as strong as chlorine and

chlorine dioxide bleached pulps. Unfortunately. due to insufficient sample, the strength

properties of this TCF fully bleached pulp could not be determined. It should be noted that

this TCF sequence started with a conventional softwood kraft pulp of kappa number 30

without using any extended cooking or oxygen pre-delignification. Another characteristic of

Residual H20, (% on charge)

3 1.3

27.3

O

49.1

Brightness (% ISO)

86.5

88.2

89.1

86.5

Viscosity (mPa.s)

19.4

19.9

15.4

22.6

the present TCF sequence is that no chelants are used in the bleachinç sequence.

3, Further Brightening of the ZnEoPaEop Treated Pulp by a Final Pa Treatment

As show in Section 2.1, the brightness of the ZnEoPaEop pulp was limited to about

89%. However, the brightness can be further increased by a find Pa treatment at a charge of

0.5% while the other conditions being the same as those of the first Pa stage. The resulting

bleached pulp has a brightness of 90.4% ISO and a viscosity of 20.5 rnPa.s. Apparently the

sequential treatment of pulp by peracetic acid and hydrogen peroxide leads to an increased

brightness ceiling due to the complementary brightening chemistry of these two chemicals.

4. Brightness Stability of the TCF Fully Bleached Pulp

TCF firlly bleached pulp samples of 88.1 and 90.4% ISO brightness obtained by the

ZnEoPaEop and ZnEoPaEopPa sequence respectively, were subjected to rapid aging at

105OC for 1 hour. The brightness reversion of both pulps was about 1.1% ISO, which

compares favourabiy with a reversion of 1.9% ISO for a conventionally chlorine and chlorine

dioxide bleached pulp [ 1 O].

5. Chernical Cost Evaluation of the ZnEoPaEop Sequence

An economic evaluation was made of the chemical cost of the ZnEoPaEop sequence

which produced a fùlly bleached Hernlock kraft pulp with a brightness of 88.1% 1SO and a

viscosity of 21.8 mPa.s. Table 3-7 shows that the total cost of the bleaching chemicals used

in the process is about 55 US.$ per tonne of pulp. This is comparable to that of an ECF

bleaching process [ 3 6 ] .

Table 3-7. Estimation of Bleaching Chernical Cost of the ZnEoPaEop Sequence

1 Total chemical cost (U.S.$/ton 0.d. pulp) 55-82 I From reference [5 1 ]

Stage

Zn

Eo

Pa

EOP

Other chernicai costs From reference 1201

CONCLUSION

Chernicals

0 3

NaOH

0 2

Peracetic acid

NaOH

H A

MgSO4

A conventional sofiwood kraft pulp with kappa number of 30 has been bleached to

88% ISO brightness while maintaininç a viscosity of more than 20 mPa.s by a ZnEoPaEop

TCF sequence with 2% ozone, 1.5% Pa and 0.5% hydrogen peroxide charge. The brightness,

Cost (U-S.$)

22.0

6.0

0.6

17.7

4.5

4.5

0.53

Charge (% on 0.d- puIp)

2.0

2.0

1 .O

1.5

viscosity and brightness stability of this TCF filly bIeached kraft pulp are similar to or better

Unit Price (U,S.%/kg)

1.1.

0.3

0.06

1.18

t han those of chlotine-containing bleached pulp.

This chapter focuses on the final brightening of ZnEo pulp by peracetic acid and

1.5

0.5

O. 1

hydrogen peroxide. The optimum conditiom for the peracetic acid and Eop stages have been

0.3

O. 9

O. 53

determined. Those for the Pa stage are 20% pulp consistency ,60°C, a neutral pH and 3 hours.

The peracetic acid charge for the first and second Pa stage is 1.5% and 0.5% respectively.

The optimum conditions for the reinforced Eop stage are 20% pulp consistency , 95"C, 60

psi oxygen pressure, 90 min with 0.5% H202 , 1.5% NaOH and 0.1% MgSO,. The unique

charactenstic of this TCF sequence is that it can be applied to conventional sofiwood kraft

pulp with a kappa number of about 30 . Also, no chelation chernicals are required for this

TCF bleaching sequence.

ACKNOWLEDGMENT

The author thanks NSERC for financial support in the form of a Strategic Grant and

MacMillan Bloedel Ltd. for supplying the ZnEo pulp.

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CHAPTER 4

KINETICS OF PERACETIC ACID DECOMPOSITION

PART 1: SPONTANEOUS DECOMPOSITION AT

TYPICAL PULP BLEACHING CONDITIONS

ABSTRACT

Peracetic acid is one of the alternative non-chlorine bleaching chernicals for

production of fiilly bleached chemical puips. In this paper, the stability of peracetic acid was

studied in an aqueous solution under conditions most likely encountered in industry. It was

found that three reactions, narnely i) spontaneous decomposition, ii) hydroiysis and iii)

transition metal catalysed reactions, are potentially responsible for the consumption of

peracetic acid. Furthemore, the kinetics of the spontaneous decomposition were developed.

It was found that the reaction follows second-order kinetics with a maximum rate at pH 8.2,

which is the pKa of peracetic acid. The developed kinetic equation provides a very good

description of the experimental results obtained in this study.

Keywords: kinetics, peracetic acid, spontaneous decomposition, hydrolysis, pulp bleaching

INTRODUCTION

Environmenta1 concems and market pressure are forcing the pulp and paper industry

to explore alternatives to conventionaI chlorine bleaching. Recent results have s h o w that

peracetic acid is a less capital intensive, easily retrofitted and highly selective Total Chlorine

Free (TCF) bleaching agent when used under optimum conditions [ Il.

Peracetic acid is the mono-acetyl derivative of hydrogen peroxide, and has a single

acidic proton with a pKa of 8.2 at 25°C [SI. It is usually prepared by mixing acetic acid and

hydrogen peroxide with sulfùric acid added as a catdyst [3]. Since the reaction is reversible,

an equilibrium mixture of reactants and products is normally available. High purïty peracetic

acid can be obtained by distillation.

Peracetic acid decomposes spontaneously in an aqueous sohtion to give acetic acid

and oxygen [3]. Transition metal ions such as cobalt and manganese catalyze peracetic acid

decomposition. However, the catalytic effect can Mrtuaily be eliminated by the addition of

a suitable chelating agent [Z]. In addition, peracetic acid cm be hydrolyzed to form hydrogen

peroxide and acetic acid [33. Since hydrogen peroxide and oxygen are ineffective bleaching

agents under typical peracetic acid bleaching (Pa) conditions, both the decomposition and the

hydrolysis are undesirable. Therefore, a better understanding of the mechanism and kinetics

of peracetic acid decomposition and its hydrolysis is of findamental importance for a

commercial Pa stage.

There are a few studies reporied in the literature on the decomposition kinetics of

peroxyacids. It was found [4] that the decomposition kinetics of caro's acid in phosphate and

carbonate buffers exhibit a second order dependence on the total caro's acid concentration.

Recently, Francis et al. [ 5 ] reported a kinetic study of the decomposition of caro's acid under

conditions more similar to those encountered during pulp bleaching, and confirmed that the

decomposition follows a second order dependence on the total caro's acid concentration.

However, Francis et al. [ 5 ] found that for each pH unit increment the decomposition rate

increased by a factor of 6 . This is different from the results obtained in phosphate and

carbonate buffers by Bal1 and Edwards [4] who reported that an increase of one pH unit lads

to an increase in the decomposition rate by a factor of 10.

The decomposition kinetics of peracetic acid were first studied by Koubek et al. 121

over the pH range of 5.9-10.2 in carbonate, phosphate and borate buffers with

ethylenediaminetetraacetic acid (EDTA) as stabilizer. He found that the dependence of the

kinetics on the total peracetic acid concentration is second-order. The reaction rate at 2S°C

reaches a maximum of 2.7 x 1 V3 Umo1.s at pH 8.2, which is the pKa of peracetic acid. The

rate constant increases proportionally with pH until it reaches 7.5, and then decreases

proportionalIy with pH when the pH is higher than 9.0.

A mechanism of peracetic acid decomposition was proposed which is compatible with

the above kinetics. It was postulated that the decomposition reaction involves a nucleophilic

attack of a peracid anion on a peracid molecule as:

However, in preiiminary experiments we found that Koubek's mode1 181 couid not accurately

predict our experimental data obtained under typical conditions encountered during pulp

bleaching. Also, we found that at a pH higher than 8.2, the peracetic acid hydrolysis is not

negligible, and therefore a correction must be made to account for the peracetic acid

consumption due to hydrolysis. Furthemore, Koubek's kinetics did not include the pH range

fiom 7.5 to 9.0. The presence of buffers in his study, which leads to an increase in the ionic

strength of the solution, might influence the decornposition rate of peracetic acid, similar to

that observed by Francis et al. [SI when they studied the kinetics of caro's acid

decomposition. In addition, the presence of buffer solutions may change the nature of how

peracetic acid is consumed, as was found by Rucker and Cates 163. They reported that the

peracetic acid consumption in a borate buEer in the pH range of 7.0 to 8.9 is mainIy due to

hydrolysis and not spontaneous decomposition. This is due to the fact that borate reacts

rapidly with hydrogen peroxide to form peroxyborates so that the hydrolysis of peracetic acid

is favoured.

The objective of this project is to investigate the stability of an aqueous solution of

peracetic acid without the prescnce of buffers under typical conditions encountered during

pulp bleaching. In this chapter, the kinetics of spontaneous decomposition of peracetic acid

are reported in a pH range of 5.5 to 9.0. In the next chapter, the effect of pH, stabilizers and

hydrogen peroxide on the decomposition of peracetic acid as well as the kinetics of the

peracetic acid hydrolysis will be discussed.

EXPERIMENTAL

Equilibrium peracetic acid and ultra-pure grade NaOH were purchased fkom Aldrich

(Milwaukee, USA). The equilibrium peracetic acid consists of about 34% peracid, 5%

hydrogen peroxide, 40% acetic acid with the remainder being water (al1 in mass%). Reagent

grade MgSO, and DTPA fiorn Fisher Scientific (Nepean, Ontario) and DTMPA from

Buckman (Memphis, USA) were used in the experirnents without fùriher purification . Ail the

experiments were carried out in a 500 mL four-neck round-bottom flask immersed in a

constant temperature bath. The flask was cleaned by washing successively with glassware

cleaner , deionized water, 5% (w/w) sodium hydroxide solution and finally distilled and

deionized water.

The aqueous solution containing al1 required chernicals except peracetic acid was

preheated to the desired temperature. The reaction was initiated by addition of the

concentrated peracetic acid soIution to the reaction flask. The initial peracetic acid

concentration was always 3.75 g/L (equivalent to 1.5% charse on oven dried pulp at 20%

bleaching consistency). The pH was controlled at a constant value by an automatic titrator

through addition of 10 N NaOH . Samples were withdrawn at predetermined time intervals

and analysed in accordance with the iodometric method deveioped by Greenspan and

Mackellar [7]. Ail reported values were the results of at least duplicate experiments.

APPROACH

In the pH mse of 5.5 to 9.0, peracetic acid may be consumed in the following three

reactions:

1. Spontaneous decomposition:

2 CH3C03H - 2 CH3C02H + O,

3 .Transition metai catalyzed reactions:

In the spontaneous decornposition (Reaction (l)), peracetic acid is decomposed to form acetic

acid and O,, thus representing a loss in oxidation power. In Reaction (2), peracetic acid is

hydrolyzed to form acetic acid and hydrogen peroxide. This reaction becomes more important

as the alkaiinity of the solution increases 181. Since hydrogen peroxide at neutral pH is not

reactive in terms of bleaching pulp, Reaction (2) is also considered a wastefiil reaction.

Finally, the presence of transition metals in a peracetic acid solution may induce fûrther

decompositions as indicated by Reactions (3) and (4). The experimental strategy of this

project is to eliniinate the transition metal catalysed reactions by choosing an appropriate

chelating agent. In addition. by measuring both the peracetic acid and hydroçen peroxide

concentrations during the course of the reaction, it is possible to determine the peracetic acid

consumed in both spontaneous decomposition and hydrolysis respectively.

The effect of the addition of a chelating agent on the consumption rate of peracetic

acid at pH 7.0 is shown in Figure 4-1, where the intesral form of the kinetics for a second-

order reaction is plotted versus reaction time 191. For the blank system one can observe that

afler an initial linear behaviour the integral form increases faster than the reaction tirne.

Figure 4-1 also shows that the presence of chelating agents l a d s to a reduced rate of

peracetic acid consurnption and a straight line behaviour of the data. The latter is a

confirmation of the second order behaviour of the kinetics in total peracetic acid

concentration. The reduced consumption of peracetic acid with the chetant addition may be

attributed to the transition metai catdyzed reactions, Le., Reactions (3) and (4). It was found

that a concentration of 0.5 g/L diethylenetriaminepentaacetic acid (DTPA) or diethylene

triaminepentarnethylenephosphonic acid (DTMPA) is sufficient to give reproducible second-

order rates at pH of 5.5 to 7.5. The fact that DTPA gives the same result as DTMPA

indicates that at this pH range both DTPA and DTMPA are effective cheiating agents.

However, as the pH inaeases further t O the range of 7.5 t O 9.0, we found t hat only DTMP A

is an efficient stabilizer while DTPA is not. Therefore, at a pH of 7.5 to 9.0, O. 5 g/L DTMPA

was added to the peracetic acid solution to eliminate the transition metal catalyzed reactions.

As report4 earlier [SI, the peracetic acid hydrolysis increases at a higher pH. It was

found that at pH 8.2 the hydrogen peroxide concentration after 2 hours at 40°C is about 9%

higher than the initial hydrogen peroxide concentration. This accounts for about 4% of the

total peracetic açid consumption in accordance with the stoichiometry of Reaction (2). Since

hydrogen peroxide is very stable at the present conditions, the increase in the hydrogen

peroxide concentration can be used to quanti@ the extent of peracetic acid hydrolysis

(Reaction (2)). Thus it can be concluded that the peracetic acid hydrolysis is negligible in the

pH range of 5.5 to 8.2 since the hydrogen peroxide concentration remained almost constant.

At pH 9.0 and 40°C, about 15% of the peracetic acid is consumed after 2 hours due to

hydrolysis. However, with the hydrolysis kinetics developed in next chapter the disappearance

of peracetic acid cm be corrected for the hydrolysis so that the spontaneous decomposition

rate of peracetic acid can be determined over the pH range of 5.5 to 9.0 under othenvise

commercial peracetic acid bleaching condition.

RESULTS AND DISCUSSION

Using the approach described above, the obsewed second-order rate constant k, was

obtained fiom the dope of straight line plots of the reciprocal of the total peracetic acid

concentration [CH,CO,H], , versus time. As s h o w in Table 4- 1, k,, increases with pH up

to the pK, of the peracetic acid of 8.2 183. This is in agreement with Koubek's results in

carbonate and phosphate buffers. As the pH increases beyond 8.2, Table 4- 1 shows that k,

is decreased. As discussed above, k, at pHs of 8.5 and 9.0 in Table 4-1 is corrected for

peracet ic acid hydrolysis.

The second-order kinetics in total peracetic acid concentration ( [CH,CO,H], ) for

spontaneous decomposition in the pH range of 5.5 to 9.0 can be written as :

where the total peracetic acid concentration is the sum of both undissociated and dissociated

peracetic acid, i.e. :

[CH,CO,H], = [CH3C03H] + [CH,CO,']

Table 4-1. The Observed Second-order &te Constant as a Function of pH at 40°C

The dissociation of peracetic acid is controlled by the equilibrium:

CH&O,H + CH,CO; + H '

with K,=[CH,CO;J w+]/[CH,CO,H].

Rearrangement of the equilibrium constant yieIds:

[CH,CO,HJ/[CH,COJ = CH+]& = M (8)

where M is a constant at fixed pH and temperature. By combining Equations (6) and (8) one

obtains:

[CH,COJ = [CH,CO,H](( 1 +M)

and [CH,CO,H] = M [CH,CO,H] J( I +M)

Based on Koubek's mechanism [8] it can be assumed that the nucleophilic attack of

a peracetic acid anion on a peracetic acid rnolecule is the rate-determining step. Therefore,

the rate equation becomes:

In tems of disappearance rate of total peracetic acid, the rate equation is:

Substitution of Equations (9) and (1 0) into Equation (1 2) yields:

By comparing Equation (5) with Equation (13), it is obvious that k, is related to the rate

constant of spontaneous decomposition, k, as:

When the pH is 8.2, i.e. at the pKa of peracetic acid, the ratio of the proton concentration to

the dissociation constant, M, is equal 1. As a result, k, reaches its maximum, and Equation

(14) reduces to k, = 0.5k .

The rate constant of the spomaneous peracetic acid decomposition, k, can be obtained

from Equation (14) by plotting k, versus 2M/(1 + M ) ~ . This plot is shown in Figure 4-2 for

the data obtained at 40°C . As discussed earlier, since the peracetic acid hydrolysis becomes

important when the pH is higher than 8.2, the two data points at a pH of higher than 8.2 have

been corrected using the kinetics of the peracetic acid hydrolysis.

It is important to note that Equation (14) produces a theoretical description of the

effect of pH on the observed rate constant of spontaneous decomposition of peracetic acid,

k, . The good agreement between the experimental and predicted values of 16, as a function

of pH in Figure 4-3 provides strong support for the reaction mechanism. Koubek's data and

mathematicai model predictions are given in Figure 4-4. It is apparent fiom Figure 4-4 that

Koubek's model prediction of his own data is rather poor at pH less than 7 and higher than

9.5. Reanalysis of his data using Equation (14) also shows that there is a considerable

deviation between the experimental data and that predicted by Equation (14). Koubek [SI

attributed this deviation to the fact that the EDTA used in his experiments reduces the

transition metal catalysed reactions (Reactions (3) and (4)), but does not eliminate them

entirely. Therefore the cataiytic reactions would be expected to form a larger proportion of

the total reaction rate at pH less than 7 or larger than 9.5 where the rate of spontaneous

decomposition is relatively low.

Using the data analysis technique descnbed above, the rate constant, k, at 25, 35 and

60°C were obtained. They are Iisted in Table 4-2 together with the experimental data of k,

at pH of 8.2. A cornparison of k and k, confirms that k,, = 0Sk which is predicted by

Equation (1 4). Furthemore, compareci with a 16, of 2.7 x 1 O-3 Umo1.s at 25°C and pH of 8.2

obtained by Koubek [SI, one cm see that the presence of buffers in Koubek's study increases

the spontaneous decomposition rate by a factor of about 2.

The activation energy of the spontaneous decornposition can be obtained fiom the

Arrhenius plot s h o w in Figure 4-6. An activation energy of 22.5 kcaVmol is found. This

compares with an activation energy of 30.0 kcal/mol reported by Koubek [SI for the

spontaneous decomposition of peracetic acid in carbonate and phosphate buffers. However

this value can only be considered as an estimate since his experimental data was influenced

by the transition metal catalysed reactions.

Table 4-2. Rate Constant of the Spontaneous Decomposition

Therefore, the rate of spontaneous decomposition of peracetic acid can be written as:

as a Function of Temperature

where [CH3COjm is the total peracetic acid concentration; T is the temperature in degrees

Kelvin and M is the ratio of the proton concentration to the dissociation constant of peracetic

acid.

CONCLUSION

The stability of an aqueous solution of peracetic acid has been studied under typical

puIp bleaching conditions. The consumption of peracetic acid without the presence of pulp

occurs via three routes: i) spontaneous decornposition; ii) hydrolysis; iii) transition metal

catalyzed reactions. The addition of chelating agents, such as DTMPA, can effectively

eliminate the third reaction route. In the pH range of 5.5 to 8.2, the hydrolysis is negligible

and the peracetic acid consumption is mainly due to spontaneous decomposition, whiIe

h, at pH 8.2 Temperature ("C) l

k

between pH of 8.2 to 9.0, the peracetic acid consumption is due to both spontaneous

decomposition and hydrolysis.

The kinetics of the spontaneous decomposition of peracetic acid were fùrther studied.

It was found to be second-order with respect to the total peracetic acid concentration

([CH,C03w), and that the maximum rate occurs at a pH of 8.2, the pKa of peracetic acid.

Based on the earlier mechanism proposed by Koubek 181, we deveIoped a kinetic equation

for the spontaneous decomposition of peracetic acid as :

where M is the ratio of the proton concentration and the dissociation constant of peracetic

acid. The activation energy is 22.5 kcahol . The present kinetic equation gives a very good

description of the experimental results obtained in this study.

ACKNOWLEDGMENT

Financial support fiom NSERC in the fom of a Strategic Grant is greatly appreciated.

REFERENCES

Troughton, N. A.. F. Desprez and J. Devenyns, Peracids: The Pathway to High Brightness TCF Kraft Pulps, Proceedinçs of 1994 International Non-chlorine Bleaching Conf, Sponsored by Puip & Paper and Engineering TechnologyTransfer Inc. ArneIia Island, FL, March 6- 10, (1 994), sec. 1 0- 1

Koubeiq E., M. L. Haggett, C. J. Battaglia, K. M. Ibne-Rasa, H. Y. Pym, and J. O. Edwards, Kinetics and Mechanism of the Spontaneous Decomposition of Some Peroxoacids, Hydrogen Peroxide and r-butyl Hydroperoxide, J. Amer. Chem. Soc., 85,2263-2268 (1 963)

Swem, D. E., Organic Peroxides, VoL.1, John Wiley and Sons, New York, 1970, p.362

Bali, D.L. and J. O. Edwards, The Kinetics and Mechanism of the Decomposition of Caro's Acid, 1, J. Amer. Chem. Soc., 78, 1 125- 1 129 (1 956)

Francis, R.C., X. Zhang, P. M. Froass, and 0. Tamer, Aikali-and Metal-induced Decomposition of Peroxymonosulfate, Tappi J., 77(6): 133 (1 994)

Rucker, J. W. and D. M. Cates, 2,2' -bipyridine Catalysed Bleaching of Cotton Fibers with Peracetic Acid Part 1, Textile Res. J., 58(13): 148 (1 988)

Greenspan, F. P. and D. G. MackeUar, Analysis of Aliphatic Per Acids, Analy. Chem., 20, 1061- 1 O63 (1948)

Koubek, E., Ph.D thesis, The Kinetics and the Mechanism of the Decomposition of Aliphatic peroxyacids in Aqueous Solutions, Brown University , New York, (1 964)

Levenspiel, O., Chernical Reaction Engineering, znd Ed., John Wiley & Sons, New York, 1972

-Blank d B l a n k + 0.5 g/L DTMPA

-= t +Blank + 0.5 g/L DTPA

Time (min)

Figure 4-1. The effmt of stabilhers on the consumption of peracetic acid at 40°C, pH of 7.0

Figure 4-2. Plot of 16, versus 2M/(l+M)' at 40°C, pH = 5.5 -9.0

4.0 i Exnerimental Data -- - - - - . - - . - - - - - - - - - 4 r e d i c t e d from k,=2M W(l +M)~

4 . 5 I I I f t

Figure 4-3. Cornparison of the predicted and experimental relationship between bbS and pH at 40°C

Figure 4-4. Koubek's plot of hgkba versus pH at 2S°C, with IOa m o n EDTA [S]

Figure 4-5. Reandysis of Koubek's data [8] with the kinetic mode1 developed in this study at 25T, with 10J moVL EDTA

Figure 4-6. Arrhenius plot of the sponbneous decomposition of peracetic acid

CHAPTER 5

KINETICS OF PERACETIC ACID DECOMPOSITION

PART LI: ALKALINE HYDROLYSIS

ABSTRACT

The consumption of peracetic acid in an aqueous solution was studied. It is shown

that three reactions are responsible for the disappearance of peracetic acid, namely i)

spontaneous decomposition, ii) hydrolysis and iii) transition metal catalysed reactions. The

spontaneous decornposition reaches its maximum at pH 8.2, while both the hydrolysis and

metal ion catalysed reactions increase as the pH increases. At pH 10.5 or higher, the

hydrolysis becomes dominant when the rnetal ion catalysed reactions is minimized by the

addition of DTMPA. Therefore the kinetics of peracetic acid hydrolysis were determined at

the latter conditions. A theoretical kinetic equation is deveioped which gives an accurate

description of the development of peracetic acid and hydrogen peroxide as a result of the

hydrolysis reaction. Finally it was found that the addition of MgSO, at pH 9.5 leads to the

formation of a precipitate of Mg(OH), which reduces the rate of transition metal catalysed

reactions, but enhances the rate of hydrotysis of peracetic acid.

Keywords: kinetics, peracetic acid, hydrogen peroxide, decomposition, hydrolysis, transition

metal ions

INTRODUCTION

Peracids, such as peracetic acid and monoperoxysulfûric acid (Caro's acid), are

presently considered as alternatives for conventional chlorine containing chernicals in an effort

to develop more environmental friendly bleaching processes. A number of technical papen

have been published which deal with the application of peracetic acid, Caro's acid. or their

mixture in the delignification stage and final brightness stages [I-41. However, fundamental

knowledge of interest for the application of peracids, such as the stability of peracetic acid

under vanous bleaching conditions, is still incomplete. In chapter 4, we reported on the

kinetics of the spontaneous decomposition. in this chapter. the stability of peracetic acid as

a function of pH and the presence of chelants as well as its hydrolysis kinetics will be

discussed.

D'Ans and Mattner [6] reported that the hydrolysis of peracetic acid increases with

increasing pH and MgSO, addition, and becomes the dominant degradation route in a 0.9

moUL NaOH solution at 12°C when the transition metal catalysed reaction of peracetic acid

and hydrogen peroxide is suppressed. However. the kinetics of the alkaline hydrolysis were

not established. Koubek [7J included the effect of pH on the peracetic acid decomposition

in his doctoral snidy. Furthemore, he confirmed that the hydrolysis of peracetic acid is

enhanced at a higher alkaii concentration.

The e f i of pH on the stability of Caro's acid was studied by Bal1 and Edwards [8]

and recently by Francis et al. [9] . Both spontaneous and catalytic decomposition paths for

Caro's acid were observed. The spontaneous decomposition increases with increasing pH up

to 9.4, the pKa value for the peroxidic proton of Caro's acid, while the catalytic

decomposition is more pronounced at a higher pH.

A review ofthe Iiterature showed that the kinetics of alkaline hydrolysis of peracetic

acid are not yet established. Therefore, the objective was to develop the kinetics of peracetic

acid hydrolysis. Furthemore, the interaction between peracetic acid and hydrogen peroxide,

its dependence on pH and the effect of the presence of chelants, such as diethylenetriarnine

pentaacetic acid (DTPA) and diethylenetnaminepentamethylene phosphonic acid (DTMPA),

were studied.

EXPERIMENTAL

Equilibrium peracetic acid and ultra-pure grade NaOH were purchased fiom Aldrich

(Milwaukee, USA). The equilibrium peracetic acid consists of about 34% peracid, 5%

hydrogen peroxide, 40% acetic acid with the remainder being water (al1 in mass%). Ragent

grade H202, DTPA and ethanot fiom Fisher Scientific (Nepean, Ontario), and DTMPA from

Buckman (Memphis, USA) were used in the experiments without fiirther purification. All the

experiments were camed out in a 500 mL four-neck round-bottom flask immersed in a

constant temperature bath.

An aqueous solution containing al1 required chernicals except peracetic acid was

mixed and preheated to the desired temperature. The reaction was initiated by addition of

the concentrated peracetic acid sohtion to the reaction flask. The pH was controlled at a

constant value by an automatic titrator through the addition of I O rnol/L NaOH. Samples

were withdrawn at predetemined time intervals and analysed following the iodometric

method developed by Greenspan and Mackellar [IO]. Ail reported values are the result of at

least duplicate experiments.

RESULTS AND DISCUSSION

Establishment of Conditions for Determination of Hydrolysis Kinetics

Since it was reported [6,8,9,1 13 that trace amounts of transition metal ions, such as

cobalt, manganese, copper and iron, catalyse the decomposition of peroxy acids, the effect

of the addition of metd chelants such as DTPA and DTMPA on the stability of peracetic acid

waç first studied. The results at 40°C and pH 9.5 are presented in Figure 5- 1. Also included

is a control experiment without the addition of any chelants, and an experiment perFormed in

the presence of a large concentration of a radical scavenger, in this case ethanol.

It can be seen that the addition of DTPA, with or without the presence of ethanol,

leads to a more rapid and extensive loss of peracetic acid than that obtained in the control

experiment. However, the presence of 0.5 glI DTMPA results in a strong reduction in the rate

of disappearance of peracetic acid, while the hydrogen peroxide concentration now increases.

DTMPA is known to be a very good chelant, with metal binding properties better than DTPA

[14]. As a result, it is reasonable to assume that the disappearance of peracetic acid is only

due to Reactions (1) and (Z), respectively spontaneous decomposition and hydrolysis.

2 CH3C0,H -, 2 CH,CO,H + O, (1 )

CH3C03H + H20 -, CH,CO,H + H70, ( 2 )

Further evidence for this assurnption will be presented in the later part of this chapter. Also,

the increase in hydrogen peroxide concentration (by 27% after 3 hours) can only be explained

by hydrolysis (Reaction (2)).

As noted above, the peracetic acid consumption rate in Figure 5- 1 is higher with the

addition of DTPA than that in the control experiment. As explanation it is proposed that the

catalytic effect of transition metal ions can not entirely be eliminated by the presence of

DTPA. A similar explanation was given by Koubek [7]. In addition, it is known [15] that

DTPA is oxidized by peracetic acid in a reaction with a stoichiometry of 3 moles of peracetic

acid per mole of DTPA. This reaction then may account for the fact that with the addition of

0.5 g/L (equivalent to about 1 mmollL) DTPA the peracetic acid concentration is about 3 to

5 mmoVL lower than that in the control experiment.

Most likely, there are two types of transition metal ion catalysed reactions which lead

to the consumption of peracetic acid, in both the control experiment and the experiment with

the presence of DTPA:

In the fint reaction only peracetic acid is involved. As proposed by Koubek [7], the

Mn@), Co@), Fe@) and Cu(II) catalysed decomposition of peracetic acid proceeds through

a radical chah mechanism. The overall reaction is represented by Reaction (3).

M "+

2CH $0 3H - 2CH 3C02H + 0 2 (3)

Both peracetic acid and hydrogen peroxide are involved in the second reaction.

Several workers have reported that the reactions between peracids and hydrogen peroxide

are catalysed by transition metal ions [8,15- 181. Radical mechanisms have been proposed by

McDonough [15] and later by Evans and Upton [18] to account for the disappearance of

peracetic acid and hydrogen peroxide. The overall reaction [ 181 could be written as:

It was also reported [S. 181 that a direct, uncatalysed reaction between peroxy acids and

hydrogen peroxide (i.e. without transition metal ion catalysis) may take place as:

RCO; + HzOz -+ RCO; + H 2 0 + O2 ( 5 )

The mechanism proposed by Evans and Upton [18] is :

This mechanism is very similar to that of the spontaneous decomposition of peracetic acid [7].

However, Reaction (5) is much more difficult than the nucleophilic attack of a peroxy acid

anion on peroxy acid since hydrogen peroxide is a much poorer electrophile than peracetic

acid. It was fiirther tested the peracetic acid consumption due to Reaction (5) by increasing

the initial hydrogen peroxide concentration from 0.612 g/L to 2.612 g/L. In the presence of

DTMPA at 40°C for both pH 8.2 and 9.5 it was found that the consumption rate of peracetic

acid was not affected by this more than 4 fold increase in hydrogen peroxide concentration.

Therefore, it cm be concluded that Reaction (5) is negligible under these conditions.

As discussed, Reactions (3) and (4) may proceed through radical pathways.

Therefore, one would expect that the presence of radical scavengers, such as alcohol, wîll

suppress these reactions, and result in a lower disappearance rate of both peracetic acid and

hydrogen peroxide. This was tested by following the peracetic acid and hydrogen peroxide

concentrations in a 30% w/w ethano1 solution with 0.5 g/L DTPA present. Frorn the results

shown in Figure 5-1, it can be observed that the presence of 30% w/w ethano! increases the

stability of both peracetic acid and hydrogen peroxide. This fùrther confims that Reaction

(4) is partialiy responsible for the consumption of both peracetic acid and hydrogen peroxide

at the present conditions.

The addition of DTPA and DTMPA on the consumption rate of peracetic acid was

fùrther studied at a pH of 8.2 and 40°C. The results are presented in Figure 5-2. Again, with

0.5 g/L DTMPA present, the disappearance rate of peracetic acid is the slowest, and the

hydrogen peroxide concentration increases by 9% after 2 hours. The formation of less

hydrogen peroxide in this case compared to that at pH 9.5 (27% increase) can be explained

by the fact that the peracetic acid hydrolysis, Reaction (2), is more pronounced at a higher

pH, as will be dernonstrated later. Comparison of Figure 5-2 with Figure 5- 1 also shows that

with 0.5 g/L DTPA addition, both peracetic acid and hydrogen peroxide are more stable at

pH 8.2 than at pH 9.5. This can be explained by a decrease in both Reactions (3) and (4) with

decreasing pH, as is supported by the results of Bal1 and Edwards [8] that the metal ion

catalysed reaction between Caro's acid and hydrogen peroxide is more pronounced at a pH

of 9.26-1 1.65 than that at 6.04-8.37. However, it is also possible that the oxidation of DTPA

by peracetic acid at pH 8.2 is not as extensive as that at pH 9.5 [ 151. Comparison of the two

control experiments in Figures 5- 1 and 5-2 reveals that at pH 9.5 the initial consumption rate

of both hydrogen peroxide and peracetic acid are higher than those at pH 8.2. This again can

be explained by the fact that Reaction (4) is favoured at a higher pH. However, since

hydrogen peroxide is the limiting reactant in Reaction (4). The importance of this

decomposition route becomes smail as the hydrogen peroxide concentration approaches zero.

Therefore, the higher consumption rate of peracetic acid in the later phase of the reaction at

pH 8.2 than that at pH 9.5 for the two control experiments rnust be due to the fact that the

spontaneous decomposition is much faster at pH 8.2 than that at pH 9.5 [5,7].

We examined Reaction (4) again by increasing the hydroçen peroxide concentration

fiom 0.612 g/L to 2.6 12 g/L. The results obtained at pH 8.2 and 40°C are plotted in Figure

5-3 according to second-order reaction behaviour with respect to the peracetic acid

concentration. Only in the presence of 0.5 g/L DTMPA is a straight-line behaviour obtained,

confirming that the disappearance rate of peracetic acid at pH 8.2 is dominated by

spontaneous decomposition in Reaction (1) (see Chapter 4). It can also be seen that the

increase in hydrogen peroxide concentration fiom 0.6 12 g/L to 2.6 1 2 g/L in the presence of

0.5 g/L DTMPA does not affect the reaction rate. This is consistent with the elirnination of

the metal ion catalysed reactions (Reactions (3) and (4)) by the addition of DTMPq and the

earlier conclusion that the uncatalysed reaction between peracid and hydrogen peroxide

(Reaction (5)) is negligible. However, the increase in initial hydrogen peroxide concentration

from 0.612 g/L to 2.612 g/L leads to a substantial increase in the disappearance rate of

peracetic acid when DTMPA is replaced by DTPA. The behaviour can be explained by

insufficient suppression of the metal ion cataiysed reactions (Reactions (3) and (4)) by DTPA,

and that a higher hydrogen peroxide concentration favours Reaction (4). Once again Figure

5-3 codrms that the presence of ethanol results in a lower rate of disappearance of peracetic

acid by suppressing Reactions (3) and (4).

The effect of pH on the consumption of peracetic acid at 40°C in the presence of

DTMPA is show in Figure 5-4. Since the metal ion catalysed reactions (Reactions (3) and

(4)) are minimized, peracetic acid is consumed either by spontaneous decomposition

(Reaction (1)) or hydrolysis (Reaction (2)). With the increase in hydroçen peroxide

concentration resulting from Reaction (2). the contribution of each of reactions (1 ) and (2)

to the overall consumption of peracetic acid can be calculated. At pH 8.2, about 96% of

peracetic acid is consumed via Reaction (1) after 2 h; at pH 9.5, the peracetic acid

consumption due to Reaction (1) is about 80% after 2 h. At pH 10.5 and 12.0, Reaction (1)

is negligible, while the peracetic acid consumption is almost fully accounted for by the

increase in hydrogen peroxide concentration.

Kinetics of the Aikaline Hydrolysis of Peracetic Acid

After establishment of the appropriate reaction conditions described above, the

kinetics of peracetic acid hydrolysis at pH 10.5 to 12.0 in the presence of 0.5 g/L DTMPA

were determined. First it was confinned that the DTMPA concentration does not affect the

reaction rate, since the results obtained at a DTMPA concentration of 2 g/L were identical

to those at a DTMPA charge of 0.5 a. The integral method was used to analyse the kinetic

data at constant pH and temperature. It was found that the hydrolysis of peracetic acid is

first-order in total peracetic acid concentration, [CH,CO,H],, as:

- d [CYCO3HI

dt = k, ~CH,CO,Hl,

where the total peracetic acid concentration is the sum of concentrations of both

undissociated and dissociated peracetic acid, i. e.

[CH,CO,H], = [CH3C03H] + [CHJOJ

The dissociation of peracetic acid is controlled by the equilibrium:

CH3C0,H s CH3C0, + H *

with K,=[CH,CO,'] [H+]/[CH3C03H]

Rearrangement of the equilibrium constant yields:

with M being a constant at a fixed pH and temperature. By combining Equation (IO) with

(8), one obtains :

Since the mechanism of the hydrolysis o f the peracetic acid and peracetic acid ion may be

written as:

the overall rate o f hydrolysis is given by:

Substitution of Equations (1 1 ) and (12) into Equation (15) yields:

By comparing Equation (1 5) with Equation (7), it is obvious that k, is related to the rate

constants, k, and k, as:

Substitution of M = [H']/K, and [OH'] = Y, /[H+] into Equation (17), followed by

mathematicai rearrangement yields:

This equation can be used to obtain the rate constants k, and k, by plotting k0&, +[H'])/K,

versus KJ[H+]. The results at pHs of 10.5, 11.0, 1 1.5 and 12.0 and temperatures of 40, 50

and 60°C are shown in Figure 5-5. It will be assumed that the effect of temperature on K, is

negiigible over the narrow range of temperatures fi-om 40 to 60°C, while the dependence of

& on temperature was taken into account. It is evident that a straight line relationship exists

between k,,& +w])/K, and KJH']. The slope and the intercept of straight lines,

represent the rate constants k, and k, respectively, and are given in Table 5-1.

The activation energies of the hydrolysis (Reactions (1 3) and (14)) obtained fi-om the

corresponding Arrhenius plots of the data shown in Table 5-1 are 14.9 and 1 1.7 kcaVmol,

respect ively .

Table 5-1. b t e Constant of Peracetic Acid Hydrolysis as a Function of Temperature

Therefore. the hydrolysis rate of peracetic acid can be expressed as:

Prediction Obtained with the Hydrolysis and Decomposition Kinetics

k2 ( Wmo1.s )

7.38

14.5

Temperature ( OC )

40

50

We fûrther tested the above kinetic expression by comparing the predicted and

k, ( L/mol.s )

0.00934

0.0206

experimentally determined peracetic acid and hydrogen peroxide concentrations. The results

obtained at pH 9.5.40°C and 0.5 g/L DTMPA are shown in Figure 5-6. At these conditions,

peracetic acid is consumed either by spontaneous decomposition, whose kinetics were

reported earlier [5], or by hydrolysis. Therefore, the rate expressions used are:

Figure 5-6 shows that these kinetic equations adequately describe the development of both

peracetic acid and hydroçen peroxide during the course of the reaction.

It was proposed [2] that peracetic acid and hydroçen peroxide bleaching can be used

in a sequence without washing , the so called (PaP) sequence. Usually, a significant amount

of peracetic acid is present at the completion of the peracetic acid treatment. Thus, it is

expected that upon addition of caustic and hydrogen peroxide to continue the (PaP) sequence,

the residual peracetic acid wilI be hydrolysed to hydrogen peroxide. With Equation (1 9), the

time required to convert 95% of the residual peraceiic acid to hydrogen peroxide in the (PaP)

stage can be predicted. As an exarnple, the following conditions were taken: 1.5% peracetic

acid charge, 12% pulp consistency and 33% of residual peracetic acid, and an initial pH of

1 1 .O &er addition of caustic. The results are show in Table 5-2. It can be concluded that at

80-90°C the hydrolysis of peracetic acid to hydrogen peroxide is very fast.

Table 5-2. The Required Time to Convert 95% of Residual Peracetic Acid to Hydrogen f eroxide

The Effect of MgSO, on the Consumption of Peracetic Acid

Magiesium sulfate is usually added to peroxide bleachinç to minimize the transition

metal catalysed decomposition of hydrogen peroxide [19]. In the present study the effect of

magnesium sulfate addition on the consumption of peracetic acid at 40°C and a pH of 5.0 to

9.5 in the absence of chelants was investigated. It was found that the addition of MgSO, up

90

3.2

Temperature ("C)

Time to hydrolyse 95% of the residuai ~eracetic acid (min)

70

23.4

60

67.4

80

8.4

to concentrations as high as 6 g/L has no effect on the peracetic acid consumption at pH 8.2

or lower. However, at pH 9.5, the peracetic acid consumption does change with the addition

of more than 2.0 p/L MgSO,. althouçh a MgSO, concentration of Iess than 0.5 g/L has a

negligible effect as s h o w in Figure 5-7. It is proposed that at pH 9.5 and a MgSO,

concentration of less than 0.5 g/L, the consumptions of both peracetic acid and hydrogen

peroxide are mainly due to transition metal catalysed reactions. However, the dominant

mechanism changes as the MgSO, concentration increases. Thus, at 2 g/L MgSO, both the

hydrolysis and the transition metal catalysed reactions are important, while at 6 g L MgSO,

the hydrolysis is dominant. as supported by the fact that the peracetic acid disappearance in

the latter case is accompanied by the formation of an equivalent amount of hydrogen

peroxide. A further increase in the MgSO, concentration to 9 g/L enhances the hydrolysis

of peracetic acid.

The following two mechanisms have been proposed to account for the observed

stabilisation effect of MgSO, on an alkaline hydrogen peroxide solution: (a) transition metals

are absorbed and deactivated by the magnesium hydroxide precipitate [20,21]; (b)

stabilization of superoxide anion radicals by Mg'. thereby reducinç the propagation of the

fiee radical chah decomposition reaction [22,23]. The same two mechanisms were considered

in the present study to explain the effect of MgSO, on the peracetic acid consumption. Table

5-3 lists the concentration of solid maçnesium hydroxide upon MgSO, addition as calculated

from the sdubility product of Mg(OH), [24,25] at 40°C. It appears that at pH 8.2 or lower,

no Mg(OH)? precipitate cm be formed if the MgSO, concentration is less than 94 g/L, which

explains why at pH 8.2 or lower, the addition of MgSO, has no effect on the peracetic acid

consumption. However, at pH 9.5 the arnount of precipitated Mg(OH)Z increases rapidly with

increasinç MçSO, addition. The close correlation between the precipate formation of

Mg(OH), and the peracetic acid consumption mechanism provides strong support that the

stabilizing effect of MçSO, is the absorption and deactivation of transition rnetals by a

precipitate of Mg(O& . The above results also explain that at practical bleaching conditions

performed in the neutrd pH region the addition of MgSO, will not improve peracetic acid

bleaching .

Table 5-3. The Composition of Magnesium Species in Aqueous Solution at 40°C Various pH and MgSO, Addition

CONCLUSION

The stability of equilibrium peracetic acid containing a significant amount of hydrogen

peroxide was studied at different pH levels. At pH 8.2, both peracetic acid and hydrogen

peroxide are rapidly consumed in three reactions, namely spontaneous decomposition,

hydrolysis and metal ion catalysed reactions. Addition of chelants, such as DTMPA, DTPA,

suppresses the metal ion catalysed reactions and thus improves the stability of both peracetic

acid and hydrogen peroxide. When the pH is higher than 8.2, the spontaneous decomposition

decreases while both the hydrolysis and the metal ion catalysed reactions increase. With

DTMPA present at pH 10.5 or higher, the spontaneous decomposition and metal ion

catalysed reactions are negligible compared to hydrolysis. The kinetics of peracetic acid

hydrolysis were developed at the latter conditions. I t was found that the hydrolysis follows

a first-order behaviour with respect to the total peracetic acid concentration [CH,CO,H],. The

rate expression is given by Equation (1 9).

It was show that the kinetic equation deveioped in this study for the hydrolysis can

very well predict the development of peracetic acid and hydrogen peroxide during the course

of the reaction when the metal ion catalysed reactions are minimized and the kinetics of

spontaneous decomposition are taken into account.

Finally it was found that the MgSO, addition at pH 9.5 leads to formation of a

precipitate of Mg(Om which reduces the rate of transition metal catalysed reactions but

enhances that of the hydrolysis of peracetic acid. With 6 g/L MgSO, addition at pH 9.5, the

hydrolysis is mainly responsible for the consumption of peracetic acid.

ACKNOWLEDGMENT

Financial support fiom NSERC in the fonn of a Strategïc Grant is greatly appreciated.

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Allison, R. W., Peroxide Bleaching of Mechanical Pulp fiom Pinus Radiata, Appita, 6(5), 362-370 (1 983)

McDonough, T. J. M., Ph.D Thesis, Peracetic Acid Decomposition and Oxidation of Lignin Mode1 Phenols in Alkaline Solutions, University of Toronto, Toronto, Canada (1 972)

Goodman, J. F. and P. Robson, Decomposition cf Inorganic Peroxyacids in Aqueous Alkali, J. Chem. Soc., 278 1-2875 (1 963)

Akiba, K. and 0. Simamura, Decomposition of Sodium Peroxybenzoate in Sodium Hydroxide Solution, Tetrahedron, 26,25 19-253 1 ( 1 970)

Evans, D. F. and M. W. Upton, Studies on Singlet Oxygen in Aqueous Solution, Part 3, J. Chem. Soc., Dalton Trans., 1 15 1 - 1 153 (1 985)

Rapson, W. H., M. Wayrnan and C. B. Anderson, Hydrosulphite and Peroxide Bleaching of Nine Pure Species Groundwoods, Tappi J., 48(2): 1 13 (1965)

Sjostrom, E. and 0. Valttila, Inhibition of Carbohydrate Degradation during Oxygen Bleaching, Papen ja Puu, 54(11):695 (1 972)

Ericsson, B. B., O. Lindgren and 0. Theander, Factors lnfluencing the Carbohydrate Degradation under Oxygen Aiakiine Bleaching, Svensk Papperstidn, 74:75 7 ( 1 97 1 )

IsBell, H. S. and H. L. Frush,, Carbohydr. Res. 59:C25 (1977)

Colodette, J. L., S. Rothenberg, and C. W. Dence, Factors Mecting Hydrogen Peroxide Stability in the Brightening of Mechanical and Chemimechanical Pulps, Part III: Hydrogen Peroxide Stability in the Presence of Magnesium and Combination of Stabilizers, JPPS, 15(2):545 ( 1989)

anon, CRC Handbook of Chernistry and Physics, 72 nd Edition, C RC Press Inc., Ann Arbor, USA, p.8-43 (1 992)

Stephen, H. and T. Stephen, Solubilities of Inorganic and Orsanic Compounds, Oxford Pergamon Press, New York, Macmillian, No.583,1963

O 20 40 60 80 100 120 140

Time (min)

Figure 5-1. Addition of DTPA or DTMPA on the peracetic acid decomposition at pH 9.5 and 40°C

A -P CH,CO,H. Control -0- H202. Control

2 -I- CH,CO,H. 0.5 glL DTPA -0- H,O,. 0.5 g/L DTPA O

-0- CH,CO,H. O S glL DTMPA * H,0,. 0.5 g1L DTMPA

0.00 1 I I I

O 20 40 60 1 I

80 100 120

Time (min)

Figure 52. Addition of DTPA or DTMPA on the peracetic acid decomposition at pH 8.2 and 40°C

h - -6 0.5 glL DTPA. 2.612 g/L H,02 O 70 - -U- 0.5 glî DTPA, 2.6t 2 gii H202, 10% w/w EiOH

-31t- 0.5 giL DTPA, 2-61 2 giL H,O,, 30% wlw ELOH

w -1- 0.5 g/L DTPA, 0.61 2 giL H,O, 60- -0-0.5 g lL DTMPA, 0.612 g1L H,O,

9 - - v - O . S ~ K D T M P A . ~ . ~ ~ ~ ~ ~ H , O ,

O 50-

O 40- e

O 1 O 20 30 40 50 60

Time (min)

Figure 5-3. Effect of H,O, concentration on the disappearance rate of peracetic acid at pH 8.2 and 40°C

Figure 5-5. Determination of the rate constants of the alkaline hydroIysis of peracetic acid (40-60°C, 10.5-12.0 pH, 0.5 g/L DTMPA)

Predicted [CH,CO,H] or [H,O,] m Determined [CH,CO,H] - o Deîermined [H,O,]

7

5 û ô O 7 0 8 û

Time (min)

Figure S. Cornpanson of determined and predicted peracetic acid and hydrogen peroxide concentrations (pH 9.5, 40°C, 0.5 g/L DTMPA)

O I O 20 30 40 50 70

Time (min)

Figure 5-7. Effect of MgSO, on the consumption of peracetic acid at 40°C and pH of 9.5

CHAPTER 6

TRANSITION METAL ION CATALYSED DECOMPOSITION OF PERACETIC

ACXD AND ITS EFFECT ON THE CHEMICAL PULP BLEACHING

ABSTRACT

Peracetic acid (Pa) has emerged as a potential alternative for chlorine containing

chemicals used for bleaching of chemical pulps. WhiIe transition metal ions have received

much attention dunng alkaline peroxide bleaching, limited results are available with respect

to their consequences in peracetic acid treatment. In this chapter, we further exarnined the

role of transition rnetal ions such as manganese, iron and copper during the course of

peracetic acid bleaching. Oxygen delignified and ozone delignified softwood kraft pulps were

used. The fûndamental phenornena associated with the presence of additives such as DTPA,

DTMPA, Na,P,O,, and MgSO, on peracetic acid decomposition and bleaching were also

investigated.

The reslilts indicated that the negative effect of some transition metal ions can be

rninirnized by proper control of the process parameters. As expected, a close relationship

between peracetic acid decomposition and pulp brightness development was observed and

codmed at constant bleaching conditions . A chelation pretreatment stage for pulp with high

metal ion contents, especially manganese, leads to an improved subsequent peracetic acid

bleaching. The addition of sodium pyrophosphate effectively deactivates the metal ions and

consequently improves the bleaching performance. However, the addition of some chelants,

such as DTPA in the peracetic acid treatment may result in poor bleaching due to the fact

that more peracetic acid is wasted in side reactions.

Keywords: peracetic acid bleaching, transition metal ions, decomposition, chelation

INTRODUCTION

Peroxy acids, such as peracetic acid (Pa) and rnonoperoxysulphuric acid (Px), have

recently been identified as promising alternatives to chlorine containing chemicals for

bleaching of chernical puIps [ 1-9 1. The impact of transition metai ions on alkaline peroxide

bleaching has been addressed extensively [ IJ O- 163. It is believed 11 I,14- 163 that these metal

ions catalytically decompose peroxide, leadins to Iess effjcient bleaching, while the radical

species generated in the decomposition reactions result in more carbohydrate degradation.

Management of transition metals is therefore crucial in hydrogen peroxide bleaching [ 1 0- 1 61.

On the other hand, studies on the influence of metai ions on peracetic acid decomposition are

rather scarce. Koubek [ I 71 studied the metal ion catalysed decomposition of peracetic acid

at pH 5.4 in the absence of pulp fibres, and found that it obeyed first-order kinetics in

peracetic acid concentration, and the order of decreasing catalytic activity is Co', Mn2+, Fe2+,

and Cu2+. Vasyutyn [18] reported that in an aqueous solution of peracetic acid without pH

control Mn2+ is the most hamiful metal ion followed by Co2+ and Fe)', and confimed that the

kinetics are first order with respect to the peracetic acid concentration. On the contrary,

Allen and Agulio [19] reported second-order kinetics in peracetic acid concentration when

he studied the manganese catalysed decomposition of peracetic acid in an acetic acid

solution. The above authors did not extend their study to neutral pH, the range which is of

interest for peracetic acid bleachin~ of pulps.

Our project is airning to gain fiindamental understanding of peracetic acid bleaching,

including peracetic atid decomposition reactions. As discussed in Chapters 4 and 5, peracetic

acid may be consumed in i) the spontaneous decomposition, ii) hydrolysis, and iii) the

transition metal ion catalysed decomposition [17,20-211. The mechanism and kinetics of the

spontaneous decomposition were studied before [17]. In Chapter 4, we established the rate

expression for the spontaneous decomposition under the typical peracetic acid bleaching

conditions. In Chapter 5, we reported the kinetics for the hydroiysis and the pH effect on the

peracetic acid decomposition mictions. In this chapter, we investigated the metaf ion induced

peracetic acid decomposition at a pH range of 5.0 to 7.0. The effects of additives such as

DTPq DTMPA, Na,P,O,, and MgSO, on the metal ion induced peracetic acid

decornposition were studied by adding known arnounts of transition metal ions to the

equilibrium peracetic acid. The underlying mechanisms were proposed, and the results were

related to peracetic acid bleaching of both oxygen delignified and ozone delignified softwood

kraft pulp.

EXPERIMENTAL

Equilibnum peracetic acid and ultra-pure grade NaOH were purchased fiom Aldrich

(Milwaukee, USA). The equiIibrium peracetic acid consists of about 34% w/w peracetic acid,

5% W/W hydrogen peroxide and 40% wlw acetic acid with the remainder being water.

Reagent grade chernicals, DTPA from Fisher Scientific (Nepean, Ontario) and DTMPA fiom

Buckman (Memphis, USA) were used in the experiments without fùrther purification. The

following sdts were used in the experiments: FeCI, CuS0,- MnSO, and CoSO,. Distilled and

deionized water was used in a11 experiments. Its metal ion content was below the detection

limits of the atomic absorption (AA) spectroscopic analysis method used in this study.

An oxygen delignified softwood kraft pulp of 37.7% ISO brightness, 13.8 kappa

number and 23.4 mPa-s viscosity was used in this study. Ozone bleaching (Zn) was camed

out on a hemlock kraft pulp with 70% methanol-water of pH 2 as impregnation liquor

following an earIier describeci procedure [22]. The ozone charge was 1.8% (by weiçht on 0.d.

pulp) of which 90% was consumeci. A subsequent alkali extraction stage (E) was perfonned

at pH 11.0,77"C, 10% pulp consistency for 45 minutes. The properties of the ZnE pulp were

53.8% ISO brighmess, 5.1 kappa number and 22.8 mPa-s viscosity. Chelation (Q) with 0.5%

DTPA was done at 50°C, pH 5 .O, 2.5% pulp consistency for 30 min. The metal ion contents

of the pulps and chemicals determined by AAS analysis are shown in Table 6- 1.

Table 6-1. Meta1 Ion Contents of the Materials Used in This Study

ZnE Pulp

ZnEQ Pulp 1 0.40 -- -

O Pulp 1 61.5

OQ Pulp 1 2.95

40% DTPA 1 0.80 - - -

80 g/L DTMPA 1 0.10

Equilibrium Peracetic Acid 1 <0.01

Peracetic acid decomposition experiments were camed out in a 500 mL four-neck,

round-bottom flask immersed in a constant temperature bath. The aqueous solution

containing al1 the required chemicals except peracetic acid was preheated to the desired

reaction temperature. The reaction was initiated by addition of a concentrated peracetic acid

solution to the reaction flask. The pH was kept constant with an automatic titrator by addition

of 10 moVL NaOH. Samples were withdrawn at predetermined times and analysed following

the iodometric method developed by Greenspan and Mackellar [23]

Peracetic acid bleaching was canied out in polyethylene baçs containing the required

quantity of NaOH and water to provide the desired initial pH and consistency. M e r the Pa

treatment, the pulp was washed thoroughly with deionized water and the spent liquor was

titrated to determine its residual peracetic acid and hydrogen peroxide concentration as well

as final pH. The bleaching expenments at constant peracetic acid concentration were

conducted in a 500 mL four-neck, weii stirred, round-bottom flask at 1.5 % pulp consistency.

Preliminary bleaching runs were carried out to determine the rate of peracetic acid

consumption. Subsequentiy, the desired amount of peracetic acid was charged to the reaction

mixture at regular time intervals to maintain a constant peracetic acid concentration.

Pulp brightness, viscosity and kappa number were determined in accordance with

TAPPI methods T452 om-92, T230 om-89, and T236 cm-85, respectively.

RESULTS AND DISCUSSION

1.The Transition Meta1 Catalysed Decomposition of Peracetic Acid

We first studied the systern in the absence of pulp fibres. The effect of 0.75 ppm each

of Cu". Fe3+ and Md' additions on the stability of a peracetic acid solution at pH 7.0 is

presented in Figure 6-1. Cobalt was not included due to its negligible amount present in the

chemical pulps as shown in Table 6- 1. Obviousiy, the presence of these metal ions enhances

the peracetic acid decomposition, i.e. are responsible for the so called metal ion induced

/catalysed decomposition [ 171. Note that not only peracetic acid but also hydroçen peroxide

consumption rates are much higher when metal ions are added.

A number of reactions may lead to the consumption of peracetic acid in the presence

of metal ions:

Only peracetic acid is involved. As proposed by Koubek [17], the Mn", CoZ+, Fe2'

and Cu2+ catalysed decomposition of peracetic acid proceeds through a radical chah

mechanism. The overall reaction can be writen in Equation (1):

Both peracetic acid and hydrogen peroxide are involved. McDonough [20) and

Evans and Upton [24] studied the transition metal catalysed reaction of peracetic acid and

hydrogen peroxide, and proposed that the reaction proceeds through a radical mechanism in

which the perhydroxyl radicals are involved. The overall reaction [24] is:

CH3C03H + H202 -' CH3C02H + H,O + 0,l (2)

Our data suggests that redox reactions also take place. For example, we found that

permanganate is generated and then consumed during the course of the reaction when 0.75

ppm Mn2+ (as MnSO,) was added. MnO,' is produced fiorn the oxidation of ~ n " by

peracetic acid as described in Reaction (3). This is supported by the observation that

peracetic acid may oxidize Mn2+ to MnO; [23,25]. Permanganate, in tum, reacts with

hydrogen peroxide and is reduced to Mn2+ (Reaction (4)). This oxidation-reduction cycle

explains why only a small amount of manganese is effective in enhancing the disappearance

of both peracetic acid and hydrogen peroxide. The overall reaction shows that peracetic

acid reacts with hydrogen peroxide to form oxygen:

5 CH,CO,H + 2 ~ n " + 3H,O - 2 MnOi + 6H' + 5 CH3CO2H (3)

2 MnO, + 6H' + 5 H202 - 5 O1 1 + 2 ~ n ' + + 8H20 (4)

CH,C03H + 40- -' CH,C02H + H20 + O2 T (2)

We fùrther confirmed the above oxidation-reduction cycle by the addition of MnO,

instead of Mn2+ to the equilibrium peracetic acid solution, and found that the rates of the

peracetic acid and hydrogen peroxide consumption are similar to those with the addition of

~ n " . It should be pointed out that in the absence of trace amount of these metal ions,

Reaction (2) is extremely slow under the condition studied.

2. The Effect of Additives on the Metal Catalysed Decomposition of Peracetic Acid

Chelants such as DTPA and DTMPA are known stabilisers that rninimize the metal

ion catalysed peroxide decornposition dunng peroxide bleaching [26-291. Magnesium sulfate

is usually added to oxygen delignification and peroxide bleaching to rninimize carbohydrate

degradation andlor peroxide decomposition [30]. Tetrasodium pyrophosphate (Na,P20,) was

also reporteci as a good stabiliser for alkaline peroxide solution [3 11 and peracetic acid [2 Il.

Therefore, in this study we investigated whether the presence of additives, including DTPA,

DTMPA, MgSO, and Na,P20, can rninimize the metal ion induced peracetic acid

decomposition. The results at pH 7.0 and 40°C are presented in Figures 6-2, 6-3 and 6-4 for

the addition of 0.75 ppm each of copper, iron and rnanganese respectively.

We can observe in al1 three cases that the addition of tetrasodium pyrophosphate

stabilises the peracetic acid solution. We fbrther confirmed that the rates of peracetic acid and

hydrogen peroxide decompositions in the presence of Na,PIO, can be adequately predicted

with the kinetics of the spontaneous decomposition developed in Chapter 4, supporting that

the metal ion induced decomposition is neçligible with the addition of Na,P20,. As expected,

the addition of magnesiurn sulfate does not affect the rate of oxidant consurnptions under this

condition, which is in agreement with our finding in Chapter 5.

2.1 The effect of DTPA and DTMPA on FeY and Cu" catalysed decomposition of

peracetic acid

Figures 6-2 and 6-3 show that the addition of DTPA or DTMPA can almost eliminate

the copper and femc induced peracetic acid decomposition as indicated by the similar profiles

of the peracetic acid and hydrogen peroxide concentrations to those when Na,P,O, was

present. We propose that both copper and femc ions can be strongly chelated by DTPA as

indicated by the conditional stability constants of metal ion to DTPA complexes (Table 6-2).

DTMPA is even stronger cheiant than DTPA [27]. The DTPA or DTMPA complexed copper

or femc ion is largely unreactive towards metal ion induced decomposition reactions. This

accounts for the fact that the catalytic effect of both Cu2' and Fe3' on the peracetic acid

decornposition is eliminated with the addition of either DTPA or DTMPA.

Table 6-2. Conditional Stability Constants of Metal Ion to DTPA Complexes at pH 7.0 and 20°C [32]

Metal Ion 1

In an earlier study [20] on bleaching of groundwood pulp with peracetic acid,

McDonough found that the presence of DTPA in the bleaching stage drastically reduced the

brightness gain, which was explained by the oxidation of DTPA by peracetic acid to its

corresponding N-oxide. He found that the oxidation occurs rapidly in a pH range of 6- 10 and

established that the stoichiornetry is 3 moles peracetic acid per mole of DTPA. Furthemore,

it was found [ZO] that in spite of its susceptibility to rapid oxidation, DTPA can still stabilise

a peracetic acid solution. The metal ion content in the solutions was not quantitatively

deterrnined. In Figures 6-2 and 6-3, 0.5 g/L of DTPA (1 mmoVL) was present. If the sarne

stoichiometry holds, the peracetic acid concentration decrease due to oxidation of the DTPA

should be about 3 rnrnol/L. This rnay explain the difference in peracetic acid concentration

between the addition of DTPA and that of Na&û, in Figures 6-2 and 6-3. The effectiveness

of DTPA in stabilizing peracetic acid solutions, despite its susceptibility to oxidation, rnight

be explained by the assumption that chelation occurs much faster than oxidation, so that a

small amount of metal ions present in the reaction solution may be immediately chelated upon

the addition of DTPA [20]. Altematively, it might be that the oxidized DTPA still exhibits

some chelating properties and consequently elirninates the Cu2+ or ~ e ) + induced peracetic acid

decomposition.

2.2 The effect of DTPA and DTMPA on Mn2+ catalysed peracetic acid decomposition

The addition of DTPA or DTMPA to an equilibrium peracetic acid solution of pH 7.0

containing 0.75 ppm Mn" has several consequences as can be seen in Figure 6-4. The

peracetic acid decomposition is enhanced with the addition of either DTPA or DTMPA,

although effect is much greater with the former. It can also be noted that a sharp decrease in

peracetic acid occurs when the reaction time exceeds 20 minutes. Similarly, it was reported

that the addition of DTPA enhances the Fe3+ cataIysed decomposition of hydrogen peroxide

[33]. On the other hand, the addition of DTMPA stabilises hydrogen peroxide, while the

addition of DTPA lads to a much higher consumption rate of hydrogen peroxide relative to

that of the control.

Table 6-2 shows that the DTPA-MnZ' cornpiex is much less stable than the complexes

of DTPA with either ~ e " or Cu", indicating that ~n'+ is not as strongly chelated.

Consequently, the experimental result that the addition of DTPA does not stabilise the Mn2+-

containing peracetic acid solution (Figure 6-4) could be explained by the hypothesis that the

weaker DTPA-h4n2+ complex is capable to catalyse peracetic acid decornposition.

The addition of DTPA on the manganese catalysed decomposition of peracetic acid

was fûrther studied by adding peracetic acid and hydrogen peroxide to the reaction solution

d e r 42 min of reaction so that their concentrations were the same as those at the start of the

reaction. The results in Figure 6-5 show that the disappearance profiles of both peracetic acid

and hydrogen peroxide dunng the second cycle closely resembles those of the first cycle.

Figures 6-6 and 6-7 show the addition of DTPA on the manganese catalysed

decomposition of peracetic acid at pHs of 5.0 and 6.0. It shows that a lower pH leads to a

higher decomposition rate of peracetic acid in the presence of DTPA and ~ n * ' .

Included in Figures 6-4,6-6 and 6-7 is also the effect of DTMPA on the manganese

catalysed consumption of peracetic acid. The addition of 0.5 @ DTMPA increases the rate

of peracetic acid decomposition at pHs 6.0 and 7.0 (Figures 6-7 and 6-4). but has a negligible

effect at pH 5.0 (Figure 6-6). Hydrogen peroxide. on the other hand, is stabilised by the

addition of D W A in al1 three cases. Therefore it is proposed that Mn" is oxidized to MnO;

by peracetic acid (Reaction (3)) at pHs 6 and 7 but not at pH 5. Furthemore, hydrogen

peroxide may not be involved in the oxidation and reduction cycle since the oxidation of

DTMPA by MnO,' could be much faster than Reaction (4).

3. The Role of Transition Metal Ions d u h g Peracetic Acid Bleaching of Chernical

PuIps

The following section describes the results of pulp bleaching with peracetic acid. First

we performed peracetic acid bleaching of oxygen delignified pulp with and without DTPA

pretreatment. The results in Table 6-3 show that a higher degree of delignification is achieved

both in terrns of brightness gain and kappa number reduction when the oxygen delignified

pulp is pretreated with DTPA. By adding manganese (as MnSOJ at a charge corresponding

to the arnount of manganese removed in the chelation stage, we found that peracetic acid

bleaching of the chelated pulp showed a delignification and brightness gain comparable to

those obtained for peracetic acid bleaching of the unchelated pulp. We therefore conclude

that manganese present in the unchelated O pulp is mainly responsible for the enhanced

peracetic acid decomposition, and thus causes the less efficient bleaching.

We also studied whether the addition of chelants / additives such as DTPA, DTMPA,

MgSO, or Na,P,O, to the bleaching stage, instead of a pretreatment stage, can effectively

deactivate the metal ions present in pulp fibres (Table 6-3). Cornparison of these results with

those obtained by peracetic acid bleaching of unchelated pulp indicates that the presence of

either DTPA or DTMPA leads to a iower brightness gain and a smaller kappa number

reduction, while the presence of tetrasodium pyrophosphate improves the bleaching

Table 6-3. Peracetic Acid Bleaching of Oxygen Delignified

Sarnple and condition

Unchelated O P U ~ P

Chelated O P U ~ P

Unchelated O pulp, 1.8% Na4P207

added

Unchelated O pulp, 0.3%

DTPA added

--

Unchelated O pulp, 0.3% DTMPA

added

Unchelated O pulp, 0.3%

MgSO.4 added

Softwood Kraft Pulp (O Pulp)

Treatment time (min)

Bright. (%ISO)

3leaching Conditions: 10% consistency, 60°C, lnitial pH of 7.0, 1.5% peracetic acid

Viscosity (mPa-s)

23.4

21.8

23.4

23.0

:harge on 0.d. pulp

47.9 90.2 3 8.5 85.2 26.0 83.7

22.6 9.6 77.2

3.2 5.8 2.9 4.3 2.1 2. I

20.2 O. 1 1.4

8.7 87.3 5.5 83.1 3.7 80.2

22.3 0.2 73.9

21.7 4.8

Residual Chernicals (% on charge)

CH3C03H

- 18.7 10.4 6.0 2.0

- 59.6 5 1.4 35.4 12.3

H20, -

9.1 8.7 7.2 5.1

- 92.5 86.8 83.6 79.8

performance, although not to the level obtained with the chelated pulp. This is in agreement

with the results obtained by McDonough [ZO], who observed that the addition of DTPA

during peracetic acid bleachins of goundwood pulp resulted in a smaller b right ness gain. This

may be attributed to the loss in peracetic acid as a result of oxidation by DTPA, ancilor the

enhanced oxidation-reduction cycle observed in the decomposition studies, leading to the

wastefid consumption of peracetic acid without any bleaching effect.

Table 6-3 shows that the residual peracetic acid and hydrogen peroxide concentrations

are lower with the addition of DTPA. As shown in Table 6-1, the oxygen delignified

softwood kraft pulp contains 6 1.5 ppm manganese which corresponds to 6.8 ppm Mn in the

bleaching liquor. Therefore, the manganese induced decomposition of both peracetic acid and

hydrogen peroxide must occur, as was discussed in the previous section. When DTMPA was

added to the peracetic acid bleaching system, the residual peracetic acid concentration was

lower than that of the control experirnent, while the residual hydrogen peroxide concentration

was much higher. These results are similar to those in Figures 6-4 and 6-7 where 0.75 ppm

manganese was added to a peracetic acid solution, suggesting that the same mechanism

prevails in the pulp bleaching systern: i.e. permanganate is fonned from the oxidation of Mn"

by peracetic acid, which then oxidizes DTMPA rather than hydrogen peroxide.

It appears that both the deligiification degree and brightness gain are closely related

to the residual peracetic acid afier bIeaching (Columns 3, 4 and 6). This is not unexpected

since poor bleaching results are obtained when more peracetic acid is consumed in wastefui

reactions. We fùrther tested this hypothesis by comparing the bleaching results obtained with

a constant peracetic acid concentration of 1.679/ L at 40°C and pH 7.0, using both chelated

and unchelated pulps at 1.5% pulp consistency. We found that the delignification degee and

brightness gain were similar. However, much more peracetic acid was required to maintain

the constant peracetic acid concentration for the unchelated pulp.

The decrease in pulp viscosity during peracetic acid bleaching is very small, as shown

in Table 6-3. This is consistent with the generat perception that peroxy acids are selective

bleaching chemicals when used under optimum operating conditions. It appears that the pulp

viscosity a h r Pa treatment is related to the residual hydrogen peroxide present; i.e. a Iower

pulp viscosity is accompanied by a smaller amount of residual hydrogen peroxide. This

suggests that the loss in pulp viscosity is mainly due to wastefùl reactions involving hydrogen

peroxide whereby highly reactive radical species are generated.

The addition of magnesium suKate has a negiigible effect on the delignification degree

and brightness gain. This is consistent with our earlier decomposition results which showed

that presence of magnesium sulfate does not affect the stability of a peracetic acid solution.

Furîhermore, Table 6-3 shows thar the cellulose degradation is not af5ected by the addition

of magnesium suIfàte. These results are not inconsistent with the protective effect of MgSO,

in &O, bleaching and CX_ delignification, since the latter two are performed at a high pH while

a neutrd pH is used for peracetic acid bleaching.

We iùrther studied the effect of different additives on peracetic acid bleaching with

a hemlock kraft pulp, which was first ozone-delignified (see experimental section for details)

followed by an alkaline extraction stage. The results are presented in Table 6-4. Comparison

of Table 6-3 and 6-4 shows that the difference in bleaching results with or without DTPA

pretreatment of the ZnE pulp is much smaller than that for the O pulp. This is attributed to

the fact that the transition metal ion content, especially manganese, in the ZnE pulp is much

smaller than that in the O pulp, as was show in Table 6-1. Again, Table 6-4 shows that the

addition of DTPA during peracetic acid bleaching of both the chelated and unchelated pulps

is not beneficial, as evidenced by a lower brightness gain and a higher viscosity loss. Again,

the addition of MgSO, during peracetic acid treatment has a negligible eFect on the bleaching

response.

The results in Table 6-4 were obtained with a peracetic acid charge of 2%. When only

0.5% peracetic acid was charged to the ZnE pulp, the difference with or without DTPA

pretreatrnent was even smaller. Thus it is possible that peracetic acid bieaching may be done

without a preceding chelation stage for pulps with a low transition metal ion content.

However, a chelation stage as a pretreatment is strongly recommended for peracetic acid

bIeaching of puIp with a high transition metai ion content, especially manganese.

Table 6-4. Peracetic Acid Bleaching of Ozone Deiignified Softwood Kraft Pulp

Sample and conditions

Unchelated ZnE pulp 1 Chelated ZnE pulp 1

Treat Bright. time (%ISO) (min)

-

Viscosity Residual Chernicals (m ~ a - s ) I (% on original)

Unchelated ZnE pulp 0.3% DTPA added in Pa

Unchelated ZnE pulp 0.3% MgSO, added in Pa

Chelated Z n . pulp 0.3% DTPA added in Pa

Bleaching Conditions: 10% consistency, 60°C, Initial pH of 7.0, 2% peracetic acid charge on 0.d. pulp

CONCLUSIONS

The transition metal ion induced decomposition of peracetic acid, and the effect of the

addition of chelantdadditives on the decomposition were studied under typical peracetic acid

pulp bleaching conditions. The results show that a significant amoünt of peracetic acid may

be wasted when metal ions are present, thus resultins in less efficient bleaching. The metal

ion induced decomposition may take place foliowing a radical mechanism. However, an

oxidation-reduction cycle involvinç ~ n ~ ' and MnO; is aiso present when rnanganese is

present in the peracetic acid solution.

It was found that the presence of chelants such as DTPA or DTMPA minimizes the

Cu2' and Fe3+ induced peracetic acid decomposition but increases the Mn" induced

decomposition. This is explained by assuming that the more stable complexes of DTPA or

DTMPA with Cu" and ~ e ) ' do not catalyse the decomposition reactions, while the less stable

DTPA-Mn" complex can, and by the fact that only manganese participates in an oxidation-

reduction cycle, while Cu" and Fe3' do not.

As expected, a close relationship between peracetic acid decomposition and pulp

brightness development was observed and confirmed at constant bleaching conditions. A

chelation pretreatment stage for pulps with a high metai ion content, especially manganese,

lads to improved peracetic acid bleaching. For pulps with a 1ow metal ion content, cheiation

pnor to Pa treatrnent may not be necessary. The addition of sodium pyrophosphate effectively

deactivates the metal ions and consequently improves the bleaching performance. However,

the addition of some chelants, such as DTPq in the peracetic acid bleaching may result in

poor bleaching due to the fact that more peracetic acid is wasted in side reactions.

ACKNOWLXDGMENT

The author thanks NSERC for financial support in the form of a Strategic Grant and

F. Munro of E. B. Eddy Forest Products LTD for supplying the oxygen delignified softwood

krafl puIp.

RIEFERENCES

Springer, E. L. and J. D. McSweeny, Treatment of Sofiwood Kraft Pulps with Peroxymonosulfate Prior to Oxygen Delignification, Tappi J., 76(8): 194(1993)

Springer, E. L., Delignification of Aspen Wood using Hydrogen Peroxide and Peroxyrnonosulfate ,Tappi J., 73( 1 ): 175 (1 990)

Hili, R T., P. B. Walsh and J. A. Hollie, Peracetic Acid, an Effective Alternative for Chlorine Compound Free Delignification of Kr& Pulp, TAPPI 1992 Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p. 12 19.

Bailey, C.W. and C.W. Dence, Peroxyacetic Acid Bleaching of Chemical Pulp, Tappi J., 49(1):9 (1966)

Devenyns, J., F. Desprez and N. Troughton, Peroxygen Prebleaching and Bleaching Technologies for Step-Wise Conversion corn Conventional Chlorine Bleaching via ECF toTCF, TAPPI 1993 Pulping Conkence Proceedings, TAPPI PRESS, Atlanta p. 341.

Devenyns, J., F. Desprez and N. Troughton, Peracetic Acid as a Selective Prebleaching Agent: an Effective Option for the Production of Fully Bleached TCF Kraft Pulps, Proceedings of 1996 International Non-chlorine Bleaching Conference, Sponsored by Pulp & Paper and Emerging Technology Transfer Inc., HHI, SC, March, 1993

Troughton, N., F. Desprez and J. Devenyns, Peracids: The Pathway to High Brightness TCF Kraft Pulps, Proceedings of 1994 International Pulp Bleaching Conf , Vancouver, June, 1994

Amini, B. and J. Webster, On-site Peracids: Tools for Bleaching Strategies to Meet the Cluster Rule, and Considerations on How to Select Among Them, 1994 TAPPI Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p. 37 1.

Liebergott, N., Peracid Delignification and BIeaching of Chemical Pulp Part II: Oxidation, Pulp & Paper Canada, 97(3):73 (1996)

Devenyns, J., F. Desprez, N. Troughton, and L. Plumet, MetaIs Control in TCF Bleaching - Implications for Kraft Bleaching Sequence Design, 1994 TAPPI Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p.3 8 1.

Chirat, C.and D. Lachenal, Benificid and Adverse Effect of Meta1 Ions in ZP Bleaching Sequences, 1994 TAPPI Pulping Conference Proceedings, TAPPI PRESS,

Atlanta, p. 1239

Lapierre, L., J. Bouchard, R. M. Berry, and B. van Lierop, Chelation Prior to Hydrogen Peroxide Bleaching of Kraft Pulps: an merview, Preprints, 8 1 st CPPA annuai meeting, Montreal, Jan., 1995, pl3233

Dick, R.H. and D.H. Andrews, The Bleaching of Groundwood Pulp with Peroxide, the Influence of Certain Metals on Bleach Response, Pulp and Paper Magazine of Canada, 66(3):T20 1, 1965

Jayarnant, M. D. and E. J. DeGraw, Practical Implications of Met& Management in Totdy Chlotine-Free (TCF) Pulp Production, Proceedings of 1 994 International Pulp Bleaching Conf, Vancouver, June 1994

Gierer, J., K. Jansbo, E. Yang, B.H. Yoon, and T. Reitberger, On the Participitation of Hydroxyl Radicals in Oxygen and Hydrogen Peroxide Bleaching Process, Procdmgs 6th Int. Symp. On Wood and Pulping Chernistry, Melbourne, Australia, Vol. 1, p.93 (199 1)

Hobbs, G.C. and J. Abbot, The Role of Radical Species in Peroxide Bleaching Process, Appita J., 45(5):344 (1 992)

Koubek, E., The Kinetics and the Mechanisrn of the Decomposition of Aiiphatic Peroxyacids in Aqueous Solutions, Ph.D thesis, Brown Univ. , 1964

Vasyutyn, Ya. M., V. O. Gavenko and 1. A. Garbuzyuk, Influence of Minera1 Salts on the Decay Rate of Peroxyacetic Acid in Aqueous Solution, Ukrainskii Khirnicheskic Zhumal, 55(6):584 (1 989)

Allen, G.C. and A. Aguilo, Metal Ion Catalysed Oxidation of Acetaldehyde, Advances in Chemistry Series, 76, p.363 (1 968)

McDonough, T. J. M., Peracetic Acid Decomposition and Oxidation of Lignin Mode1 Phenols in Alkaline Solutions, Ph.D Thesis, University of Toronto (1972)

Swem, D., Ed., Organic Peroxides, Vol.1, John Wiley and Sons, N. Y., 1970 , p.362

Gang, K. J., Y. Ni and A.R.P. van Heiningen, Mechanism of Cellulose Protection in a Novel and Selective Ozone Pulp Bleaching Process, Preprints, 82nd CPPA Annual Meeting, Montreal, Jan., 1996, p. B303

Greenspan, F.P. and D. G. Mackellar, Analysis of Aliphatic Peracids, Analy. Chem., 20(11):1061 (1948)

Evans, D. F. and M. W. Upton, Studies on Sin~let Oxygen in Aqueous Solution, Part 3, J. Chem. Soc. , Dalton Trans., 1985, p. 1 15 1

Kagan M. J. and G. D. Lubarslq, The Intermediate Stages of Aldehyde Oxidation, J. Phys. Chem., 39, 837-847 (1935)

van Lierop, B., N. Liebergott and M. G. Faubert, Using Oxygen and Peroxide to Bleach Kr& Pulps, Preprints. 79th CPPA Annual Meeting, Montreal, 1993, p.B2 1

Allison, R. W., Peroxide Bleaching of Mechanical Pulp fiom Pinus Radiata, Appita, 36(5):362 (1 983)

Solines, M., Brightening Response of Western Hemlock Refiner Groundwood to Hydrogen Peroxide, Pulp Paper Can., 77(3):59 (1 976)

Anderson, R, E. Stenberg and B. Sjogren, Optimised Hydrogen Peroxide Bleaching in Closed White-water Systems, Tappi J., 63(4): 1 1 l(1980)

Rapson, W. H., M. Wayman and C. B. Anderson, Hydrosulphite and Peroxide Bleaching of Nine Pure Species Groundwoods, Tappi J., 48(2): 1 13 (1 965)

Thiessn, F. I., Finlay Forest Uses Flash Dryer as Reaction Tower for Bleaching Pulp, Pulp & Paper, SS(1): l6l(l979)

The Dow Chernical Company, Versene Chelating Agent, Midland, Michigan, p 4.12 (1 993)

Colodette, J. L., S. Rothenberg and C.W. Dence, Factors Mecting Hydrogen Peroxide Stabiiity in the Brightening of Mechanical and Chernimechanical Pulps. Part III: Hydrogen Peroxide Stability in the Presence of Magnesium and Combinations of Stabilizers, JPPS, 1 5(2): 545 ( 1989)

0.07 -i-CH,C03H, 0.5 S/L DTMPA -0- H,O,, 0.5 S/L DTMPA z : -A-CH,CO,H, C~(l1) -A- H20, C~( l l )

0 10 20 30 40 50 60

Time (min)

Figure 6-1. Effect of transition metal ions on the stability of a peracetic acid solution (40°C, pH 7,0,0.75 ppm metal ion concentration)

-a-CH,CO,H, Control -d- H20,, Control J : -HH,CO,H, Na4P20, + H,02, Na4P20,

E "O6 - . <H,CO,H, DTPA -O- H,O,, DTPA

w -mUH,CO,H, OTMPA -n- H,02, DTMPA c &H3C03H, MgSO, 4 H202, MgSO, .O 0.05 c. e u c 8 0.04 C

O 0; 043 I L

O 0.02 =, O 3 0.01 O

0.00 O 10 20 30 40 50 60

Tirne (min)

Figure 6-2. Effect of additives on copper catalysed peracetic acid decomposition (40°C, pH 7.0,0.75 ppm Cuz+, 0.5 g L DTPA, DTMPA or MgSO,, 2 g/L Na,P,O,)

O 10

Time (min)

0.07

Figure 63. Effect of additives on iron catalysed peracetic acid decomposition (40°C, pH 7.0,0.75 ppm Fe*, 0.5 0.5 DTPA, DTMPA or MgSO,, 2 g/L Na,P,O,)

î 5 E 0."- w

s n ---

-A- CH,CO,H, Control -A- H202, Control - CH,C03H, Na4P20, -o- H202, Na4P,0, - - CH3C03H, DTPA -O- H202, DTPA -i-CH3C03H,DTMPA -0-H,O,,DTMPA + CH,CO,H. MaSO. + H-O-. MaSO-

I --

A -A4H,CO3H, Control -A- H202. Cont rol - H H 3 C 0 3 H , Na,P20, -XP- H202, Na4P207 --4H,C03H, DTPA + H,O,, DTPA -i-CH,C03H, DTMPA -0- H~O; DTMPA ,H3C03H, MgSO, -O- H20,, MgSO,

O 10 30 40

Time (min)

Figure 6-4. Effect of additives on manganese catalysed peracetic acid decomposition (40°C, pH 7.0,0.75 ppm Mn2', 0.5 g/L DTPA, DTMPA or MgSO,, 2 g/L Na,PzO,)

O 10 20 30 40 50 60 70 80 90

Time (min)

Figure 6-5. Effkct of DTPA on the manganese catalysed decomposition of peracetic acid (40°C, pH 7.0,0.75 ppm Mn2+, 0.5 g/L DTPA )

-A- CH,CO,H, Control -A- H20,, Control -r- CH,CO,H, DTMPA -D- H20,, DTMPA -0- CH,CO,H, DTPA -O- H,02, DTPA

-13 O O

+-L* --O -

0 1-0 20 30 40 50 60

Time (min)

Figure 6-6. Effect of DTPA or DTMPA on the manganese catalysed decomposition of peracetic acid (40°C, pH 5.0, 0.75 ppm Mn", 0.5 g/L DTPA or DTMPA)

Time (min)

Figure 6-7. Effect of DTPA or DTMPA on the manganese catalysed decomposition of peracetic acid (40°C, pH 6.0,0.75 ppm ~ n " , 0.5 g/L DTPA or DTMPA)

CHAPTER 7

THE FORMATION OF GASEOUS PRODUCTS AND ITS RELATION TO PULP

BLEACHING DURING THE PERACETIC ACID TREATMENT

When used in pulp bleaching, peracetic acid is consumed by two cornpetitive

pathways: oxidation of pulp and wasteful reactions such as decomposition and combination

with peroxide. In this chapter, the peracetic acid consumed in wastefbl reactions is quantified

by determining the gaseous products evolved. It is found that aithough carbon dioxide is

formed under certain conditions, oxygen is the dominant product of the wastefül reactions.

The oxygen formation is directly proportional to the consurnption of oxidants (as peracetic

acid and hydrogen peroxide) in the wastefùl reactions with 2 moles of oxidant consumed per

mole of oxygen formed. Therefore, the brightening efficiency during peracetic acid treatment

can be determined by measuring the oxygen formation. The peracetic acid brightening results

for an oxygen delignified s o ~ o o d kraft pulp are reported. It is found that a significant

amount of peracetic acid is consumed by the wastehl reactions during the brightening. The

effect of process parameters on the peracetic acid brightening efficiency is determined. It is

shown for an oxygen delignjfied sofiwood kraft pulp that there is a unique relationship

between pulp brightness gain and peracetic acid consumption due to the pulp oxidation

irrespective of the brightening conditions. This suggests that the oxygen formation can be

used for control of an industrial peracetic acid treatment.

Keywords: peracetic acid, bn~htening, decomposition, gas formation, oxygen delignified softwood kraft pulp

INTRODUCTION

Peracids, such as peracetic acid (Pa) and monoperoxysu1phuric acid (Px), have been

identified as prornising alternatives to chlorine containing chemicals for bleaching of chemical

pulps [l-81. Under typical peracetic acid bleaching conditions, in addition to the desirable

Iignin oxidation reactions, peracetic acid may be consumed by spontaneous and transition

metal catalysed decomposition reactions [9- 141. Therefore, it is important to understand and

monitor the peracetic acid decomposition in order to minimize these wastefùl side reactions

during peracetic acid bleaching.

Martin [ I 51 and later Gonzalez-Siena [ 1 61 studied hydrogen peroxide decomposition

by monitoring the gas formation dunng peroxide bleaching of groundwood pulps. It was

found that oxygen is the only gas formed from hydrogen peroxide decomposition, and that

two moles of hydrogen peroxide are decomposed per mole of oxygen formed. It was also

established [Io] that under typical hydrogen peroxide bleaching conditions the oxygen

produced does not contribute to pulp bteaching.

Gaseous products are also formed during peracetic acid decomposition [ 1 O- 131.

Kagan and Lubarsky [IO] found that carbon dioxide and oxygen are the main gaseous

products evolved during manganese catalysed decomposition of peracetic acid in acetic acid

and water respectively. Later, Allen and Aguilo Cl 11 confirmed that carbon dioxide is the

main gaseous product generated fiom manganese cataIysed decomposition of peracetic acid

in acetic acid solution. Vrbaski and Bnhta [12] investiçated the cobalt catalysed

decomposition of peracetic acid in an aqueous solution without pH control and claimed that

oxygen was the oniy gas formed. However, it was found by Koubek [13] who studied the

cobalt catalysed decomposition of peracetic acid at pH 5.4 in an ammonium acetate buffer,

that both oxygen and carbon dioxide were formed in a ratio of about one to one.

Furthemore, Koubek [ 131 also established that roughly 80% of the carbon dioxide formed

originated fiom the acetate ion of the buffer.

The above authors did not extend their studies to a neutd pH range, which is optimd

for peracetic acid brightening [1,8,9,17]. Furthermore, the relationship between the gas

formation and efficiency of peracetic acid brightening has never been exarnined. In this

chapter, the relationship between gas formation and peracetic acid consumption is determined

for typical peracetic acid brightening conditions. Subsequently it is attempted to relate the gas

formation to the brightening efficiency during peracetic acid treatment of an oxygen

deIignified softwood kraft pulp.

EXPERIMENTAL

Peracetic acid and ultra-pure grade NaOH were purchased ftom Aldrich (Milwaukee,

USA). The peracetic acid consists of about 34% w/w peracetic acid, 5% wlw hydrogen

peroxide and 4û% w/w acetic acid with the remainder being water. Reagent grade chemicals,

DTPA fiom Fisher Scientific (Nepean, Ontario) and DTMPA from Buckman (Memphis,

USA) were used in the experiments without hrther purification. Distilled and deionized

water was used in ail experiments. Its metal ion content was below the detection limits of the

atomic absorption (AA) spectroscopic analysis method used in this study.

The oxygen delignified softwood kraft pulp used for peracetic acid bleaching had a

37.7% ISO brightness, a kappa number of 13.8 and a viscosity of 23.4 mPa-S. The chelation

stage (Q) with 0.5% DTPA was performed at SOT, pH 5 , 2.5% pulp consistency and 30

minutes.

The experimental setup was s h o w in Figure 7-1. Experiments without pulp fibres

were carried out in a well sealed 500 mL four-neck round-bottom flask immersed in a

constant temperature bath. The aqueous solution containing al1 the required chernicals except

peracetic acid was preheated to the desired reaction temperature. The reaction was initiated

by addition of a concentrated peracetic acid solution to the reaction flask. The pH was kept

constant with an automatic titrator by addition of 10 m o n NaOH. Mixing was provided by

a submersed 41 mm long magnetic stirrer bar driven by a magnetic drive placed undemeath

the constant temperature bath. Ultra pure helium was bubbled through the solution to

transport the gases generated by the reactions fiom the flask to a gas sampling loop of a gas

chromatography (Fisher model 1200 Gas Partitioner ). Any leaks of the system can be

detected by the appearance of nitrogen in the gas sarnple. The gas partitioner employs a dual-

column, dual-detector chrornatographic system to separate and measure the gaseous

products. The first column is packed with 80-100 rnesh Columpakm PQ and the second with

60-80 mesh Molmlar Sieve 13X. The flow rate of the helium transport gas was controlled

by a mass flow controller (Edwards Model 825). The gas in the sampling loop was injected

at predetermined time intervals into the gas partitioner for analysis. SampIes of peracetic acid

solution were also withdrawn with a syringe, weighted and analysed following the iodometric

method developed by Greenspan and Mackellar [ l 81.

The volume concentrations of oxygen and carbon dioxide in the helium gas Stream are

related to the different gas flow rates as:

where F(O3 and F(C0J represent the volumetric flow rates of oxygen and carbon dioxide

respectivefy generated by the reaction solution, and F(He) is the helium fi ow rate. By solving

Equations (1) and (2), one can calculate F(O3 and F(C02) as:

By integrating

WJ = W e ) . [O,]

1 O0 - [O2] - [CO,]

the flowrates of 0, and CO, fiom time t, to t, the volume of 0, and CO,

evolved during t i i e intemal of t, to t, can be determined. The volume of 0, and CO, evolved

was then converted into moies using the ideal gas faw. The molar quantities of peracetic acid

and hydrogen peroxide consumed were calculated fiom the difference in the concentrations

of peracetic acid and hydrogen peroxide at determined times t, and t,.

Peracetic acid brightening of pulp was conducted in a 1000 mL four-neck round

bottom flask at 0.5 and 1.5 % consistency, with mixing provided during the course of

brightening by a mechanical stirrer. Again ultra pure helium was bubbled through the

suspension to cany the gaseous products out of the reaction. However, now al1 the gas was

collected in a I O L Tedlar gas sampiing bag. The helium gas flow rate was kept around 50

mL/min with the mass flow controller. The total gas volume collected was measured by

water displacement and the gas composition was determined by anaIysis with the gas

partitioner. After the peracetic acid treatment, the pulp was thoroughiy washed with deionized

water and made into handsheets. Samples of the spent liquor were withdrawn and titrated

for the residual peracetic acid and hydrogen peroxide concentrations.

Pulp brightness, viscosity and kappa number were determined in accordance with

TAPPI methods T452 om-92, T230 om-89, and T236 cm-85, respectively.

RESULTS AND DISCUSSION

The Formation of Gaseous Products During the Decomposition of Peracetic Acid

The result ofgas formation and decomposition of peracetic acid at pH 8.2 and 25°C

with 0.5 g/L DTMPA present was presented in Figure 7-2. The flow rate of helium was 100

mL/min and the mixing speed was 400 rpm. It was established in Chapter 4 that the

disappearance of peracetic acid under this condition is mainly due to the spontaneous

decomposition, as iliustrated in Equation (5):

2 CH3C0,H -+ 2 CH,CO,H + 0, (5)

Figure 7-2b shows that a negligible amount of CO, is released and that O, is the only

ças generated in the system at this condition. The experiments were repeated at helium

flowrates of 70 and 300 &min, and stimng speeds of 200 and 800 rpm. It was found that

the oxygen formation is not af5ected by the helium gas flowrate or the mixing speed. This

means that there are no significant transport limitations for the release of oxygen from the

reaction system.

By plottins the peracetic acid consumption against the oxygen formation in Figure 7-

3, it c m be seen that the molar ratio of peracetic acid consumed and the O2 formed is quite

close to 2 stipulated by Equation (5). Similar experiments at pH of 7.0 and 6.0 under

otherwise the Same conditions confirmed the above conclusions that only oxyzen is generated

and that the stoichiometry ofthe peracetic acid consumption to the oxygen formation is very

close to 2.

It was found [ 10- 13,191 that trace amounts of transition metal ions, such as cobalt,

manganese, copper and iron catalyse the decomposition of peracetic acid. Among these

metals, manganese is the main catalytic species responsible for enhanced peracetic acid

decomposition during pulp bleaching as shown in Chapter 6. Therefore, for an aqueous

solution of equilibrium peracetic acid the reIationship between the peracetic acid

decomposition and the gas formation was examinec? in the presence of 0.75 ppm Mn(I1)

without the addition of DTMPA at pH 8.2 and 25°C (Figure 7-4). As expected, the presence

of Mn(I1) enhanced the disappearance rates of both peracetic acid and hydrogen peroxide

(Figure 7-4a). Similady, the 0 formation was also increased with the addition of Mn(Il), but

the amount of CO, released was still negligible (Figure 74b). It was further confirmed that

the sum of the nurnber of moles of peracetic acid and hydrogen peroxide consumed was twice

that of the number of moles of oxygen formed.

Kagan and Lubarsky [IO] and later Vrbaski and Brihta 1121 identified the

stoichiometry of the manganese catalysed decomposition of peracetic acid in an aqueous

solution as:

McDonough [14] and later Evans and Upton (201 reporteci that the reactions of peracetic acid

and hydrogen peroxide were catalysed by transition metal ions. The overall reaction was

reported as [20]:

Furthermore, it was established in Chapter 6 that there is also an oxidation-reduction cycle

in the reaction system with the addition of Mn" . The overall stoichiometry of the oxidation-

reduction cycle is the same as Reaction (7). As is evident fiom Reactions (5), (6) and (7),

the molar ratio of the consumption of oxidants and the formation of oxygen is always 2

consistent with the experimental evidence.

The effect of pH on the formation of gaseous products in the presence of 0.75 ppm

Mn" was firther investigated by decreasing the pH eom 8.2 to 5.0. As shown in Figure 7 3 %

the disappearance rates of both peracetic acid and hydrogen peroxide are much slower at pH

5.0 than at pH 8.2. This is due to the fact that the reaction rates of Reactions (9, (6) and (7)

are al1 decreased at a Iower pH, as discussed in Chapter 5 . Figure 7-5b shows that both

oxygen and carbon dioxide are generated at pH 5.0. Further experiments showed that the

formation of COz is increased by the addition of acetic acid in the presence of ~ n ' + . This is

in agreement with the earIier result by Kagan and Lubarsky [ 1 0 3 as well as that of Allen and

Agulio [ l I l that carbon dioxide is the main gaseous product fiom the manganese catalysed

decomposition of peracetic acid when it was carrieci out in an acetic acid solution. In addition,

it was reported [13] that both carbon dioxide and oxygen are the gaseous products when the

cobalt catalysed decomposition of peracetic acid was performed in an ammonium acetate

bufer of pH 5.4. As will be shown below, the present experimental evidence as well as that

reported in literature support that carbon dioxide is the fkrther oxidation product of acetic

acid, rather than a direct product fiom the decomposition of peracetic acid.

In the previous section it was shown that two moles of the oxidants, peracetic acid

and hydrogen peroxide, are consumed per mole of oxysen generated. Therefore for the

experiments at pH 5.0 described above, the amount of oxidant consumed was corrected for

the oxygen formation according to the stoichiometry of Reactions (S), (6) and (7).

Subsequently the molar carbon dioxide formation was plotted against this corrected rnolar

amount of oxidant consumed. As can be seen in Figure 7-6, the stoichiometry in terms of

moles of corrected percetic acid and hydrogen peroxjde consumed per mole of carbon dioxide

formed is approximately 3. This is consistent with the suggestion of Kagan and Lubarsky [IO]

that the stoichiometry of the oxidants consumed to carbon dioxide formed is 3 when

manganese catalysed decomposition of peracetic acid is camed out in an acetic acid solution.

Therefore, the mechanism in Figure 7-7 is proposed to account for the formation of carbon

dioxide. The singlet oxygen, formed in Reactions (S) , (6) and (7) oxidizes acetic acid to

produce a-peroxide compound O), which undergoes decarboxyiation under acidic conditions,

generating d o n dioxide and fomaldehyde. Subsequently. fomaldehyde is further oxidized

by peracetic acid to formic acid as identified experimentally by Kagan and Lubarsky [IO].

Thus, the proposed mechanism satisfactorily accounts for the observed stoichiometry of the

oxidant consumption and the carbon dioxide formation. In addition, it explains why a lower

pH favours the formation of carbon dioxide.

The Effect of DTPA and DTMfA on Gas Formation and Peracetic Acid Decomposition

The effect of DTPA and DTMPA addition on peracetic acid decomposition and

oxygen formation at pH 8.2 was examined by adding either DTPA or DTMPA at the mid

point ofthe reaction (at 30 minutes). As s h o w in Figures 7-8 and 7-9 respectively, there is

a sharp increase in peracetic acid consumption rate when 0.02 mol/L DTPA was added with

or without the addition of 0.75 ppm Mn(I1) (Figure 7-8a). This is attributed to the fact that

peracetic acid is being rapidIy consumed in a reaction with DTPA as was reported earlier by

McDonough [14]. He found that DTPA was rapidIy oxidized by peracetic acid at ciifferent

pH's varying fiom 6 to 10 in the absence of metal ions and obtained a stoichiometry of 3

moles peracetic acid per mole DTPA. We confirmed this stoichiometry by comparing the

initial quantity of DTPA added with the peracetic acid loss calculated as the difference

between the total oxidant consumed and the oxidant decomposed as detennined by the

formation of oxygen. It is also shown in Figures 7-8a and 7-8b that the addition of 0.02 moVL

DTPA at 30 minutes l a d s to the decrease in the disappearance rate of peracetic acid and

hydrogen peroxide as well as the formation rate of oxygen. This couid be explained by the

hypothesis that part of the DTPA still chelates with the metal ions but that any excess is

rapidly consumed by peracetic acid [ 1 41.

Figure 7-9a shows that the oxidation of DTMPA by peracetic acid is negligible as

evidenced by the absence of sharp decrease in the peracetic acid concentration after the

DTMPA addition. The much slower rate in oxygen formation shown in Figure 7-9b is due to

the fkct that DTMPA stabilizes both peracetic acid and hydrogen peroxide [9]. Furthemore,

the stoichiometq of two moIes of oxidant consumed (both hydrogen peroxide and peracetic

ahd) per mole of oxygen fomed in these experiments supports that the oxidation of DTMPA

by peracetic acid and /or hydrogen peroxide can be neglected.

The Formation of Gaseous Products and Its Relation to Peracetic Acid Bleaching

The effect of chelation, pH, pulp consistency, peracetic acid charge and reaction time

on the gas formaion and its relation to pulp brightening during peracetic acid treatment were

studied. The results of the peracetic acid treatment of an oxygen delignified pulp are

presented in Table 7-1. Listed are the development of the residuaI amount of peracetic acid

and hydrogen peroxide, the amount of Oz and CO1 formed, and the bnghtness and viscosity

of the bleached pulp. The amount of peracetic acid and hydrogen peroxide consumed in

wastefiil reactions is calculated fiom the formation of O2 and CO2 using the appropriate

stoichiometry. Since it is reasonable to assume that at the present conditions hydrogen

peroxide is ody consumed in wastehl reactions, i.e. does not contribute to pulp oxidation,

the amount of peracetic acid consumed in wastefùl reactions can be calculated fiom the gas

fornation results and the amount of hydrogen peroxide consumed. Subsequent Iy the amount

of peracetic acid consumed in pulp oxidation reactions can be calculated as the difference

between the total amount of peracetic acid consumed and that consumed in wastefùl

reactions. Both the development of peracetic acid consumed in pulp oxidation reactions and

that in wastefbl reactions are given in Table 7-1 expressecl in tems of percentage on 0.d. pulp

as well as of percentage of total arnount of peracetic acid consumed. The results in Table 7- 1

show that only oxygen is produced when peracetic acid brightening is performed at pH 7.

However, at pH 5 (Run 5) a small amount of carbon dioxide is formed. It can also be seen

that the total amount of peracetic acid consumed in wasteful reactions is much less at pH

5 than that at pH 7. These results are consistent with those obtained at the same conditions

Table 7-1. The formation of gaseous products and its relation to bleaching during peracetic acid treatment of an oxygen delignified pulp

(Initial brightness of 37.7

1 Experimental Runs 1

6ISO; initial

II Samples

h l p consistency (%)

Pa Charge (% on 0.d. pulp)

II Temperature (OC) 1 60

Bleaching tirne (min) 120

Residual Pa 1 O-O9

Il Residual H202 (% on 0.d. pulp)

O formation

CO, formation (mmol CO, / g Pa charged)

II Viscosity (rn~a .~) 1 20.6

II Pa consumed in pulp oxidation (% on 0.d. pulp) 1

II I Pa consumed in wastefùl 2.36 reactions (% on 0.d. pulp)

II H,02 consurned in wastefid 0.43 reactions (% on 0.d. pu~p) I

Pa decomposed 81

TotaI Pa consumed (%)

Pa consumed by oxidation Total Pa consumed (%) 1 l9

* pH was kept constant dunng the blëaching

but without the presence of pulp fibres, presented in the previous sections. A surpnsing result

ofthe analysis in Table 7-1 is that the majority of the peracetic acid is consumed in wastehl

reactions, accounting for 59 to 87% of the total peracetic acid consumed. The amount of

peracetic acid wasted was plotted against the hydrogen peroxide wasted for al1 the

experiments listed in Table 7-1. Figure 7-10 shows that the molar ratio of peracetic acid

wasted and hydrogen peroxide wasted is about 2. Since hydrogen peroxide is only wasted in

Reaction (7) with a molar ratio of 1 between peracetic acid and hydrogen peroxide 1141,

Figure 7-10 suggests that about half of the peracetic acid is wasted by Reaction (7) while

spontaneous decomposition (Reaction (5)) and metal catalysed decomposition (Reaction (6))

are responsible for the other half of the peracetic acid wasted.

Cornparison of Run 1 with Run 2 confirrns that DTPA pretreatment leads to Iess

peracetic acid decomposition and a higher brightness gain, obviously because the DTPA

chelated pulp contains less residuai transition metals as shown in Chapter 6. The effect of pH

on peracetic acid brightening is seen by comparing Run 5 and 2 at pH 5.0 and 7.0

respectively. The analysis shows that a higher pH leads to more oxidation of pulp and

subsequently a higher brightness gain. This is consistent with literature results [ 1,8,9,, 171 that

peracetic acid brightening is more effective at a neutral pH than at a sliçhtly acidic condition.

However, a higher pH also results in less efficient use of peracetic acid since 7 1 % of the total

peracetic acid consurned was lost by the wasteiùl reactions at pH 7 compared to 63% at pH

5. An important result is that the data in Table 7- 1 shows that a higher consistency increases

the pulp oxidation while the consurnption of peracetic acid in wastetùl reactions is slightly

iower @uns 3 and 4). Therefore a higher brightness gain and especially a higher brightening

efficiency can be expected when peracetic acid brishtening is performed at hiçh consistency.

As expected a h i ~ e r brightness sain is obtained at higher peracetic acid charse under

otherwise the same brightening conditions (see Runs 8, 2 and 3). However, an increase in

peracetic acid charge results in a siçnificant lower briçhteninç efficiency. This could be

explaùied by the fact that at a higher Pa charge the reactive lignin chromophores are rapidly

consumed, while the peracetic acid concentration is still hi& so that the wastefûl reactions

remain substantial. The same argument explains why the peracetic acid brightening efficiency

worsens when the peracetic acid treatment is extended fkom 30 to 120 minutes (Runs 7 and

8).

The brightness of the peracetic acid bleached pulps were plotted against the peracetic

acid consumption due to the pulp oxidation reactions for al1 the experiments listed in Table

7-1. Figure 7-1 1 shows that there is a unique relationship between these two parameters,

independent of the process conditions. This confirms that the peracetic acid consumption in

wastefil reactions and by pulp oxidation are two completeiy independent phenornena,

therefore, the practical importance of Figure 7- 1 1 is that the bnghtness gain and brightening

efficiency can be predicted for a particular pulp independent of the process conditions, Frorn

the peracetic acid consumption (Le. charçe minus residual) and the gas (O2 and CO2 )

generation. This constitutes a powefil tool for control and optimization of an industrial

peracetic acid stage.

It is evident fiom Figure 7-12a that the viscosity of the resuIting pulp is closely related

to the hydrogen peroxide consumption in wastefbl reactions; a higher Ioss of hydrogen

peroxide in wastefiil reactions is always accompanied by a lower pulp viscosity. This may be

explained by the hct that highly reactive oxyçen species, such as singlet oxyçen and radicals,

are generated in the wasteful reactions involved hydrogen peroxide. This is in agreement with

the findig by Devenyns et ai. [21] that distilled peracetic acid (almost no hydrogen peroxide)

is more selective than equifibnum peracetic acid in pulp delignification. Since Figure 7- 10

shows that hydrogen peroxide and peracetic acid consumed in wastefùl reactions is closely

related, one can expect that the pulp viscosity l o s is also closely related to oxidant (peracetic

acid and hydrogen peroxide) consumed in wasteful reactions. Figure 7- 12b shows that this

is indeed the case. The practical implication of Figure 7- 12b is that the pulp viscosity after

peracetic acid treatment can also be predicted for a particular pulp independent of the process

conditions, fiom the measurement of gas generation.

CONCLUSIONS

Oxygen is the main gaseous product produced fiom the decomposition of peracetic

acid under typical brightening conditions, i.e. pH 7 and 60°C. One mole of oxygen is

generated per two moles of oxidants (peracetic acid and hydrogen peroxide) consumed. At

a relatively low pH (-5) CO, may also be formed fiom the wasteful reactions at a

stoichiometry of 3 moles of oxidant consumed per mole of COS released. Thus, based on the

stoichiometry of formation of O? and CO2 the amount of oxidant consumed in wasteful

reactions can be calculated from the generation of these two gases. Furthemore, the latter

information allows the detemination of the amount of peracetic acid used in pulp oxidation

as the difference of the peracetic acid consumed (Le. charge minus residual) and that what is

wasted.

It is found that the pulp brightness gain is directly reiated to the peracetic acid

consumption in the oxidation of pulp, independent of the process conditions. As a result the

brightness gain of the pulp and brighteninç efficiency can be predicted fi-om the chemical

consurnption (peracetic acid and hydrogen peroxide) and the 0, and CO, çeneration.

Similady, the pulp viscosity loss is uniquely related to the loss in oxidant as measured by the

O2 and C O generation. These findings constitute a powerfùl tool for control and optirnization

of an industriai peracetic acid stage.

ACKNOWLEDGMENT

The author thanks NSERC for financial support in the forrn of a Strategic Grant and

F. Munro of E. B. Eddy Forest Products LTD for supplying the oxygen delignified softwood

kraft pulp.

REFERENCES

Bailey, C. W. and C. W. Dence, Peroxyacetic Acid Bleaching of Chemical Pulp, Tappi J., 49(1):9 (1966)

Hill, R. T., P. B. Walsh and J. A. Hollie, Peracetic Acid, an Effective Alternative for Chlorine Compound Free Ddjgnjfication of Kr& Pulp, TAPPI 1992 Pulping Conference Proceedings, TAPPI PRESS, Boston, p. 12 19.

Springer, E. L. and J. D. McSweeny, Treatment of Softwood Kr& Pulps with PeroxymonosuIfate Pnor to Oxygen Delignification, Tappi J . , 76(8): 1 94 (1 993)

Geng, 2. P., H-m. Chang, H. Jarneel, B. Evans, B. Amini, and J. R. Webster, Mixed Peracids: Manufacture and Use as Non-Chlorine Delignification and Bleaching Agents, 1993 TAPPI Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p.353.

Troughton, N., F. Desprez and J. Devenyns, Peracids: The Pathway to High Brightness TCF Kraft Pulps, Proceedings of 1994 International Non-chlorine Bleaching Conference, Sponsored by Pulp & Paper and Emerging Technology Transfer Inc. Amelia Island, FL, sec. 10- 1.

Amini, B. and J. Webster, On-site Peracids: Tools for Bleaching Strategies to Meet the Cluster Rule, and Considerations on How to Select Among Them, 1994 TAPPI Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p. 371.

Allison, R. W. and K. G. McGrounther, Improved Oxygen Delignification With Interstage PeroxymonosulfÙnc Acid Treatment. Tappi J., 78(10): 134 (1 995)

Liebergott, N., Peracid Delignification and Bleaching, 1994 TAPPI Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p. 357.

Yuan, Z., Y. Ni and A-RP. van Heiningen, Kinetics of Peracetic Acid Decomposition, Part 1 and Part II, Canadian Journal of Chemical Engineering, 75,37-47 (1 997)

Kagan, M.J. and G. D Lubarsky. The Intermediate Stages of Aldehyde Oxidation, J. Phys. Chem., 39, p.837 (1935).

Allen, G. C. and A. Aguilo, Metal ion catalyseci oxidation of acetaldehyde, Advances in Chemistry Series, 76, p.363 (1 968).

Vrbaski, T. and 1. Brihta The Kinetics of Oxidation of Ndehydes to Acids and Acid

Anhydrides, Arhiv Za Kemiju, 26, p.267 ( 1954).

Koubek, E., The Kinetics and the Mechanism of the Decornposition of Aliphatic Peroxyacids in Aqueous Solutions, Ph.D thesis, Brown Univ. , 1964. Martin, D. M., The Bleachinç of Eastern Spruce Groundwood with Alkaline Peroxide 1. Reaction Kinetics, Tappi J., 40(2):65 (1 957)

McDonough, T. J. M., Peracetic Acid Decomposition and Oxidation of Lignin Model PhenoIs in Alkaline Solutions, Ph.D Thesis, University of Toronto (1 972)

Martin, D. M., The Bleaching of Eastern Spruce Groundwood with Alkaline Peroxide 1. Reaction Kinetics, Tappi J., 40(2):65 (1957)

Gonzalez-Sierra, G., The Effect of Process Variables on the Decomposition of Hydrogen Peroxide in the Bleaching of Groundwood Pulp, MSc. thesis, State Univ. Of New York, College of Environmental Science & Forestry, Syracuse, N.Y., (1 977)

Rapson, W. H., The RoIe of pH in Bleaching, Tappi J., 39(5):284 (1956).

Greenspan, F. P. and D. G. Mackellar, Analysis of Aliphatic Peracids, Analy. Chem., 20(11): 1 O6 1 (1 948).

Vasyutyn, Ya. M., V. O. Gavenko and 1. A. Garbuzyuk, lnfluence of mineral salts on the decay rate of peroxyacetic acid in aqueous solution, Ukrainskii Khimicheskic Zhumal, 55(6):584 (1 989)

Evans, D. F. and M. W. Upton, Studies on Singlet Oxygen in Aqueous Solution, Part 3, J. Chem. Soc., Dalton Trans., p. 1 15 1, 1985

Devenyns, J., F. Desprez and N. Troughton, Peracetic Acid as a Selective Prebleaching Agent: an Effective Option for the Production of Fully Bleached TCF Kraft hlps, Proceedings of 1993 International Non-chlorine Bleaching Conference, Sponsored by Pulp & Paper and Emerging Technology Transfer Inc., HHI, SC, sec. 8- 1

f'7- Mass Flowmeter

Auto Reaction Titrator Vessel in

Bath

u 2-way Swagelok s O Whitey Valve 1, 2 Calibration Gases

Figure 7-1. The experimentrl setup for the determiiiation of grseous products during peracetic rcid decomposition

5 3 O 15

O, Formation (mmol)

Figure 7-3. The stoichiometry of the peracetic acid consumption and the oxygen formation at 25*C, pH 8.2, F(He) of 100 mL/min and with 0.5 g/L DTMPA addition

2 4 6

CO, Formation (mmol)

Figure 7-6. The stoichiometry of peracetic acid consumption and carbon dioxide formation in the presence of 0.75 ppm Mn(ïï) and at 25OC, p H 5.0, F(He) of 100 mllm in)

1 I I "T-\ oz*

H O-H -

Figure 7-7, Plausible mechanism of the carbon dioxide formation during peracetic acid decomposition

Time (min)

(a)

O 20 40 60 80

Time (min)

(b)

Figure 7-8. The effect of DTPA addition on peracetic acid consumption and the oxygen formation nt 25OC, pH 8.2, F(He) of 100 mllmin and with 0.02 niol/L DTPA added at 30 min

O 1 2 3 4 5 6 7 8

H,O, wasted (mmol)

Figure 7-10. The relationship between the wasted peracetic acid and hydrogen peroxide (O or OQ pulp, 40-60°C, pH 5-7, 1.5-596 peracetic acid charge, 0.5-1.5% pulp consistency, 30-120 min)

CH,CO,H consurnption in pulp oxidation (% on 0.d. pulp)

Figure 7-11. The relationship between the peracetic acid consumption due to the pulp oxidation and the pulp brightness (O or OQ pulp, 40-6O0C, pH 57, 1.55% peracetic acid charge, 0.5-1.5% pulp consistency, 30-120 min)

0.0 0.2 0.4 0.6 0.8 1 .O

H202 wasted (% on 0.d. pulp)

(a)

O 2 4 6

CH3C03H and H,02 wasted (% on 0.d. pulp)

(b)

Figure 7-12. The relationship between the pulp viscosity and the oxidant coiisunied in wasteful reactions (O or OQ pulp, 40-60°C, pH 5-7,1.5-5% peracetic ricid charge, 0.5-1.5% pulp consistency, 30-120 min)

CHAPTER 8

A KINETIC MODEL FOR PERQCETIC ACID BRIGHTENING OF

AN OZONE DELIGNIFIED SOFTWOOD KRAFT PULP

ABSTRACT

A kinetic model of peracetic acid brightening of a solvent-assisted ozone delignified

puIp was developed. The model consists of three paralle1 and one consecutive reaction, and

is based on reaction pathways reported for lignin model compounds with peracetic acid. An

optimization search aigorithm was used to find the kinetic parameters of the model. The

kinetic model provides an adequate description of the disappearance rate of the chromophores

in pulp at constant pH and peracetic acid concentration.

Keywards: Peracetic acid brightening, kinetic model, chromophore, light absorption

coefficient, ozone deIignified softwood kraft pulp

INTRODUCTION

Peracids have recently emersed as a potential alternative to chlorine containing

chemicais for bleaching of chemid pulps [ I - 131. It has been shown [l -2,13- 1 71 that peracetic

acid is a good delignifier when used in the prebleaching s tqes . In addition, peracetic acid (Pa)

can also be applied in the Iater brightening stages as a substitute for chIorine dioxide [2,8- 1 O].

As shown in Chapter 3, an optirnised Pa stage can effectively and selectively increase the

bnghtness of a conventional soflwood kraft pulp as part of a ZnEoPaEop bleaching sequence,

in which Zn represents a solvent-assisted ozone stage. For example, it was reported in

Chapter 3 that a ZnEo delignified Hedock kraft pulp with a bnghtness of 56.5% and

viscosity of 28.4 mPa.s is bIeached by the Pa stage to a brightness of 82.7% while maintaining

a viscosity of 26.6 mPas with only about 1.5% peracetic acid by weight as peracetic acid, or

0.66% as hydrogen peroxide. In the present chapter, a kinetic model is developed for the

peracetic acid bnghtening of a methanol-assisted ozone delignified and subsequently alkaline

extracted (i-e. ZnE) pulp.

The constant condition method of kinetic experimentation in which the bleaching

chemicai concentrations, pH and temperature are held constant during the course of reaction

was used in the present study . The kinetic data was obtained by following the light absorption

coefficient of the bleached pulp. A similar experimental approach has been used to develop

the kinetics of hydrogen peroxide bleaching of mechanical pulps [18-221 as well as that of

chlorine dioxide bleaching of chernical pulps [23,24].

The kinetic model presented in this chapter consists of three parallel reactions and one

consecutive reaction, al1 representing reaction pathways identified in lignin mode1 cornpound

studies with peracetic acid. First order reaction behaviour with respect to the chromophore

and peracetic acid (or peracetate anion) concentrations were assumed. This approach is

different fiom that of other studies [IS-20, 23,241 for which the prime objective was to

develop an empirical equation based on its ability to represent the experimental data rather

than on fiindamental understanding of the detailed reaction mechanism.

The Hooke and Jeeves search algorithm [25] was used to find the kinetic parameters

of the model by minirnizing the sum of the squares of the differences between expenmental

and model predicted values of the light absorption coefficient. The initial values were

estimated fiom the data at conditions when only one or two reactions dominated the

brightening development.

EXPERIMENTAL

Equilibrium peracetic acid and ultra-pure grade NaOH were purchased fi-om Aldrich

(Milwaukee, USA). The equiiibnum peracetic acid consists of about 34% wlw peracetic acid,

5% W/W hydrogen peroxide and 40% wlw acetic acid with the rernainder being water.

Distilled and deionized water was used in al1 experiments. Its metal ion content was below

the detection limits of the atomic absorption (AA) spectroscopic anaIysis method used in this

study.

An ozonated and alkaline extracted softwood kraft pulp of 2.8 1 m'/kg absorption

coefficient, 47.5% ISO brightness, 6.7 kappa number and 27.1 mPa-s viscosity was used in

this study. Ozone bleaching (Zn) was carried out on a Hernlock kraft pulp impregnated with

70% methanol-water of pH 2, following an earlier descnbed procedure 1261. The ozone

charse was 1.60/0 @y weight on 0.d. pulp) of which 90% was consumed. A subsequent alkali

extraction stage (E) was perforrned at pH 1 1 .O, 77"C, IO% pulp consistency for 45 minutes.

The peracetic acid bleaching experiments at constant peracetic acid concentration

were canied out in a 1000 mL four-neck, well stirred, round-bottom flask immersed in a

constant temperature bath. 3 g r a m o.ci. pulp was used in each experiment and the pulp

consistency was 0.3%. The pulp s l u q was preheated to the desired temperature. A suitabie

amount of NaOH was then added to adjust pH to the desired level. Subsequently, the

required amount of peracetic acid solution was added to initiate the bleaching. The pH was

kept constant with an automatic titrator by addition of 10 m o n NaOH. Preliminary bleaching

nins were conducted to determine the rate of peracetic acid consumption. Subsequently, the

desired amount of peracetic acid was charged to the reaction mixture at regular time intervals

to maintain a constant peracetic acid concentration. The maximum change in peracetic acid

concentration was les than 10%. The bleaching reaction was quenched by injecting an excess

amount of KI dissolved in dilute sulfiiric acid. The pulp was immediately washed with

deionized water, and two hand sheets were made according to Tappi standard method T272

orn-92.

The peracetic acid and hydrosen peroxide concentrations were determined foltowing

the iodometric method developed by Greenspan and Mackellar [27]. The absorption

coefficient was determined at 557 nm wavelength with Technjbrite Micro TB- 1 C. The pulp

bnghtness was determined following TAPPI method T452 0111-92. The viscosity and kappa

number were deterrnined in accordance with T330 om-89 and T236 cm-85 respectively.

RESULTS AND DISCUSSION

Since the light absorption coefficient of pulp is directly proportional to its

chromophore concentration, the kinetics of peracetic acid brightening are described in tenns

of the rate change in the light absorption coefficient. Figure 8-1 shows the results obtained

at different pHs but a constant peracetic acid concentration of 0.0493 mol/L and temperature

of 50°C . It is evident that the light absorption coefficient decreases rapidly at the beginning,

but that the decrease becomes increasingly slower as the reaction continues. A similar

behaviour has been observed for chlorine dioxide bleaching of chemical pulps and hydrogen

peroxide bleaching of mechanical pulps [18-241. As in these two other cases, this pattern

could be explaineci by large differences in reactivity of the different chromophore structures.

It is noted that pH has a significant eEect on both the chromophore disappearance rate

and the final degree of chromophore removal. When the pH increases fiom 1 to 7, both of the

above two parameters increase. However, as the pH increases hrther fiom 7 to 10, the initial

chromophore disappearance rate is unchanged while the final degree of chromophore

rernoval is reduced. These results are consistent with the general knowledge t hat the optimum

pH of a peracetic acid brightening stage is in the neutral range [8,14,28].

The development of the light absorption coefficient at pH 6.0 and different peracetic

acid concentrations varying fiom 0.0329 to 0.0658 moi/L was also determined. The results

are listed in Table 8-1. It shows that the chromophore disappearance rate and the final degree

of chromophore removal are increased at a higher peracetic acid concentration. The results

listed in Table 8-1 as well as those in Figure 8- 1 forrn the data base for the determination of

the kinetic parameters during peracetic acid brightening of the ZnE pulp.

Table 8-1. The Development of the Light Absorption Coefficient at pH 6.0,50°C and Different Peracetic Acid Concentrations

! Tirne (min) 1 C, (m'kg) at different peracetic acid concentrations

1. Development of the Kinetic Mode1

Typid chromophoric structures in pulps include conjugated lignin macromolecules,

a d o n y l aromaîic moieties and quinones [29-301. The oxidation of lignin-like material by

peracetic atid has ben studied, mostly with model cornpounds [29-38 1. The overall reaction

pathways which occur during peracetic acid bleaching of wood pulps can be sumrnarized by

the following three general categories:

1. Hydroxylation of the aromatic rings of lignin by electrophilic substitution, resulting in the

formation of hydroquinones [30-341. The hydroquinones can either be destroyed to colourIess

water soluble products via electrophilic ring opening [30,3 11 or be further oxidized to

conjugated quinones, which represent another chromophoric structure [3 1-34]. Kawamoto

et al. [3 11 studied the oxidation of biphenyl lignin model compounds with peracetic acid at

pH 5.5. They found that highly conjugated quinones were formed and that tiirther destruction

O

[fa]: 0.0329 moVL

2.8 1

[Pa]: 0.0493 mol/L

2.8 1

[Pa]: 0.0658 moVL

2.8 1

of these conjugated quinones was a relatively slow process. These hishly conjugated quinones

may be formed by condensation of quinonoid intermediates via a Diels-Alder mechanism f 391.

2. Conjugated quinones, both originally present, and formed fiom Route 1 above can be

fûrther oxidized to usually colourless and water soluble carboxylic acids via the Baeyer-

Villger reaction 13 1-33 1.

3. Lignin structures containing an a-carbonyl group are oxidized via the Baeyer-Villiger

reaction 135-381. It has been found [35] that esters and their hydrolysis products were formed

during the peracetic acid oxidation of a-carbonyl containing lignin mode1 compounds. It was

also found that a ketone is somewhat less reactive than an aldehyde. For example,

acetoguaiacone is signi£icantly less reactive than vanillin due to the rnethyl substituent on the

carbonyl carbon which stencally hinders the Baeyer-Villiger oxidation andor disfavours the

nucleophilic attacking by peracetic acid. The etherïfication of a phenolic hydroxyl group or

the presence of an extra methoxyi in the aromatic nucleus appears to have little or no

influence on the reaction rate [35].

It is important to point out that depending on the process conditions both nucleophilic

and electrophilic rnechanisrns may contribute to the brightness increase during peracetic acid

treatment. Peracetic acid (CH,CO,H) is a strong electrophile [29,30,34,39]. When the pH is

in the range of 1 to 6, peracetic acid , rather than peracetate anion (CHJO,') is the dominant

species (the pKa of peracetic acid is 8.2). Therefore, electrophilic attack is the main

mechanism. When the pH is substantially higher than 8.3, the strongly nucleophilic peracetate

anion (CH,COJ [33,35,4042] is the dominant species. As a result, a nucleophilic attack is

expected to be the main reaction mechanism at high pH. It was established that

hydroxylation of the aromatic rings proceeds via the electrophilic mechanism 130-341, while

the Baeyer-Villiger reaction takes place through a nucleophilic attack [35-381. Because of

these facts, it is understandable that the best brightening is obtained at a neutral pH range

where both peracetic acid and peracetate anion are present in significant quantities.

In principle, the Baeyer-VilIiger oxidation could take place at general acidic

conditions. However, experimentd results show that a high concentration of strong acid is

required, For exarnple, it was reported f43-461 that high concentrations of suIfùnc acid and

perchloric acid catalyse the Baeyer-Villiger reaction. However, in the absence of a high

concentration of strong acid, the Baeyer-Viiliger oxidation is expected to be slow under acidic

conditions due to the low nucleophilicity of the undissociated peracid. This is supported by

the results that the oxidation of benzophenone by peracetic acid in glacial acetic acid proceeds

very slowly and that the addition of concentrated sulfùric acid greatly increases the reaction

rate [44]. Sarkanen and Suniki 1471 found that the amount of side-chah oxidation via the

Baeyer-ViIliger oxidation remains smd at a low pH when they treated Douglas-fir meal with

10% peracetic acid in an aqueous solution. Similarly, Ishikawa et al. [37] treated

acetoguaiacone with 8% peracetic acid at 35°C and found that the oxidation of

acetoguaiacone is quite slow. However, at a high peracetic acid concentration, high

temperature, and an extended reaction time, the Baeyer-Villiger reaction may account for part

of the substrate consumed, as was observed by Nirnz and Schwind 1351. Based on these

results, we assume that peracetate anion, not peracetic acid, is mainly responsible for the

Baeyer-Villiger oxidation in the system we studied.

Based on these identified reaction pathways, the kinetic mode1 shown in Figure 8-2

is proposed for peracetic acid brightening of ozone delignified sofkwood kraft pulp.

The model in Figure 8-2 shows that the chromophoric Iignin macromoIecule C,, is

eIectrophilicdly attacked by peracetic acid CH,CO,H via the hydroxylation route, leading to

the formation of either colourless products (Cp) or conjugated quinones (Ca, as discussed

previously. Therefore, C, is consumed in two cornpetitive reactions, with k, and k2 as their

rzte constants respectively. The conjugated quinones, C,, both the newly formed and those

originally present, are oxidized to water soluble carboxylic acids via the Baeyer-Villiger

reaction by peracetate anion CH,CO;, represented by the rate constant k,. FinalIy, the a-

carbonyl containhg chromophores, C,, are destroyed via the Baeyer-Villiger mechanism by

CH,CO;, with a rate constant of k,.

It is assumed that the reactions discussed in Figure 8-2 are first order with respect to

the peracetic acid concentration and the chromophore concentration. This hypothesis is

supporteci by the results of McDonough [33] that the kinetics of the oxidation of creosol by

peracetic acid is fint order both in peracetic acid and creosol concentrations. Therefore, based

on the model described in Figure 8-2, the following rate equations of peracetic acid

brightening are obtained:

- -- dC1 - (k, + kJ C, [C%CO,H] d t

- -- dC2 - $ C, [CH&O;] - \ C , [CH$O,H] dt

-- dC3 = k4C, [ C H p , ] dt

Peracetic acid concentration, [CH,CO,H], and peracetate anion concentration, [CH,CO,],

are related to the total titrable peracetic acid concentration [CH3C0,H], as in Equations (4)

and (5), with M beinç the ratio of the proton concentration to the dissociation constant of

peracetic acid, i.e. [H+]/K,.

Since the concentration of peracetic acid, [CH,C03H], and that of peracetate anion,

[CH$%], are constant at a fixed pH and temperature, Equations (1) to (3) can be simplified

as:

M where k; = k, - [CH-,C03H], 1 +M

Equations (6) to (8) can be solved analyticaily as:

AIso the sum of a11 the different chromophore concentrations at any time during the

course of the peracetic acid treatment equals to the total chromophore concentration. C,

which is known experimentally, i.e.

With the knowledge of the disappearance rate of total chromophore concentration

under different conditions, it is possible to determine the rate constants and the initial

concentration of each of the three chromophores by an optimization technique which utilizes

the Hooke and Jeeves search algorithm [25]. The sarne technique was applied to determine

the kinetics of chlorination.[48,49].

Equations (1 3) to (16) show that there are seven unknown variables: ki, ki, kj, k;,

C,, C , and C,, 3 equations, (13), (14) and (15) and 56 data points in the fom of equation

(16). in order to start the search algorithm, it is needed to provide initial values for six of the

variables, with the seventh being fixed by equation (16).

2, Estimation of the Initial Values

2.1, Estimation of CSiand k,

Figure 8- 1 shows that the initial rate of the chromophore disappearance is strongly

dependent on the pH Since the concentration of peracetate anion increases dramatically while

that of peracetic acid is relatively constant when the pH increases fi-om 1 to 7, the higher

initial reaction rate in this pH region must be related to the higher peracetate anion

concentration. Since it has ben show that a-carbonyl compounds, represented by C,, react

easily with peracetate anion accordinç to the Baeyer-Villiger mechanism [35-381, while the

oxidation of conjugated quinones proceed slowly in aqueous solutions [3 11, it was assumed

that in the initial phase the Baeyer-Villiger oxidation of a-carbonyl chromophoric structures

is the dominant reaction at a higher pH.

In order to better quanti@ the initial reaction rate, fùrther cxperiments were done to

characterize the light absorption coefficient development during the first 4 minutes at pH 7.0

and 50°C. The results in Figure 8-3 suggest that the Baeyer-Villiger oxidation is very fast

since the first data point obtained at 5 seconds, the shortest time possible with our

experimental set-up, shows that the light absorption coefficient decreases rapidly fiom 2.81

to 1.75 m2/kg. Therefore, the initial a-carbonyl group concentration, C,i , was estimated as

0.96 m'kg fiom the difference of the initial total chromophores, Ci , of 2.8 1 m2/kg and the

sum of C,i and C, of 1.85 m2kg estimated by extrapolating the initial four points in Figure

8-3 to zero time.

Only a vety rough estimate of the rate constant k; can be obtained fiom the data in

Figure 8-3, because the reaction of the cc-carbonyl chromophores with peracetic anion appears

to be essentially complete at the first data point of 5 seconds. However, in order to generate

a first order estimate of the initial value of k; for the optimization procedure it was assumed

that 99% of the a-carbony1 chromophores initially present, C,,, disappeared in the first five

seconds. Following Equation (1 5). we estimated a k; of 6x1 0 min-'. Furthemore, we

calculateci that k, is approximately 2x 10' LImoLmin, by taking into consideration of the pKa,

pH and the total peracetic acid concentration. Since it is realized that the calculated value of

k, of 2x10" L/mol.min is only a first order estimate, the initial value of k, for the optimization

procedure will be varied by two orders of magnitude around 2x10' Llmo1.min in order to

evaluate the sensitivity of the procedure to this initiai guess of k,.

2.2. Estimation of CZi and k,

Since oxidation of both conjugated quinones, Cr and a-carbonyl containing

chromophores, C,, at pH 1.0 is insignificant due to the very low peracetate anion

concentration, the hydroxylation, which is achieved by peracetic acid and represented by

reactions k, and k2, can be considered the ody reactian pathway tu remove chromophures at

this pH. Figure 8-1 shows that the chromophore removal is negligible after 50 minutes of

reaction at pH 1 .O, so it is reasonable to assume that the hydroxylation reaction is completed

within 50 minutes at this condition. Also because the actual concentration of peracetic acid

is essentially the sarne when the pH is increased fiom 1.0 to 6.0, it is assumed that the

hydroxylation reaction at pH 6.0 is also completed within 50 minutes. As discussed earlier,

the Baeyer-Villiger oxidation of a-carbonyl containing chromophores is a fast reaction with

k, being about 2x10' L/mol.min, so that it takes less than one minute to complete this

oxidation at pH 6.0. Based on the above analysis, it can thus be concluded that after 50

minutes at pH 6.0 the decrease in the light absorption coefficient is mainly due to the

destruction of conjugated quinones, C, by reaction with peracetate anion, CH,CO,: i.e.

Equations (6), (7) and (8) reduce to:

Therefore, the initial concentration of conjugated quinones, C,. and the rate constant k; can

be estimated by plotting 1nC against time t at pH 6.0 for the data points obtained beyond 50

minutes. From this plot shown in Figure 8-4 it follows that C, is 0.57 m'kg and k; is 0.00 145

min1. Substitution of k; to Equation (1 1) yields k, of 4.7 L/mol.min. With C, of 0.96 m'/kg

(shown in the previous section) and C , of 0.57 m'kg, the estimate for C,i is calculated as

2.3. Estimation of k, and k,

The values of k , and k2 were estimated using the results obtained at pH 1 .O, shown

in Figure 8-1. Since the peracetate anion concentration, [CH,CO,], is extremely low at pH

1 .O, both of the Baeyer-Villiger pathways c m be neglected, as discussed earlier. Therefore,

both k; and k; wili be set equal to zero at pH 1 .O. As a result, Equation (1 6) can be simplified

An application of the exponential decay data fitting technique yields Equation (1 9) to mode1

the data points at pH 1 .O in Figure 8- 1 as:

C = 1.602 + 1.205 e'0-'06' (19)

From a cornparison of Equations (1 8) and ( 1 9), it follows that:

k; and (1 --) CIi = 1.205 k;+ k;

By solving Equations (20) and (21), one obtains a k; of 0.100 min-' and a k; of 0.0062 min-'.

Taking into consideration the pH and the total concentration of peracetic acid in Equations

(9) and (IO), k, and k2 are estimated as 2.03 and 0.126 L/mol.min, respectively.

3. Modelling Results

With the estimation of the initial values of C,i, CZi, C,i, k,, k,, k, and k,, it is now

possible to determine the optimal value of these kinetic parameters using the data base shown

in Figure 8-1 and Table 8-1 following the Hooke and Jeeves search algorithm [25] . The

approach is to minimize the sum of squares of the difference between the experimental values

of the total chromophore concentration and that of the model predictions. The procedure was

considered converged when, for each data set at a fixed pH and peracetic acid concentration,

the surn of squares changed by less than O. 1% between iterations. To ensure that the

optirnised set of parameters indeed represented the unique solution of the kinetic modei for

the present data, the most uncertain parameter, k,, was varied by two orders of magnitude

around the estimated value, while the other rate constants were varied by a factor of two. It

was found that the d u e s of the optimised pararneters were essentially the same as initial

estimates of the kinetic parameters.

The optimized results are Listed in Table 8-2. It shows that k, is much higher than the

other rate constants, confhing that the oxidation of a-carbonyl containing chromophores,

C,, is a fast reaction. This is in agreement with the findings by McDonough [333 that

acetoguaiacone reacts much faster than apocynol and vanillyl alcohol during peracetic acid

treatment at a pH of 6 to 9. Table 8-2 also shows that k, is comparable to k , so their

reaction rates largely depend on the concentrations of CH,CO,H and CH,CO;. Thus it is

expected that the oxidation of conjugated quinones, CL proceeds slowly due to the lower

concentration of CH,CO,- at a pH lower than 7.0, as was supported by Kawamoto et al. [3 11

who showed that fùrther oxidation of the conjugated quinones fomed via the hydroxylation

pathway to colourIess products is quite slow at pH 5.5 [3 Il.

An example of a companson between calculated and measured light absorption

coefficients at pH 7.0 is given in Figure 8-5. It is apparent that the proposed kinetic model

gives a good description of the development of the Iight absorption coefficient dunng

peracetic acid brightening of the ozone detignified pulp used in this study.

Table 8-2. The Kinetic Parameters Obtained by the Hook and Jeeves Search Algorithm at 50°C for the Brightening of ZnE Pulp

With the optirnized kinetic parameters, the concentration-tirne profiles of the three

different chromophores, C,, G, C, and the total chromophore concentration C was predicted

at a pH of 3.O,7.O and 10.0. The results are plotted in Figures 8-6 to 8-8 respectively. At pH

3 in Figure 8-6 the destruction of chromophores C , is completed d e r about 50 minutes while

around 40% of the a-carbonyl containing chromophores, C,, are destroyed at 90 minutes.

However, the concentration of the conjugated quinones, G, increases slightly over the course

of the reaction. The behaviour of the C, chromophores is in agreement with the results by

Nimz and Schwind [35] that about 42% acetoguaiacone is oxidized via the Baeyer-Villiger

reaction at pH 3 and 60°C over a period of 60 minutes. At pH 7.0 in Figure 8-7, the oxidation

of the a d o n y l containing chromophores, C,, is accomplished within a few seconds, while

the CI chromophores are also removed within one hour. The degradation of conjugated

quinones, however, is still incomplete at 90 minutes at pH 7. At a higher pH of I O in Figure

8-8, it c m be seen that now the oxidation of conjugated quinones, Cb is complete in almost

20 minutes. A higher pH favours the degradation of conjugated quinones relative to at pH 3

which is in agreement with the finding by Nimz and Schwind [35]. It can also be noticed that

Estimated

Optimized

Initial chromophore concentration (m" /kg)

Rate constants (L/mol. min)

C3i

0.96

1-01

CIi

1.28

1.26

C3

0.57

0.54

kt

2.03

k3

4.7

3.33

kz

0.126

k4

20000

17700 0.17

at pH 1 0 the destruction rate of chromophores C, is significantly reduced due to a much

lower concentration of peracetic acid. Thus, although both quinones, C2, a-carbonyl

containing chromophores, C,, are efficiently removed at pH 10, the overalI degree of

chromophore removal is much less than that at pH 7.0 because the initial concentration of C,

chromophores is larger than that ofthe C, chromophores. This explains the experimental fact

that optimum brightness gain can be obtained at a neutral pH ranse.

CONCLUSIONS

Based on reaction pathways identified in literature, a kinetic model for peracetic acid

brightening of a solvent-assisted ozone delignified pulp has been developed. The kinetic

equations can adequately describe the developrnent of the light absorption coefficient of pulp

under conditions of constant pH and peracetic acid concentration. The model includes three

parailel reactions, namely hydroxylation, oxidation of conjugated quinones, and oxidation of

carbonyl containing iignin stmctures, which are responsible for the overall removal of

chromophores, and thus for the decrease in the light absorption coefficient of the bleached

pulp. The reaction rate of each of the three reactions is dependent on the pH during the

peracetic acid treatment. At pH 3 only hydroxylation is significant. At pH 7, both the

hydroxylation and oxidation of carbonyl containing lignin structures is complete, while oniy

the conjugated quinones are rernaining when the treatment is extended beyond 50 minutes

at 50°C. A fitrther increase in pH to 10.0 significantly retards the hydroxylation rate, which

explains the smalter removal of chromophores at this pH as compared to that at pH 7.0.

ACKNOWLEDGMENT

NSERC's financial support in the form of a Strategic Grant is greatly appreciated.

REFERENCES

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Anderson, J. R., 8. Amini and W. Wilkinson, On-site Generation and Use of Peroxy Acids in Chemical Pulp Bleaching, Prepnnts, the 8 1'' CPPA Annual Meeting, Tech. Sect., CPPq Montreal, 1995, p. B59.

Basta, J., L. Holtinger, P. Lundgren, H. Fasten, and R. Fredriksson, Alternatives for Achieving f igh Brightness TCF Pulps, Proceedings, 1 994 International Non-chlorine Bleaching Conference, Sponsored by Pulp & Paper wid Emerging Technology Transfer Inc. Amelia Island, FL, sec. 1 1-3

Amini, B. and J. Webster, On-site Peracids: Tools for Bleaching Strategies to Meet the Cluster Rule, and Considerations on How to Select Among Them, TAPPI 1994 Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p. 37 1

Liebergott, N., Di-per process: Time for a peracid delignification and bleaching, Proceedings, Preprints, SI* CPPA Annual Meeting, Tech. Sect., CPPA, Montreal, 1995, p. BI57

Song, J., Peroxyacid Bleaching of Conventional, MCC and RDH Kraft Pulps, Ph.D thesis, North Caroline State University, USA, 1992

Springer, E. L. and J. D. McSweeny, Treatment of Softwood Kraft Pulps with Peroxymonosulfate Prior to Oxygen Delignification, Tappi J., 76(8): 194 (1 993)

Liebergott, N., Peracid Delignification and Bleaching of Chemical Pulp Part II: Oxidation, Pulp & Paper Canada, 97(3):73 (1996)

Troughton, N., F. Desprez and J. Devenyns, Peracids: The Pathway to High Brightness TCF Kraft Pulps, Proceedings, 1994 International Non-chlorine Bleaching Conference, Sponsored by Pulp & Paper and Emerging Technolog Transfer Inc. Amelia Island, FL, sec. 1 0- 1.

Devenyns, J., F. Desprez and N. Troughton, Peroxygen PrebIeaching and Bleaching Technologies for Step-Wise Conversion fiom Conventional Chlorine Bleaching via ECF toTCF, TAPPI 1993 Pulping Conference Proceedings, TAPPI PRESS, Atlanta, p. 341.

I l . Swan, B., Sustainable Development in STORA's Chemical Pulp Mills, Proceedings, 1996 International Non-chlorine BIeaching Conference, Sponsored by Pulp & Paper and Emerging Techno lo~ Transfer Inc. Hyatt Orlando, FL, sec. 13-2.

Nirnrnerfi-oh, N. and H. U. Sus, Generation and Application of Peracids for Chemical Pulp Bleaching- A Cost Cornparison, Proceedings, 1996 International Non-chlorine Bleaching Conference, Sponsored by Pulp & Paper and Emerging Technology Transfer Inc. Hyatt Orlando, FL, sec. 5-4.

Devenyns, J., F. Desprez and N. Troughton, Peracetic Acid as a Selective Prebleaching Agent: an Effective Option for the Production of Fully Bleached TCF Kraft Pulps, 1993 International Non-chlorine Bleaching Conference, Sponsored by Pulp & Paper and Emerging Technology Transfer Inc., HHI, SC, sec. 8-1

Bailey, C. W. and C. W. Dence, Peroxyacetic Acid Bleaching of Chernical Pulp, Tappi J., 49(1):9 (1966).

Chnstiansen, C. B., and W. F. Parker, Laboratory Studies of Acetic Acid Bleaching, Preprint, TAPPI Annual Meeting, 1965, p 132.

Rapson, W. H., and C. B. Anderson, BIeaching in Five Stages to Asymptotic Limit Using Only One Oxidant and Sodium hydroxide, 1985 International Pulp Bleaching Conference, Quebec, 1985, p. 227 .

GeUerstedt, G., Peracid Technology Review, Preprint, 1993 Workshop on Emerging Pulping and Chlorine-Free Bleaching Technology, Raleigh, N.C., March 3, 1993.

Lundqvist, M., Kinetics of Hydrogen Peroxide Bleaching of Mechanid Pulp, Svensk Papperstidn. 82(1): 16(1979)

Moldenius, S. and B. Sjogren, Kinetics Models for Hydrogen Peroxide Bleaching of Mechanical Pulps, J. Wood Chem. Tech. 2(4):447(1982)

Allison, R.W. and K. L. Graham, Peroxide Bleaching of Mechanical Pulp Fractions fiom Radiata pine, JPPS 1 5(4):.Jl45(1989)

Wright, P. and J. Abbot, Kinetic Models for Peroxide Bleaching under alkaline Conditions. Part 1 : One and Two Chromophore Models, J. Wood Chem. Tech. 1 l(3): 349(1991)

Wright, P., Y. Ginting and J. Abbot, Kinetic Models for Peroxide Bleaching under Alkaline Conditions. Part 2: Equilibrium Models, J. Wood Chem. Tech. IS(1): 1 1 l(1992)

Teder, A. and D. Tormund, Kinetics of Chlorine Dioxide Bleaching, Trans. Tech. Sect. CPPA, 3,41(1977)

Germgird, U. and A. Teder, Kinetics of Chlorine Dioxide Prebleaching, Trans. Tech. Sect., CPPq 6, TR3 1 ( 1980)

Hooke, R. and T.A. Jeeves, Direct Search Solutions of Numerical and Statistical Problems, 3. Assoc. Comp. Mech., 8(2):2 12(196 1 )

Kang, G. J., Y. Ni and A. R. P. van Heiningen, Mechanism of Cellulose Protection in a Novel and Selective Ozone Pulp Bleaching Process, Preprints, the 82nd CPPA Annual Meeting Preprints, Tech. Sect., CPPq Montreal, 1996, p. B303

Greenspan, F. P. and D. G. Mackellar, Andysis of Aliphatic Peracids, Analy. Chem., 20(I 1): 1061 (1 948)

Rapson, W. H., The Role of pH in Bfeaching, Tappi J., 39(5):284 (1956).

Sarkanen, K. V-and C. H. Ludwig, Lignins, Wiley Interscience, N. Y., 197 1, p.459

Boeseken, J. and R. Engleberts,, Proc. Acad. Sci. Amsterdam, 34, 1292(193 1); 35 750(1932)

Kawamoto, H., H. Chang and H. Jameel, Reaction of Peroxyacids with Lignin and Li& Model Compounds, Proceeding, the 8& International Symposium on Wood and Pulping Chemistry, Helsinki, Finland, June, 1995, p.3 83.

Strumila, G. and H. Rapson, Reaction Products of Neutra1 Peracetic Acid Oxidation of Model Lignin Phenols, Pulp & Paper Canada, 76(9):72(1975)

McDonough, T. J. M., Peracetic Acid Decomposition and Oxidation of Lignin Mode1 Phenols in Aikaline Solutions, Ph.D Thesis, University of Toronto, Canada (1 972)

Henderson, G.G. and R. Boyd, Oxidation of Monohydric Phenols with Hydroçen Peroxide, J. Chem. Soc., 97, l659(l9 10)

Nimz, H. H. and H. Schwind, Oxidation of Monomeric Lignin Model Compounds with Peracetic Acid, CelluIose Chem. Technof ., 13, 3 5 (1 979)

Hatakeyarna, H., J. Nakano, and N. Migita, Lignin, XLVII. Origin of Succinic Acid as An Oxidation Product of Lignin with Peracetic Acid, J. Chem. Soc. Japan, Ind. Chem. Sect., 68, 972(1965)

Ishikawa, H., T. Oki and K. Ohkubo, Oxidative Decomposition of Lignin, V. The Degradation by Peracetic Acid of Aromatic Compounds Stmcturally Related to Vanillyl Alcohol Elements Present in Sofbood Lignin, J. Japan Tappi, 20, 435(1966)

Kinoshita, Y., T. Oki and H. Ishikawa, Oxidative Degradation of Guaiacylglycerol with Peracetic Acid, J. Japan. Wood Res. Soc., 13, 3 19 (1 967)

Farrand, J. C., Peroxyacetic Acid Oxidation of 4-Methylphenols and Their Methyl Ethers, Ph.D. Thesis, Institute of Paper Chemistry, Appleton, Wisconsin, USA, 1969

Jencks, W. P. and J. Camuolo, Reactivity of Nucleophilic Reagents toward Esters, J. Am. Chem. Soc., 82, 1778 (1960)

HassalI,C. H., Organic Reactions, Vol. 9, p.73, A. Roger (ed.), John Willey & Sons Ltd., New York, 1957.

Yasuhiko, S., Organic Peroxides, p.437, W. Ando, (ed.), John Willey & Sons Ltd., New York, 1992.

Lefiler, J. E, Hydroperoxide Oxidation of a-dicarbonyl Compounds,J. Org. Chem., 16, 1785 (1951)

Doering, W. von E. and L. Speers, The Peracetic Acid Cleavage of Unsymmetrical Ketones, J. Am. Chem. Soc., 72(12):% 15 (1 950)

Friess, S. L., Reaction of Per Acids II: The Reaction of Perbenzoic Acid with Simple Cyclic Ketones, J. Am. Chem. Soc., 7I (7):257l (1 949)

Marker, R. E., E. Rohmann, H. M. Crooks, E. L. Whittle, E. M. Jones, and D. L. Turner, Oxidation Products of Sarsapogenin Pregnanetriol-3,16,20, J. Am. Chem. Soc., 62(2): 5 25 ( 1 940)

Sarkanen, K. V. and J. Suzuki, Delignification by Peracetic Acid, Studies on Oxidative Delignification Mechanisms, Tappi J. 48(8):459 (1 965)

Mackinnon, J., Dynamic Simulation of the First Two Stages of a Kraft Softwood Bleach Process, M. Eng. Thesis, McGill Univ., Canada, 1989

Ni, Y., G. J. Kubes and A. R. P. van Heiningen, Chlorination Kinetics of Kraft Pulp, JPPS, 21(1):J30 (1 995)

Figure 8-1. The e f f i of pH on the light absorption coefficient development during Pa brightening of ozone delignified softwood kraft pulp (50°C and [CH,CO,Hl of 0.0493 m o n )

Figure 8-2. The proposed kinetic mode1 of peracetic acid brightening

3.0

0 . 0 ~ . , I [ i = 1 ~ l ~ l ' ~ ' 0!0 0-5 1.0 1.5 2 0 2 5 3.0 3.5 4.0 4.5

Time (min)

Figure 8-3. Estimation of C, and k, (50°C, p H 7.0, [CB,CO,H] of 0.0493 m o n )

O 100 150 200 250 300 350

Time (min)

Figure 8-4. Estimation of CIi and k, (50°C, pH 6.0, [CH,CO,H] of 0.0493 moVL)

3.0 1

O 10 M 30 40 50 ô0 70 80 90 100

Time (min)

Figure 8-5. Cornparison of mode1 predictions and experimental results (SOT, pH 7.0, [CH,CO,HJ of 0.0493 m o a )

O 20 40 60 80 100

Time (min)

Figure 8-6. Predictions of the development of C,, C,, C, and C (5OUC, pH 3.0, [CH3C03H] of 0.0493 moVL)

O 20 40 60 80 1

Time (min)

Figure 8-7. Predictions of the development of C,, C,, C, and C (50°C, pH 7.0, [CH,CO,H] of 0,0493 moVL)

Time (min)

Figure 8-8. Predictions of the development of C,, C,, C, and C (SOT, pH 10.0, [CH3C03H] of 0.0493 m o n )

CHAPTER 9

CONCLUSIONS

GENERAL SUMMARY

This thesis focuses on fundamental and practicai aspects of peracetic acid brightening

of softwood kraft pulps.

A final TCF brightening sequence of PaEop has been developed for a solvent assisted

ozone delignifieci and oxidative extracted (ZnEo) Hemlock kraft pulp. With the ZnlEoPaEop

bleaching process a conventional softwood kraft pulp with a kappa number of 30 could be

bleached to 88% ISO brightness while rnaintaining a viscosity of more than 20 mPa.s with

charges of 2% ozone, 1.5% Pa and 0.5% hydrogen peroxide. The brightness, viscosity and

brightness stability of this TCF fully bleached kraft pulp are similar to or better than those

bleached 4 t h chlorine-containing chemicals. The optimum conditions for the peracetic acid

and Eop stages have been detemined. Those for the Pa stage are: a peracetic acid charge of

1.5%, 20% pulp consistency, 60°C, a neutral pH and 3 hours. The optimum conditions for

the reinforced Eop stage are: 20% pulp consistency , 95"C, 60 psi oxygen pressure, 90 min

with 0.5% H202 , 1.5% NaOH and 0.1% MgSO,. No chelation chemicals are required for

this TCF bleaching sequence.

Tt has been shown that under typical peracetic acid bleaching conditions, peracetic acid

is consumed by three wasteful side reactions in addition to the desired oxidation reactions

with pulp. These unwanted reactions are: i) spontaneous decomposition, ii) hydrolysis and

iii) transition meta1 catalysed reactions.

The addition of the chelating agent DTMPA to a dilute peracetic acid solution

effectively suppresses the metal catalysed reactions of peracetic acid. ln a pH range of 5.5 to

8.2 the hydrolysis is negligible and the peracetic acid consumption is mainly due to

spontaneous decomposition, while between pH of 8.2 to 9.0 the peracetic acid consumption

is due to spontaneous decomposition and alkaline hydrolysis.

The kinetics of the spontaneous decomposition of peracetic acid were developed. It

was found that the spontaneous decomposition is second-order with respect to the total

peracetic acid concentration ([CH,CO,H]J. The present experimental resuIts are well

described by the developed kinetic equation.

pH is one of the important factors which determine how peracetic acid is consumed.

It was found that the spontaneous decomposition reaches its maximum rate at pH 8.2, while

both the hydrolysis and metal ion catalysed reactions increase as the pH increases. At pH 1 0.5

or higher the hydrolysis becomes dominant when the metal ion catalysed reactions are

minimized by the addition of DTMPA.

The kinetics of the alkaline hydrolysis of peracetic acid were also developed. When

these kinetics are combined with those of the spontaneous decomposition developed earlier,

the consumption of peracetic acid and hydrogen peroxide by decomposition reactions can

be accurately predicted. It was also found that the addition of MgSO, at pH 9.5 leads to

formation of a precipitate of Mg(OH), which reduces the rate of transition metal catalysed

reactions but enhances that of the hydrolysis of peracetic acid. With the addition of 6 g/L

MgSO, at pH 9.5, the hydrolysis is rnainly responsible for the consumption of peracetic acid.

The transition metal ion induced peracetic acid decomposition and the effect of the

addition of cheiants/additives on the decomposition were studied under the typical peracetic

acid pulp bleaching conditions. The results show that a significant amount of peracetic acid

c m be wasted when metal ions are present, resulting in Iess efficient bleaching. The metal ion

induced decomposition may proceed via a radical mechanism; however, the oxidation and

reduction cycle involving Mn(I1) and MnO,- prevails when manganese is present in the

peracetic acid solution at a pH of around 6.

In-situ addition of chelants such as DTPA or DTMPA to the peracetic acid solution

minirnizes the Cu(II) and Fe(II1) induced peracetic acid decomposition but increases the

M n 0 induced decomposition. This is explained by assuminç that the more stable complexes

of DTPA or DTMPA with Cu2+ and ~ e ) ' do not catalyse the decomposition reactions, while

the less stable DTPA-M~I~' complex does, and by the fact that only rnanganese participates

in an oxidation-reduction cycle, while Cu2+ and ~ e ~ ' do not.

A close inverse relationship between peracetic acid decomposition and pulp brightness

development was observed and confinned at constant pH and peracetic acid condition. A

chelation pretreatment stage for pulps with a high metal ion content, especially manganese,

lads to improved subsequent peracetic acid bleaching, while for pulps with a low content of

metal ions chelation prior to Pa treatment may not be necessary. The addition of sodium

pyrophosphate effdvely deactivates the metal ions and consequently improves the bleaching

performance. However, the addition of some chelants, such as DTPA, to the peracetic acid

bleach liquor may rmlt in poor bleaching due to the fact that peracetic acid is consumed by

wastefùl reactions with DTPA.

Oqgen is the main gaseous product produced fiom the decomposition of peracetic

acid under typical brightening conditions, Le. pH 7 and 60°C. One mole of oxygen is

generated per two moles of oxidants (peracetic acid and hydrogen peroxide) consumed. At

a relatively low pH (-5) CO2 rnay also be formed fiom the wastefùl reactions at a

stoichiometry of 3 moles of oxidant consumed per mole of CO2 released. Thus, based on the

stoichiometry of formation of OZ and COZ the arnount of oxidant consumed in wastehl

reactions can be calculated fiom the generation of these two gases. Furthermore, the latter

information allows the determination of the amount of peracetic acid used in pulp oxidation

as the difference of the peracetic acid consumed (i.e. charge minus residual) and that what is

wasted.

It is found that the pulp bnghtness gain is directly related to the peracetic acid

consumption in the oxidation of pulp, independent of the process conditions. As a result the

brightness gain of the pulp and brightening efficiency can be predicted fiom the chernical

consumption (peracetic acid and hydrogen peroxide) and the O3 and CO2 generation.

Simiiarly, the pulp viscosity loss is uniquely related to the loss in oxidant as measured by the

0, and CO, generation. These findings contribute a powerfbl tool for control and

optimization of an industrial peracetic acid stage.

Finaliy, a kinetic model for peracetic acid brightening of an ozone delignified pulp was

developed based on the reaction pathways identified in the literature. The kinetic equations

adequately describe the development of the light absorption coefficient of pulp at conditions

of constant pH and peracetic acid concentration. The model is based on three parallel

reactions, namely hydroxylation, oxidation ofconjugated quinones, and oxidation of carbonyl

containing lignin structures, which together are considered to be responsible for the overall

removal of chromophores, and thus for the decrease in the light absorption coefficient. The

reaction rate of each of the three reactions is dependent on the pH during the peracetic acid

treatment. At pH 3 only hydroxylation is significant. At pH 7, both the hydroxylation

reactions and oxidation of carbonyl containing lignin structures go to complet ion, while the

conjugated quinones are the only residual chromophores when the treatment is extended

beyond 50 minutes at 50°C. A fiirther increase in pH to 10.0 sigiificantly retards the

hydroxylation rate, resulting in a reduced removal level of chromophores compared to that

at pH 7.0.

RECOMMENDATIONS FOR FUTUlRE WORK

to study the kinetics of manganese catalysed decomposition of peracetic acid

to test the feasibitity of monitoring peracetic acid bleaching in a commercial bleach

plant by detemining the generation rate of gaseous products from peracetic acid

decomposition

to identifi alternative stabilisers for peracetic acid bleaching other than sodium

pyrophosphate

to investigate the stability and consumption of an aqueous solution of distilled

peracetic acid under typical peracetic acid bleaching conditions

to determine the activation energies of the reactions which are responsible for the

chromophore consurnption and generation during peracetic acid bleaching

to determine the kinetics of peracetic acid brightening for different pulp types

to develop a complete kinetic mode1 capable of describing industrial peracetic acid

brightening

APPENDIX

THE COMPUTER OPTIMIZATION PROGRAM FOR THE KINETIC STUDY OF PERACETIC ACID BIUGHTENING

C ******************************************************************* C * MAIN PROGRAM FOR THE MODELLING OF BLEACHING PROCESS * c ******************************************************************* C C ******************************************************************** C * THE ROLES OF THE MAIN PROGRAM ARE : * C * - READ IN AND MANIPLLATE DATA * C * - EXECUTE HOOKE AND JEEVES SEARCH ALGORITHM * C * - PRn\ST OUT TEMPORARY AND FINAL PARAMETER VALUES * C * - P W OUT FINAL COMPARISONS * c ******************************************************************** C

MAL CKE(12,l O),CKC(12,1 O),TIM( 12,l O),PAO( 12),PHV(I 2),TEMP(12) REAL CK 1 ( 12,l O), C D ( 1 2,l O), CKC0( 1 2,l O) REAL PARVAL(1 Z),PARSTP(I 2), STPSZL(12) REAL NEWStTM,CURSUM,PAROLD INTEGER STUMP,NLTMPAR,NUMSET,NUMPT,LSTONE,PARCNT MTEGER FLAG(lZ),SETNUM( 12), SETPTS( 1 2), TAG,FTAG,SET

C C c C # VARIABLE DICTIONARY # C # # C # CKE -- ARRAY OF EXPERTMENTAL ABSORPTION COEFFICIENTS # C # CKC -- ARRAY OF CALCULATED ABSORPTION COEFFICIENTS # C # TIM -- ARRAY OF THE SAMPLING TLME # C # PA0 -- ARRAY OF TOTAL PERACETIC ACID CONCENTRATION # C # PHV - ARRAY OF pH # C # TEMP -- ARRAY OF TEMPERATURE # C # CKI - ARRAY OF CK ACCOUNTED BY CARBONYL STRUCTURE # C # CK2 -- ARRAY OF CK ACCOUNTED BY HYDROXYLATION # C # CKCO -- ARRAY OF CK ACCOUNTED BY QUINONE STRUCTURE # C # NUMSET-- NUMBER OF DATA SETS # C # SETNUM-- ARRAY OF SET NUMBERS OF SIZE NUMSET # C # SEPTS-- ARRAY OF NT-JMBER OF POINTS IN EACH DATA SET # C # NUMPT -- NUMBER OF DATA POINTS # C # NUMPAR-- NUMBER OF PARAMETERS TO BE FIT # C # PARVAL- ARRAY OF CURRENT PARAMETER VALUES # C # PARSTP- ARRAY OF CURRENT STEP SIZES # C # STPSZL- ARRAY OF STEP SIZES USED TO KEEP TRACK OF # C # PROPER SEARCH METHOD # C # CURSUM-- REFERENCE SUM OR THE LOWEST SUM OF SQUARES # C # OF DIFFERENCE IN LIGNIN CONTENT # C # NEWSUM-- SUM OF SQUARES CALCULATED USING NEW #

C # PARAMETER C # C # VALUES, TO BE COMPARED WITH CURSUM # C # FLAG -- ARRAY OF TAGS IDENTIFYING WHICH PARAMETERS # C # HAVE CHANGED VALUE IN THE ITERATION # C # P, STUMP,LSTONE,PARCNT,TAG,FTAG # C # - COUNTERS OR TAGS # C C# C C TAG TO BYPASS ITERATION SCHEME C

TAG=O C C SET NLRLiZBER OF ITERATIONS C

WRITE(* ,12) 12 FORMAT(1 X,'NtTMBER OF ITERATIONS :')

RE AD(*, *) SET C C READ EXPERIMENTAL DATA FROM INPUT DATA FILE C

OPEN(2,FILE='CKSO. DAT', STATUS='OLD1) READ(2, *) NUMSET,NUMPT READ(2, *) ((TIM(I, J),CKE(I,J), J= 1 ,NUMPT), I= 1 ,NUMSET) READ(2,*) (PAO(I),PHV(I),TEMP(I), I= 1 ,NUIUSET)

C C READ INITIAL PARAMETER VALUE GUESSES AND STEP S E E S AND PRINT C

READ(2,*) NLTMPAR DO 30 I= 1 ,NUMPAR

READ(2, *) PARVAL(I), PARSTP(1) FL AG(I)=O WRITE(*,50 1) I,PARVAL(I)

50 1 FORMAT(2X,'INITIAL GUESS OF PARAMETER',IZ,'=',ZX,F 18.5) WRITE(*, 503) PARSTP(1)

503 FORMAT(2XtINITIAL STEP SUE=', 1 2X,F 1 5.7) 30 CONTINUE

CLOSE(2) C C WRITE INITIAL PARAMETERS INTO OUTPUT FILE C

OPEN(3,FILE='OUTSO. DAT',STATUS='NEW1) DO 777 I=l,NUMPAR WRITE(3.723) I,PARVAL(I),PARSTP(I)

723 FORMAT(1 X,'INITIAL GUESS FOR PAR4METERS',I3,3X'=',F l5.4,2X,F 15.4) 777 c o m m

WRITE(3,121) LSTONE= 1 PARCNT= 1

C C SET SUMTOO C

CURSUM=O. O C C CALL SUBROUTINE TO CALCULATE CK AND DETERMINE SUM OF C SQUARES C FOR THE INITIAL BASE CASE C

C ALL SUMCwSET,NUMPT,TMCKE,PAO,PW,TEMP,PARVAL, PARSTP, + CKC,CURSUM)

C C GO TO THE END OF PROGRAM IF TAG = 1 C

IF (TAG .EQ. 1) GOTO 999 570 CONTINUE

PARCNT= 1 C C START SEARCH PATTERN C

DO 5 15 I= 1 ,NUMPAR PARCNT=I

C C INCREASE THE PARAMETER VALUE BY CORRESPONDING STEP SEE C

PAROLD=PARVAL(PARCNT) PARVAL(PARCNT)= PAROLD+PARSTP(PARCNT)

C C CALCULATE THE NEW SUM OF SQUARES C 57 1 CALL SUMCAL(NUMSET,NUMPT,TIM,CKE,PAO,PHV,TEMP,PMVAL, PARSTP,

+ CKC,NEWSUM) C C CHECK IF NEW MINUMUNI H M BEEN FOUND C

IF (NEWSUM .GT. CURSUM ) GOTO 550 CuRSUM=NEWSUM FLAG(I)= 1

PAROLD=PARVAL(l) STPSZL(I)=PARSTP(I)

C C DECREASE THE PARAMETER VALUE BY STEP SIZE C

PARVAL(I)=PAROLD-STPSZL(1)-PARSTP(1) 531 C0NTNL.E C C CHECK IF NEW PARAMETER VALUE IS GREATER THAN O C

IF (PARVAL(1) .GT. 0.0) GûTO 532 STPSZL(I)=STPSZL(I)/8 PARVAL(I)=PAROLD- STPSZL(1)-P ARSTP(1) GOTO 53 1

532 CONTINUE GOTO 55 1

550 CONTINUE STP SZL(I)=P ARSTP(1)

C C DECREASE PARAMETER VALUE BY STEP SIZE C

PARVAL(PARCNT)=PAROLD-PARSTP(PARCNT) 533 CON- C C CHECK IF NEW PARAMETER VALUE IS GREATER THAN O C

IF (PARVAL(1) .GT. 0.0) GOTO 534 STPSZL(I)=STPSZL(I)/8 P ARVAL(I)=P AROLD-STPSZL(1) GOTO 533

534 C ONTrn 551 CONTINUE C C CALCULATE NEW SUM OF SQUARES C

CALL SUMCAL(NUMSET,NUMPT,TIM,CKE,PAO,PHV,TEMPARVAL,PARSTP, + CKC,NEWSUM)

C C CHECK IF NEW MTNlMUM HAS BEEN FOUND C

IF (NEWSUM .LT. CURSUM ) GOTO 180 PARVAL(I)=PAROLD GOTO 560

180 CONTTNUE PARSTP(I)=STPSZL(I) CURSUM=NEWSUM FLAG(I)= 1

560 CONTINUE C C EXAMINE NEXT PARAMETER IF NPT ALL HAS BEEN EXAMINED C 515 CONTINUE C C INCREASE ITERATION COUNTER AND CHECK IF LIMlT OF ITERATION H M C BEEN MET C

LSTONE=LSTONE+ 1 IF (LSTONE .GT. SET) GOTO 100

C C CHECK IF MINIMUM IS ENCLOSED C

DO 575 I=l,NUMPAR IF (FLAG(1) .EQ. 1 ) GOTO 5 74 PARSTP(I)=PARSTP(I)*0.90

574 CONTINUE FLAG(I)=O

575 CONTrNuE C C PRINT MTERMEDIATE PARAMETER VALUES AND SUM OF SQUARES C

STUMP=LSTONE- 1 WRITE(*,80 1) STUMP

80 1 FORMAT(ZX,' ITERATION NUMBER = ',2X,I4) DO 802 I=1 , M A R

WRITE(*, 803) I,PARVAL(I), PARSTP(1) 803 FORMAT(IX,' BEST VALUE OF PARAMETER ',12,'= ', 1 X,F18.5,3X,F10.4) 802 CONTINUE

WRITE(*,804) CURS UM 804 FORMAT(2X,'SUM OF SQUARES AT THIS STAGE = ',2X,F15.7)

GOTO 570 331 CONTINUE 100 CONTINUE C C PRINT FINAL PARAMETER VALUES AND SUM OF SQUARES C

WRITE(*, 120)

C C DETERMTNE MINIMUM S U M OF SQUARES AND FWAL PARAMETER VALUES C

CALL SCIMCmSET,NllMPT,TIM,CKE,PAO,PHV,TEMP,PARVAL,PARSTP, + CKC,NEWSUM) M'RITE(*, 122) NEWSUM

122 FORMAT(2X,'SUM OF SQUARES OF BEST FIT EQUALS ',FI 5.9) M'RITE(*, 120) WRITE(*, 12 1) WRITE(*, 120) DO 977 I=I,NUMPAR

WRITE(*, 123) I,PARVAL(I),PARSTP(I) 123 FORMAT(lX,'BESTFITFORPARAMETER',I3,3X'=',F18.5,2X,F15.4) 977 CONTINUE 999 CONTINUE

PAUSE C C PRINT COMPARISON OF EXPERLMENTAL AND PREDICTED VALUES C 124 WRITE(*, 1 20)

WRITE(*, 121) WRITE(*, 1 20) W T E ( * , 125)

125 FORMAT(IX,' TIME(MTN) CKC CKE') WRITE(*, 1 28)

1 28 FO RMAT(1 X,' --------- -------- ---------- ') WRITE(*, 120) DO 130 I=I,NUMSET

DO 132 J=l,NUMPT WRITE(*, 13 1) TIM(I,J),CKC(I,J),CKE(I,J)

13 1 FORMAT(4X,F8.2, I3X,F7.4,11 YF7.4) 132 CONTINUE 130 CONTINUE C

c WRITE OUTPUT DATA INTO FILE C

WRITE(3,122) NEWSUM WR..ITE(3,120)

WRITE(3,121) WRITE(3,120) DO 987 I=l ,NUMPAR WRITE(3,123) 1, PARVAL(I), PARSTP(1)

987 CONTINUE C C P W T COMPAFUSON OF EXPERIMENTAL AND PREDICTED VALUES C

WRITE(3,120) WRITE(3,121) WFUTE(3,120) WRITE(3,125) WRITE(3, I 28) WRITE(3,120) DO 133 I=l,NUMSET

DO 134 J=l,NUMPT WRITE(3,13 1) TIM(I,J),CKC(I,J),CKE(I,J)

134 CONTINUE 133 CONTTNUE

WRITE(3,lZO) WRITE(3,121) WRITE(3,120) WRJTE(3,165) WTE(3,168) WRITE(3,lZO)

165 FORMAT(1 X,' TIME(MIN) CK 1 CK2 CKX') 1 68 FORMAT(1 X,' ---------- ---------- ---------- --------- '1

DO 163 I=l,NUMSET DO 164 J= 1 ,NUMPT WRITE(3,169) TIM(I,J),CK 1 (I,J),CKZ(I,J),CKCO(I,J)

169 FORMAT(3X,F5 .Z,SXF7.4,S?C,F7.4,8X,F7.4) 164 CONTTNUE 163 CONTINUE

CLOSE(3) STOP END

C C c ******************************************************************** C * SUBROUTINE SUMCAL TO CALCULATE THE SUM OF SQUARES OF * C * THE DIFFERENCE OF EXPERIMENTAL AND MODEL PREDICTED DATA * C **************************************************#***************** C

SUBROUTINE SUMCALO\IUMSET,NUMPT,TIM,CKE,PAO,PHV,TEM, +PARSTP,CKC,SUM) REAL CKE(12,l O),CKC( 12, I O),TIM(12,1 O),PAO( I2),PHV( 12),TEMP( 1 2) REAL PARVAL(I2),PARSTP(12),CURSUM,CKO,C 10,C20,C 1 ,C2,C3,C4 REAL M,K 1 ,K2,K3,K4,R,CKALPHA,CK 1 CK2 RVTEGER NUMSET,NUMPT

C C SETSUMSTOO C

SUMl=O.O SUM=O.O

C C SET KNOWN PARAMETERS C

CKO=2.8 1 R=8.3 14 CKALPHA=PARVAL(6) CK1 CK2~2.8 1 -CKALPHA

C C CALCULATE CKC FOR ALL DATA SETS C

DO 840 I= 1 ,NUMSET PA=PAO(I) CKC(I,1)=2.8 1 M= 1 O* *(8.2-PHV(1)) DO 850 J=2,NUMPT

C 1 O=PARVAL( 1 ) C20=CK 1 CK2-C 1 O K 1 =PARVAL(2)*M*PN(I +M) K2=PARVAL(3)*M*PAI(l +M) K3 =PARVAL(4)*PA/( 1 +M) K4=PARVAL(S)*PA/( 1+M) C l=C 1 O*EXP(-(KI +K2)*TIM(I,J)) C2=C 1 0iK2*EXP(-(K 1 +K2)*TIM(I,J))/(K3-K 1 -K2) C3=(C2O+C 1 O*K2/(K 1 +K2-K3))*EXP(-K3 *TIM(I, J)) C4=CKALPHA* EXP(-K4*TIM(I, J)) CKC(I,J)=C 1 +C2+C3+C4

C C SUM THE SQUARES OF DIFFERENCES OF CKC AND CKE C

SUM I=SUM 1 +(CKC(I,J)-CKE(I,J))* *3 SUM=StTMl

850 c o w m

840 CONTINUE RETURN END

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