Embed Size (px)
DESCRIPTION
adr
Citation preview
January 2011
Preliminary Assessment of Alternative Alkalis for Brukunga Acid Water Treatment Plant
Report of vacation
student project
Prepared by
Mr Gerry Xu Supported by
Mine Completion Program, Minerals & Energy
Resources, Primary Industries &
Resources SA
Disclaimer • This document was prepared by and for the Mine Completion Program,
Department of Primary Industries and Resources (PIRSA). No other party should rely on this document without the prior written consent of PIRSA.
• PIRSA accepts no responsibility to any third party who may rely on information within this document.
• All prices quoted in this report are estimates only, and were provided to PIRSA for the purpose of this project only. Prices quotes in this report include freight and other costs and may not be relied upon by any third party.
Content 1. Introduction ................................................................................................................ 3
2. Overview of Chemicals Available to Treat AMD ..................................................... 4
3. Proposed Alternative Alkalis ..................................................................................... 5
4. Neutralisation Capability ........................................................................................... 6
4.1 Lime based Reagents Results ............................................................................ 7
4.2 Limestone based Reagent Results .................................................................... 7
4.3 Fly ash Results .................................................................................................... 8
4.4 Dense Soda Ash Results ..................................................................................... 8
4.5 Neutralisation Rate .......................................................................................... 10
5. Sludge Settling Experiment ..................................................................................... 16
5.1 Settleability and Sludge Volume ..................................................................... 16
5.2 Surface Water Quality in Settling Experiment ............................................... 20
6. Slurry preparation .................................................................................................... 24
6.1 Quicklime slaking ............................................................................................. 24
6.2 Limestone Grinding .......................................................................................... 32
6.3 Slurry Experiment ............................................................................................ 32
7. Results summary and Cost evaluation ..................................................................... 39
8. Recommendations .................................................................................................... 42
9. Reference ................................................................................................................. 43
Page 3
1. Introduction
Mining for pyrite and pyrrhotite commenced at Brukunga in 1955 and concluded in
1972. Mining was open-cut and has resulted in mine wastes of 8 million tonnes of
waste rock and 3.5 million tonnes of tailings. Following the completion of mining
activities, oxidation of the exposed rock and mine wastes has produced acid and
metalliferous drainage (AMD) at the site, with a leachate below pH 3. The AMD
leachate enters the main creek that travels through the site, Dawesley Creek. AMD
has made the creek water unsuitable for potable use, irrigation and stock watering, up
to 55 km downstream of the mine site.
The South Australian state government took up responsibility for remediation of the
site in the mid 1970s, and an acid water neutralisation plant was installed in 1981.
Ownership was transferred to PIRSA from SA Water in 1997. In 2003, Dawesley
Creek was diverted around the AMD generating areas of the mine site through a large
diameter pipe and an open channel. This resulted in a significant improvement in
water quality.
Water collected from the site is pumped through an acid neutralisation treatment plant
in order to raise the pH and precipitate out heavy metals. The reagent historically used
by PIRSA to neutralise the lime was carbide lime, a by-product of the acetylene
manufacturing process used by BOC Gases Australia based in Adelaide. The carbide
lime was supplied to Brukunga in 10-30 weight. % slurry form, in 20,000L tankers. In
2008, BOC gases moved production to Victoria and were no longer able to supply
PIRSA. The treatment process was subsequently converted to receive hydrated lime,
sourced from Adelaide Brighton Cement (ABC), Angaston Quarry, and transported
by Kalari to Brukunga at a total cost of $307.63 per tonne (Jan 2011 pricing). The
hydrated lime is stored on site in a mobile ISO storage facility rented from Kalari at
$44,200 per year. The cost associated with the use of hydrated lime in 2010 (assuming
800 tonnes/ year) was $246,104 (ex GST). The key risks associated with the use of
hydrated lime is that ABC is currently the only commercial South Australian supplier,
and that PIRSA is at risk of price increases or the product becoming unavailable
To explore opportunities for cost reduction and risk mitigation at the Brukunga acid
water neutralisation plant a range of lime reagents were tested to determine their
Page 4
effectiveness (i.e. pH increase, removal of heavy metals, sludge characteristics) and
financial viability.
2. Overview of Chemicals Available to Treat AMD
The trials of lime alkalis for the chemical treatment of AMD included most
commercially available neutralising agents in South Australia. It was found that few
alkalis can be applied practically and economically for AMD neutralisation at
Brukunga. Some alkalis have been successfully utilized in water treatment processes
elsewhere, and their main physical and chemical properties have been summarised in
Table 1. Commercially available alkalis usually contain additional substances (ie.
secondary alkalis and impurities) to the key active alkalis listed in table 1 (column 1).
High calcium lime may contain small amounts of magnesium oxide or magnesium
hydroxide (<5%). Dolomitic lime typically contains 35 to 46 percent of magnesium
oxide or magnesium hydroxide and also Ammonia (Anhydrous ammonia, NH3)
(Skousen et al. 1990).
Table 1 Physical and chemical properties of theoretically pure alkali component
Alkali Formula Molecular wt, g
Solubility, g/L Solubility product,
Ksp
Heat of solution at 18 deg C
Density, g/cm3
Calcium neutralizers Calcium
hydroxide Ca(OH)2 74.10 1.73
(20 deg C) 4.68×10- 6 +2.79 kg-
cal 2.21
Calcium oxide
CaO 56.08 React N.A. + 18.33 kg-cal
3.35
Calcium carbonate
CaCO3 100.08 0.015 (25 deg C)
4.8×10- 9 N.A. 2.71
Magnesium neutralizers Magnesium hydroxide
Mg(OH)2 58.30 0.0098 (18 deg C)
1.5×10- 11 -0.0 kg-cal 2.34
Magnesium oxide
MgO 40.32 0.08 N.A. N.A. 3.58
Sodium neutralizers Sodium
hydroxide NaOH 39.99 1110 N.A. N.A. 2.13
Sodium carbonate
Na2CO3 105.99 220 N.A. N.A. 2.54 (anhydrous)
Source from US EPA (1983)
The optimal choice for a neutralising reagent depends on both technical and economic
considerations. The technical factors include the acidity concentration, flow rate,
composition and concentration of heavy metals in the untreated acid water, the speed
Page 5
of the neutralising process, the volume of reagent needed for effective neutralisation
and the final treated water quality. The economic factors include the costs of reagents,
labour and machinery, the equipment required, the number of years that treatment will
be needed, interest rates and risk factors (e.g. OHSW or supply risks).
3. Proposed Alternative Alkalis
Preliminary investigations on alternative alkalis were conducted in January 2011. This
involved identifying and contacting a range of product manufacturers around the
country (predominantly in South Australia) including known suppliers of lime
products as well as waste licensing and regulatory organisations such as EPA and
ZeroWaste who might be aware of companies producing alkaline waste streams.
Product information sheets and Material Safety Data Sheets (MSDS) were obtained
and reviewed for each supplied product. Further information regarding the properties
of specific reagents is provided in Appendix A. A summary of alternative alkalis
proposed for this trial is shown in Table 2.
Table 2 List of alternative alkalis
No Supplier Product name
Effective alkali component Purity %
Price, $/tonne
#
Delivery inclusive
Packing option
1 Penrice Aglime CaCO3/MgCO3 94.81/1.25 39.03 Yes N.A
2 Penrice Nutrilime grits CaO/CaCO3 3.5/50-60 27 YES N.A.
3 Penrice Calsilt* CaCO3/Mg(OH)2 56-60/8-11 21 YES N.A.
4 Penrice Dense soda ash Na2CO3 99.2 378 YES Bulk
5 ABC Calfines* CaO/MgO 30-65/0.2-2 70 YES Bulk
6 ABC Quick lime CaO/MgO 88 283.30 YES Bulk 7 ABC Fly ash* CaO 5 150.55 YES Bulk
8 ABC Hydrated lime
Ca(OH)2 /Mg(OH)2
85-95 323.82 YES Bulk
9 Agricola Superfine lime CaCO3/MgCO3 65.1/17 32 YES N.A.
10 Agricola Fine Cal lime CaCO3/MgCO3 93/5 55 YES N.A.
* These products are considered waste or secondary products and no warrantee is made by the supplier as to their availability or product quality and/or consistency. Price or technical enquiries should be made with the supplier in the first instance. # at date of enquiry.
Following the desktop investigations, a laboratory study was undertaken to assess
basic performance characteristics of the reagents under the direction of the Mine
Completion Program. This study looked at neutralisation capacity, neutralisation rate,
Page 6
sludge settling rate and slurry preparation. A preliminary chemical cost and capital
cost assessment was also undertaken and is included in this report.
4. Neutralisation Capability
A laboratory neutralisation titration (without aeration) was performed using a 20L
bucket containing 5L of acid water from the Brukunga acid holding dams. To enable
precipitation of metal contaminants, alkali dosing of the acid water is aimed at
producing a target pH > 9.5. An incremental amount of reagent was added to the
water every 10-15 minutes. The buckets were agitated at 70-100 rpm. The efficiency
of the neutralizing agent and dosage is dependant on several factors including: particle
size, contact time, acid water properties (iron content, pH) and reagent reactivity
(Potgieter-Vermaak et al, 2005). All reagents were added to the acid water without
any pre-treatment (such as sieving, crushing or pre-mixing). It is possible that the
inconsistency of particle sizes between the reagents has affected the results. Results of
the neutralisation capacity testing are shown in Figure 1 and Table 3.
Table 3. Results for neutralisation titration experiment
No Supplier Product Formula of
major composition
Purity %
Calculated reagent demand for acid water
titration, g/L#
Actually consumed reagent in acid water titration (pH=8.2)
g/L
pH in water
solution (water-to-solid
ratio 4:1)
1 Penrice Aglime CaCO3 94.81 15.8 80 (pH=6.30) 7.50
2 Penrice Nutrilime grits CaCO3 35 31.7 86 12.20 3 Penrice Calsilt CaCO3 50 30.0 112 9.49 4 Penrice Dense soda ash Na2CO3 99.2 14.2 20 11.57 5 ABC Calfines CaO 40 20.3 26 11.65 6 ABC Quick lime CaO 88 9.2 16 11.05
7 ABC Fly ash CaO 5 162.0 750 (pH=4.37) 9.50
8 ABC Hydrated lime Ca(OH)2 85 13.1 18 12.2
9 Agricola Superfine lime CaCO3 65.1 23. 128 (pH=5.98) 8.20
10 Agricola Fine cal lime CaCO3 93 16.1 90 (pH=5.89) 8.10
# calculation based on acidity of AMD water: 15000 mg CaCO3/L
Page 7
4.1 Lime based Reagents Results
All lime (CaO or Ca(OH)2 products) (Quicklime, Hydrate lime, Cal fines) are capable
of raising the pH value over 11 with a relatively low dosage (<30 g/L).
The actual consumption of lime is higher than the theoretical calculated value,
possibly due to incomplete slaking and reaction of the reagents. The consumption of
quicklime is approximate 76% higher than the calculated value, 28 % for Calfines and
and 46% for hydrated lime by volume. All 3 lime reagents had small suspended lime
particles. . This indicates that the lime reagent reactions were kinetically limited rather
than thermodynamically limited. The optimal effectiveness of the lime appears
dependant on the slaking and feeding patterns (as these preliminary trials did not
include a pre-mixing / slurry input stage). Results show that Calfines were closest to
the theoretical calculated value which could be caused by the presence of additional
alkaline impurities (e.g. Mg(OH)2), not considered in the theoretical calculation, and
efficient mixing due to fine particle size. The accuracy of these results is limited by
the small scale of the experiment. The optimum dosage rates requires further
investigation prior to application.
4.2 Limestone based Reagent Results
Most reagents with a high content of calcium carbonate and magnesium carbonate
could not sufficiently increase the pH in AMD water to the required level (pH 9.5).
Aglime, Superfine lime and Fine Cal lime failed to raise the pH>6.0 in the
experiments (duration 1 hour) even with large dosages of 80-100 g/L. Only Nutrilime
achieved pH of 11.41 at the end of titration experiment with a dosage of 127 g/L.
Other components such as Mg(OH)2 in Calsilt and CaO in Nutrilime are likely to
contribute proportionately more to the pH increase. Calsilt achieved pH of 8.3 after
adding 112 g/L. The optimum dose for those reagents could be significantly impacted
by reaction time, particle size, solubility, etc. Reaction time should be measured to
identify the optimal dosage for limestone reagents, according to Potgieter-Vermaak et
al. (2005). A minimum dosage of 10g/L limestone with 120 minutes contact time was
required to reach a pH of 5.5, 100g/L with 30 minutes of contact time. These reagents
will be investigated further in the ‘neutralisation rate’ part of the experiment (section
4.5).
Page 8
4.3 Fly ash Results
Due to a low concentration of CaO, fly ash was unable to raise the pH to the required
level. Even with a high dosage (over 750 g/L) fly ash was incapable of raising the pH
to 5.
4.4 Dense Soda Ash Results
Dense Soda ash was highly soluble with 20 g/L of dense soda ash raising the pH value
to 8.5 which was higher than the theoretical calculation values.
pH response from neutralization of Brukunga acid water with various reagents
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
8.50
9.00
9.50
10.00
10.50
11.00
11.50
12.00
12.50
13.00
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 102 104 106 108 110 112 114 116 118 120 122 124 126 128 130 132 134 136 138 140 142 144 146 148 150
reagent mass (g/L)
pH
Quicklime(ABC) Hydratelime (ABC) Aglime (Penrice Nutralime (Penrice) Calsilt (Penrice) Dense Soda Ash (ABC)
Calfine (ABC) Flyash(ABC) Surfine lime(Agricola) Cal lime(Agricola) Target pH
Figure 1
AMD acidity= 15000 mg CaCO3/L
Page 10
4.5 Neutralisation Rate
The neutralisation reaction rate was measured by determining the change in pH over
time for a fixed dosage of the reagent. A summary of the results shown in Figure 4
illustrates the dependence of pH on the reaction time. The dosage to be added was
determined by the neutralisation titration experiment described previously in Chapter
4.0. In order to examine the best neutralisation rate both a large dosage (80-100 g/L)
and a low dosage (10-30 g/L) were applied. The results are in Table 4.
Table 4 Results for Neutralisation Rate Testing
No Supplier Product Dosage Results
1 Penrice Aglime 80 g/L Achieved pH>5.5 after 30 min, achieved
best pH=6.5 after 90 min. 20 g/L 2 hours, approach pH 4.45
2 Penrice Nutrilime grits 100g/L Achieved pH>5.5 after 30 min, achieved
best pH=11.26 after 50 min. 20 g/L 3 hours, approach pH 9.99
3 Penrice Calsilt 100g/L Achieved pH>5.5 after 48 min, achieved
best pH=7.75 after 111 min. 30 g/L 2 hours 12 mins, approach Ph 5.5
4 Penrice Dense soda ash 30g/L Achieved pH>9.5 in2 mins 5 ABC Calfines 30g/L Achieved pH>11.5 in2 mins 6 ABC Quick lime 16g/L Achieved pH>11.5 in2 mins 7 ABC Fly ash 600 g/L Achieved pH>5.5 after 60 min, 8 ABC Hydrated lime 18.5g/L Achieved pH>11.5 in2 mins.
9 Agricola Superfine lime 100g/L Achieved pH>5.5 after 67 min, pH<6.2
after 2 hours 30g/L 2 hours, approach pH 6.1
10 Agricola Fine cal lime 100g/L Achieved pH>5.5 after 60 min, pH<6.2
after 2 hours 30 g/L 2 hours, pH approach pH5.93
Hydrated lime, Quick lime, Calfines and Dense soda ash were capable of rapidly
neutralising the acid water and achieving a pH > 9.5. All limestone reagents required
at least 30 minutes to raise the pH to 5.5. Of these, only Nutrilime was capable of
raising the pH to 11 (after 50 min). Calsilt could reach a pH of 7.75 after about 2
hours. Other limestone reagents (Aglime, Super Fine Lime and Fine Cal Lime) were
not able to increase the pH over 6.5.
In order to examine the effects of dosage and contact time on pH, both large and small
dosage of Superfine lime, Fine cal lime, Ag lime, Nutrilime and Calsilt were tested
Page 11
(see Figures 2-1 to 2-5). These figures indicate that a larger dosage of limestone
corresponds to a reduced contact time to reach a given pH value (ie. shortens the
reaction time). It was found that 100 g/L of superfine lime with 15 minutes contact
time can increase pH >5, compared to 30 g/L with 50 minutes contact time to reach
pH >5. Both large and small dosages of limestone are capable of reaching pH 5.5 but
not over 6.5. This result was also observed for Nutrilime, Aglime and Calsilt.
Superfine Lime neutralization rate at different dosage
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
0 20 40 60 80 100 120 140
Reation duration
pH v
alue
Superfine lime(Agricola): 30g/L Superfine Lime(Agricola): 100g/L
Figure 2-1
Page 12
Fine Cal lime neutralization rate at different dosage
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
0 20 40 60 80 100 120 140
Reation duration
pH v
alue
Fine Cal lime(Agricola): 30g/L Fine Cal Lime(Agricola): 100g/L
2.00
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
0 50 100 150 200
pH v
alue
Reation duration
Penrice Aglime neutralization rate at different dosage
Penrice Aglime 80g/L Penrice Aglime 20g/L
Figure 2-2
Page 13
For limestone based reagents with low neutralising capacity the dosage and
neutralisation rate required to reach pH 5.5 are summarised in Figure 3. For all tested
reagents a higher dosage corresponds to an increased neutralisation rate.
22.5
33.5
44.5
55.5
66.5
77.5
88.5
99.510
10.511
11.512
12.5
0 50 100 150 200
pH v
alue
Reation duration
Penrice Nutrilime neutralization rate at different dosage
Penrice Nutrilime 100g/L Penrice Nutrilime 20g/L
2.00
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
8.50
0 20 40 60 80 100 120 140 160
pH v
alue
Reation duration
Penrice Calsilt neutralization rate at different dosage
Penrice Calsilt 100g/L Penrice Calsilt 30 g/L
Page 14
The results obtained for limestone reagents (Figure 4) are consistent with values
reported by Potgieter-Vermaak et al (2005). The efficiency of limestone neutralisation
is strongly influenced by the iron content of the acid water. This is a two stage process,
the first stage is associated with the neutralisation of pure sulphuric acid and the
second stage can be attributed to the increasingly thick layer of ferric hydroxide
precipitate forming on the limestone particles. The second stage is much slower than
the first. This is illustrated by the data in Figure 4, where a slight decrease in pH is
observed after the initial fast increase.
0
20
40
60
80
100
120
0 20 40 60 80 100 120 140
Dos
age,
g/l
Reaction time, min
Figure 3 Dosage and contact times required to reach a pH of 5.5 after treatment of the acid mine water
Penrice Aglime
Penrice Nutrilime grits
Agricola superfine lime
Agricola Fine Cal lime
Penrice Calsilt
Page 15
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
8.50
9.00
9.50
10.00
10.50
11.00
11.50
12.00
12.50
13.00
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180
pH v
alue
Reaction duration (min)
Neutralization Rate for 10 reagents
Quicklime(ABC): 16g/L
Hydratelime(ABC):18.5g/L
Aglime(Penrice): 80g/L
Nutrilime(Penrice): 100g/L
Calsiilt (Penrice): 100g/L
Dense Soda As(Penrice): 30g/L
Calfine(ABC):30g/L
FlyAsh (ABC): 600g/L
Superfinelime (Agricola) 100g/L
Fine Cal lime(Agricola) 100g/L
Figure 4
AMD acidity= 7000 mg CaCO3/L
Page 16
5. Sludge Settling Experiment
Precipitation by-products of AMD treatment commonly involve the formation of
gypsum sludge and hydroxide precipitates with heavy metals. The residence time of
the treated water, which is dictated by treatment volume and flow, is important for
adequate metal precipitation. The amount of sludge generated by AMD neutralisation
depends primarily on the quantity of water being treated, which in turn determines
how often the sludge must be transported from the treatment plant to the mine site for
storage. The chemical composition of the reagent used in the treatment process will
determine the sludge properties and should provide an estimate of the stability of the
various metal compounds in the sludge.
Ackman (1982) investigated the chemical and physical characteristics of treated AMD
derived sludge and concluded that the physical and chemical properties of the sludge
varied depending on the properties of acid water, the neutralising reagent and the
mechanical mixing or aeration device used during treatment. The most important
factor is the reagent’s settleability, which includes the settling rate and final sludge
volume. Ackman found that calcium hydroxide and sodium carbonate produced a
granular, dense sludge compared to a more gelatinous, loose sludge generated by
sodium hydroxide and ammonia. The chemical compositions of sludge were generally
composed of hydrated ferrous or ferric oxyhydroxides, gypsum, hydrated aluminium
oxides, calcium carbonate and bicarbonate, with trace amounts of silica, phosphate,
manganese, copper, and zinc.
5.1 Settleability and Sludge Volume
In this experiment, suspended solids were generated from the addition of lime
reagents to 2L of acid water and allowed sufficient time to complete the reaction
without addition of flocculants (polymer). The suspended solids were then poured into
a 1000 mL volumetric cylinder for determination of the settling rate and final sludge
volume (see Figures 5 and 6).
Quicklime, Hydrated lime and Nutrilime produced relatively large volumes of sludge.
The final sludge volume (>95% settled) was observed after 5 hours. Nutrilime and
hydrated lime generated a slower settling sludge than Quicklime, a final sludge
Page 17
volume of 98%-99% was observed after 24 hours of settling. Currently, the treatment
plant uses polymer as a flocculent to assist in sludge settlement.
Most limestone grit was insoluble in acid water. The residual grits were found in the
bottom of buckets for all three reagents. The settling of these reagents was fast as they
largely consisted of insoluble residuals. Aglime achieved a sludge settling volume of
27% (ie. 270mL sludge settled out from 1000mL of reaction mixture) after 5 hours.
Super fine lime achieved a sludge settling volume of 22% after 2 hours.
Fly ash recorded a high final sludge volume of 66% after 3 hours. This high sludge
volume can be attributed to the large dosage (750 g/L) that was added to the acid
water during the neutralisation experiment (which was still incapable of raising pH >
5.5).
Calfines was the only reagent capable of both (i) raising the pH to 11 and (ii)
generating a small volume of sludge without the addition of flocculants (as shown in
Figure 5). The final sludge settling volume of 32% was produced after approximately
5 hours.
Key findings in the experiment:
1. More sludge was produced over time as the pH of the acid water was increased by
reagent addition.
2. The amount of sludge produced as a percentage of the chemical added (termed as
it’s efficiency) remained the same across all pH ranges for calcium hydroxide, sodium
hydroxide, and sodium carbonate. For some reagents the amount of chemical dosage
changed both settling rate and sludge volume. High and low dosage of Nutrilime
generated significantly different sludge volume and settling rates. Low dosage sludge
had less volume and settled very quickly, while high dosage sludge settled slower
with a larger volume.
Page 18
3. Sludge volumes were lowest with sodium carbonate or calcium carbonate and
highest with calcium hydroxide after 5 hours of settling.
4. Longer settling time corresponded to greater sludge consolidation.
5. Limestone grits have the fastest settling rate, followed by dense soda ash. Lime has
a very slow settling rate without the addition of polymer.
0
20
40
60
80
100
0 5 10 15 20 25 30
Slud
ge v
olum
e pe
rcet
age
in t
reat
ed w
ater
v/
v %
Reaction time
Comparison settling rate of low and high dosage Nutrilime reagent (30 mins)
Nutrilime 128g/L
Dosage: 126 g/L, 24 hour settling
Dosage: 20 g/L, 0.5 hour settling
Page 19
05
101520253035404550556065707580859095
100105110
0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300
Volu
me
of s
ludg
e as
% o
f tot
al v
olum
e of
trea
ted
wat
er
Time (min.)
Sludge settling curve (fully neutralised acid water)
Aglime (Penrice) Nutrilime(Penrice) Calsilt
Dense Soda Ash Calfine Quicklime
Flyash Hydratelime Superfine lime
Figure 5 Photos of selected reagents after 24 hour settling experiment
Nutrilime Quicklime Aglime Dense soda
ash Calfines
Figure 6
Page 20
5.2 Surface Water Quality in Settling Experiment
Treated water was kept in a bucket for 10 days, with the pH, EC and TDS analysed
after 3 and 10 days. Results are illustrated in Figures 7-1 to 7-3.
Fly ash experienced a pH value drop from 6 to 3.9. Calsilt pH dropped to 6.8 after 10
days. Agricola cal fine lime increased water pH up to 7 after 10 days. The remaining
reagents experienced a slight increase in pH.
Treated water pH value change
0.000.501.001.502.002.503.003.504.004.505.005.506.006.507.007.508.008.509.009.50
10.0010.5011.0011.5012.0012.5013.00
14/02/11 17/02/11 20/02/11 23/02/11
Date
pH v
alue
ABC QuicklimeABC hydrate limePenrice AglimePenrice NutrilimePenrice CalsiltPenrice Dense Soda AshABC CalfineABC flyashAgricola Superfine limeAgricola cal fine lime
Figure 7-1
Page 21
Penrice soda ash recorded the highest EC value due to its solubility. Calsilt was the
second highest, and an increasing trend was observed that might indicate that Calsilt
grit continued to release minerals into the solution over a longer timeframe. A small
change in EC was observed for Quicklime, hydrated lime and Calfines. Agricola
super fine lime and Agricola cal fine lime had the lowest EC values which appeared to
drop slightly over time.
Treated water EC value change
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
22000
24000
26000
28000
30000
32000
34000
36000
38000
40000
14/02/11 17/02/11 20/02/11 23/02/11
Date
EC ,
um
ABC QuicklimeABC hydrate limePenrice AglimePenrice NutrilimePenrice CalsiltPenrice Dense Soda AshABC CalfineABC flyashAgricola Superfine limeAgricola cal fine lime
Figure 7-2
Page 22
Addition of contaminants
The next table shows that impurities in the reagents could potentially introduce
contamination to the final treated water quality.
No Supplier Name Formula for main component Concern 1 ABC Hydrated lime Ca(OH)2
2 ABC Calfines CaO, MgO, SiO2, Al2O3, Fe2O3, Na2O, K2O
Na, K, Al
3 ABC Quiklime CaO, MgO, SiO2 4 ABC Fly ash CaO, SiO2, Al2O3 SiO2 , Hg 5 Penrice Calsilt CaCO3, Mg(OH)2, SiO2, NaCl Na, K, Cl, B 6 Penrice Aglime Ca(OH)2, MgCO3, SiO2
7 Penrice Dense soda ash
NaCO3 Na
8 Penrice Nutrilime grits CaCO3, Ca(OH)2, SiO2 Na
9 Lilydale (Uninim) Quicklime
CaO, MgO, SiO2
Treated water TDS value change
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
22000
24000
26000
28000
30000
32000
34000
36000
38000
40000
42000
14/02/11 17/02/11 20/02/11 23/02/11
Date
TDS,
ppm
ABC QuicklimeABC hydrate limePenrice AglimePenrice NutrilimePenrice CalsiltPenrice Dense Soda AshABC CalfineABC flyashAgricola Superfine limeAgricola cal fine lime
Figure 7-3
Page 23
10 Traralgon (Uninim) Quicklime
CaO, MgO, SiO2
11 Lilydale (Uninim) Hydratelime
CaO, MgO, SiO2
12 Agricola Superfine lime CaCO3, MgCO3
Advantages and disadvantages of contamination risk of reagents
Name Formula for main
component
Advantages Disadvantages
Quick lime
CaO Quick lime neutralisation is a simple, effective and proven technique used to treated acid mine drainage. It normally can achieve a pH value >9. Most of metal concentrations (As, Cd, Cu, Fe, Pb, Mn and Zn) were removed below expected levels.
Aluminium might begin to re-dissolve if pH is raised > 7.0. That could account for relatively high Al concentrations existing in treated water when using quicklime or hydrated lime.
Hydrated lime
Ca(OH)2 Similar treatment results to Quicklime. Easier and safer than Quicklime to operate.
Less effective than quicklime.
Calfines CaO This is the same active ingredient as Quicklime (see above)
See Quicklime. Lower purity than Quicklime and may include other contaminants.
Nutrilime CaCO3/CaO Similar treatment results as quicklime. Easier and safer than Quicklime to operate.
Less effective than quicklime and hydrated lime
Ag lime Calsilt Superfine lime, fine cal lime
CaCO3 Generally, limestone can reach pH value about 6-6.5, which is good for Al(OH)3 and Arsenic precipitation. The material is much cheaper than lime products.
Low pH levels, poor performance in removal of all other metals ( e.g. pH>10 required for Mn removal, pH>7 required for Zinc removal.
Page 24
6. Slurry preparation
The Brukunga neutralisation process involves the input of dry hydrated lime which is
pre-mixed with water to form a slurry, before adding it to the acid water. AWQC’s
report (1995) showed that lime was more efficient when dosed as slurry. In the report,
lime slurry achieved the target pH 8.5 within 40 minutes while solid raw lime took
nearly 3 hours. The addition of lime grit slurry was not successful in raising pH due to
its low solubility and its lack of alkalinity. For lime and limestone, the larger available
surface area of a finer particle size results in a more efficient reaction. Therefore, the
lime and limestone used in AMD neutralisation typically requires particle sizes for
lime or limestone 95% less than 44 microns (or 325 meshes) and fed as a slurry. For
limestone, the particle size reduction is typically undertaken by a grinding process,
which includes a feeder, a ball mill and a hydro cyclone. For lime, particles of CaO
are premixed with water, which results in a chemical reaction called “hydration” or
“lime slaking”. Both lime slaking and limestone grinding processes were reviewed as
a part of this report, and a simple experiment was conducted for proposed reagents to
examine if pre-preparation was required for each reagent.
6.1 Quicklime slaking
Quicklime is manufactured by kilning high quality limestone at high temperatures
causing volatilisation of nearly half of the limestone’s weigh as carbon dioxide. The
reaction is as follows:
CaCO3 (limestone) + Heat CaO (calcium oxide) + CO2 (carbon dioxide)
In most acid water neutralisation applications, lime is used as calcium hydroxide
therefore calcium oxide must be converted to calcium hydroxide. The chemical
formula is:
CaO (calcium oxide) + H2O (water) Ca(OH)2 (calcium hydroxide) + Heat
1 kg of CaO and 0.32 kg of water will produce 1.32 Kg of Ca(OH)2, this is the
minimum water required for chemical reaction, so calcium hydroxide contains 75.7%
CaO and 24.3% H2O. The process of adding water to calcium oxide to produce
calcium hydroxide is referred to as the ‘hydration process’ or lime ‘slaking’.
Page 25
Generally, excess water is used for hydration (/ “slaking”) ranging from 2½ parts
water to 1 part CaO to 6 parts water to 1 part CaO (Hassibi M, 1999). The resultant
hydrate is in a slurry form. The slaking process of quick lime is an exothermic process
releasing a great quantity of heat
EQUIPMENT USED FOR THE SLAKING PROCESS
Based on the review by Hassibi M (1999), there are four types of lime slakers
available:
1. Slurry detention slakers
This is comprised of two chambers, the first chamber is a slaking chamber and the
second chamber is used as a grit removal chamber. The lime slurry flows by gravity
from the first chamber to the second chamber. Cold water is added to the grit chamber
to reduce viscosity allowing the heavier grit to settle to the bottom of the chamber.
The grit is elevated and discharged by a screw.
Figure 8-1. Typical slaker flow pattern (detention type slaker) (Hassibi M, 1999).
Page 26
2. Paste Slakers
In a paste slaker, the hydroxide paste is too heavy to flow by gravity so a pair of
horizontal rotating paddles pushes the paste forward toward the discharge point.
Dilution allows grits to separate by gravity.
Figure 8-2. Typical Paste Slakers (Hassibi M, 1999).
3. Ball mill slakers
Ball mill slakers are generally used where the capacity required is too large for other
types of slakers, zero discharge is allowed at the site or the water available is too high
in sulphates for regular slakers. Ball mill slakers are also used for wet and dry
grinding of limestone grit.
Page 27
Figure 8-3. Typical ball mill Slakers (Hassibi M, 1999).
4. Batch slakers
The batch slaking process uses the desired batch to calculate how much water and
lime is needed to make one batch. Cold water of a predetermined quantity
(volumetrically or gravimetrically) is added to the batch tank. The quicklime is added
in a predetermined quantity (volumetrically or gravimetrically) to the batch tank. The
lime and water are mixed and agitated until the mixture’s temperature reaches a
temperature between 77°C to 82 °C. Once the desired temperature is reached, the
resultant slurry is dumped into a second tank for use or grit removal
Page 28
Figure 8-4. Typical batch Slakers (Hassibi M, 1999).
FACTORS AFFECTING THE SLAKING PROCESS
The most important single factor that affects the process efficiency of a slaking
system is the specific surface area of calcium hydroxide particles. The larger the
specific surface area of the lime particles, the more surface is available for reaction.
The following factors affect slaking efficiency by affecting the specific surface of the
calcium hydroxide, directly or indirectly (Hassibi M, 1999):
A. Type of limestone used in calcination
B. Calcination process to manufacture CaO
C. Slaking temperature
D. Lime to water ratio
E. Degree of agitation during slaking
F. Viscosity of slurry
G. Slaking time
H. Water chemistry
I. Air slaking
Page 29
COMPARISON OF FOUR TYPES OF SLAKING PROCESS
Table 5. Comparison of 4 types of slakers (Hassibi M, 1999, Hassibi, 2003).
Slakers Capacity Water-
lime ratio
Available lime in slurry
Particle size of lime
Reactivity Grit Separation method and
disposal
Preventative maintenance
Safety Initial investment and equipment life
(1) Detention slakers
150 lb/hr-15 tons/hr Top limitation: 25 tons/hr
3.3: 1 to 5:1
Most impurities are removed by the grit removal system, but top size limited to 25 tons per hour
very fine particles ranging in size 90% less than 20 microns, 100% less than 42 microns.
Very high due to fine particle size achieved by chemical reaction of lime and water.
This method is very simple, trouble-free and requires very little maintenance.
Daily Only ½-hour per day to inspect the slaker inlet and inside, and hose down of build-up with high-pressure water. Also cleaning dust and steam removal system. Weekly None if daily routine is done properly. Monthly None if daily routine is done properly. Six Month Drain slaking and grit removal chamber and check for any abnormal wear
Three independent control loops monitor the operation of the slaker and act as back-up if one loop fails. A safety door latch and proximity switches prevent opening access doors while the slaker is running.
Initial investment is relatively low compared to the Ball Mill slakers. Its price is about one quarter of the cost of a mill system of the same capacity. With proper maintenance, the life expectancy is 20 years.
Page 30
Slakers Capacity Water-
lime ratio
Available lime in slurry
Particle size of lime Reactivity
Grit Separation
method and
disposal
Preventative maintenance Safety
Initial investment and equipment life
(2) Paste Slakers
1000lb/hr - 8000lb/hr Top limitation: 10000 lb/hr
2.5:1 Similar to Detention slakers. Most grit and impurities are removed by the grit removal system.
Due to small physical size, it would be suitable for some retrofits - where space is very limited and final slurry quality is not critical. Due to the lack of temperature control, the slaker temperature varies. Also, hot spots develop causing agglomeration of fine particles. Final product is not uniform.
Medium to high reactivity.
This method of grit removal is a simple and compact design, maximum slurry concentration is 15%.
Daily: When the unit is operated in Batch mode, at the end of each batch the paste must be thinned to slurry by adding water. This thinning will result in larger grit to settle at the bottom and, during the next batch run, rub against the paddle shaft and cause erratic operation of the torque valve. Weekly: Must empty large grit accumulated at the bottom of the slaker by draining and flushing. Six Month Torque water valve needs adjustment every six months or so to keep the paste consistency correct
No major safety issues other than the fact that the slaker access doors do not have any provision for locking.
Initial investment is relatively low –similar to detention slakers. With proper maintenance, 20 years.
Page 31
Slakers Capacity Water-
lime ratio
Available lime in slurry
Particle size of lime Reactivity
Grit Separation method and
disposal
Preventative maintenance Safety
Initial investment and equipment life
(3) Ball mill slakers
1000lb/hr -50 tons/hr Sizes less than1000 lb/hr per hour not available.
N.A. • Final product contains 5% to 10% inert grit. • Generally no temperature control used for process, therefore, slurry particle surfaces are not uniform.
Water with high sulphate or sulphite content can be used for slaking. Particle size of hydrate is coarse compared to Detention, Paste or Batch slaker.
100% Generally none. In some cases, a hydrocyclone is used for separation of coarse grit. In this case, the final particle size of grit carried in the slurry will be finer than other types of slakers.
The mill lining wear must be checked once a year and replaced as need.
Generally, mills are safe. Sudden steam release is minimal.
Initial investment for this type of slaker is very high: • Mill cost three to four times of a Detention slaker. • Space requirements five to six times of a Detention slaker. • Foundation cost is extremely expensive. With proper maintenance and upkeep, 25 – 30 years
(40 Batch slakers
<2000lb per batch. Limited capacities are available. Higher cost than continuous slakers. Generally used with pulverized quicklime. Grit removal not readily available.
< 2.5:1 The quality of hydrate is high with fine particles. No grit removal is required. Approximately 5% to 10% of total produced slurry is inert grit. Higher raw material cost for pulverized quicklime in some areas.
N.A. Very high with particles of high surface areas.
Typically not used. If required, it can be added by the addition of a tank and a vibratory screen separator. In this case, the slaker must be elevated approximately 10’ to allow gravity feed.
Very little required. Monthly Inspection of slaker inside for build-up and cleaning by high pressure water.
Safest of all slakers with minimal attention.
Very low, and long life cycle
Page 32
6.2 Limestone Grinding
Figure 9 shows a typical process flow diagram of a limestone grinding system. The
limestone is grinded by a ball mill or other special grinding machine. Other grinding
methods are similar (i.e. a ball mill slaking system) which means relatively high initial
investment.
Figure 9. Sketch for typical grinding system
6.3 Slurry Experiment
This experiment was designed to examine the slurry forming process and examine if
pre-preparation was required for each reagent. To aid the investigation, all proposed
reagents were mixed with mains water (pH=8) at different water-to-solid ratios (9:1 to
1:1) in 20 L buckets and mechanically stirred (rpm 70-100) for 2 minutes. The
characteristics of slurry was examined and recorded.
Page 33
SPECIFIC GRAVITY (SG)
The density (and specific gravity, SG) of the slurry is determined by measuring the
mass of slurry within a 100 mL measuring cylinder. Specific gravity measurements of
lime slurries have been used as a proxy measurement of alkali concentration (e.g. in
terms of wt% CaCO3). This type of theoretical relationship (which could be developed
for each specific alkali) allows alkali concentration to be identified from a table, or
chart, relating lime slurry specific gravity to alkali concentration. For Hydrated lime,
the lime SG vs. solid Ca(OH)2 concentration table (Appendix B Table B, US EPA
1983) can be used to determine an approximate solid lime (as Ca(OH)2) concentration
in slurry form for lime reagents. Figure 10 compares estimated wt% Ca(OH)2
concentrations for Hydrated lime (ABC) and Quicklime (ABC) at different water-to-
solid ratios.
0.0%
5.0%
10.0%
15.0%
20.0%
25.0%
30.0%
35.0%
Water-to-solid ratio
Appr
ox.%
sol
ids
lime
(Ca(
OH)
2 ,%
Quicklime(ABC)
Hydratelime(ABC)
Quicklime(ABC) 3.3% 11.5% 30.8%
Hydratelime (ABC) 8.6% 15.3% 24.2%
9:1 4:1 2.3:1
Figure 1 Approx. solid concentration in Hydrate lime and Quicklime slurry Figure 10
Page 34
TEMPERATURE
During slurry preparation for three reagents (Quicklime, Calfines and Dense Soda
Ash), significant temperature increases where experienced. Reaction temperature
versus time for these reagents is shown in Figure 11. Quicklime and Calfines both
contain CaO, which reacts with water to release heat. The reactivity of Quicklime is
characterized by the time required for the reaction temperature to reach a specific
value. Both Quicklime and Calfines are capable of lifting temperature above 50oC.
Quicklime increased temperature much more quickly with a higher reactivity than
Calfines. In addition, hot steam generation should be considered carefully when
designing a feeding system or choosing an appropriate slaker.
Figure 11
Page 35
DENSITY
For ease of operation, 100 mL volumetric cylinders were used to determine the mass
and volume of slurry mixtures. The density was calculated by the following formula:
Slurry density= Slurry mass / Slurry volume (g/mL).
A comparison of slurry densities for 10 reagents at different water-to-solid ratio is
shown in Figure 12. This reflects the ability of the various reagents to be suspended in
a slurry mix (with progressive addition of solids to an initial volume of water),
however this does not reflect the respective neutralisation capacity of each mixture.
As reagents vary in their effectiveness and purity, a particular slurry density value will
not equate to the same neutralisation capacity across different reagents. A target slurry
density of around 35wt% (as CaCO3) is seen as desirable for the practical operation
of the Brukunga acid neutralisation plant (P Grindley, pers. comm. Nov 2010).
Dense soda ash and fly ash were capable of achieving the highest slurry densities,
followed by the lime products: Hydrated lime, Quicklime and Calfines. Fine cal
limestone and Superfine lime achieved a medium slurry density. The other limestone
grits (Aglime, Nutrilime, Calsilt and Fine Cal lime) all have relatively low density
slurry and leave large residuals in the bucket. Higher density slurries provide greater
alkali content, but may require more energy consumption for pumping.
RESIDUALS
Large quantities of residual solids (not able to be suspended in the slurry mixture)
were found for most limestone products. Over 75% of Aglime, Nutrilime, Calsilt and
Fine cal lime remained as insoluble residuals after mixing with water at a 1:1 water
solid ratio (50wt%). These quantities of residual solids typically built up progressively
with reagent addition but were only measured following the final addition. Compared
toother limestone products, Superfine lime performed well, with only approximately
25% remaining as an insoluble residual, indicating that the finer particle size was a
benefit to the slurry forming process.
Fewer additions (and density measurements) were conducted for Quicklime, Hydrated
lime and Calfines as these were able to produce denser (and greater neutralisation
Page 36
capacity) mixtures with less addition of solids, compared to many of the other
reagents. Residuals were rarely found in limes - only Cafines with 5% residual after
mixing with water at ration 2.3:1.
Page 37
Slurry density for 10 reagents at different water-to-soild weight ratio
0.90.920.940.960.98
11.021.041.061.081.1
1.121.141.161.181.2
1.221.241.261.281.3
1.321.341.361.381.4
9:1 4:1 2.3:1 1.5:1 1:1
Water-to-solid weight ratio
Dens
ity o
f slu
rry, g
/mL
Penrice Aglime Penrice nutrilime Penrice Calsilt Penrice Dense Soda Ash Calf ine(ABC)Quicklime(ABC) Flyash(ABC) Hydrate lime(ABC) Superfine lime (Agricola) Fine Cal lime
Figure 12
Page 38
Residuals in slurry preparation
05
1015202530354045505560657075808590
0 10 20 30 40 50 60
water-to-solid ratio (wt% reagent added)
wt%
non
-slu
rrie
d re
agen
t
Ag Lime
Nutrilime
Calsilt
Dense Soda Ash
Calfines
Quicklime
Fly Ash
Hydrated Lime
Surperfine lime
Fine Cal lime
Figure 13
Page 39
7. Results summary and Cost evaluation Table 6 Summary for neutralisation capacity and chemical cost
No Supplier Reagent name Reagent dosage
(g/L AMD treated
Final treatment
pH (max. 2 hours) Reaction time
pH of superatant
water after settling
(48 hours)
Sludge volume
percentage in
suspensions after
treatment ( 5 hour
settling)
Chemical cost,
per tonne acid
water acidity=
7000
mgCaCO3/L
1 Penrice Aglime 80 5.38 1 hour 28 mins 5.95 27% (v/v) $3.12 20 4.45 2 hour N.A. N.A. $0.78
2 Penrice nutrilime grits 86 12.05 49 mins 11.98 100% (v/v) $2.70 20 9.99 2 hour 28 mins N.A. N.A. $0.54
3 Penrice CALSILT* 112 7.67 1 hour 6.60 31% (v/v) $2.10 30 5.5 2 hour 12 mins N.A. N.A. $0.63
4 Penrice Dense soda ash 20 9.65 <2 mins 9.63 47% (v/v)
$11.34 5 ABC CALFINES* 26 11.85 <2 mins 11.84 40% (v/v) $2.31 6 ABC Quiklime 16 11.80 <2 mins 11.91 94 % (v/v) $4.99 7 ABC Fly ash* 750 6.70 1 hour 49 mins 5.40 66% (v/v) $124.21 8 ABC Hydrated lime 18 11.84 <2 mins 11.80 100% (v/v) $5.99
9 Agricola Superfine lime 128 6.15 1 hour 56 mins 6.50 22 % (v/v) $4.95 30 6.10 2 hours N.A. N.A. $1.49
10 Agricola Fine cal lime 90 5.82 2 hours 6.05 N.A. $5.50 30 6.15 1 hour 56 mins N.A. N.A. $1.65
* These products are considered waste or secondary products and no warrantee is made by the supplier as to their availability or product quality and/or consistency. Price or technical enquiries should be made with the supplier in the first instance. Note: Chemical cost is based on simply bucket experiment in section 4. For limestone reagents, chemcial cost are calculated both low dosage and high dosage, which could also depend on reaction time and wanted pH
Page 40
0
10
20
30
40
50
60
70
80
90
100
110
120
$0.00
$2.00
$4.00
$6.00
$8.00
$10.00
$12.00
Penrice Aglime, 80g/L
Penrice nutrilime grits, 100g/L
Penrice CALSILT, 100g/L
Agricola-Superfine lime, 100g/L
Agricola-Fine cal lime, 100g/L
Alk
ali c
ost p
er t
onne
aci
d w
ater
($/t
)Alkali (limestone) cost and time for approaching pH=5.5 with high dosage
(per tonne acid water; acidity= 7000 mg CaCO3/L)
Alkali cost Retention time for approaching pH=5.5
Retention time for pH
=5.5
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
$0.00
$2.00
$4.00
$6.00
$8.00
$10.00
$12.00
Penrice Aglime, 80g/L Penrice nutrilime grits, 100g/L
Penrice CALSILT, 100g/L Agricola-Superfine lime, 100g/L
Alk
ali c
ost p
er t
onne
aci
d w
ater
($/t
)
Alkali (limestone) cost and time for approaching pH=5.5 with low dosage (per tonne acid water; acidity= 7000 mg CaCO3/L)
Alkali cost Retention time for approaching pH=5.5
Retention time for pH
=5.5
Page 41
Table 7 Summary for slurry preparation and initial investment evaluation
No Supplier Reagent name Slarking or
grinding process
Initial Investment for slaking
or grinding equipment
1 Penrice Aglime Yes High
2 Penrice nutrilime grits Yes High
3 Penrice CALSILT Yes High
4 Penrice Dense soda ash No Not sure
5 ABC CALFINES Yes If choose batch slaker, Low
If choose other slaker, High
6 ABC Quiklime Yes If choose batch slaker, Low
If choose other slaker, High
7 ABC Fly ash No -
8 ABC Hydrated lime No -
9 Agricola Superfine lime No -
10 Agricola Fine cal lime Yes High
Alkali cost per tonne acid water (acidity= 7000 mg CaCO3/L)
$0.00
$2.00
$4.00
$6.00
$8.00
$10.00
$12.00
Penrice Dense sodaash
ABC-CALFINES ABC-Quiklime ABC-Hydrated l ime
Alk
ali c
ost
per
tonn
e ac
id w
ater
($/t
)
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
10.00
11.00
12.00
13.00
Alkali cost Final pH value
Fil
H
l (f
ll
lid
Page 42
8. Recommendations
1. Quicklime is cheaper than hydrated lime and a batch mode operation (section 6.3)
of lime slaking might be suitable for the current treatment plant. Treated water quality
may be similar to hydrated lime, owing to the high purity of CaO in Quicklime.
2. Based on these preliminary tests, Calfines is the most cost-efficient of all reagents
(at a third the chemical cost of Hydrated lime and half the cost of quicklime). Calfines
can be used by the current treatment plant by a batch slaking process. The treated
water quality may be of some concern. Low sludge volumes and a short settling time
show that Calfines derived sludge is different to hydrated lime and quicklime derived
sludge. Further investigations are recommended.
3. Limestone can be used in pre-treatment processes such as anoxic limestone drains,
open limestone channels or a two-stage neutralisation. Nutrilime is the most cost-
efficient limestone reagent; it can raise pH value up to 11. After Calsilt (pH 6.6-7), the
other limestone reagents are not capable to raise pH above 6.5. A grinding system is
required for most limestone (except superfine lime).
The concept of the two-step neutralisation process is to remove AMD constituents
that form hydroxides in low solubility (constituents like, Fe3+, Al, Si, As, etc.).
Raising the pH to approximately 5 is the first neutralisation step, precipitating
remaining heavy metals is the second neutralisation step. Limestone is usually used as
the first step reagent and lime as the second step reagent. In addition, reagents could
generate industrially useful material. Herrera et al. utilized two-step neutralisation:
limestone for first step neutralisation and sodium hydroxide for second neutralisation
to generate ferrite compounds (Herrera S, Uchiyama et al. 2007).
Page 43
9. Reference
ASTM C25 - 06 Standard Test Methods for Chemical Analysis of Limestone,
Quicklime, and Hydrated Lime.
Ackman, T. 1982. Sludge disposal from acid mine drainage treatment. U.S. Bureau of
Mines, Report of Invest. 8672, Pittsburgh, PA.
E.F. Hively, “Practical Lime Slaking,” Alis Mineral System Grinding Division.
Hassibi M, 1999 An overview of lime slaking and Factors that affect the process,
Chemco Systems, L.P. http://www.agtgroup.cl/mining/
Hassibi M, 2003 A Review Of Lime Slakers And Their Advantages And
Disadvantages, Chemco Systems, L.P. http://www.agtgroup.cl/mining/
Hassibi M, 2005 A New Approach for Particle Size Reduction in Lime Slaking and
Wet Limestone Grinding, Chemco Systems, L.P. http://www.agtgroup.cl/mining/
Herrera S, P., H. Uchiyama, et al. (2007). "Acid mine drainage treatment through a two-step neutralisation ferrite-formation process in northern Japan: Physical and chemical characterization of the sludge." Minerals Engineering 20(14): 1309-1314. Skousen, J., K. Politan, T. Hilton, and A. Meek. 1990. Acid mine drainage treatment
systems: chemicals and costs. Green Lands 20(4): 31-37.
US EPA, 1983. Design Manual: Neutralisation of Acid Mine Drainage, US EPA
Industrial Environmental Research Laboratory, Report EPA-600/2-83-001, Cincinnati
OH
