Neutralization of Acid Mine Water
and Sludge Disposal
Report No 1057/1/04
Acid mine waters contain high concentrations of dissolved heavy metals
and sulphate, and can have pH values as low as 2.5. Unless
treated, such waters may not be discharged into public
streams. The acid water is formed as a result of
bacterial oxidation when pyrites are exposed to oxygen and
water after or during the mining process. Currently, acid
water is neutralized with lime before it is re-used (e.g. for coal
washing in the coal mining industry) or discharged into public
streams. The following disadvantages can be linked to lime
Neutralization is generally the first step in treating acid mine water
(gold, operational and abandoned coal mines). In Gauteng a
volume of 240 Mℓ/d of acid mine water from gold mining requires
treatment. At an acidity of 3 g/ℓ (as CaCO3),
a lime (CaO) price of R360 and a purity of 93 % the neutralization cost
would amount to R57 million/a. It is therefore essential that
the most suitable and cost-effective technology should be identified or
- Scaling of equipment by the unstable water produced.
- Malfunctioning of lime dosing equipment. Settling
of lime particles in pipelines and valves often causes blockages, which
may result in under-dosage and acid corrosion.
- Lime is costly. The cost of slaked lime (HDS
process), unslaked lime (HDS process), unslaked lime (modified HDS
process) and limestone amounts to R4.93/m3,
R3.36/m3, R2.48/m3 and
R1.57/m3 respectively for the treatment of water
with an acidity of 10 g/ℓ (as CaCO3).
The aim of this project (see Objectives) is to identify the most cost
effective neutralization process which meets the following criteria for
water with a specific chemical composition:
Legislation requires that sludge from neutralization plants be
discharged into lined ponds to prevent metal leachate from
polluting underground water. If leachate studies could show
that sludge is stable with respect to leachate of metals, as long as it
is not contacted with acid water, such information could be used to
assist in formulating a strategy for sludge disposal in a less costly
way (e.g. to use open cast or underground voids for discard of sludge)
than in costly lined ponds.
- Treated water which is neutral and stable with respect to
- Sludge with a high solids content.
- Minimum alkali cost.
- Minimum capital cost of plant and
- Confidence in the selected process.
Sludge disposal in lined ponds is costly due to the following:
Other, more cost effective methods of sludge discharge, such as worked
out open cast or underground voids, may be used for this
purpose. This approach would be in line with the accepted
backfill approach in the gold, nickel and copper mining industry where
waste rock is returned to underground. With the proposed
approach, ferric hydroxide (Fe(OH)3), which is
stable, would be returned to its origin, and not pyrites (FeS2),
which could be oxidized to generate acid. Acid water in
underground voids can be pumped to the surface and treated in an
integrated plant. The amount of sludge produced from the
integrated process will amount only to 5% the volume of dischard
leachate with an acidity of 15 g/ℓ (as CaCO3).
Benefits of this approach are:
- Large masses of sludge is produced. An estimated
amount of 20 t/d of sludge is produced from 1 Mℓ/d of discard leachate
when neutralized with lime or limestone.
- Plastic lining of sludge pond. Sludge produced
from acid mine water is classified as a class 3 waste due to its metal
content and must be discharged into a lined pond.
- Cost reduction as costly sludge disposal ponds are not
- Reduced seepage to underground water. Settled
sludge has a low permeability and will reduce the rate of seepage to
- Neutralization capacity is created in terms of underground
acid water. Sludge contains unused alkali (e.g. 10% to 30 %
of the lime used for neutralization in the HDS process is not utilized
for neutralization) which can be used to neutralize underground acid
- Aesthetic benefits. No waste from water treatment
needs to be stored at the surface.
Against this background the following aims were set for the project:
- Biological iron(II) oxidation. Determine the
conditions required for rapid iron(II) oxidation under acidic
conditions within a residence time of one hour.
- Integrated neutralization process. Obtain design
criteria for the treatment of low acidity water (2 000 mg/ℓ acidity (as
CaCO3) and 300 mg/ℓ iron(II) (as Fe)) with the
neutralization and iron(II) oxidation process.
- High density sludge (HDS) process. Optimize the process
flow diagram of the HDS process to meet the following criteria for
different water qualities:
- produce sludge with a high solids content (greater than
for water containing 10 g/ℓ acidity (as CaCO3))
- produce sludge with a rapid settling rate
- achieve maximum lime utilization (greater than 95%).
- Leachate studies. Determine
the stability of mine water sludge (neutralized with lime and
limestone) with respect to re-dissolution and metal leachate as a
function of pH, for the following wastes:
- Sludge produced during treatment of
acid mine water with lime or limestone.
- Coal discard (rich in FeS2) to
that discard leachate is the main source of acid generation of the
various wastes produced during coal mining.
The following findings were made during the investigation:
Biological iron(II) oxidation
Iron(II) should be oxidized to iron(III) before the neutralization of
acid water with limestone, otherwise the oxidation will occur
downstream of the neutralization plant with the formation of acid. This
study aimed at investigating the kinetics of biological iron(II)
oxidation in a plate reactor and to identify the suitability of a plate
reactor for biological iron(II) oxidation. The study showed
that the highest achievable rate was 120 g Fe2+/(ℓ.d)
mℓ/min; T = 20.5°C; surface area = 847 m2/m3).
kinetics of the biological iron(II) oxidation in a plate reactor can be
Biological iron(II) oxidation to achieve low iron(II) concentrations is
needed as pre-treatment to enable effective limestone neutralization.
The effect of various parameters on biological iron(II) oxidation was
investigated, including oxygen transfer, iron(II) concentration,
support medium surface area, type of support medium, reactor
configurations and flow regime. The study showed that the kinetics of
biological iron(II) oxidation follow the rate equation:
where, Rf = reciprocating frequency
(oxygenation), and A = support medium surface area.
By treating acid water with a pH of 2 and an iron(II) concentration of
3000 mg/ℓ, an oxidation rate of 74 g Fe/(ℓ medium.d) and effluent
iron(II) concentration of 300 mg/ℓ was attained in a continuously
operated submersed packed-column reactor (at 24 °C).
The medium used was silica sand (particle size of 4.75 to 6.35 mm) at a
cost of R100/t. At a loading rate of 20 g Fe/(ℓ medium.d) the iron(II)
is removed to less than 60 mg/ℓ in the effluent.
Integrated neutralization process.
A novel process is described for the neutralization of acid streams
produced during coal mining and processing. The leachate from
a waste coal dump was neutralized with limestone for the removal of
iron, aluminium and sulphate. Specific aspects studied were
the process configuration, the rates of iron(II) oxidation, limestone
neutralization and gypsum crystallization, the chemical composition of
the effluents before and after treatment, the efficiency of limestone
utilization and the sludge solids content.
The study showed that the acid content was reduced from 12 000 to 300
mg/ℓ (as CaCO3), sulphate from 15 000 to 2 600
mg/ℓ (as SO4), iron from
5 000 to 10 mg/ℓ (as Fe), aluminium from 100 to 5 mg/ℓ (as Al) while
the pH increased from 2,2 to 7,0. Reaction times of 2.0 and
4.5 h are required under continuous and batch operations respectively
for the removal of 4 g/ℓ iron(II) (as Fe) . The
iron(II) oxidation rate equation is a function of the iron(II),
hydroxide, oxygen and suspended solids concentrations. The
optimum suspended solids concentration for iron(II) oxidation in a
fluidized-bed reactor is 190 g/ℓ. Upflow velocity has no
influence on the rate of iron(II) oxidation in the range 5 to 45
m/h. Sludge with a high solids content of 55% is
produced. This compares well with the typical 20% solids content that
can be achieved with the High Density Sludge process in the case of
lime neutralization. Neutralization cost of acid water can be
reduced significantly with the integrated iron(II) oxidation and
limestone neutralization process as limestone instead of lime is used
and sludge with a high solids content is produced. The alkali
cost to treat discard leachate with an acidity of 10 g/ℓ (as CaCO3)
amounts to R5.15/m3, R2.79/m3, R1.37/m3 and R1.95/m3 for slaked lime,
unslaked lime, limestone when milled on-site and purchased limestone
respectively. The expected capital cost for a 1 Mℓ/d
integrated iron(II) oxidation and neutralization plant is R1.87 million
when the alkali is purchased and R1.95 million when limestone is milled
Design criteria are provided for application on full-scale.
High density sludge (HDS) process.
Acid mine drainage (AMD) poses serious pollution problems if discharged
untreated into public streams. Up to date, the conventional and High
Density Sludge (HDS) processes are used to neutralized AMD. The
conventional neutralization process produces sludge with low sludge
solids content. Although the HDS process produces sludge with high
sludge solids content, one of the disadvantages is the difficulty to
control the process, especially where there is fluctuation in flow
rates and acid concentrations. It is thus priority to improve
the existing HDS process. Less pH fluctuation occurred during the
operation of the Modified HDS process due to better pH control. The pH
fluctuated between pH 7.47 and 7.59. Existing lime
neutralization plants can be adapted with minor changes to accommodate
the modified HDS process.
This investigation compared the HDS and modified HDS process
configurations with beaker studies and an laboratory pilot plant scale.
Results from the continuous laboratory pilot scale studies confirmed
findings from the laboratory beaker studies. The Modified HDS process
gave better lime utilization, higher sludge solids concentrations, and
faster settling rates.
The more CaCO3 added during the beaker studies,
the less lime was used;
the higher the sludge solids content; and the faster the settling rates.
Water with high sulphate concentrations is less suitable for treatment
with the HDS or Modified HDS processes due to gypsum scaling.
Coal discard, fines and high density sludge (HDS-sludge) are generated
during coal mining. Both, discard and HDS-sludge can be
classified as hazardous wastes which require special disposal
criteria. Discard dumps need to be designed in such a way
that contact between discard, water and air is minimized to ensure
minimum acid formation. For the disposal of hazardous
HDS-sludge, legislation requires that it be discharged into lined
ponds, which is costly, to prevent metal leachate from polluting
groundwater. The purpose of this study was to investigate the
benefits associated with co-disposal of HDS-sludge and coal
discard. It is argued that there is little environmental
benefit in disposal of HDS-sludge in lined ponds compared to the
co-disposal of HDS-sludge with coal discard. Co-disposal of
High Density sludge (HDS-sludge) with coal discard would offer the
following benefits: cost reduction as costly sludge disposal ponds are
not required and neutralization capacity is created as HDS-sludge
usually contains unused alkali. Permission for such
co-disposal, however, is dependant on an Environmental Impact
Assessment as required by DWAF.
The purpose of this investigation was to:
It was found that:
- Demonstrate that co-disposal
of HDS-sludge and coal discard offers an effective alternative to
disposal of HDS-sludge in lined landfills.
- Compare the efficiency of
HDS with other methods for the control of pyrite oxidation in coal
- Determine the potential
toxicity of leachate from the untreated and treated coal discard.
- HDS-sludge from Brugspruit liming
plant contains 50 g/kg alkali (as CaCO3) which can be used for the
neutralization of coal discard,
- The rate of pyrite oxidation and metal
leachate are reduced significantly when HDS-sludge is co-disposed with
coal discard, compared with that of coal discard on its own.
- Acid generation from coal discard can
also be controlled with methods such as addition of activated sludge
(to create reducing conditions) or submersion (to eliminate oxygen
A CaCO3 handling and dosing has been developed
and demonstrated on
full-scale that: (i) powdered calcium carbonate in a dump can be
slurried to a constant density and applied for treatment of acid water;
(ii) Acid water, rich in iron(II) can be treated with calcium carbonate
for neutralization, complete removal of metals (iron(II), iron(III) and
aluminium) and partial sulphate removal (to saturation level).
4. Recommendations for further research
It is recommended that further work be done in order to provide an
integrated solution to treat water to the level suitable for discharge
into public streams and for drinking water. This would entail
the following :
- Evaluate the calcium
carbonate/lime/gypsum crystallization process for partial sulphate
removal to less than 1 100 mg/ℓ. In this process sulphate is
reduced to less than 1 100 mg/ℓ through gypsum crystallization by
raising the pH with lime to 12. Increased sulphate removal is
achieved as magnesium and sulphate associated with magnesium is
removed. Due to the high calcium concentration in solution at
pH 12, sulphate is removed to lower levels due tot the solubility
product of calcium and sulphate ions.
- Evaluate the biological
sulphate removal process for the reduction of sulphate to levels less
than 500 mg/ℓ using coal gas as energy source. It has been
demonstrated on pilot-scale (400 m3/d) that
sulphate can be removed to
less than 200 mg/ℓ provided that sufficient energy source is
dosed. Ethanol was used as energy source. Ethanol,
unfortunately has the following disadvantages:
- Costly. At a
dosage of 0.8
g/ℓ and a price of R3 750/ton the ethanol cost amount to R3/m3.
- An aerobic stage is
removal of residual organic material as a portion of the ethanol is
converted to acetate and is not utilized for sulphate reduction.
- Develop a
model to identify the most cost-effective combination of processes for
a specific application. Sulphate for instance can be removed
at the lowest cost with limestone (14 c/kg SO4
for chemical cost, but
only to a level of 2 500 mg/ℓ), or at a higher cost with lime (41 c/kg
SO4 for chemical cost, to a level of 1 100 mg/ℓ)
or to low levels with
the biological process (R1.50/kg SO4, to a level
less than 500
mg/ℓ). Such a model will determine the chemical composition
of the treated water, size and cost of the various capital items, total
capital and running cost. As input the model will require the
flow rate of the various feed water streams and their chemical
- Estimate the total volume and chemical composition of
mining effluents (coal, gold and platinum)
that need to be treated.