Neutralization of Acid Mine Water and Sludge Disposal
Report No 1057/1/04
Nov 2004

EXECUTIVE SUMMARY

1.    Background

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:

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 developed.

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:
  1. Treated water which is neutral and stable with respect to gypsum crystallisation.
  2. Sludge with a high solids content.
  3. Minimum alkali cost.
  4. Minimum capital cost of plant and
  5. Confidence in the selected process.
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.  

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:
2.    Objectives

Against this background the following aims were set for the project:
  1. Biological iron(II) oxidation.  Determine the conditions required for rapid iron(II) oxidation under acidic conditions within a residence time of one hour.
  2. 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 integrated neutralization and iron(II) oxidation process.
  3. High density sludge (HDS) process. Optimize the process flow diagram of the HDS process to meet the following criteria for different water qualities:
  4. 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:
3.    Findings

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) (O2-flow= 70 mℓ/min; T = 20.5C; surface area = 847 m2/m3).  The kinetics of the biological iron(II) oxidation in a plate reactor can be described as: 
d[Fe2+]/dt = k.[Fe2+]0.5.[RSA]1.[O2]0.5

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:

 -d[Fe(II)]/dt = k[Fe(II)]0.5 Rf0.5 A1.0
    
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 on-site.

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.

Leachate studies

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:
  1. HDS-sludge from Brugspruit liming plant contains 50 g/kg alkali (as CaCO3) which can be used for the neutralization of coal discard,
  2. 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.
  3. 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 ingress).
Full-scale application

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 :