|dc.description.abstract||1.1 Background: Acid mine water containing sulphate and high concentrations of dissolved heavy metals,
including iron(II), can have pH values as low as 2.5. Environmental pollution caused by such
effluents are major contributors to the salinisation of receiving water, and may prove toxic to
both fauna and flora. Acid, sulphate-rich solutions are produced bacteriologically from pyrite
present in waste dumps from mining and metallurgical operations and from spent sulphuric acid
used in chemical or metallurgical plants. The following large mine water treatment projects are
currently receiving attention in South Africa on a national level:
1. Amanzi Water Project. The Amanzi project deals with the treatment of mine water
(potentially 240 Ml/d) for the recovery of potable water and by-products (e.g. gypsum).
Participating mines in the project are Randfontein Estates, First Wesgold, Durban
Roodepoort Deep, Rand Leases, ERPM and Grootvlei. The pH of these waters varies
fiom 2.8 to 6.0 and the sulphate concentrations h m 600 to 3 000 mg/l (SWaMP Steering
2.Olifants Forum. Polluted mine water, estimated at a volume of 130 Ml/d, is currently
discharged to water courses on the Highveld. The mine water has a pH level between 2
and 4 and contains high sulphate concentrations (> 700 mg/l) (Van Zyl, et al., 2000).
Unless neutralized, such water may not be discharged into water courses. Lime is generally used
for neutralization. Neutralization costs could be reduced significantly should lime be replaced
with limestone. The cost of limestone is currently R130/t compared to R700/t for lime.
Furthermore, increasing pressure is being exerted by the Department of Water Affairs and
Forestry to enforce sulphate removal from effluent. Extensive studies have already been carried
out by the mining industry to evaluate possible sulphate removal technologies. The high cost of
these technologies are considered a major obstacle. Therefore, efforts to develop a cost-effective
treatment process for the recovery of re-usable water from sulphate-rich effluents, is of national
1.2 Objectives: The objectives of this investigation were to develop processes whereby acid and/or sulphate-rich
water can be treated. The specific aims of the investigation were to:
1.Develop the integrated iron(II)-oxidation and limestone neutralization process where
powdered limestone is used for the neutralization of iron(II)-rich acid water in a
completely-mixed reactor (Chapters 3 and 4 and Patents 1 - 3).
2.Develop the biological sulphate removal process for treatment of sulphate-rich effluents
(Chapters 5 and 6).
3.Develop the barium sulphide process for treatment of sulphate-rich effluents (Chapter 7).
4.Develop a water flow and chemical mass balance model to identify the most cost-effective
treatment option for a water network (Chapter 8).
1.3 Findings: The following innovative processes/models were developed for neutralization and sulphate
removal from industrial effluents:
1. A limestone handling and dosing system.
2. A limestone neutralization and iron(II)-oxidation process for the removal of free acid,
iron and aluminium.
3. A biological sulphate removal stage which includes biological sulphate reduction, H2S-stripping
and aerobic treatment for the removal of residual organic material, and calcium
carbonate precipitation. The barium process, which is similar to the biological sulphate
removal process, can also be used for sulphate removal.
4. Modelling of a typical water network of a mining operation.
1.3.1 Limestone neutralization:
In order to develop the limestone neutralization technology to the stage of full-scale
implementation it was necessary to understand its limitations, study its kinetics, develop design
criteria for full-scale plants and to protect the intellectual property through patents.
22.214.171.124 The limestone neutralization process.
Limestone was not used previously on a large scale for neutralization of iron(II)-rich acid water.
The reasons were:
1. The pH of iron(II)-rich water could not be raised sufficiently with limestone to rapidly
allow iron(II) to be oxidized to iron(II). Rapid oxidation of iron(II) occurs only at pH 7
and higher. This can however be achieved with lime, while limestone only raises the pH
of iron(II)-rich water to pH 6.
2. The reactivity of limestone is too low to neutralize acid water completely within an
acceptably short residence time when stoichiometric dosages are applied.
3. Iron(II) passivates limestone particles due to Fe(OH)3 preferentially precipitating on the
surface of the limestone particles, where the pH is the highest.
126.96.36.199 Kinetics of limestone neutralization.
Stumm and Lee (1961) investigated the rate equation for biological iron(II)-oxidation and
determined that it is a function of the pH, iron(II) and oxygen concentrations. This rate equation
was investigated for the case where limestone was used as the neutralization agent. Special
attention was given to the effect of suspended solids concentration on the rate of iron(II)-
188.8.131.52 Full-scale implementation of limestone neutralization
For the present investigation a demonstration plant was constructed and evaluated for iron(II)-
Oxidation/limestone neutralization (Maree, et al., 2004). A plant with a capacity of 1 Ml/d was
constructed at BCL, a nickel and copper mine in Botswana. Ore tailings leachate, with an acid
concentration of 10 g/l (as CaC03), was treated. Limestone, available at a cost of R150/t, was
used for neutralization of the acid water. Previously, leachate with a high acid concentration was
combined with less acidic streams before it was neutralized with lime. The result of this approach
was that a large volume of product water was slightly over-saturated with respect to gypsum,
resulting in scaling of pipelines and other equipment. The leachate was neutralized separately
from the less acidic streams. The over-saturated fraction was first allowed to crystallize from
solution in the fluidized-bed reactor before being combined with the other streams.
The following patents were registered, following the investigation:
1. A patent on the integrated limestone and iron(II)-oxidation process.
2. A patent for a limestone handling and dosing system was registered where powdered
precipitated CaC03 was dumped onto a concrete slab, slurried to constant density with an
automatic control, and used for neutralization of the acid water.
3. A patent on an integrated limestone and lime process for the treatment of acid and
sulphate-rich effluents. This allows the following:
- Stage 1 : The bulk of the acid is neutralized with limestone while C02 is produced
and stripped off by aeration.
- Stage 2: Lime is added to allow precipitation of magnesium and other metals as
well as sulphate associated with these metals.
- Stage 3: The C02 that is produced in Stage 1 is used to adjust the high pH of the
water from Stage 2 to 8.3. This allows CaC03 precipitation.
1.3.2 Biological sulphate removal
A biological process was developed whereby sulphate reduction to sulphide and sulphide
oxidation to elemental sulphur occur in the same reactor. The following aspects were
investigated: the reaction rate of biological sulphate reduction, the effect of various parameters on
the reaction rate such as temperature, sulphide and sulphate concentrations and the identification
of intermediate products formed.
Pilot scale evaluation of the following stages of the biological sulphate removal process were
1. Heating stage. Feed water to the anaerobic stage was first contacted directly with hot
coal gas to raise the temperature of the water to 30 °C.
2. Anaerobic stage. A pilot plant with a capacity of 8 m³/h was operated, using ethanol or
sugar as energy source.
H2S-stripping and processing stage. A laboratory unit was operated to evaluate the
suitability of the following reactor types for H2S-stripping and processing: Venturi device
and a packed-bed reactor.
1.3.3 Integrated Bas process for sulphate removal
Laboratory studies were carried out to demonstrate that the integrated Bas-process is technically
and economically viable for sulphate removal. The Bas process consists of the following stages:
1.Thermal stage where barium sulphate is reduced to barium sulphide at 1 050°C, using coal
as the reductant.
2.Sulphate removal stage
3.Sulphide stripping and processing stage
4.Softening stage where limestone is precipitated.
The water network of a coal mine was audited and simulated by an interactive, steady state model
to determine the optimum effluent treatment process configuration. The findings from this
investigation were used to optimize the mine's water management strategy. Simulation of the
interactions in the water network was used to show the following: (i) Powdered CaC03 can be
used as an alternative to lime for the neutralization of acid water at a cost saving. (ii) The
amount of gypsum crystallization that occurred in the primary neutralization and coal processing
plants. This information was needed to plan for sludge disposal. (iii) The benefits associated
with separate treatment of the most polluted stream versus combined treatment of all streams
during mine water treatment. By treating the higher polluted streams separate from the lesser
polluted streams, higher salt removal efficiencies are achieved. (iv) The OSI (gypsum oversaturation
index) value can be controlled effectively at 1 by treating the feed water to the coal
processing, for sulphate removal. The capacity of the sulphate removal plant required was
determined as well as the associated capital and running costs.
The treatment approach outlined offers the following benefits: (i) The cheapest alkali, a by-product
from the paper industry, can be used for neutralization of the acid and for the removal of
the bulk of the sulphate concentration through gypsum crystallization. The more advanced
biological process is then used only for removal of the remaining sulphate, to low concentrations.
(ii) A robust biological process is used for sulphate removal to produce process water which is
non-scaling and suitable for discharge into public streams. (iii) This is an integrated process as
CO2 produced during limestone-neutralization is used for H2S-stripping in the biological stage.
The stripped H2S-gas is utilized in the limestone-neutralization stage for precipitation of iron as
iron sulphide. Iron is also removed as inert Fe(OH)3 together with gypsum in the limestone neutralization
stage, after oxidation.||