17    BIOLOGICAL TREATMENT OF MINING EFFLUENTS
J. P. Maree, Chief Researcher
A. Gerber, Researcher
E. Hill, Assistant
National Institute for Water Research
Council for Scientific and Industrial Research
Pretoria, South Africa 0001
A. R. McLaren, Water Quality Control Officer
Gold Fields of South Africa, Ltd.
Johannesburg, South Africa  2000
INTRODUCTION
Mining effluents are major contributors to mineralization of receiving waters and may prove toxic
to men, animals, and plants due to unacceptably high concentrations of heavy metals and cyanide. Its
reuse potential is limited due to the presence of saturated calcium sulphate, which causes scaling of
pipes and equipment. Mining effluents originate from both underground sources and metallurgical
process plants. Its sulphate content results from bacterial oxidation of pyrite, while metallurgical
processing adds sulfate in the form of spent sulfuric acid. Sulphate, calcium, heavy metals and
cyanide may be removed by well-established demineralization processes, such as reverse osmosis and
electrodialysis, but these are costly, hence the need for the development of alternative processes. A
promising new process entails the biological reduction of sulphate to sulphide by the bacterium
Desulfovibrio desulfuricans. Sulphide can, in turn, be converted to elemental sulphur.
Various researchers have studied biological sulphate reduction. Middleton and Lawrence (1) determined the kinetics of microbial sulphate reduction in complete mix reactors using acetic acid as
carbon source and observed a sulphate reduction rate of 0.29 g S04/L/d (biomass concentration not
specified). Cork and Cusanovich (2) developed a continuous purge system, using an inert carrier gas
(75% argon and 25% C02), to feed sulphide removed from actively growing cultures of Desulfovibrio
desulfuricans to cultures of Chlorobium thiosulfatophilum, for oxidation to sulphur. A sulphate
reduction rate of 6.3 g S04/L/d was observed in a completely mixed reactor containing 12,600 mg/L
lactic acid, at pH 6.5 and temperature 30°C (biomass concentration not specified). Some 91% of the
hydrogen sulphide produced was swept off into a sulphide oxidizing chamber containing Chlorobium,
where 88% was converted to sulphur. This represents an overall sulphur yield of 80%. Hilton et al. (3)
applied a similar process, known as the BIOSULFIX process, in which hydrogen sulphide was
removed in an external stripping chamber, and observed a sulphate reduction rate of 6.5 g S04/L/d.
Maree and Strydom (4) studied sulphate reduction in a packed bed reactor and observed reduction
rates of the same order of magnitude.
Oversaturated calcium carbonate levels and unutilized carbonaceous material prevent water from
being reused directly after anaerobic treatment. Maree (5) showed that these products can be removed
successfully by applying hydrogen sulphide stripping, clarification and aerobic treatment. Aerobic
treatment reduced the organic content from 1,100 to 300 mg/L COD.
The specific purpose of this study was to study calcium sulphate removal from lead mine effluent in
the presence of heavy metals and organics such as xanthates and carbamates, used during mineral
flotation. A secondary objective was to find alternative ways of effecting clarification without resorting to the addition of chemicals. Typically, 500 mg/L FeCl3. which adds unwanted salts to the water,
is required for the latter purpose.
The lead mine involved is situated at Black Mountain in the North Western Cape. Being situated in
an arid region, water is scarce and the daily requirement of 4.5 ML is pumped from the Orange River
over a distance of some 58 km. After utilization in the flotation plant, where lead, copper and zinc are
separated selectively, a side stream of 3.5 ML/d is bled off and discharged into a nearby vlei to
maintain acceptable levels of dissolved salt and organics in the process water. The discharge water
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