Abstract: Grootvlei Proprietary Mines Ltd is discharging between 80 and 100 Ml/d underground water into the Blesbokspruit. This water is pumped out of the mine to keep the underground water at such a level as to make mining possible. The water is of poor quality because it contains high TDS levels (2700-3800 mg/l) including high concentrations of iron, manganese, sulphate, calcium, magnesium, sodium and chloride. This water will adversely affect the water ecology in the Blesbokspruit, and it will significantly increase the TDS concentration of one of the major water resources if not treated prior to disposal into the stream. Therefore, alternative water desalination technologies were evaluated to estimate performance and the economics of the processes for treatment of the mine water. It was predicted that water of potable quality should be produced from the mine water with spiral reverse osmosis (SRO). It was demonstrated that it should be possible to reduce the TDS of the mine water (2000-2700-3400-4500 mg/l) to potable standards with SRO (85% water recovery). The capital costs (pretreatment and desalination) for a 80 Ml/d plant (worst-case water) were estimated at US$35M. Total operating costs were estimated at 88.1c/kl. Brine disposal costs were estimated at US$18M. Therefore, the total capital costs are estimated at US$53M. It was predicted that it should be possible to produce potable water from the worst-case feed water (80 Ml/d) with the EDR process. It was demonstrated that the TDS in the feed could be reduced from 4178 to 246 mg/l in the EDR product (65% water recovery). The capital costs (pretreatment plus desalination) to desalinate the worst-case feed water to potable quality with EDR is estimated at US$53.3M. The operational costs are estimated at 47.6 c/kl. Brine disposal costs were estimated at US$42M. Therefore, the total capital costs are estimated at US$95.3 M. It was predicted that it should be possible to produce potable water from the mine water with the GYP-CIX ion- exchange process. It was demonstrated that the feed TDS (2000- 4500 mg/l) could be reduced to less than 240 mg/l (54% water recovery for the worst-case water). The capital cost for an 80 Ml/d ion-exchange plant (worst-case water) was estimated at US$26.7M (no pretreatment). Operational costs were estimated at 60.4 c/kl. Brine disposal costs were estimated at US$55.1M. Therefore, the total desalination costs were estimated at US$81.8M. The capital outlay for a SRO plant will be significantly less than that for either an EDR or a GYP-CIX plant. The operating costs, however, of the RO plant are significantly higher than for the other two processes. Potable water sales, however, will bring more in for the RO process than for the other two processes because a higher water recovery can be obtained with RO. The operating costs minus the savings in water sales were estimated at 17.2; 6.7 and US$8.6M/y for the RO, EDR and GYP-CIX processes, respectively (worst case). Therefore, the operational costs of the EDR and GYP-CIX processes are the lowest if the sale of water is taken into consideration. This may favour the EDR and GYP-CIX processes for the desalination of the mine water.

Abstract: Acid-mine drainage can contain high concentrations of heavy metals and release of these contaminants into the environment is generally avoided by lime neutralization. However, this classical treatment is expensive and generates large amounts of residual sludge. The selective precipitation of metals using H2S produced biologically by sulfate-reducing bacteria has been proposed as an alternative process. Here, we report on experiments using real effluent from the disused Chessy-les-Mines mine-site at the laboratory pilot scale. A fixed-bed bioreactor, fed with an H2/CO2 mixture, was used in conjunction with a gas stripping column. The maximum rate of hydrogen transfer in the bioreactor was determined before inoculation. kLa was deduced from measurements of O2 using Higbie and Danckwert's models which predict a dependence on diffusivity. The dynamic method of physical absorption and desorption was used. The maximum rate of H2 transfer suggests that this step should not be a limiting factor. However, an increase in H2 flow rate was observed to induce an increase in sulfate reduction rate. For the precipitation step, the gas mixture from the bioreactor was bubbled into a stirred reactor fed with the real effluent. Cu and Zn could be selectively recovered at pH=2.8 and pH=3.5, respectively. Other impurities such as Ni and Fe could also be removed at pH=6 by sulfide precipitation. Part of the outlet stream from the bioreactor was used to regulate and maintain the pH during sulfide precipitation by feeding the outlet stream back into the bioreactor. The replacement of synthetic medium with real effluent had a positive effect on sulfate reduction rate which increased by 30-40%. This improvement in bacterial efficiency may be related to the large range of oligo-elements provided by the mine-water. The maximum sulfate reduction rate observed with the real effluent was 200 mgl-1 h-1, corresponding to a residence time of 0.9 day. A preliminary cost estimation based on a treatment rate of 5 m3 h-1 of a mine effluent containing 5 gl-1 SO42- is presented.