Treatment by sulfate-reducing bacteria of Chessy acid-mine drainage and metals recovery

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. Were, 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 H-2/CO2 mixture, was used in conjunction with a gas stripping column. The maximum rate of hydrogen transfer in the bioreactor was determined before inoculation. k(L)a was deduced from measurements of O-2 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 H-2 transfer suggests that this step should not be a limiting factor. However, an increase in II, 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 mg 1(-1) h(-1), corresponding to a residence time of 0.9 day. A preliminary cost estimation based on a treatment rate of 5 m(3) h(-1) of a mine effluent containing 5 gl(-1) SO42- is presented. (C) 2001 Elsevier Science Ltd. All rights reserved.