Evaluation of organic covers over oxidized mine tailings to control acid mine drainage
Acid mine drainage (AMD) is generated by sulphidic mineral wastes at many mining locations throughout the world. It results from the percolation of water through mine wastes where chemical and biological oxidation of residual metal sulphides occur. The oxidation results in the generation of an acidic (pH 2 to 4), sulphate-rich, metalliferous solution. If left untreated, the acidity and high metal content of AMD flowing into the environment can have a devastating effect on terrestrial and aquatic ecosystems. As AMD can continue for tens of decades after a mine closure, a low maintenance and economical containment system is desired for AMD mitigation. In the work reported in the thesis, column tests were conducted to evaluate the merit of using an organic cover layer over oxidized mine tailings as a method of AMD mitigation. The two different organic materials tested in the laboratory were domestic sewage sludge and peat. Four columns were set up. The tailings in the first column were left uncovered. Tailings in the 2nd, 3rd and 4th columns were covered with a water layer, a sewage sludge layer and a peat layer, respectively. The first two columns served as controls. All of the four columns were set up in an environmental chamber where a temperature of 25Â°C and humidity of 75% was maintained. Characterization tests on the tailings indicated that the tailings were fine grained and highly acidic. The aqueous extracts of the tailings contained high concentrations of iron and sulphates. The other metal ions present in significant concentrations in the distilled water extract were AI, Ca, Mg, Ni and Zn. Cadmium, Cu, Cr and Pb were present in concentrations less than 10 mg/1. The selected cover materials had near neutral pH, and high biochemical oxygen demand (BOD) and chemical oxygen demand (COD). The columns with these cover materials developed low hydraulic conductivity during the simulation experiments. The results of a 4 1/2 month long column-leaching experiment indicated that the water quality of the leachate from the columns with organic covers was better in comparison with the two controls. The performance of the sewage sludge cover was best. The leachate under this column had better water quality in terms of the monitored parameters. This column had the lowest relative ion concentrations in the leachate, developed a low hydraulic conductivity (2.5x10-6 cm/s) and maintained anaerobic conditions (Eh < 0). The low redox potential propelled reduction reactions and formation of metal sulphides. The formation of metal sulphides was indicated by the concurrent decrease in metal concentration, appearance of black coloration and negative redox potential. The pH of the leachate increased from 3.2 to 3.4 for this column. The performance of the peat cover was second best. This cover also maintained reducing conditions (low Eh) but less in comparison with the sewage sludge cover. The peat column had a hydraulic conductivity of 7.96x10-6 cm/s. The performances of the two control columns were similar to each other. Chemical speciation analysis using the MINTEQA2/PRODEFA2 model was performed to determine the equilibrium of Fe2+/Fe3+ ratio. It was predicted by the model that the Fe2+/Fe3+ ratio is higher for columns with organic covers. The research project demonstrated that the organic cover on top of the tailings resulted in improved leachate quality. A strong reducing environment prevailed under the organic cover layers that reduced and consequently immobilized the metals as metal sulphides. Before the installation of these covers for reclaiming a mine site, field experiments of longer duration should be carried out. It is also recommended that the effects of desiccation, freeze-thaw cycles and the effect of compaction of covers on hydraulic conductivity be analyzed.