Geochemical Mass Balance and Characterisation of Pollutants Draining from a Nickel Mine Site

Trojan Nickel Mine is the largest producer of Ni, Cu and Co in Zimbabwe. The mine has operated a nickel flotation concentrator and base metal refinery continuously since 1969, creating 28 million tonnes of sulfide bearing tailings, five million tonnes of permanent waste rock and about seven million tonnes of smelter slag. Exposure of the sulfide bearing minerals renders them susceptible to oxidation in the presence of water and oxygen, resulting in acidic solutions with elevated concentrations of total dissolved solids, sulfate, metals and residual flotation chemicals in various waters draining from the mine site. These pollutants pose a direct threat to the environment and human health, and understanding their deportment and behaviour in the various waste streams is essential in the mineÆs environmental objectives to prevent or minimise adverse impacts arising from its operations. A geochemical assessment of the pollutants was carried out to compute a mass balance between the major sources of pollution and the distribution and fate of pollutants in waste streams from the mine site. Waste rock, tailings, surface water, ground water and sediments were sampled, tested and analysed. Mineral composition and water chemistry data show that geochemical reactions are responsible for the release of pollutants from waste rock, tailings and plant spills. Drainage from the mine area is near neutral due to buffering by magnesium carbonates in the fine fraction of the tailings. The natural wetlands that exist downstream of the waste rock and tailings dams are effective in reducing metal loadings to water courses. However, solutes and heavy metals are occasionally released to surface and ground water, when their concentrations peak during the dry season. The results demonstrate the necessity to understand the natural chemical processes that characterise the distribution of pollutants in nature and how such knowledge can be used to develop cost effective methods to remediate pollution in the natural environment.