Abstract:
Kumba Iron Ore, Ltd. is the world‘s fourth largest supplier of sea-borne iron ore and currently operates two mines in South Africa namely: the Sishen mine in the Northern Cape and Thabazimbi mine in Limpopo. The Sishen mine, located at the northern end of the Maremane anticline where the bulk of the hematite ore is buried beneath younger cover lithologies, was our focus area. Here the iron resources are made up by laminated and massive ore bodies that belong to the Asbestos Hills Subgroup. These ore bodies are overlain by conglomerates, shales, flagstone and quartzite. The alkalis, potassium and phosphorous, are common constituents of iron ore, which is known to have a deleterious effect on the manufacturing of iron and steel. Therefore steel making companies charge penalties when purchasing iron ore concentrates with alkali concentrations above predetermined levels. To ensure that the export batches at the Sishen mine stay within set limits, the ores from different batches (with alkali concentration greater and below set limits) are mixed to produce a batch which meet requirements. However this solution will soon become ineffective as the low alkali ore is progressively depleted. Conventional methods used to treat high alkali ores include pyro-and hydrometallurgical methods. These approaches have several limitations such as poor product recovery, involvement of high process and energy cost and an increase in pollution load of water resources. Therefore necessitating research and development of alternative cheap and environment friendly procedures, which could supplement or replace conventional methods to ensure that mining stays economically feasible at the Sishen Iron Ore mine. The application of microorganisms to mining practices is collectively referred to as biohydrometallurgy and includes bioleaching and biooxidation processes. The phrase bioleaching refers to the conversion of an insoluble metal (typically a metal sulfide) into a soluble form (typically a metal sulfate), via microbial activity. When metals are extracted into solution, the process is referred to as bioleaching, whereas if the metal remains in the mineral, it is referred to as biooxidation. The latter term biobeneficiation refers to the selective dissolution of undesired minerals from the ores by direct or indirect action of microbes, thereby enriching the desirable mineral content. Therefore the objective of this study was to determine whether bacteria (naturally occurring on the ore or introduced species) could be used to selectively remove the alkalis from the iron ore mined at Sishen. The species evaluated were able to change the solution pH and/or form biofilms, which is assumed to have affected mineral mobilization. Data obtained during this study suggests that the composition of the ore plays a significant role in its susceptibility to bioleaching. Furthermore we also found that the indigenous cultures were more effective than the introduced species to mobilize the alkalis, which could possibly be ascribed to an adaptation of the microbes present. These preliminary results suggest that bioleaching is an effective alternative cost effective approach to treat iron ore and could possibly be implemented in future into the mining schedule at Sishen.
