As global technologies and civilisation have advanced, the utilization of diverse mineral products has increased, resulting in the progressive depletion of high-grade mineral deposits. Consequently, metal production has to rely more on the use of lower-grade or complex ores, as well as metal extraction from mining and industrial wastes. Certain substances, such as phosphorous (P) and potassium (K), contained within the iron ore have a detrimental effect on the smelting process during steel manufacturing, resulting in the international steel making companies charging penalties when purchasing iron ore concentrates containing high concentrations of P and K. It has, therefore, become necessary to develop an economically viable and environmentally friendly process to reduce the high P and K concentrations contained in the iron ore concentrate of the Sishen Iron Ore Mine, resulting in the minimizing of the penalties charged by the steel making companies. During this study no microbial bioleaching candidates could be isolated from the aquatic environment in order to develop an economical process to remove the P and K from the iron ore concentrate. The most likely candidate for the removal of P and K from the iron ore concentrate would seem to be Acidithiobacillus ferrooxidans that was isolated from the iron ore concentrate, however, this bacterium produces sulphuric acid, which is undesired in the iron ore industry due its corrosive properties. Furthermore, the P and K contained in the iron ore is in a non-sulphidic phase, and therefore, conventional bioleaching processes are not viable for their removal from the ore. Metals in certain nonsulphide minerals, such as the iron ore concentrate of the Sishen Iron Ore Mine may be solubilised by a process of complexation using microbially produced inorganic or organic acids. Chemical leaching of the iron ore concentrate using citric acid proved to be more efficient than “heap leaching”, as more P and K was removed from the iron ore concentrate, as well as in a shorter time frame. The results of the chemical leaching suggested that a 1M citric acid leaching solution be used at 60°C for 5 days for the chemical leaching process, as the most P and K is removed from the iron ore concentrate using these leaching conditions. The possibility to use A. niger as a bioleaching microorganism was also investigated, due to its ability to produce organic acids such as citric acid, which has the ability to remove P and K from the iron ore concentrate by chemical leaching. Compared to chemical leaching, which requires high concentrations of citric acid and/or high leaching temperatures, bioleaching using A. niger offers a more economical method with similar efficiency for the removal of P and K from the iron ore concentrate of the Sishen Iron Ore Mine. The most economically viable process for the removal of P and K from the iron ore concentrate, although not as efficient as chemical leaching using citric acid at high temperature, proved to be the bioleaching process using A. niger with a bioleaching pulp density of 33% at 30°C for 10 days. This is the first report of the use of A.niger for the use in any bioleaching process. / Thesis (PhD)--University of Pretoria, 2009. / Microbiology and Plant Pathology / unrestricted