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Role of ore mineralogy in optimizing conditions for bioleaching low-grade complex sulphide ores
P. A. OLUBAMBI1, 2, S. NDLOVU1, J. H. POTGIETER1, J. O. BORODE2 1. School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, South Africa; 2. Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure, Nigeria Received 25February 2008; accepted 2 July 2008
Abstract: The role that ore mineralogy plays in understanding and optimizing the conditions favouring the bioleaching of complex sulphide ore containing high amounts of siderite was studied using mixed cultures of mesophilic bacteria, with emphasis on zinc, lead and copper recoveries. The influencing parameters investigated include particle size, stirring speed,volume of inoculum, pulp density, and pH. The results show that the mixed mesophilic cultures can extract about two and a half times the amount of zinc than copper over an equivalent period of time. The highest zinc and copper recoveries of 89.2% and 36.4% respectively are obtained at particle size of 75 μm, stirring speed of 150 r/min, pulp density of 10% (w/v), 12% (v/v) inoculum concentration, anda pH of 1.6. Variations in elemental composition within different particle sizes resulting from the mineralogy of the ore account for the bioleaching behaviour at varying particle sizes. The dissolution at varying pulp density, volume of inoculum, solution pH and the low solution potential observed are also influenced by ore mineralogy. Key words: sulphide ore; ore mineralogy; mesophiles;bioleaching; processing parameters

1 Introduction
Microbially assisted recovery of base metals from low grade complex sulphide ores has become a significant research interest since the 20th century, due to the high depletion rate of the world’s deposits of high grade less complex ores. Recovering the constituent metals of these ores has in most cases been reported to be very difficult[1 2]. Thiscould be attributed to the complexities in the mineralogical associations of these ores and the close similarities in the mineralogical characteristics of the constituting minerals which make them unsuitable for the conventional hydrometallurgical method of processing. The low solubility of these ores in many leaching reagents, low quantity of metal to be recovered from the ores, and the large amountof ores to be processed, limit the conventional hydrometallurgical methods of processing of them. Pre-treatment processes including roasting, pressure oxidation and chemical oxidation are often employed prior to solubilisation. These methods have been used over a long period for

physically releasing the desired metals and making them amenable to leaching reagents through their conversion towater-soluble sulphates or oxides. They are however observed to be very complex, expensive[3 4] and mostly environmentally unsuitable[5] due to the large volume of reagents. The biohydrometallurgical process has been reported to be an effective alternative for processing these ores and recovering the valuable metals[6 8], as it has proved to be cheaper, environmentally benign and less complex[9].Despite the encouraging incentives of bioleaching as an alternative to chemical leaching, there are still a number of challenges to overcome. The heterogeneities and the complexities in the mineralogy of low grade complex sulphide ores, coupled with their general low solubility place a limitation during biohydrometallurgical processing. The slow kinetics of the reactions and high residence timeduring bioleaching, especially with mesophiles, also hinder the industrial application of biohydrometallurgical processing and limit its economic viability[1]. Moreover, bioleaching with mesophilic microorganisms has been observed to have

Corresponding author: P. A. OLUBAMBI; Tel: +27117177597, Fax: +27117177591 E-mail: polubambi@yahoo.com

P. A. OLUBAMBI, et al/Trans. Nonferrous Met. Soc....