Influence of microwave pretreatment on the bioleaching behaviour of low-grade complex sulphide ores P.A. Olubambi ⁎ School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Private Bag 2050, WITS, Johannesburg, South Africa ABSTRACT ARTICLE INFO Article history: Received 14 February 2008 Received in revised form 22 May 2008 Accepted 23 May 2008 Available online xxxx Keywords: Complex sulphide ore Mineralogy Microwave heating Bioleaching Electrochemical study Mineralogical information on the mechanisms by which microwave pretreatment improves the bioleaching behaviour of low-grade complex sulphide ore, and an interpretation of the interrelationship between mineralogy, microwave processing, and bioleaching process are provided in this study. The influence of microwave irradiation on the bioleaching behaviour and mechanisms of a low-grade complex sulphide ore subjected to microwave heating in a kitchen type microwave oven at a power output 1100 W for 5 minwas investigated in a mixed mesophilic bacterial culture through bioleaching experiments and electrochemical techniques. Results revealed that microwave treatment improved the bioleaching of behaviour of the ore, with more effect on copper and iron dissolutions than on zinc and lead. Both microwave treated and untreated samples showed similar electrochemical behaviour. However, microwaved samples displayed higher reactivity, dissolution rates, dissolution currents, current densities and a decreased polarization resistance. Increase in dissolution of the microwaved treated samples resulted from phase changes in the ore which promoted galvanic interaction within the system, decrease in the amounts of sulphur contents, and an increase in electrochemical sites resulting from an increase in the number of cracks induced by microwave heating. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The application of biohydrometallurgical techniques has become an increasingly important aspect in the recovery of base and precious metals from complex sulphide ores. However, biohydrometallurgical processing is often confronted with the problem of the low solubility of sulphide ores, especially the low-grade complex ones. The low solubility of these ores does not usually allow for the recovery of the target metal even by direct chemical leaching in many leaching reagents (Hiskey and Wadsworth, 1975; Dutrizac, 1989). Whichever form the different elements occur in low-grade complex sulphide ores, they are usually very difficult to process (Deveci et al., 2004; Rubio and Garcia Frutos, 2002). This may be due to the close similarities in their mineralogical properties, which make them unsuitable for conventional methods of processing (Olubambi et al., 2006). Due to their poor electromagnetic properties, they are unsuitable for the magnetic method of separation, while the closeness in their specific gravities limits their suitability for the gravity concentration and heavy medium separation methods. Differential flotation does not release all the constituent phases (Majima, 1969) and so the different concentrates obtained are of poor quality with a low rate of metal recovery. This makes further pyrometallurgical processing of these ores difficult and costly (Rubio and Garcia Frutos, 2002) and renders them unattractive for commercialization (Sand- strisom and Petersson, 1997). As a result, metal value is preferably extracted from sulphide ores through hydrometallurgical processes. In the hydrometallurgical process for treating and extracting metals from these ores, it is observed that sulphide ores do not allow the recovery of metal by direct chemical leaching (Dutrizac, 1989; Hiskey and Wadsworth, 1975) because the sulphides are insoluble in nearly all reagents. For the metal content to be leached therefore, the bacteria and reagent must come into direct contact with metal atoms or metal containing compounds within the mineral ore. Amongst several methods, a suitable approach to ensure that the metal content of the ore comes in contact with bacterial cells is to thoroughly grind the ore fine enough (Barbery et al., 1980) to liberate the constituent mineral phases prior to microbial. However, crushing and grinding of ore are a significant capital and operational cost in many mineral processing plants. According to Bilgili and Scarlett (2005), size reduction is an expensive and energy-inefficient process, however operated. Owing to the complexities in the mineralogical associations of low-grade complex sulphide ores, the intergrown nature of the constituent mineral usually results in a poor liberation of the associated minerals. As a means of improving the efficiency of conventional grinding circuits and the liberation of minerals, several approaches could be adopted. A combination of electrical and ultrasonic energy has been successfully used to fracture the matrix of ores causing selective liberation (Parekh et al., 1984). Chemical additives which act as Hydrometallurgy xxx (2008) xxx-xxx ⁎ Corresponding author. Tel.: +27 117177597; fax: +27 117177591. E-mail address: polubambi@yahoo.com. HYDROM-02857; No of Pages 7 0304-386X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2008.05.043 Contents lists available at ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet ARTICLE IN PRESS Please cite this article as: Olubambi, P.A., Influence of microwave pretreatment on the bioleaching behaviour of low-grade complex sulphide ores, Hydrometallurgy (2008), doi:10.1016/j.hydromet.2008.05.043