The nickel present in nickel laterites is not usually present as discrete minerals, but as cations substituted within manganese oxides, goethite, and/or clays. Because of this, it is difficult to upgrade the ore by beneficiation. As a result, nickel laterites are traditionally processed using pyrometallurgical and hydrometallurgical methods. In recent years, microbiological leaching has been found to be a promising novel technology for recovering valuable minerals from traditionally difficult-toprocess ores.Nickel is an important metal in human life and in the industry. In recent years, the world nickel demand has been driven by soaring steel production, particularly in China. With the rapid growing demand for nickel coupled with the depletion of high grade sulphide reserves, low-grade nickel ores, which cannot be economically processed by conventional metallurgical processes, become increasingly important sources of nickel. Laterite ore, which is often considered as a low-grade nickel ore, contains several kinds of metal elements including nickel, cobalt, iron, silicon, aluminium, and chromium; and thus, constitutes an alternative source of nickel.
Metal leaching by fungi generally involves an indirect process with microbial production of organic acids, amino acids, and other metabolites. Four mechanisms have been identified: (i) acidolysis, (ii) complexolysis, (iii) redoxolysis (Berthelin, 1983), and (iv) bioaccumulation (Weed et al., 1969).The following are some of the possible reactions that can take place to finally produce nickel ions (Tzeferis, 1992):
However, commercial application of these microorganisms has been less successful due to process inefficiencies such as poor metal recovery (Tang and Valix,2006). This problem is particularly prevalent in limonit ore, characterised by high iron content in the form of goethite (FeOOH) and nontronite, characterised by clay minerals consisting of illite, kaolinite, and chlorite (Tang and Valix, 2006). The effectiveness of these microorganisms was found to depend on their ability to produce hydroxycarboxylic acids (citric, lactic, gluconic, pyruvic and tartaric), and also other metabolites, which are excreted in culture media (Tzeferis, 1994; Castro et al., 2000; Le et al., 2006), and their resistant to heavy metals (Burgstaller and Schinner, 1993; Le et al., 2006).
The results of these studies showed that microbiological leaching is more effective compared to chemical leaching. The more favourable results obtained in the bioleaching process suggest that microbiological activity, apart from bio-acid production is participating in the leaching process (Valix et al., 2001a). It is suggested, for example, that fungal hyphae physically attach onto surfaces of minerals with possibility of high acid concentration formed at hyphal tips reacting directly with adjacent mineral surfaces without greatly affecting the pH of the bulk medium (Alibhai et al., 1993).
Attached Table shows a summary of some of the studies conducted, showing the types of ore or mineralogical makeup of nickel laterite, fungi species, types of acid produced, and a range of recoveries obtained. It is also clear from attached table that citric and oxalic acids were the two well established products of fungal metabolism. Citric acid was the most effective and oxalic acid was the least effective leaching agent (Tzeferis, 1994, Tzeferis, 1995). A possible explanation for this could be that oxalic acid precipitates the leached nickel as nickel oxalate, which is known to have a very low solubility (Tzeferis, 1994).
Available from:Geoffrey S. Simate:The fungal and chemolithotrophic leaching of nickel laterites—Challenges and opportunities (PDF Download Available)
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