Potential utilisation of micro-organisms in gold processing: A review

Using iron and sulphur oxidising bacteria to catalyse the breakdown of sulphides that host the gold is an important biological method for the pretreatment of refractory gold ores. Following this biological treatment, a combination of chemical and physical methods is used for leaching (such as the cyanide process) and concentration (such as carbon in pulp or electrowinning) of the gold. Although these methods are well accepted by industry, they harbour limitations in the processing of low grade refractory ores and regulatory agency/public acceptance of cyanide use. Thus, it is beneficial for industry to develop environmentally friendly, as well as cost efficient, leaching and concentration techniques that are based on micro-organisms. This may soon be possible by adapting the results of recent multidisciplinary research, which has shown that micro-organisms are capable of driving a biogeochemical cycle of gold dispersion, transport and re-concentration in the supergene environment. The indigenous microbiota in biologically active soil microcosms from a number of Australian sites are capable of solubilising up to 80 wt-% of the gold contained in soils and deeper regolith materials. Studies using molecular microbial techniques have shown that a metallophilic bacterium, Cupriavidus metallidurans, is present in biofilms on gold grains from a number of Australian sites. Cupriavidus metallidurans is capable of actively accumulating gold from solution and therefore contributes to the formation of secondary gold grains and nuggets. Identifying the biochemical and physiological pathways that lead to the dispersion and accumulation of gold in regolith and quantifying the reaction kinetics of these processes may thus lead to the development of industrial bio-processing capacities for gold containing ores.