The role of microorganisms in gold processing and recovery—A review
Introduction
Gold (Au) ore grades in Australia show long-term declining trends over time (Mudd, 2009; Fig. 1). As the quality of gold deposits continues to decrease, it is expected that processes which can economically extract gold from low grade ores will grow in importance to the minerals industry. Biotechnology has the potential to transform uneconomic gold reserves into resources. Bioprocessing can be attractive for: 1) low grade gold ores that are too expensive to process using conventional processes and 2) ores that contain impurities that foul conventional processing equipment (e.g. arsenic in gold ore). Microorganisms can mediate gold solubilisation by oxidising the sulphide matrix of refractory gold ores making the gold more accessible to leaching by chemical lixiviants. Microorganisms can also excrete ligands which are capable of stabilising gold by forming gold-rich complexes and/or colloids (Reith et al., 2007a). The solubilisation of gold can be facilitated by biologically produced amino acids, cyanide and thiosulphate (Reith et al., 2007a). Moreover, microorganisms can participate in the redox cycling of iodine (Amachi, 2008), which is a potential alternative lixiviant for gold leaching. Microorganisms can also decrease gold solubility by consuming the ligands that have bound gold, or by biosorption, enzymatic reduction and precipitation, and by using gold as a micronutrient (Fig. 2) (Reith et al., 2007a). Additionally, microorganisms can influence gold solubilisation indirectly by enhancing the permeability of ore bodies (Brehm et al., 2005, Burford et al., 2003, Ehrlich, 1998, Jongmans et al., 1997, Kumar and Kumar, 1999). Understanding the possible activities of microorganisms is important, especially when considering leaching applications, where the control of operational conditions may be challenging. This literature review aims to identify microbial processes which may be relevant or hold potential for the processing and recovery of gold.
Section snippets
Principles of biooxidation
Many gold deposits are sulphidic in nature and contain gold in a form that is inaccessible to lixiviants. Refractory gold ores often contain finely disseminated gold particles encapsulated by a sulphide mineral matrix containing arsenopyrite, pyrite and pyrrhotite (Bosecker, 1997). The inaccessibility of gold to lixiviant has been overcome by biooxidising the sulphides contained in the ore, thereby liberating gold particles from the sulphide matrix and rendering the gold amenable to dissolution
Gold solubilisation through biooxidation and complexation
A number of chemical and biologically produced lixiviants have been assessed for their ability to oxidise and complex gold as alternatives to chemical cyanide solubilisation.
Gold recovery and/or loss through bioprocesses decreasing gold solubility
In contrast to most other metals, gold is extremely rare, inert, and unstable as a free ion in aqueous solutions under atmospheric conditions (Reith et al., 2007a). Gold complexes can be highly toxic to microorganisms. Hence, microorganisms have many mechanisms to deal with toxic gold complexes and are able to precipitate gold intra- and extracellularly, and in products of their metabolism, such as exopolysaccharide (EPS) and sulphide minerals (Reith et al., 2007a). Some of these mechanisms may
Conclusions
Microorganisms play many roles in the biogeochemical cycling of gold and can be utilised in a number of ways for gold processing and recovery. Biooxidation of refractory gold bearing sulphide ores with acidophilic iron and sulphur-oxidising microorganisms has been already commercially practised since the 1980s, first in bioreactors and subsequently as heap leaching operations. Although in situ or in place biooxidation of gold ores has not yet been industrially practised, the indirect oxidation
Acknowledgements
The support of the CSIRO Minerals Down Under National Research Flagship, sponsors of MERIWA project M409 and ORICA is gratefully acknowledged.
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