Bioleaching: A microbial process of metal recovery; A review

The present review describes the historical development and mechanisms of bioleaching. Recent development has shown commercial application of the process and, concurrently, details pertaining to the key microorganisms involved in these processes have been described. Bioleaching of metal sulfides is caused by diverse groups of bacteria. The dissolution biochemistry signifies two types of pathways, which are specifically determined by the acid-solubility of the sulfides, the thiosulfate and polysulfate pathways. This sulfide dissolution can be affected by 'direct' and 'indirect' mechanisms. In the 'indirect' mechanism bacteria oxidize only dissolved iron (II) ions to iron (III) ions and the latter can then attack metal sulfides and then be reduced to iron (II) ions. The 'direct' mechanism requires the attachment of bacteria to the sulfide surfaces. In the case of thiobacilli, bacteria secrete exopolymer that facilitates attachment of the bacteria to a metal surface, thus enhancing the leaching rate. In terms of eco-friendliness and process economics, within the field of biohydrometallurgy the technology is considered robust.