EXTRACTION OF METALS FROM ORES BY BACTERIAL LEACHING: PRESENT STATUS AND FUTURE PROSPECTS.

Abstract

The principal organism effecting bacterial leaching of ferrous and sulphide ores is Thiobacillus ferrooxidans, though other thiobacilli and other bacteria may be involved. The process depends on (a) direct solubilization of metal sulphides by bacterial oxidation; (b) dissolution of metal sulphides or oxides by ferric iron produced by bacterial pyrite oxidation. Mining spoil dumps and low grade ores can be leached for copper or uranium by cheap low-level technology. Dump leaching enables maximum recovery of valuable metal from any ore, but makes possible exploitation of very low grade Cu and U ores. Continuous extraction processes are possible where a continuously growing bacterial culture is fed with pyritic ores (or FeSO4 or other sulphide) and continuous metal solubilization proceeds. Intimate contact between the bacteria and the ore to be leached (especially with uranium oxide ores) is not always necessary: leaching of UO2 ores probably depends only on ferric iron reaction with the ore. Degradation of pyrite-containing rocks may also be developed as part of future recovery processes for petroleum from oil shales. Two-stage leaching systems present the best prospect for developing a higher-level technology for metal extraction. Stage 1: bacterial generation of Fe3+ from pyrite or a Fe2+ source; Stage 2: chemical leaching of ore by Fe3+ in acid solution. Two-stage processes can be surface processes using crushed or milled ores or can be applied to underground solution mining, when an ore (e.g. uranium) can be leached by pumping Fe3+ solutions through shattered underground deposits, metal recovered (e.g. solvent extraction) and Fe3+ regenerated by bacterial oxidation at the surface. The use of controlled continuous microbial cultures to generate either bacteria or ferric iron is outlined.