The aim of this study was to investigate the iron- and\ud sulphur-oxidizing activities of thermophilic bacteria with\ud reference to the possible use of such bacteria in the extraction of metals from mineral sulphides. The initial characterization of a range of isolates was based on growth studies with iron and sulphur substrates and on the comparison of whole cell protein electrophoresis patterns. Three groups of bacteria were isolated and studied: moderately thermophilic iron- and mineral sulphide-oxidizing bacteria, moderately thermophilic sulphur oxidizers and extremely thermophilic Sulfolobus-like organisms. Both moderately and extremely thermophilic acidophiles were isolated from hot spring and coal pile samples. The moderately thermophilic iron-oxidizing bacteria and the extreme thermophiles which were examined were sub-divided into three and four sub-groups respectively.\ud In a comparative study of continuous flow iron-oxidation\ud reactors, moderate thermophiles did not produce higher rates of ferric iron production than the mesophile T. ferrooxidans but iron oxidation was less sensitive to inhibition by chloride in a vessel containing a thermophile than in a vessel operating with the mesophile.\ud Iron oxidation during autotrophic growth of moderately\ud thermophilic acidophiles and the rapid dissolution of mineral sulphides during the autotrophic growth of both the moderate and the extreme thermophiles were demonstated, thus considerably increasing the potential industrial significance of these bacteria. The yield of soluble copper from a chalcopyrite concentrate was shown to increase with temperature from relatively low yields with the mesophile T. ferrooxidans, through moderate yields with the moderately thermophilic bacteria to almost complete mineral solubilization with the newly isolated Sulfolobus strains
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