... and 30% of the world's gold reserves occur as refractory minerals - microscopic particles of gold encapsulated in a mineral matrix. A traditional method of extracting this gold is to use froth flotation to separate the refractory minerals from any unwanted oxide ores and non-metallic minerals present. This produces a sulphide concentrate, which is then roasted to liberate the gold. The gold is extracted by treating the resulting mixture with an aerated solution of sodium cyanide, a process called cyanidation. However, roasting converts any sulphur in the refractory minerals to sulphur dioxide and any arsenic to arsenic (III) oxide, both of which have environmental pollutant qualities. Cyanidation is also difficult. The mineral matrix acts as an impervious physical barrier and shields most of the gold particles from attack by cyanide ions. The biological approach is to treat the refractory sulphide concentrate with the thermophilic bacterium Suipholobus acidocalderius. These bacteria catalyse the ...

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