Biooxidation as pre-treatment for a telluride-rich refractory gold concentrate

Biooxidation is an attractive alternative pre-treatment to roasting for refractory gold concentrates because it eliminates toxic gaseous emissions. Biooxidation utilises the ability of acidophilic chemolithotrophic bacteria to oxidise pyrite and arsenopyrite and release the gold or gold telluride trapped within their grains. The gold can be extracted by cyanidation and recovered using CIL/CIP technologies. Gold telluride may require further oxidation to render it amenable to cyanidation. In this study chemical and bio-oxidative treatments of a refractory, telluride-rich gold concentrate have been compared. It was shown that mesophilic iron- and sulphur-oxidising bacteria enhanced the dissolution of pyrite and arsenopyrite significantly, compared with chemical (ferric ion) oxidation. The gold telluride (calaverite) trapped within the pyrite grains was exposed during the pretreatment and underwent rapid oxidation by ferric ions in solution. Bacteria did nor oxidise the telluride directly but contributed to the process by oxidising ferrous ions to ferric ions, the so-called indirect mechanism of bacterial oxidation.