The biological and chemical leaching of an auriferous pyrite/arsenopyrite flotation concentrate: A microscopic examination

Abstract Sulphide minerals can be leached both chemically and bacterially. The effects of acidic‐ferric leaching and bacterial leaching by a mixed culturé of thiobacilli and leptospirilli on a pyrite/arsenopyrite flotation concentrate were examined microscopically. A time sequence of leaching was recorded and the etching patterns examined. Pits, grooves, holes, and jagged edges were observed. Bacterial leaching progressed from fine cracks to pits, to progressively wider and longer grooves until the mineral was destroyed. Chemical leaching produced characteristic pits. The two types of etching were distinct in appearance. It was found that mineral defects and weaknesses and crystallographic orientation influenced etching. Both types of leaching were selective in that the arsenopyrite was destroyed before the pyrite. Bacterial attachment occurred and the pitting also strongly suggested direct mineral attack at the point of cell contact. Bacterial leaching was far more extensive than acidic‐ferric leaching, since cyanidation yielded 97% and 55% gold dissolution, respectively. We conclude that certain bacteria play a major role in the geochemical changes occurring in pyrite and arsenopyrite, not withstanding the chemical leaching that also takes place.