Mineralogical factors affecting arsenopyrite oxidation rate during acid ferric sulphate and bacterial leaching of refractory gold ores

Abstract

The acid ferric sulphate and bacterial leach behaviour of compositionally distinct gold-bearing arsenopyrite concentrates from four different ore deposits were compared. The changes during leaching were monitored microscopically, and post-leach samples were examined by means of electron microprobe analysis and Auger Electron Spectroscopy. Results show that the oxidation rate of arsenopyrite is determined by its major and trace element content. Arsenopyrite types which show strong zoning into As-rich and S-rich zones leach rapidly under both ferric sulphate and bacterial leach conditions. Arsenopyrite crystals with finely-interspaced zones leach more quickly than those with a coarse zone distribution. The presence of a cobalt content of as little as 0.1 mass per cent increases the arsenopyrite oxidation rate. During both bacterial and acid ferric sulphate oxidation, arsenic appears to be removed first from the surface of the arsenopyrite crystals, followed by iron, and eventually a thin, non-passivating sulphur layer forms, which persists until the crystal is leached out. The relative leach behaviour of the four arsenopyrite types does not differ between bacterial and non-bacterial leaching. The major difference observed is the strong dependence on crystal orientation during ferric sulphate leaching, and the stronger galvanic effects present during bacterial leaching. The leach rate is also accelerated by the mere presence of the catalytic bacteria. Since the orientation effect would be minimal during fine powder leaching, it is clear that cheaper and more controllable ferric sulphate leach amenability tests can safely be used to predict relative leach behaviour of an ore under bacterial conditions. A prior mineralogical examination of the ore could provide a great deal of information on its subsequent leach behaviour.