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

The acid ferric sulphate and bacterial leach behaviour of compositionally d istinct gold-bearing arsenopyrite concentrates from four different ore depo sits were compared. The changes during leaching were monitored microscopica lly, and post-leach samples were examined by means of electron microprobe a nalysis and Auger Electron Spectroscopy. Results show that the oxidation rate of arsenopyrite is determined by its m ajor 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 pr esence 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 t o be removed first from the surface of the arsenopyrite crystals, followed by iron, and eventually a thin, nonpassivating sulphur layer forms, which p ersists 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 observe d is the strong dependence on crystal orientation during ferric sulphate le aching, 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 le ach amenability tests can safely be used to predict relative leach behaviou r of an ore under bacterial conditions. A prior mineralogical examination o f the ore could provide a great deal of information on its subsequent leach behaviour.