Oxidation behaviour of arsenopyrite during acid ferric sulphate and bacterial leaching

Abstract

The acid ferric sulphate and bacterial leach behaviour of gold-bearing arsenopyrite concentrates from four different ore deposits in Southern Africa was compared. The arsenopyrite crystals, which were compositionally distinct, were isolated and their major and trace element contents were determined on the electron microprobe. The four arsenopyrite types all proved to be sulphur-rich overall, as is most naturally occurring arsenopyrite, but they possess widely differing patterns of zonation into arsenic-rich and sulphur-rich compositional zones. These zone patterns range from weak to strong and from coarsely to finely interspaced depending on the source of the arsenopyrite. Trace elements such as gold were found to be associated with arsenic-rich zones, antimony with sulphur-rich zones, and nickel and cobalt apparently substituted for iron irrespective of zone composition. The four crystal types were then subjected to ferric sulphate leaching, and to leaching in a mixed bacterial culture. The changes during leaching were monitored microscopically, and post-leach samples were characterised by electron microprobe analysis and Auger Electron Spectroscopy. A link between the reflectivity and colour and the depth of oxidation of the arsenopyrite crystals was observed, as has been suggested by previous authors. Particles of pyrite, pyrrhotite, gersdorffite, chalcopyrite, sphalerite, galena, tetrahedrite and gold were also analysed on the electron microprobe and observed during leaching. This provided information on the leach rate of these minerals relative to arsenopyrite, and on the effect of galvanic interaction between sulphide minerals in contact. Results showed the oxidation rate of arsenopyrite to be determined by its composition, both of major and trace elements. Arsenopyrite types that showed strong compositional zonation leached rapidly, under both ferric sulphate and bacterial leach conditions. Arsenopyrite with finely interspaced zones leached more quickly than that with a coarse zone distribution. The presence of trace elements can also accelerate leach behaviour, and in this study a cobalt content of up to two mass per cent has been shown to increase the arsenopyrite oxidation rate. Arsenopyrite in contact with pyrite showed accelerated leach rates due to galvanic interaction, where the more passive pyrite is protected by the arsenopyrite. During both bacterial and acid ferric sulphate oxidation, arsenic appeared to be removed first from the surface of the arsenopyrite crystals, followed by iron, and eventually a non-passivating sulphur layer built up which remained until the crystals were completely leached away. The leach rate accelerated when bacteria were present, but the relative leach rate of the four arsenopyrite types did not change between bacterial and sterile ferric sulphate leaching. Apart from the accelerated leach rate, the major differences observed between bacterial and non-bacterial leaching were the stronger dependence on crystal orientation during ferric sulphate leaching, and the stronger galvanic effects present during bacterial leaching. Since the orientation effect would be minimal during fine powder leaching, it is clear that cheaper and more controllable ferric sulphate leach amenability tests could safely be used to predict the relative leach rate of an ore under bacterial conditions. It has also been established that a prior mineralogical examination of the ore could provide a great deal of information on its subsequent leach behaviour.