MINERALOGICAL CONTROLS ON THE BACTERIAL OXIDATION OF REFRACTORY BARBERTON GOLD ORES

The effect of mineralogical characteristics of gold ore minerals on th e nature of sulphide oxidation during a bacterial leaching process was investigated. Three different ore types from the South African goldmi nes were used, i.e. an arsenopyritic-pyritic ore (Sheba goldmine), a p yritic ore (Agnes goldmine) and a loellingitic-arsenopyritic ore (New Consort goldmine). Detailed mineralogical characterization of each ore was performed. Thereafter, polished sections of the sulphides were su spended in a bacterial leach pulp in an air-stirred vessel for various periods of time. The effect of bacterial oxidation on the sulphides w as monitored. Different types of gold-bearing arsenopyrite exist, each type having its own characteristic behaviour during the bacterial oxi dation process. The rate of oxidation is controlled by the amount of d efects in the crystal structure. and the amount of defects is again co ntrolled by the composition of the arsenopyrite crystal. The distribut ion of refractory gold in the sulphide minerals can be correlated with the presence of compositional zones and structural deviations. These same mineralogical features also control the sites and rates of bacter ial oxidation. Thus, refractory gold occurs at sites which are prefere ntially leached by the bacteria. The rate of gold liberation from sulp hides is therefore being enhanced during the early stages of bacterial oxidation. Defects in a crystal structure influence the rate of bio-o xidation, and can be related directly to the crystal structure of the sulphide mineral, the crystallographic orientation of the exposed surf aces, and differences in chemical compositional and mechanical deviati ons in the crystals. A combination of all of these mineralogical facto rs influences the bacterial oxidation process. To optimize and to unde rstand the leaching of an individual ore it is important to establish its controlling factors.