An assessment of oxygen availability, iron build-up and the relative significance of free and attached bacteria, as factors affecting bio-oxidation of refractory gold-bearing sulphides at high solids concentrations

Abstract

Bacterial oxidation is currently finding significant application for the oxidative pretreatment of refractory gold-bearing sulphides. Plants processing sulphide concentrates have commonly been operated at solids concentrations of between 18 and 20 per cent (m/v) (Le 180 and 200 kg.m-3). At higher concentrations, a decline in the bio-oxidation rate has been observed. Other metallurgical processes, such as chemical leaching and cyanidation, are performed at higher solids concentrations of between 40 and 50 per cent (400 and 500 kg.m-3), providing an incentive to increase the solids concentration at which bio-oxidation plants are operated. A review of literature indicated the following factors to be potential causes of reduced bio-oxidation rates at high solids concentrations: oxygen and carbon dioxide mass transfer; a low bacteria-to-solids ratio; mechanical damage of the bacte.ria; and the build-up of inhibitory oxidation products. Interaction of these factors in the completely-mixed reactors that are commonly used for biooxidation, has confounded the interpretation of the effects of individual factors. Analysis of literature data revealed a link between the sulphide grade of a particular material and the highest solids concentration at which the bacterial oxidation rate was maximal. The oxygen demand is directly proportional to the sulphide concentration in the reactor. Correlations were used to predict the oxygen transfer potential in the experimental reactors and it was found that as long as the oxygen transfer potential exceeded the oxygen demand, the biooxidation rate was proportional to the solids concentration for a specific material. Wh~n the oxygen demand equalled or exceeded the oxygen transfer potential, then the bacterial oxidation rate was limited by oxygen availability. The sulphide grade is characteristic of a particular ore or concentrate and from the data analysis oxygen availabiiity appeared to be the underlying reason why low grade materials could be oxidised at the maximum specific bio-oxidation rate at far higher solids concentrations than high-grade f!laterials. Abstract ii The experiments performed in this study were designed to further investigate the apparent relationship, identified by analysis of literature data, between sulphide grade and the solids concentration at which the bacterial oxidation rate was maximal. The effect of both solids concentration and sulphide grade on the biooxidation rate was investigated and related to the oxygen availability in the reactor.