Microbiological oxidation of sulphide minerals from Agnes, New Consort and Sheba goldmines

Abstract

Recently the gold mining industry has been forced to treat leaner and more refractory gold ores. Pre-treatment by bacterial oxidation is one of the most promising alternative methods of recovering refractory gold. The bacterial oxidation process occurs at moderate temperature and ambient pressure. Under these conditions, oxidation kinetics are comparatively slow, and the effect of mineralogical characteristics on sulphide oxidation and gold recovery relationships assumes greater importance. Ore samples from the Sheba, Agnes and New Consort goldmines from the Barberton Mountain land were studied. Sulphides were liberated and concentrated from whole rock samples. Polished sections were prepared from selected crystals and were studied mineralogically. Bacterial oxidation test work was conducted in air-stirred Pachuca’s. Polished sections were suspended and submerged in the bacterial leach pulp (containing an arsenopyrite-pyrite flotation concentrate and Thiobacillus ferrooxidans bacteria) for various periods of time. The effect of oxidation was monitored by observations of the leached sections using an optical microscope and a Scanning Electron Microscope (SEM). The Sheba ore comprises a S-rich mineral assemblage consisting mainly of arsenopyrite and pyrite. These sulphides are chemically zoned, with respect to As-content. Both the arsenopyrite and pyrite host submicroscopic gold (up to o. 7 wt% and 0.1 wt% respectively) in the relatively As-rich zones. Particulate gold occurs mostly at the contacts of different zones in arsenopyrite. More than one generation are present in the Agnes pyrite. The core generation is compositionally zoned, with respect to As-content. The As-rich zones contain the highest amount of sub microscopic gold. A second generation of pyrite (As-poor) developed after a period of resorption. The New Consort ore consists of As-rich arsenopyrite (unzoned) and loellingite, with particulate gold being the dominant gold-type in these minerals. During bacterial oxidation of the Sheba sulphides the arsenopyrite was more rapidly oxidized than the pyrite. Preferential oxidation of the As-rich zones in the arsenopyrite took place. During the initial bacterial oxidation period of both Sheba and Agnes pyrite, dissolution channels developed along grain boundaries, zone and generation contacts and along pre-existing cracks. Square, rectangular, elongated and hexagonal shaped dissolution pits developed at different rates of pyrite oxidation. The loellingite inclusions within the New Consort arsenopyrite were oxidized extremely rapidly. This is due to the reactive nature of loellingite towards ferric sulphate. The oxidation of arsenopyrite was, however, slow. It can be concluded that knowledge of the mineralogical characteristics of gold-bearing sulphides (such as the modes of occurrence and distribution of gold, compositional zoning and generational differences), can aid in the understanding of gold recovery-sulphide oxidation curves. Furthermore, the sites of preferential bacterial attack are in most cases determined by the presence of defects in crystal structures (chemical deviations as reflected by zoning and generations or mechanical deviations such as zone contacts, grain boundaries, etc.). Detailed mineralogical information can thus provide an understanding of the behaviour of different ore types during the bacterial leaching process.