Fungal-Transformation of Surrogate Sulphides and Carbonaceous Matter in Refractory Gold Ores: Revisited

Abstract

In the recovery of gold from refractory gold ores, pretreatment is required to decompose sulphides and liberate occluded gold before cyanidation, and to deactivate carbonaceous matter and prevent it from adsorbing dissolved gold. Until the past three decades, most commercial pretreatment processes had been by abiotic means. Biological pretreatment methods on commercial basis is therefore a recent phenomenon, and several researches are underway to assess the ability of different biological agents in the breakdown of sulphur and carbonaceous matter (CM) in gold ores. This paper, which is a revisit of an earlier one, presents an overview of on-going research aimed at assessing the capability of the fungus, Phanerochaete chrysosporium, to degrade sulphides and CM. Surrogate carbonaceous materials (lignite, bituminous and anthracite coals) and pure sulphides (pyrite and arsenopyrite) were used to model the behavior of CM and sulphides in refractory gold ores. To monitor the extent of biotransformation, preg-robbing test was conducted on the as-received and treated CM, and sulphide sulphur in the residual sulphidic materials was also determined. From an initial preg-robbing effect of 125 µg of gold per gram of CM, the ability of CM to preg-rob gold reduced by 70-95% in the order of lignite<bituminous <anthracite within 21 days of treatment, whereas there were 18% and 39% oxidation of sulphide sulphur in pyrite and arsenopyrite respectively. XRD examination of the treated anthracite confirmed reduction in the graphitic structure of carbon following fungal transformation. Similarly, there was a decline in the major sulphide peak after microbial pretreatment. The results indicate that the fungus biotransforms through destruction of the ordered structure, followed by introduction of oxygen groups. The amorphous nature, thus generated, inhibits the uptake of aurocyanide ions by CM, while enhancing the affinity of cyanide for the oxidised sulphide material in subsequent cyanidation treatment. The findings contribute to knowledge on novel and technically viable alternative methods for oxidative pretreatment of refractory gold ores.

Keywords: Phanerochaete Chrysosporium, Carbonaceous Matter, Metal Sulphides, Biotransformation