Process parameters for bio-oxidation of sulphur in the pre-Treatment of bioleaching residues destined for cyanide gold extraction

A common problem faced during the recovery of valuable metals is the presence of impurities/unwanted elements. In order to retain a high concentration of a desired metal, all impurities which affect the metal's recovery should be removed before the final stage of recovery. In this work, research is conducted on the removal of residual sulphur from refractory gold bearing bioleach residue through the application of bio-oxidation, where sulphur is oxidized at different temperatures and pH levels to form sulphuric acid. Refractory sulphide ores and concentrates often consume large quantities of cyanide during leaching of gold using cyanide solution. Sulphides, elemental sulphur and many base metals react readily with cyanide, reducing the amount of cyanide available for leaching of the desired metal. So sulphur causes problems by increasing cyanide consumption and decreasing the environmental quality of the residue. In this research bacterial oxidation of sulphur was simulated in volumetric and Erlenmeyer flasks where inoculums were added to sulphur suspension to catalyse the oxidation of sulphur. Some of the flasks were operated at different pH levels and were controlled using a NaOH solution to investigate the optimum conditions of bacterial oxidation of sulphur. The controlled pH of 2.6 and a temperature of 50oC were found to be the best conditions for bacterial oxidation of sulphur. So this problem of sulphur consuming cyanide could possibly be overcome by controlling the pH at 2.6 and temperature at 50oC in the final stage of bacterial leaching gold plant using NaOH or some other neutralizing agent depending on the cost, which happens to enhance the kinetics of bacterial oxidation of sulphur. The operational conditions of moderate temperature and atmospheric pressure make this method very attractive from economic and environmental point of view.