Preg-robbing phenomena in the thiosulphate leaching of gold ores

Abstract

The ability of various sulphide and gangue minerals and gold ores to adsorb gold during thiosulphate leaching was examined. The effect of free thiosulphate concentration, metal species, gold thiosulphate concentration, ammonia concentration, particle size and cupric addition on preg-robbing was examined in detail, and likely preg-robbing mechanisms were proposed as well. It was found that all the gold ores were highly preg-robbing in thiosulphate deficient systems and that the preg-robbing process was fairly fast. The presence of free thiosulphate could greatly decrease or eliminate preg-robbing in both mineral and gold ore systems, depending on its concentration. Physical adsorption of gold was reversible in the presence of thiosulphate. Lead and zinc ions could reduce gold adsorption to some extent, and competitive adsorption of the gold thiosulphate with the lead and zinc thiosulphate species on the minerals or ores was expected. The gold elution tests demonstrated that preg-robbing was dominated by physical adsorption for the gangue minerals and was controlled by both chemical and physical adsorption for the sulphide minerals. The ability of an ore to adsorb the gold thiosulphate species from solution was found to be intimately associated with the free thiosulphate concentration. Surface topological studies by SEM coupled with EDS showed that gold was adsorbed at defect sites on the surfaces of the minerals.