Thiosulphate Leaching of Gold/Silver from a Copper-Bearing Pyritic Gold Concentrate

"There are increasingly stricter environmental regulations imposed for use of cyanide leaching for the extraction of gold from ores. Alternative lixiviants to cyanide, preferably with low toxicity, have been extensively studied. Thiosulphate has emerged as one of the most suitable alternative leaching agent in place of cyanide due to its low toxicity and technical attributes e.g. fast leaching kinetics, high gold extractions from carbonaceous ores in particular. High solubility of most copper minerals in cyanide solutions results in low gold extractions due to excessively high consumption of cyanide. In contrast, low reactivity of copper minerals in thiosulphate solutions makes thiosulphate leaching potentially suitable alternative for treatment Cu-bearing gold ores. The extraction of gold/silver from a copper-bearing pyritic gold concentrate (62 g/t Au, 110 g/t Ag and 0.3% Cu) in thiosulphate solutions was investigated in this study. The concentrate is composed of predominantly quartz (25%) and pyrite (32.7%) as well as sphalerite (5.37%), galena (2.12%) and anglesite (1.13%). Other minor minerals are arsenopyrite, chalcopyrite, covellite and bournonite. Previous studies by the authors indicated that the concentrate did not respond well to cyanide leaching (i.e. 45-46% Au/Ag at 1 g/L NaCN). The effects of concentration of ammonia (0.2-1.5 M NH3), thiosulphate (0.1-0.75 M S2O3) and copper (0-20 mM CuSO4) on the extraction of gold and silver over 8 h. was investigated. Increasing the concentration of thiosulphate and ammonia substantially improved the rate and extent of gold leaching while high copper levels produced an adverse effect particularly at low concentration of thiosulphate (i.e. 0.1 M). Complete extraction of gold was achieved under suitable conditions. In contrast to gold, silver extractions remained consistently at low levels (i.e. 28%). These findings demonstrated that thiosulphate leaching could be used to achieve acceptable levels of gold extraction from the copper-bearing pyritic gold concentrate."