Treatment of a refractory gold—copper sulfide concentrate by copper ammoniacal thiosulfate leaching

Abstract

The potential for leaching gold directly from a refractory AuCu sulfide concentrate, to replace the current treatment of roasting and cyanidation, was investigated using ammoniacal thiosulfate with copper. Leaching the concentrate with a solution containing 0.8M S 2O 3 2−, 0.05M Cu 2+ and 4M NH 3 resulted in over 90% gold and 60% silver recovery. Evidence of dissolution of gold tellurides was observed and contributed to the high gold extraction from the ore. However the leaching of gold tellurides was slow and high concentrations in a concentrate would be expected to reduce recovery. There was no evidence of partial alteration of pyrite in the leach which would suggest that gold inclusions present in pyrite grains will not be recovered until some pretreatment process is carried out to liberate these inclusions. Pre-treating ore by ultra fine milling followed by thiosulfate leaching resulted in 94% gold and 60% silver recovery after 48 hours. Thiosulfate leaching of roasted concentrate resulted in 98% gold and 93% silver recovery after 48 hours. Under the conditions used in this study, reagent consumption was high ranging from 36 to 70 kg/t for thiosulfate, ∼ 5 kg/t for ammonia and 1 to 2 kg/t for copper after leaching for 24 hours. Extended leaching resulted in higher reagent consumption and loss of gold and silver from solution. Preliminary investigation into generating thiosulfate in-situ were undertaken using sulfur dioxide, together with sulfur and ammonia to leach gold directly from concentrate or roasted products. Up to 1.2 M of thiosulfate was generated with gold extraction of 80% for highly refractory concentrate and >98% for roasted samples. The cost of high reagent consumption observed could be overcome by using sulfur dioxide gas produced from the roaster, sulfur and ammonia to leach gold directly from ore or roasted products.