Pressure Oxidation of Arsenopyrite Concentrates at New England Antimony Mines

New England Antimony Mines has operated the Hillgrove Gold Plant at Hillgrove in New South Wales since 1969, producing stibnite and arsenopyrite products by selective flotation. Gold is present in the ore as fine free gold, gold associated with stibnite and gold associated with arsenopyrite, with some ten to 20 per cent of the gold reporting to the antimony stream and 35 - 50 per cent reporting to the arsenopyrite stream. Opportunity to process the arsenopyrite concentrate for onsite recovery of the gold values to improve upon the ten per cent recovery achieved by conventional cyanidation was recognised and subject to comparison with biological oxidation, roasting, nitric acid oxidation, low temperature oxidation (Activox«) and conventional pressure oxidation. Following further testwork and a feasibility study, a 7500 tonnes per annum concentrate feed pressure oxidation autoclave circuit was constructed at Hillgrove and commissioned in October 1999. The plant has operated at the close to design capacity since commissioning whilst optimising various operational and metallurgical aspects. The last three years has seen the process achieve an overall gold recovery of greater than 90 per cent for oxidised concentrates.