Gold leaching from oxide ores in alkaline glycine solutions in the presence of permanganate

Abstract In this study, the effect of using strong oxidants with alkaline glycine solutions on the leach rate and recovery of gold was studied in the absence of cyanide. Prior research indicated that when using only alkaline glycine and air at ambient temperature in the absence of catalysts, leaching times would be too long for conventional agitated tank leaching of ores. The ores used in this study were from oxidised geological domains, which would not be expected to excessively consume oxidant. Ores with reactive sulfides or other reducing minerals would be expected to rapidly consume an oxidant, making the process ineffective. The influence of the oxidants such as potassium permanganate, hydrogen peroxide, sodium chlorate, potassium iodide, and potassium ferricyanide were initially evaluated in a screening test. The highest gold extractions were observed with potassium permanganate, where a recovery of 85.1% was achieved at 1.5 kg/t glycine and 3 kg/t permanganate at 30% solids, pH 10.5 and ambient temperature using a bottle roll process. This compared to a 87.4% recovery against a conventional cyanidation benchmark with 30% solids, pH 10.5 and ambient temperature using the bottle roll. Other oxidants only achieved a recovery of 11% at best, so permanganate was the focus of the optimisation work. The solution pH had very little effect on gold recovery between 9.5 and 11.5. The ratio of glycine to permanganate concentration is significant, if potassium permanganate is in a molar excess, the recovery is greatly reduced due to oxidation of the glycine. The main products of this reaction, when lime was used as pH modifier, were determined to be calcium carbonate, calcium oxalate, trace amounts of ammonia and manganese dioxide. Gold is recoverable from leach solutions by conventional adsorption onto activated carbon.