A fundamental study of ferric oxalate for dissolving gold in thiosulfate solutions

A fundamental study was undertaken of the dissolution of gold using a novel thiosulfate leaching system that utilises ferric oxalate as the oxidant in place of the traditional cupric ammine. The key advantage of the ferric oxalate system is that the thiosulfate consumption is negligible compared to the cupric ammine system. The gold leaching kinetics were measured using the rotating electrochemical quartz crystal microbalance (REQCM), and it was found that fast gold dissolution rates can be obtained using ferric oxalate in the presence of thiourea as a catalyst. The ratio of oxalate to ferric is also very important to gold leaching as this ratio governs the speciation and reactivity of the oxidant. At low Ox/Fe ratios, the gold dissolution rate was found to be initially high, but decays due to the homogeneous reduction of the ferric complex. Electrochemical studies of the reduction of ferric oxalate showed that the more oxidizing Fe(C2O4)2− is reactive towards thiosulfate and/or thiourea, whereas the Fe(C2O4)33− complex is stable, and hence at an Ox/Fe mole ratio of 3, the gold dissolution rate does not change with time. This study also presents the effect of other parameters such as thiosulfate concentration, ferric concentration, and pH on the gold dissolution kinetics. It will also be shown that gold dissolution rates obtained with the ferric oxalate system are higher than for the copper-ammonia system under similar leach conditions.