Improved thiourea leaching of gold from a gold ore using additives

Abstract

Thiourea (TU) is a low-toxicity gold lixiviant that has the promise to replace the highly toxic cyanide for gold extraction. The significant issue of high thiourea consumption and the suspected carcinogenic nature, however, severely restricts its industrial application. In this paper, oxalate and citrate were used as additives to enhance the thiourea leaching of gold from a gold ore. The leaching results showed that the thiourea consumption was significantly reduced whilst the gold extraction improved to be comparable to that by cyanidation. The presence of additive was found to achieve a faster and higher gold adsorption by activated carbon due likely to the much-reduced side products from thiourea decomposition. In addition, citrate has a better performance than oxalate under the same additive dosage. Analytical results demonstrate the possible mechanisms for the additive-improved thiourea leaching of the ore. Oxalate and citrate can complex with the oxidant Fe3+ forming its complexes mainly of Fe(ox)+/Fe(ox)2− and Fe2(cit)2(OH)22−, which stabilizes Fe3+ and lowers the solution potential, thus decreasing thiourea consumption. The additive-improved thiourea leaching also significantly reduced the formation of the passivating product, i.e., calcium sulfate hydrate, and thus effectively weakened the impediment of gold leaching caused by calcium sulfate hydrate. The cyclic leaching results further suggested that high gold extraction with relatively low thiourea consumption was achieved in five cycles of the citrate-improved thiourea leaching. Thus, citrate-improved thiourea leaching technology appears promising for the extraction of gold from gold ores.