Cyanide-free gold leaching in exceptionally mild chloride solutions

Abstract

All the industrially applied gold leaching methods (historical chlorine gas based leaching, dominating state-of-the-art cyanide gold leaching, processes at precious metals plants) suffer from the characteristics related to aggressive and even toxic leaching media and high chemical consumption. This study targets environmentally sound cyanide-free gold leaching in mild chloride media in terms of minimizing chemical consumption. In the current study, it was investigated whether providing instant gold recovery (carbon-in-chloride-leach, CICl) could allow high gold recovery in a mild and non-toxic leaching environment. The investigated leaching parameters were S/L ratio, T, type of oxidant i.e. [Cu2+]/[Fe3+] and [Cl−]. The results showed that gold could be dissolved in exceptionally mild conditions, when an appropriate adsorption/reduction (activated carbon) site was provided immediately after leaching. It was found that impurity metals iron and copper originating from the gold ore (Fe 1.6% and Cu 0.05%), were advantageous self-initiating oxidants and 87% of gold could be dissolved in pure calcium chloride (2.8 M) solution. In addition, no bromide, which is a commonly added aggressive additive in modern cyanide-free processes, was required. The lowest chloride concentrations applied were comparable (0.6 M) or even milder (0.3 M) than those typical of seawater chloride concentrations, and could still result in gold recovery, 72% and 64%, respectively, with copper as oxidant. Conventionally gold extraction is assumed to require highly aggressive leaching media, high redox potentials, and high gold complex stability in the solution. The findings presented can provide a competitive environmental and economic edge and therefore new horizons for future cyanide-free gold chloride technologies, suggesting that in future, even seawater can act as the basis for cyanide free gold leaching.