Dissolution of gold with cyanide replacingreagents

Cyanide is used in the leaching phase of hydrometallurgical recovery of gold. The toxicity and environmental risks have created a need for safer alternatives. There is a vast amount of information about gold extraction and a selection has been collected and formalized to be used in a decision support tool. The tool Auric Advisor uses case-based reasoning (CBR) to provide process alternatives for the user queries. The aim of this paper is to study leaching of gold with cyanide replacing alternatives such as ammonium thiosulfate, chloride-hypochlorite, thiourea and thiocyanate to provide new cases for the tool. The behaviour of gold was examined with electrochemical tests and weight loss measurements a using quartz crystal microbalance (QCM) were used to determine the dissolution rate. Based on literature, the dissolution rate determined for cyanide solution was 2.5 mgcm-2h-1. Based on polarisation experiments and potential measurements, the ammonium thiosulfate system with cupric ion as oxidant was estimated to dissolve gold slower than cyanide. In the QCM tests the chloride-hypochlorite alternative showed highest dissolution rate of 8.6 mgcm-2h-1 at the redox potential of 900 mV vs. SHE. The thiocyanate system with ferric ion dissolved gold at a rate of 3.4 mgcm-2h-1 at the redox potential of 620 mV vs. SHE. Thiourea system with ferric ion showed the dissolution rate of 1.65 mg cm-2 h-1 at the redox potential of 450 mV vs. SHE. The results of the rapid tests were comparable with literature and were included in the case base of the Auric Advisor decision-support tool.