Cyanide leaching vs. thio-compound leaching

In recent years thiourea and thiosulphate leaching gained a certain attractivity because it was believed that less toxic or non-toxic reagents are involved. An additional argument for thiourea or thiosulphate are their high leaching rates that can reach 250mg/cm² per hour exceeding cyanides almost by a factor of 100. The technical implementation of a thio-compound leaching process, however, implies a series of problems and therefore we see very few operations that apply thiourea as an alternative lixiviant. Another adverse factor are the high costs for the reagents because of high reagent consumptions as compared to sodium cyanide. The gold leaching reaction by thiourea and thiosulphate is very sensitive to pH and redox potential and both chemicals are intrinsically unstable decomposing readily to substances that are unable to leach gold. This means that process control is difficult and reagent recycling is very limited if not impossible. The seemingly low toxicity of the two compounds as compared to cyanide is only valid regarding their lethal toxicity. Decomposition of thio-compounds generates a series of partly highly toxic products that must not be released into the ecosystem. In addition to this thiourea is suspected to be carcinogenic.