Enhancement of gold extraction from biooxidation residues using an acidic sodium sulphite-thiourea system

In this paper, a gold extraction process from an acidic biooxidative residue was studied. Pretreatment by washing the biooxidation residues with a pH = 2 sulphuric acid solution was needed to eliminate the excessive thiourea consumption by iron (III). By adding Na2SO3 powder into the acidic thiourea medium, the consumption of thiourea decreased significantly from 12 g/kg to 3 g/kg of ores and the gold extraction time was shortened remarkably from 6 hours to 1 hour. The optimum conditions for efficient gold extraction were: a lixiviant of 3 g/kg of thiourea and 4.5 g/L of sodium sulphite in a pulp density of 33% (w/v) in a pH = 2 medium with stirring for 1 hour at room temperature. A gold extraction of more than 95% was achieved with a non pre-concentrated low-grade ore obtained after biological oxidation. The direct gold extraction technique in the low pH Na2SO3 — thiourea system is of great advantage over the traditional cyanide process as it eliminates the costly neutralization process and greatly reduce the risks of environmental pollution.