Reduction of thiourea consumption in gold extraction by acid thiourea solutions

The thiourea leaching of gold and silver from the gold-bearing silicate ore of the Hishikari mine (Kagoshima, Japan) was studied. The apparent activation energy of the extraction of gold and silver was smaller than that of the reaction rate of thiourea consumption by ferric iron. We carried out the leaching after the leaching solution was preserved for several hours. The gold recovery at 288 K was greater than that at 308 K when the pretreatment period was over 3 h. This is because the concentration of thiourea was decreased by the oxidation at 308 K, while the thiourea was not consumed by ferric ion at 288 K. It is considered that the lower temperature operation is preferable when the leaching solution is recycled.