Improving the cementation effect of copper on gold in thiosulfate solution by pre-regulating the existing form of Cu2+

The effective recovery of Au(S2O3)23− from leachates using thiosulfate gold extraction techniques remains challenging. Metal cementation can be efficiently used to recover Au(S2O3)23−, but is easily affected by the oxidant (Cu(II)) in solution, leading to problems such as high consumption of the reduced metals and copper-gold co-precipitation. Copper powder cementation can prevent the co-precipitation of copper in solution to improve the grade of the gold product and also supplement the oxidant in the solution, thereby enhancing the cyclic leaching of gold into the solution, which can reduce production costs. In this study, copper powder was used to recover gold ions and the results showed that the gold cementation rate was negatively correlated with the Cu(NH3)42+ concentration. In order to weaken the negative influence of Cu(NH3)42+, this study proposes the application of a reduction voltage to convert Cu(NH3)42+ in solution to Cu(S2O3)35− before adding copper powder. After applying a reduction voltage of 0.14 V to a solution of 5 mmol/L CuSO4, 0.5 mol/L NH3, 0.1 mol/L Na2S2O3, and 10 mg/L Au(S2O3)23− for 20 min, the concentration of Cu(NH3)42+ in the solution changed from 5 mmol/L to 0.88 mmol/L. Under these conditions, gold ions were recovered, and the consumption of copper powder was reduced by 60%. Scanning electron microscopy and X-ray photoelectron spectroscopy characterization showed that under a applied reduction potential, the gold content on the surface of the copper powder was higher, and the Cu(II) content in the surface copper species was lower than that achieved in the direct cementation of copper powder. After cementation was completed, stirring the residual solution for 20 min restored the concentration of Cu(NH3)42+ to 4.963 mmol/L. At this point, the leaching rate of gold ore into the solution reached 89% of the first leaching, which maximized the circulation of the solution.