Study on selective dissolution of gold in alkaline thiourea media

Abstract

Gold dissolves easily in an alkaline thiourea solution with Na₂SO₃ and Na₂S₂O₈, where the accompanied elements of Au, such as Ag, Cu, Ni and Fe can hardly dissolve. It was considered that electrochemical reduction and catalytic action of SO₃²⁻ prevent thiourea from decomposing irreversibly and accelerates dissolution of gold. The S₂O₈²⁻ as an oxidant can control the potential so that formamidine disulfide may form efficiently. The dissolution of gold is mainly through oxidation of Au by formamidine disulfide and compexation of thiourea with Au⁺, and not direct oxidation of S₂O₈²⁻. Thus, sufficient formamidine disulfide is necessary to accelerate the dissolution of gold. The reason for Ag, Cu hardly being dissoluble is that the black layers of Ag₂S and Cu_1.92S form on the surface of Ag and Cu. The metal Ni and Fe do not dissolve in the alkaline media due to their sulfide films passivation samely.

Keywords

mechanism / selective dissolution of gold / alkaline thiourea / electrochemical reduction and catalytic action / formamidine disulfide / mix potential

References

[1] GroenewaldT. Electrochemical studies on gold electrodes in acidic solutions of thiourea containing gold (I) Thiourea complex ions. J of Applied Electrochem, 1975, 5: 71-78

[2] GroenewaldT. The dissolution of gold in acidic solutions of thiourea. Hydrometallurgy, 1976, 1: 277-290

[3] HuangL H. Elementary investigation of mechanism on thiourea dissolution of gold. Gold, 1980, 3: 64-68

[4] ChenC K, LungT N, WanC C. A study of the leaching of gold and silver by acidothioureation. Hydrometallurgy, 1980, 5: 207-212

[5] SanB J, ZhangF Y, ChengH. Dissolution of gold in alkaline thiourea solution. Gold, 1984, 3: 45-47