The gold chloride, being soluble, is extracted from the mixture with water, and the gold is precipitated from the solution, usually by adding ferrous sulphate:
AuCl3 + 3FeSO4 = Au + FeCl3 + Fe2(SO4)3.
3. Cyanide process. This process depends upon the fact that gold is soluble in a solution of potassium cyanide in the presence of the oxygen of the air. The powder from the stamping mills is treated with a very dilute potassium cyanide solution which extracts the gold:
2Au + 4KCN + H2O + O = 2KOH + 2KAu(CN)2.
From this solution the gold can be obtained by electrolysis or by precipitation with metallic zinc:
2KAu(CN)2 + Zn = K2Zn(CN)4 + 2Au.
Refining of gold. Gold is refined by three general methods:
1. Electrolysis. When gold is dissolved in a solution of potassium cyanide, and the solution electrolyzed, the gold is deposited in very pure condition on the cathode.
2. Cupellation. When the gold is alloyed with easily oxidizable metals, such as copper or lead, it may be refined by cupellation. The alloy is fused with an oxidizing flame on a shallow hearth made of bone ash, which substance has the property of absorbing metallic oxides but not the gold. Any silver which may be present remains alloyed with the gold.