The recovery of gold by electrolysis is important both in the initial stages on the mine and in final purification in the refinery. The original process of electrolytic refining was developed by Dr Emil Wohlwill in the late nineteenth century. His process is based on the solubility of gold but the insolubility of silver, in an electrolyte solution of gold chloride (AuCl3) in hydrochloric acid. Impure gold or ore is cast into anodes of about 100 ounces each which are suspended in porcelain cells, while the cathodes are thin strips of pure gold. By passing an electric current from anode to cathode through the electrolyte solution, the anodes are gradually dissolved and the gold therein is deposited on the cathodes; any silver is precipitated as insoluble silver chloride to the bottom of the cells along with any platinum metals present. The gold-coated cathodes are removed, melted and cast into bars.

Gold-coated cathodes being removed from their cells after elect-
rolysis to produce 999.9 fine gold (Credit: courtesy PAMP S.A.)

The initial process can produce gold up to 999.5 fine, with further treatment bringing it up to 999.9. The disadvantage of the process is that it ties up gold in the cells for two days or more and so if gold of only 995 is required, it will usually be refined by the chemical Miller process.

Electrolysis is also used on gold mines using carbon-in-pulp recovery for ‘Electrowinning’. The gold (and any silver) is separated from the carbon into a solution of sodium cyanide and caustic soda, which is placed in electrowinning cells through which a current is passed. The gold, still with silver, is deposited on steel wool cathodes, from which it can be melted off as gold foil ready for fire-refining into doré before being sent to the refinery. See also Refining.