This process is employed to refine gold and was developed and introduced in the gold mining industry by Emil Wohlwill. The basic electrolyte consists of an aqueous solution of auric chloride containing 55 to 35 g/L gold, mixed with 25 to 55 g/L hydrochloric acid and chlorine gas. Sodium chloride can be employed instead of hydrochloric acid. Basically the amount of sodium chloride must not be higher than 100 g/L due to silver chloride is highly soluble in strong solutions of sodium chloride and the excess of chloride ions will promote the deposition of silver on the cathode. The anode consists of impure gold bullion containing lead, silver, platinum, iridium and other impurities. The cathodes are made of thin sheets of electrolytically deposited gold of the same length as the anodes, but narrower for during the course of the electrolytic process, gold deposits grow very fats in every direction. There is not formation of arborescent or loose crystalline growths at the cathode even when the current density is as high as 100 amp/ft2.
Ideally the distance between anode and cathode is not more than 30 mm and there is no danger of short circuit due to metallic growths at the cathode. At the beginning of the process the electrolyte is a solution of auric chloride with hydrochloric acid and the cathode deposit is looser and less dense than it if the soluble impurities from the anode have been impregnated with chlorides of lead, platinum and other metals. It has been noted that palladium could be a problem and the solution must not content more than 5-8 g/L.
The impurities present in the anode partly pass into the solution and partly are deposited as anode sludge. Also, the anode sludge contains silver chloride and 10-15% gold. The gold is deposited in the anode sludge in its metallic state. The presence of gold is not due to the mechanical disintegration of the anode. Basically, there is a different mechanical condition on the gold anode and the gold in the sludge. The latter is aurous chloride, which is converted by a secondary reaction into auric chloride and gold.
A high current density at the anode and a high temperature of the electrolyte are unfavorable to the formation of aurous chloride, and these conditions are conductive to a small amount of gold present in the anode sludge and must, therefore, be as far as possible attained. With a current density of 130-140 amp/ft2 and a temperature of 50-70 oC the gold present in the anode sludge is reduced to a minimum. With lower amperes, aurous chloride is formed at the anode by the direct action of the current, and a high amount of gold appears in the sludge.