The electrolytic method of gold refining was first developed by Dr. Emil Wohlwill of Norddeutsche Affinerie in Hamburg in 1874. Dr. Wohlwill’s process is based on the solubility of gold but the insolubility of silver in an electrolyte solution of gold chloride (AuCl3) in hydrochloric acid.
The impure gold 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, which is insoluble in the electrolyte, and any platinum group metals are precipitated to the bottom of the cells. The sequence takes about two days, following which the gold-coated cathodes are removed, melted and cast into bars. The initial process can produce gold up to 999.5 fine, with further treatment bringing it up to 999.9.
The disadvantage of the Wohlwill process is the time that the gold is ‘locked up’. Initially this led to most gold being refined by the quicker Miller Process, which can take gold to 995; as long as gold was essentially a monetary metal that was all that was required. But the modern demands of jewellery, industry, coin and investors in many regional markets for gold that is at least 999, if not 999.9, has led to the extension of electrolytic facilities at many refineries.