Wohlwill process
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The Wohlwill process is an industrial-scale chemical procedure used to refine gold to a high degree of purity (99.999%). The process was invented in 1874 by Emil Wohlwill. This electrochemical process involves casting a dore ingot of 95%+ gold to serve as an anode. Lower percentages of gold in the anode will interfere with the reaction, especially when the contaminating metal is silver or one of the platinum group elements. The cathode(s) for this reaction are small sheets of pure (24k) gold sheeting. Current is applied to the system, and electricity travels through the electrolyte solution of gold chloride and hydrochloric acid. Gold and other metals are dissolved at the anode, and pure gold (coming from the dissolved gold chloride) is plated onto the gold cathode. When the anode is dissolved, the cathode is removed and melted or otherwise processed in the manner required for sale or use. The resulting gold is 99.999% pure, and of higher purity than gold produced by the other common refining method, the Miller process, which produces gold of 99.95% purity.
The Wohlwill process is necessary for highest purity gold applications, and when lower purity gold is required refiners often utilize the Miller process due to its relative ease, quicker turnaround times, and because it does not require a large inventory of gold, in this case gold chloride, on site at all times.
A variant of the Wohlwill process is the fizzer cell, of which the Simplicity system of Shor International is an example. In a fizzer cell, the decaying anode is surrounded by a ceramic cell through which current can pass, but dissolved ions cannot. To recover gold from a fizzer cell, one removes the gold chloride laden solution and selectively precipitates the gold using a selective reducing agent such as copperas (ferrous sulfate), hydrazine, or several other reagents. In a fizzer cell, because no plating of gold occurs, the cathode can be as simple as a graphite rod.
