Wohlwill process
The Wohlwill process is an industrial-scale chemical procedure used to refine gold to the highest degree of purity (99.999%). The process was invented in 1874 by Emil Wohlwill. This electrochemical process involves using a cast dore ingot, often called a Doré bar, 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 of chloroauric acid. Gold and other metals are dissolved at the anode, and pure gold (coming through the chloroauric acid by ion transfer) 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.[1][2]
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 the form of chloroauric acid, on site at all times.[1][2]
A variant of the Wohlwill process is the fizzer cell. In a fizzer cell, the decaying anode is separated from cathode by a ceramic cell or nanofilter through which electric 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 ferrous sulfate, hydrazine (which is highly dangerous and explosive), or other lixiviants. A fizzer cell which utilizes a carbon rod instantly introduces contaminants into the solution. Carbon rods through which electrical current passes, begin to break down immediately upon application of current. This perceptibly worsens contamination of cell over time.
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- ^ a b Noyes, Robert (1993). Pollution prevention technology handbook. William Andrew. p. 342. ISBN 0815513119. http://books.google.com/books?id=__lqGczo9TwC&pg=PA342.
- ^ a b Pletcher, Derek and Walsh, Frank (1990). Industrial electrochemistry. Springer. p. 244. ISBN 0412304104. http://books.google.com/books?id=E_u9ARrm37oC&pg=PA244.
