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How do the Miller process and the Wohlwill process refine impure gold?

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A worker in safety gear walks beside a conveyor belt carrying ore at the Tanami gold mine in the arid, expansive landscape of the Tanami Desert. — gold processing The Tanami gold mine, Northern Territory, Australia.

Gold extracted through methods like cyanidation often contains impurities such as silver, copper, iron, and zinc. Two primary methods are used to refine this metal:

The Miller Process

The Miller process is a rapid, simple technique that relies on chemical reactivity. Impure gold is melted, and gaseous chlorine is blown into the liquid. Because impurities combine with chlorine more readily than gold does, they form chloride compounds that rise to the surface as a separate layer. This method typically achieves a purity level of approximately 99.5 percent.1

The Wohlwill Process

The Wohlwill process utilizes electrolysis to achieve a higher purity of about 99.99 percent. An impure gold anode is placed in an electrolyte solution of hydrochloric acid and gold chloride. When an electric current is applied, the anode dissolves; impurities either settle as insoluble slime or dissolve into the solution, while pure gold migrates to the cathode. While highly effective, this process is costly due to the required inventory of gold in the electrolyte.1