The Miller process is an industrial-scale chemical procedure used to refine gold to a high degree of purity (99.5%).
This chemical process involves blowing chlorine gas through molten, but (slightly) impure, gold.
The other metals volatilize or form a low density slag on top of the molten gold.
[1][2] The Wohlwill process is commonly used for producing high-purity gold, such as in electronics work, where exacting standards of purity are required.
When highest purity gold is not required, refiners use the Miller process due to its relative ease, quicker turnaround times, and because it does not tie up the large amount of gold in the form of chloroauric acid which the Wohlwill process permanently requires for the electrolyte.