The Miller process is an industrial-scale chemical procedure used to refine gold to a high degree of purity (99.95%). It was invented by Francis Bowyer Miller ( patented 1867). This chemical process involves blowing a stream of pure chlorine gas over and through a crucible filled with molten, but impure, gold. This process purifies the gold because nearly all other elements will form chlorides before gold does, and they can then be removed as salts that are insoluble in the molten metal.
When all impurities have been removed from the gold (observable by a change in flame color) the gold is removed and processed in the manner required for sale or use. The resulting gold is 99.95% pure, but of lower purity than gold produced by the other common refining method, the Wohlwill process, which produces gold to 99.999% purity.
The Wohlwill process is commonly used for producing high-purity gold, such as in electronics work and the manufacture of some silicates, where exacting standards of purity are required. When highest purity gold is not required, refiners often utilize 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.