The Miller process
is an industrial-scale chemical procedure used to refine gold
to a high degree of purity (99.95%). 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 and can thereby 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, and of lower purity than gold produced by the other common refining method, the Wohlwill process
(which produces gold of 99.999% purity).
The Miller process is commonly used for producing high-purity gold when exacting standards of purity are not required (such as in electronics work and the manufacture of some silicates). 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 case of the Wohlwill process as gold chloride) on site at all times.