Miller process
Encyclopedia
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. 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 chloride
s 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 Miller 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 not 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
.
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
to a high degree of purity (99.95%). It was invented by Francis Bowyer Miller. This chemical process involves blowing a stream of pure chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
gas over and through a crucible
Crucible
A crucible is a container used for metal, glass, and pigment production as well as a number of modern laboratory processes, which can withstand temperatures high enough to melt or otherwise alter its contents...
filled with molten, but impure, gold. This process purifies the gold because nearly all other elements will form chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
s 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
Wohlwill process
The Wohlwill process is an industrial-scale chemical procedure used to refine gold to the highest degree of purity . 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...
, which produces gold to 99.999% purity.
The Miller 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 not 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
Chloroauric acid
Chloroauric acid is a inorganic compound with the formula HAuCl4. This pale yellow compound is a common precursor to gold in a variety of purposes. The term chloroauric acid is also sometimes used to describe other gold chlorides....
which the Wohlwill process permanently requires for the electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
.