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This method uses chlorine, the resulting gases from the melt are very dangerous. Do not try this at home!
Chlorine gas has been used as a weapon during wars. A concentration of 60 ppm is enough to damage the lungs and a few breaths of 0.1% can be fatal.
The Miller chlorine process is a refining process used to refine large amounts of gold with chlorine gas. When chlorine gas is injected into molten gold the chlorine forms chlorides with less noble metals. The chlorides can then be easily removed as they will not alloy with the gold.
The process works well if the impurities are all base metals and silver, but it can't remove platinum and palladium as the resulting chlorides breaks down into metal and chlorine at the melting point of gold. Often the Miller process is used as an initial stage, taking the gold to 99,95% purity before running the gold through a Wohlwill gold cell. Rhodium will form a volatile trichloride and be removed.
The process is fast and cheap in large volumes, but not an easy process for the home refiner.
The process was first proposed by Lewis Thompson, and in 1867 F. B. Miller installed it in the Sydney Mint.
- GRF : A short discussion.
- GRF : Description of a setup.
- Wikipedia : The Miller process.
- Hoke : REFINING PRECIOUS METAL WASTES, page 251-253. "THE MILLER CHLORINE PROCESS FOR GOLD"
- Hoke : REFINING PRECIOUS METAL WASTES, page 266-271. "THE MILLER PROCESS AT THE HOMESTAKE MINE"
- Francis Bowyer Miller, F.C.S., J. Chem. Soc., 1868,21, 506-513