What Is the Miller Process?

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  • Written By: Mary McMahon
  • Edited By: Shereen Skola
  • Last Modified Date: 14 January 2020
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The Miller process is a gold refining process that produces gold of approximately 99.95% purity, sufficient for many applications. It is faster and less costly than other refining options used to produce purer gold, which makes it a popular choice at some refineries. This technique involves passing chlorine gas through molten told to trigger a chemical reaction that separates impurities. If a refinery needs gold of a higher purity, it can send the processed bullion to a facility with other refining techniques available for additional treatment.

Refinery starts with basic smelting to extract gold and remove as many impurities as possible. Smelted gold can be poured into a crucible where it is kept hot while chlorine gas bubbles through it. The gas reacts with impurities, causing them to precipitate to the surface in the form of chlorides which can be skimmed away. Some also form gases that vent from the top of the crucible.

After approximately an hour and a half, the gold is pure enough that it can be poured into solid bullion or other storage formats. Gold treated with the Miller process is assayed to confirm the purity so the final product can be stamped to record it. For industrial processes like gold contacts on electrical equipment, gold of 99.95% purity is often acceptable. Using gold produced by Miller process can be less expensive than 99.99% gold mixtures, which require more time and money.


Environmental controls can be important during gold processing. Mining to smelting and final purification can generate a number of harmful chemicals that need to be controlled. Techniques for managing the environmental impact of gold production can include filtering industrial waste, storing chemicals in hazardous material containers for disposal, and making refinery processes more efficient to reduce overall waste and usage. Firms using the Miller process may control their chlorine to prevent environmental contamination and worker injury while also limiting waste, because chlorine can get expensive when used on an industrial scale.

Markets where gold is traded typically have strict regulations on the content and weight of gold products. This ensures consistency and prevents attempts to take advantage of traders and consumers with products that may not contain pure gold or could be underweight. Bullion can be randomly assayed and weighed to confirm that it meets the advertised specifications. Substitution of Miller process gold for other golds with a higher purity rate can be grounds for penalties.


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