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High Purity Gold and the Miller Process

Here at the MGS blog, we’ve gone on and on about the importance of gold purity – from how it is measured, to record setting levels of gold purity. But how are certain levels of purity actually achieved? You may think that one just needs to be lucky enough to find an untainted vein of gold and mine it. However, such happenstance would be impractical and perhaps even impossible. Certain stores of gold become very pure because refiners make them that way. One such method of doing so is known as the Miller Process.

After gold is mined, it is usually riddled with impurities – typically lesser metals like zinc, copper, iron, and silver. These metals are obviously all very different from one another. But, they do have one key property in common: they will readily combine with pure chlorine gas to form chlorides. To take advantage of this trait, the impure gold sample is melted down in a crucible. Once the sample is liquefied, pure chlorine gas is blown across it. This causes the impurities to form chlorides – which rise to the top of the crucible. The chlorides can be skimmed off of the top of the crucible, leaving pure gold behind. With that, the Miller Process is complete and the gold is then refined in the manner specific to it’s intended use (jewelry, industry, bullion, etc.).

Invented by Dr. Francis Bowyer Miller, the Miller Process was a game changer in the world of gold refining. It is popular among metal refiners all over the world because – in a nut shell – it’s cheap, easy, and produces high-purity gold samples – about 99.95% pure gold. While that level is considered very pure by many standards, some industries require even purer gold samples. Naturally, a higher purity sample requires a more expensive and complex process. We’ll discuss that in a future blog post.

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