How does the Miller process purify impure gold?

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A diagram showing the periodic table entry for gold, with the symbol Au, atomic number 79, atomic weight 196.96657, electron configuration, and labels for its properties as a solid transition metal with a face-centered cubic crystal structure. — gold The precious metal gold is a chemical element with the symbol Au. Its square on the periodic table details some of its properties, including its atomic weight, atomic number, and crystal structure.

The Miller process purifies gold by leveraging the chemical reactivity of impurities compared to gold itself.

Because impurities such as zinc, copper, silver, and iron combine with gaseous chlorine more readily than gold does at or above the melting point of gold, the process follows these steps:

Melting: The impure gold is melted into a liquid state.1
Chlorination: Gaseous chlorine is blown directly into the molten metal.1
Separation: The impurities react with the chlorine to form chloride compounds, which then rise to the surface of the molten gold as a distinct layer that can be removed.1

While this method is both rapid and simple, it typically yields gold with a purity of approximately 99.5 percent.1