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Why does chlorine remove impurities during the Miller process?

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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 removes impurities from gold because of the chemical reactivity of those impurities relative to gold. When the impure metal is heated to or above its melting point, gaseous chlorine is introduced into the molten mixture.1

Because most impurities—such as zinc, copper, iron, and silver—combine with chlorine more readily than gold does, they react to form chloride compounds. These resulting compounds are less dense than the molten gold and separate into a distinct layer on the surface, allowing them to be removed. This method is valued for being rapid and simple, though it typically achieves a purity level of approximately 99.5 percent.1