Why can the Wohlwill process produce purer gold than 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 and the Wohlwill process are both used to refine gold, but they achieve different levels of purity due to their underlying chemical and physical mechanisms.1

The Miller process relies on the chemical reaction between gaseous chlorine and impurities in molten gold. While efficient and rapid, it typically yields gold with a purity of approximately 99.5 percent.1

In contrast, the Wohlwill process utilizes electrolysis to achieve a higher purity of about 99.99 percent. In this method, an impure gold anode is dissolved in an electrolyte solution of hydrochloric acid and gold chloride. Under an electric current, the gold migrates through the solution to a cathode, leaving impurities behind either as dissolved substances or as insoluble "slime" at the bottom of the tank. This electrolytic separation is more precise than the chlorination method, allowing for the recovery of gold in a highly pure metallic state.1