Gold Refining | From Gold Ore to Doré | World Gold Council
The Miller process uses gaseous chlorine to extract impurities when gold is at melting point; impurities separate into a layer on the surface of the molten purified gold. The Miller process is rapid and simple, but it produces gold of only about 99.5 percent purity. The Wohlwill process increases purity to about 99.99 percent by electrolysis.