The gold yielded by ore-dressing processes remains alloyed with silver and trace impurities. It may be refined, or purified, by several methods. Electrolysis is employed at all of the United States mints. Anodes of crude gold from the mines and cathodes of pure gold are suspended in an electrolyte of gold chloride and hydrochloric acid. When current is applied, gold dissolves from the anodes and deposits on the cathodes.
Impurities remain behind.
Although a slow process, electrolysis yields nearly pure gold as much as 999.9 fine. In chlorination, chlorine gas is passed through molten crude gold. The chlorine combines with impurities, and the resulting compounds rise to the top. The gold begins to react with the chlorine only when virtually all the impurities are gone. In other refining processes, gold is treated with acids that react with impurities but not with gold itself.