When powdered zinc is added, the gold is precipitated. In certain ores, gold and gangue may be separated by flotation. Finely ground ore is placed in a solution of chemicals and water. 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.