Less is more: removing impurities from gold
The extraction of gold from ore and refining it into ‘pure’ gold involves physical and chemical processes conducted on an industrial scale. Most major gold mines process their gold-bearing ore on site to produce ‘doré’ bars of up to 92% purity.
Mines send doré bars to refineries around the world, where the remaining impurities are removed to create gold of 99.5% purity or greater.
Modern refineries use chemical or electrolytic processes to separate pure gold from other substances. In the Miller Process, chlorine gas is bubbled into the molten gold. The impurities within the gold separate from it and form chlorides, which concentrate on the surface of the molten metal. It is then relatively easy to pour off the purified gold, which is about 99.5% pure.
Gold of even greater purity (99.99%) is created by more complex and expensive electrochemical Wohlwill Process. In this, an electric current is passed through an electrolyte solution of hydrochloric acid and gold chloride. Gold of 99.5% purity is used as the anode and 99.99% pure gold plates out onto the cathode.