Refining/Refineries

The separating and purifying of gold from other metals is called refining, as distinct from smelting which is the separation of gold from non-metallic impurities.

Gold going through refineries may either be recycled scrap being purified and upgraded, or on the final stage of its transformation from ore in the mine to bullion bars.

In major refineries two basic processes are used: the Miller Process, employing chlorine as the purifying agent; or electrolysis, a technique originally developed in the 1870s by Dr Emil Wohlwill and since then, much modernised. Using chlorine, gold can be refined up to a fineness of 995 parts per thousand but for fineness up to 999.9, electrolysis is essential. Markets do not recognise bars stamped with a fineness higher than 999.9 although a 999.99 (five nines) gold wire is refined for the electronics industry.

Pouring a 999.9 good delivery bar at the Pamp Refinery
(Credit: courtesy PAMP S.A.)

Refineries traditionally produced most of their bars by the chemical chlorine process simply because it was quicker and the gold was not tied up for several days as it is if electrolysis is used. But the shift in the demand for gold from monetary applications (for which central banks will take 995 gold) to jewellery, industry or coin has meant that since the 1960s electrolytic facilities have been installed increasingly in refineries.

More recently, solvent extraction technology has been developed and is finding application in primary refining of ore. In small refineries, processing scrap from jewellery, dental production and similar, other processes may be used, particularly the aqua regia process. This involves dissolving the gold scrap in aqua regia acid and then selectively precipitating pure gold with a chemical reducing agent such as ferrous sulphate or sodium bisulphate. This can produce gold of up to 99.99 per cent purity.

The consistent technical ability required for a refinery to maintain the quality of its bars means that achieving international acceptability is relatively difficult. About sixty refineries worldwide have their bars accepted as good delivery by the London Bullion Market Association. The largest refinery is the Rand Refinery in South Africa.