Gold parting is the separating of gold from silver (and other metallic impurities). Gold and silver are often extracted from the same ores and are chemically similar. Their separation therefore is the usual focus of parting the alloy of gold and silver, called electrum.
Contemporary technology Four technologies are dominant. The first two start with relatively pure gold, while the third start with alloys even when there is high silver content.
The Miller process affords gold up to 99.5% purity. The process involves blowing a stream of chlorine through molten gold. Impurities in the gold form chlorides, which form a slag that floats on the molten gold. Invented by Emil Wohlwill in 1874, the Wohlwill process produces the highest purity gold (99.999%). It is an electrolytic process using pure gold for the cathode (or titanium as a starter cathode) and chloroauric acid (gold chloride-hydrochloric acid) as the electrolyte; this is made by dissolving gold with chlorine gas in the presence of hydrochloric acid. Gold is dissolved at the anode, and pure gold, traveling through the acid by ion transfer, is plated onto the cathode. Silver forms an insoluble chloride slime and copper and platinum form soluble chlorides that are removed. This procedure is used on a very large industrial scale and has a large set up cost due to the amount of gold that needs to be permanently dissolved in the electrolyte. The Aqua Regia process has been used since ancient times . It works by dissolving the sample in Aqua Regia. Silver precipitates as chloride while other impurities stay in solution. After filtration a reducing agent is added which selectively precipitates gold as metal, with 99.99% purity achievable. It is fast, cost-effective and preferable for small operations. The Acidless Separation (ALS), developed in 2014, has been designed to pre-refine doré and jewelry alloys even when there is high silver content, which is normally a problem for existing chemical pre-refinement processes. The metals are separated by vacuum distillation. It is currently adopted by the most important international refineries.
Other Alternative methods exist for parting gold. Silver can be dissolved selectively by boiling the mixture with 30% nitric acid. Affination is a largely obsolete process of removing silver from gold using concentrated sulfuric acid. Cupellation removes gold and silver from mixtures containing lead and other metals, but silver cannot be separated from gold by this process alone.
History Gold parting as a process was invented specifically to remove silver. The advent of coinage required methods to remove impurities from the gold. Over the centuries special means of separation have been invented. The main ancient process of gold parting was by salt cementation, of which there is archaeological evidence from the 6th century BC in Sardis, Lydia. In the post-medieval period parting using antimony, sulfates and mineral acids was also used.
Early history The very earliest attempts at refining gold can be shown by the surface enhancement of gold rings. Gold quality was increased at the surface by 80–95% gold compared to 64–75% gold at the interior found in Nahal Qanah Cave dated to the 4th millennium BC. Further evidence is from three gold chisels from the 3rd Millennium BC royal cemetery at Ur that had a surface of high gold (83%), low silver (9%) and copper (8%) compared with an interior of 45% gold, 10% silver and 45% copper. The surface was compacted and heavily burnished and indicates early use of depletion gilding.
Ancient and medieval world
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