What is parting process?

Answer 1

Gold parting or parting process is the separating of gold from silver. Gold and silver are often extracted from the same ores and are chemically similar and therefore hard to separate.

Processes involving in parting of Gold.Salt cementationThis process was used from Lydian to post medieval times. It is a solid state process relying on common salt as the active ingredient but it is possible to use a mixture of saltpetre (KNO3) and green vitriol (FeSO4). The basic process involved the mixing of argentiferous gold foil (in later periods granules were used), common salt and brick dust or burnt clay in a closed and sealed container. Theophilus mentions the addition of urine to the mix. With heating, the silver reacts with the salt to form silver chloride which is removed leaving a purified gold behind. Conditions needed for this process are below 1000°C as the gold should not melt. Silver can be recovered by smelting the debris.[17] Heating can take 24 hours. Hoover and Hoover[18] explains the process thus: under heating salt (sodium chloride, NaCl) decomposes in the presence of silica and alumina (from the brick dust or clay) to produce hydrochloric acid and also some chlorine. This reacts with the silver to produce silver chloride (AgCl). The urine is acidic and aids decomposition. Silver chloride is volatile and would be removed from the metal. And the container is sealed to stop the escape of the silver which can be recovered later. Notton in experiments found that with one heating the gold content could be taken from 37.5% to 93%[19]Sulphur and antimony processesThis is similar to the salt cementation process but creates sulphides instead of chlorides. Finely divided impure gold and elemental sulphur are reacted together under moderate heat in a sealed crucible. The impurities form metal sulphides and the gold is left unreacted. The gaseous sulphide condenses on the crucible fabric. The antimony process is the same but uses stibnite (Sb2S3) instead of sulphur because stibnite is stable at a higher temperature than sulphur. This is much quicker than the salt process and gave a purer gold, but it could dissolve some of the gold as well. This process is first described in the Probierbuchlein.[20]Acid partingPure gold precipitate produced by theaqua regia refining process

The distillation was used in the 12th century Europe after its introduction from the East[21] and after that period more powerful acids could be created. Nitric acid (aqua valens) could be made by the distillation of saltpetre (KNO3) with alum (KAl(SO4)2) or vitriol (FeSO4) with water.[16][15]

2KNO3 + H2O + FeSO4 → FeO + K2SO4 + 2HNO3

Nitric acid, after distillation to increase the acid strength, was capable of dissolving the silver from a finely divided mixture of silver and gold, leaving the gold. Aqua regia was also used for parting. It was made by adding sal ammoniac to nitric acid which produced a mixture of hydrochloric acid and nitric acid. This acid dissolved the gold to a soluble chloride and the silver was attacked and precipitated as an insoluble chloride. Silver was removed by filtering and gold was then recovered by evaporating the liquid and heating the residue. Nitric acid was suitable for separating small quantities of gold from silver and Aqua regia used to separate small quantities of silver from gold.[14]

Miller processThe Miller process was invented by Francis Bowyer Miller in 1860's Australia. It is able to refine gold up to 99.5% purity. The process involves blowing a stream of chlorine gas over and through a crucible filled with molten impure gold. Impurities in the gold form chlorides before the gold does and these insoluble salts are removed from the melt by skimming the surface.[22][23] Wohlwill processInvented by Emil Wohlwill in 1874 this 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.[22][23] ElectropartingElectroparting is recent gold refining procedure where gold is separated from other metals in electrolytic cell in a way that dissolves all metals except gold and platinum. This procedure refines gold directly, producing up to 99.5% purity gold powder in a single step.