Dore is a mixture of gold and silver generally with less than 5% impurities. Its composition varies depending on its source and its organizational history. Gilders, when deciding whether to refine their gilt or not, can design a design around a particular resource. On the other hand, purveyors will process different foods. The amount of gold and silver and the type and number of impurities determine the most effective refining process.
The most common method adopted by large refiners is to remove the base metals followed by the separation of gold and silver (called fractionation), resulting in fine gold production. Platinum copper is obtained and separated after gold.
Melting
After melting the incoming gold for homogenization and sampling (discussed in another section), most of the lower metals are removed before separating the gold and silver. This step is usually carried out pyrometallurgical, sometimes in the same equipment used for the initial melting. In some cases, this step can be performed at the mine rather than at the refinery (e.g. retorting on site to remove zinc or mercury). Refining in “gold ovens” as practiced by coppersmiths are typical of these pyrometallurgical operations. The sludge tank that is removed is placed in a small reverberatory furnace to create a layer that is discharged.
Antimony is converted and collected as combustible dust. Selenium and tellurium change in some way, but it is collected in alkaline slag. A low silver, high copper alloy may contain enough copper to allow an oxidative melting process known as cupelation to form a copper oxide slag. This can be done in a reverberatory furnace with oxygen injection until the batch contains less than 10% copper. Slag is primarily made up of base metal oxides, but contains enough precious metals to require further processing, usually by leaching.
Silver refining
The ingots from cupellation are cast in anodes for electrolytic refining. In this process, the silver dissolves in the nitric acid electrolyte and is deposited on the cathodic surface (stainless steel or graphite) and the dendritic deposit is highly concentrated. Gold and heavy platinum metals form a sludge that is collected in the anode bag.
After cleaning, this sludge is sent to the gold refining area. Copper, a major contaminant in the electrolyte, can build up to high levels before the solution is removed from the process for silver recovery. Silver crystals are finer than 0.999 and, after washing and drying, are poured into 1,000-ounce bottles for commercial use.
Refinement of gold
Two methods have been developed for the treatment of gold-plated tails. The first is advanced chlorination of the molten metal (Miller process), followed by electrolysis of gold in an aqueous chloride solution (Wohlwill process). The second method is hydrometallurgical involving aqua regia dissolution of granulated metal followed by filtration of silver chloride and precipitation of gold. In both cases, the silver is separated from the gold in the form of silver chloride, which requires reduction in metal and electrorefining.
Additionally, platinum group metals are kept in chloride solution. As a general rule, the Miller-Wohlwill process compares economically with aqua regia refining only for large-scale production. Small refiners inevitably choose to spread because of lower capital and commodity costs.
Chlorination-Electrowinning
Chlorination is used to improve the quality of ingots to 0.95 or higher. Electrolytic cleaning, known as the Wohlwill process, produces a quality of more than 0.9995, suitable for trade and international markets. The Miller process is based on selective chlorination. The charge is melted under a borax/silica reflux at about 1100°C and chlorine gas is introduced through silica tubes fitted into the bath. Zinc, iron, tin and lead form volatile chlorides. On the other hand, copper and silver form molten chlorides which are associated with slag (Miller’s salt).
Dissolution/Precipitation
The second gold refining process involves dissolving the gold in aqua regia, a mixture of nitric and hydrochloric acid. This process is used in many small factories and, to some extent, in every refinery that uses a Wohlwill cell to make fresh electrolyte. Gold bars are fed in granular form into the reactor. Aqua regia, in the proportion of one-part nitric acid to four parts hydrochloric acid, is added. Washing with nitrogen oxide represents a significant cost for this process.
Gold and PGMs dissolve in solution, leaving silver chloride particles. When the silver chloride is carefully filtered from the solution, the gold is precipitated, usually in the form of sulfur as sulfur dioxide gas or sodium metabisulphite. An important consideration is the type of gold being raised. If it is too good, problems arise in filtering and washing. If it is too thick, the solution can be trapped. These features are controlled by the nature of the reducer and the quantity that is added to the solution. The deposited gold is collected, cleaned and melted to produce ingots with a fineness of at least 0.9995 and, with proper care, can exceed 0.9999.
Recovery and separation of PGMs
Platinum group metals are a small but important source of revenue for most refiners. These metals include platinum, palladium, iridium, ruthenium, osmium and rhodium, of which platinum and palladium are by far the most important. Only these two will be discussed here.
Platinum and palladium are soluble in aqua regia so follow the gold to the refinery. As they are less valuable, gold is best reduced, either in the Wohlwill process or in the precipitation of gold from the aqua regia process. In both cases, solutions containing platinum and palladium in the form of chlorides must be treated for their recovery. This is done by adding ammonium chloride to the solution, thereby raising the platinum as ammonium chloroplatinate. This solid is analyzed and turned into a platinum sponge.
The palladium is introduced by the addition of dimethylglyoxime which is screened and injected into a palladium sponge. On the other hand, by oxidizing palladium to its highest state with nitric acid or sodium chlorate (or even by electrolysis), ammonium chlorosalt of palladium can be formed. In general, a lot of rain and showers are needed to produce the metal with the required purity.
Alternatively, platinum and palladium can be extracted and separated by solvent extraction or ion exchange.