Gold chemical refining is an interesting option to separate alloys containing precious metals and base metals. The alloy must content at least 70% of gold and less than 10% silver. The presence of platinum of platinum group metals is usually higher that 8%. Essentially, all the components of the alloy are dissolved by hydrochloric acid in presence of an oxidizing agent such as nitric acid, hydrogen peroxide or any halogen. For many years, the refiners are employed aqua regia preparing a mixture of hydrochloric acid and nitric acid in the proportion 4:1.
It is important to have the material in small grains in order to accelerate the dissolution process; otherwise the time and acid consumption will be higher than necessary. First the material is oxidized and silver chloride is separated from the solution and gold can be reduced and precipitated as a fine powder. The typical reducing agent is sulphur dioxide or a chemical compound able to generate SO2 once dissolved in water. Platinum group metals and copper are separated from the remaining solutions using specific processes. For a long time the gold refining process has been considered as a preferential process to treat precious metals containing high degree of impurities. The process does not need a large weight of material; the residence time in the reactor is relatively short and can be employed in small and large scale with low operating costs. The main disadvantage of the process is the emanation of acid gases and the special equipment required to control the generation of gases.