Parting is a traditional process employed to refine gold and comprises the separation precious metals and base metals by the addition of nitric acid. The main stages of the parting process are granulation of the alloy, addition of nitric acid, treatment of the gold residue, and reduction of silver. The process was employed in Germany in secret and some years later the method was introduced in England. Basically, the gold to be refined must be almost free of base metals, especially from those with low solubility (e.g. tin, arsenic, antimony). If the insoluble compounds are present, the formation of insoluble oxides is a problem due to they will be present with gold until the end of the process and a new reefing stage would be required. Other problem associated to the presence of base metals is the excessive consumption of nitric acid. The presence of highly soluble elements such copper and lead is not a problem due to they can be separated successfully during the reduction of silver. Although, the presence of copper favors the dissolution process, its presence must be controlled.
Since the refinery receives several types of Dore bars, they must be melted in order to form an alloy with 25% gold and 75% silver. This is the ideal proportion, but some refineries use a less proportion of silver (e.g. 70%). Dore bars with low content of gold are preferred due to the final balance between gold and silver can be obtained easily. Once the alloy is obtained, it is introduced in the crucible, melted and when the molten charge is homogeneous, the metal in poured in a vessel with water. It is important to add the metal properly in order to avoid the formation of metallic lumps. The idea is to form small granules that will react with nitric acid.
During the dissolution process, granules are heated with nitric acid using vessel made of platinum or porcelain. The addition of nitric acid (25%) is approximately 6 kg per kilogram of gold. This is the ideal dosage, which is influenced by the copper content. For this reason is important to add the acid in two parts, the first addition of acid is 50% of the total. The bath is heated until all the silver has been dissolved. The solution is allowed to settle and the hot supernatant liquid is removed. The second addition is 25% of the total and the solution is heated and the supernatant liquid is removed again. The last addition of nitric acid is similar to the previous additions. It is important to mention that part of the silver nitrate is trapped by the fumes and scrubber system must be able to minimize the losses of silver. It has been noted that silver, copper and zinc and the main elements dissolved by the acid.