The process starts with the basic operations of mass inventory, homogenization, sampling and assaying of the material to be processed at the refinery. The information must be provided quickly in order to plan the previous processes required. In this way, competence, modernity and sophisticated plants are important. The basic Pre-Refining processes are copper refining, nitric acid parting and chlorination. Copper refining is appropriate to recover low grade metallic materials from mine operations or secondary sources, which are rich in copper and gold.
Initially copper materials are cast into anodes and electrorefined in a copper sulphate solution. During the process, copper and other non-ferrous metals dissolve at the anode and copper is plated on the cathode, which are converted to wirebars, ingots or billets for additional fabrication. Precious metals remain behind at the anode or fall to the bottom of the electrolytic cells as slimes, a mixture of gold, silver with other insoluble materials (e.g. arsenic, antimony). These slimes are processed for recovering precious metals using pyrometallurgical treatments for removing minor elements. Then silver is recovered by electrolysis and finally gold. It is important to bleed the electrolytic solution to eliminate soluble compounds. This is obtained through crystallization or other process. At the end the only waste materials are slags from melting and pyrometallurgical processes and some hydroxides from neutralization of the acid bleed stream.
Nitric acid treatment is suitable to treat low-grade gold alloy in silver. Out of the alloy cast into grains and the silver is selectively dissolved by the nitric acid and the gold phase is left in the form of skeleton. The solution is basically silver nitrate and is employed in the silver refining process. During this acid process, nitrous gases are produced and they must be recycled at high cost by reoxidation to nitric acid. The chlorination process also known as Miller Process is employed to treat gold alloys of high gold concentration. The process is based on the selectivity of the reaction between chlorine and metals different than gold. The order of impurity elimination is as follow: zinc, iron, antimony, tin, arsenic, copper, lead, bismuth, silver, tellurium, selenium and gold. This high temperature process is usually employed in the production of monetary grade bars. The gold content is 99.5 to 99.8%.
The high content gold alloy or Dore is loaded into a furnace and melted. Ceramic or graphite tubes are immersed in the melted bath and gaseous chlorine is sent at the bottom of the crucible. At the beginning volatile base metal chlorides are formed and the bath is characterized by the high turbulence. If the addition of chlorine is not controlled, some losses of material are expected. Silver and copper start forming liquid chloride slags and the turbulence decreases and the addition of chlorine is increased. In this way, when almost all copper and silver are removed, the turbulence appears again because gold chloride is formed. Basically, the formation of red-brown gold chloride fumes is considered indication of the end of the process and the reaction can be stopped. The Chlorination product is treated in the refining process.