A gold refinery receives gold with impurities and processes it to generate pure gold for sale or alloy with other metals for specific purposes. Some refineries may accept lots directly from members of the public, while others work only with metal dealers. The facility has an assay station on the premises to assess the quality and value of the gold it processes on behalf of clients. Some gold refineries buy lots outright, while others may pay people based on the value of the assay at the end of the process.
The first step at a gold refinery typically involves weighing the lot. Lots can include recycled gold jewelry and product components, metal scraps, and so forth. If the lot contains combustible impurities like paper and cardboard, the refinery may put it in a low-heat furnace to burn these off and make the lot easier to work with during the smelting process. During smelting, the refinery adds a flux material to the gold and melts it in a furnace. The gold will sink to the bottom of the furnace, while impurities float to the top, bound to the flux in the form of slag.
At the end of smelting, the gold refinery has a solid piece of gold bullion. It may still contain some impurities like trace metals, requiring it to go through a final refining stage where the facility melts the metal again and treats it to force silver and other materials to the top. One method, the Miller process, involves pumping bubbles of chlorine gas through the molten gold to force out the impurities, leaving behind a solid block of pure gold.
During the assay at the end of the process, the company will weigh the gold and test it to determine the level of purity. Pure gold obtains the highest prices, even if it is destined for an alloy with another metal. The gold refinery can sell the gold to other companies like recycled metal suppliers, jewelers, and electronics manufacturers.
The processes at a gold refinery can vary. Processing precious metals requires the use of caustic and toxic chemicals, and historically pollution was a problem with many refineries. Some companies pride themselves on environmentally friendly practices and may specifically advertise to customers interested in gold produced by environmentally conscious refineries. These companies control waste, use nontoxic materials when possible, and submit to independent audits to confirm that they are operating cleanly and safely.