Gold starts out as tiny flakes and nuggets.
A gold refinery takes gold with impurities and processes it to generate pure gold for sale or alloy with other metals for specific purposes. Some refiners may accept batches directly from the public, while others work only with metal traders. The facility has an on-site testing station to assess the quality and value of the gold it processes on behalf of customers. Some gold refiners buy lots right away, while others may pay people based on the trial value at the end of the process.
Some refineries may accept lots of gold directly from the public.
The first step of a gold refinery typically involves batch weighing. Many may include recycled gold jewelry and product components, scrap metal, and so on. If the batch contains combustible impurities such as paper and cardboard, the refinery can place it in a low-temperature furnace to burn them off and make it easier to work with the batch during the smelting process. During smelting, the refinery adds a flux to the gold and melts it in a furnace. The gold will sink to the bottom of the furnace, while the impurities float to the top, bound to the flux in the form of slag.
At the end of the smelting, the gold refinery has a solid piece of gold bars. It may still contain some impurities like trace metals, requiring it to go through a final refining stage, where the facility re-melts the metal and treats it to force the silver and other materials to the top. One method, the Miller process, involves pumping bubbles of chlorine gas through molten gold to force out the impurities, leaving behind a solid block of pure gold.
During the end-of-process testing, the company will weigh the gold and test it to determine the level of purity. Pure gold fetches the highest prices, even if it is destined for an alloy with another metal. The gold refinery may sell the gold to other companies, such as recycled metal suppliers, jewelry stores, and electronics manufacturers.
Processes in a gold refinery can vary. Processing precious metals requires the use of caustic and toxic chemicals, and historically, pollution was a problem for many refineries. Some companies pride themselves on eco-friendly practices and may advertise specifically to customers interested in gold produced by environmentally conscious refiners. These companies control waste, use non-toxic materials when possible, and undergo independent audits to confirm they are operating cleanly and safely.