Gold is purified through a process of high temperature heating or chemical exposure, depending on the purity of the mined gold, according to ResponsibleGold.org.
Initial Processing of DifferentTyes of Ore
If the gold is a low grade ore, then it is broken up into chunks that are then put in carefully lined pads and treated with a dilute cyanide solution, which dissolves the gold. For high grade ore, the metal is sent to a grinding mill and made into a powder. Refractory ore contains carbon and is heated to over 1000 degrees F, which removes sulfide and carbon. The resulting oxide ore is directed to the leaching circuit. Sulfide refractory ore that contains no carbon is oxidized in an autoclave to free gold from sulfide minerals, then it is sent to the leaching circuit.
At this point, treated high grade ore is leached with cyanide and gold is collected onto activated carbon with the cyanide solution being recycled. The gold-carbon mixture is put in a vessel where the gold is removed chemically. The carbon is then recycled. The gold is then extracted from the solution by electrolysis or chemical substitution.
Purifying the Gold
At this point, the gold is melted into dore bars composed of 90 percent gold. The bars are then sent to an external refinery to make them 999.9 parts per thousand pure gold.
Other Means of Refining Gold
According to Hoover and Strong, a refiner and manufacturer of precious metals, they produce 98 percent pure gold using the Miller process. After a sample of treated impure gold has been tested in a lab for purity, the gold is melted in a furnace, then chlorine is bubbled through the liquid. The chlorine attaches to elements in the gold that then become solid and move toward the top of the furnace. They are skimmed off. Electrolysis is ultimately used to purify the gold.