Gold ore, when discovered in nature, is usually found combined with stones and other metals, such as silver and lead. This natural state does not display gold’s aesthetic virtues, nor does it easily permit workers in gold to fashion jewelry. Gold refiners use several different methods to remove these impurities and achieve a high level of purity in the final product. Removing impurities through melting the ore is a method they have used for thousands of years. Other methods have been developed over time as technology has advanced and the refiners’ understanding of chemical elements improved.
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Place the gold ore in a crucible. Put the crucible in a furnace. Heat to 1,100 degrees Celsius. Dross will rise to the surface. Periodically remove the crucible from the furnace and skim the impurities off the surface of the molten gold. Stir the gold after each removal of dross and before returning it to the furnace. Repeat this process until dross stops rising to the surface.
Set the gold ore and a small quantity of lead in a cupel. Place the cupel in a blast furnace. Heat the mixture until the lead and gold ore liquefy. The blast furnace batters the cupel with hot air. The lead combines with other metals and forms oxide compounds that separate from the gold. This process works best with small quantities of gold ore.
Submerge the unrefined gold in a mixture of nitric and hydrochloric acids. This method will dissolve the ore and separate the gold from the impurities, which can later be washed away. The remaining substances will be just water and gold, the latter having a purity level of more than 99.99 percent. Dry this residue until only gold powder remains. This method is not useful if large quantities of silver are present in the ore.
Add some copper and silver to the crucible before melting the gold in the furnace. Remove the crucible and allow the metallic mixture to cool and harden. Pour nitric acid over the metallic mixture. The acid will dissolve the silver and any base metals, and pure gold will remain. This method is called inquartation and parting.
References and ResourcesGold Traders: How to Refine Gold
Encyslopedia Brittanica: Cupellation
Ganoksin: Small-medium scale refining by inquartation and parting
Chemical Elements.com: Periodic Table-Gold