It is believed that the average gold bullion produced in gold mining operations could assay 70-90% gold and 5-28% silver. For example in South Africa, the average bullion reports 89% gold and 10% silver. The base metals encountered are mainly copper, lead, zinc and iron. In addition traces of platinum group metals, notably osmiridium, occur, generally together with some nickel values. One of the most refining processes employed was developed by F. B. Miller in which chlorine gas is passed through molten gold contained in a suitable crucible. A separation effected between the gold and alloying elements since the latter are transformed to their respective chlorides which float on the surface of the molten gold. The chlorides are bailed or skimmed into separated containers, leaving the refined gold to be poured into suitable moulds. In this way, gold acceptable for monetary purposes, i.e., at least 99.5% pure is readily obtained. The gold refining department is divided into five areas: melting, refining, de-golding, electrolyzing and alloying. See next figure for a better idea of the process.
Gold Refining Flowsheet