Gold when
extracted from ores has impurities contained in it. The primary impurities contained are copper,
silver and iron. This entire if not removed has considerable impact on its
properties such as malleability, conduction and shine. There are several methods, but two are most
famous, the Millers process and the Wohlwill process.
Miller’s
process involves the oxidation of metals at a very high temperature. When
molten at about two thousand degrees where all the metals in the gold ore melt,
chlorine gas, is blasted through it at a very high pressure. This when comes
into contact with those metals which can be considered as impurities, react
very quickly. High temperature also speeds up the process and the resulting
combination produces oxides of various metals such as silver fluoride, iron
fluoride etc. This process is simple and
not very expensive but the level of purity which can be achieved is around
99.95%. It might sound very pure, but still for several uses, gold needs to be
more pure.
The metal
chlorides which are formed during the blast of gas, are mostly in molten liquid
form. Because of low density, the solution rises to the surface from where they
are skimmed. They are further processed to extract other metal from them.