Crude gold obtained by MacArthur-Forrest cyanide process and chlorination process contains Ag, Cu, Zn, and sometimes Pb as impurity.
(A) Zn and Pb are removed by cupellation process.
(B) Cu and Ag are removed by parting process.
(1) Parting with sulfuric acid or nitric acid:Gold is not attacked by these acids while Cu and Ag dissolve. If, however, the Au content in an impure sample is more than 30%, the Cu and Ag are also not attacked by the acid of any strength. Hence, before the acid treatment, the impure sample is melted with necessary amount of Ag to reduce its gold content to about 25% (quartation). The resulting alloy, after being granulated in water, is boiled with H2SO4 or nitric acid when Cu and Ag pass into solution, leaving Au undissolved . Au is separated and
fused again with borax and nitre when 100% Au is obtained.
(2) Parting with CI2 : Sometimes chlorine is used for the purification of Au. The impure sample of Au is fused with borax and CI2 gas is forced through it. The base metals are converted into chlorides that pass out as fumes at this high temperature, and AgCI forms a separate layer between the fused layer of Au and borax, which is skimmed off and the Au left behind cast into ingots.
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