Gold Thiourea Complex
Thiourea. Strong base resins are more difficult to elute than weak base resins, but elution can be effected either by displacement of the aurocyanide i on with another ion, or by conversion of the gold to a non-anionic complex. The latter method has been extensively exploited in Russia and involves the neutral thiourea ligand. In an acid environment, thiourea reacts with aurocyanide to form a stable gold thiourea complex.
] - +NR3Au(CN)2-+ 2(NH2)2CS +2HCl = ] - +NR3Cl + [Au((NH2)2CS)2]Cl + 2HCN
The complex is cationic and is readily eluted from the anion ion exchange resin. Alternatively, the aurocyanide anion can be displaced from the resin by another anion such chloride or sulphate. However, aurocyanide is absorbed far more strongly than chloride or sulphate anions and the elution becomes viable only when the activity of chloride or sulphate is markedly increased by the addition of a polar organic solvent such as acetone to the eluant.
The use of thiourea in acidic solutions also presents some problems: steel wood cathodes in electrowinning cells and other metal process components are readily corroded in the acid media required for thiourea elution, and the osmotic shock produced by repeated elution in acidic media and adsorption in alkaline media degrades the resin structure, resulting in resin loss.