Solvent extraction and ion exchange

Solvent extraction is established technology for copper and uranium, but the extraction of gold has been hampered by the lack of a selective reagent. In 1984 however, it was reported that mixtures of secondary amnes and TBP were selective for aurocyanide at pH 9. These reagents are widely used in the uranium industry and ODen up the prospect of solvent extracting heap leach eluates or developing solvent-in pulp shows that the solvent mixture is quite selective for gold over copper. Gold is extracted at pH 9 and stripped at pH 11.

Resins, however, have attracted most attent on because they offer high loadings of gold without fouling by organics and are relatively easy to regenerate .Some Russian plants have used resins for years but the main disadvantages are related to their selectivity, stripping and screening. As recently discussed, strong base resins are difficult to strip and require reagents like thiocyanate or thiourea. Weak base resins on the other hand are simply stripped by pH adjustment, but most such resins are not pure and have strong base impurities. Hence they do not strip efficiently when the pH is raised. These resins also load copper zinc and nickel cyanide complexes just as readily as gold and show less discrimination than carbon. Thus the ideal resin has yet to be developed. It is therefore interesting to note that an entirely new approachs being developed which uses ion-exchange fibres woven into cloth. These are polyacrylonitrile fibre with imidazole groups or polybenzimidazole functional groups which represent a new concept in substrate and functional group design.