Co-extraction of cations and anions in base metal recovery

Solvent extraction is used increasingly in extractive hydrometallurgy. Conventionally, for base metal recovery, metal cations are transferred selectively to a water-immiscible phase using ligands as ion-exchangers which release an equivalent number of cations, usually protons, back to the aqueous feed solution. This review focuses on a new type of extractant, polytopic zwitterionic ligands, which transfers metal salts into a water-immiscible phase. The systems discussed have metal-coordinating sites based on salicylaldimines or salicylaldoximes and the anion-binding sites are provided by pendant alkyl ammonium groups which sometimes also contain amido moieties to enhance the anion-uptake. These new reagents provide an alternative strategy for the concentration and separation of the metal values which eliminates the necessity of proton transfer, thus opening new flowsheets with good materials balances. In many cases anion- and cation-loading prove to be co-operative processes.