Search

The thesis considers the development of new reagents which could transport transition metal salts in extractive hydrometallurgy and addresses the ligand design features which are needed to control the strength and selectivity of binding of both a particular metal cation and its attendant anions(s). Extractive metallurgy of base metals is surveyed in chapter 1 and suggests that more efficient recovery processes are needed. One new approach, which could lead to much better materials balances in many cases, is to use hydrometallurgical techniques which involve co-extraction and transport of metal cations and their attendant anion(s). The problems in obtaining the selective extraction of anions makes the development of such a process very challenging, especially for hydrophilic anions such as sulphate which would be present in many pregnant solutions generated in processing sulfides ores. The rapidly emerging field of anion coordination chemistry and approaches to the development of selective ligands are discussed. Chapter 2 focuses on the pH dependence of sulfate-loading from an aqueous solution into chloroform solutions of a selection of zwitterionic ditopic ligands containing two 3-dialkylaminomethylsalicylaldimine units. These “salen-type2 ligands have quadridentate N2O22- binding sites for divalent metal cations and the cis-coordination of the phenolate aligns the pendant protonated 3-dialkylaminomethyl groups to bind to a sulfate dianion. Studies of the pH dependence of sulfate-loading confirm that sulfate binding is enhanced significantly by the incorporation of a divalent cation such as Cu2+ in the salen N2O22- site. Metal dication loading is very dependent on the nature of the bridging group between the two imines in the “salen” unit; Cu2+ loading follows the order ortho-phenylene > 1,2-ethane > 2,2’-biphenyl. The ethane-bridged ligand, 4,4’-di-tert-butyl-6,6’-bis(dihexylaminomethyl)-2,2’-(ethylenedinitrilodimethylidyne) diphenol, was found to have a nearly ideal loading profile for CuSo4. Stability to hydrolysis and oxidation is a key requirement for commercial metal solvent extractants. The stability of the imine bond in the “salen-type” extractants investigated in chapter 2 was tested in a two phase chloroform: water systems under conditions likely to be used for loading or stripping of metal salts. Stability is dependent on the nature of the bridging unit between the two imines varying in this order 2,2’-biphenyl > ortho-phenylene > 1,2-ethane. Two new ligands, 4,4’-di-tert-butyl-6,6’-bis(dihexylaminomethyl)-2,2’-(ethylenedinitrilo-1,1’-phenyldimethylidyne) diphenol and N,N’-dimethyl-N,N’-bis(2-hydroxy-3-[(E)-phenyliminomethyl]-5-tert-butylbenzyl)hexane-1,6-diamine, which exhibit improved stabilities at low pH are also discussed.