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MICHAEL COX

metal-extractant bonds are strongj morę specific reagents are required. As the stripping process involves the substitution of one ligand (strip reagent) for anothen (extractant) a study of relative stability or formation constants can provide the desired information for a good strip reagent. The commer-cial use of organophosphorus esters to datę has been largely limited to the use of tri-n-butyl phosphate, and then mainly in the nuclear industry where it is the preferred extractant for fue] reprocessing (Naylor and Wilson 1983; Naylor and Eccles 1988). TBP has also been used for zirconium/hafnium (Nandi et al. 1983) niobium/tantalum separations (Ritcey and Ashbrook 1979), where it competes with MIBK and amines (Murthy et al. 1986); and in the Falconbridge nickel process where iron in the leachate is extracted as FeClj (Thomhill et al. 1971). TBP does suffer some degradation problems, being hydrolysed to di-n-butylphosphoric acid, which is of course an acidic extractant. In spite of this potential disadvantage TBP is still widely used and studied, as a recent comprehensive text shows, (Schulz and Navratil 1987). The phosphine oxide, tri-n-octylphosphine oxide, although having the highest extractive power, is rriore expensive and only currently features in one commercial operation aą a component with di-2-ethylhexylphosphoric acid in the syneęgistic extractant mixture for uranium from wet-process phosphoric acid. However, a number of other processes, including niobium/ tantalum separation, where the niobium purity compares very favourably with that from the MIBK process (Hahn and Retelsdorf 1984; Eckert and Bauer 1984), rhenium recovery from catalyst wastes (Bright 1984) and arsenie removal from copper electrolyte Solutions (Marr et al. 1984), have been patented.

Tri-n-octylphosphine oxide is a solid and some problems can be found with solubility in common hydrocarbon diluents, especially at Iow tempera-tures. To overcome these problems American Cyanamid have produced Cyanex 923 which a mixture of four trialkylphosphine oxides where the alkyl groups are combinations of n-hexyl and n-octyl. This reagent is completely miscible with all common diluents at all temperatures. A further develop-ment has been the manufacture of a branched chain derivative, Cyanex 925, with 2,4,4,trimethylpentyl substituents. This is reported (American Cyanamid) as being morę selective than the straight chain analogue with advantages in extraction of tin, arsenie, and gallium over TOPO .

In addition to these oxygen donor extractants, other solvating reagents are available commercially. For example, sulphur analogues of the ketones and organophosphine oxides have recently been produced for metal extraction. The rationale behind this development is the ‘h£rd and soft acid and base theory’ (Pearson 1963) which concludes that some metal ions termed ‘soft' would form stronger complexes when associated with ‘soft’ donors like sulphur, rather than ‘hard’ donors like oxygen. -These ‘soft’ metal ions include Cu(I), Ag(I), Cd(II), Hg(II), Pd(II), etc. in contrast to ions such