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LIQUID-LIQUID EXTRACTION IN H YDROMETALLU RGY 11

be used to extract cadmium from, for example, wet process phosphoric acid (Bierman et al. 1985).

Cyanex 302 has a similar performance to the dithio compound but has a higher hydrolytic stability (American Cyanamid). Hoechst also supply two similar extractants in HOE F 3541, a dialkyldithiophosphoric acid ester, and HOE F 3787, di-2-ethylhexyl monothiophosphoric acid ester.

1.2.1.2    Chelating acid extractants

During the past 20 years the commercial availability of these extractants has provided a particular stimulus to the use of liquid.-liquid extraction in hydro-metallurgy. This was largely due to the development of reagents which were selective for copper over ferric iron in acidic leach Solutions. The back-ground to this development has been widely documented (Ritcey and Ashbrook 1984) and many papers have been published especially on the copper-hydroxyoxime system (Hanson and Whewell 1981). In addition to hydroxyoximes other reagents have been proposed for copper extraction from both acidic and ammoniacal Solutions. The former category includes extractants based on 8-quinolinoI, for example: Kelex 100 and 108, LIX 26; 8-sulphonamidoquinoline, LIX34, whereas the latter are mainly derivatives of 1,3-diketones, LIX54, DK16. Ali these chelating reagents (Table 1.3) are capable of extracting other metals, but under conditions which normally allow adequate separation factors for copper to be achieved. The separation of copper and iron with the 8-quinolinol derivatives is interesting in that it is based on kinetic rather than equilibrium factors (Fleming et al. 1980). Ali these extractants rely on a combination of an acidic function and a specific chelation effect with the metal to achieve selectivity and extraction. In addition to these groupings other substituents are incorporated to convey com-patability with the organie phase and satisfy particular steric requirements (Dalton and Price 1982). Thus this type of reagent offers the greatest poten-tial for the design and production of selective extractants for any desired element; whether or not these will become available commercially in the futurę will depend mainly on economic factors.

1.2.2    Basic, anionie extractants

In hydrometallurgical processes these basie extractants are limited to the alkylamines and quaternary ammonium compounds. Although it is possible to use other cationic ’onium species in analytical applications (Sekine and Hasagawa 1977) their cost and potential toxicity make them unsuitable for hydrometallurgical use.

Extraction depends on the ability of metal ions to form anionie species in the aqueous phase which are then extracted as ion-pairs by aminę salts in an ion-exchange process (eqn(2.!2)).