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LIQUID-LIQUID EXTRACTION IN HYDROMETALLURGY

tridecanol modified aldoxime P50. Oximes are prone to hydrolysis especially when in contact with the strip acid; for commercial operation such degrada-tion should be minimized. Generally aldoximes are less stable than ketox-imes, and it has been suggested that the presence of modifiers can accelerate this degradation (Sudderth and Kordowsky 1986). This generalisation should be viewed with caution because it has been shown (Dalton and Seward 1986) that the precise naturę of the modifier can have a significant effect on the level of degradation. Finally in an extraction circuit the prob-lems of entrainment and crud are very important for efficient operation. Once again it has been shown that modifiers can have an effect on both these parameters. Clear differences were found between types of modifier and also between different molecules within the same class of reagents (Dalton and Seward 1986).

Although discussion has been concentrated on this one example of reagent/modifer combination, similar examples can be found in other systems, for example selectivity, rare earth/DEHPA (Ritcey and Lucas 1974), and phase separation, copper/Kelex 100 (Ritcey and Lucas 1974). The modę of action of modifiers is not yet fully understood and a lot morę basie knowledge is required; however, it is seetns.that in the aldoxime/modifier system the modifiers are involved in hydrogen bonding via the oxygen lone pairs to hydroxy-protons on the extractant (Dalton 1979). It is also likely that interfacial properties of the modifiers will be important for aspects such as phase separation, entrainment, and crud control. The above examples indicate that the choice of whether or not to use a modifier is not simple and that benefits other than elimination of third phase deveiopment can accrue from their use. Until the role of the modifier in extraction circuits is better understood this choice can only be madę by a series of trials which, as shown by the work above, may have to be quite extensive to attain the desired performance.

1.2.6 Diluents

The major component in the organie phase is the diluent, for, except in very few situations like the use of alcohols, ethers, and ketones, the extractant does not possess suitable properties to be used by itself. Therefore a diluent is required to:

(1)    reduce the viscosity of the extractant;

(2)    provide a suitable concentration of extractant for the metal in the feed;

(3)    reduce the tendency of extractants to emulsify with the aqueous feed, and improve dispersion and coalescence.