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LIQUID-LIQUID EXTR ACT10N IN HYDROM ET ALLURGY 5

considering these extraction types in morę detail it is appropriate to list some generał reąuirements that a reagent should satisfy for hydrometallurgical operation. The following alliterative list (Price and Tumilty 1975) sum-marizes the main features.

1.    Strength — a measure of the ability of an extractant to transfer the metal both in extraction and stripping. This includes such properties as metal loading capacity, which relates the mass of metal extracted to the mass of reagent used and determines the number of stages to meet the design capacity, and also the amount of reagent required.

2.    Selectivity —the ability to provide a pure product stream free from dele-terious impurities.

3.    Solubility—high solubility in the chosen organie diluent of both the extractant and the metal complex. This determines the required volume of the organie phase and hence the size of the plant. Also, Iow aqueous solubility of the extractant is desirable to minimize loss from the Circuit to the environment.

4.    Speed —kinetics of extraction and stripping which determine mixer size and stage efficiency.

5.    Separation —good coalescing properties of the aqueous and organie phases after contacting either in extraction or stripping, with little tendency to form st^ble emulsions.

6.    Stability —the extractant should have good stability with regard to, for example, hydrolysis and oxidation, thereby allowing extensive recycling without degradation. In addition, any degradation products should preferably be inactive towards extraction, especially of impurities.

7.    Synthesis —the extractant should have a consistent quality with little batch-to-batch variation, thereby allowing consistent plant performance following any make-up operations.

8. ‘ System —the ability to interface with the remainder of the hydrometal

lurgical flowsheet and provide a product in the form and purity required by the industry.

An addition should be madę to this list, that is, Sterling or $, the cost of the reagent which is important for the initial outlay and for make-up. Also, because it is unrealistic to assume that no organie phase will be released into the environment (Flett et al. 1983), the extractant and diluent should be non-toxic and biodegradable.