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LIQUID-LIQUID EXTRACTION IN H YDROMETALLURGY 15

manganese. However, most systems invo!ve complex anions of the minerał acids and their salts used in leaching. The extent of complexation by these common anions is nitrate < chloride < sulphate. Thus nitrate only forms weak complexes in aąueous solution but with some cations, e.g. uranyl (VI), they are sufficiently stable to allow extraction. In this particular case it has been suggested that extraction occurs via the neutral complexes such as (R₃N.HN03)U02(N0₃)2 (Lloyd and Mason 1964). Sulphato complexes, although morę common than nitrato, are still rather uncommon and suggested processes are confined to uranium(VI), thorium, hafnium, and rare earths (Cattrall and Slater 1973). In this system the sulphate-bisulphate eąuilibrium becomes important and tends to impose a limit on the rangę of sulphate concentration and pH for extraction. Thus at high acid concen-tration bisulphate is the dominant species and extraction is reduced, presumably because bisulphate forms a weaker complex and also the HSO; ion competes for the aminę extractant. Conversely, as the acid concentration decreases the sulphate concentration increases but the pH now becomes too high for the aminę to protonate readily and again extraction is reduced. Finally, in the sulphate system the extraction depends strongly on the type of aminę with the order primary > secondary > tertiary being followed. This is the reverse of that found for nitrato- and halo-complexes but is the same as that for sulphuric acid extraction.

The majority of hydrometallurgical applications of ion-pair extraction involves the formation of chloro-anions. Extraction from alkali chloride media is usually preferred to minerał acid not only because metal chlorides are used as lixiviants, but also the HC1₂" ion formed in high minerał acid concentrations competes for the extractant.

The selectivity series for metals depends on the ease of formation of stable aąueous chloroanions, and for base metals follows the order: zinc > iron (III) > copper > cobalt > iron (II) > manganese (II). Because nickel does not form chlorocomplexes under these conditions easy separation of cobalt from nickel can be achieved with this system. Again it should be emphasized that as very high aąueous concentrations of salts are used, activity cannot be eąuated to concentration, and also, because of non-ideal behaviour of the extractant in the organie phase, great care must be taken in extrapoIating from laboratory data obtained in dilute Solutions. Conseąuently work on these systems should rely on experimental data using the actual operating conditions of the proposed process rather than predictions from published data.

The stripping of the loaded organie phase can be achieved by a wide variety of salt Solutions such as chloride, sulphate, or carbonate, the choice depending on the subseąuent recovery process used for the metal. An alter-native, which can be used for primary, secondary, and tertiary amines, is to use basie Solutions which reverse the aminę protonation step. This generally gives a morę complete strip in fewer stages.