scan0043 2

30

MICHAEL COX

hydrodynamic parameters but are difficult to set up and operate. Trouve et al. (1982) deveIoped a falline-film device whefe the organie phase flows over a hydrophobic rod immersed in the aąueous phase. A considerable number of earlier studies used single-drop methods as approximating to the condi-tions within a stirred tank contactor. Although this eąuipment is easy to operate, detailed analysis of the data is morę difficult and its use has decreas-ed in favour of the stirred celi and other methods.

Studies on the kinetics of metal extraction have been published using both Lewis-type cells and single drop methods. Probably the most extensive are those on the copper-hydroxyoxime system with both commercial and pure synthesized reagents (Hanson and Whewell 1981; Danesi and Chiarizia 1980). These have included an extensive study by Szymanowski et al. (1984) in which they concluded that for molecules where the total carbon chain substituents on the basie molecular structure consisted of less than three atoms then morę than 90 per cent of the reaction occurred in the aąueous phase. However, as the carbon atoms inereased above a total of 11 atoms this bulk reaction accounted for less than 10 per cent of the total. This observation was linked to the hydrophilic-lipophilic balance which is a measure of the interfacial activity. Similar conclusions werę reached by Miyake et al. (1986).

Extraction reagents all exhibit interfacial activity arising from the presence of both polar groups and non-polar alkyl carbon chains. This activity can be estimated by the lowering of the interfacial tension caused by the penetra-tion of the interfacial surface by the hydrophilic groups. The extent of this lowering depends on reagent type and geometry, bulk phase concentration, diluent and water solubility of the reagent. Values of interfacial concentration which feature in the kinetic eąuations can be obtained from interfacial tension measurements by the application of the Gibbs isotherm. However, because of limitations to the use of the single solute form often ąuoted (Cox and Flett 1979) and possible variation in the molecular geometry of the reagent between the bulk and adsorbed species, great care must be taken in the use of the isotherm to obtain quantitative data for solvent extraction systems.

Other morę direct methods are available to determine reagent concen-trations close to the interface (Hanna and Noble 1985) but these have not yet been used for metal extraction systems. For all these interfacial studies extreme purity of the reagents is reąuired which often limits their application with commercial reagents. When considering hydrometallurgical appli-cations the concentration of reagents employed ensures that either the interface is saturated by adsorbed species or the reagent is above its critical micelle concentration, so that unless the reagent impurities are orders of magnitude morę interfacially active than the reagent they should not affect the overall kinetics too much. It should be noted that for reagents that are only weakly adsorbed, these can become desorbed from the interface by vigorous agita-

J

V