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LIQUID-LIQUID EXTRACTION IN HYDROMETALLURGY

On the other hand, the second model considered that the bulk aąueous phase was the site of the Chemical reaction (Freiser F984). The first model has been widely adopted (Danesi and Chiarizia 1980; Tarasov and Yagodin 1988) for extractants used commercially which normally have Iow aqueous solu-bilities. Recently, Aprahamian and Freiser (1987) have shown by a careful study of the extraction of nickel by 8-quinoIinol extractants, including Kelex 100, that even when large interfacial areas, up to specific interfacial areas (i.e. interfacial area/phase volume), >200cm^-1, aregenerated by yigorous stirring, the ratę of extraction may be influenced by the interface under some pH conditions. Thus a controversy which has existed for 20 years concerning the mechanism of extraction in these Systems has now been largely resolved with the acceptance that in regimes of Iow agitation diffusion will be import-agt: even with yigorous stirring, which effectively eliminates diffusion layers around droplets, the organic/aqueous interface does have an effect on the reaction under certain conditions.

The interfacial areas generated by these very high stirring speeds are much higher than those normally experienced in commercial contacting equipment so that the majority of experimental laboratory studies still use equipment with defined interfaces (Hanna and Noble 1985). The results of these studies show that depending on the experimental conditions and hydrodynamics of the system, regimes exist where either Chemical reaction or diffusion contrpl dominate. The latter becomes important, as expected, in regimes where fast Chemical kinetics are found, that is, with high reagent concentrations and, where appropriate, high pH values. Thus it is very important in studies of kinetics of metal transfer to distinguish between Chemical reaction and diffusion control. This can be achieved either by using techniques which operate in a kinetic-controlled regime or by allowing for the diffusion component of the overall mass transfer ratę.

Several techniques have been deyeloped over the years for this purpose. The stirred quiescent interface celi as originally designed by Lewis has under-gone many modifications and provides one of the simplest devices to operate. But as indicated by Hanna and Noble (1985) there are still many limitations which need to be recognized. One of the problems associated with these cells is maintaining a quiescent interface while stirring both bulk phases. This can be overcome in the rotating diffusion celi where a porous membranę is situated at the interface and adapts the well-known hydrodynamics of the rotating disc electrode to a liquid-liquid system (Albery et al. 1976). One of the obvious difficulties with this celi is the presence of the membranę with its unknown free porę area for mass transfer and the effect of diffusion of the extracted species through the membranę. This system has been used both for diffusion-controlled measurements (Guy and Fleming 1979) and also for interfacial Chemical reactions (Albery and Fisk 1981). Liquid jet contactors (Freeman and Tavlarides 1980) also possess known