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The kinetics of zinc extraction in the di(2-ethylhexyl) phosphoric acid, n-heptane-zinc perchlorate, perchloric acid, water system MacLean, Donald William John


The kinetics of zinc extraction from perchlorate solutions with di(2-ethylhexyl) phosphoric acid in n-heptane have been measured using the rotating diffusion cell technique. The extraction of zinc is controlled by the mass transfer of reactants (Zn²⁺ and D2EHPA) to the interface. At low zinc concentrations, the system is controlled by the aqueous transport of Zn²⁺ to the interface; at higher zinc concentrations transport of D2EHPA becomes rate controlling. For the range of D2EHPA concentrations examined, the transport of D2EHPA is rate controlling. Bulk pH has a negligible effect, except perhaps at the lowest pH values examined, where there may be a slight decrease in extraction rate. This decrease was attributed to less favourable thermodynamics at low interfacial pH values. It appears that the chemical reaction rate is fast enough that it has a negligible effect on the overall extraction rate. A basic mathematical model was developed which is adequate for predicting the extraction rate under variable conditions of zinc concentration, D2EHPA concentration, and pH. The effect of using a partially loaded organic extractant was also investigated, and the system was found to be mass transfer controlled. An extended mathematical model was developed which predicts that the speciation of organic complexed zinc changes with increasing preload, and at high loadings the direction of ZnL₂HL and ZnL₂(HL)₂ flux reverses, with these species providing extractant to the interface. At very high loadings, ZnL₂HL provides almost all the extractant to the interface. Experimental studies of the effect of temperature on the rate of zinc extraction resulted in a calculation of the activation energy which was consistent with a diffusion controlled mechanism. Finally, the effect of different filter pore sizes on extraction was examined. The extraction rate decreases significantly with a very small filter pore size, while there appeared to be little or no effect for larger filter pore sizes. For the filter pore size used in this study, it was therefore concluded that the filter pores do not pose an additional resistance to mass transfer.

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