Emulsion liquid membranes Part ll: Modelling mass transfer of zinc with bis(2-ethylhexyl)dithiophosphoric acid

Abstract

Emulsion liquid membranes (ELMs) are liquid membranes where the membrane is the continuous phase of an emulsion. Dispersed in a third continuous phase the emulsion globules are very well suited for various concentration processes. The present work investigates the recovery of zinc from an aqueous solution with water/oil/water emulsion liquid membranes. The carrier reagent in the organic membrane is the liquid ion exchanger bis(2-ethylhexyl)dithiophosphoric acid (DTPA). A mass transfer model for carrier-facilitated transport in emulsion liquid membranes is developed which is based on the extraction and stripping reaction between the Zn 2+ ions and the carrier molecules as well as on the transient diffusion of the loaded carrier molecules into an emulsion globule. Mass transfer experiments were carried out in a single-drop apparatus to test model predictions for actual transfer rates. Results indicate that the model is well suited for describing mass transfer phenomena in emulsion globules. Experiments show that it is essential to account for both reaction kinetics and transient diffusion in an emulsion globule. The purpose of this paper was to show new developments in the modelling of complex transport effects in emulsion liquid membranes. A mass transfer model for zinc permeation with DTPA was development and critically compared to experimental results for a single-drop apparatus. It was shown that a rigorous description of diffusion and reaction desribes the experimental results well. The importance of reliable chemical kinetics of the extraction involved was demonstrated. Results show that all of the model parameters can be estimated with existing correlations, although more work has to be done to obtain correlations with better estimative capability, especially for the effective diffusivity. As a practical result for emulsion liquid membrane contacting devices, it can be concluded that it is important to achieve large surface areas and small drop sizes as the diffusion resistance inside an emulsion globule becomes very important with increasing emulsion/continous-phase contact time. Another possiblity to enhance mass transfer would be the forced coalescence of emulsion globules, thus averaging the globule concentration within an emulsion globule and reducing the fraction of the globule where equilibrium of the stripping reaction has already been established.