Mass transfer based on chemical potential theory: ZnSO4/H(2)SO(4)D2EHPA

A fundamental model based on chemical reaction kinetics and diffusional mass transfer for the extraction of zinc sulfate with di(2-ethylhexyl) phosphoric acid (D2EHPA) in n-heptane at 25 degrees C was developed Gradients of the chemical potential were used as the driving force for diffusion. Activity coefficients and their derivatives were calculated from the Fitter equation in the aqueous phase, while the organic nonideality was considered by the Hildebrand-Scott treatment. The Nerst-Planck equation was chosen for describing the diffusion of aqueous ions, paying special care to the formation of hydrogen sulfate. It was assumed that this fast ionic reaction can be described in terms of the mass action law in the bulk and film. For the multicomponent mass transfer of the organic species, the Maxwell-Stefan theory was adopted. A kinetic equation for the extraction of zinc with D2EHPA, which considers the adsorption of the D2EHPA molecules at the interface based on the Langmuir law, was suitable for our experimental data. Organic zinc concentration vs. time was obtained in a type of Lewis cell with vibrational mixing. Molar fixtures were calculated by fitting it with rational functions, which were analytically differentiated Initial conditions of the experiments cover a wide range of the zinc (0.1 mM to 0.05 M), D2EHPA, and sulfuric acid concentration. Experimental fluxes could be well described by this model when chemical kinetics and mass transfer were considered. In particular, the resistance to mass transfer in the organic film is important.