In solvent extraction, a concentrated solution of mixed salts is contacted in order to obtain equilibrium with a reverse (w/o) micelle or microemulsion. Desextraction corresponds to the reverse process. Selectivity is obtained via cascades. The common point of all three two-phase equilibrium is that they correspond to a precise point in tie-lines of a pseudo-five component phase diagram.
Selectivity between targeted and non targeted in an efficient separation, due to a difference in chemical potential of the species between the concentrated ionic solution and the reverse micelles: a strong supra-molecular complexation mechanism is in counter-balanced by entropic terms combined in an adsorption isotherm on a divided flexible interface, in order to understand the apparent distribution “constants” and their variation, with formulation and temperature, determination of phase diagram is necessary.
Selectivity between iron and uranium is observed, with up to a factor of 50 increase of the selectivity of extraction. The selectivity is yet an unpredictable effect and depends on the nature of the solvent. The experimental dependence of the solvent is puzzling, since standard “theory” used by engineers does not involve the solvent, but only supra-molecular assembly and competing equilibrium.
We will describe in this talk how the free energy of extraction, i.e. the apparent equilibrium constants depend on the w/o micellisation, i.e. loss of entropy associated to cmc, extractant film bending constant, film extension, image charges and solvent dielectric properties. As a consequence, the selectivity in synergic systems should always show the apparently paradoxical solvent-sensitive peak in selectivity observed and used in applications.
thomas.zemb@icsm.fr
