Modelling of uranium(VI) extraction by D2EHPA/TOPO from phosphoric acid within a wide range of concentrations

- A thermodynamic model based on equation of state is presented.
- Modelling of U(VI) distribution vs. phosphoric acid concentration
- Good agreement between experimental and calculated D-values

Uranium(VI) extraction from 0.73-7.08 mol · L− 1 phosphoric acid by a mixture of 0.5 mol · L− 1 D2EHPA and 0.125 mol · L− 1 TOPO in Isane IP 185 has been modelled at room temperature as a function of phosphoric acid concentration. This model takes into account the non-ideal behaviour in concentrated phosphoric acid by means of the Fürst-Renon Equation of State which is an original extension of a classical cubic equation of state (EOS) valid for non-electrolyte systems. A significant improvement of the thermodynamic model is obtained by considering an exchange of water molecules during the extraction of the uranyl cation by the mixture of D2EHPA and TOPO. Apparent extraction equilibrium of uranium(VI) by D2EHPA-TOPO, complexation factor of uranium(VI) by phosphoric acid, hydrogenophosphate and phosphate ions and activity coefficients of uranyl cation as a function of apparent phosphoric acid concentration are deduced from the thermodynamic model.