Modeling of the extraction of uranium (VI) from concentrated phosphoric acid by synergistic mixtures of bis-(2-ethylhexyl)-phosphoric acid and tri-n-octylphosphine oxide

A thermodynamic model has been developed to describe the extraction of uranium (VI) from 5.3 mol L− 1 phosphoric acid (a typical concentration encountered in wet phosphoric acid) by mixtures of bis-(2-ethylhexyl)-phosphoric acid (D2EHPA, also denoted hereafter HL) and tri-n-octylphosphine oxide (TOPO). A good agreement between experimental and calculated distribution coefficients of uranium (VI) was obtained by taking into account the following species in the organic phase: (HL)2TOPO, (HL)5TOPO, (HL)2(TOPO)2, UO2(HL2)2, UO2(HL2)2TOPO, and UO2L2TOPO where L is the deprotonated form of D2EHPA (HL). The speciation of uranyl, TOPO and D2EHPA in the organic phase was derived from this model as a function of the concentrations of the latter species.