Modeling of the Equilibria of Yttrium(III) and Europium(III) Solvent Extraction from Nitric Acid with PC-88A

Abstract

The equilibria of yttrium(III) and europium(III) (RE) extraction with PC-88A dissolved in Shellsol D70 from nitric acid solutions were studied under nonideal conditions in order to develop a chemically-based model enabling the engineering prediction of the equilibrium distribution ratios. The distribution of the extractant between the nitrate solution and the diluent was also examined. As a result, it was found that (i) the majority of PC-88A is dimerized in the organic phase, (ii) the cation exchange extraction is predominant in the low-acidity region, and (iii) solvation extraction becomes appreciable in the high-acidity region. By using a model that considers these two extraction equilibria together with the complexation of the RE with NO₃⁻, the nonideality of the PC-88A dimer, and the competing extraction of HNO₃, the extraction equilibrium constants and the coefficient for the effective concentration of the extractant were determined by the nonlinear least-squares method. The proposed model can reproduce the experimental data with good accuracy for single-metal systems over wide feed concentration ranges of RE up to 0.1 kmol·m⁻³, nitric acid (0.05 to 3 kmol·m⁻³), and the extractant (0.25 to 1 kmol·m⁻³). Its applicability to the prediction of the extraction from a binary metal system was also confirmed.