Kinetic investigation and synergistic effects in the separation of neodymium from praseodymium and lanthanum using organophosphorus extractants

The synergistic solvent extraction behavior and kinetic mechanism of light rare earth elements using a mixed Cyanex 572/Cyanex 302 system were investigated. The binary extractant system showed substantially higher extraction efficiency than either extractant used alone. Slope analysis confirmed the participation of both Cyanex 572 and Cyanex 302 in the formation of a mixed-ligand complex. The extraction experiments were performed using a Cyanex 572/Cyanex 302 synergistic system diluted in kerosene as the organic phase. A synthetic nitrate solution containing Nd(III), Pr(III), and La(III) at initial concentrations of 0.015, 0.004, and 0.014 M, respectively, was contacted with the organic phase at an aqueous-to-organic phase ratio of 1:1. The extraction behavior was investigated over a pH range of 1–6, and the optimum condition was obtained at approximately pH 5. The separation performance of the system was evaluated, and separation factors of βNd/Pr = 3.67 and βNd/La = 14.29 were obtained under the optimized conditions. Kinetic analysis revealed positive reaction orders for Cyanex 572 and Cyanex 302 and an inverse dependence on hydrogen ion concentration. The apparent reaction orders were 0.75 for Cyanex 572, 1.14 for Cyanex 302, and −1.05 for hydrogen ions, indicating the important role of proton exchange. A diffusion-controlled mechanism was proposed, involving dissociation of the dimeric extractant, rapid formation of a neutral interfacial metal complex, and subsequent diffusion of this complex into the organic phase. Similar extraction rate constants were obtained for La(III), Pr(III), and Nd(III), ranging from 2.24 × 10⁻² to 2.57 × 10⁻² s⁻¹. The obtained extraction and separation results demonstrate the potential of the Cyanex 572/Cyanex 302 system for Nd(III) recovery; however, further counter-current studies are required for process-scale validation.