Effect of Structural Modification in β Diketone on the Mutual Separation of Nd and Pr into Room-Temperature Ionic Liquid

A novel approach for mutual separation between Nd and Pr has been demonstrated by the selective dissolution of Nd2O3 into an ionic liquid (C4mim·NTf2) containing two β diketones, viz., acetyl trifluoroacetone (ATA) and benzoyl trifluoroacetone (BTA), directly from a physical mixture of their most stable oxides, i.e., Nd2O3 and Pr6O11, avoiding their cumbersome aqueous phase dissolution and the subsequent aqueous phase chemistry. The controlled release of the H+ ion by the β diketones to form the enolate conformation has been used to hold the dissolved Nd3+ ion in the ionic liquid phase as Nd (enolate of β diketone)4– species. The selective dissolution of Nd2O3 over Pr6O11 can be achieved by utilizing one of the prime driving forces of their difference in lattice energy of formation (UNd2O3 ∼ −3292.86 kJ/mol and UPr6O11 ∼ −7360.45 kJ/mol, respectively). The extent of dissolution was found to be greater with BTA for both Nd and Pr as compared to that seen with ATA. However, the selectivity was much better with ATA. A maximum separation factor of βNd/Pr ∼ 217 was achieved using 0.1 M ATA in C4mim·NTf2 containing 1.5% water for 15 min of biphasic equilibration with a solid-to-liquid ratio of 5.33 g L–1 from the physical mixture of Nd2O3/Pr6O11 ∼ 2:1. The separation factor, reported in this study, was found to be more than 100 times better than that reported for the conventional route of hydrometallurgical separation. The exothermic nature of Nd dissolution was evidenced from the calorimetric experiments, with the estimated enthalpy of dissolution for ATA and BTA being −169.23 and −126.46 J/g, respectively. The subsequent quantitative back extraction of Nd by 0.5 M HNO3 provides the opportunity for recycling of the ionic liquid for subsequent dissolution. The effects of water content, ATA/BTA concentration, relative ratio for Nd2O3/Pr6O11, dissolution temperature, etc., have been studied to understand their separation characteristic and hence to achieve the best mutual separation of the two rare earth metal ions.