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 (U
Nd2O3
∼ −3292.86 kJ/mol
and U
Pr6O11
∼
−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.
