Solubility of niobium(V) and tantalum(V) under mild alkaline conditions

The solubility of both Nb(V) and Ta(V) has been investigated as a function of pH at 25 degrees C in alkaline media. Solid phase analysis revealed that a phase transition between Nb2O5 center dot nH(2)O((s)) and Na7HNb6O19 center dot 15H(2)O((s)) occurs even under mild conditions (8 <= pH <= 12). Similar phase transition is also expected to take place between Ta2O5 center dot nH(2)O(s) and Na(8)Ta(6)O(19)mH(2)O(s), but at pH higher than 12. The observations, supported by Raman spectroscopy and XRD analysis of the solid phases and UV spectrophotometry of the solutions, show that hexaniobate ions HxNb6O19x (-) (8) (0 <= X <= 3) play a major role in the solubility of Nb(V) in alkaline media. The solubility of Na7HNb6O19 center dot 15H(2)O((s)) was investigated at 25 degrees C and strongly depends on the interactions with the background electrolyte. Modeling of the solubility data suggests that ion-pairs are not formed between Na-(aq)(+) and HNb6O19(aq)7-. The solubility product measured for Na7HNb6O19 center dot 15H(2)O((s)) is Ks = 2.275 * 10(-12) at 25 degrees C. Oppositely, ion-paring effect occurs in K+-containing media leading to an enhancement of the solubility of Na7HNb6O19 center dot 15H(2)O(5). This effect could be of interest for the development of Nb recovery processes. (C) 2015 Elsevier B.V. All rights reserved,