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

• Nb2O5·nH2O(s) and Ta2O5·nH2O(s) solubilities were measured at 8 ≤ pH ≤ 13 at 25 °C.
• Ta2O5·nH2O(s) was found more stable than Nb2O5·nH2O(s) in alkaline media.
• Nb2O5·nH2O(s) is transformed to Na7HNb6O19·15H2O(s) even under mild conditions.
• Na7HNb6O19·15H2O(s) solubility is enhanced in the presence of K+ ions.

The solubility of both Nb(V) and Ta(V) has been investigated as a function of pH at 25 °C in alkaline media. Solid phase analysis revealed that a phase transition between Nb2O5·nH2O(s) and Na7HNb6O19·15H2O(s) occurs even under mild conditions (8 ≤ pH ≤ 12). Similar phase transition is also expected to take place between Ta2O5·nH2O(s) and Na8Ta6O19·mH2O(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 Na7HNb6O19·15H2O(s) was investigated at 25 °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 HNb6O197 −(aq). The solubility product measured for Na7HNb6O19·15H2O(s) is Ks = 2.275 ∗ 10− 12 at 25 °C. Oppositely, ion-paring effect occurs in K+-containing media leading to an enhancement of the solubility of Na7HNb6O19·15H2O(s). This effect could be of interest for the development of Nb recovery processes.