Niobium and tantalum processing in oxalic-nitric media: Nb2O5·nH2O and Ta2O5·nH2O precipitation with oxalates and nitrates recycling

Abstract

Oxalate-based aqueous media represent one of the rare options for solubilizing weighable amounts of niobium (Nb) and tantalum (Ta) without using toxic fluoride-based mixtures. Recent progress in the hydrometallurgy of Nb and Ta also highlighted the potential of oxalate-nitrate media for the separation of Nb and Ta. Nonetheless, the resulting purified aqueous solutions, containing Nb or Ta in HNO3-H2C2O4 mixtures, need to be further processed in order to yield Nb and Ta solid products that can be commercialized. Furthermore, oxalic acid is relatively expensive in the frame of the hydrometallurgy of Nb and its recycling can significantly increase the process economy. In this study, the precipitation behavior of Nb, Ta, and their usual minor impurities (Ti and Fe), from oxalic-nitric acid solutions has been investigated for the first time. Neutralization of Nb-HNO3-H2C2O4 mixtures to pH 7–8 by concentrated NaOH or NH4OH was found to be effective in decomposing the oxalate complexes ([NbO(C2O4)3]3− and [NbO(C2O4)2(H2O)2]−) and precipitating Nb in the form of amorphous Nb2O5·nH2O(s) while leaving the oxalate and nitrate ions in the filtrates. The same phenomena take place with the Ta-HNO3-H2C2O4 solutions with precipitation of Ta2O5·nH2O(s). Taking advantage of the solubility difference between nitrate and oxalate salts, the subsequent concentration of the nitrate-oxalate filtrates by evaporation yields to the quantitative and selective recovery of the oxalates as crystalline Na2C2O4(s) or (NH4)2C2O4·H2O(s) (purity > 95%) and a concentrated solution of sodium or ammonium nitrate. The developed method exhibits high precipitation yields (>99.9%) for Nb and Ta and high recovery yields (>99%) for the nitrates and oxalates. The process was optimized at the laboratory scale and then validated on industrial Nb (∼10 g/L) and Ta (∼400 ppm) solutions with successful production of purified Nb2O5·nH2O(s) (purity of 99.5 wt%) and a Ta2O5·nH2O(s) concentrate (>20 wt% Ta), followed by the recovery and separation of the oxalates and nitrates. Taken together, the proposed precipitation method and the fluoride-free liquid-liquid Nb-Ta separation recently reported pave the way for more sustainable hydrometallurgical processes for Nb and Ta.