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 macroscopic amount 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 is necessary. In this study, the precipitation of Nb and Ta, as well as 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 at decomposing the oxalate complexes ([NbO(C2O4)3] 3-and [NbO(C2O4)2(H2O)]-) and precipitating Nb in the form of Nb2O5·nH2O while leaving the Page 2/27 oxalate ions in solution. Taking advantage of the solubility difference between nitrate and oxalate salts, the subsequent concentration of the filtrates yields to the recovery of the oxalates as Na2C2O4 or (NH4)2C2O4·H2O 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 (~50 mL) and then validated on industrial Nb and Ta solutions (~225 L) with successful production of purified Nb2O5·nH2O and Ta2O5·nH2O, recovery of the oxalates in the form of Na2C2O4 or (NH4)2C2O4·H2O and a concentration solution of nitrates. Taking together, the proposed precipitation method and the fluoride-free liquid-liquid Nb-Ta separation recently reported pave the way for more suitable hydrometallurgical processes for Nb and Ta.
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Gauthier J.-P. Deblonde, David Bengio, Denis Beltrami, Sarah Bélair, Gérard Cote, et al.. Niobium and tantalum processing in oxalic-nitric media: Nb2O5·nH2O and Ta2O5·nH2O precipitation with oxalates and nitrates recycling. Separation and Purification Technology, Elsevier, 2019, 226, pp.209-217. ⟨10.1016/j.seppur.2019.05.087⟩. ⟨hal-02182964⟩

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