Niobium-containing solutions

Based on a comprehensive investigation of solubility isotherms and of conductometric and potentiometric titration in the NbF5 - HF - H20 system, Nicolaev and Buslaev [288] concluded that F NbOFs is the predominant niobium-containing form present in such solutions. 

Raman spectra of fluoride solutions containing niobium were investigated by Keller [171]. Solutions were prepared by dissolving niobium fluoride compounds in solutions of hydro fluoride acid, HF, of different concentrations. 

Complexes in fluoride solutions - niobium-containing solutions 

The type of complex present in each solution depends on the acidity of the solution. In particular, HF solutions with concentrations lower than 25% contain niobium only in the form of NbOF52 complexes, whereas significant amounts of NbF6 ions are found in solutions containing 35% HF and higher. Table 44 shows the composition of complex ions in solutions of different concentrations, as found by Keller [171]. 

Howell and Moss [289] confirmed, by 19F NMR spectroscopy, that NbOF52 complex is present in hydrofluoric acid solutions of up to 30%. Increase in HF concentration leads to the formation of NbF6 complex ions. 

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Crystals of Rb7TaF7 were prepared from relatively diluted solutions of HF, while re-crystallization of rubidium heptafluorotantalate, Rb7TaF7, using a 33% HF solution resulted in the precipitation of rubidium hexafluorotantalate, RbTaFe [56]. Under the same conditions, niobium-containing solutions yielded rubidium oxyfluoroniobate, Rb2NbOF5 [29]. 

Table 5. Compounds precipitated from niobium-containing solutions Reproducedfrom [61], D. V. Tsikaeva, A. I. Agulyansky, Y. I. Balabanov, V. Y. Kuznetsov, V. T. Kalinnikov, Zh. Neorg. Khim. 34 (1989) 3046, Copyright 1989, with permission of Nauka (Russian Academy of Sciences) publishing.

The addition of alkali metal or ammonium fluorides reduce the acidity of the system and shift the equilibrium between the two ions toward the formation ofNbOFs2 ions [60,61]. The shift depends on the alkalinity of the cation. The more alkaline the cation is (higher atomic weight), the stronger the shift toward NbOF52 ion formation. Fig. 48 shows typical Raman spectra of niobium-containing solutions before and after such additions were made. 

Preparation of the solutions was similar to that of niobium-containing solutions, i.e. by dissolving tantalum metal powder in hydrofluoric acid, HF, at a concentration of about 40% weight. 

The second solution that results from the liquid-liquid extraction process is a high-purity niobium-containing solution. This solution is used in the preparation of niobium oxide, Nb205. The process is similar to the above-described process of tantalum oxide preparation and consists of the precipitation of niobium hydroxide and subsequent thermal treatment to obtain niobium oxide powder. 

Tantalum containing solution Niobium containing solution... 

H2TaF7(liquid) + 5 H20(liquid) —-> Ta205(solid) + 14 HF(gas) (160) as well as for niobium-containing solutions ... 

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