Niobium heptafluorotantalate

Two niobium chemicals are industrially used as feedstock to prepare the niobium metal niobium heptafluorotantalate (K NbF,) and niobium pentoxide (Nb Os)- Niobium hepta-fluorotantalate is obtained by adding potassium fluoride, KF, to the purified stripping solution in order to precipitate the insoluble crystals. The settled crystals of K NbF, are easily removed from the solution by centrifugation and filtration. Once separated, the crystals are dried. Niobium pentoxide is prepared by precipitation of niobium hydroxide, NbjOj.xHjO, by adding ammonia gas, NH, to the stripping solution containing niobium. The settled precipitate is then filtrated, washed with deionized water, dried, and calcinated, giving off water, to obtain the anhydrous niobium pentoxide. 

Solvent Extraction. The industrial separation of tantalum from niobium was carried out historicahy by the Marignac process of fractional crystallization of potassium heptafluorotantalate and potassium heptafluoroniobate (15,16) or the long-estabhshed Fansteel process (17), which involved the decomposition of the ore by a caustic fusion procedure. Processors have replaced these expensive processes by procedures based on solvent extraction. This technique was developed in the United States at Ames Laboratory and the U.S. Bureau of Mines (18). Figure 2 shows the flow sheet of an industrial instahation for the hydrometahurgical processing of tantalum—niobium raw materials. 

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]. 

Potassium heptafluorotantalate, K2TaF7, precipitates in the form of transparent needles. The precipitated particles must not be too fine, since fine powder usually promotes co-precipitation and adsorption of some impurities from the solution. Even niobium can be adsorbed by the surface of K2TaF7 developed during precipitation, as shown by Herak et al. [535]. On the other hand, the precipitation of large K-salt crystals should not be strived for either. Laboratory and industrial experience indicates that excessively large crystals usually contain small drops of solution trapped within the crystals. This occluded solution can remain inside of the crystal until drying and will certainly lead the hydrolysis of the material. 

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