Precipitation from solutions niobium fluoride compounds

Potassium-containing tantalum and niobium fluoride compounds can be precipitated from HF solutions as described previously (see Fig. 3 and 4). Ritchie and Mitra [59] described the synthesis of K.2TaF7 in an HF solution, based on the following interaction (5), using TaCl5 as a precursor ... 

An irreversible extinction of the SHG signal at 150-200°C is observed for a number of other fluoride and oxyfluoride compounds of tantalum and niobium that crystallize in centrosymmetric space groups. This phenomenon is especially typical for the compounds prepared by precipitation from solutions [206]. The appearance of the weak SHG signal for such compounds is related to imperfections in their crystal structure and the creation of dipoles. Nevertheless, appropriate thermal treatment improves the structure and leads to the disappearance of dipoles and to the irreversible disappearance of the corresponding SHG signal. 

Modem refining technology uses tantalum and niobium fluoride compounds, and includes fluorination of raw material, separation and purification of tantalum and niobium by liquid-liquid extraction from such fluoride solutions. Preparation of additional products and by-products is also related to the treatment of fluoride solutions oxide production is based on the hydrolysis of tantalum and niobium fluorides into hydroxides production of potassium fluorotantalate (K - salt) requires the precipitation of fine crystals and finishing avoiding hydrolysis. Tantalum metal production is related to the chemistry of fluoride melts and is performed by sodium reduction of fluoride melts. Thus, the refining technology of tantalum and niobium involves work with tantalum and niobium fluoride compounds in solid, dissolved and molten states. 

Precipitation of fluoride compounds from solutions of hydrofluoric acid, HF, is performed by the addition of certain soluble compounds to solutions containing niobium or tantalum. Initial solutions can be prepared by dissolving metals or oxides of tantalum or niobium in HF solution. Naturally, a higher concentration of HF leads to a higher dissolution rate, but it is recommended to use a commercial 40-48% HF acid. A 70% HF solution is also available, but it is usually heavily contaminated by H2SiF6 and other impurities, and the handling of such solutions is extremely dangerous. 

No available data was found on the precipitation from fluoride solutions of niobium and tantalum fluoride compounds containing tri- and tetravalent metals. 

The process of separating the intermediate products from the purified solutions, in the form of solid complex fluoride salts or hydroxides, is also related to the behavior of tantalum and niobium complexes in solutions of different compositions. The precipitation of complex fluoride compounds must be performed under conditions that prevent hydrolysis, whereas the precipitation of hydroxides is intended to be performed along with hydrolysis in order to reduce contamination of the oxide material by fluorine. 

Niobium(V) chloride. Hydrous niobium(V) oxide (0.75 g. Nb) is precipitated from acid solution by the addition of ammonium hydroxide, thoroughly washed by centrifugation with water (two 15-ml. portions), 0.5 M nitric acid (two 10-ml. portions) to remove adsorbed ammonium ion, and acetone (three 20-ml. portions) and vacuum-dried at room temperature. If the initial hydroxide precipitation is carried out from hydrofluoric acid solution, an appreciable quantity of the hydrous oxide may dissolve in the nitric acid washes, presumably because of the presence of traces of fluoride. However, reprecipitation and treatment as above reduces losses at this stage. The dried hydrous oxide is placed in a 40-ml. centrifuge tube fitted with a standard-taper outer joint, and 10 to 15 ml. of freshly distilled thionyl chloride is added slowly, since the initial reaction may be vigorous. The vessel is stoppered loosely, and the reaction is allowed to go to completion at room temperature (24 to 48 hours). Any traces of undissolved hydrous oxide, usually very small, and any yellow crystalline compound (see Discussion) are removed by centrifugation, and the penta-chloride is isolated by vacuum evaporation of the thionyl chloride at room temperature and pumping for several hours at 10 mm. If necessary, the product is further purified by vacuum sublimation in a sealed tube ( 150°). The yield, based on dried hydrous oxide, is 90 to 95%. Anal. Calcd, for NbCU Nb, 34.39 Cl, 65.61. Found Nb, 34.27 Cl. 

The double fluoride can be produced by wet predpitation methods from a pure solution, after removal of the corresponding tantalum compound, e.g. by solvent extraction. The salt is produced in crystalline form by the addition of potassium fluoride solution to a solution of niobium fluoride containing a free acidity in excess of 4N. The readion has been carried out successfully on a 20 kg (of niobium) scale. The yield depends upon the concentration and acidity conditions but, for example, can be about 95 per cent if precipitation is carried out at 0°C from a solution 8N in free hydrofluoric acid. 

The cake is leached with water in order to dissolve tantalum and niobium (and other related compounds) in the form of fluoride salts of ammonium. Ammonium fluoroferrate and fluoromanganate are unstable in aqueous solutions of low acidity. It is assumed that iron and manganese will form precipitates of insoluble fluorides or oxyfluorides that can be separated from the solution by filtration. 

---