HF solution

The existence of diamagnetic salts of AgF was first reported in 1957 (26), butHttle was known about their properties. In 1988 (27) it was claimed that AgF3 was prepared by a reaction of Ag metal and O2F2 iu CIF. Silver trifluoride [91899-63-7], AgF, has since been prepared (28) from anhydrous HF solutions of AgF 4 salts by addition of BF, PF, or AsF. ... 

Zinc Fluoride Tetrahydrate. Zinc fluoride tetrahydrate [13986-18-0] is prepared by reaction of ZnO and aqueous HF. ZnF2 4H20 has a water solubihty of about 1.6 g/100 mL solution at 25°C. Addition of HF increases the solubihty to 11.8 g/100 mL in a 29% HF solution. The tetrahydrate loses water at temperatures above 75°C. 

Iron(III) fluoride ttihydrate [15469-38-2] FeF3-3H2 0, crystallizes from 40% HF solution ia two possible crystalline forms. At low temperature the a-form, which is isostmctural with a-AlF 3H2O, is favored. High temperatures favor P-FeF 3H2O, the stmcture of which consists of fluoride-bridged octahedra with one water of hydration per unit cell. 

Hydrofluoric acid [7664-39-3] M 20.0, b 112.2"(aq azeotrope, 38.2% HF), d 1.15 (47-53% HF), pK 3.21. Freed from lead (Pb ca 0.002ppm) by co-precipitation with Srp2, by addition of lOmL of 10% SrCl2 soln per kilogram of the cone acid. After the ppte has settled, the supernatant is decanted through a filter in a hard-rubber or paraffin lined-glass vessel [Rosenqvist Am J Sci 240 358 1942. Pure aqueous HF solutions (up to 25M) can be prepared by isothermal distn in polyethylene, polypropylene or platinum apparatus [Kwestroo and Visser Analyst 90 297 7965]. HIGHLY TOXIC. 

It was pointed out earlier that the low nucleophilicity of fluoride ion and its low concentration in HF solutions can create circumstances not commonly observed with the other halogen acids. Under such conditions rearrangement reactions either of a concerted nature or via a true carbonium ion may compete with nucleophilic attack by fluoride ion. To favor the latter the addition of oxygen bases, e.g., tetrahydrofuran, to the medium in the proper concentration can provide the required increase in fluoride ion concentration without harmful reduction in the acidity of the medium. 

Perbromyl fluoride, FBr03, is made by fluo-rinating the corresponding peibromate ion with AsFj, SbFj, BrFj or [BrF6]" [AsF6] in HF solutions. The reactions are smooth and quantitative at room temperature ... 

The compound tends to dissociate into AUF3 and, when treated with Xe 2 in anhydrous HF solution below room temperature, yields yellow-orange crystals of the complex [Xe2F3][AuFfi] ... 

The HF method determines the best one-determinant trial wave function (within the given basis set). It is therefore clear that in order to improve on HF results, the starting point must be a trial wave function which contains more than one Slater Determinant (SD). This also means that the mental picture of electrons residing in orbitals has to be abandoned, and the more fundamental property, the electron density, should be considered. As the HF solution usually gives 99% of the correct answer, electron correlation methods normally use the HF wave function as a starting point for improvements. 

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. 

Tantalum and niobium oxides dissolve very slowly in HF solutions. Thus, it is recommended to use a high concentration of HF or a mixture of HF and H2SO4 at a temperature of about 70-90°C. The best precursors for the preparation of fluoride solutions are hydroxides. Both tantalum hydroxide, Ta205 nH20, and niobium hydroxide, M Os-nHjO, dissolve well, even in diluted HF solutions. 

Using metallic precursors, HF solutions with higher concentrations of tantalum or niobium can be achieved. It is possible to prepare solutions that have maximum concentrations of about 1000 g/1 tantalum oxide and about 600 g/1 niobium oxide (Me205). 

Synthesis of the compounds from such HF solutions is performed by adding soluble fluoride compounds to the tantalum or niobium solution or by recrystallization of prepared fluoride compounds from water or HF solutions of different concentrations. In the first case, the composition of the compounds obtained depends on the ratio between Ta/Nb and the added metal and on the initial concentration of the HF used, whereas in the second case, it depends only on the HF concentration. 

Fig. 3. Potassium fluorotantalate, KfTaFy, solubility in HF solutions at 25°C Reproduced from [53], G. S. Savchenko, I. V. Tananaev, Zh. Prikl. Khim 20 (1947) 385, Copyright 1947, with permission of Nauka (Russian Academy of Sciences) publishing.

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 ... 

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]. 

CsNbF6 can be prepared from a 35% HF solution, while CsTaF6 can be precipitated from more diluted solutions [29]. 

Systematic investigations of the compounds that can be precipitated by adding alkali metals fluorides to HF solutions containing tantalum or niobium are discussed in [60, 61]. Compositions of the precipitated compounds and of their corresponding mother solutions are given in Table 4. 

Table 7. Niobium-containing compounds prepared from HF solutions. Reproduced from [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.

RbsNbaOFig is precipitated and re-crystallized from 20-30% HF solutions (Fig. 6). Lower concentrations of HF yield Rb5Nb3C)3F14, while RbNbF6 precipitates at concentrations over 35%. 

Single crystals of RbsNbgOFig can be grown successfully by dissolving the prepared compound in a 20-30% HF solution at increased temperature and subsequent slow cooling of the solution down to room temperature. The solubility of RbgNbjOFjg in 20% HF solution versus the temperature of the solution is given in Fig. 7. 

Fig. 6.Crystallization field and solubility (45°C) ofRbsNbsOFis in HF solutions (after Tsikaeva et al. [60, 61]).

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]. 

Based on an analysis of the initial dissolution rate in different solutions at different temperatures, several very useful conclusions and recommendations were made. It was found that the apparent activation energies for the dissolution of niobium and tantalum in 10 mol/1 HF solution are 56.5 and 65.5 kJ/mol, respectively for columbite, and 42.7 and 61.1, respectively, in the case of tantalite. It was also concluded that the mechanism of dissolution is the same for both columbite and tantalite. In addition, the initial dissolution rate of niobiuth (RNb) from columbite is controlled primarily by the activities of the... 

Fig. 136. Nb205 concentration versus pH of solutions prepared by dissolution of (NH4)3NbOF6 (1) and (NH4)2NbOF5 (2) in water or Nb in HF solution (3) (after Agulyanskaya et al., [492, 493]).

Increasing the pH to 10-11 (in the case of Nb - HF solution - pH = 8) reduced the Nb205 concentration in the solutions to 0.5-0.3 g/l and the solid precipitate was identified as a pure amorphous substance, which after thermal treatment was identified as niobium oxide. 

The unique advantage of the plasma chemical method is the ability to collect the condensate, which can be used for raw material decomposition or even liquid-liquid extraction processes. The condensate consists of a hydrofluoric acid solution, the concentration of which can be adjusted by controlling the heat exchanger temperature according to a binary diagram of the HF - H20 system [534]. For instance, at a temperature of 80-100°C, the condensate composition corresponds to a 30-33% wt. HF solution. 

Additional purification of the product and improvement of particle size and shape can be achieved by re-ciystallization. The process consists of sequential dissolutions of potassium heptafluorotantalate in appropriate solutions at increased temperatures, filtration of the solution to separate possible insoluble parts of the product and cooling of the filtrated solution at a certain rate. The precipitated crystals are filtrated, washed and dried to obtain the final product. Re-crystallization can be performed both after filtration of the preliminary precipitated salt or after drying if the quality of the product is not sufficient. HF solutions of low concentrations are usually used for re-ciystallization. In general, even water can be used as a solvent if the process is performed fast enough. Nevertheless, practical experience suggested the use of a 30—40% HF solution within the temperature interval of 80-25°C, and a cooling rate of about 8-10°C per hour. The above conditions enable to achieve an acceptable process yield and good performance of the product. 

The reduction of K2TaF7 can also be performed using sodium vapors [584]. This process is conducted at an Na pressure as low as 0.1 torr, which enables the removal of interferring gases such as N, O and H20. The interaction begins at 350°C. The temperature further increases up to 800°C to prevent the condensation of sodium and the formation of colloidal tantalum powder. The product of the interaction is removed from the reactor after cooling and treated with boiled HC1 and HF solutions. The method enables the production of coarse grain tantalum powder with 99.5% purity. 

Sodium reduction development directions, 336 diluted melts, 331-332 of K-Salt, 327-328 principals, 326 Solid-phase interaction mechanism, 34-37 niobium oxyfluorides, 26-31 tantalum oxyfluorides, 32-34 Solubility diagrams (NH4)5Nb3OF18, 22 K2NbF7 in HF solutions, 14 K2TaF7 in HF solutions, 14 RbsNbjOF,, 22-23 Solubility of peroxides, 307 Specific conductivity, 153, 164 Spontaneous polarization, 223 Structural characteristics for X Me=8, 61,... 

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