Ammonium niobium fluoride

Ammonium Niobium Fluoride.—(See below under Potassium Niobium Fluoride.)... 

The synthesis of tantalum and niobium fluoride compounds is, above all, related to the fluorination of metals or oxides. Table 3 presents a thermodynamic analysis of fluorination processes at ambient temperature as performed by Rakov [51, 52]. It is obvious that the fluorination of both metals and oxides of niobium and tantalum can take place even at low temperatures, whereas fluorination using ammonium fluoride and ammonium hydrofluoride can be performed only at higher temperatures. 

The compound (NIDsNbaOFig can be prepared by adding ammonium fluoride, NH4F, to a solution containing Nb (3.20 M/l) and F (27.10 M/l). The solubility isotherm (25°) of this compound is presented in Fig. 5. The minimum point on the solubility isotherm approximately corresponds to the stoichiometrical ammonium-niobium ratio of the compound (NfLOsNbsOFig. 

Ammonium hydrofluoride is relatively stable, even in the molten state. In addition to being in contact with tantalum or niobium oxide, the compound will initiate the fluorination process yielding complex tantalum or niobium fluoride compounds. There is no doubt that thermal treatment of the hydroxides at high temperatures and/or at a high temperature rate leads to the enhancement of the defluorination processes, which in turn results in an increase in fluorine content of the final oxides. 

NIOBIC ACID. Any hydrated form of Nb Os. It forms as a white, insoluble precipitate when a potassium hydrogen sulfate fusion of a niobium compound is leached with hot water or when niobium fluoride solutions are treated widi ammonium hydroxide. Soluble in concentrated sulfuric acid, concentrated hydrochloric acid, hydrogen fluoride, and bases. Important in analytical determination of niobium. See also Niobium. 

Interaction between niobium oxide and fluorides, chlorides or carbonates of alkali metals in an ammonium hydrofluoride melt, yielded monooxyfluoroniobates with different compositions, MxNbOF3+x, where they were subsequently investigated [123-127]. According to DTA patterns of the Nb205 - 6NFL HF2 - 2MF system, (Fig. 18) a rich variety of endothermic effects result from the formation of ammonium monooxyfluoroniobate, its thermal decomposition and its interaction with alkali metal fluorides. The number of effects decreases and separation of ammonium ceases at lower temperatures and when going from lithium to cesium in the sequence of alkali metal fluorides. 

Simultaneous fluorination of niobium oxide and oxides of trivalent metals using an ammonium hydrofluoride melt leads only to oxide-type compounds, MinNbC>4 due to low thermal stability of fluoride or oxyfluoride compounds that contain both niobium and trivalent metals. 

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. 

Decomposition of lithium niobate, LiNb03, by molten ammonium hydrofluoride can be performed even at temperatures as low as 130-260°C [122]. The process also enables the separation of niobium and lithium, yielding ammonium oxyfluoroniobate and lithium fluoride. The interaction can be represented by Equation (132) ... 

The optimal temperature range for the interaction was found to be 150-230°C. The cake resulting from the fluorination process was also successfully leached with water, dissolving ammonium oxyfluoroniobate, (NH4)3NbOF6. The solution was separated from the precipitate of lithium fluoride. The main parameters of the solution were a niobium concentration of about 75 g/1 Nb205, pH = 3—4. 

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. 

Another way of applying the selective extraction method directly on the initial solution is to produce a solution of low acidity. This can be achieved by using the hydrofluoride method for fluorination and decomposition of raw material. As was discussed in Paragraph 8.2.2, the raw material is fluorinated by molten ammonium hydrofluoride yielding soluble complex fluorides of ammonium and tantalum or niobium. The cake obtained following fluorination is dissolved in water, leading to a solution of low initial acidity that is related for the most part to the partial hydrolysis of complex fluoride compounds. The acidity of the solution is first adjusted to ensure selective tantalum extraction. In the second step, the acidity of the raffinate is increased to provide the necessary conditions for niobium extraction. 

S Fluorination of tantalum and niobium oxides by hydrofluorides of ammonium or alkali metals yields fluorotantalate or monooxy-fluoroniobate compounds. Fluorination of tantalum or niobium oxides in the presence of oxides of other metals yields complex fluoride compounds containing both tantalum or niobium and added metals. 

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. 

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