Ammonium tantalum fluorides

Increasing the temperature to 260-300°C leads to the decomposition of the ammonium-tantalum complex 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 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. 

Fig. 137. Tantalum oxide content versus pH. Curve 1 - solution containing no ammonium fluoride. Curves 2-5 - solutions with increased initial ammonium fluoride content (after Agulyanskaya et al., [492, 493]).

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. 

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. 

Lanthanum in purified metallic state may be obtained from its purified oxide or other salts. One such process involves heating the oxide with ammonium chloride or ammonium fluoride and hydrofluoric acid at 300° to 400° C in a tantalum or tungsten crucible. This is followed by reduction with alkali or alkaline earth metals at 1,000°C under argon or in vacuum. 

Uranium Z, also thought to be a product of uranium X, was discovered by Hahn in 1921, who isolated it in the following manner. The mother-liquors from repeated fractional crj stallisations of uranous nitrate, containing uranium X and uranium Z, w ere treated with ferric chloride solution, and the iron precipitated by means of ammonia and ammonium carbonate. The precipitate, which contained both the uranium Xj and uranium Z, was treated with a solution of tantalum in hydrofluoric acid, lanthanum nitrate added, and the mixture digested on a water-bath with dilute hydrofluoric and sulphuric acids. Lanthanum fluoride was precipitated, and carried do%vn with it uranium The filtrate was evaporated to dryness and the residue ignited. The tantalum was thus rendered insoluble, whilst the iron could be removed by means of concentrated hydrochloric acid. The uranium Z remained with the tantalum. By this means Hahn obtained specimens of uranium Z which were 99-5 per cent, radioactively pure. 

Derivation From tantalum potassium fluoride by heating in an electric furnace, by sodium reduction, or by fused salt electrolysis. The powdered metal is converted to a massive metal by sintering in a vacuum. Foot-long crystals can be grown by arc fusion. Corrosion resistance 99.5% pure tantalum is resistant to all concentrations of hot and cold sulfuric acid (except concentrated boding), hydrochloric acid, nitric and acetic acids, hot and cold dilute sodium hydroxide, all dilutions of hot and cold ammonium hydroxide, mine and seawaters, moist sul-furous atmospheres, aqueous solutions of chlorine. 

Mo, W, Sb, U, and fluoride interfere seriously in the determination of Ta with pyrogallol. Fluoride can be masked with boric acid. The tantalum-pyrogallol complex may be extracted in the presence of quaternary ammonium bases [88,89]. 

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