Double tantalum fluorides

The metal may be obtained from its fluoride salt, promethium(lll) fluoride by heating with lithium metal in a double tantalum crucible at 700 to 800°C in vacuum and then increasing the temperature to 1,100°C. 

Tantalum can be separated from niobium by recrystallization of the double potassium fluorides. In the commercial process the ore is fused with caustic soda. The insoluble sodium niobate, sodium tantalate, and... 

The estimation of small quantities of tantalum in niobium compounds is more difficult, and cannot be carried out colorimetrically. The usual method is to convert the material into the potassium double fluoride, and then to take advantage of the fact that a white precipitate of potassium tantalum oxyfluoride, K4Ta405F14 (see p. 132), is thrown down when a solution of potassium tantalum fluoride, KaTaF7, is boiled.7 Powell and Schoeller 8 find this test imperfect, and have modified the procedure (based on the differential hydrolytic dissociation of oxalo-niobic acid and oxalo-tantalic acid in the presence of tannin in slightly add solution) for the detection and estimation of traces of tantalum in niobium compounds. 

Tantalum Pentafluoride, TaFs, is the only known fluoride of tantalum, and has been successfully isolated by methods that avoid hydrolysis (1) Tantalum and fluorine are brought into reaction exactly as in the preparation of niobium pentafluoride.1 (2) Tantalum penta-chloride is treated in the cold with dry hydrofluoric add the hydrochloric acid liberated and excess of hydrofluoric add are evaporated oft, and the resulting tantalum pentafluoride is purified by redistillation in a platinum crucible between 300° and 400° C.2 (3) The double barium tantalum fluoride, 3BaF2.2TaF5, is very strongly heated in a platinum tube, one end of which is kept cold. ... 

Fio. 9.11, Extraction of tantalum (fluoride dissolution, re rystallization of double fluoride, sodium reduction process). 

Fluorides are nonhygroscopic, and their melting points are higher than those of the corresponding chlorides. Besides, the fluoride reduction reactions are considerably more exothermic. The prime examples of the use of fluorides as intermediates are the reduction of uranium tetrafluoride by calcium or magnesium the reduction of rare earth fluorides by calcium, reduction of beryllium fluoride by magnesium and the reduction of potassium tantalum double fluoride by sodium. 

In the case of molten salts, the functional electrolytes are generally oxides or halides. As examples of the use of oxides, mention may be made of the electrowinning processes for aluminum, tantalum, molybdenum, tungsten, and some of the rare earth metals. The appropriate oxides, dissolved in halide melts, act as the sources of the respective metals intended to be deposited cathodically. Halides are used as functional electrolytes for almost all other metals. In principle, all halides can be used, but in practice only fluorides and chlorides are used. Bromides and iodides are thermally unstable and are relatively expensive. Fluorides are ideally suited because of their stability and low volatility, their drawbacks pertain to the difficulty in obtaining them in forms free from oxygenated ions, and to their poor solubility in water. It is a truism that aqueous solubility makes the post-electrolysis separation of the electrodeposit from the electrolyte easy because the electrolyte can be leached away. The drawback associated with fluorides due to their poor solubility can, to a large extent, be overcome by using double fluorides instead of simple fluorides. Chlorides are widely used in electrodeposition because they are readily available in a pure form and... 

Ruff and Schiller 8 determined the solubilities of the double fluorides of niobium and tantalum, K2NbF7 and KaTaF7, in varying quantities of hydrofluoric add and potassium fluoride, and based a method of fractional separation on the results, which showed that the solubility of both fluorides diminishes with increasing concentration of potassium fluoride and decreasing concentration of hydrofluoric acid the solubility increases rapidly with rising temperature, and is always... 

Conversion of the separated fluorides into the corresponding oxides is effected by boiling with concentrated sulphuric acid until free from fluorine, and then hydrolysing the product by boiling with water. Alternatively, the hydrated acids are precipitated by the addition of ammonia to the solutions of the double fluorides.4 Niobium pentoxide, Nbg05, or tantalum pentoxide, TaaOs, is obtained on ignition of the precipitated hydrate. 

A method of separation which avoids the preparation of the double fluorides consists in fusing the mixed niobic and t an tali c acids with sodium carbonate and nitrate, the product is digested with warm water and a current of carbon dioxide is passed through the solution. It is claimed that only tantalic acid is precipitated.5 This process has, however, been the subject of adverse criticism.6 Partial separation of niobium from tantalum can be effected by warming the mixed, freshly precipitated, hydrated oxides with a mixture of hydrogen peroxide and hydrochloric acid the niobium dissolves readily, while the tantalum dissolves only sparingly.7... 

Despite the facts (a) that the figures for individual determinations differed by several whole units and (b) that the results as a whole were obviously discordant, Marignac s work formed the basis for the accepted atomic weight of tantalum (namely, 183) for forty years. The suitability of the double fluorides for use in the determination of the atomic weight has been questioned.9... 

Double Fluorides of Tantalum Pentafluoride.—When solutions of tantalum pentoxide in hydrofluoric acid are treated with solutions of the fluorides of the alkali (and other) metals, double fluorides are obtained which possess the general formula R F.TaF5, where n usually varies between 1 and 3 in the most important series n=2. These double fluorides are much more stable than tantalum pentafluoride. They were among the first tantalum compounds to recdve examination,5 and still form an important class of tantalum compounds. A study of their isomorphism with the corresponding compounds of niobium... 

The following double fluorides with tantalum pentafluoride are known ... 

Hydrates of Tantalum Pentoxide, Colloidal Tantalum Pentoxide, Tantalic Acid.—When tantalum pentachloride or pentabromide is treated with water, or when a solution of a tantalate is boiled with dilute acids, a gelatinous predpitate of more or less hydrated tantalum pentoxide is thrown down. Insoluble tantalates on fusion with potassium hydrogen sulphate and extraction of the melt with water give the gel. In dealing with double fluorides of tantalum it is necessary to remove all the hydrofluoric add by evaporation with concentrated sulphuric acid, otherwise double fluorides are obtained. 

Fluorine forms tantalum pentafluoride, TaF6, which resembles the columbium salt, being known only in solution. It forms important double fluorides. 

Syn. Tantalum.—It occurs very sparingly, in the minerals tantalite and yttro-tantalite, as columbic acid. The metal is obtained by the action of potassium on the double fluoride of columbium and potassium, as a black powder, which, when compressed, exhibits metallic lustre and when heated bums in air, yielding columbic acid. 

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. 

It was not xmtil 1865 that Mangnac separated niobium and tantalum by using the differences in solubilities of their double fluorides of potassium. In 1905, Dr. W. von Bolten introduced both tantalum and niobium to industry. 

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