Double niobium fluorides

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... 

In 1905 Hall and Smith 2 investigated all the then known methods for the removal of titanium, and tried various other processes they were unable, however, to improve on Marignac s method of fractional recrystallisation of the double potassium fluorides.3 This method has the disadvantage that in the case of the niobium salt protracted and tedious repetition is necessary before it is obtained free from titanium, and the method becomes impossible with small quantities of material.4... 

Niobic acid displays a much less pronounced tendency than vanadie acid to form heteropoly-compounds with other acids, but oxaloniobates are known. It reacts with hydrogen peroxide to form perniobic add, HNb04.a,H20, salts of which are known. The double niobium oxy-fluorides also take up active oxygen. 

Rubidium Niobium Fluoride, RbNbF6 or RbF.NbF5, is obtained by repeated crystallisation of rubidium niobium oxyfluoride, RbjNbOFg, from hydrofluoric acid.8 The double salt Rb2NbF7 or 2RbF.NbFs has also been reported.4... 

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. 

Once purification of the niobium has been effected, the niobium can be reduced to the metallic form. The double fluoride salt with potassium, K2NbFy, can be reduced using sodium metal. The reaction is carried out in a cylindrical iron vessel filled with alternating layers of K NbF and oxygen-free sodium ... 

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... 

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. 

Double Fluorides of Niobium Pentafluoride.—Niobium pentafluoride shows a strong tendency to form stable double fluorides with the fluorides of other metals. These are conveniently prepared by the action of carbonates of the metals on solutions of niobium pentoxide in a large excess of hydrofluoric add, or by the addition of a large excess of hydrofluoric acid to solutions of the oxyfluorides of the metals. In the absence of excess of hydrofluoric add hydrolysis takes place as usual with the formation of niobium oxytrifluoride, NbOF3. The precipitation of these double fluorides indicates the probable existence in solution of niobium pentafluoride stability is imparted by the formation of complex anions containing several fluorine atoms. 

When dewed from the point of view of the Werner co-ordination theory, it is observed that the co-ordination number seven is frequent in the double fluorides as well as in the niobium oxyhalides. Many members of the series 2R F.NbFs or [NbFJRa, and of the series... 

The following double fluorides of niobium have been prepared 1... 

Double chlorides of niobium pentachloride with chlorides of other metals to correspond with the double fluorides given by niobium penta-fluoride are not known. A double compound mth piperidine, NbCl5. 

The alkali niobates are most conveniently prepared by the action of caustic alkalis on niobic acid or on solutions of niobium oxytri-fluoride. Other compounds of niobic acid and bases are generally prepared by fusing niobic add with the oxide, hydroxide, carbonate, or other salt of the metal. Occasionally double decomposition of a soluble alkali niobate and a soluble salt of the metal has been employed. Larsson s method1 consists in predpitating a solution of potassium niobate with a salt of a metal the dried predpitate is fused for thirty-six hours at a high temperature with boric acid, and the melt is boiled with water to which hydrochloric acid has been added. The residue consists of crystals of the insoluble niobate of the metal, usually the metaniobate. 

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. ... 

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... 

Fig. 5. Idealized, close-packed structures for (a) niobium and (b) rhodium penta-fluorides. Atoms in the first, second, third, and fourth layers are shown as single, double, crossed, and hatched circles, respectively. The symbols for overlapped atoms are shown dashed. The bridge bonds are shaded.

The dry double fluoride is best reduced on a 2 kg niobium scale, either the closed bomb or the open reactor technique being suitable, i.e. 

The yield of niobium is about 90 per cent from the double fluoride, although the loss is dependent principally upon the care exercised in the handling operations. Clearly, this could be improved if the throughput of the plant was sufficiently high to justify more careful attention to the design of equipment for handling operations. 

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