ON THE CHEMISTRY OF TANTALUM AND NIOBIUM

SEPARATION OF TANTALUM FROM NIOBIUM

M. E. Efendi, I. P. Onufrienok

(Presented by Prof. B. V. Tronov, Doctor of Chemical Sciences)

The separation of tantalum from niobium is usually associated with great difficulties; these difficulties are further aggravated by the fact that the chemical properties of these two elements are very similar and that the results of separations are affected by their usual genetic companions—titanium, tin, zirconium, and tungsten. Usually, one first attempts to free the earth acids from these accompanying elements, and then carries out the separation of tantalum from niobium.

The first of the known methods for separating tantalum from niobium was based on the different solubilities of \(K_2TaF_6\) and \(K_2NbOF_5\); niobium and tantalum are separated in this case by fractional recrystallization of these salts [1]. In practice, certain variants of this method have been used [2, 3, 4].

Subsequently, a method appeared for separating tantalum from niobium based on the solubility of the oxides of niobium and titanium in a mixture of sulfuric acid and selenium oxychloride on boiling; tantalum oxide does not dissolve under these conditions [5]. This method was also laborious and did not find wide application in analytical practice.

The tannin method of separating tantalum from niobium has found the greatest practical application; it is based on the different stability of the oxalate complexes of the earth acids in the presence of tannin [6]. The tannin compound of tantalum is formed at \(pH = 2\), while the formation of the tannin compound of niobium begins at \(pH = 3\). The narrow pH range for the formation of the tannin compounds of tantalum and niobium does not allow this method to be carried out sharply; on the whole, the method is also very laborious, and even its simplified variants [7, 8] are nevertheless complex.

Less laborious is the hypophosphite method [9], based on the precipitation from oxalic-acid solutions of the earth acids of the complex acid \([(PO_2H_2)(Ta_2O_5)]\); niobium, in this case, gives no precipitate. A disadvantage of the method is that the reagent—phosphorous acid—must be free from the product of its oxidation, phosphoric acid, since the latter also precipitates niobium.

Working in 1947 with tantaloniobates, we used the tannin method for separating tantalum and niobium; at the same time, we became interested in the properties of the tannin compounds of tantalum and niobium washed free of oxalate ion with a 2-percent solution of ammonium chloride. In 1948 we studied the solubility of these compounds in oxalic, hydrochloric, and

sulfuric acids. As a result of this work it became possible to propose a new, simpler method for separating tantalum from niobium, based on the solubility of the tannin compound of niobium and the insolubility of the tannin compound of tantalum in 6 N sulfuric acid [9].

Experimental Part

For the investigation, stock solutions of tantalum and niobium were prepared with a titer equal to 1 mg/ml, calculated as the pentoxide. In preparing the stock solutions, a weighed portion of tantalum or niobium pentoxide was fused with a 10–12-fold amount by weight of potassium bisulfate. The melt was dissolved, with heating, in 50 ml of a saturated solution of ammonium oxalate and 50 ml of a 0.5 N solution of oxalic acid; after dissolution of the melt, the solution was filtered into a volumetric flask and diluted with water to the mark. Solutions prepared in this way are very stable on storage. It should be noted that, during storage, gels of the earth acids appear only if less ammonium oxalate is taken than is required to replace the free sulfuric acid with oxalic acid.

Method for Investigating the Solubility of Tannin Compounds of Tantalum and Niobium in Acids

Into a 100 ml beaker, 10 ml of the stock solution is introduced with a pipette; to it are added 10 ml of a freshly prepared 2% tannin solution and 20 ml of a saturated solution of ammonium chloride. The resulting solution is neutralized with 2 N ammonia to an alkaline reaction on Congo-red paper, but still acid on litmus paper. After neutralization, the solution is heated to boiling, boiled for 5 minutes, and then kept for 30 minutes on a boiling water bath.

The resulting precipitate of the tannin compound is filtered through a blue-band filter 7 cm in diameter, transferred onto the filter, and washed on the filter 3 times with a hot 2-percent solution of ammonium chloride. The precipitate is then washed from the filter with the same washing liquid back into the beaker, stirred well, again filtered through the same filter, and washed on the filter 4 times with the same washing liquid. After such washing the precipitate no longer contains oxalate ions.

The washed precipitate of the tannin compound is washed from the filter into the same beaker with the acid in which the solubility of this compound is being tested,¹ using 30 ml of acid solution for this purpose. The washed-off precipitate is thoroughly stirred with the acid solution using a glass rod. The walls of the beaker are rinsed with the same acid so that the volume of solution in the beaker does not exceed 40 ml.

Next, the precipitate together with the solution in the beaker is heated to boiling, boiled for 5 minutes, and kept for 30 minutes on a boiling water bath. The precipitate is then filtered through the same filter, transferred quantitatively onto the filter with the use of paper pulp for this purpose, washed on the filter 3 times with the same acid in which the dissolution was tested, and then finally the precipitate is washed on the filter with a 2-percent hot solution of ammonium chloride.

¹ Tannin compounds are readily washed from filters with a jet from a wash bottle. Experiments carried out by us to determine unwashed precipitates showed that their amount does not exceed 0.0002 g, calculated as the pentoxide.

After washing, the precipitate is dried, incinerated in a weighed porcelain crucible, and ignited to constant weight. The solubility of the tannin compound under these conditions is calculated as % pentoxide, as the difference between the amount of pentoxide taken and that obtained from the undissolved residue.

It must be emphasized that in the present work we determined the solubility in acids of the tannin compounds of tantalum and niobium, as well as of their mixtures, not as solubility in the generally accepted sense (i.e., a physicochemical constant), but as the solubility dictated by the above-described testing procedure, arising from certain analytical conveniences.

Solubility of the Tannin Compounds of Tantalum and Niobium in Oxalic Acid

We established qualitatively that a 0.1 N solution of oxalic acid dissolves the tannin compounds of the earthy acids on boiling. A 0.5 N solution dissolves the greater part of the precipitate in the cold and rapidly dissolves the entire precipitate when brought to boiling; a 1 N solution of oxalic acid dissolves almost the entire precipitate in the cold, and the remainder of the precipitate dissolves when the solution is still far from being heated to boiling. No qualitative difference is observed in the rates of dissolution of the tannin compounds of tantalum and niobium in oxalic acid. A series of quantitative experiments carried out according to the above-described method gave the following results: 0.1 N oxalic acid dissolves the tannin compound of tantalum within the range 97–98%; 0.5 N oxalic acid, within the range 99–100%; 1 N oxalic acid, within the range 99–100%. Figures of the same order were also obtained for the tannin compound of niobium. The practically complete solubility of the tannin compounds of the earthy acids in oxalic acid can be used in the analysis of tantalo-niobates for the purpose of purifying the earthy acids from iron, alkalis, and indifferent impurities.

Solubility of the Tannin Compounds of Tantalum and Niobium in Hydrochloric Acid

The tannin compounds of tantalum and niobium are fairly resistant to the action of hydrochloric acid. Quantitative experiments showed that neither 0.1 N, nor 0.5 N, nor 1 N solutions of hydrochloric acid dissolve these compounds; higher concentrations, up to 6 N hydrochloric acid, do not dissolve the tannin compound of tantalum. Up to 3% of the tannin compound of niobium passes into solution under the action of 3 N hydrochloric acid, and up to 6% under the action of 6 N hydrochloric acid. Hydrochloric acid solutions from 3 N to 6 N, on boiling, change the yellow color of the tannin compound of tantalum to reddish-brown; the same change in color is observed in the cold under the action of 9 N hydrochloric acid.

Solubility of the Tannin Compounds of Tantalum and Niobium in Sulfuric Acid

The tannin compound of tantalum is also fairly resistant to the action of sulfuric acid. Low concentrations of sulfuric acid, including a 6 N solution, do not act on this compound and do not even change its color on boiling. Boiling with 9 N sulfuric acid leads

leads to appreciable dissolution of this compound; up to 14% of it passes into the soluble state under the conditions of the procedure described above.

The tannin compounds of niobium are considerably less stable with respect to the action of sulfuric acid solutions. Low concentrations of sulfuric acid, up to and including 1 N, have no dissolving action; 3 N sulfuric acid dissolves this compound to 18%; 6 N, to 85%; and 9 N, to 90%. The undissolved tannin compound of niobium, after boiling with 6 N and 9 N sulfuric acid solutions, is dark-colored—most probably the color of resinified tannin.

If this resinified residue is ignited, converted into a soluble state by fusion with KHSO₄, and the tannin compound is again formed, then, upon repeated treatment with 6 N and 9 N sulfuric acid, about 90% of the weight of this residue passes into solution.

The tannin compound of niobium that has passed into solution in 6 N sulfuric acid can again be quantitatively precipitated upon neutralization with ammonia; the color of the compound thus precipitated is meat-red, i.e., it differs in no way from the usual tannin compound of niobium.

The sharp difference in the behavior of the tannin compounds of tantalum and niobium with respect to treatment with 6 N sulfuric acid prompted us to carry out experiments on the quantitative separation of tantalum from niobium. For the experiment, a mixture of the tannin compounds of tantalum and niobium (1:1), with a total weight of 0.0200 g calculated as the pentoxides, was taken. After the first treatment with 6 N sulfuric acid, 0.0060 g of niobium, calculated as the pentoxide, passed into solution. The insoluble residue was dissolved in 1 N oxalic acid, except for a small resinified residue which, after ignition and calcination, weighed 0.0027 g. The latter was fused with KHSO₄, leached with a solution of ammonium oxalate, and added to the oxalic-acid solution, from which the tannin compounds were again precipitated. A second treatment with 6 N sulfuric acid gave a residue of niobium in tantalum pentoxide of only 0.0009; a third gave a niobium residue of 0.0003 g and, finally, a fourth treatment, 0.0001 g. It should be noted that repeated experiments with mixtures of the tannin compounds of tantalum and niobium in the ratio (1:1) always gave a remarkable constancy of extraction after each treatment with 6 N sulfuric acid.

Thus, a new method for separating tantalum from niobium was revealed, in which, instead of a very laborious fractional precipitation of tantalum, one dissolves the tannin compound of niobium, which achieves the objective much more simply and 2–3 times faster. It should also be noted that if the tantalum and niobium compounds contain titanium, the latter, after the first two treatments with 6 N sulfuric acid, passes quantitatively into the filtrate.

Conclusions

1. The good solubility of the tannin compounds of the earthy acids in oxalic acid can be widely used in the analysis of tantalo-niobates for the purpose of purifying the latter from indifferent impurities and from alkali metals, and also makes it possible to avoid many fusions with potassium bisulfate.

2. The complete insolubility of the tannin compounds of tantalum and niobium in 1 N hydrochloric acid can be used for purifying these compounds from iron, aluminum, and alkalis. Hydrochloric acid—

at higher concentrations is capable of dissolving only the tannin compound of niobium.

1. The insolubility of the tannin compound of tantalum and the considerable solubility of the tannin compound of niobium in 6 N sulfuric acid on boiling enabled us to develop a method for separating tantalum from niobium.

REFERENCES

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