On the Chemistry of Tantalum and Niobium

I. P. Onufrienok, M. E. Effendi

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

Separation of Titanium from Tantalum and Niobium

Titanium is one of the most common and constant companions of tantalum and niobium in their natural compounds. Therefore, in studying mineral raw materials containing tantalum and niobium, the analytical chemist always has to deal with the separation of titanium from tantalum and niobium. The existing methods for separating titanium from tantalum and niobium were either very laborious or did not give reliable separation. The following methods were the most deserving of attention and were used.

1. The Ditrich and Freund method [1].
   The separation is based on the solubility of titanic acid in salicylic acid. The separation is carried out by boiling freshly precipitated acids with an excess of salicylic acid. Boiling is conducted under a reflux condenser for 3–4 hours.

2. The Schoeller method, or the pyrosulfate–tannin method, for separating small amounts of tantalum and niobium from titanium by leaching the pyrosulfate melt with a mixture consisting of 1 g of tannin and 5 ml of concentrated sulfuric acid in 100 ml of solution; the earthy acids thereby remain in the insoluble residue [2].

3. The oxalate–salicylate method of Schoeller and Jahn [3], based on the fact that titanium gives a soluble complex compound with salts of salicylic acid; tantalum and niobium, in this process, pass into the precipitate.

4. A modification of the pyrosulfate–tannin method is the method proposed by V. S. Bykova, in which the leaching of the pyrosulfate melt is carried out with a hydrochloric-acid tannin mixture [4].

When these methods were compared, it turned out that none of them gives complete separation of titanium from tantalum and niobium, and for complete purification of the earthy acids from titanium it is necessary to carry out repeated fusions with potassium pyrosulfate and to perform the separations again. The last of the listed methods is less laborious, but even when it is used, small amounts of titanium are separated with great difficulty from large amounts of the earthy acids. Therefore it was decided to undertake a study of the solubility of tannin compounds of titanium, especially since we were prompted to do so by the successes achieved by us in studying the solubility of tannin compounds

tantalum and niobium, which led to the development of a new method for separating tantalum from niobium [5].

For the experiments, standard stock solutions were prepared containing 1 mg in 1 ml of \(TiO_2\), \(Nb_2O_5\), and \(Ta_2O_5\), separately for each of the oxides, in volumes of 0.5 liter. The standard solutions were prepared by fusing weighed portions of the corresponding oxides in a platinum or quartz crucible with a 6–10-fold amount by weight of potassium pyrosulfate, leaching the resulting melt in a saturated solution of ammonium oxalate, transferring the resulting solutions into 0.5-liter flasks, and diluting to the mark with 0.1 N oxalic acid solution. The solutions obtained in this way are fairly stable; signs of hydrolysis in these solutions appeared no earlier than after 6 months; the most stable was the standard \(Nb_2O_5\) solution, which showed no signs of hydrolysis after 12 months.

The tannin compounds of titanium, tantalum, and niobium, as well as their mixtures having different quantitative ratios of \(TiO_2\) to the earth acids, were prepared under the conditions for precipitating the tannin compounds of tantalum and niobium as follows.

1. A definite volume of the standard solution was taken, or a mixture was made up from definite volumes of different standard solutions.

2. To the selected mixture there was added an equal volume of a solution of ammonium chloride saturated in the cold, and a freshly prepared 2.5% tannin solution, calculated as 1 ml per 1 mg of the oxides taken.

3. The resulting solution was heated to boiling and gradually neutralized with concentrated ammonia, added dropwise with stirring, to an alkaline reaction to Congo red and an acid reaction to litmus, after which the solution was kept at boiling for no more than 5 minutes, and then for 30 minutes on a boiling water bath.

4. The precipitate of the tannin compound was filtered off while still hot through a red-ribbon filter and washed with hot 2% ammonium chloride solution until the reaction of the oxalate ion disappeared (test with \(CaCl_2\) solution).

To accelerate washing, the precipitate was washed off the filter with the washing liquid back into the beaker, and then filtered again and washed. The precipitate of the tannin compound, washed in this way, was tested for solubility.

The solubility of the tannin compound of titanium was tested by us: in 0.1 N, 0.5 N, and 1 N oxalic acid; in 0.1 N, 0.5 N, 1 N, and 3 N hydrochloric acid; in 0.1 N, 0.5 N, 1 N, and 3 N sulfuric acid.

For the solubility test, the precipitate of the tannin compound of titanium was washed from the filter into a beaker (as cleanly as possible), in which precipitation had been carried out, with the acid in which solubility was being studied; the volume in the beaker was brought with a solution of this same acid to approximately 50 ml. The solution with the precipitate was heated with stirring, boiled for 5 minutes, and filtered through the same filter. On the filter the precipitate was washed 3–4 times with a hot solution of the same acid, then 2–3 times with hot 2% ammonium chloride solution.

The washed insoluble residue was ignited together with the filter in a weighed porcelain crucible and calcined to constant weight. The solubility of the tannin compound of titanium was calculated as the difference between the weight of the amount of \(TiO_2\) taken and that obtained in the insoluble residue.

Study of the solubility of the tannin compound of titanium by the above-mentioned method showed that this compound is quantitatively soluble in all the tested concentrations of oxalic acid, readily soluble in 0.5 N solutions of hydrochloric and sulfuric acids, quantitatively soluble in 1 N and 3 N solutions of sulfuric and hydrochloric acids, and far from completely soluble in 0.1 N solutions of sulfuric and hydrochloric acids.

Subsequently, in order to develop a method for separating titanium from tantalum and niobium, we used the property of the tannin compound of titanium to dissolve in 1 N hydrochloric acid, since 1 N hydrochloric acid does not affect the tannin compounds of tantalum and niobium even upon prolonged boiling.

To check the solubility of the tannin compound of titanium in the presence of the tannin compounds of tantalum and niobium, mixtures of standard solutions were prepared with the ratios \(\mathrm{TiO_2} : \mathrm{Nb_2O_5} : \mathrm{Ta_2O_5}\)—\((8 : 1 : 1)\), \((6 : 2 : 2)\), \((4 : 3 : 3)\), and \((2 : 4 : 4)\). From these mixtures tannin compounds were prepared and tested for solubility in 1 N hydrochloric acid by the methods described above.

The insoluble residues were then ignited in weighed platinum or quartz crucibles and calcined to constant weight. The residue of \(\mathrm{TiO_2}\) that had not passed into solution was determined by subtracting the sum of the \(\mathrm{Ta_2O_5}\) and \(\mathrm{Nb_2O_5}\) taken, and was subsequently checked colorimetrically.

For the colorimetric determination of \(\mathrm{TiO_2}\), the weighed insoluble residue was fused with a 6–10-fold amount of potassium pyrosulfate; the melt was leached with a cold saturated solution of ammonium oxalate, and the resulting solution was then treated with several drops of 3% hydrogen peroxide. Comparison was carried out by the method of colorimetric titration with a standard \(\mathrm{TiO_2}\) solution.

The maximum residue of \(\mathrm{TiO_2}\) that did not pass into solution did not exceed 0.0010 g. The minimum residue was 0.0002 g.

After the colorimetric determination of titanium, the solutions were decolorized by adding a solution of \(\mathrm{Na_2SO_3}\) dropwise, and were then again used to precipitate tannin compounds from them. The tannin compounds obtained were once more tested for solubility in 1 N hydrochloric acid as before. The insoluble residues were weighed, and the amount of residual \(\mathrm{TiO_2}\) in them was again determined colorimetrically. After secondary treatment of the tannin compounds with 1 N hydrochloric acid, the residue of \(\mathrm{TiO_2}\) in none of the mixtures weighed more than 0.0002 g.

Subsequently, after repeated colorimetry of the \(\mathrm{TiO_2}\) residues, tannin compounds were again obtained from the solutions as before; these were used for separating tantalum from niobium by treating these compounds with 6 N sulfuric acid\(^1\), with the \(\mathrm{TiO_2}\) residue passing into solution together with \(\mathrm{Nb_2O_5}\).

The \(\mathrm{Ta_2O_5}\) obtained as a result of such separation is always free from \(\mathrm{TiO_2}\).

Conclusions

1. The solubility of the tannin compound of titanium in oxalic acid and in 0.1 N, 0.5 N, 1 N, and 3 N hydrochloric and sulfuric acids has been studied.

2. The quantitative solubility of the tannin compound of titanium in 1 N hydrochloric acid has been used to develop an effective method for separating titanium from tantalum and niobium.

\(^1\) See the article in this volume, “On the Analysis of Tantaloniobates.”

References

1. M. Dittrich, S. Freund, Z. anorg. Chem., 56, 344, 1908.
2. W. R. Shoeller, Analyst, 641, 453, 1929.
3. W. R. Shoeller, C. Jahn, Analyst, 671, 72, 1932.
4. V. S. Bykova, Materials on the geochemistry of the Khibiny tundras. Kola Base of the USSR Academy of Sciences, II, p. 39, 1936.
5. M. E. Efendi, I. P. Onufrienok. Author’s certificate No. 12886, 1951.