SEPARATION OF TITANIUM AND ZIRCONIUM BY ION EXCHANGE

V. N. Neustrueva, G. M. Kuznetsova, A. G. Borodulina

(Presented by Prof. A. G. Stromberg, Doctor of Chemical Sciences)

A number of works have been devoted to the study of the state of zirconium and titanium in hydrochloric-acid solutions [1, 2, 3, 4, 5]. The authors of some of these works consider that in solutions of hydrochloric and nitric acids with concentrations of 0.3 M and higher, titanium and zirconium are present in the form of complex ions. The absorption of chloride complexes by ion-exchange resins has been studied by most investigators by the static method, by determining the distribution coefficient of the element between the ion-exchange resin and the solution [1, 2, 4, 6].

On the basis of these studies it was established that: a) in hydrochloric-acid solutions with HCl concentrations from 0.1 to 6 M, titanium and zirconium are present in cationic form; b) the intensity of absorption of titanium and zirconium by a cation exchanger depends on the concentration of hydrochloric acid in the solution. The greatest differences in the behavior of these elements during ion exchange appeared when 1 M HCl was used. Under these conditions titanium was not absorbed at all by the KU-2 cation exchanger, whereas zirconium was sorbed completely. All studies were carried out with milligram quantities of substances [1, 2].

It was of interest to verify whether this regularity would be maintained in the case of higher concentrations of titanium and zirconium and other ratios of the concentrations of these elements in a mixture, and also to determine the possibility of separating them in the course of sorption.

Results of the investigation

The separation of a mixture of titanium and zirconium was carried out on columns packed with a layer (28 × 1 cm) of KU-2 resin in the H-form, with grain size 0.25–0.5 mm. The resin with which the columns were packed was preliminarily treated with 1 M HCl. 100 ml of the solution under investigation was filtered through the column at a rate of 0.8–1 ml/min. The solution passing through the column was collected as separate 25 ml fractions and analyzed for titanium and zirconium content. The analyses were performed photocolorimetrically [7]. Titanium was determined by reaction with hydrogen peroxide in an acid medium, and zirconium—with the use of the organic dye sodium alizarin sulfonate (alizarin red) [7]. The volume, rate, concentration of hydrochloric acid, and temperature of the solutions passed through the column were kept constant.

through the column remained constant for all experiments. Thus, the contact time of the solution with the resin also remained constant. According to our calculations, it was 0.36 h (column diameter = 1 cm, volume of resin in the column 28 cm³, volumetric rate 60 ml/h, cross-sectional area of the column \(w = 0.785\ \text{cm}^2\). The linear velocity

\[ \frac{60}{0.785} = 0.764\ \text{m/h}, \]

the contact time

\[ \tau = \frac{0.280}{0.764} = 0.36\ \text{h}. \]

To determine the possibility of separating titanium and zirconium in the process of sorption from 1 M HCl, their uptake from a mixture was studied at different ratios of the concentrations of these elements in solution.

The titanium concentration remained constant in all experiments, equal to 12 g/l TiO₂. This is approximately 80% of the total dynamic capacity of the ion exchanger for the calcium ion. The zirconium concentration in the mixture was varied. As can be seen from the table, complete uptake of zirconium by the cation exchanger at this titanium content is possible only at concentration ratios ZrO₂/TiO₂ = 1 : 100; 2 : 100; 4 : 100; 6 : 100. Under these conditions titanium is absorbed by the resin by 96.6%, while zirconium is absorbed completely. With a further increase in the concentration of zirconium in the mixture, uptake by the cation exchanger of both titanium and zirconium is incomplete. While the sorption capacity of the resin remains constant, regardless of the composition of the incoming solution, the amount of zirconium absorbed by the cation exchanger depends on the presence of titanium ions in the solution, just as the uptake of titanium depends on the concentration of zirconium ions in this solution.

Separation of titanium and zirconium on the cation exchanger KU-2 from 1 M HCl solutions

In the second series of experiments, the intensity of uptake of titanium and zirconium by the KU-2 resin was studied as a function of the concentration of hydrochloric acid in the solution. The results of the study are presented in Figs. 1 and 2. As can be seen from Fig. 1, zirconium is best absorbed by the cation exchanger at hydrochloric acid concentrations of 0.5 M and 1 M. When the acid concentration is increased or decreased, the percentage of absorption falls. The decrease in the absorption capacity of the cation-exchange filter at low hydrochloric acid concentration in the solution is explained by an increase in the degree of hydrolysis of the salt. The decrease in capacity at high

concentration of hydrochloric acid can be explained by a shift in the established equilibrium between the exchanging ions as a result of the increase in the concentration of \( \mathrm{H}^+ \) ions in the solution.

\[ 2\mathrm{R{-}SO_3H} + \mathrm{Me}^{2+} \rightleftharpoons (\mathrm{R{-}SO_3})_2\mathrm{Me} + 2\mathrm{H}^+ . \]

The low percentage of zirconium absorption from solutions with a salt concentration of \(18\ \mathrm{g/l}\ \mathrm{ZrO_2}\) is due to the fact that, evidently, the salt concentration exceeds the sorption capacity of the resin for the zirconium chloride complex.

Fig. 1. Absorption of zirconium by the KU-2 cation exchanger from 100 ml of hydrochloric-acid solution.
Content in the solutions: No. 1—\(10\ \mathrm{g/l}\ \mathrm{TiO_2}\), No. 2—\(14\ \mathrm{g/l}\ \mathrm{TiO_2}\), No. 3—\(18\ \mathrm{g/l}\ \mathrm{TiO_2}\).

Titanium is absorbed best by the cation exchanger from 0.1, 0.5, and 1 M hydrochloric-acid solutions (Fig. 2). With an increase in acid concentration, the degree of titanium absorption decreases sharply. As both series of experiments showed, in 1 M hydrochloric-acid solutions titanium and zirconium behave identically with respect to their sorption capacity. Consequently, in the sorption process separation of these elements is impossible.

The difference in the behavior of titanium and zirconium in hydrochloric-acid media appears only during their desorption from the resin. It was established experimentally that a 1 M solution readily washes the titanium ion from the cation exchanger, whereas zirconium is practically not washed out by acid of this concentration. Moreover, comparatively small volumes of 1 M hydrochloric acid are required for eluting titanium (Fig. 3). Hydrochloric acid of higher concentration, 1.5 M and 2 M, washes out not only titanium but also partially zirconium. Therefore, for purposes of separation, hydrochloric acid with a concentration not higher than 1 M may be used as the eluting solution. This different behavior of titanium and zirconium adsorbed on the resin toward 1 M HCl is a consequence of the unequal strength of bonding of their chloride complexes with the resin. Evidently, the titanium complexes

Fig. 2. Absorption of titanium by KU-2 cation exchanger from 100 ml of hydrochloric acid solution.
Content in the solutions: No. 1—10 g/l TiO₂, No. 2—14 g/l TiO₂, No. 3—18 g/l TiO₂.

Fig. 3. Desorption of titanium with 1 M hydrochloric acid. Loading: 1.0 g TiO₂.

are less strongly bound to the cation exchanger than zirconium complexes and therefore are washed out by an excess of 1 M HCl. For eluting zirconium, 1 M hydrochloric acid is unsuitable. In practice, 6 M HCl is used for its elution [4]. The difference in the strength of binding of the chloride complexes of titanium and zirconium to the resin can also explain the difference in their distribution coefficients on the KU-2 cation exchanger when 1 M hydrochloric acid is used [1, 2]. Experiments under static conditions were carried out with a large excess of acid and a low salt concentration. Under these conditions, the weak bonds of the titanium complexes with the resin were destroyed and all the titanium was desorbed from the cation exchanger, whereas zirconium sorbed by the resin was not washed out.

Conclusions

1. The sorption of gram quantities of zirconium and titanium by the KU-2 cation exchanger from hydrochloric-acid solutions has been studied. It has been shown that the degree of absorption of these elements by the resin depends on the concentration of hydrochloric acid in the solution.

2. By elution with a 1 M solution of hydrochloric acid, separation of titanium and zirconium adsorbed on KU-2 resin is achieved.

References

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2. T. A. Belyavskaya, M. K. Chmutova, Nauchnye doklady Vysshei shkoly, 2, 305, 1958.
3. K. A. Kraus, F. Nelson, Chemistry of Nuclear Fuel, 353, 1955.
4. V. A. Lister, Chem. Soc., 11, 3123, 1951.
5. J. T. Benedikt, W. C. Schamb et al., J. Am. Chem. Soc., 76, 2036, 1954.
6. Tompkins, G. Mayer, J. Am. Chem. Soc., 69, 2859, 1947.
7. Analysis of Mineral Raw Materials. Gostekhizdat, 1956, Leningrad.