Distribution of Chromium Oxide in Ilmenite Processing Products

V. M. Vityugin, O. A. Fuks

(Presented by the scientific seminar of the Department of Mineral Beneficiation)

Titanium dioxide, used as a white pigment, must have a minimum content of impurities, since even traces of impurities impart various shades to white paint. Chromium-oxide impurities have an especially strong effect on the color of titanium dioxide; even when present in insignificant quantities, they already impart to the white pigment a brownish-yellow coloration visible to the eye. For this reason, in practice, for the production of pigment titanium dioxide, ilmenite concentrates are used that contain only a minimal amount of chromium oxides—no more than 0.3% relative to TiO₂.

We studied the distribution of chromium oxides among the products of decomposition of ilmenite concentrate, and also investigated the possibility of obtaining white titanium dioxide from ilmenites containing various amounts of chromium oxide.

For the study, ilmenite concentrates of three different compositions were taken:

Ilmenite A contained TiO₂ — 51.7%, Cr₂O₃ — 2.39%;
Ilmenite B contained TiO₂ — 47.0%, Cr₂O₃ — 1.55%;
Ilmenite C contained TiO₂ — 43.5%, Cr₂O₃ — 0.061%.

Decomposition was carried out by the solid-phase sulfuric-acid method [1]. The content of chromium oxides was determined in the residue after decomposition, in the titanium dioxide obtained by hydrolysis of the sulfuric-acid solution, and also in the solid product obtained by precipitating the titanium filtrate with ammonia.

In the study of ilmenite A it was established that from 92.9 to 97.7% of Cr₂O₃ remains in the undecomposed residue; from 0.08 to 0.34% of Cr₂O₃ passes into the commercial titanium dioxide, which corresponds to a content in it of ~0.04% Cr₂O₃. The filtrate after separation of metatitanic acid H₂TiO₃ is precipitated with ammonia, and in the resulting solid product, after its calcination, the chromium-oxide content was also determined. In processing ilmenite A, it amounted to from 2.4% to 8.4% of its initial quantity.

Thus, in the sulfuric-acid decomposition of a high-chromium ilmenite concentrate, a very insignificant amount of chromium oxides passes into titanium dioxide, not preventing the use of TiO₂ as a white paint after additional purification and

processing, which is always carried out in the industrial production of pigment titanium dioxide. Nevertheless, sulfuric-acid decomposition as a method for opening up this ilmenite cannot be proposed. This is explained by the fact that, owing to the considerable leucoxenization of the mineral under study, its decomposition by sulfuric acid is hindered and amounts to only 50–55%.

Ilmenite B has an even higher degree of leucoxenization; its decomposition by sulfuric acid is still more difficult and amounts to 47–49%. The distribution of chromium oxides among the products of its decomposition proceeds in the same manner as for ilmenite A. In the undecomposed residue, 94–97.4% of the $\mathrm{Cr_2O_3}$ remains from its initial amount; 0.24–0.8% $\mathrm{Cr_2O_3}$ passes into the $\mathrm{TiO_2}$ product, which corresponds to a content in it of $\sim 0.035\%$ $\mathrm{Cr_2O_3}$, i.e., in this case as well titanium dioxide is obtained from which, as a result of subsequent treatment, pigment $\mathrm{TiO_2}$ can be obtained. The solid product obtained from the filtrate contained 2.6–5.6% of the initial amount of chromium oxide.

Ilmenite C, containing the minimum amount of chromium, showed a degree of decomposition of 72–75%. The distribution of chromium oxides among the decomposition products of this ilmenite proceeded as follows: 52.2–67.0% $\mathrm{Cr_2O_3}$ remained in the undecomposed residue; 10–18% $\mathrm{Cr_2O_3}$ passed into the $\mathrm{TiO_2}$ product, which corresponds to a content in it of 0.013–0.03% $\mathrm{Cr_2O_3}$; and 25.4–29% $\mathrm{Cr_2O_3}$ passed into the solid product obtained from the filtrate. The results obtained are given in Table 1.

Table 1

Content of chromium oxide in the products of sulfuric-acid processing of ilmenites

As a result of the investigation carried out, it was established that the titanium dioxide obtained in the sulfuric-acid decomposition of different ilmenites contains approximately the same amount of chromium oxide. Consequently, after additional treatment and purification, the titanium dioxide obtained from ilmenites with different chromium oxide contents can be used as a white pigment.

The low degree of sulfuric-acid decomposition of ilmenite A requires the development of a more rational method for opening up this concentrate. It was decided to test the method of chalk decomposition and acid leaching of the resulting sinter, which is used in industry for opening up zircon concentrates [2].

For the study, ilmenite concentrate A was taken. Its decomposition was carried out by sintering with chalk and calcium chloride at a temperature of 1100°. In this process the reaction proceeds according to the equation:

\[ 2\mathrm{FeTiO}_3 + 3\mathrm{CaO} + \tfrac{1}{2}\mathrm{O}_2 \to 2\mathrm{CaTiO}_3 - \mathrm{CaO}\cdot\mathrm{Fe}_2\mathrm{O}_3. \]

The resulting clinker was subjected to two-stage leaching. The first leaching was carried out with 5% hydrochloric acid; in this process iron, excess chalk, calcium chloride, a certain amount of chromium compounds, and an insignificant part of the calcium titanate passed into solution.

The second leaching was carried out with 25% hydrochloric acid, with heating to 80–90°, in order to dissolve the calcium titanate formed during sintering. The titanium compounds that passed into solution were subjected to hydrolysis by dilution and boiling of the solution. The metatitanic acid formed during hydrolysis was filtered off, washed, and calcined. The titanium dioxide obtained as a result of calcination contained chromium oxides and had a yellowish tint.

It was established that chromium oxides enter the titanium dioxide during the hydrolysis process from the solutions of the second leaching. On this basis, the task was set of transferring chromium into solution during the first leaching or leaving it in the undecomposed residue.

It is known from the literature that decomposition of chromites is carried out by sintering them with soda or with a mixture of soda and magnesium oxide. Therefore, in order to form more readily soluble chromium compounds passing into solution during leaching with 5% hydrochloric acid, soda was added to the charge for sintering [3], [4], [5].

In order to determine the optimal conditions for removing chromium, the influence of additions of sintering reagents and the influence of the sintering temperature were studied. For this purpose, charges of seven different compositions were prepared and subjected to sintering at temperatures of 500, 600, 700, 800, 900, 1000, and 1100°C. The resulting sinters were analyzed for chromium oxide content and then subjected to further processing. In this process the chromium oxide content was determined in the solutions of the first and second leaching and in the undecomposed residue after its calcination.

As a result of the study it was established that the optimal temperature is 1100°C, since at a lower temperature complete decomposition of the ilmenite itself is not achieved.

The influence of soda additions to the charge on the distribution of chromium oxide among the products of ilmenite decomposition at 1100°C is shown in Table 2.

Table 2

¹) The missing amount of chromium oxide remains in the undecomposed residue.

Conclusions

To obtain pigment-grade titanium dioxide from leucoxenized ilmenite concentrates, it is proposed to introduce an addition of soda into the sintering charge in an amount of 0.7–0.8 parts by weight relative to the weight of ilmenite in the charge.

References

1. E. F. Belen’kii, I. V. Riskin. Chemistry and Technology of Pigments. State Scientific and Technical Publishing House of Chemical Literature, 1960.

2. G. A. Meerson, A. N. Zelikman. Metallurgy of Rare Metals. Metallurgizdat, 1955.

3. A. D. Dymchishin. Production of Chromium Salts. ONTI Goskhimtekhizdat, 1934.

4. Finkel’shtein and Boretskaya. Methods for the Analysis of Mineral Raw Materials. Gosgeoltekhizdat, 1958.

5. Analysis of Mineral Raw Materials. Edited by Knipovich and Moracheskii. GKhI, 1956.