COLD DESULFURIZATION OF IRON-ORE CONCENTRATES

I. V. Gebler, V. M. Vityugin, O. A. Fuks, N. Ya. Negodina

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

Concentrates obtained even by deep beneficiation of ores contain up to 1.0% sulfur in the form of iron sulfides. Ore intended for blast-furnace smelting, however, must contain no more than 0.03% sulfur. Usually sulfur is removed from ores by oxidative roasting at a temperature of 1300–1400°C. The high cost of the roasting process necessitates the search for cold methods of desulfurization.

Cold desulfurization of iron-ore materials is based on the possibility of converting metal sulfides into water-soluble sulfates. In particular, pyrite and pyrrhotite contained in iron-ore concentrates must be converted into ferric and ferrous sulfates by oxidizing the former in an aqueous medium.

The strongest oxidizing agents capable of converting metal sulfides into sulfites are hydrogen peroxide, ozone, and chlorine. On this basis, the study of the desulfurization process was carried out in three directions, using each of the indicated oxidizing agents.

Oxidation of Iron Sulfides by Hydrogen Peroxide

Iron-ore concentrate powder introduced into a hydrogen peroxide solution promotes its decomposition. The active oxygen released oxidizes the sulfides. The oxidation process may be expressed by the following equations:

\[ 2\mathrm{FeS}_{2} + 15\mathrm{O} + \mathrm{H}_{2}\mathrm{O} = \mathrm{Fe}_{2}(\mathrm{SO}_{4})_{3} + \mathrm{H}_{2}\mathrm{SO}_{4}, \tag{1} \]

\[ 6\mathrm{FeS} + 27\mathrm{O} + 3\mathrm{H}_{2}\mathrm{O} = 2\mathrm{Fe}_{2}(\mathrm{SO}_{4})_{3} + 2\mathrm{Fe}(\mathrm{OH})_{3}, \tag{2} \]

\[ 2\mathrm{Fe}(\mathrm{OH})_{3} + 3\mathrm{H}_{2}\mathrm{SO}_{4} = \mathrm{Fe}_{2}(\mathrm{SO}_{4})_{3} + 6\mathrm{H}_{2}\mathrm{O}. \]

Since in iron ores the pyrite content is much greater than that of pyrrhotite, the principal reaction will be that proceeding according to equation (1).

For the investigations, a Sokolovsko-Sarbaiskii magnetite concentrate of particle size less than 0.074 mm, containing 0.9% sulfur, was taken. For oxidation of the sulfides contained in one gram of concentrate, theoretically 1.27 ml of 3% hydrogen peroxide is required. When the ore concentrate was treated with a hydrogen peroxide solution taken in eightfold excess at room temperature for 72 hours, the degree of desulfurization was 13.3%. Treatment under the same conditions with vigorous shaking did not increase the degree of removal.

sulfur. When a fourfold excess of hydrogen peroxide was used at a temperature of 60–65°C for 70 minutes, desulfurization amounted to 22.2%, while under the same conditions an eightfold excess of hydrogen peroxide made it possible to obtain 37.7% desulfurization.

Thus, treatment of iron-ore concentrates with weak solutions of hydrogen peroxide gives more or less significant results only when large excesses of peroxide are used and with simultaneous heating of the suspension, which leads to the practical unprofitability of the process.

Oxidation of Iron Sulfides with Ozone

For the investigations an iron-ore concentrate of particle size less than 0.05 mm, containing 0.37% sulfur, was taken. Ozone was obtained in a laboratory ozonizer from air. Its content in the ozonized air was approximately 2%. When the ozone–air mixture was passed through the pulp (S:L = 1:10) for various times, with heating and without heating, the degree of desulfurization ranged from 6.7 to 13.6%.

During testing of the installation for oxidizing sulfur with ozonized air, the favorable influence of increasing the acidity of the medium was noted. Taking this into account, a series of experiments was carried out with the introduction of various amounts of hydrochloric acid into the pulp. The degree of desulfurization in this case ranged from 14.3 to 28.3%. A further increase in the degree of sulfur removal to 40.5% was achieved by passing ozonized air for 1.5 hours through an acidified pulp heated to 70–75°C. Thus, desulfurization of Sokolovsko-Sarbaisk concentrates with ozonized air also proved to be of low effectiveness.

Oxidation of Iron Sulfides with Chlorine

For the investigations an iron-ore concentrate with a sulfur content of 0.37% and grain size less than 54 μ was taken. To oxidize the sulfur, chlorine water containing 5.6 g of chlorine per liter was prepared. Theoretically, for complete removal of sulfur from one gram of ore, 4.6 ml of chlorine water is sufficient. However, because of the reduced utilization coefficient of chlorine, the amount required for complete desulfurization proved to be higher.

The influence of the amount of chlorine water on the degree of desulfurization of the concentrate is shown in Table 1. When treating an iron-ore concentrate of particle size less than 0.1 mm and sulfur content equal to 1.46%, the degree of desulfurization ranged from 15 to 26%.

Table 1

Thus, for more complete removal of sulfur when treating concentrates with chlorine water, concentrates of fine-

classes. Proceeding from the fact that the oxidizing action of chlorine is manifested only in freshly prepared solutions, it was decided to carry out oxidation by directly passing gaseous chlorine into the pulp.

For the experiments, Sokolovsko-Sarbai concentrates of different particle sizes and with different sulfur contents were taken. The effect of chlorination time on the degree of their desulfurization is shown in Table 2.

Table 2

The experimental data show that treatment of the pulp with gaseous chlorine gives a high degree of desulfurization for fine-size concentrates even with a short chlorine-passing time. The effectiveness of the treatment will increase with an increase in the activity of chlorine.

Electrochemical Oxidation of Sulfur

During electrolysis of a hydrochloric acid solution, atomic chlorine is liberated at the anode; it is a strong oxidizing agent capable of oxidizing sulfide sulfur. Part of the atomic chlorine is converted into molecular chlorine, which is also an oxidizing agent. Molecular chlorine partially combines with water, forming hydrochloric and hypochlorous acids. The latter decomposes with the liberation of atomic oxygen, which also oxidizes sulfur. The following reactions occur in the electrolyzer:

\[ \mathrm{Cl'} - \bar{e} = \mathrm{Cl}, \]

\[ \mathrm{S''} + 8\mathrm{Cl} + 4\mathrm{H_2O} = \mathrm{SO_4''} + 8\mathrm{HCl}, \]

\[ \mathrm{Cl} + \mathrm{Cl} = \mathrm{Cl_2}, \]

\[ \mathrm{S''} + 4\mathrm{Cl_2} + 4\mathrm{H_2O} = \mathrm{SO_4''} + 8\mathrm{HCl}, \]

\[ \mathrm{Cl_2} + \mathrm{H_2O} = \mathrm{HCl} + \mathrm{HClO}, \]

\[ \mathrm{HClO} = \mathrm{HCl} + \mathrm{O}, \]

\[ \mathrm{S''} + 4\mathrm{O} = \mathrm{SO_4''}. \]

Therefore, the oxidation of sulfur proceeds due to atomic and molecular chlorine and atomic oxygen.

For the studies, an iron-ore concentrate with a particle size of 0.1 mm, containing 0.68% sulfur, was used, and the dependence of the degree of sulfur oxidation on current strength and time was studied.

Fig. 1. Diagram of the apparatus for electrochemical oxidation of sulfur contained in the concentrate.
1 — electrolyzer body; 2 — anode; 3 — cathode; 4 — refrigerator; 5 — rheostat; 6 — stirrer; 7 — thermometer; 8 — stand; 9 — motor; 10 — autotransformer.

Research Method

Iron-ore concentrate in an amount of 20 g was placed in the electrolyzer (Fig. 1); 100 ml of a 5% hydrochloric acid solution was poured in there as well, and a constant current of specified strength was passed through. The pulp was continuously stirred by means of a stirrer. The electrolyzer was cooled with water. After a certain time had elapsed, the contents of the electrolyzer were transferred to a beaker, the concentrate was filtered off, and the sulfate ions that had passed into the filtrate were precipitated with a barium chloride solution.

The degree of oxidation of the sulfur contained in the iron-ore concentrate was calculated from the amount of barium sulfate formed.

The results of the study are presented in Table 3.
It is evident from the table that the degree of sulfur oxidation during electrochemical oxidation is proportional to the current strength and time and can be brought to 99–100%.

Table 3

Since hydrochloric acid is consumed only through chlorine losses, the consumption of hydrochloric acid can be reduced by using the electrolyte filtered after each successive treatment of the ore.

Electrochemical oxidation can be made cheaper by using the hydrogen obtained at the cathode and by utilizing the impurities of nonferrous metals that pass from the concentrate into solution.

Conclusions

1. The oxidation of sulfur contained in iron-ore concentrate has been studied by oxidizing it with hydrogen peroxide, ozone, chlorine, and by an electrochemical method.

2. It has been established that treatment of concentrates with chlorine and by the electrochemical method gives a high degree of sulfur oxidation and may find industrial application.