ON THE DESULFURIZATION OF BLAST-FURNACE PELLETS FROM SOKOLOV–SARBAI CONCENTRATES DURING OXIDIZING ROASTING

V. M. VITYUGIN, V. A. PROKHOROVICH, A. S. BOGMA

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

The exceptionally deep beneficiation of the Sokolov–Sarbai ores predetermines the preparation of concentrates for blast-furnace smelting by the pelletizing method. Particular attention in developing pelletizing technology must be devoted to achieving a very high degree of desulfurization (95–97%) of the pellets during their roasting. Meeting such stringent requirements for desulfurization makes it necessary to carry out a preliminary study of the theoretical foundations of the desulfurization process; knowledge of these foundations will aid in the development of a rational technology for desulfurizing roasting.

Studies by the Uralmekhanobr Institute [1, 2] have analyzed questions of the desulfurization of pellets prepared only from a mixture of concentrate with comparatively coarse-grained limestone. Meanwhile, it is of great interest to study the desulfurization of pellets prepared from mixtures of concentrate with finely ground dolomite and hydrated lime. In addition, in developing a technology for roasting pellets in rotary kilns, it is necessary to study the process of pellet desulfurization both at the low-temperature stage of oxidizing roasting and at the high-temperature stage.

Sulfur in green pellets made from Sokolov–Sarbai concentrates is represented mainly by pyrite and pyrrhotite. In analyzing the behavior of sulfur during roasting, as applied to the burden composition recognized as the most rational for pelletizing, it is necessary to consider the interaction of pyrite and pyrrhotite with magnetite, iron oxide, and the oxides of calcium and magnesium. When pyrite and pyrrhotite are heated in an oxidizing atmosphere at temperatures below ignition (for pyrite with a particle size of less than 0.1 mm—325°C, for pyrrhotite—430°C), they are oxidized to sulfate [3]. Inside the pellets at these temperatures, owing to a deficiency of gaseous oxygen, decomposition of pyrite and pyrrhotite will begin. Ferrous sulfates formed in the peripheral layers of the pellets in the presence of a sufficient amount of oxygen are oxidized to ferric sulfates (when heated to temperatures of 450–500°C).

When the pellets are heated, beginning at a temperature of 350°C, dehydration of the hydrated lime occurs. This process ends at temperatures of 500–550°C and, thus, calcium oxide appears in the system under consideration.

Free calcium oxide, already at a temperature of 400°C, can react in the solid phase in pellet cores with ferrous sulfide [4]. Almost simultaneously with this, free calcium oxide will react with vapors of elemental sulfur [5]. Further heating of the pellets leads to intensive oxidation of iron sulfides. In the temperature interval 735–905°C, dolomite, and in the temperature interval 900–921°C, limestone, will decompose completely with the formation of free oxides of calcium and magnesium [6]. At the same time, part of the sulfurous gas, in the presence of excess oxygen, will be bound into calcium and magnesium sulfates.

At the low-temperature stage of roasting, the main component of the pellets—magnetite—is oxidized quite completely to iron oxide. Iron oxide and magnetite, in the solid phase at temperatures from 550 to 1000°C, interact with ferrous sulfide [7].

To determine the distribution of sulfur remaining in the pellets after heating in an oxidizing atmosphere to 900°C, the following experiment was carried out. Sokolov-Sarbai concentrate, separately and in a mixture with limestone, magnesite, and burnt lime, was calcined in an oxidizing atmosphere for 30 minutes at 900°C. The roasting products were analyzed for the content of silica, calcium oxide, magnesium oxide, total sulfur, and sulfate sulfur. The results of the experiment are given in Table 1.

Table 1

From the table it is evident that the presence in pellets of fine-grained inclusions of calcium oxide and magnesium oxide substantially affects both the degree of low-temperature desulfurization and the character of the distribution of sulfur in the roasting product. It is obvious that in pellets made from the recommended burden, similar in composition to the third mixture shown in the table, approximately 90% of all sulfur after low-temperature roasting will be represented by calcium and magnesium sulfates, and the remaining 10% by iron and calcium sulfides.

Thus, in analyzing the high-temperature stage (1000–1350°C) of pellet desulfurization, it is necessary to consider mainly the behavior of calcium and magnesium sulfates.

By the beginning of the high-temperature stage of pellet roasting, magnetite will have practically completely transformed into iron oxide. The latter will interact both with sulfides and with sulfates.

When pellets are heated to a temperature of 1150°C, complete decomposition of magnesium sulfate occurs and weak decomposition of calcium sulfate begins. These processes, favorable for desulfurization, are substantially improved in the presence of oxides of silicon, aluminum, and iron. However, despite this, calcium sulfate decomposes very slow—

slowly, and only when the roasting temperature is raised to 1300–1350°C is its decomposition accelerated.

A very important factor in the high-temperature desulfurization of pellets is the parallel occurrence of the process of dissociation of iron oxide, accompanied by the release of atomic oxygen. This oxygen can ensure complete removal of sulfur from iron sulfide, despite possible melting of the material.

This circumstance once again indicates that, in the process of high-temperature desulfurization of fluxed pellets, the main attention must be devoted to sulfur bound in the form of calcium sulfate. In this respect, the solution to the problem of pellet desulfurization proposed by the Uralmekhanobr Institute deserves attention. Fluxing pellets with coarse-grained limestone significantly impedes the formation of calcium sulfate at the low-temperature stage of oxidizing roasting.

In light of the theoretical analysis of the desulfurization process presented above, the significance of magnesium oxide becomes clear. This component improves pellet desulfurization at the high-temperature stage, mainly because it forms a readily decomposable sulfate, and not because it raises the softening temperature of the pellets.

The role of the individual stages of pellet desulfurization during oxidizing roasting, as is evident from the analysis, is determined to a considerable extent by the composition of the pellets. If unfluxed pellets are subjected to roasting, the principal role in desulfurization belongs to the low-temperature stage. When highly basic pellets are roasted, especially those fluxed with fine-grained lime, desulfurization proceeds mainly at the high-temperature stage. Hence it follows that, in order to ensure complete desulfurization of pellets, the roasting time at the low-temperature and high-temperature stages of roasting must be established in accordance with their composition.

In establishing the most advantageous regime for desulfurization of fluxed pellets during roasting, an equally important aim of roasting is the strengthening of the pellets and an increase in their degree of oxidation. Excessive overheating of the pellets, while beneficial for desulfurization, may lead to the fusion of individual pellets and to disruption of the roasting process, as well as to a decrease in the degree of oxidation due to dissociation of iron oxide. On this basis, experimental verification of the desulfurization of fluxed pellets from Sokolov-Sarbai concentrates was carried out for such roasting conditions as correspond to the operating regimes of shaft furnaces, rotary kilns, and belt roasting machines.

To evaluate the influence of pellet composition and roasting temperature on the degree of their desulfurization, a series of experiments was conducted in a tubular electric furnace. A batch of dry pellets, 3–5 pieces in number, was introduced into the working space of the furnace at a specified constant temperature and held at this temperature for 30 minutes in a stream of hot air with a superficial velocity of 0.4 m³/m²·s. The results of the experiments are presented in Table 2.

Oxidizing roasting of pellets made from pure concentrate with a natural basicity (ratio of calcium to silicon) of about 0.35 showed that the degree of desulfurization increases in proportion to the roasting temperature. Intensive decomposition and oxidation of pyrite and pyrrhotite begins in the temperature interval from 300 to 500°C. The absence of active calcium and magnesium oxides in the pellets leads to the result that, already at low roasting temperatures, up to 85% of the sulfur is removed. At tempera-

temperatures above 1250°C, practically complete desulfurization of the pellets occurs.

During oxidizing roasting of pellets with inclusions of coarse-grained limestone, the mechanism of desulfurization changes somewhat. In the low-temperature stage of roasting (from 300 to 700°C), when the limestone has not yet decomposed, its effect is practically not manifested. With a further increase in roasting temperature, the degree of desulfurization of pellets with coarse-grained limestone drops sharply because sulfur-containing gases are bound by the active calcium oxide formed into difficult-to-decompose calcium sulfate.

Table 2

Thus, if roasting of pellets at a temperature of 700°C showed a desulfurization degree of 73,8%, then roasting at 900°C reduced the degree of desulfurization

to 46.5%. The retarding effect of limestone on the degree of desulfurization of pellets is manifested during roasting up to a temperature of 1250°C. Only at temperatures of 1300–1350°C, when calcium sulfates begin to decompose, does the degree of desulfurization of the pellets during oxidizing roasting reach the standard.

Roasting of pellets with additions of finely ground lime and magnesite shows retarded desulfurization already at the low-temperature stage. In this case, the retarding effect of the lime additions, and then also of magnesite, is manifested up to a temperature of 1250°C. A further increase in the roasting temperature leads to progressive desulfurization of the pellets up to the standard. It should be noted that at elevated roasting temperatures the desulfurization of pellets with additions of lime and magnesite proceeds more intensively than for pellets with limestone additions. This confirms the previously indicated positive influence of magnesium oxide on the process of high-temperature desulfurization.

To clarify the effect of magnesite additions, experiments were carried out on roasting powdered concentrate without additions and with additions of limestone and magnesite. Oxidizing roasting was conducted at 1250°C for 10 minutes. The roasting temperature was selected taking into account the dissociation temperatures of magnesium sulfates (1155°C) and calcium sulfates (1450°C).

As the results of these experiments show (Table 3), the addition of magnesite ensures a high degree of desulfurization, in contrast to the limestone addition, which sharply inhibits desulfurization at the experimental temperature.

Table 3

Analyzing the experimental results presented in Tables 2 and 3, one may conclude that the optimum charge composition for producing low-sulfur fluxed pellets should include magnesium compounds, preferably in the form of dolomite. The final roasting temperature of the pellets should be about 1300°C.

Conclusions

1. The process of desulfurization of pellets prepared from mixtures of Sokolov-Sarbai concentrate with finely ground dolomite and pulverized lime has been analyzed at the low-temperature and high-temperature stages of oxidizing roasting.

2. It has been shown that introducing dolomite into the charge for pellets promotes their desulfurization during roasting.

REFERENCES

1. D. G. Khokhlov, I. E. Ruchkin. Production of fluxed pellets from Sokolov-Sarbai concentrates. In: Mining Industry of Kazakhstan, Alma-Ata, 102, 1959.

1. D. G. Khokhlov. Sulfur burnout during the roasting of fluxed pellets. Stal’, No. 7, 592, Metallurgizdat, Moscow, 1962.
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3. Zh. L. Wert, M. V. Kamentsev. Study of the interaction of oxides comprising bauxite with sulfides. Abrasives, issue 15, 3, Moscow, TsBTI, 1956.
4. V. A. Vanchikov, A. P. Lyuban, V. G. Manchinskii. Absorption of sulfur by components of the blast-furnace burden. Stal’, No. 6, 508, Moscow, Metallurgizdat, 1954.
5. A. A. Baikov. Collected Works of the Academy of Sciences of the USSR, Moscow–Leningrad, Vol. IV, 286, 1949.
6. V. I. Smirnov, A. A. Veselovskii. Interaction of higher iron oxides with sulfides of heavy metals. Proceedings of the Ural Industrial Institute, ONTI-NKTP, Sverdlovsk–Moscow, collection 5, 39, 1938.