On the Effect of Secondary Heating on the Strength of Coke Substance

A. V. Vikharev, N. M. Smolyaninova, V. D. Chetvergov

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

Strength is one of the principal and most rigorously and systematically monitored indicators of coke quality. However, it is generally known that a high value of this indicator does not always correspond to the actual quality of coke, as is indicated by an unsatisfactory course of blast-furnace smelting. It has repeatedly been suggested that such a discrepancy is due to the sharp difference between test conditions and the real conditions of a blast furnace. This led to the development of methods for assessing coke strength in the hot state. Work in this direction continues up to the present time [1, 2, 3], but the problem remains unresolved. Moreover, researchers have no common opinion on the question of the effect of secondary heating on coke strength. The experimental material obtained is highly contradictory. In some works a sharp deterioration in the mechanical properties of coke with increasing temperature was noted [4, 5], while in others an increase in strength was observed [6, 7].

An interpretation of the causes that produced one or another behavior of coke is either entirely absent or, more often, is conjectural in character.

A major contribution to the resolution of the problem under consideration was made by K. I. Syskov, who, proceeding from the contradictory nature of the processes taking place, gave a general explanation of the behavior of coke during secondary heating, but without a detailed specification of the causes that produced these processes.

In order to obtain new experimental data on the effect of secondary heating and to establish a possible mechanism of this effect, we carried out work to determine the abrasion of coke and the compressive strength limit over a wide temperature range.

The experimental procedure adopted was based on the following considerations. It is known that in the longitudinal direction any part of a piece of coke differs sharply in its properties and has different porosity, fissuring, specific gravity, etc.

Each of these factors affects the strength values obtained in a definite way. The influence of porosity and fissuring is especially great; moreover, the latter most often introduces an element of randomness into the values obtained and makes it difficult to reveal true relationships of a more general character.

Determination of the structural strength of coke, i.e., the strength of pieces of coke free of cracks, makes it possible to eliminate to some extent...

the influence of this factor. In addition, the strength of a porous body and its substance ultimately determine the resistance of coke to fracture; therefore, the study of the influence of secondary heating specifically on this index is of undoubted interest. A detailed study by A. S. Bruk [8] has already been devoted to these same questions of changes in coke cracking under secondary heating.

It is also of considerable interest to study the change in strength under the influence of secondary heating not of an entire piece of coke, but of its individual most characteristic parts, which makes it possible, to a certain extent, to differentiate the influence of the factors mentioned above and to obtain not averaged dependences characterizing the whole piece as a whole, but specific ones—for material that is more homogeneous in structural terms.

In accordance with the views set forth above, the material was prepared for testing. The object of the study was industrial coke from the Kuznetsk Metallurgical Combine. A 50 kg sample was taken directly on the ramp after quenching. All the pieces of coke were obtained in one chamber with a normal technological regime and were located approximately in its center, at a short distance from one another. This permits the assumption that the conditions under which each piece was obtained were identical.

The selected pieces were divided into three most characteristic parts, known as the “cauliflower,” the “center,” and the “seam.” All subsequent experiments were carried out, respectively, on the three batches of coke thus obtained.

The structural strength of the coke, or its abrasion resistance, was determined on a previously created apparatus [9] by an improved method [10].

The compressive strength limit was determined on a high-temperature apparatus. It consists of a kryptol furnace, in the working space of which a cube of the material under test is placed. The load on the specimen was produced by a graphite punch through a lever, at one end of which a container was placed that was gradually filled with shot.

A coke cube measuring \(10 \times 10\) mm was prepared on an emery wheel. The parallelism of the faces was strictly controlled, as were the linear dimensions and the magnitude of the angles at the vertices of the cube.

The data obtained are presented in Figs. 1 and 2 in the form of graphs of the dependence of abrasion resistance and compressive strength limit on the secondary-heating temperature for the three parts of a coke piece.

As can be seen from the graphs (Fig. 1), the dependence of the structural strength of coke on the temperature of secondary heating for the “cauliflower” and the “seam” is expressed by curves with a characteristic minimum in abrasion resistance in the temperature interval 400–600°C. For the abrasion resistance of coke taken from the center of the piece, an increase in temperature up to 400°C had no effect, but with a further increase in temperature the abrasion resistance steadily increases.

The change in the compressive strength limit (Fig. 2) also indicates an increase in coke strength with an increase in temperature up to relatively small values. The maximum increase in strength is observed in the “cauliflower,” the minimum in the “seam,” and the strength maxima for each investigated part of the coke piece appear at different temperatures. For the “seam” this is 400°C, for the center 600°C, and for the “cauliflower” 800°C. The specimens from the center of the piece have the greatest strength over the entire temperature range, while those from the “seam” have the least; the “cauliflower” occupies an intermediate position.

Fig. 1. Dependence of the structural strength of coke on the temperature of secondary heating.
1—“seam,” 2—“cabbage,” 3—“center.”

Fig. 2. Dependence of the compressive strength limit on the temperature of secondary heating.
1—“seam,” 2—“cabbage,” 3—“center.”

The observed fact of an increase in coke strength at relatively low temperatures, which are considerably below the coking temperature, is apparently associated with the presence of internal stresses in the coke structure, with the principal role belonging to the stresses that arise in coke during wet quenching. The increase in strength occurs as a result of stress relief during coke heating. It is clear that the greater the stresses in the initial coke structure, the greater the absolute increase in strength will be when they are relieved.

The results obtained give grounds for assuming that the stresses reach their greatest magnitude at the “cabbage,” and their smallest at the “center.” The slight increase in the compressive strength limit at the “seam” is explained by its high porosity, which in this case determines the strength of the coke cube, and not by an absence of internal stresses. This is confirmed by the data on structural strength (Fig. 1), which is determined mainly by the strength of the coke substance and depends to a lesser degree on porosity; therefore, when internal stresses are relieved in this case, an increase in structural strength occurs.

Conclusions

1. The effect of secondary heating on the mechanical properties of coke has been studied.

2. Coke samples obtained from the center of the piece have the greatest strength, and those from the “seam” have the least.

3. The observed increase in coke strength is explained by the relief of internal stresses, which occurs under the influence of heating.

Literature

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2. Humphreys Kenneth K. Methods of assessing the quality of blast-furnace coke. Blast Furnace and Steel Plant 51, No. 12, 1065—1069, 1963.

3. Das Gupta N. N., Raja K., Sarkar P. K. Coke tests at elevated temperatures. Journal of Mines, Metals and Fuels, 12, No. 8, 253—258, 1964.

4. G. V. Speranskaya. Thermal destruction of coke. Zavodskaya laboratoriya, No. 3, 97, 1950.

5. A. S. Bruk. Study of industrial coke in the hot state. DUKhIN Fund, 1933.

6. P. A. Shchukin. On the thermomechanical stability of metallurgical coke. Paliva. 38, No. 9, 299—301, 1955.

7. L. V. Bardina. Resistance of coke to compression at various temperatures. Proceedings of the A. A. Baikov Institute of Metallurgy, issue 4, 42, 1960.

8. A. S. Bruk, R. E. Leibovich. Behavior of coke at high temperatures. Koks i khimiya, No. 3, 24, 1953.

9. N. M. Smolyaninova, A. V. Kaplin, and A. M. Vasil’eva. Testing the strength of coke in the hot state. Koks i khimiya, No. 5, 23, 1961.

10. N. M. Smolyaninova, A. M. Stukalov, and A. V. Vikarev. Improved method for testing coke strength. Izvestiya TPI, vol. 136, 52, 1965.

11. K. I. Syskov, O. V. Verbitskaya. Basic regularities in the behavior of coke during secondary heating. Moscow, Metallurgizdat, 1962.