INVESTIGATION OF CHANGES IN THE STRUCTURE OF TMP DURING HEAT TREATMENT

S. I. Smolyaninov, G. G. Krinitsyn, N. G. Antonov

(Presented by the scientific-methodological seminar of KhTF)

A conclusion about the possibility of using various solid fuels in the metallurgical process is made by comparing their qualities with standard coke. However, such a comparison is difficult when a material fundamentally different from coke is used. A typical example is the evaluation of the quality of molded peat-ore material (TMP). The presence of a large amount of volatile carbon compounds, mineral additives, and the amorphous structure of the starting material—all this creates noticeable differences between the properties of TMP and the properties of hard-coal coke.

Thermal decomposition of peat mass, leading to loss of volatiles, relative enrichment of the residue with mineral components, and the appearance of an ordered carbonaceous structure, makes the quality of the briquette after heat treatment closer to coke. The course of the listed processes in a blast furnace affects the mechanical properties of the material; therefore it is of interest to trace the character and temperature zones in which the briquettes undergo significant changes.

Lowland Tagan peat with a moisture content of 85%, ash content of 8.27%, and volatile yield of 62.4% was taken as the starting material; concentrate from the Abagur agglomeration plant [1], ground to pass a 0.1 mm sieve, was added to it in an amount of 10% of the wet peat mass.

The material was studied in the air-dry state and after heating at a rate of 5 deg/min to temperatures of 400, 800, and 1200°C. In parallel, an analysis was carried out of samples prepared without ore addition. Determination of true and apparent specific gravity, the amount of volatiles remaining in the organic matter, and the content of mineral impurities was performed in accordance with methods adopted for characterizing the quality of peat (for non-coked samples) and hard-coal coke [2, 3]. The results are summarized in Table 1.

An increase in the treatment temperature leads to an increase in differences in the structure of TMP and peat without additive. This is especially evident when considering the porosity of the substances studied (Table 2). In the initial state, the porosity of TMP is somewhat higher than that of peat. With increasing temperature, in connection with the maximum of gas evolution and an increase in the number of pores of the largest diameter [4], it rises sharply in peat and, after heating to a temperature of 400°C, exceeds in absolute value the increase in porosity of TMP.

This phenomenon indicates a shift of the gas-evolution maximum in the peat–ore material toward higher temperatures.

A sharp increase in the porosity of the TPM briquette occurs in the temperature range 400–800°C, and with further heating it exceeds the value determined for peat. Here it is necessary to note that, beginning at a temperature of 500–600°C, intensive processes take place that are associated with changes in the structure and chemical composition of the iron oxides. The transformation of the oxides and their interaction with the carbonaceous compounds in close contact with them lead to the development of pores that are absent in peat without additives and are located along the contact surface between the iron ore and the peat mass. Pore formation on this surface should be most intense at tempe-

Table 1

Technical analysis of the samples studied

Table 2

Change in the porosity of TPM and peat during heat treatment

ratures of 700–800°C, during the transition of iron protoxide into the form of reduced iron. According to the data obtained, in the temperature range 800–1200°C the porosity of the material containing an ore additive increases almost threefold in comparison with peat without an additive. The absolute value of the porosity attained as a result of heat treatment of TPM is 12% higher than that of peat.

An X-ray study of the materials (Fig. 1) made it possible to trace the development of the processes of transformation of iron oxides in TPM briquettes and showed the presence of a noticeable amount of reduced iron in the material after heating to 800°C, and at a temperature of 1200°C a complete transition of iron to the metallic state. Whereas after heating to a temperature of 800°C, for peat without additives it was possible to establish the appearance of the 002 line and to measure an interplanar spacing equal to 3.35 Å, in the case of TPM the high concentration of mineral components did not make it possible to determine the presence of interference

... of the diffraction maximum corresponding to a graphite-like structure.

The change in the structure of the peat-ore material proceeds continuously, and scattered determinations cannot give a complete characterization of them; therefore it was of interest to trace the change in structure over the entire course of heating up to 1200°C.

As an indicator by which the course of changes in the structure of the carbonaceous substance was judged, the shrinkage of the material upon heating was taken (Fig. 2). Shrinkage reflects the change in the mean pore radius and makes it possible to trace the uniformity of changes in the material under investigation. For the study, TPM and peat were taken in an air-dry state. The specimens were heated in a neutral atmosphere. The change in dimensions was judged from the readings of a mechanical indicator connected to the specimen by means of a rod.

Fig. 1. X-ray diffraction patterns. 1 — peat treated to 400°C, 2 — 800°C, 3 — 1200°C, 4 — 1600°C, 5 — graphite.

From Fig. 2 it is seen that at the beginning of the experiment the shrinkage is approximately the same for both specimens; after the temperature reaches 260°C, the shrinkage rate of peat is higher than that of TPM. This can probably be explained by the presence in peat with additives of a large number of inclusions that do not change volume, which retards the course of shrinkage in the mixture.

On curve 2 for the shrinkage of the peat-ore material, in the temperature range 700–850°C, a delay in the change of specimen size is observed. It is known [5] that in this range there is a change in the structure both of iron oxides and of reduced iron, accompanied—

is accompanied by a change in volume. At these temperatures, apparently, the most intensive reduction process occurs, with the formation of iron. According to off-gas analysis data, it is precisely this temperature interval that accounts for the maximum release of carbon dioxide.

Thus, the presence of iron ore in peat leads to an intensification of the process of thermal decomposition of its organic mass. At the same time, owing to the close contact of iron oxides with the organic mass of the peat, reduction processes proceed intensively.

Fig. 2. Dynamics of shrinkage during heating: 1—TPM, 2—peat

REFERENCES

1. Blast-Furnace Production. Handbook edited by I. P. Bardin. Moscow, Metallurgizdat, 1963.

2. N. E. Semenskii. Technical Analysis of Peat. Moscow, “Nedra,” 1966.

3. A. D. Gluzman, I. I. Edelman. Laboratory Control of Coke-Chemical Production. Kharkov, Metallurgizdat, 1957.

4. A. S. Fialkov. Formation of the Structure and Properties of Carbon-Graphite Materials. Moscow, “Metallurgiya,” 1965.

5. M. A. Pavlov. Metallurgy of Cast Iron. Moscow, Metallurgizdat, 1945.