STUDY OF THE THERMAL DECOMPOSITION OF PEAT AND THE REDUCIBILITY OF IRON ORE BY DERIVATOGRAPHY

O. P. Lobas, S. I. Smolyaninov

The shortage of coking coals has forced many countries to seek ways of replacing them with other types of fuel, by changing ore-processing technology (blast-furnace-free production of iron, the use of natural gas and fuel oil in blast furnaces, etc.), making it possible to save coke.

Both in the USSR and abroad, many attempts have been made to bypass the blast-furnace process and to obtain iron and steel directly from ore. At present there are more than 1000 patents for direct-reduction processes in all industrial countries.

The large amount of work in the field of direct reduction of iron is due to a number of positive factors of these processes as compared with the blast-furnace process, namely:

1. Complete replacement of coal-based metallurgical coke by other types of reducing fuel.
2. Production of high-quality metallurgical charge materials (artificial scrap, sponge iron for powder metallurgy).
3. Comparatively low capital investment.
4. Use of fine iron-ore concentrates or dusty ores.

The requirements imposed on the reducing agent with respect to strength are not especially regulated; practically any carbon- and hydrogen-containing fuel can serve as the reducing agent, and therefore peat may also be used as a reducing agent.

Although peat has not played a major role in the metallurgical industry, certain attempts to use it at some plants are known.

The use of peat in metallurgy proceeded mainly along two independent lines. The first line was the use of peat coke; the second, the direct use of air-dry, unprocessed peat.

The main obstacle to the use of peat in metallurgical processes was its low mechanical strength. Work carried out in recent years has made it possible to eliminate this substantial drawback by obtaining from peat fuel-smelting materials, i.e., materials containing in their composition fuel (peat), iron ore, and, if necessary, fluxes in the required proportions [1, 2, 3].

The close contact of the reducing agent (peat) with iron ore in fuel-smelting materials, as well as the considerable amount of volatile-

…containing carbon and hydrogen oxides, favorably affect the rate of reduction of iron ore.

Publications devoted to the study of the reduction of iron ores by peat and in fuel-smelting materials are not encountered in the literature.

In order to study the reduction of iron ore in peat–ore granules under slow heating, an investigation was carried out on a derivatograph.

Derivatography, as one of the objective methods of physicochemical investigation of substances, is being widely applied in our country and abroad.

The method of derivatography is based on the principle of simultaneous photographic recording of the thermophysical properties (DTA), change in weight (TG), rate of change in weight (DTG), and temperature ($T$) of a substance during its heating (see Fig. 1).

On the basis of preliminary experiments with peats and peat–ore granules, an optimal heating rate of 5 degrees per minute was selected. With an increase in the heating rate, the character of the curves (DTA) and (DTG) did not change, but the distinctness of the effects decreased. When the heating rate was decreased (to less than 5 degrees per minute), the distinctness of the effects increased insignificantly, but the analysis time was considerably lengthened. Comparison of the (DTA) curves obtained using aluminum oxide and low-ash coke as the inert sample showed no significant differences; the character of the effects did not change.

Fig. 1. Derivatogram of peat–ore granules based on Vasjugan peat (top degree of decomposition 20%) and Kachkanar concentrate; weight ratio peat (dry): ore (concentrate) 35:15.

All experiments were carried out in a nitrogen atmosphere, at a heating rate of five degrees per minute, and aluminum oxide was used as the inert sample.

Derivatograms were taken of Kachkanar concentrate, peats from the Vasjugan deposit, peat–ore granules, and mechanical mixtures of peat with ore.

The thermal decomposition of peats during heating is characterized by the presence of three maxima in the rates of volatile release (DTG). The first maximum, at a temperature of 100–150°C, corresponds in all samples to the removal of water. The second and third maxima are located in the temperature intervals 230–290°C and 285–370°C, respectively. The complex character of the DTG curve is explained by the stepwise pyrolysis of the organic part of the peat. During heating, water is first released; then the thermally unstable constituents of the peat begin to decompose (water-soluble, vol-

cogydrolyzable, etc.), after which the more thermally stable constituents decompose and the bitumens are distilled off.

Comparison of the thermal decomposition of peats from the derivatograms shows that raised bog peats have the lowest thermal stability, and lowland peats the highest. In peats with a greater degree of decomposition, the thermal stability is higher.

During the thermal decomposition of peat-ore granules, one more maximum appears on the curve of the rate of weight change (DTG) and on the DTA curve (an endothermic effect) in the temperature intervals 820–1020°C. In addition, the weight loss of peat-ore granules up to a temperature of 600°C differs from the weight loss of the initial peat; specifically, the weight change, recalculated to peat, is the greater the more iron ore there is in the initial peat-ore granules or in the mechanical mixture peat : ore. For raised bog peat (Vasyugan deposit), the decrease in weight upon heating at a rate of five degrees per minute to a temperature of 600°C is 66.8%, while for peat-ore granules based on this peat and Kachkanar concentrate, at weight ratios peat (dry) : ore of 35 : 65 and 40 : 60, it is respectively 77.8% and 71.1%. This means that additions of iron ore affect peat pyrolysis.

Noticeable reduction (according to the rate of weight change) of iron ore in peat-ore granules begins at temperatures above 620°C. The reduction of iron ore in peat-ore granules is indicated by the total endothermic effect (DTA).

Confirmation of the occurrence of reactions of iron-ore reduction in peat-ore granules and of gasification reactions of the organic mass of peat during slow heating is provided by the characteristics of the solid residue with respect to total iron, metallic iron, and carbon.

Thus, in peat-ore granules based on raised bog peat from the Vasyugan deposit and Kachkanar concentrate, at a ratio peat (dry) : ore of 55 : 45, up to 900°C the degree of reduction is 79.0%, and the carbon content in the solid residue is 27.8%. Upon heating to 1000°C, the degree of reduction reaches 96.9%, and the carbon content in the solid residue decreases to 16.7%.

It has been established that, with an increase in the peat content in the initial peat-ore granules and with an increase in the final temperature, the degree of reduction increases. Moreover, the more peat there is in the initial peat-ore granules, the lower the temperature at which reduction ends. For peat-ore granules based on raised bog peat from the Vasyugan deposit and Kachkanar concentrate, maxima on the DTG curve appear at ratios peat (dry) : ore of 36 : 65, 55 : 45, and 65 : 35, respectively at temperatures of 910°C (Fig. 1), 860, and 830°C.

From the derivatograms one can judge the role of the initial peat in the reducibility of the ore. The best reduction conditions are provided by lowland peats, as well as by raised bog peats with a high degree of decomposition.

REFERENCES

1. I. V. Gebler et al. Izv. TPI. Tomsk, vol. III, pp. 86–91, 1961.
2. I. V. Gebler et al. Izv. TPI. Tomsk, vol. 126, pp. 8–12, 1964.
3. O. P. Lobas, S. I. Smolyaninov. Izv. vuzov, “Ferrous Metallurgy,” Metallurgizdat, Moscow, No. 10, pp. 19–21, 1966.