REACTIVITY AND PROPERTIES OF PEAT-ORE MATERIAL

G. G. Krinitsyn

(Presented by the scientific-methodological seminar of the organic departments of the Faculty of Chemical Technology)

The rate of oxidation of carbonaceous materials depends on a variety of factors, the most important of which are the chemical structure and the size of the surface on which this process occurs. Determination of reactivity, based on the oxidation of a carbonaceous substance, thus provides an idea of the chemical and physical structure of the solid products of the thermal decomposition of peat. The presence of iron oxides in the solid residue of the thermal decomposition process will lead to a change in the ratio between $\mathrm{CO_2}$ and $\mathrm{CO}$ in the reaction products [1, 2].

When determining the reactivity of peat coke according to N. P. Chizhevsky, it was shown that the addition of 1% limestone has practically no effect on the reactivity with respect to $\mathrm{CO_2}$. The addition of 3% Bakchar iron ore noticeably increases it. This increase is especially significant at temperatures above 600°C. With respect to water vapor, the reactivity is increased over the entire range of tested temperatures, although at 900°C the difference between samples without additives and with an admixture of iron ore levels out. In connection with this, to determine the reactivity of peat coke with mineral additives, the wet-oxidation method was used, consisting in the treatment of carbonaceous materials with a mixture of orthophosphoric acid and potassium dichromate [4].

Fig. 1. Change in the activation energy of the peat-coke oxidation process as a function of the coking temperature.
1 — peat coke; 2 — peat coke with an addition of 20% $\mathrm{Fe_2O_3}$; 3 — peat coke with an addition of 10% $\mathrm{Fe_2O_3}$.

The method is based on the interaction of the oxidant with carbon according to the reaction

\[ 6\mathrm{K_2Cr_2O_7} + 9\mathrm{C} + 16\mathrm{H_3PO_4} = 24\mathrm{H_2O} + 9\mathrm{CO_2} + 12\mathrm{CrPO_4} + 4\mathrm{K_3PO_4}. \]

This reaction proceeds at temperatures above 40°C; the only volatile product containing carbon is carbon dioxide. Measuring the concentration of carbon dioxide over time makes it possible to construct curves characterizing the kinetics of the process. Carrying out the reaction at different temperatures makes it possible to determine the activation energy of the oxidation process. The absence of an effect from additions of iron oxides on the process makes it possible to reveal the influence of the structural factor on the oxidation process of coke with a filler.

The study of the oxidation process of peat material in a liquid medium was carried out with samples of sedge–lowland peat, the degree of decomposition of which was 25–30%, the ash content 8.4%, and the yield of volatiles 53.88% on a dry basis. The coking temperature was 400, 600, 800, and 120°C. As the filler, reactive iron oxide was used, with a grain size not exceeding 0.06 mm. As a result of the experiments, values were obtained for the activation energies of the oxidation process for various solid residues of the coking process (Fig. 1).

Fig. 2. Plot of the dependence \(\lg \rho = f\!\left(\frac{1}{T}\right)\) for peat (1) and peat-ore material (2)

In the coking-temperature range up to 800°C, a solid residue is formed whose activation energy for the oxidation process in the absence of a filler is higher than that of the material with additions of iron oxides. After 800°C, the activation energy for oxidation of the material without additives

drops sharply. The decrease in the reactivity of the material with additives in this temperature range is gradual.

The magnitude of the activation energy of the iron-coke oxidation process is substantially lower than for coke without mineral impurities. Since, in our case, the activation energy is inversely proportional to the surface of interaction with the oxidizing agent, it may be assumed that up to temperatures of 800°C and higher, the surface area of the carbonaceous material with additives is greater than that of ordinary peat coke. With a further increase in temperature, the reduction process develops, together with the associated burning-out of carbon on the pore surface. This leads to a decrease in the pore surface area and to a reduction in the reactivity of the material.

The change in the nature of the structure at the moment of reduction of the filler can be recorded by other methods. Measurement of electrical resistivity shows that a substantial change in the characteristics of the peat-ore material is observed in the temperature range 800–900°C (Fig. 2).

A sharp change in the electrophysical characteristics of peat coke with additives probably also indicates a substantial change in the structure [5]. As a result of chemical interaction between iron oxides and the carbon of the coke, the solid residue is enriched with highly condensed components, while the active components enter the process as reducing agents and are removed in the form of gaseous products. The consequence of this is a lowering of the reactivity of the material.

REFERENCES

1. N. D. Drozhzhina, V. E. Rakovskii. In: Chemistry and Genesis of Peat and Sapropel. Minsk, Publishing House of the Academy of Sciences of the BSSR, 1962, 61–64.

2. A. G. Khvachiya. Proceedings of Higher Educational Institutions of the USSR, “Ferrous Metallurgy,” Moscow, 1960, No. 2, 21–24.

3. L. Mott, R. Wheeler. Coke Quality. Metallurgizdat, 1949.

4. K. K. Stramkovskaya, V. K. Borodin, V. M. Dinel’t. Proceedings of the Tomsk Polytechnic Institute, Tomsk, Tomsk State University Publishing House, vol. 111, 1961, 26.

5. B. A. Onusaitis. Formation and Structure of Hard-Coal Coke. Moscow, Publishing House of the Academy of Sciences of the USSR, 1960.