Investigation of the Structural and Mechanical Properties of Peat-Ore Materials during Heat Treatment
Abstract Generated abstract
This study examines structural and mechanical changes in peat and peat-ore materials during heat treatment, with particular attention to the reduction of iron oxides added to peat. High-moor peat containing powdered iron oxide was heated in an inert medium and compared with peat without additives and with graphite, using X-ray diffraction, electrical resistance measurements, and mercury porosimetry. The results indicate that volatile products of peat pyrolysis promote iron oxide reduction at substantially lower temperatures than graphite, with most iron reduced by about 800°C in the peat-ore material. Heating also increases ordering of the peat carbon structure above 1000°C, lowers electrical resistance, and develops porosity with several predominant pore-size ranges.
Full Text
Study of the Structural and Mechanical Properties of Peat-Ore Materials during Heat Treatment
S. I. Smolyaninov, G. G. Krinitsyn, N. G. Antonov, L. I. Bobrovskaya
(Presented by the scientific-methodological seminar of the organic departments)
Recently, many works have been published devoted to the study of the complex of properties of a number of carbonaceous materials in order to assess the possibility of their practical use [1—4]. A special place among such materials may be occupied by solid residues after the heat treatment of peats, which have high reactivity and low sulfur and phosphorus content.
This paper presents the results of a study of the structural features of solid products of the thermal decomposition of peat containing additions of powdered iron oxide, and a comparison of these data with the characteristics of peat without additives. The results obtained make it possible to trace the process of iron reduction from oxides and its effect on the structure of the material.
As the starting material, high-moor peat from Peschanoye Lake was taken, with a degree of decomposition of 10—15%, moisture content of 81.5%, and ash content of 5.1%; 20% iron oxide, calculated on the combustible mass, was added to it. The material was studied in the air-dry state and after heat treatment. Heating was carried out in an inert medium at a rate of 5°/min. Upon reaching the specified final temperature, the samples were held for one hour.
When the peat-ore material is heated, along with the process of thermal destruction of the organic mass of the peat, reduction of iron oxides by the decomposition products takes place.
In order to obtain an idea of the nature of the reduction process without the participation of gaseous and liquid pyrolysis products, the transformation of iron oxides during heating in a mixture with graphite was studied; at high temperatures graphite does not undergo decomposition. X-ray diffraction analysis showed that the process of reduction of iron oxides by graphite begins at 550°C, and metallic iron is formed at 1000°C. Complete reduction of the oxides does not occur, and at a temperature of 1600°C part of the iron is present in the form of various oxides.
In the peat-ore material, reduction processes begin to develop at temperatures above 200°C, and at 800°C almost all the iron is in the metallic state. The decrease in the temperature interval during the reduction of iron oxides in the composition of the peat-ore material is due to the reducing gases CO, H₂, CH₄, released during pyrolysis of the organic mass of the peat.
An increase in the treatment temperature of peat moldings leads to greater order in the structure of the organic mass. Their X-ray diffraction...
... study (Table 1) showed that up to a temperature of 400° peat has an amorphous structure, and a noticeable rate of formation of a graphite-like structure of the organic mass is observed above 1000°C. In the temperature range 1000–1600°C, the ratio of the interference maximum of the 002 band of peat to that of graphite decreases by 15.7% (Table 1).
Table 1
Degree of graphitization of peat during heat treatment
| Sampling temperature, °C | Initial peat | 200 | 400 | 600 | 800 | 1000 | 1200 | 1400 | 1600 |
|---|---|---|---|---|---|---|---|---|---|
| Degree of graphitization | 1.5 | 1 | 1.2 | 2 | 1.7 | 2.25 | 2.9 | 3 | 8 |
The decrease in specific electrical resistance observed with an increase in the peat treatment temperature (Table 2) indicates an increase in the content of highly condensed substances in the solid coking products [5].
Table 2
Specific electrical resistance of peat during heat treatment
| Treatment temperature, °C | $\rho$, $\dfrac{\Omega \cdot \text{mm}^2}{\text{m}}$ |
|---|---|
| 400 | $4.7 \cdot 10^9$ |
| 600 | $3 \cdot 10^6$ |
| 800 | 11.8 |
| 1000 | 3.5 |
| 1400 | 2.5 |
| 1600 | 0.7 |
An increase in the treatment temperature of the peat–ore material leads to an increase in the porosity of the solid residue due to the release of volatiles. The transformation of oxides and their interaction with the carbonaceous compounds in close contact with them leads to the additional development of pores absent in peat and located along the contact surface of iron oxide–peat mass.
A study of the macroporosity of peat in the range of effective radii 300000–250 Å by the mercury porosimetry method indicates an increase in porosity (Table 3) and a predominating number of pores of three sizes: 1) 10500 Å–95000 Å, 2) 7000–9000 Å, 3) 400–700 Å.
Table 3
Porosity of peat during heat treatment
| Treatment temperature, °C | 800 | 1000 | 1200 | 1600 |
|---|---|---|---|---|
| Porosity, $\dfrac{\text{cm}^3}{\text{g}}$ | 0.22 | 0.551 | 0.639 | 0.756 |
Conclusions
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The process of reduction of iron oxides in a peat-ore briquette and in a mixture of graphite with iron oxide was studied. Owing to the release of volatile products of peat pyrolysis, the temperature at which reduction of iron oxides begins is 200–250° lower than in a mixture of graphite with iron oxide.
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The temperature at which the greater part of the iron is in the reduced form is 200° lower in peat-ore briquettes than in a mixture of graphite with $\mathrm{Fe_2O_3}$.
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An appreciable rate of transition of the organic mass of peat into a graphite-like structure is observed at temperatures above 1000°C.
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In heat-treated peat briquettes, a predominant number of pores of three sizes was found: a) 95000–105000 Å, b) 7000–900 Å, c) 400–700 Å.
References
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