National University of Science and Technology ‘MISIS’, Moscow, Russian Federation:

K. V. Agarkov, PhD student
S. A. Epstein, Head of Scientific and Educational Testing Laboratory of Physics and Chemistry of Coals, D.Sci. (Engineering), apshtein@yandex.ru
E. L. Kossovich, Senior Researcher, Scientific and Educational Testing Laboratory of Physics and Chemistry of Coals, Ph.D. (Physics and Mathematics)
N. N. Dobryakova, Researcher, Scientific and Educational Testing Laboratory of Physics and Chemistry of Coals, Ph.D. (Engineering)

The effect of freezing-thawing on the mechanical properties of coals at the microscale, the character of their destruction during cyclic nanoindentation, and the release of airborne dust have been studied. For these purposes, three hard coals from the Apsatsky deposit, two hard coals from the Pechora coal basin, and two lignites from the Kangalassky deposit were studied. It has been established that low-temperature impacts lead to a decrease in the content of airborne dust for all the coals. It was revealed that after freezingthawing of coals, their strength decreases, however, the character of destruction under mechanical impacts do not change. The exception is the coal of the middle stage of metamorphism of the Pechora basin, selected from a pack potentially prone to coal and gas outbursts. For this coal, low-temperature effects lead to a change in the character of its destruction during cyclic nanoindentation. Namely, there was a switch from the local (in the zone of contact with the indenter) character of destruction of the initial coal to the bulk one (outside the zone of contact with the indenter) for coal after low-temperature treatment. To assess the coals propensity to form fine dust particles, an index characterizing the degree of compaction of coal matter during cyclic nanoindentation with increasing load was introduced. It has been established that this index is proportionally connected with the content of airborne dust in coals (with sizes of less than 10 microns). It is shown that the coals proneness to form airborne dust under low-temperature impacts does not depend on the stage of coal metamorphism. For bituminous coals of the Apsatsky deposit with similar metamorphism degree the propensity to form dust is different. Variations in the propensity to form dust in coals of the Pechora basin, which also have a similar metamorphism degree, are apparently due to the peculiarities of the vitrinite structure of these coals.
This work was carried out with the financial support of the Russian Science Foundation (Grant No. 18-77-10052).

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