Effects of cyclic freeze-thaw by liquid nitrogen on the coal breakage properties and fragment size distribution

Transforming intact coal seams into broken coals that are convenient to transport is one of the technical problems of fluidized coal mining. In this paper, cyclic freeze-thaw by liquid nitrogen (CFT-LN2) was adopted to improve the fragmentation degree of coal seams. The coal pore structure evolutions and coal breakage properties were explored using the acoustic emission (AE) parameter method (the kernel density estimation method), the fractal theory, and the nuclear magnetic resonance spectra. The main factors discussed include the number of freeze-thaw cycles, the LN2 freezing time and the moisture content of the coal sample. Variations of these factors alter the mechanical and physical characteristics and pore structures of coal samples treated with CFT-LN2. The results reveal that the kernel density estimation method can distinguish the density region determined based on the rise angle (RA) and average frequency (AF) values and visualize the rupture modes of coal samples formed after CFT-LN2. As the number of freeze-thaw cycles grows, the tensile rupture mode gradually transits to shear rupture mode; the fractal dimension gradually increases; the equivalent average size gradually decreases; and the mass of different fragment sizes tend to be more uniform. Only freezing in LN2 without CFT-LN2 cannot notably change the rupture mode or fragmentation degree of a coal sample. During CFT-LN2, the coal matrix is subjected to repeated action of contraction in LN2 and expansion at room temperature. As a result, new micro-pores are formed easily in the matrix, and micro-pores incline to connect into larger pores. Meanwhile, increasing the moisture content is conducive to coal fracturing through CFT-LN2.