Quantitative analysis of layered re-crushing of crushed coal particles during compression based on CT scanning

Due to the compaction of the overburden strata, the crushed coal in caved zone will be re-crushed. The compaction and re-crushing degree of crushed coal differ at different heights, which is called layered re-crushing. Quantitatively revealing the layered re-crushing characteristics of crushed coal particles is the key to elucidating pore structure evolution of caved zone and coalbed methane enrichment pattern in the gob. In this paper, quantitative characterization of layered re-crushing of coal particles during compression is achieved using self-designed CT visual compacting apparatus. The results showed that: (1) the degree of particle re-crushing decreased from the loaded side (the upper layer) to the fixed side (the lower layer). In the early stage of compression, the overall skeletal structure is looser, which is not conducive to the transfer of force chains. Thus, the stress has the greatest effect on the particles in the upper layer, with the proportion of particle increase reaching 49.3% from 0 Mpa to 2.14 Mpa. There was a pattern of becoming larger first and then smaller in the difference of average diameter among different layers, and the final average diameter decreased by about 60% in three layers. (2) The particle diameter distribution transferred to a smaller diameter fastest in the upper layer and slowest in the lower layer, which is due to the gradual transfer of external load from the top to the bottom. After compression, the number of particles smaller than 4 mm predominated in the upper layer (40.3%) and the number of particles larger than 5 mm predominated in the lower layer (47.1%). (3) With the increase of stress, the void volume gradually decreases regardless of the whole sample or the three layers, while the void number increases instead. And at both 2.14 MPa and 23.24 MPa, the void volume shows the law of upper layer < middle layer < lower layer.