Crushing behavior and mechanism of bottom groove coal in physical fluidized mining

Physical fluidized mining, as an innovative coal extraction technology, enables underground unmanned operation and remote surface control by breaking coal with high-pressure water jets and lifting the coal–water mixture through drill pipes. However, large coal lumps which fail to be broken by water-jets tend to accumulate in the bottom groove of drill wells, hindering mixture lifting and reducing mining efficiency. To address this issue, a secondary crushing method using mechanical drill bits was proposed, and a confined-space physical simulation system was established to investigate particle size distribution and crushing mechanisms. Results indicate that under the combined action of axial pressure and rotation, particles with a diameter of less than 2 cm account for more than 76% of the crushed products. A Weibull-based model (R² = 0.985) accurately predicts the cumulative mass distribution. The crushed particles exhibit relatively high sphericity, with a power-function negative correlation between particle size and sphericity. Mechanical analysis demonstrates that, under drill bit loading, bottom groove constraints, and inter-particle interactions, the crushing process results from the synergistic effects of axial compression, tangential rotation, and frictional disturbance, characterized by multi-scale energy transfer and crack evolution. This study provides theoretical support for optimizing drill bit structural parameters and improving fluidized coal mining efficiency.