Acoustic Emission-Based Multi-Parameter Optimization of Remolding Conditions for Tectonic Coal: An Orthogonal Experimental Study

Tectonic coal, highly fragmented by geological stresses, cannot be sampled as intact specimens, making remolded (briquette) samples essential for mechanical testing. However, remolding conditions, particle grading, molding pressure, and moisture content, are typically selected empirically, lacking objective evaluation criteria. This study proposes an acoustic emission (AE)-based multi-parameter framework to optimize these conditions. Using an orthogonal design (three factors at three levels), we prepared remolded tectonic coal samples with varying gradings (1:1:1, 1:4:1, 1:8:1), pressures (15 MPa, 20 MPa, 25 MPa), and moisture contents (8%, 10%, 12%). Uniaxial compression tests were conducted, and five evaluation parameters were extracted: compressive strength, cumulative AE count, cumulative AE energy, energy conversion ratio k, and shear crack proportion r. Results show that AE parameters effectively reflect remolding quality in terms of signal activity, damage mode, and energy efficiency. Grading and pressure dominate cumulative AE metrics, while moisture content and pressure control k and r. Increasing medium-particle proportion or pressure enhances AE activity, k, and r; increasing moisture content suppresses them. The optimal remolding conditions for the studied coal are 1:8:1 grading, 25 MPa pressure, and 8% moisture content, with different condition priority sequences depending on optimization goals. This AE-based approach provides an objective, quantitative tool for tectonic coal remolding optimization, benefiting subsequent mechanical and permeability studies.