Experimental method for optimizing the molding conditions of hot-pressed briquette

In response to the problems of low strength and high permeability of coal materials in the current physical simulation test of coal mine gas dynamics, a set of experimental research methods for optimizing the molding conditions of hot-pressed briquette. Firstly, a hot-pressed briquette test system was independently established and the advantages and future improvement directions of the test system were summarized. At the same time, based on the Horsfield dense stacking theory, the optimal preparation plan for coal briquette materials was formulated. Finally, a molding condition optimization method was developed that combines the Markov distance measurement method and the golden section method. To verify the effectiveness of the experimental method, the secondary carbonization experiments of briquette were conducted under different molding pressure conditions by controlling the molding temperature to 311.8 ℃, heating rate to 5 ℃/min, and holding time to 5.3 h. The response characteristics of the microstructure, physical and mechanical properties, and permeability characteristics of the hot-pressed briquette under different molding pressure conditions were studied. The results show that with the increase of molding pressure, the total porosity gradually decreases, and the uniaxial compressive strength shows a trend of first increasing and then decreasing. The main forms of failure are block spalling and longitudinal fracture. The initial permeability shows a trend of first decreasing and then increasing, while the minimum permeability shows a trend of first decreasing, then increasing and then decreasing. Using the specific values of each molding condition as the test points, and the key parameters of hot-pressed coal and raw coal as the evaluation parameters, a sample matrix was constructed to calculate the Mahalanobis distance between hot-pressed briquette and raw coal under each molding condition, and then optimize the experimental interval using the golden section method. The optimized final molding pressure is 80 MPa. Under these molding conditions, the density, uniaxial compressive strength, and initial permeability of the hot-pressed briquette produced are 1.137 g/cm³, 12.21 MPa, and 1.32 × 10⁻¹⁵ m², respectively. They are highly similar to the 1.132 g/cm³, 12.83 MPa, and 1.08 × 10⁻¹⁵ m² of the raw coal, achieving the goal of improving the strength of the briquette and reducing the permeability of the briquette.