Progressive microstructural evolution characteristics and weakening mechanism of coal exposed to acid mine drainage (AMD)

This study aims to analyze the extent of softening structural deterioration of coal pillars over time when exposed to acid mine drainage (AMD) from abandoned pits. The research involves using microscopic recognition techniques on coal samples and hydrochemistry tests in solution to identify changes in mineral compositions and microstructure parameters quantitatively. The study examines the micro-scale changes in microstructural hydro-chemical damage processes in coal samples. Based on the findings, the decrease in pH of the acidic pit effluent leads to degradation in the physical microstructural characteristics of the coal. Specifically, there is a 6.90% increase in the fractal dimension and a 23.33% rise in the area porosity. Similarly, the chemical parameters of the coal also undergo significant changes. The thickness of the aromatic layer in the coal decreases significantly, while the average diameter of the aromatic layer increases by 16.9%. The aromaticity of the coal increases, indicating noticeable deterioration of the matrix layer, particularly fragmentation. Moreover, the solution characteristics reflect the progressive coal deterioration, including increased metal ions in the soaking solution, the transformation of divalent iron to trivalent iron, and a continuous rise in sulfate content. These results have implications for assessing and maintaining load-bearing structures in underground abandoned pits subjected to long-term immersion.