Study on the effect of type and content of hardness-indicator ions within fluids on the wettability and microstructure of coal dusts

The traditional evaluation system for coal dust wettability does not adequately account for highly mineralized water environments. Therefore, it is essential to systematically investigate the effects of hardness-indicator ions (Ca2+/Mg2+) in aqueous solutions on coal dust wettability and microstructure. This study employed wettability experiments (surface tension, contact angle measurements, and sedimentation tests) to analyze the influence of ion type and concentration on coal–water interfacial behavior. Results demonstrate that the surface tension of solutions containing only Ca2+ or Mg2+ initially decreased and then increased with rising hardness levels, with hardness thresholds observed at 500 and 700 ppm, respectively. This trend was consistently reflected in parameters such as droplet contact angle, spreading coefficient, and coal dust immersion time. Furthermore, microscopic characterization techniques, including low-temperature nitrogen adsorption, Fourier-transform infrared, and x-ray photoelectron spectroscopy, revealed significant impacts of ion type and concentration on coal dust pore characteristics, functional group distribution, and hydrophilic/hydrophobic element content. Specifically, low-hardness levels [≤500 ppm (Ca2+) and ≤700 ppm (Mg2+)] synergistically enhanced wettability, whereas high-hardness levels [>500 ppm (Ca2+) and >700 ppm (Mg2+)] suppressed it. The divergent effects of Ca2+ and Mg2+ were attributed to differences in their charge-to-ionic radius ratios. These findings provide theoretical support for the effective mitigation of coal dust pollution in mining environments.