The coal structure is extensively used for studying the properties of coal, and the construction of accurate and reliable coal structure models can promote these researches. In this study, Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) were used to analyze the changes in the coal structure as a function of the coalification degree, and a semiquantitative model construction method based on FTIR, XRD, and X-ray photoelectron spectroscopy (XPS) analyses was proposed. With an increase in the coalification degree, the size of the aromatic cores in the coal increased. During the conversion from lignite to bituminous coal, the content of aliphatic structures increased, while the content of oxygen-containing functional groups (OFGs) significantly decreased. Conversely, during the conversion from bituminous coal to anthracite, the content of aliphatic structures decreased while the content of OFGs slightly increased. The calculated FTIR spectra of the coal structure models were consistent with the experimental FTIR spectra, which confirmed the accuracy of the models. Furthermore, the models successfully explained the microscopic mechanism underlying the differences in the wettability of the coal samples with varying coalification degrees. The construction method and coal structure models in this study will be more widely applied in future research.

In this paper, a relationship between the roundness, roughness and induction time of low-rank coal particles was investigated. With the increase of grind time from 1 to 5 minutes, the roundness of low-rank coal particles slightly increased. The results of induction time measurements indicated that the particle surface roughness played a significant role in the surface hydrophobicity of low-rank coal particles. Moreover, it was also found that the roundness and roughness of low-rank coal particle surfaces presented a certain degree of change after both grinding and abrasion processes. Contrary to roundness change, during the grinding process, the roughness of low-rank coal particles significantly decreased. It was also observed that the roundnesses of low-rank coal samples after the grinding process significantly increased with the increase of abrading time. However, unlike the roundness increase with the abrading time increase, the roughness of low-rank coal samples significantly decreased. Therefore, the induction times of low-rank coal samples showed an increased trend after the grinding and abrading processes, which may be attributed to the difficult rupture of wetting film on the smooth solid surface. The grinding and abrading processes may result in a further decrease in surface hydrophobicity and adversely impact on the flotation performance of low-rank coal.