Insight into the separation mechanism of coal macerals by micro characterization and density functional theory calculation

[Display omitted] •The purity of vitrinite and inertinite separated by hydrothermal float-sink reached more than 75% and 60%.•The asynchronous graphitization development of coal macerals resulted in the density difference.•The combination of experiment and calculation updated the separation mechanism of macerals. It is of great significance to reveal the specific separation behavior of macerals for realizing the classification and utilization of coal resources. In this article, the separation mechanism of macerals during coal upgrading was investigated experimentally and computationally based on an improved hydrothermal float-sink method combined with various characterization approaches. The apparent morphology, endogenous mineral and aromatic structural differences of macerals were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM). Materials Studio was applied to calculate the adsorption conformation, energy, charge density difference and bonding tendency of heavy media with functional groups in coal to elucidate the interaction between macerals and heavy liquids. The results show that the float-sink procedure with water bath heating instead of the previous strong acid inhibitor can achieve a purity of more than 75% and 60% for vitrinite and inertinite, respectively. Further refinement of coal particle size can enhance the yield of vitrinite with lightweight, which is less dense due to its lower aromatization than inertinite. In addition, the partial distribution of characteristic functional groups in coal on the surface of inertinite enhances the adsorption between it and heavy liquid, thereby affecting the separation behavior. Therefore, the mechanism of separation and enrichment of macerals by the float-sink method lies in the density difference of individual components and the interaction between the functional groups of each maceral and ZnCl2 solution.