Insight into the self-aggregation behavior of lignite and anthracite in water: Atomic-level research using experiments and molecular dynamics simulations

Abstract

To enable effective treatment of coal slime and prevent its release as an environmental pollutant, the interaction between coal particles in aqueous solutions must be well understood to enhance flocculation and sedimentation of the coal. In this study, the interaction between coal particles was explored using experiments and molecular simulations, which revealed the self-aggregation behavior of lignite and anthracite at different coalification levels in water. Atomic force microscopy results indicated that the anthracite–anthracite adhesion force is greater than that of lignite–lignite, and the adhesion force dominates particle interactions. The surface electrostatic potential range of lignite (-85.57–87.67 kcal/mol) is much higher than that of anthracite (-63.68–59.60 kcal/mol) and the lignite surface provides more polar regions for adsorbing water molecules. Lignite and anthracite rely on π–π accumulation to form a dimer skeleton structure. The dispersion interaction, dominated by π–π interactions, is key to promoting cluster formation and maintaining a stable cluster. The coal molecules proceed through three stages of movement, forming a cluster structure by dissipating the surrounding water molecules. The hydrophobic surface of the cluster molecules is exposed, gradually concealing the hydrophilic groups, which maintains the stability of coal self-aggregates. This process affects the water-molecule distribution, damaging the water bridge structure formed between the water molecules in coal molecule interlayers. This results in a decrease in the average number of water molecules surrounding a single lignite or anthracite molecule, and a change in the water-molecule orientation. Therefore, to enhance the flocculation of coal particles in slime water, it is necessary to increase the hydrophobicity of low-rank coal particles.