Micro-mechanism of improving low-rank coal flotation by using carboxylic acid collector: A DFT calculation and MD simulation study

The poor flotation behavior of low-rank coal is mainly caused by the high content of functional oxygen groups present on the coal surface. Polar collectors, especially oxygenated collectors, play a significant role in the flotation enhancement of low-rank coal. In this study, to improve the low-rank coal flotation performance, octanoic acid was selected as a polar collector and the corresponding promotion mechanism was investigated at the molecular scale theoretically. The flotation results showed that an increased low-rank coal flotation response can be obtained by using an octanoic acid collector compared to a nonpolar dodecane collector. To obtain a molecular-level explanation of this phenomenon, a method of density functional theory (DFT) combined with molecular dynamics simulation was employed to explore the involved micro-mechanism. DFT calculation results suggested that the octanoic acid collector can interact with benzoic acid and phenol (representatives of oxygencontaining groups on the low-rank coal surface) through hydrogen bonding and repel water molecules present on benzoic acid and phenol. Based on theoretical calculations, in the presence or absence of water molecules, hydrogen bonding and electrostatic interactions are responsible for the strong affinity of octanoic acid to carboxyl group and hydroxyl group on the low-rank coal surface. Therefore, it was favorable for octanoic acid to form a stable adsorption layer on the low-rank coal surface, thus contributing to low-rank coal flotation significantly.