Estimation of hydrophilicity of coals by using the quantum chemistry calculation

- The quantum chemistry calculation was used to quantify the hydrophobicity.
- The calculated wetting energy of forming hydrogen bond was verified by the calorimetric experiments.
- The real reason of hard-to-float of lignite was dominated by the complex hydrophilic sites.

The hydrophilicity is an important parameter in characterizing the floatability of coals in floatation. To quantify the hydrophilicity, the quantum chemistry calculation was used to calculate the hydrogen bonding energies in the wetting process by DFT B3LYP 6-31G. It is known that the floatability of lignite is much lower than that of the high-volatile bituminous coal from the natural floatability test. Besides, the hydrophilic sites were mainly oxygen functional groups. From XPS results, it is indicated that there were four kinds of oxygen functional groups, including OH, CO, CO and COOH in lignite, while there were three kinds of oxygen functional groups, containing OH, CO and CO in the high-volatile bituminous coal. On the other hand, the simulation results showed that the hydrogen bonding energy of these hydrophilic sites are in the following order − COOH > − OH > C − O > C = O. The released hydrogen-bonding energies were calculated to be 2.36 kcal/mol for lignite and 1.31 kcal/mol for high-volatile bituminous coal, respectively. In addition, the calorimetric experiments also showed that the wetting heats were 105.37 J/g (lignite) and 47.00 J/g (high-volatile bituminous coal). The hydrogen bond energy distribution of each hydrophilic sites showed that all the four kind hydrophilic sites impacted the lignite hydrophilicity remarkably whereas only OH and CO affected the hydrophilicity of high-volatile bituminous coal. It was revealed that the difficulty in lignite flotation was dominated by the complex contribution to hydrophilicity of the oxygen functional groups.