Effect of acidification on microscopic properties and pore structure of coal

Abstract

Acidification is a common method for removing minerals from coal. In this study, four groups of acidification samples were evaluated: raw coal (Raw), hydrofluoric acid (HF), hydrochloric acid (HCl), and mixed HF–HCl acid (Mix) groups. The concentrations of ash and various elements in the sample groups were measured by industrial and elemental analyses. The Mix solution most effectively removed the minerals from coal, with an ash removal fraction of 87.87%. The microscopic characteristics of the four groups were studied using Fourier-transform infrared (FT-IR) and carbon nuclear magnetic resonance spectroscopy (13C NMR). The oxygen-containing functional groups of the coal samples increased after acidification by 76.07%, 14.49%, and 51.06% for the HF, HCl, and Mix groups, respectively. In addition, the content of aliphatic structures decreased during acidification. The adiposity of the Mix, HCl, and HF groups decreased by 16.36%, 8.50%, and 12.73 %, respectively. The results of CO2 and liquid nitrogen adsorption showed that the cumulative pore volume of the HF and Mix groups increased by 24.90% and 21.39%, respectively, while that of the HCl group decreased by 3.23% compared to the Raw group. The changes in the coal microstructures were observed for both micropores and mesopores. The aliphatic content was inversely proportional to the pore volume, whereas some oxygen-containing functional groups enhanced the pore volume. These findings contribute to the body of knowledge related to coal-seam acidification.