The influence of mineral matter on moisture adsorption property of Shengli lignite

A typical Chinese lignite was subjected to H2O, CH3COONH4/HCl and HCl/HF/HCl leaching procedures, to study the influence of water-soluble salts, alkali and alkaline earth metals (AAEMs) organically bound to the coal substance or occur as acid-decomposable minerals and discrete mineral matters on moisture adsorption property of lignite. Variations of pore structure and chemical functional groups of coal samples during leaching procedures were determined by the nitrogen adsorption measurements and Fourier transform infrared spectroscopy (FTIR), respectively. Possible interactions between different types of minerals and moisture in lignite were also discussed. The results showed that the specific surface area and porosity of the coal treated by HCl/HF/HCl method increased significantly, while other leaching procedures had slight effect on coal pore structure. The FTIR spectrum of treated samples displayed similar bands of the organic part as those of raw coal. Water-soluble minerals in coal have adversely influence on the sorption behaviour of water molecules. The ionic cross-links forces and the hydration of ions, gradually disappeared with the removal of AAEMs result in the evident decrease of the capacity of moisture adsorption of lignite. The increase of specific surface area and the concentrations of oxygen-containing functional groups by removing discrete mineral particles could cause the increased equilibrium adsorption moisture content (EMC). On the other hand, a poorer cohesive structure of adsorbing moisture resulted from the presence of discrete mineral matters, raw lignite revealed lower EMC.