THE ASSOCIATIVE NATURE OF LOWER RANK COAL

Initial volumetric swelling in tetrahydrofuran of pyridine-unextracted Parts from subbituminous coal and lignite showed no dependence of their concentration, and was smaller than that of their pyridine extracts. These results are opposite to those obtained from high volatile bituminous coals and coincide with predictions for the cross-linked network model of coal. However, when ionic forces in these coals were reduced by acid washing or 0-alkylation, these coals showed the same associative nature as did high volatile bituminous coals. Swelling kinetics were analysed on the basis of associative equilibria controlled by the ionic forces. It was concluded that solvation of the ionic forces was the rate determining step of volumetric swelling of lower rank coal rather than solvent diffusion into the coal, although diffusion has been proposed to be the most important factor in swelling.