Mass-transfer plays an important role in the solvent extraction of coals. The studies on the kinetics of diffusion of the solvent in coals help in understanding the process of mass-transfer in coal. The coal molecules get dissociated by the penetration of solvents in the solid coal and associative forces between coal molecules and solvent molecules are formed. The activation energy for solvent (N-methyl-2-pyrrolidone,NMP) swelling of coal decreased as the swelling was increased. The relaxation way of diffusion became Fickian in nature as the swelling of
coal by NMP increased. It was found that the unstirred refluxing conditions of extraction of Samla coal suffers from external mass-transfer limitations. The role of internal mass-transfer in solvent extraction is discussed.