STUDY ON REACTION MECHANISM AND KINETICS OF NH_4OH-KOHPULPING OF RICE STRAW

Reaction mechanism and kinetics of NH4OHKOH pulping of rice straw were studied by pulping experiments carried out under stable and welldefined conditions.Results of study showed that three distinct delignification stages occurred during this pulping process,namely,the initial delignification stages from beginning of cooking to 100 ℃,the bulk delignification stage from 100 to 155 ℃ for 45 min and the residual delignification stage at 155 ℃ for 45 min to the cooking end.There were two silica removal stages,namely,the first stage from the beginning of cooking to 100 ℃ and the second stage from 100 ℃ to the cooking end.It was approved by experiment that removal of silica was helpful for removal of lignin.During pulping process,hydroxyls of lignin reacted with KOH and NH4OH one after another,which resulted in degradation of lignin into small molecules and dissolution in alkali solution to form ammoniacal lignin.Delignification of the initial delignification stage was found to be of pseudo first order with respect to residual lignin.Activation energy of delignification reaction was 35.60 kJ/mol.Silica removal during the first silica removal stage was found to be of pseudo first order with respect to residual silica.Activation energy of silica removal reaction was 30.91 kJ/mol.