Abstract
A systematic study on kinetics of delignification and silica removal is necessary for optimization and a control of rice straw pulping process. Delignification and silica removal from four fractions (internode, sheath, blade and ear) of rice straw were kinetically investigated under soda pulping conditions in the temperature ranges from 30 to 70°C and from 100 to 140°C. Both delignification and silica removal by sodium hydroxide treatment were suggested to be a pseudo first order reaction. The delignification progress of rice straw was separated clearly into at least two phases. About 80% delignification was achieved in the first phase even under a mild condition at 80°C under which the degradation reaction of wood lignin was almost impossible to occur. The delignification rates of the internode, sheath and blade are larger than that of ear. It was confirmed that silica removal is a kinetically easier process than delignification under alkali pulping conditions. The rate of silica removal is greater than that of delignification, and the former is easier to increase with increasing temperature. Therefore, it was concluded that complete delignification from rice straw without serious silica removal can not be achieved as long as sodium hydroxide is employed as a cooking reagent. Other components of cell wall (i.e. hemicellulose) had virtually no effect on the kinetic parameters of silica removal.
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