Abstract
Relationship between the progress of delignification and silica removal from rice straw was studied under various pulping conditions. Alkali pulping and oxygen-alkali pulping were found to remove lignin and silica significantly from pulp, while sulfite and ammonia pulping were found to retain a certain amount of silica in pulp with low lignin content. Delignification with higher carbohydrate yield can be achieved by the multi-stage process of 0.05 M sodium alkali treatment. It was confirmed that there is a strong relationship between silica removal and the pH of cooking liquor. Comparison between oxygen-alkali pulping and conventional alkali pulping at the same degree of delignification shows that oxygen-alkali pulping can keep higher silica content than conventional alkali pulping probably because of lower pH in the former case. Oxygen-alkali pulping can, however, remove more silica under the same pH condition than conventional alkali pulping, which suggests the silica removal is not only dependent upon pH condition. Silica removal up to 30% was observed at any pH with various cooking liquor. However, it depends on alkali concentration and cooking temperature whether further removal of silica proceeds or not.
