Stepwise removal of lignin and silica from rice straw by weak alkali -oxygen cooking

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Silica removal from the rice straw during pulping was dependent on the pH of the cooking liquor. As far as strong alkali is used for the delignification, dissolution of silica into the cooking liquor cannot be avoided. In order to control the silica removal from rice straw during pulping, oxygen-sodium sulfite pulping and oxygen-ammonia pulping were examined. It was found that a high retention of silica in pulp with very low lignin content could be achieved by these pulping methods. The applicability of weak alkali for delignification from rice straw created an interest in examining the structure of what was determined as lignin in rice straw. It was suggested that typical methods for the determination of lignin in wood such as Klason method and others, when applied to non-woody cell wall, could overestimate the amount of lignin significantly.Delignification with higher carbohydrate yield can be achieved by oxygen-sodium sulfite pulping. This was attributed to a less degradation of hemicellulose during the pulping by this method. Based on these results, two stage pulping method consisting of delignification stage followed by silica removal stage was proposed.