Effect of alkaline pre-extraction of hemicelluloses and silica on kraft pulping of bamboo (Neosinocalamus affinis Keng)

•Silica mass distribution along the bamboo stem was determined.•All silica and up to 50% of hemicelluloses in bamboo raw chips could be pre-extracted by alkaline pretreatment.•Alkaline pre-treatment benefited the subsequent pulping process.•Information here might provide a novel way to solve silica problems in pulping bamboo.

Commercial bamboo chips were pre-treated with sodium hydroxide (NaOH) solutions to completely extract silica and partially extract hemicelluloses prior to kraft pulping. Reaction temperatures of 80–100 °C, times of 1–5 h, and NaOH charges of 6–18% were explored. With increasing pre-extraction severity, all silica and up to 50% of hemicelluloses in raw chips could be extracted without degrading cellulose and lignin. The chips from select extractions were cooked using the kraft process with varying effective alkali (EA) charges. Pre-extraction resulted in significant improvement in the delignification of chips during subsequent kraft pulping, offering an option to reduce the EA charge or the H-factor. The pulp yield was similar to the control while the drainage resistance of pulp from pre-extracted chips was slightly improved. Physical strength properties of pulps made from pre-extracted chips showed lower tensile index and higher tear index as compared with the control runs. Moreover, silica was no more a problem for chemical recovery and production of high-grade pulp. Extracted silica and hemicelluloses in the alkaline extraction liquor (AEL) can be used as a potential raw material for value-added products.