Production of cellulosic pulp by subcritical water extraction followed by mild alkaline pulping

The potential of hot water extraction of birch wood chips followed by mild soda-anthraquinone pulping is investigated for the production of pure cellulosic pulp. Delignification in wood during a hot water extraction increases with temperature and extraction time, before condensation of lignin prevails. Furthermore, the removal of lignin appears to be governed by solubility, as indicated by the effect of the liquid-to-wood (L:W) ratio of the process. On the other hand, the L:W ratio has little effect on the removal of carbohydrates. A hot water treatment at 240 °C and a few minutes of extraction time, with a L:W ratio of 10:1 g/g, removes about 40% of the original lignin in wood. Under such operational conditions, all the hemicelluloses are extracted while the cellulose is hardly affected. The wood residue after the hot water extraction is further subjected to soda-anthraquinone pulping. The final pulp is composed of more than 95% pure cellulose, with a kappa number around 10. The final pulp yield, however, is less than 20% of the initial dry wood, owing to significant degradation of cellulose during the alkaline treatment.