Hemicelluloses extraction from birch wood prior to kraft cooking : extraction optimisation and pulp properties vestigation

Wood – a raw material for pulp and paper industry - contains large amount of hemicelluloses, which are partly dissolved in black liquor in the chemical pulping process. Hardwood is particularly rich in hemicelluloses, i.e. pentosans. This work investigates a possibility of extraction of hemicelluloses prior to kraft cooking in order to find applications for the isolated polysaccharides and increase mill capacities. The response of the cooking process and pulp properties is analysed. The optimal conditions for targeted 90-95% yield of wood after pre- treatment were determined to be at temperature 150°C, liquid-to-wood ratio 3:1 and dilute sulphuric acid charge of 0,18% on wood weight. Water pre- treatment was found to be less efficient due to the need for longer dwell times of wood chips. The degree of the extraction and the yield of the extracted hemicelluloses in combination with liquid-to-wood ratio should be sufficient to provide high concentrations of pentosans in the extraction liquor. Target Kappa number of the pulp cooked from pre-treated chips was obtained with half of the H-factor compared with the standard pulp. The yield of pulp decreased equally to the amount of the material removed in the pre- treatment stage. The limiting viscosity number was higher, than that of the standard pulp due to much lower content of low-molecular hemicelluloses and high content of cellulose. Strength properties of the pre-treated pulps were found to be surprisingly lower, than that of the standard pulp. Lower extent of the hemicelluloses removal in the pre-treatment stage gives increase in strength properties.