Behavior of pine wood material during displacement kraft batch cooking.


Pine wood (Pinus sylvestris) was delignified using a modern displacement kraft cooking method. A series of laboratory-scale cooks (stop-cooks) was carried out to different cooking degrees representing i) the end points of warm black liquor impregnation (temperature 80°C, total yield 93.3%) and hot black liquor pretreatment (temperature 155°C, total yield 77.5%) and ii) varying chlorine numbers (1.8-7.0) in the cooking stage (temperature 170°C, H factor 370-1820, total yield 40.7-48.6%). The resulting black liquors were analysed in terms of carbohydrate-derived aliphatic carboxylic acids and the fraction of lignin degradation products including its molecular mass distribution. Total yield, residual lignin content, viscosity and brightness were determined from the corresponding pulps. Based on these results the dissolution of different wood carbohydrates and lignin was monitored as a function of total yield. The chemical stability of lignin material in black liquor was studied separately by treating the corresponding liquor at temperatures of 160°C and 175°C for 15-480 minutes. At both temperatures a slight degradation of lignin was detected during the treatment.