Alkalikulutuksen pienentäminen havupuun sulfaattikeitossa mustalipeäimeytyksellä

Besides the extracted wood components, black liquor contains residual cooking chemicals, sodium hydroxide and sodium hydrogen sulfide, which are typically recovered via a recovery cycle. However, such residual chemicals from the black liquor could be better utilized as impregnation liquor for wood chips prior to a kraft cook, which could potentially reduce the chemical consumption during the kraft cook. This thesis aimed to investigate the possibility of reducing the effective alkali charge in kraft cooking of softwood by black liquor impregnation, and to study the effect of such pretreatment on the pulp properties and bleachability. The experimental work of this thesis was completed in three phases. First, the effect of impregnation time and temperature on the pulp and liquor properties was investigated. Second, the possible reduction in the effective alkali charge to the kraft cook was studied. Lastly, upscaled kraft cooks were performed in a more industrially relevant setting. The pulp and liquor samples produced in the experiments were comprehensively analysed. Both the alkali and sulfide consumption of the kraft cook can be reduced by a black liquor impregnation before the kraft cook. The EA charge from white liquor to the kraft cook was reduced by up to 2.5% on wood by black liquor impregnation, while maintaining comparable pulp properties to the reference kraft cook without impregnation and a higher EA charge from WL to the cook. Also, the total alkali consumption of the upscaled kraft cook preceded by impregnation (including alkali from BL and WL) was lower by ca. 0.5% on wood, when compared to the upscaled reference cook without impregnation. The total sulfide consumption of the kraft cook was also reduced by ca. 2% on wood by black liquor impregnation. The bleachability of pulp was not affected by black liquor impregnation within the scope of the experiments performed in this thesis.