Impact of hardwood black liquor addition on the chemical and physical properties of kraftliner - a lab study

Kraft pulping, also known as sulphate pulping is a complex process where the wood components are liberated from each other facilitated by chemical reactions. During the pulping process some dissolved and degraded compounds end up together with the spent cooking chemicals in the black liquor. In kraft pulping of hardwood, the black liquor contains substantial amounts of dissolved hemicellulose, especially xylan due to the wood composition. The xylan content is of high value due to its ability to re-deposit by adsorption on cellulose fibres and its tendency to increase pulp strength and yield. In this work, the possibility of introducing xylan rich, hardwood black liquor from the hardwood digester into the softwood digester at the process of SCA – Munksund was studied. The objective was to investigate if hardwood black liquor addition in a softwood cook can increase the yield and pulp strength. The theoretical feasibility of altering the process was investigated by a literature review and a brief process investigation. During the study a total of eight lab cooks were conducted to obtain information on how hardwood black liquor addition changes the pulp quality of regular softwood cooks. The wood chips, white liquor and hardwood black liquor used were collected at Munksund and the process conditions in the lab cooks were chosen to simulate the actual process in the Munksund mill. In addition to the cooking, chemical and physical evaluation on pulps, cooking liquors and wood chips were performed. The theoretical evaluation indicates that the process in Munksund is well suited for introducing the hardwood black liquor in to the softwood digester. The chemical analysis of the black liquors show a xylan content that is approximately four times higher in the hardwood black liquor compared to the softwood black liquor. In the kappa number range of approximately 80-90, a total yield increase of 0.4%-units was obtained for two cooks with hardwood black liquor addition compared to their corresponding reference cooks. A carbohydrate analysis showed a higher xylan content in the pulp cooked with addition of hardwood black liquor compared to the corresponding reference pulp. One pulp with hardwood black liquor addition and its corresponding reference cook were refined in an Escher Wyss lab mill. The pulp with hardwood black liquor addition exhibits an increase or retained strength for all strength evaluation tests made on handmade pulp sheets. Strength increases of approximately 5% were obtained for tensile index and ISO-Z strength for a pulp with hardwood black liquor addition. The results conducted in this study shows that it can be possible to slightly increase both the yield and some pulp strength parameters when adding hardwood black liquor in to the softwood cook in lab scale. It is also shown that the increase in the total yield most likely depends on xylan adsorption on the cellulose fibres. The calculated increased revenue for this process change could be over 400 000 Euro per year.