Licentiate seminar: Vijaya Lakshmi Vegunta – On calcium-related problems in kraft pulping of Eucalyptus dunnii
Kraft pulping is the most widely used pulping process for producing chemical pulp today. It has been in use for more than 130 years. During the pulping process most of the lignin and part of the hemicellulose are removed from the wood, resulting in an unbleached chemical pulp which is mainly composed of cellulose, some hemicellulose with smaller amounts of modified lignin. Several studies have been conducted to ensure effective delignification during the kraft cooking process. Nonetheless, due to the highly complex chemistry and anatomy of wood, there is a need to understand pulping chemistry in more detail.
The principal inorganic component in wood is calcium. A considerable share of the calcium in wood is released during the chemical pulping process. The calcium released initially during kraft pulping is mainly soluble in cooking liquor which passes through a thermodynamically stable limit before reacting with carbonate to form calcium carbonate. The calcium in the pulping process causes precipitation and scaling problems in the pulping process lines, especially in the black liquor evaporation sections.
The focus of this work is to increase the knowledge of kraft cooking of Eucalyptus dunnii which has high calcium content and cooking chemicals reactions during delignification. In addition, the aim was to investigate the influence of different calcium levels in E. dunnii wood chips on the kraft pulping process. Chips with different calcium levels were pulped in lab-scale digesters.
The results obtained in this work showed that the rate of delignification decreased and polysaccharide degradation increased during kraft pulping of E dunnii when the calcium content was high. E dunnii with low calcium content showed no adverse effect on the delignification rate and polysaccharide degradation during kraft pulping.
The uronic acid content in wood and the hexuronic acid content in pulp after kraft cooking showed no significant impact on the delignification rate. The detrimental effect of calcium during kraft pulping was studied using green liquor introduction in kraft pulping. Using green liquor during the kraft pulping of high calcium E dunni wood chips and extending the impregnation time resulted in good delignification and lower degradation of polysaccharides. Unbleached pulps produced from high calcium content wood could be bleached with good results, i.e., equal brightness and increased strength properties. The drainage resistance of bleached pulp was slightly increased. The calcium present in the wood chips follows the fibre line with the pulp after kraft cooking and is less soluble in kraft black liquor. These phenomena are probably related to the carbonate present in green liquor “inactivating” the adverse effects of calcium ions. Possible scientific mechanisms and hypotheses are discussed in this work. Preliminary studies using industrial green liquor compared to synthetic green liquor on a lab scale are also discussed in this work.
Keywords: kraft pulping, delignification, polysaccharide degradation, viscosity, H-factor, kappa number, calcium content