Changes Within the Modified Kraft Process Aiming to Improve TCF Bleachability, 1996 International Pulp Bleaching Conference Proceedings

Attempts have been made to improve the modified kraft pulping process further in order to obtain a more selective delignification and a more easily removed and/or brightened lignin in the subsequent TCF-bleaching stages. A modified kraft pulping process, consisting of HS pretreatment as well as co-current and counter-current zones at low temperatures and with multiple alkali charges, has been the reference process. This rather advanced process has been further modified in a number of ways that could be expected to produce a pulp with a more easily bleached lignin. The process modifications were: l presulfonation; replacing the HS - pretreatment l postsulfonation in the latter part of the cook . polysulfide pretreatment replacing the HS - pretreatment, followed by cooks of different alkalinity l polysulfide pretreatment, combined with postsulfonation in the latter part of the cook l the reference pulp cooked with a lower alkalinity, followed by two-stage oxygen delignification. All the investigated process variants showed an improvement in the bleaching selectivity (pulp viscosity at a given brightness) compared to the reference. In the case of the process modifications that included a postsulfonation and in the case of the reference pulp with a lower alkalinity, but followed by two-stage oxygen delignification, a slightly higher brightness was observed than for the reference pulp at a given consumption of bleaching agents, expressed as OXE/kappa number. The PS-pretreated pulp showed a lower tear/tensile index relationship than the other investigated kraft variants, but a better beatability.