Chip Thickness vs. Kraft Pulping Performance, Part 2: Effect of Chip Thickness Screening on Cooking, Oxygen Delignification and Bleaching of Softwood Kraft...", 1993 Pulping Conference Proceedings

Chip thickness has been acknowledged as one of the key factors towards more efficient pulping and better pulp quality. The present paper describes a study, where the chip screening system was switched to the conventional gyratory screening for two days in a pulp mill normally operating with a modern thickness screening system. During two days of gyratory and thickness screening chip and pulp samples were taken from the fiber line. Simultaneously, differently screened chips and the slicer input and output chips were cooked in baskets inside the mill’s batch digesters. In the laboratory, basket pulps were bleached and reference pulps cooked from chip samples. The results show that thickness screening is advantageous throughout the entire fiber line: Rejects could be cut by 60%, knots by 70%. The kappa number decreased by one point. Total pulping yield increased by 0,4 %-points at a given kappa number and brown stock tear-tensile strength by 1 tear unit. Viscosity increased by 5%. In oxygen delignification the same kappa reduction could be obtained by a 10 % lower alkali charge. Results from DEoDED bleaching indicated better brightness at a given active chlorine consumption, better final yield and higher tear-tensile strength of the fully bleached pulp. The chip material after the slicer resulted in a pulp with 80 % less rejects, ten points lower kappa number and increased tear-tensile strength.