CHIP THICKNESS vs. KRAFT PULPINGPERFORMANCE, PART 2: EFFECT OF CHIP THICKNESS SCREENING ON COOKING, OXYGEN DELIGNIFICATION AND BLEACHING OF SOFTWOOD KRAFT BATCH PULP

摘要

Chip thickness has been acknowledged as one of the keyrnfactors towards more efficient pulping and better pulp quality.rnThe present paper describes a study, where the chip screeningrnsystem was switched to the conventional gyratory screening forrntwo days in a pulp mill normally operating with a modemrnthickness screening system. During two days of gyratory andrnthickness screening chip and pulp samples were taken from thernfiber line. Simultaneously, differently screened chips and thernslicer input and output chips were cooked in baskets inside thernmill’s batch digesters. In the laboratory, basket pulps werernbleached and reference pulps cooked from chip samples. Thernresults show that thickness screening is advantageousrnthroughout the entire fiber line: Rejects could be cut by 60%,rnknots by 70%. The kappa number decreased by one point.rnTotal pulping yield increased by 0,4 %-points at a given kapparnnumber and brown stock tear-tensile strength by 1 tear unit.rnViscosity increased by 5%. In oxygen delignification the samernkappa reduction could be obtained by a 10 % lower alkalirncharge. Results from DEoDED bleaching indicated betterrnbrightness at a given active chlorine consumption, better finalrnyield and higher tear-tensile strength of the fully bleached pulp.rnThe chip material after the slicer resulted in a pulp with 80 %rnless rejects, ten points lower kappa number and increased teartensilernstrength.