BLEACHING PRESSURIZED GROUNDWOOD PULPS IN THE GRINDER PIT WITH HYDROSULFITE

The pressurized grinding process has progressed rapidly and has earned widespread acceptance and application. Pressurized groundwood (PGW) is used in all major grades that contain mechanical pulp; most goes into supercalendered and lightweight-coated grades. Strength properties of super pressurized groundwood (PGW-S) are 10-30% higher than conventional PGW, and PGW is stronger than atmospheric groundwood (GW). However, the higher temperatures and pressures used in PGW-S and PGW pulping have a negative impact on the brightness of the unbleached pulp. This study presents the results of pilot-and mill-scale trials that were carried out to explore the use of sodium hydrosulfite in the grinder pit to increase pulp brightness. The results confirmed that brightness of PGW-S and PGW pulps can be increased to levels comparable with conventional GW while retaining their superior strength properties.