PRACTICAL EXPERIENCES IN REDUCING THE ECF BLEACHING COSTS OF AN OXYGEN DELIGNIFIED FULLY-BLEACHED SOFTWOOD KRAFT PULP

摘要

Pope & Talbot, Inc. produces fully-bleached softwood market kraft pulp at Halsey, OR using an ECF bleaching sequence. Brownstock cooked in continuous digesters utilizing either wood chips or sawdust as their feedstock is processed after washing through a medium consistency, single-stage oxygen delignification system and enters the bleach plant having a 16-20 Kappa number. Since October, 2000 the mill has successfully started-up and optimized a new bleaching sequence including multiple stages of chlorine dioxide (ClO_2) and hydrogen peroxide (H_2O_2), replacing multiple stages of stabilized hypochlorous acid (Monox L) utilized prior to October, 2000. The new bleaching sequence was required to meet Cluster Rule regulations related to AOX and chloroform. After initial startup, laboratory and production trials were completed to reduce ECF bleaching costs without sacrificing pulp quality. Improvements included rebalancing ClO_2 between the D_0, D_1 and D_2 stages, lowering the lignin content of pulp from the (EOP) stage by optimizing H_2O_2 application, operating temperature/pressure and control of metal ions into the stage and reducing demand for ClO_2 at the D_1 stage by improving washing and hence reducing soda carryover from the (EOP) stage washer and better controlling inlet pH. As well, it was found that a pitch fixant being added on bleached pulp was bonding darker fines from pulp machine Whitewater to the fiber and limiting the final brightness of pulp achievable. This issue was resolved by applying talc to properly disperse pitch.