The use of a laccase mediator stage in bleaching eucalypt kraft pulps

An oxygen-bleached eucalypt kraft pulp (Kappa number 10.5) was used in a study of oxidative delignification with laccase and the mediator 1-hydroxybenzotriazole. The laccase treatment, followed by alkali extraction, lowered the Kappa number to 6.6. Addition of xylanase to the reaction liquor resulted in a further reduction to 5.9. Spent liquor from the laccase treatment was not effective when used a second time. The hexenuronic acid groups in the treated pulps contributed to more than half of the measured Kappa numbers. The pulp was bleached with an L(EO)LQ(PO) sequence (where L is a laccase-mediator stage, (EO) an oxygen reinforced alkali extraction, Q a chelation stage and (PO) a pressurized hydrogen peroxide stage). Brightness of 89% ISO was attained and the tearing resistance was the same as that of pulp bleached with a D(EO)DD sequence. An extended delignification pulp (ITC™ procedure) was bleached to 90% brightness with this sequence, but the tearing resistance was about 15% lower than when the pulp was bleached with an ECF sequence.