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 st udy of oxidative delignification with laccase and the mediator 1-hydroxyben zotriazole. The laccase treatment, followed by alkali extraction, lowered t he Kappa number to 6.6. Addition of xylanase to the reaction liquor resulte d in a further reduction to 5.9. Spent liquor from the laccase treatment wa s 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-me diator stage, (EO) an oxygen reinforced alkali extraction, Q a chelation st age and (PO) a pressurized hydrogen peroxide stage). Brightness of 89% ISO was attained and the tearing resistance was the same as that of pulp bleach ed with a D(EO)DD sequence. An extended delignification pulp (ITC(TM) proce dure) was bleached to 90% brightness with this sequence, but the tearing re sistance was about 15% lower than when the pulp was bleached with an ECF se quence.