HIGH-BRIGHTNESS BLEACHING OF SOFTWOOD KRAFT PULPS WITH OXYGEN, OZONE AND PEROXIDE

The OQPZP sequence bleached a softwood kraft pulp to an ISO brightness of 89.9%, which was 6 points higher than that obtained with the OZQPP sequence, and 12 points higher than the brightness observed after the OQPP sequence. The delignification and brightening capabilities of th e peroxide and ozone stages were best exploited in the OQPZP configura tion when compared on the basis of chemical consumptions and oxidation equivalents in each stage. The best distribution of H2O2 in the P sta ges of the OQPZP sequence to produce a brightness higher than 90% was 2.0-2.5% H2O2 in P-1 and 1.0% H2O2 in P-2. Reversing this order of H2O 2 addition between P-1 and P-2 (1.0% H2O2 in PI and 2.5% H2O2 in P-2) resulted in a brightness of 86% after OQPZP at optimum alkalinity in t he P stages.