PEROXIDE AND SODIUM HYDROSULFITE BLEACHING OF BIRCH-HARDWOOD AND BIRCH-SOFTWOOD MIXTURES

One stage hydrogen peroxide (P) and sodium hydrosulfite (T) and two st age (PT, TP, PP) bleaching sequences were applied to CTMP pulps made f rom mixtures of birch/aspen, and birch/spruce/balsam. Impregnation of birch/softwoods with a liquor of sulfite and sodium hydroxide decrease d the initial brightness by 10 to 17 ISO units, when compared with an impregnation liquor containing sulfite alone. One-stage bleaching with sodium hydrosulfite gave a maximum brightness gain of 8.7 ISO units, while hydrogen peroxide gave a gain of 21.6 ISO units. The two-stage s equences studied gave higher brightness gains. The incorporation of bi rch to aspen or spruce/balsam in CTMP pulps can lead to a brightness o f 80% ISO using an optimum bleaching sequence.