THE ROLE OF OZONE IN CHEMICAL PULP BLEACHING

Ozone is a very effective bleaching agent which is suited for a prebleaching and final bleaching. Ozone reactions are not limited to lignin but carbohydrates are attacked simultaneously. Carbohydrate degradation must be surpressed by application of appropriate parameters. This is possible by pre-acidification of the pulp, low temperature bleaching, slow addition of ozone to the pulp and proper mixing. By application of these conditions 0.1% o.d/pulp is required to reduce the pulp kappa number by one unit. The total ozone charge should be limited to about 1%/pulp to keep carbohydrate degradation within acceptable limits. Viscosity can be stabilized by a NaBH4 reduction of ozone treated pulp. Sulfite as well as ASAM hardwood and softwood pulps can be bleached in chlorine free sequences to high brightness levels applying OZ(E)P. Even kraft hardwood pulps can be bleached in a OZP sequence. Strength losses are acceptable in chlorine free bleaching totalling to about 10%. Tear strength is more or less stable but tensile strength is reduced. Kraft softwood bleaching requires more bleaching stages. By OZCaEP bleaching 87% ISO can be obtained but strength losses exceeding 20% have to be accepted. Ca corresponds to the potassium salt of the caroic acid.