PROCESS FOR DELIGNIFICATION OF LIGNOCELLULOSE-CONTAINING PULP

摘要

The present invention relates to a process for delignifying and bleaching lignocellulose-containing pulp, in which the pulp is delignified with a peracid or a salt thereof, treated with a complexing agent, and subsequently bleached with a chlorine-free bleaching agent. Suitably, delignification is carried out with the strongly oxidising peracetic acid, giving a considerable increase in brightness and a considerable reduction of the kappa number after bleaching with a chlorine-free bleaching agent comprising at least one of a peroxide-containing compound, ozone or sodium dithionite, or optional sequences or mixtures thereof. The brightness-increasing effect is highly selective, i.e. the viscosity of the pulp is maintained to a comparatively great extent. Both the delignification and the treatment with a complexing agent are advantageously carried out at a close to neutral pH, thus minimising the need of pH adjustment and making it possible to use spent bleach liquor internally, e.g. for washing the pulp. By final bleaching with ozone and hydrogen peroxide, it is possible with the inventive process to produce softwood pulps having a brightness exceeding 90 % ISO, and to remove practically all lignin in the pulp while maintaining sufficient pulp strength.