PROCESS FOR DELIGNIFICATION OF LIGNOCELLULOSE-CONTAINING PULP

The present invention relates to a process for delignifying and bleaching lignocellulose-containing pulp; in which the pulp is delignified kith 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.