Process for treating oxygen delignified pulp using an organic peracid or salt, complexing agent and peroxide bleach sequence

摘要

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 oxidizing 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 minimizing 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 to produce softwood pulps having a brightness exceeding 90% ISO, and to remove practically all lignin in the pulp while maintaining sufficient pulp strength.