Patent ID: 6258208
Filing Date: 2001-07-10
Classification: D21C

Abstract:
Method for the manufacturing of bleached cellulose pulp, in conjunction with which lignocellulose material that contains transition metals is digested to form cellulose pulp by means of an alkaline digestion liquor, and then the cellulose pulp in the form of a suspension is subjected in series to at least a stage of chlorine dioxide bleaching (D) and a stage of bleaching with a non-chlorine containing, oxidative bleaching agent;with said stages of bleaching interspersed with at least washing with a liquid and/or reconcentration of the cellulose pulp, and wherein a complexing agent that complexes the transition metals is added to the cellulose pulp in conjunction with the chlorine dioxide bleaching (D),wherein the liquid removed from the pulp in conjunction with the washing and/or reconcentration of the cellulose pulp is conveyed in essentially strict counter-current, in such a way that the pulp manufacturing process is essentially totally closed with regard to the liquid circuit; andthe pH value of the suspension liquid, is maintained at .ltoreq.10, in the absence of a reduction agent, from said chlorine dioxide bleaching stage of the cellulose pulp following digestion and onwards throughout each of the cellulose pulp treatment stages prior to the bleaching stage with the non-chlorine containing, oxidative bleaching agent; andwherein a carbonate content of the suspension liquid is maintained at or in excess over a minimum value for each stage of the method, said minimum value determined by the stage in cellulose pulp treatment,said pH value and said carbonate content controlled in a manner to prevent breakdown of complexes of the complexing agent and the transition metals in the suspension liquid in every position backwards through a washing stage that occurs in the direction of pulp flow before the chlorine dioxide bleaching stage.