Patent ID: 6126782
Filing Date: 2000-10-03
Classification: D21C

Abstract:
Method for producing bleached cellulose pulp comprising:digesting, by means of an alkaline digestion liquor, a transition metal containing lignocellulose material to form cellulose pulp;subjecting the cellulose pulp in the form of a suspension to a cellulose pulp treatment chain comprising the following stages:1) treatment stages including;an oxygen gas delignification stage (O),a complexing stage (Q) involving treatment with complexers to form a water soluble transition metal complex,and a bleaching stage involving bleaching with a non chlorine-containing oxidative bleaching agent;2) at least one stage of washing and/or reconcentrating interspersed between said treatment stages in said cellulose pulp treatment chain;conveying a suspension liquid essentially in strict counter-current to pulp flow into a thick waste liquor, which is combusted;wherein the pH value of the suspension liquid is maintained at .ltoreq.10, in the absence of a reducing agent, from just after oxygen gas delignification and onwards with respect to direction of pulp flow into the cellulose pulp treatment chain as far as the bleaching stage with the non chlorine-containing oxidative bleaching agent, andwherein the carbonate content of the suspension liquid is maintained at or in excess over a minimum value for each stage of the chain, said minimum value determined by the stage in the cellulose pulp treatment chain,said pH value in the stages concerned and said minimum value of carbonate content, at each stage of the process where complexer is present, being sufficient to keep the formed water soluble transition metal complexes intact in the suspension liquid as said suspension liquid flows backwards with respect to direction of pulp flow in the whole cellulose pulp treatment chain,with the water-soluble transition metal complex intact up to and through a washing stage that occurs in the direction of pulp flow before the oxygen gas delignification stage;with the result that the pulp manufacturing process is essentially totally closed with regard to the liquid circuit.