Process for the delignifying bleaching of cellulose pulp by activating the pulp with NO.sub.2 and oxygen

A process for delignifying bleaching lignin-containing cellulose pulp which comprises:

• (1) activating cellulose pulp by reacting the pulp with a gas comprising NO.sub.2 and oxygen in the presence of water at a temperature within the range from about 30.degree. to about 120.degree. C. while so controlling the amount of nitrogen oxides, the pulp consistency, the temperature, and the residence time so that the reduction in intrinsic viscosity of the pulp at the end of this stage is limited to within the range from 2 to 35% less than at the beginning of the stage;
• (2) washing the activated pulp with water or an aqueous solution;
• (3) treating the activated washed pulp with an aqueous alkaline solution comprising an alkaline carbonate of which a major proportion is in the form of HCO.sub.3.sup.- at a temperature within the range from about 90.degree. to about 170.degree. C., in the presence of oxygen gas at an average oxygen partial pressure within the range from 0.001 to about 0.2 MPa, until the lignin content of the pulp is so reduced that the Kappa number of the pulp is within the range from about 10 to about 60% of the Kappa number of the pulp entering the activating stage (1), and releasing carbon dioxide gas liberated;
• (4) treating the activated washed pulp from step (3) with an aqueous alkaline solution comprising an alkaline carbonate of which a major proportion is in the form of CO.sub.3.sup.-- at a temperature within the range from about 90.degree. to about 170.degree. C., in the presence of oxygen gas at an average oxygen partial pressure within the range from about 0.1 to about 3 MPa;
• (5) withdrawing from stage (4) alkaline liquor comprising HCO.sub.3.sup.- and recycling said liquor to stage (3) as a source of HCO.sub.3.sup.-.