Process for the oxygen bleaching of cellulose pulp

A two stage process for the delignification of cellulose pulp, including chemical pulp prepared from the digestion of lignoscellulosic material, including an activation stage followed by an oxygen gas bleaching stage, the activation stage leading to a quick delignification in the oxygen gas bleaching stage, and also to a deactivation of the pulp in such a manner that degradation of the carbohydrates during the oxygen gas bleaching stage becomes slower, which comprises bringing the cellulose pulp in the activation stage in the presence of water in contact with a gas phase containing nitrogen dioxide and modifying the lignin content of the cellulose pulp to increase the rate of delignification in the oxygen gas bleaching stage by reaction with nitrogen dioxide over a reaction time within the range from about 5 seconds to about 30 minutes;

• adding oxygen gas to the activating reaction in an amount within the range from about 0.1 to about 5 moles per mole of NO.sub.2 and in an amount within the range from about 0.6 to about 5 moles per mole of NO, nitrogen monoxide formed in the activation being utilized in the activation reaction;
• and then in the oxygen gas bleaching stage, subjecting the pulp to an oxygen gas bleaching in the presence of an alkaline-reacting neutralization medium or neutralizing agent, thereby obtaining an increased rate of delignification and a reduced degradation of the carbohydrates during the oxygen gas bleaching.