Method for controlling the addition of active chemical for delignifying and/or bleaching cellulose pulp suspended in a liquor containing chemicals reactive with the delignifying and/or bleaching chemical

A process for controlling the addition of delignifying chemical for delignifying cellulose pulp suspended in an aqueous liquor containing spent chemical other than the cellulose pulp that is reactive with such delignifying chemical, which comprises determining firstly prior to the addition of delignifying chemical the amount of delignifying chemical consumed by the chemical reactive with the delignifying chemical in the liquor in the absence of the cellulose pulp;

• adding a known amount of the delignifying chemical to the cellulose pulp suspension, and then, during the delignification after a predetermined time interval following addition of the delignifying chemical to the cellulose pulp suspension, determining secondly in the absence of cellulose pulp fibers the residual amount of delignifying chemical in the liquor;
• from these first and second determinations and the known amount of delignifying chemical determining the amount of delignifying chemical consumed by the pulp;
• and then adding an amount of delignifying chemical adjusted according to the amount of delignifying chemical consumed by the pulp, and carrying out the delignification.