Patent ID: 7156952

Claim:
A method for alkaline oxygen delignification of cellulose pulp, comprising: cooking the cellulose pulp according to a sulphate method; completing the cooking of the cellulose pulp so that the cellulose pulp has a kappa value of at least 15, performing oxygen delignification in a reactor system with one or many oxygen reactors (O 2 ) in series between a main charge position (M 1 ) for alkali and oxygen and a subsequent wash (W 2 ) with a predetermined dwell time of the cellulose pulp in the reactor system between the main charge position and the subsequent wash, adding alkali to the cellulose pulp to obtain an initial pH over 9.0, adding oxygen in an amount of between 5–50 kg per ton pulp in the main charge position before a first oxygen reactor in the reactor system, providing the cellulose pulp with a predetermined total dwell time of at least 60 minutes in the reactor system, detecting a prevailing pH value, by a pH sensor disposed between a last reactor and the subsequent wash, in connection with a final phase of the reactor system when less than 20% of the dwell time remain for the cellulose pulp in the reactor system, and when the prevailing pH value, detected by the pH sensor, falls below a pre-determined pH threshold value, charging an amount of alkali to the reactor system at a point that is after the last reactor of the reactor system and the pH sensor but before the subsequent wash, the amount of alkali charged being sufficient to increase the detected pH value to the pH threshold value prior to washing the cellulose pulp in the subsequent wash.