Method and apparatus for the continuous oxygen delignification of fibrous materials

Method and apparatus for the good temperature and pH control as well as the uniform mixing of pulp or other fibrous materials, achieved in a medium consistency oxygen delignification system. Alkaline chemicals (30) are sprayed, optionally using oxygen gas (40) as the atomizing agent, into the gas space above the level of pulp maintained in one or more substantially horizontal tubular reaction vessels (10). At least a part of the steam requirement of the reaction is added only after the major portion of alkaline chemicals has been added to the system. The fibrous materials are combined with a first alkaline chemical (30,34) and then introduced at a consistency of from 8-20% into the reaction zone (10). Oxygen gas (40) and a second alkaline chemical (30,32) which optionally may be different from the first one (30) are then introduced into the reaction zone (10). The partially delignified mixture is introduced into a second reaction zone (22) through a substantially vertical conduit (26), whereby a second portion of alkaline chemicals (30,32,52) is added to said mixture as it falls through the conduit (26).