Process for producing alkali pulp

In a process for producing alkali pulp by using a single, cylindrical pressurized reaction vessel having therein a liquor-impregnating zone, a cooking zone, a washing zone and a diluting zone in that order, and having a motor-driven scraping or agitating device and a pulp discharging outlet in the end portion of the diluting zone, which process comprises continuously introducing chip-form cellulosic raw materials into the reaction vessel, subjecting the cellulosic materials to impregnation of alkali cooling liquor, alkali cooking, washing and diluting, in succession, during their transfer inside the reaction vessel, and continuously discharging resulting pulp out of the vessel through the pulp discharging outlet, the improvement comprising:

• continuously introducing alkaline aqueous medium containing dissolved oxgygen into the end portion of the diluting zone;
• contacting a part of the introduced alkaline aqueous medium containing dissolved oxygen in a countercurrent manner with cooked cellulosic materials transferred from the washing zone to the diluting zone to proceed with oxygen-alkali delignification in the diluting zone, waste liquor produced during delignification due to the countercurrent contact being transferred to the washing zone where it is used as washing liquor and being finally discharged out of the vessel from the end portion of the cooking zone;
• and
• contacting the remainder of the introduced alkaline aqueous medium containing dissolved oxygen with the cooked cellulosic materials in the diluting zone in a scraping or agitating manner by means of the motor-driven scraping or agitating device to proceed with oxygen-alkali delignification, said remainder of the alkaline aqueous medium being finally used as diluting liquor to be discharged out of the vessel accompanying the resulting pulp from the pulp discharging outlet.