PRETREATMENT OF OXYGEN BLEACHED PULP

PROBLEM TO BE SOLVED: To reduce both the unit requirement for a bleaching chemical and the amount of discharged pollutants by forming an unbleached washing filter into a hermetically sealed type and feeding a gas at a higher oxygen concentration than that of air thereinto. SOLUTION: The upper part of an unbleached washing filter 1 in the former process of an oxygen bleaching tower 6 is covered with a hood to form a hermetically sealed type in oxygen bleaching of a pulp when producing a kraft pulp. A waste gas 12 of a deaerating tower 7 at a higher oxygen concentration than that of air is fed into the hood so as to provide a higher pressure in the interior than the atmospheric pressure. When the unbleached pulp is disintegrated with a repulper 14 to afford a pulp slurry, the oxygen concentration in the pulp slurry after an oxygen mixer 5 is raised with the waste gas 12 at the high oxygen concentration to promote the reaction in the oxygen bleaching tower 6. Thereby, the delignification is advanced to reduce the Kappa number.