OXYGEN BLEACHING OF KRAFT PULPS PRETREATED WITH NITROGEN-OXIDES - INFLUENCE OF THE CHARGE OF NAOH AND OF THE OXYGEN-PRESSURE

The pulps were impregnated with black liquor to simulate carry-over in integrated recovery systems. After S3-pretreatment with 2% NO2 at 60 degrees and dilution from 27 to 8% consistency with a weakly acid sodi um nitrate solution simulating recovery of pretreatment liquor, the pu lp suspension was heated to 80 degrees and ripened for 3 hours. During oxygen bleaching at 8% consistency with 4% NaOH (on dry pulp) at low partial pressure of oxygen, e.g. 0.35 MPa, the loss in viscosity at a kappa number of about 5 was less than that generally accepted during c onventional oxygen bleaching to kappa number 8-10. High oxygen pressur es and large NaOH-additions result in rapid delignification and seriou s losses in viscosity and should be avoided when high selectivity is a imed at. The losses of carbohydrates in alkaline peeling reactions are , however, suppressed by high oxygen pressures. This offers attractive alternatives during production of pulps for certain end-uses.