USE OF A REDUCING STAGE TO AVOID DEGRADATION OF SOFTWOOD KRAFT PULP AFTER OZONE BLEACHING

During ozone bleaching (Z) some carbonyl groups are created on the cellulose chains. These groups are responsible for the rapid depolymerization of the chains during a subsequent peroxide stage (P). Sodium borohydride (R stage) reduces these groups very efficiently and makes the cellulose quite stable during a P stage. The best conditions in the R stage have been determined and are very mild (room temperature, less than 10 minutes, pH around 9). The NaBH4 charge will depend on the lignin content after Z, and on the ozone consumed during Z: it can be as low as 0.05 to 0.1% on pulp.