Decreased Brightness Reversion with Powerful Ozone Bleaching, 2002 International Pulp Bleaching Conference Proceedings

Maria Wennerström

The focus on brightness reversion has increased lately due to the use of new cooking methods and bleaching sequences, but also because of customer demands for higher brightness. One cause of brightness reversion is the hexenuronic acids, HexA, in the pulp. Ozone reacts very selectively with HexA, and therefore eucalyptus pulps with a high HexA content are especially well suited for bleaching with ozone. Comparing ozone bleaching with hot chlorine dioxide bleaching (D/A) in prebleaching shows that the ozone stage is more efficient than the D/A stage for reducing the HexA content.

The use of an ozone stage for lower brightness reversion by removal of HexA is most efficient when ozone is used in prebleaching. As ozone bleaching form carbonyl groups in the pulp, which also cause brightness reversion, it is important to establish a bleaching sequence that decomposes these groups. Efficient final bleaching alternatives are, for example, (Dn)D or DP.