Brightness reversion and brightness loss in fully-bleached kraft pulp: a case study

The mill at La Tuque has experienced a significant brightness loss problem with its bleached softwood kraft pulp There were two phenomena contributing to the problem: a brightness loss from deposition of a dark complex onto t he sheet and a true chemical reversion. Our investigation of the white-wate r showed that this brightness loss originated from a complex formed from ro sin size, lignosulphonate and the blue dye that then deposits on the sheet. Our investigation of the brightness reversion allowed us to identify carbo nyl groups as the source. Borohydride reduction or hydrogen pet-oxide oxida tion of these group, under conditions that can be applied in the last reten tion tower, successfully increases brightness and decreases reversion.