Delignification and Bleaching Efficiency of Different Bleaching Agents on Chemical and Mechanical Pulp Mixtures. Basic and Applied Aspects

Pulp mixtures containing mechanical and chemical pulps were prepared and treated by conventional bleaching agents used for chemical pulps. In a preliminary study, the significance of kappa number for pulp mixtures containing some mechanical pulp was validated. The relationship between lignin content and kappa number was found to be very close to the relationship observed for chemical pulp. A mixture of pulps containing 20% unbleached TMP and 80% bleached chemical pulp was delignified with D, Z and O treatments. Efficiencies of ClO2 and O3 were good even though the extent of delignification was in both cases lower than for a kraft pulp of the same kappa number. O2 delignification was very poor with only 20% delignification in comparison to 50% for a kraft pulp. Brightness development was studied in the case of O, P and (OP) stages applied on a similar blend containing 30% TMP. (OP) was not significantly better than P, whereas O alone was very detrimental to the brightness of the blend containing mechanical pulp. Differences in lignin structures between TMP and kraft pulp may explain these results.