Effects of Pulping Conditions on the Bleachability of Hardwood Kraft Pulps: 1. Effects of Effective Alkali Charge in the Pulping of Birch and Maple, 1998 Pulping Conference Proceedings

The objective of this work was to study the effects of digester alkali charge on the chlorine dioxide bleachability of hardwood kraft pulps. We prepared birch and maple pulps with unbleached kappa numbers in the 19-20 range at two effective alkali (EA) levels, 14% and 20% (Na2O). These were bleached in the D0(EO)D1ED2 sequence. A kappa factor of 0.2 was used throughout and various combinations of ClO2 charges in D1 and D2 stages were investigated. The response to ClO2 in the D1 and D2 stages was characterized in terms of the values of the parameters in a simple mathematical model, which were then subjected to statistical analysis. The result was a model that describes and predicts the effect of EA on ECF bleachability.

In the case of birch, increasing EA gave an unbleached pulp of higher brightness, lower total yield, higher rejects and lower viscosity. The high-EA maple pulp, on the other hand, had much higher brightness than its low-EA counterpart but similar yield, rejects and viscosity levels. Increasing EA substantially increased the brightness ceiling of both the hardwood species. After the D1 stage, the brightnesses of both high-EA pulps were about 5 points higher than those of their low-EA counterparts and the brightness advantage of maple over birch was maintained. It was possible to prepare pulps of 93% ISO brightness in three stages from high-EA maple pulp. After the 5-stage sequence, the high- and low- EA pulps exhibited brightness ceilings of 94% ISO and 92% ISO, respectively. The corresponding birch pulps had brightness ceilings close to 90% ISO, and brightnesses near the ceiling were reached with lower consumption of ClO2 in the case of high-EA pulp.

At moderate levels of ClO2 consumption, increasing EA allowed substantially higher brightness to be reached after three stages of bleaching in the case of maple, and after five stages in case of birch. Much of the beneficial effect of increasing EA can be attributed to an unbleached pulp brightness advantage that was retained through the bleaching sequence, except in the case of five-stage bleaching of birch with high ClO2 charges.