BLEACHING MIXTURES OF HARDWOOD AND SOFTWOOD KRAFT PULPS

In separate bleaching of selected hardwood and softwood kraft pulps with a CED sequence, the initial pulp Kappa number had a greater influence on the bleaching response than did species. Species did, however, influence the final brightness plateau and hardwood pulps show a 2 to 3 unit higher brightness than their softwood counterparts when compared at the same first-stage chlorine multiple (CM). In mixed bleaching of hardwood and softwood kraft pulps in a 1:1 ratio with a CED sequence, the hardwood and softwood lignins at equal Kappa number appear to be equally reactive to chlorine and, as the Kappa number decreases, the residual lignins of both species become more reactive to chlorine. Evidence suggests that chlorinated aspen lignin was more readily extracted by alkali than chlorinated spruce lignin. In most cases, when hardwood and softwood kraft pulps are bleached as a mixture, the gap in the brightness plateaus of the two species is wider than that for separate bleaching.