The influence of softwood kraft cooking conditions on the brightness development in ECF bleaching

The extent to which the Kappa number of the pulp as well as the alkali profile and organic content of the cooking liquor during softwood kraft pulping influence bleaching was investigated. Kraft cooks with high and low alkali levels and organic contents in the cooking liquors were performed to produce pulps at two Kappa number levels. The bleachability was evaluated with and without oxygen delignification in a D(EO)D(EP)D-sequence and the Kappa number, brightness and light-absorption coefficient (k) was monitored. At all Kappa number levels (non-oxygen delignified similar to17.7, similar to27.6 and oxygen delignified similar to8.4, similar to13.4), the pulps cooked with high alkali level showed higher brightness (lower k/Kappa number ratio). However, it was only for the low Kappa number nonoxygen delignified pulps that this effect had an impact on the chlorine dioxide consumption to reach full brightness. In all cases the variations in organic content had little or no, effect on ECF-bleachability.