Brightness and kappa number - important variables to secure appropriate control of chemical charges in TCF- and ECF-bleaching sequences

Different combinations of kraft cooking and oxygen delignification stages w ere studied, with respect to the quality as well as the economics of the fu lly bleached pulp. A brighter pulp at the same kappa number and viscosity c an be achieved after oxygen delignification if the cook is ended at higher kappa number levels (30 instead of 17). When using the investigated TCF-ble aching sequence, this is a major advantage, since the bleachability in a Q- OP-Q-PO-sequence is shown to be dependent on the brightness rather than the kappa number of the pulp entering the bleach plant. Considering the econom ics of the different alternatives studied, the yield as well as the bleachi ng chemical costs of the fully bleached pulp were taken into account. The c ombination of cooking to kappa number 30 instead of 17 followed by oxygen d elignification to kappa number 9 results in a cost effective pulp of high q uality regarding both ECF- and TCF-bleaching. However, for the TCF-sequence it is important to keep in mind, that the yellowing tendency (heat induced ), of the fully bleached pulp increases with increasing kappa number after cooking, probably due to a higher hexenuronic acid content in the pulp.