Extension of a steady-state chlorine dioxide brightening model for Z-ECF bleaching of softwood kraft pulps

Earlier studies developed a steady-state model to predict the brightness and/or bleach consumption during the chlorine dioxide brightening (D-1) of softwood pulps produced by conventional elemental-chlorine-free (ECF) sequences. This model relates the chlorine dioxide consumed to the brightness gains predicated upon an asymptotic D-1 brightness limit, an incoming D-1 pulp brightness, and an equation parameter (beta(11)). The current investigation examines the application of this model to ECF sequences that use ozone delignification (Z-ECF). Literature D-1 data from various Z-ECF bleaching studies, which investigated OZ, OD0/Z, and OZ/D-0 delignification, were fitted to the model. The beta(11) parameter was found to be linearly correlated to the entering kappa number. Interestingly, this linear relationship was found to be identical to the relationships observed when modeling the D-1 stage for conventional ECF and chlorine-based bleach sequences. Subtle differences in D-1 brightening response in the model among the various bleach sequences are reflected by incoming pulp brightness (at the same kappa number). The current model is used to illustrate how alterations to Z-ECF delignification affect D-1 brightening and chlorine dioxide consumption.