Kinetics of the First Chlorine Dioxide Bleaching Stage (D1) of a Hardwood Kraft Pulp

In the present work, kinetic studies were performed to investigate the first chlorine dioxide bleaching stage of a Eucalyptus globulus kraft pulp. The bleaching reaction can be described by an initial short but very fast step, followed by a slower step. The proposed model for the D 1 stage is based on both the light absorption coefficient and the chlorine dioxide concentration. A set of two depletion factors for the chlorine dioxide concentration and for the light absorption coefficient was described as a function of temperature for the fast and short steps of the reaction. The longer and slower periods were described by a homogeneous model. A nonlinear relationship between the chlorine dioxide consumption and the decrease in the light absorption coefficient, which is dependent on the extent of delignification but independent of temperature, was also established. Moreover, a linear relationship between the upper brightness limit and the temperature was observed. The fit of the experimental results obtained for different temperatures, initial chlorine dioxide concentrations, and initial light absorption coefficients is very good, revealing the ability of the model to predict typical mill operating conditions.