Generalized steady-state model for chlorine dioxide brightening of hardwood kraft pulps

Abstract

A generalized, steady-state model for the first chlorine dioxide brightening (D1) stage is proposed for North American hardwood kraft pulps. The model predicts the post-D1 brightness obtained for a given chlorine dioxide (ClO2) charge based on three equation parameters. Data from laboratory D1 studies were examined with the model to determine the relationships of the parameters to extracted pulp properties. The asymptotic D1 brightness limit at extremely high bleach charges was ~93% ISO for most hardwood pulps. This parameter can be calculated from the limiting light absorption coefficient of D1 kinetic studies (≈ 0.10 m2/kg) and the unbeaten light scattering coefficient of the pulp. A second parameter was linearly correlated to the extracted kappa number of the hardwood pulp. This relationship seems to be affected by the chosen kraft pulping process (e.g., liquor or dissolved solids profiling). The final parameter is the extracted pulp brightness. The hardwood D1 model forecasted the brightness values of laboratory pulps that differ from measured values of ±0.5 to ±0.6 points. Daily averages of bleach plant data for a southeastern U.S. hardwood fiber line were examined with the model. Predictions were generally 0.5 points higher than the measured mill brightness values. This observation implied that the D0(EOP) pulp contained a small amount of extraction washer carryover (~0.4 kappa units on pulp) that was consuming ~1.2 kg ClO2/ton pulp, which was not accounted for in the hardwood D1 model.