Two Stage Bleaching with Chlorine Dioxide and Mixtures of Chlorine/Chlorine Dioxide in the D Stage, 1990 Pulping Conference Proceedings

A process to improve the efficiency of chlorine dioxide (C102) bleaching was previously developed to produce pulps with higher brightness by using a two stage process. The process involved reacting the chlorine dioxide with pulp at a pH of 5 - 10 for 5 to 15 minutes (optimum pH is 6 to 7.5), and then subsequently acidifying the mixture to a pH between 4.2 to 1.9 (optimum is pH 3.8). Recent data on (DC) (EO)D, D(EO)D, and O(DC) (EO)D sequences indicate that up to 40 % chlorine dioxide in the final D stage can be saved using the high/low pH scheme.

In addition, it has been found that chlorine can be substituted for chlorine dioxide in the D stage of the bleaching process to decrease chlorine dioxide usage and lower the pH of the slurry, eliminating the need for acid addition. Results indicate that chlorine can be substituted for up to 60 % of the total available charge on pulp. Optimum conditions are found using all of the chlorine dioxide and 20 % of the chlorine at a high pH between 7.5 and 9 for 5 to 10 minutes. Subsequent chlorine addition decreases the pH to about 2 where the reaction time is kept up to 2.5 hours. Brightness, viscosity, and AOX results are given.