Total Chlorine Dioxide Substitution in Short Sequence Bleaching and Its Effect on Pulp Brightness and Effluent Characteristics, 1993 Pulping Conference Proceedings

This paper discusses the results of the optimization of process variables to switch to 100% chlorine dioxide substitution in a short sequence (C+D)(EOP)D bleaching. The process parameters employed at each stage are critically examined to evaluate the changes required for 90 %ISO brightness pulps with less than 2.0 points reversion. The parameters that effect the pulp brightness and effluent characteristics are, the black liquor carry-over with the brown stock, low consistency first-stage, and low temperature Dl-stage. Changing to the total substitution of chlorine dioxide at the first-stage would increase the chemical cost by as much as 35%. Such a switch would be expensive particularly in a short-sequence bleach plant attempting to bleach conventional HW and SW pulps to 90 %ISO brightness. Post-peroxide bleaching of the final D-stage pulp may be necessary to reach the target 9O%ISO brightness. For a low consistency first-stage, the operation of alkaline extraction under optimum conditions becomes the only conduit to decrease the chlorine dioxide demand at the first-and the final D-stage. Hydrogen peroxide reinforced oxidative extraction (EOP) at 95°C results in low DE-Kappa number to Dl-stage. This helps to decrease the chlorine dioxide charge at the final stage to reach the target brightness. The savings in hydrogen peroxide realized from the hot alkaline extraction can be helpful in the post-peroxide treatment of bleached pulp. This would provide not only higher pulp brightness but also stable brightness. It is recommended that the D-stage bleaching be stopped at 87 to 88 %ISO brightness and the balance in brightness be gained by hydrogen peroxide bleaching in the HD storage chest (Post-Peroxide Treatment). Achieving 90 %ISO brightness pulps by post-peroxide treatment of Dl-pulps should limit the brightness reversion to less than 2.0 points. The single biggest advantage in switching to 100% chlorine dioxide substitution at the first-stage is the decrease in AOX emission from the bleach plant by 75%.