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Summary

Near-neutral chlorine dioxide brightening (NNB), a technology proven to improve the performance of the final chlorine dioxide stage in the bleaching of chemical pulps, has been implemented in several pulp mills in North America. This paper presents the results of another successful NNB application at the Domtar kraft mill in Johnsonburg, Pennsylvania. The implementation’s results showed that the historical brightness loss of as much as 0.8% ISO unit has been eliminated in the HD tower due to the persistent brightening effect of chlorite (NaClO2), which is the brightening agent generated in the NNB process. The NNB implementation at this mill resulted in an average savings of 3 to 4 lb chlorine dioxide per ton of pulp while maintaining the target final pulp brightness. The elimination of gas off issues at the D2 washer, realized by maintaining a higher pH (greater than five) at the end of the D2 stage, offered an additional safety benefit. A composite D2 ClO2 dosage control equation, specifically designed for this mill, was also developed which, in conjunction with NNB technology, improved D2 final pulp brightness stability to the HD tower. The implementation of this process requires very little capital expenditures (small piping, valves, pressure gauge and flow meter), making it an attractive technology to many fully-bleached chemical pulp mills.

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