Effect of Brownstock Washing, Split Addition of Chlorine, and pH Control in the C-Stage on Formation of AOX and Chlorophenols During Bleaching, 1992 Environmental Conference Proceedings

The effectiveness of brownstock washing has an impact on the chemical recovery cycle, on bleaching chemical costs, and on environmental parameters associated with bleach plant effluents. Improved brownstock washing has been shown to decrease the amount of dissolved organic material carried over to the bleach plant and reduce the amount of chlorinated organics produced during subsequent bleaching operations. Laboratory studies have quantified the impact of carryover on formation of TCDD/Fs, AOX, and tetrachlorinated phenols. Splitting the chlorine and chlorine dioxide in the first bleaching stage into several smaller charges and controlling the C-stage pH to higher than normal levels were shown to lower dramatically the formation of all highly chlorinated phenolics as well as PCDD/Fs, even as the carryover increased. Formation of AOX was lowered by 15-30% by using this technique with preferential reduction of higher levels of chlorination. Methods to improve brownstock washing and reduce carryover also are discussed briefly.