ECF Bleaching of Hardwood Pulps: Evaluation of Oxidant-Reinforced Extraction Variables on Overall Bleaching Optimization, 2010 TAPPI PEERS Conference

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This investigation evaluates how higher reaction temperatures and/or oxidant reinforcement of caustic extraction affect chlorine dioxide consumption during the ECF bleaching of hardwood pulps. A review of the available scientific literature is presented and the data obtained was critically analyzed. The addition of peroxide to an (EO)-stage roughly displaces an equal amount of ClO2 (by weight). Increasing the (EO) temperature from 70 to 90°C lowers the ClO2 usage by ≤0.5% on pulp. Unlike what is observed for ECF softwood bleaching, the presented findings suggest that aggressive oxidant-reinforced extraction stages do not result in significantly lower bleaching costs when compared to milder alkaline extraction stages. Likewise, the optimum amount of ClO2 used in delignification versus brightening varies from 40 to 80% of the total ClO2 bleach demand, and is dependent upon the brightness target and the hardwood kappa number.