The Effect of Process Variables in Chlorine Dioxide Prebleaching of Birch Kraft Pulp. Part 1. Inorganic Chlorine Compounds, Kappa Number, Lignin, and Hexenuronic Acid Content.


This study investigated the effects of temperature, chlorine dioxide dosage, and filtrate circulation on various filtrate and pulp properties in chlorine dioxide prebleaching of birch kraft pulp. Also the effect of a preceding A-stage was examined. Kappa number, delignification, and hexenuronic acid removal were unaffected by the used dilution media and temperature. Increasing ClO2 charge resulted in a lower kappa number, increased hexenuronic acid removal, and more efficient delignification. With a preceding A-stage the kappa number was lower throughout the D0-stage, but the kappa number reduction and the removal of hexenuronic acid were slightly decreased. Chlorine dioxide consumption was practically independent of the tested variables; only dosage had an effect. The formation of chlorate and chloride was increased with higher dosages of ClO2, but the stoichiometry remained unchanged. The concentration of chlorite + chlorous acid during bleaching depended on temperature, used dilution medium, and the presence of an A-stage.