Formation of chlorinated organic material and chlorate during chlorine dioxide prebleaching of kraft pulp: Effects of sodium chloride, charge of chlorine dioxide and pH

The presence of sodium chloride in the chlorine dioxide stage affects the formation of chlorinated organic material (AOX) as found in spent bleach liquor from a D(EP) bleaching sequence of oxygen-predelignified kraft pulp. It is shown in this work that the effect of chloride to a large extent interacts with the effect of the amount of charged chlorine dioxide. At a high charge of chlorine dioxide, AOX tends to decrease but, at lower charges, AOX tends instead to increase. The formation of chlorinated organic material in the pulp (OCl) is increased by the presence of chloride under all conditions and is decreased only by using a higher end pH (pH 4). The chlorate formation is decreased by the addition of chloride and also by a higher end pH. The mechanisms of these effects of chloride and pH on AOX, OCl and chlorate formation are complex and involve several reactions which are discussed.