Inorganic Reactions in Chlorine Dioxide Bleaching of Softwood Kraft Pulp, 1999 Pulping Conference Proceedings

Douglas R. Svenson, Hasan Jameel, Hou-min Chang & John F Kadla

Department of Wood and Paper Science

North Carolina State University

Pre-bleaching a 27 kappa softwood kraft pulp with chlorine dioxide was performed to various end pH levels. During high pH bleaching, the majority of bleaching chemicals was lost to chlorite formation. The rapid oxidation of the phenolic lignin results in the formation of chlorite and hypochlorous acid. But the oxidation of non-phenolic lignin at high pH forms only chlorite. Without the presence of the hypochlorous acid formed, due to the reaction with the non-phenolic, no more chlorine dioxide is generated to further react with lignin and the residual chlorite. At low pH, the reaction of the non-phenolic lignin with chlorine dioxide formed both chlorite and hypochlorous acid. Hypochlorous acid further reacted with chlorite to regenerate chlorine dioxide, which further reacted with the lignin. Chlorate formation increased with decreasing end pH during chlorine dioxide bleaching. At high pH, chlorate formed through radical quenching by chlorine dioxide. At low pH, chlorate formation resulted from chlorous acid disproportionation. Increases in chlorate levels are not very significant at end pH below 3.4. The amount of residual chlorite + chlorate decreases with end pH until about 3.4. Below end pH 3.4, the quantity of residual chlorite + chlorate levels out. This is because no residual chlorite remained and residual chlorate was relatively constant at end pH from 3.4 to 1.8. Brightness and permanganate number trends correlate with the amount of residual chlorite + chlorate. At a low pH more reaction with hypochlorous acid and lignin occurs resulting in the formation of more organic chlorine. When pre-bleaching with chlorine dioxide, more flexibility is allowed with end pH (3.5-1.5) compared to a D1 bleaching stage where the optimum pH is about 3.5.