The fundamentals of chlorine dioxide bleaching of softwood kraft pulp are studied by measuring the development of the inorganic chlorine species: chloride, hypochlorous acid, chlorite, chlorine dioxide and chlorate, as well as organically bound chlorine in the pulp and bleach solution. The pulp delignification is characterized by determining the development of the concentration of methanol and methoxyl group containing material in the bleach liquor and that of the functional groups in the residual lignin. A complete chlorine mass balance was obtained throughout the bleaching reaction. Based on the development of the chlorine containing species, a series of reactions are formulated which give a complete description of the transformation of atomic chlorine during ClO$ sb2$ bleaching. It is found that all organic chlorine, defined as the sum of AOX in the spent bleach liquor and the organically bound chlorine in the chlorinated pulp, is formed within the first 10 minutes of reaction. Subsequently, the increase in AOX in solution is matched by an equivalent decrease of organically bound chlorine in the pulp. The organic chlorine during ClO$ sb2$ bleaching is mainly formed from the reaction between lignin and hypochlorous acid. It is also shown that chlorate is mainly formed by the reaction of two intermediates generated during chlorine dioxide bleaching, chlorite and hypochlorous acid. A theory is presented which explains the effect of operating variables on chlorite formation. The methoxyl group loss from softwood kraft pulp during chlorine dioxide treatment can be accounted for by the methanol produced and the methoxyl content of dissolved organic material in the bleach filtrate. It was found that ClO$ sb2$ rather than the in-situ generated hypochlorous acid is mainly responsible for the methanol formation. The experimental evidence suggests that demethylation and delignification are closely related during ClO$ sb2$ treatment of softwood kraft pulp. Based on the present experimental evidence the ClO$ sb2$ delignification mechanism presented earlier by Gierer and others is modified. Finally, two techniques are evaluated to decrease the AOX formation in a bleach plant based on respectively the removal of hypochlorous acid by reaction with chlorite, and on decreasing the dissolved lignin concentration. The former can be achieved either by mixing chlorite with pulp prior to the addition of chlorine dioxide, or by converting part of chlorine dioxide to chlorite with hydrogen peroxide. The latter is accomplished by introducing a washing step between the D and C stages of a DC bleach sequence.

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