Studies of Chloride Dioxide Delgnification: Vapor Phase Bleaching of Hardwood and Softwood Kraft Pulps, 1998 Environmental Conference Proceedings

Laboratory vapor phase ClO2 delignification experiments were performed on hardwood pulps made from sweetgum by laboratory simulations of both conventional and RDH kraft pulping. A smaller number of experiments were conducted on pulps made from southern pine by laboratory simulations of conventional kraft pulping. All experiments consisted of D0 (EO) bleaching followed by determinations of kappa number and analysis of the individual stage filtrates for AOX, as well as chloride, chlorate and chlorite ions. For comparison, control experiments were done in which the CIO2 was applied at low consistency, in the conventional manner. In bleaching the hardwood pulps, vapor phase delignification was remarkably more efficient, but generated markedly larger quantities of AOX than the conventional process, which itself generated much more AOX than expected on the basis of earlier softwood pulp bleaching data. These observations, together with analyses of the bleaching filtrates, are consistent wit the hypothesis that ClO2 partially decomposes to Cl, in the vapor phase before reacting with the pulp. In the case of southern pine kraft pulps, there was a much smaller improvement in delignification efficiency in going from low consistency to vapor phase bleaching. Qualitatively similar observations were made with regard to the softwood pulps, but the trends were generally less pronounced than in the case of the hardwood pulps. Differences in the structure of hardwood pulp residual lignin after conventional and vapor phase bleaching were slight. This may indicate that the superior bleaching efficiency of the vapor phase process is due to diminished importance of reactions that occur between active chlorine compounds and dissolved organic byproducts.