Aalto University

OF DOCTORAL DISSERTATION AALTO UNIVERSITY SCHOOL OF SCIENCE AND TECHNOLOGY P.O. BOX 11000, FI-00076 AALTO http://www.tkk.fi Author Tuula Lehtimaa Name of the dissertation Reactions of Chlorine (III) and Their Kinetics in the Chlorine Dioxide Bleaching of Kraft Pulps Manuscript submitted February 9, 2010 Manuscript revised May 12, 2010 Date of the defence June 21, 2010 Monograph Article dissertation (summary + original articles) Faculty Faculty of Chemistry and Material Sciences Department Department of Forest Products Technology Field of research Forest Products Chemistry Opponent(s) Professor Dominique Lachenal Supervisor Professor Tapani Vuorinen Instructor Professor Tapani Vuorinen Abstract The reactions of Cl(III) were investigated utilizing iodometric titration in combination with DMSO and EDTA to prevent undesired side reactions. Among a large group of suggested Cl(III) decomposition reactions, two reactions were found to be adequate to describe the Cl(III) decomposition in the absence of metals and HOCl. The rate parameters (k – rate coefficient, Ea – activation energy) were determined for these reactions. The reaction between Cl(III) and HOCl is known to start with the formation of a Cl2O2 intermediate. The Cl2O2 intermediate was found to be formed through both, hypochlorous acid and chlorine. The kinetic parameters were obtained for the reaction between chlorous acid and hypochlorous acid. The reaction rate between chlorine and chlorite was found to be restricted by the kinetics of the HOCl/Cl2 equilibrium. The extremely fast Cl2O2 consuming reactions were identified among a group of suggested reactions and their relative rates were determined. Kinetic parameters were identified for several aldehydes with Cl(III). Aldehyde groups were shown to be present in kraft pulps and a considerable amount of aldehydes was found to be formed during chlorine dioxide bleaching. These aldehydes seemed to originate both from carbohydrates and lignin and the amount was dependent on the ClO2 dosage used. According to the results obtained, it was concluded that the reaction between Cl(III) and aldehydes may contribute substantially to the overall Cl(III) consumption during chlorine dioxide bleaching. Chlorine dioxide prebleaching (D0-stage) experiments were conducted to gain more knowledge of the rate of the different reactions and on the time-dependent behavior of various components. Kappa number reduction, hexenuronic acid depletion, chlorination and chlorine dioxide consumption were all found to be fast reactions, with almost everything occurring during the first minute of bleaching. Cl(III) was an exception. The amount of Cl(III) first increased rapidly and then decreased slowly.