Tertiary Amines in Catalytic Pulp Bleaching

The motif of the current study arises from the importance of the paper in Finland as one of the core industries. The latest released statistics reported by the Confederation of European Paper Industries (CEPI) confirms that Finland, by producing 28.5% of pulp production, possesses the second rank in terms of the production of pulp among all members of the CEPI. Pulp bleaching as one of the subcategories of the tremendous paper industries aim at producing high quality paper with three fundamental goals: the pulp maintains its color over the time; it does not revert color by exposing to the sunlight; and retains its strength. With these goals, sodium hypochlorite bleaching was initiated in the 18th century which was later replaced by chlorine in 1930s. Environmental concerns prompted this industry to opt the Elemental Chlorine Free (ECF) and the Totally Chlorine Free (TCF) bleaching processes, which alleviate the concerns about the release of organochlorine compounds. Nowadays, the ECF with chlorine dioxide is the dominant technology worldwide. Consequently, in this thesis we present a couple of catalysts which introduce a significant reduction in the retention time of the ECF bleaching processes and concurrently bring in a considerable decrease in the chemical consumption. In modern ECF hardwood pulp mills the bleaching sequences (A/D-EOP-D-P or D/A-EOP-D-P) initiate with a long, hot acidic stage which a primary target is to remove most of hexenuronic acid in the pulp. Therefore, active chlorine can be consumed from the traditional 40 kg/t to the level of 20-25 kg/t. The whole bleaching sequence lasts typically 4-5 h. The nucleophilic nature of hypochlorous acid formed in situ in chlorine dioxide bleaching leads to the secondary reactions of hexenuronic acid. Indeed, the nucleophilicity of hypochlorous acid could be enhanced through formation of a highly reactive, electrophilic quaternary chloroammonium cation in the presence of a tertiary amine. After discovering a catalyst, triethylenediamine that is both an industrially used and stable enough chemical, catalytic bleaching (Hcat), utilizing hypochlorite (H), triethylenediamine (DABCO) and its derivative N-carboxymethyltriethylenediamine (CM-DABCO), has the potential to improve the chemical and energetic efficiency of bleaching processes in chemical pulp mills, e.g., through reducing the reaction time of the bleaching processes. Indeed, this thesis studied to clarify if new kraft pulp bleaching sequences with not only Hcat as the first stage (HcatZP and HcatZ/DP) but also as an initial stage of chlorine dioxide (D) and an intermediate stage of Hcat (D0EHcatP) could provide fully bleached pulps. The bleaching sequences of the studied eucalyptus pulps include HcatZP, HcatZ/DP and D0EHcatP which attained a final brightness of 88, 89, and 88% ISO, respectively. In this thesis, it was shown that the HcatZP bleached pulps had low carbonyl group content. Interestingly it was shown that cellulose is not damaged at all in the ozone stage that typically depolymerizes cellulose. In the last part of the thesis, the efficiency and stability of the applied catalysts were studied which made the CM-DABCO as the supreme catalyst. Thus, it can already be claimed that production of almost fully bleached pulp with an exceptionally efficient system is feasible. Softwood pulp bleaching technology has not undertaken major developments in comparison with hardwood pulp bleaching during the last generation. The reason is the content of hexenuronic acid in softwood pulps is relatively low which makes the use of the long, hot acid treatment uneconomic. The catalytic bleaching offers here a solution which extracts hexenuronic acid very fast without depolymerizing cellulose.