Modern manufacturing processes assume efficient utilization and recycling of natural resources whenever possible. Over the past decade paper recycling has progressed from 33.5% in 1990 to just above 48% in 2002.1 Indeed, for certain select grades, (newspaper and old corrugated containers) greater than 70% is currently being recycled. In contrast, mixed office waste and colored directory papers are often underutilized. A major difficulty in recycling these grades of paper is the problems associated with decolorizing the dyes present in the paper.2 Of the commonly used paper dyes, the stilbene dye Direct Yellow 113 and methine dye Basazol 46L are notorious4 for poor bleachability with the commonly used chemical bleaching agents including chlorine dioxide, oxygen, hydrogen peroxide and sodium dithionite. The ability of white-rot fungi to decolorize colored effluents containing textile dyes is currently the subject of intensive research efforts. The secreted enzymes involved in dye decolorization include manganese peroxidase, lignin peroxidase and laccase. Laccase, a lignolytic enzyme, has also been studied for many years for the biobleaching of wood pulps. The ability of laccase to delignify pulp is greatly enhanced by the addition of small molecule mediators such as 2-2´ azinobis (3-ethylbenzthiazoline-6-sulfonate) (ABTS) and 1-hydroxybenzotriazole (HBT). This research project focused on applying laccase combined with a mediator to decolorize C.I. Direct Yellow 11 and Basazol 46L. Three mediators were tested: ABTS, HBT and violuric acid. Laccase/ABTS was most effective with 60% of the color being removed. The level of color removal was maintained at 60% even when ABTS concentration was lowered from 5 mM to 0.01 mM. When laccase/1 mM ABTS was applied to Direct Yellow 11 in solution, the majority of color loss occurred within 60 minutes. The ability of soybean (SBP) and horseradish (HRP) peroxidases and laccase to decolorize Direct Yellow 11 and Basazol 46L in solution was also examined. The results demonstrated that these two recalcitrant dyes could be effectively decolorized by enzymatic treatments by horseradish peroxidase, soybean peroxidase, and laccase with ABTS as mediator. SBP is effective from pH 4.5 to 8.5. The stilbene dye Direct Yellow 11 responded to both SBP and laccase/ABTS. For the methine dye Basazol 46L, SBP was a more effective treatment than HRP or laccase/ABTS. Basazol 46L responded quickly to SBP treatment with 74% reduction in signal intensity within 5 minutes. To evaluate the effectiveness of laccase/ABTS treatment, pulp dyed with Direct Yellow 11 and three commercial colored pulps were subjected to seven different bleaching treatments. These treatments consisted of 1)laccase/ABTS; 2)laccase/ABTS followed by alkaline extraction; 3)laccase/ABTS followed by bleaching with sodium dithionite; 4)oxygen bleaching; 5)oxygen bleaching followed by dithionite treatment; 6)alkaline hydrogen peroxide bleaching; and 7)alkaline peroxide bleaching followed by dithionite treatment. The best results were obtained by including reductive bleaching with sodium dithionite. For Direct Yellow 11 dyed pulp, laccase/ABTS followed by dithionite yield comparable reduction in color to oxygen or peroxide followed by dithionite.

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In the early 1990’s, a lot of work was focused on extending the kraft cook to a low lignin content (low kappa number). The driving force was the need to further reduce the environmental impact of the bleaching, as less delignification work would be needed there. However, the delignification during the residual phase of a kraft cook is very slow and, due to its poor selectivity, it is a limiting factor for the lignin removal. If the amount of lignin reacting according to the residual phase could be reduced, it would be possible to improve the selectivity of the kraft cook. In the work described in this thesis, special attention has been given to the activation energy of the slowly reacting residual phase of a kraft cook on softwood raw material and to the influence of different cooking parameters on the amount of the residual phase lignin. The activation energy of the residual phase delignification of the kraft cook was shown to be higher than that of the bulk phase delignification. In order to decrease the amount of residual phase lignin, it was essential to have a high concentration of hydrogen sulphide ions when cooking with a low hydroxide concentration. It was also important to avoid a high sodium ion concentration when cooking with low hydroxide and low hydrogen sulphide ion concentrations. Furthermore, it was demonstrated that dissolved wood components had a positive effect on the delignification rate in the bulk phase of a kraft cook. The influence of different cooking parameters in the extended softwood kraft process on the bleachability (i.e. the ease with which the pulps can be bleached to a target brightness) of the manufactured pulp was also investigated. If variations in bleachability were seen, an attempt would also be made to find chemical reasons to explain the differences. It was difficult to establish clear relationships between the chemical structures of the residual lignin and the bleachability of the pulp. However, it was seen that the higher the content of β-aryl ether structures in the residual lignin after cooking, the better was the QPQP*-bleachability. In the middle/end of the 1990’s, the focus moved from extended cooking to efficient utilisation of the wood raw material, e.g. by interrupting the kraft cook at higher kappa number levels and choosing appropriate cooking conditions to maximise the cooking yield. A high cooking yield often leads to a somewhat higher hexenuronic acid (HexA) content of the pulp at a given kappa number. Therefore additional attention was devoted to how the HexA content and carbohydrate composition were affected, e.g. by a set of cooking parameters. Performing these studies it was also important to investigate the effects of a low HexA (after cooking) strategy on such vital factors as the cooking yield, the bleachability and the yellowing characteristics of the pulp obtained. It proved to be difficult to significantly reduce the HexA content in a kraft pulp by altering the cooking conditions for both softwood and the hardwood Eucalyptus Globulus. A reduction in HexA content can be achieved by extending the cook to lower kappa numbers, or by using a high hydroxide concentration, a low hydrogen sulphide concentration or a high sodium ion concentration. However, neither of these strategies is attractive for industrial implementation since they would result in an extensive loss of yield, viscosity and strength. / <p>QC 20100825</p>

The aim of this work is to contribute to the process development of the ECF light bleaching sequence Q(OP)D(PO) by providing a better understanding of the bleaching of softwood kraft pulp. There are few published studies on this type of sequence, and more knowledge is needed on how to reduce environmental impact, improve cost-effectiveness and ensure a high pulp quality. Firstly, this research showed that using a pH buffer to stabilize the pH at a near-neutral level during bleaching with chlorine dioxide can lower the formation of adsorbable organically bound halogens (AOX) without altering the bleaching efficiency. A near-neutral pH decreases the formation of strongly chlorinating species so that the AOX content in the bleaching effluents is reduced by up to 30%. The increased pH in the near-neutral pH D stage compared with the reference lowered the chlorine dioxide consumption, resulting in a higher kappa number and viscosity. A lower degradation of hexenuronic acid correlated well with a lower AOX content in the effluents, affirming earlier theories that hexenuronic acid has an important impact on AOX formation. Secondly, this research aimed at studying bleachability in (OO)Q(OP)D(PO). The bleachability was defined as delignifying and brightness gain bleachability, which is suited to ECF light bleaching sequences. The delignifying and brightness gain bleachability were used to evaluate the stages in (OO)Q(OP)D(PO) in regard to the unbleached kappa number. When comparing different stages, it is possible to distinguish the bleaching efficiency in each stage. For this ECF light sequence, the bleachability depends on the purpose of the stage. It was found that in the stages that are mainly delignifying (OO and D), a higher unbleached kappa number is beneficial for the delignifying bleachability, although the brightness gain is not improved. However, in the stages that are mainly brightness increasing (OP and PO), the brightness gain bleachability is improved by a lower unbleached kappa number. Thirdly, the bleaching of the pulp samples in this study with the ECF light sequence was most effective with a kappa number around 32 after cooking. Although an even higher kappa number resulted in a higher yield after cooking, it seemed that this bleaching sequence cannot preserve the yield gain. Kappa number 32 also gave the best results in regard to brightness ceiling and viscosity. On the other hand, kappa number 27 was the most favourable with regard to yellowing and chemical charge. / <p>Examinator: Helena Håkansson, lektor, Karlstads universitet</p>