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On the nature of residual lignin
KTH, Superseded Departments, Fibre and Polymer Technology.ORCID iD: 0000-0002-2900-4713
KTH, Superseded Departments, Fibre and Polymer Technology.
2004 (English)In: Cellulose chem. techmol., ISSN 0576-9787, Vol. 35, 321-331 p.Article in journal (Refereed) Published
Abstract [en]

In a recent study, the results of oxygen delignification trials on kraft, prehydrolysis kraft and magnesium sulphite pulps were compared, thus providing the basis for a theory on the nature of the lignin residual lignin remaining in pulp after cooking, and particularly after oxygen delignification, the so-called residual lignin. It is suggested that, at least after oxygen delignification, the residual lignin effectively consists of LCC-bound monolignins, a theory relating to the efficiency of oxygen delignification and bleaching, and indicating possible strategies for improving the efficiency of such processes. An efficient means of breaking the LCC bonds would lead to a more efficient bleaching, known as requiring either a complete oxidation of the LCC-bound monolignins or a peeling off the carbohydrate binding to the LCC. In any case, a mere breaking of one unsaturated bond in an aromatic ring is not sufficient to achieve depolymerisation of the residual lignin or an efficient kappa reduction.

Place, publisher, year, edition, pages
2004. Vol. 35, 321-331 p.
Keyword [en]
residual lignin; lignin-carbohydrate complex (LCC); oxygen delignification; bleaching; oxygen; chlorine dioxide; ozone; hydrogen peroxide; peeling
National Category
Materials Engineering
URN: urn:nbn:se:kth:diva-6619ISI: 000229625200002ScopusID: 2-s2.0-18944405373OAI: diva2:11376
QC 20100825 QC 20110923Available from: 2006-12-13 Created: 2006-12-13 Last updated: 2011-09-23Bibliographically approved
In thesis
1. On the Interrelation Between Kraft Cooking Conditions and Pulp Composition
Open this publication in new window or tab >>On the Interrelation Between Kraft Cooking Conditions and Pulp Composition
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

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.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 89 p.
Trita-FPT-Report, ISSN 1652-2443 ; 2006.39
Delignification, Kraft pulping, Residual phase lignin, Hydroxide, Hydrogen sulphide ion, Ionic strength, Temperature, Bleachability, Hexenuronic acid, Carbohydrates
National Category
Paper, Pulp and Fiber Technology
urn:nbn:se:kth:diva-4232 (URN)
Public defence
2006-12-15, Sal F3, KTH, Lindstedtsvägen 26, Stockholm, 14:00
QC 20100825Available from: 2006-12-13 Created: 2006-12-13 Last updated: 2012-03-16Bibliographically approved

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