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The influence of cooking conditions on the bleachability and chemical structure of kraft pulps
KTH, Superseded Departments, Pulp and Paper Technology.
KTH, Superseded Departments, Pulp and Paper Technology.
KTH, Superseded Departments, Pulp and Paper Technology.
1999 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 14, no 1, 71-81 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this investigation was primarily td investigate how variations in cooking conditions in the kraft pulping of softwood influence the subsequent bleaching, and secondly to study the relationship between the bleaching response and the chemical structure of the pulp. The cooking variables studied were hydroxide ion concentration, hydrogen sulfide ion concentration and cooking temperature. The pulps had the same kappa number after the cook, about 20, and were oxygen delignified to about kappa number 8 before bleaching. The influence of the cooking variables on the TCF-bleachability was studied in an AZQP*- and in a QPQP*-sequence (A=acid treatment, Z=ozone stage, Q=chelating agent stage, P*=peroxide stage with the addition of magnesium ions). All three cooking variables studied influenced the TCF-bleachability, but to different extents. The bleachability was improved by increased temperature for low chemical charges, but not at higher chemical charges. When [HS-] was increased the QPQP*-bleachability was improved but the AZQP*-bleachability was not affected. When [HO-] was varied a bleachability maximum was seen for the intermediate hydroxide ion concentration. The content of hexenuronic acid in the pulp after cooking could be reduced by using high initial [HO-], low initial [HS-] and a long cooking time. A high content of beta-O-4 structures in the unbleached residual lignin was found to contribute to a better bleachability of the pulp. However, the phenolic hydroxyl content could not be related to the bleaching response.

Place, publisher, year, edition, pages
1999. Vol. 14, no 1, 71-81 p.
Keyword [en]
bleachability, chlorine-free bleaching, softwoods, kraft pulps, hydrogen sulfide ion, hydroxide ion, temperature, beta-O-4, hexenuronic acid, chemical structures, HEXENEURONIC ACID GROUPS, TCF-BLEACHABILITY, LIGNINS, XYLAN
National Category
Materials Engineering Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-13904ISI: 000079757500009OAI: oai:DiVA.org:kth-13904DiVA: diva2:328074
Note
QC 20100701Available from: 2010-07-01 Created: 2010-07-01 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Some aspects on TCF-bleachability of softwood alkaline pulps
Open this publication in new window or tab >>Some aspects on TCF-bleachability of softwood alkaline pulps
1999 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The bleachability of softwood oxygen delignified pulps has been studied, i.e. the ease (requirement of bleaching chemicals) with which the pulps can be brightened to a target brightness in totally chlorine free (TCF) bleaching stages, including hydrogen peroxide, ozone and chelating agent stages. Different cooking processes have been compared and the influence of different pre- and post-treatments on the kraft process has been investigated. The influence of different cooking parameters in the kraft cook on the bleachability has also been studied, as well as the influence on the bleachability of the kappa number of the pulp after the cook and after the oxygen stage.

Pulps produced by alkaline sulfite processes, ASAM and MSSAQ, showed better bleachability and process selectivity(viscosity at a given brightness) than pulps produced by the modified kraft process. The bleachability of the modified kraftpulps could be improved by a post sulfonation.

The bleachability was improved by terminating the cook at a higher kappa number level, when oxygen delignifying to the same kappa number level before bleaching. Improvements are also achieved by starting the oxygen stage from the same kappa number and extending the oxygen delignification to a lower kappa number level. The process selectivity and the yield are improved in the same way.

The QPQP*-bleachability (P*=peroxide stage with the addition of magnesium ions) was improved by changing the cooking conditions in a kraft cook leading to a shorter cooking time, i.e. by increasing the hydroxide ion concentration, the hydrogen sulfide ion concentration, or the cooking temperature or by decreasing the sodium ion concentration. Exceptions could be seen for very high [HO-], where the bleachability even deteriorated, and when the temperature was increased at very high chemical charges, where no more improvement was achieved.

The pulp with the best QPQP*-bleachability in a series of pulps had a lower light absorption coefficient (k)/lignin content (kappa number corrected for the hexenuronic acid contribution (kappa**)) ratio already after the cook as well as after the oxygen stage and a higherβ-O-4 content after the cook. A decreasedk/kappa** ratio after the cook is most probably due to less redeposition of dark dissolved organic material from the black liquor when the residual hydroxide ion concentration is increased. The degree of delignification in a subsequent oxygenstage becomes lower for the pulp with a lowerk/kappa** ratio after the cook and the metal ion content in the pulps was lower. Additives like, for example sulfite reduce the redeposition of lignin and lead to a brighter pulp.

The changes leading to improved bleachability for a pulp are not always followed by improved yield and process selectivity. The hydrogen sulfide ion concentration and the sodium ion concentration influenced the yield and selectivity positively in the same way as the bleachability. Increased hydroxide ion concentration and temperature, however, lead to lower yield andpoorer selectivity. Thus for an optimal product a compromise between a good bleachability and high yield and good strength properties must be chosen.

 

Place, publisher, year, edition, pages
Stockholm: KTH, 1999. 69 p.
Series
Trita-PMT, 1999:9
Keyword
Bleachability, chlorine-free bleaching, softwoods, kraft pulps, alkaline sulfite pulps, oxygen, hydrogen peroxide, ozone, hydrogen sulfide ion, hydroxide ion, temperature, sodium ion.
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-2801 (URN)99-2962708-1 (ISBN)
Public defence
1999-05-27, 00:00
Note
QC 20100701Available from: 2000-01-01 Created: 2000-01-01 Last updated: 2010-07-01Bibliographically approved
2. 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.
Series
TRITA-FPT-Report, ISSN 1652-2443 ; 2006.39
Keyword
Delignification, Kraft pulping, Residual phase lignin, Hydroxide, Hydrogen sulphide ion, Ionic strength, Temperature, Bleachability, Hexenuronic acid, Carbohydrates
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-4232 (URN)
Public defence
2006-12-15, Sal F3, KTH, Lindstedtsvägen 26, Stockholm, 14:00
Opponent
Supervisors
Note

QC 20100825

Available from: 2006-12-13 Created: 2006-12-13 Last updated: 2016-12-21Bibliographically approved

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