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A comparison of bleachability in TCF sequences for alkaline sulphite and kraft pulps
KTH, Superseded Departments, Pulp and Paper Technology.
KTH, Superseded Departments, Pulp and Paper Technology.
1996 (English)In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 22, no 8, J296-J300 p.Article in journal (Refereed) Published
Abstract [en]

The bleachability in different totally chlorine-free (TCF) bleaching sequences was investigated for two types of alkaline sulphite pulps, ASAM and MSSAQ, and compared with that of pulps produced by different new and industrially applied modified kraft pulping processes. The TCF sequences included oxygen, ozone, peroxide and complexing agent stages. The alkaline sulphite pulps were found to be easier to bleach in terms of the consumption of oxidative agents required to reach a given brightness than the pulps produced by the different modified kraft pulping processes. The relationship between pulp brightness and pulp viscosity was also much better for the sulphite pulps, although the new types of kraft pulps are gradually approaching the level of the alkaline sulphite pulps in this respect. There was no significant difference between the ASAM (alkaline sulphite anthraquinone methanol) and the two-stage MSSAQ (minisulphide sulphite anthraquinone) processes.

Place, publisher, year, edition, pages
1996. Vol. 22, no 8, J296-J300 p.
Keyword [en]
kraft pulps, oxygen, delignification, chlorine free bleaching, multistage process, selectivity, brightness, viscosity, softwoods, ozone, hydrogen peroxide, alkaline pulps, sulfite pulps
National Category
Paper, Pulp and Fiber Technology Materials Engineering
URN: urn:nbn:se:kth:diva-13899ISI: A1996VD41400007OAI: diva2:328085
QC 20100701Available from: 2010-07-01 Created: 2010-07-01 Last updated: 2010-07-01Bibliographically 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.
Trita-PMT, 1999:9
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
urn:nbn:se:kth:diva-2801 (URN)99-2962708-1 (ISBN)
Public defence
1999-05-27, 00:00
QC 20100701Available from: 2000-01-01 Created: 2000-01-01 Last updated: 2010-07-01Bibliographically approved

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