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Kraft cooking with varying alkali concentration: Influence on TCF-bleachability
KTH, Superseded Departments, Pulp and Paper Technology.
1998 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, Vol. 13, no 1, 57-63 p.Article in journal (Refereed) Published
Abstract [en]

The TCF-bleachability of softwood pulps produced by kraft pulping processes with varying alkali concentrations was investigated. The cooks began with a hydrogen sulfide ion pretreatment at low alkalinity, i.e. simulating a black liquor preimpregnation. The alkalinity in the cooks was varied in three different ways: already at the beginning of the cook, both at 40 % sulfidity and at a HS- concentration of 0.3 mol/l (2-stage cooks), and in the final part of a modified kraft cook (4-stage cooks) simulating an ITC (Isothermal cooking) cook. The pulps had the same kappa number after the cook, about 17, and were oxygen delignified before bleaching. The influence of the alkali concentration profile in the cook on the bleachability was studied with a QPQP*-sequence (Q=chelating agent stage, P*=peroxide-stage with the addition of 0.05 % Mg ions). The bleachability (pulp brightness achieved for a given consumption of bleaching agent per kappa number) was found to reach a maximum for pulps with a residual alkali concentration in the vicinity of 0.5 mol/l. The bleachability was however affected to only a minor extent by the alkali concentration during the cook. The hexeneuronic acid content was found to decrease with increasing alkali concentration and this was taken into consideration when calculating the chemical consumption per amount of lignin. The process selectivity (pulp viscosity at a given brightness) decreased with increasing residual alkali. The bleachability was not improved by conducting the cook with a levelled out active cooking chemical profile, when the comparison was made at the same residual alkali concentration, but the selectivity was improved.

Place, publisher, year, edition, pages
1998. Vol. 13, no 1, 57-63 p.
Keyword [en]
kraft pulp, softwood, chlorine-free bleaching, bleachability, effective alkali, oxygen, hydrogen peroxide, pulp brightness, pulp viscosity, hexenuronic acid, PULPS
National Category
Paper, Pulp and Fiber Technology Materials Engineering
URN: urn:nbn:se:kth:diva-13902ISI: 000075150100008OAI: diva2:328079
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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