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Changes in light absorption coefficient spectra as a result of TCF bleaching
KTH, Superseded Departments, Pulp and Paper Technology.
1999 (English)In: Paperi ja puu, ISSN 0031-1243, Vol. 81, no 2, 130-137 p.Article in journal (Refereed) Published
Abstract [en]

Light absorption coefficient, k, spectra in the wavelength range from 200 to 700 nm have been determined for a number of modified kraft and alkaline sulfite pulps after O, OAZ and OAZQP* bleaching (O=oxygen, A=acid treatment, Z=ozone, Q=complexing agent, P*=peroxide stage with Mg addition). The different pulp types have earlier shown differences in bleachability and selectivity when bleached in an OAZQP* sequence and also in other sequences. However, the reason for the differences in bleachability are not fully understood. In this paper, the changes in k spectra after the different bleaching stages for the different pulp types are shown, and the differences are discussed, The accuracy in the determination of k is also discussed and why it is preferable to measure k rather than brightness. There were no significant differences between the shape of the k spectra for the different pulp types that could explain the known differences in bleachability between the pulp types. However, the levels of the k spectra after the oxygen stage could be correlated to the known differences in bleachability between the pulps, The k spectra for the ASAM pulp, the pulp that has shown best bleachability, was significantly lower compared to the other investigated pulps. Ozone and peroxide bleached pulps showed k spectra of approximately the same shape as the oxygen-delignified pulps. In this study, k values as high as about 10 m(2)/kg were correctly measured. At higher k values, the light scattering coefficient (s) began to deviate due to too high a light absorption. Dilution, i.e. mixing the pulp with a brighter pulp of lower and known k value so that the k value of the mixture does not exceed 10 m2/kg is a way of circumventing the problem. However, there are also some limitations with the pulp mixing method, which are discussed.

Place, publisher, year, edition, pages
1999. Vol. 81, no 2, 130-137 p.
Keyword [en]
Kubelka-Munk equation, light absorption, light scattering, softwood, kraft pulps, alkaline sulfite pulps, oxygen, azone, hydrogen peroxide, DIFFUSE-REFLECTANCE, PULPS, SPECTROSCOPY
National Category
Paper, Pulp and Fiber Technology Materials Engineering
URN: urn:nbn:se:kth:diva-13901ISI: 000079200500010OAI: diva2:328082
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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