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|EFPG DAYS 2005||cerig.efpg.inpg.fr|
|Last update : December 12, 2005|
session - Improvement of bleaching response
of chemical pulps
|6 - Reasons for bleachability problems and ways to resolve them|
Dominique Lachenal , Christine Chirat, Nicolas Benattar, Yahya Hamzeh, Nathalie Marlin (EFPG),
Christelle Mateo (Tembec-Tartas), Bernard Brochier (CTP)
Chemical pulps do exhibit bleachability differences. This is particularly the case with pulps obtained by
non-sulphur processes such as Novacell, IDE or soda-AQ. However even among kraft pulps substantial
differences may be observed.
L a*b*measurements carried out on unbleached pulp indicate that the non-sulphur pulps exhibit higher a* and b* components. The higher a* value reflects the reddish coloration taken by these pulps as soon as they are put in contact with air the case with pulps and suggest that quinone groups are formed. There are no reliable methods to measure quinines in pulp. 19F NMR spectroscopy and voltametry were used to approach the quinone content. Even though these methods may not give absolute quantitative values the ranking was the same with both, which gives some confidence in the conclusion that the higher the quinone content in the unbleached pulp the more difficult to bleach by ClO2-based ECF sequences.
This observation is supported by the well known fact that ClO2 is not very reactive with quinones. Actually more quinones are found after a D stage than before, which indicates that not only ClO2 forms new quinones but also cannot destroy them efficiently.
If this hypothesis is valid several process modifications could be contemplated. Adding hydrogen peroxide at several points in the bleaching sequence should be beneficial since H2O2 is known to be very reactive with quinones. Another approach could be to apply some reductive steps to transform quinones into phenols which then could be destroyed by ClO2.
It was also shown that a hot E stage (NaOH treatment at 90°C) carried out prior to bleaching, improved the bleaching response of pulps. One proposed explanation is the destruction of quinones by the alkaline treatment. Indeed, according to literature quinines are transformed into polyphenols when submitted to alkaline conditions. This treatment would be desirable in the case of pulps which are particularly difficult to bleach.