Influence of pulp colour on bleachability. Ways to improve the bleaching response of alkaline pulp.

Chemical pulps were produced by the conventional kraft process and by alternative alkaline sulphur-free processes. They were bleached by ClO2-based ECF sequences. The kraft pulps were easier to bleach. However all the kraft pulps were not equal and some were harder to bleach than others. It was observed that the pulps which were more difficult to bleach were richer in quinone groups. They were also more coloured, with a more pronounced reddish hue (a*coordinate of the L* a* b* values) which must be given by the quinone groups. Since quinones are not easily degraded by ClO2 it was proposed that pulp bleachability would be related to the amount of quinone groups, or to the level of a* coordinate. The origin of quinone groups is not known.When AQ was used in the process, some of the detected quinones could be adsorbed residual AQ or AQ derivatives and possibly AQ condensation products with lignin. However since the simple NaOH cooking gave rise also to high contents of quinones, other origins have to be looked for. In the case of kraft cooking the reducing character of HS- might hinder the formation of quinones. Ways to deal with the problem caused by the presence of quinones are proposed.