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It is known from the literature that sulfur-free pulps obtained by an alkaline process are usually more difficult to bleach than the corresponding kraft pulps. It seems to be related to the presence of higher quinone contents. This bleaching ability problem was confirmed in the case of Novacell pulps delignified with oxygen, both for softwood and hardwood pulps. It was thus important to study and optimise the bleaching of such pulps to obtain optimised brightness and brightness stability with the lowest bleaching chemical charges.
In the case of the bleaching of the Novacell softwood pulps, the following results were obtained:
- The bleaching ability seems to be dependant upon some cooking parameters like the amount of AQ and the effective alkali.
- The use of ozone or hydrogen peroxide toward the end of the bleaching sequence enables to increase the final brightness rather easily. This confirms the presence of quinone type chromophores in the Novacell pulps, which was also supported by the analysis of the residual lignin.
-The following parameters improve brightness ability and /or the final brightness of the Novacel pulps:
- As low kappa as possible after cooking
- The use of ozone in a OO(ZD)EpD sequence enables to bleach the Novacell softwood pulps to higher brightness and yield values than with a OOQ(OP)DQ(PO) sequence.
The Novacell eucalyptus pulps delignified with oxygen were also more difficult to bleach than the corresponding kraft pulp, but the difference with the softwood case was that high brightness levels (higher than 90%) could be reached easily. One way to improve the brightness level for a given charge of bleaching chemical (expressed as OXE) is to introduce an ozone stage. The best results were obtained with a (DZ)QP or (DZ)Q(PO) sequence. Furthermore the use of ozone in (DZ)Q(PO) enables to improve the brightness stability upon heat exposure of the bleached pulps compared to the "control" DQ(PO) sequence. This can be explained by the fact that ozone is very efficient to remove hexenuronic acid groups and residual lignin compared to (PO) which has only a bleaching effect. Thus the recommendation for the bleaching of the Eucalyptus Novacell pulp is
- either to use a sufficient amount of chlorine dioxide and apply classical DED or DEpD sequences
- or to introduce ozone in the sequence if the amount of chlorine dioxide is to be limited, the best sequence being then (DZ)Q(PO).
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