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        Last update : December 12, 2005
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            4 - Ozone as final bleaching stage: basic aspects and process performance            
Guillaume Pipon, Christine Chirat, Dominique Lachenal (EFPG)

Site Web de l'EFPGOzone is a highly efficient bleaching reagent for chemical pulps. Since 1992, the ozone bleaching process has been set up in 28 bleaching lines. In most of these lines, chlorine dioxide is still used, mainly because ozonation may degrade cellulose if used too extensively.

It was shown that the degradation of cellulose during ozonation of chemical pulps could be caused by some radicals produced in the reaction between ozone and lignin. This has led to the idea of applying ozone on almost fully bleached pulps, with very low lignin content, but which still contain coloured groups. In most cases 1 to 2 kg ozone per ton of pulp produce an instant bleaching, increasing the brightness by several points. The process is working equally well at acidic as well as neutral pH. No cellulose degradation takes place under these conditions.

The objectives of the present work were first to explain the reasons why the efficiency of a final ozone stage varies depending on the pulp type, and second to recommend the best ways to apply ozone as a last stage from a process point of view.

In this work, a final ozone stage was applied to different types of pulps. The tested pulps were softwood and hardwood kraft pulps, ECF and TCF bleached. It was confirmed that the nature of the pre-bleaching sequence, the nature of the wood, but also the starting brightness before ozonation could influence the efficiency and selectivity of the final ozone stage.
In particular it was shown that pulps pre-bleached with a TCF sequence of the OOQ(OP)PaaQ(PO) type were hardly bleached by a final ozone stage contrary to ECF sequences for which several points could be gained.
Two main hypotheses were made to explain this behaviour. The first one was that new chromophores might be formed during the reaction of ozone with pulp components. The ozonation of lignin model compounds in aqueous solutions were then carried out and it was shown that coloured groups could be created by this way. The second hypothesis was that residual hexenuronic acid groups or white residual lignin fragments, which might remain in the pulp after pre-bleaching, reacted with ozone very readily and consumed ozone without any effect on brightness. This second hypothesis was also correct since after removal of the hexenuronic acids from the pulp, the efficiency of the final ozone stage could be significantly improved.

The second aspect of this work dealt with the way to perform a final ozone stage: a comparison was made between the injection of gaseous ozone in the reactor, and the use of ozonated water. The type of process water (pure water or white water from the mill) was also considered. Recommendations as to the application of ozone as a final stage will be given.

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