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You are here : Home > Training > EFPG Days > 8-  Why using ozone in mechanical pulping? Tutorial (abstract)
        Last update : December 12, 2005
                  Third session - Ozone in mechanical pulping              
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            8 - Why using ozone in mechanical pulping? Tutorial            
Dominique Lachenal (EFPG)

Site Web de l'EFPGThe ozone treatment of mechanical pulp results in a dramatic increase of the pulp strength properties. Since ozone reacts with all the pulp components, understanding the reason for the strength improvement is not straightforward.

During pulp ozonation new carboxylic groups (COOH) are formed which should increase the hydrophilic character of the fibres and favour fibre bonding. However a simple NaOH treatment also creates new COOH groups without any positive effect on strength. One major difference with ozonation is that the COOH groups created by NaOH are exclusively located on the carbohydrates. Moreover comparison with ClO2 treatment indicates that both ozone and ClO2 form new COOH groups on lignin. It was found that the content of new COOH groups was much higher after ClO2 than after O3 treatments, showing again that the formation of COOH groups is not the explanation for the beneficial effect of ozone.

Other factors were looked for. The decrease in pulp yield during treatment was not responsible for the effect either, since other treatments like NaOH extraction produces the same yield abatement without any similar beneficial effect. 

Two other explanations are proposed for the strength improvement: the decrease in the lignin content and the degradation of the extractives into hydrophilic substances. The first explanation is supported by the further strength gain brought about by a successive caustic extraction. The second makes also sense since the extractives, due to their chemical structure, are likely the most reactive components with ozone. Their oxidation should make them much more hydrophilic and thereby favour fibre bonding.

Practically, the burst index and the breaking length of a TMP pulp can be increased by 40% by applying 20 kg ozone per ton of pulp. Another way to take benefit of the effect of ozone is to decrease the energy input so as to keep the same final quality. In this case ozone has to be added before the last refining stage. 20% energy savings should be easily attainable.

Ozonation has an adverse effect on pulp brightness. It is shown that most of this effect is directly due to better bonding (lower light scattering). However some new chromophores are also formed. Adding some extra peroxide should easily destroy these new coloured groups. Since the main contributor to the brightness loss is the lower scattering coefficient, the problem will be automatically solved if more TMP is introduced in the paper furnish.

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