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You are here : Home > Training > EFPG Days > 2- Why using ozone for the bleaching of chemical pulps (abstract)
        Last update : December 12, 2005
 
                  First session - Ozone treatment of chemical pulps              
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            2 - Why using ozone for the bleaching of chemical pulps ? Tutorial            
Dominique Lachenal, Christine Chirat, Guillaume Pipon (EFPG)

Site Web de l'EFPGChemical pulp bleaching consists in the removal of 100% of the residual lignin left after cooking, and in the degradation of the remaining coloured groups when the lignin has gone

In both cases ozone appears to have a high efficiency. Chemistry considerations explain why the displacement ratio between ozone and chlorine dioxide is higher than1.5 in delignification (by weight). One reason relates to the higher number of electrons that ozone can exchange (on a weight basis). Another reason is due to the fact that although both ozone and chlorine dioxide participate in secondary reactions with the primary lignin oxidation products (muconic acid-type structures), ozone degrades them in smaller molecules which are soluble whereas chlorine dioxide does not. Consequently substitution of ozone for chlorine dioxide must lead to cost saving.

At the end of bleaching coloured groups are still present. These chromophores necessarily contain conjugated carbon-carbon double bonds and carbonyl groups. Again chemistry suggests that ozone must be more efficient than chlorine dioxide, since ozone is far more reactive with double bonds than chlorine dioxide.

The risk of pulp damage during ozone treatment must be taken into consideration and discussed. Contrary to chlorine dioxide, ozone may react with pulp carbohydrates. The most probable mechanism is the direct attack of molecular ozone on hemicellulose and cellulose. However several considerations indicate that in proper conditions pulp degradation must be marginal. In fact (1) lignin is far more reactive with ozone than carbohydrates and acts as a protector, (2) among carbohydrates, the amorphous and more accessible hemicelluloses should be attacked well before cellulose and (3) even if ozone molecules reach the cellulose chains, then the formation of carbonyl groups will precede the depolymerisation of cellulose.

Another concern is the effect of temperature on cellulose degradation during an ozone treatment. Nothing indicates that at moderate temperature (50-70C), cellulose should be more degraded than at room temperature. On the contrary literature data suggest that cellulose depolymerisation would be actually less pronounced at 60C than at 20C.

Overall ozone possesses all the requirements to become a universal bleaching agent of chemical pulp.

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