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        Last update : June 19, 2003
 
                  Third Session : Bleaching                  
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            11 - Effect and future of the bleaching chemicals            
D. Lachenal (EFPG)

Logo EFPGThe examination of the chemical mechanisms given by the most common bleaching reagents (chlorine, chlorine dioxide, oxygen, ozone, hydrogen peroxide) reveals that they all have the effect of grafting carboxyl groups on the lignin molecules. This results in an increase of the lignin hydrophilic character and thereby in its solubilization. The formation of carboxyl groups is due to the oxidation of the lignin phenolic rings. This reaction necessitates the exchange of 4 electrons.

Knowing that all the bleaching chemicals are giving the same global reaction with lignin, one may conclude that their delignification power will depend on the number of electrons that they can exchange and on the ease these electrons are actually exchanged. Among the bleaching chemicals which have the capacity of oxidizing any phenolic group, ozone is the most efficient (on a weight basis) followed by chlorine dioxide and chlorine. Oxygen should have a great potential according to the number of electrons possibly exchanged. However it reacts only with some of the phenolic units in lignin (free phenols).

Supposing that all the phenolic units in lignin need to be oxidized to achieve full solubilization (which must not be the case) it is easy to calculate the theoretical amount of bleaching reagent which is required. The conclusion is that in conventional ECF bleaching, based on the use of ClO2, at last half the charge of chemicals is wasted in useless reactions. It is proposed that ClO2 is consumed by further reaction with the primary oxidation products of lignin.

The potential of cost savings in ECF bleaching is therefore very high. One way is to minimize the concentration of reagent by splitting the charge in small quantities and by removing the reaction products as often as possible. The example of (de)(de)(de)... bleaching where d and e stand for chlorine dioxide and alkaline stages with low chemical charge, respectively, is given.

Even though lignin removal is the main reaction taking place during bleaching, achieving very high brightness necessitates the destruction of all the colored groups in pulp. The structure of the chromophores in unbleached pulp is not well known. However they must contain conjugated carbon-carbon double bonds and carbonyl groups. Only ozone and hydrogen peroxide can easily react with these structures or with part of them. Consequently it is thought that ozone or hydrogen peroxide should be advantageously introduced in any ECF sequence.

 

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