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|EFPG DAYS 2005||cerig.efpg.inpg.fr|
|Last update : December 12, 2005|
session - Improvement of bleaching response
of chemical pulps
|7 - Getting the best from chlorine dioxide bleaching|
Dominique Lachenal, Christine Chirat, Gérard Mortha, Yahya Hamzeh (EFPG)
Although chlorine dioxide has become the universal bleaching agent of chemical pulps, there are some indications
that the process conditions are not properly optimised. This appears clearly when one compares its total consumption
in ECF sequences with the theoretical quantity needed to open all the aromatic rings in lignin and make it soluble.
One can conclude that two thirds of the ClO2 is actually lost in useless reactions.
Our hypothesis is that most of the loss is due to the reaction of ClO2 with the muconic acid derivatives which are the primary products of lignin oxidation. This is supported by the results of a study on the behaviour of a mixture of lignin models compounds consisting of vanillin (phenolic model), 3,4- dimethoxy benzilic alcohol (non phenolic model) and 2,5-dimethyl muconic acid, submitted to chlorine dioxide. It appears that the muconic acid derivative starts to be consumed well before the non phenolic model has disappeared.
It is then proposed to perform the first D stage with a charge of ClO2 small enough to keep the consumption of the produced muconic acid derivatives at a marginal level (d), and to extract them by caustic soda addition (e) and washing before continuing with successive similar procedures, till most of the lignin has been removed. This process is described as (de) (de) (de)...
At least 30% ClO2 can be saved by applying this concept. This is obtained at the expense of some extra consumption of NaOH and a higher number of bleaching vessels. However a net cost saving should be obtained considering the relative cost of chemicals and the lower or equal number of expensive washing steps. Moreover some hydrogen peroxide can be advantageously added to the e stages.
Another cause of ClO2 loss is the consumption of ClO2 by the hexenuronic acids present in the hardwood kraft pulps. It is postulated that the reaction of ClO2 with the hexenuronic acids has lower activation energy than the reaction of ClO2 with lignin. Although no values are available, we found a difference of that sort with the muconic acid derivatives, the structure of which presents similarities with hexnuronic acids. This is the basis of the high temperature (95°C) D process (D*) proposed a few years ago by our group. This process favours the reaction of ClO2 with the lignin and the destruction of the hexenuronic acids by the acidity of the D liquor, provided that the retention time is long enough (2-3 hours). Up to 20% of the ClO2 can be saved, depending on the wood species.