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        Last update : July 29, 2003
 
                  Second Session : The Yield Issue                  
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            5 - Reasons for yield loss in Kraft cooking. Possible strategies for yield increase            
D. Lachenal (EFPG)

Logo EFPGDuring kraft cooking most of the hemicelluloses are dissolved. Among them, the galactoglucomannans are much more affected than the xylans. Dissolution of the hemicelluloses results from the combination of peeling and alkaline hydrolysis reactions. The peeling process starts already at low temperature (< 100C) (primary peeling), whereas alkaline hydrolysis is significant at higher temperatures (> 140C). After hydrolysis peeling is reactivated (secondary peeling). In principle, cellulose undergoes the same series of reactions. However due to the size of its molecular chains, yield loss of cellulose remains low.

Close examination of yield loss profile versus delignification in kraft cooking shows that most of the hemicelluloses are already removed before the start of bulk delignification i.e. during the heating-up period. Another severe carbohydrate dissolution, relative to delignification, is observed at the end of the cook, during the so-called final delignification. Consequently actions against yield loss must be advantageously taken either at the start or toward the end of the cook.

The extent of peeling at the start of the cook can be minimized by the oxidation or the reduction of the aldehyde (hemi-acetal) end-groups of carbohydrates. Polysulfide and anthraquinone can oxidize these end-groups, which are transformed into stable carboxyl end-groups. Yield can then be increased by up to 3 % (on wood) depending on the charge of the additives.

Polysulfide is much less efficient than anthraquinone, on a molecular basis, because it is decomposed at high temperature (T> 130C) and is therefore ineffective against secondary peeling. Antraquinone is not only stable but also continuously maintained in its oxidized form due to reaction of its reduced form with lignin and finally behaves as a catalyst.

Reduction of the aldehyde end-groups is feasible but not economically viable. Very few reagents can be used for this purpose. Sodium borohydride is one candidate. Unfortunately, yield saving does not cover the extra chemical cost.

Blocking the peeling reaction remains a challenge and is still the subject of numerous investigations.

Another approach is to stop the cook earlier, so as to avoid the poorly selective final delignification. Kappa number of around 50 and 30 for softwood and hardwood kraft pulps respectively appear to be the appropriate levels to start the bleaching phase, provided that this latter phase is performed with chlorine-free chemicals with good selectivity. Potential yield saving is 2 to 3 % on wood.

Finally it seems that some of the latest technologies developed for kraft cooking such as ITC offer some benefits in terms of yield, likely because lower temperatures and alkali charges are used, which minimizes the secondary peeling.

Whatever the means used for yield increase it must be reminded that pulp strength will be adversely affected.

 

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