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        Last update : June 18, 2003
 
                  Third Session : Bleaching                  
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            13 - Factors affecting the bleaching ability of pulps            
C. Chirat, D. Lachenal (EFPG) et C. Mateo (CTP/EFPG)

Logo EFPGThe bleaching ability of unbleached pulps can be defined, for a given bleaching sequence, by the amount of chemicals needed to reach a target brightness value, divided by the starting kappa number. Some people use the OXE (oxidation equivalents) when they want to compare different sequences (1 OXE is equal to the quantity of substance which receives 1 mol of electrons when the substance is reduced): for example when chlorine dioxide is used in the sequence, one kg of chlorine dioxide represents 74.12 OXE. For hydrogen peroxide, the value is 58.79 OXE/kg. The bleaching ability of pulps from several mills is given: sulfite pulps are more easy to bleach than kraft pulps, and some hardwood kraft pulps are more difficult to bleach than softwoods. However as many factors vary among the mills, it is difficult to conclude very precisely. For a given mill, problems of bleaching ability sometimes occur: in most cases it is related to modification in the process conditions (like pH or T problems, carry-over, ...), but in some cases variations in bleaching ability are not understood.

Several factors can affect bleaching ability. First, the cooking type: sulfite pulps are known to be more easy to bleach which is due to different lignin structures. The sulfite lignin is sulfonated which facilitates its solubilization. Soda AQ pulps are usually more difficult to bleach than kraft pulp. One reason for that could be that the lignin contains more quinone groups, which are not easily eliminated by usual bleaching chemicals. The kraft cooking parameters like the residual alkali, the sulfidity, the extent of delignification or the alkali profile, also play a role in bleaching ability.

Second, the presence of precipitated lignin onto the fibers (at the end of the cook or during the washing after cooking or after bleaching stages) also seems to influence bleaching ability. The pH and the presence of some metals (from calcium or aluminum for example) influence lignin precipitation. The precipitated lignin from black liquor for example, not only increases the kappa number of the pulp, but also its brightness for a given kappa number, and leads to a lower bleaching ability.

Thirdly, the presence of metal ions (iron for example) can have a direct effect on brightness (it lowers the brightness), and on the bleaching efficiency (iron, copper and manganese can decompose hydrogen peroxide, thus wasting the bleaching reagent, and in some cases (iron and copper), harmful radical species can be formed leading to cellulose degradation). Most of the bleaching stages can be sensitive to metal ions: oxygen, hydrogen peroxide, ozone, chlorine dioxide (for high levels of iron). The presence of extractives would also play a role in bleaching ability.

To conclude, the relevance of these different parameters to pulp mills is given. One key issue when one is sure that these parameters are not involved into the bleaching ability of a mill, is to control the chromophore content of a pulp ("its color") for a given kappa number.

 

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