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        Last update : July 21, 2003
 
                  Third Session : Bleaching                  
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  Logo EFPG     XIII - Factors affecting the bleaching ability of pulps (slides 16-21)

C. Chirat, D. Lachenal (EFPG) et C. Mateo* (CTP/EFPG)		  
* Now at Tembec R&D Tartas
 
16 - Effect of the presence of metal ions (5)
Metal ions (like iron, copper or manganese) can react with the bleaching chemicals:
        bleaching chemicals are wasted. This is the case with iron, copper and manganese ions in P stages
  in some cases harmful radical species (OH°, Cl°) are formed, which can de-polymerise cellulose. It is the case when iron or copper ions are present during P, O, Z or D stages. Peroxide stages are the more sensitive to metal ions
  Example: catalytic decomposition of hydrogen peroxide by metal ions (Mn+ =Fe2+ for ex.)
      Mn+ + H2O2     --> M(n+1)+ + OH- + OH°
      M(n+1)+ + H2O2 --> Mn+ + H+ + HOO°
   
17 - Effect of the extractives
    The extractives can influence the color of unbleached pulps. This could be one explanation why some pulps from eucalyptus or chestnut are more difficult to bleach than other pulps (poplar for example)
 
 
18 - Relevance to pulp mills
    control of the metal ion profile
  measurement of brightness toward the end of the sequence, and after SO2 washing. Usually, the brightness after SO2 is 0.5 to 3 points higher than regular brightness. If this difference varies or is higher, it might indicate a metal ion pollution
  control of the kappa number / brightness relationship, after washing of the unbleached pulp and after O2 washing
  if the factors "metal ions" and "washing" can be eliminated, the color of the starting pulp for a given kappa number should be a good indication of the bleaching ability. In a kraft pulp, a darker color might arise from the presence of quinone type structures
  oxygen and chlorine dioxide do not eliminate quinone easily, they can even create some quinone compounds. Hydrogen peroxide and ozone should react with these substances
   
19 - Effect of bleaching chemicals on a
quinone compound (1)
    impregnation of a quinone (p-benzoquinone) on a high brightness pulp (sulfite pulp, brightness = 85.1% ISO)
 
 
20 - Effect of bleaching chemicals on a
quinone compound (2)
    Effect of hydrogen peroxide, ozone or chlorine dioxide on a quinone model compound (p-benzoquinone) impregnated on a high brightness pulp
   
21 - Conclusion
Several factors can affect bleaching ability:
     from the process side:
    the cooking temperature
  the washing
  from the raw material:
  metal ions from wood or process water  
  wood content (e.g. extractives content)
A key issue is the presence of the chromophores in the pulp. Research is going on on chromophores characterization. Among these, quinone type structures are thought to play an important role. Some bleaching agents are known to react with these structures (hydrogen peroxide, ozone), and this might explain why final (PO) or Z stages can be very efficient.
 
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