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        Last update : July 11, 2003
 
                  Third Session : Bleaching                  
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  Logo EFPG     XIII - Factors affecting the bleaching ability of pulps (slides 10-15)

C. Chirat, D. Lachenal (EFPG) et C. Mateo* (CTP/EFPG)		  
* Now at Tembec R&D Tartas
 
10 - Effect of lignin precipitation (1)
    The lignin dissolved during kraft cooking can precipitate onto the fibres either at the end of the cook, or during the washing of the unbleached pulp, or in the bleaching line
  A pH lower than 11 and the presence of metal ions like Ca2+ or Al3+ favor lignin precipitation
From : N. Maximova et al., Int. Pulp Beaching Conf. (Portland, 2002)
   
11 - Effect of lignin precipitation (2)
    The precipitated lignin increases lignin content and thus the kappa number of the pulp
  Furthermore it leads to a more colored pulp for a given kappa number, and to a lower bleaching ability (Christelle Mateo thesis, November 2002)
 
 
12 - Effect of the presence of metal ions (1)
    Some metal ions can color a pulp and
thus lower the brightness (particularly iron)   		 Chromophores in the pulp

   Examples of chromophores and effect of the presence of iron
(Wavelength of maximum absorption in nm)
  Origin of metal ions in pulps :
   - wood chips and bark
   - chemicals
   - corrosion of the equipment
   - process water
 
 
13 - Effect of the presence of metal ions (2)
    Hexenuronic acids, which are present in significant quantities
in hardwood kraft pulp, can chelate metal ions and make it
difficult to remove them from the pulp, even with a chelating stage   		    
  Chlorine dioxide and ozone are known to be able to eliminate
hexenuronic acids from a pulp, thus favoring the elimination
of metal ions linked to them
 
 
14 - Effect of the presence of metal ions (3)
    Effect of the addition of iron II and iron III on pulp brightness:
ISO brightness vs Fe content
   
15 - Effect of the presence of metal ions (4)
    Effect of the addition of iron II and iron III on pulp brightness
  Effect of an acid washing (sulfuric or SO2): it removes the metals and restore the brightness
 
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