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You are here : Home > Training > EFPG Days > 13- Factors affecting the bleaching ability of pulps (slides 1-9)
        Last update : July 03, 2003
 
                  Third Session : Bleaching                  
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  Logo EFPG     XIII - Factors affecting the bleaching ability of pulps (slides 1-9)

C. Chirat, D. Lachenal (EFPG) et C. Mateo* (CTP/EFPG)
 
* Now at Tembec R&D Tartas
 
1 - Introduction
    The bleaching ability of unbleached chemical pulps can be defined, for a given bleaching sequence, by the amount of bleaching chemicals needed to reach a given brighteness value divided by the starting kappa number.
Some people use the oxidation equivalents (OXE) when they want to compare different sequences:
  1 OXE = quantity of substance which receives 1 mol of electrons when the substance is reduced
   
2 - Conversion factors to OXE
  Molecular weight e-/mol for reduction to
Cl(-1) and O(-2) resp.
g/mol e- OXE/kg
Cl2 70,914 2 35,46 28,20
ClO2 67,457 5 13,49* 74,12**
NaClO 74,448 2 37,22* 26,86**
O2 32,00 4 8,00 125,00
H2O2 34,018 2 17,01 58,79
O3 48,000 6 8,00 125,00
* 35,46 g active chlorine/mol e-
** 28,20 OXE/kg active chlorine
From : R. Grundelius, Int. Pulp Bleaching Conf. (Stockholm, 1991)
 
 
3 - Bleaching ability of industrial pulps
Bleaching ability of industrial pulps by DEDD (% ClO2/kappa number)
 
 
4 - Bleaching situation
    The bleaching ability can vary a lot from one mill to another, as many factors vary.
For a given mill, problems of bleaching ability sometimes occur. In most cases, the problem is related to modifications in the process conditions (carry-over, problem of pH or temperature control) rather than an effective bleaching ability problem. But in some other cases, the reasons for bleaching ability variations are not understood.
   
5 - Example of bleaching ability (1)
Two pulps from the same mill: one "easy" to bleach (kappa 13,3) and one difficult to bleach (kappa 12,9). Sequence DEDD.
 
 
6 - Example of bleaching ability (2)
Calculation of bleaching ability to reach 89,6% ISO brightness:
  -- easy:      2,82/13,3 = 0,21% ClO2 / kappa unit
  -- difficult:     3,2/12,9 = 0,25% ClO2 / kappa unit
   
7 - Effect of the type of cooking (1)
Kraft versus sulfite
It is for example well known that sulfite pulps are easier to delignify and to bleach than kraft pulp due to the structure of the lignin. The lignin of a sulfite pulp is sulfonated, which facilitates its solubilization.
 
 
8 - Effect of the type of cooking (2)
Kraft versus soda AQ
    Soda AQ pulps are usually more difficult to bleach than kraft pulps.
Example : kraft cooking versus NaOH/AQ for a birch pulp. The brightness after cooking is much lower for the NaOH/AQ for the same kappa number. This could be explained by a higher amount of quinonic groups.
Cooking Kraft Soda AQ
Kappa number
Brightness, % ISO
% quinones in lignin (number per aromatic unit)
18,7
37,2
0,021
18,9
31,1
0,065
ODEDD
Brightness, % ISO
% ClO2
 
89,3
2,85
 
84,4
3,10
   
9 - Effect of the type of cooking (3)
Kraft cooking parameters
Kraft cooking parameters can affect bleaching ability:
    an increase in residual alkali improves bleaching ability
an increase in sulfidity improves bleaching ability
an increase in delignification (lower kappa after the cook) tends to lower the bleaching ability
the use of levelled-out profile improves the bleachability compared to conventional cooking (where all the alkali is added at the beginning of the cook)
the removal of dissolved organic substance toward the end of the cook improves bleachability
 
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