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        Last update : September 12, 2003
 
                  Third Session : Bleaching                  
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  Logo EFPG     XI - Effect and future of the bleaching chemicals (slides 1-8)

Dominique Lachenal (EFPG)
 

A - Bleaching chemistry (slides 1-8)

 
1 - Introduction
During cooking, lignin is either depolymerized (Kraft process) or made hydrophilic (sulphite process).
As a consequence:
    Residual lignin in kraft pulp is not sufficiently depolymerized to become soluble
  Residual lignin in sulphite pulp is not enough sulphonated to become soluble
Bleaching chemistry can be summarized by the grafting of COOH on lignin.
   
2 - Lignin in kraft process
Lignin in wood Lignin in kraft pulp Dissolved lignin during bleaching
 
 
3 - Lignin in sulphite process
Lignin in wood Lignin in sulfite pulp Dissolved lignin during bleaching
   
4 - Bleaching general reaction
All the bleaching chemicals give the same global reaction on lignin, as shown by the following figure:
This global reaction is an oxidation involving four electrons per aromatic ring.
 
 
5 - Bleaching with chlorine dioxide
Chlorine is formed and is responsible for part of the delignification.
   
6 - Bleaching with chlorine
 
 
7 - Bleaching with ozone
Hydrogen peroxide is formed. Its contribution to delignification is not known.
   
8 - Bleaching with oxygen
 
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