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        Last update : August 05, 2003
 
                  Second Session : The yield issue                  
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  Logo EFPG     V - Reasons for yield losses during kraft cooking.
Possible strategies for yield increase

D. Lachenal (EFPG)		  

B - Ways to improve yield (slides 21-30)

 
21 - Ways to improve yield
It is clear that yield loss is particularly important:
    at the start, and
  at the end of the cook
Thus, to improve yield, we can act either at the start or at the end of the cook.
   
22 - Operating at the start of the cook
At the start, one can hinder the peeling reaction by destruction of the end groups.
 
 
23 - Oxidation or reduction of the end groups
The end groups can be destroyed by oxidation or by reduction:
Oxidation

(polysuflide, anthraquinone...)
Reduction

(NaBH4...)
 
 
24 - Manufacture of polysulfide
Polysulfide (PS) is obtained by oxidation of HS¯ containing liquors:
2 HS¯ + O2    2 OH¯ + 2 S
HS¯ + 2 S    S—S—SH¯ (polysulfide)
Polysulfide reacts as if it were S.
   
25 - Oxidation by polysulfide
S is a good oxidant for aldehydes:
+ S + 2 OH¯ + SH¯ + H2O
With 1% S on wood, yield is increased by 1.5%
Why isn't it better ?
 
 
26 - Thermal destruction of polysulfide
Polysulfide is destroyed at temperatures higher than 130 °C:
4 S + 4 OH¯    S2O3¯ ¯ + 2 HS¯ + H2O
Thus the secondary peeling is not affected by polysulfide, contrary to the primary peeling.
   
27 - Alkaline hydrolysis
Alkaline hydrolysis operates at high temperatures (>130°C) in alkaline conditions:
Peeling can start again (secondary peeling)
 
 
28 - Anthraquinone as the oxidant
The redox reaction of anthraquinone (AQ):
+ 2 e-  
AQ AQ¯ ¯
 
 
29 - Oxidation by anthraquinone (1)
Anthraquinone is a good oxidant for aldehydes:
+ AQ + 3 OH¯ + AQ¯ ¯ + 2 H2O
With 0.03% AQ on wood, yield is increased by 1%
Why is AQ better than PS ?
   
30 - Oxidation by anthraquinone (2)
AQ is not destroyed at 130 °C, and is rather stable even at 170 °C.
AQ¯ ¯ regenerates AQ when reacting with lignin.
 
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