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Bleaching
Caustic soda (sodium hydroxide) solubilizes degraded lignin products.
Oxygen, hydrogen peroxide or both are often added to the caustic solution to enhance the removal of organic waste.
E |
alkali extraction |
E O |
Alkali extraction reinforced with oxygen |
E P |
Alkali extraction reinforced with hydrogen peroxide |
E OP |
Alkali extraction reinforced with oxygen and hydrogen peroxide |
Chlorine, chlorine dioxide and ozone work best when they are run in acidic conditions at pHs that range from 1.5 to 4. Once the bleaching reactions are finished, the pulp is washed to remove the degraded lignin or other organic waste that has been dissolved in the waste water. Much of the organic waste, however, consists of organic acids and alcohols. These compounds are not soluble in acidic water so they remain with the pulp during washing.
Alkali Extraction Process |
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R-COOH |
+ |
NaOH |
=> |
R-COO-Na |
+ |
H 2O |
organic acid |
|
caustic soda |
|
organic sodium salt |
|
water |
In the alkali extraction stage, these organic acids and alcohols react with the caustic (sodium hydroxide) to form organic sodium compounds and water. These organic substances do dissolve in water. In fact, most of the color and organic waste in mill effluent comes from this first extraction stage. There is much less organic waste in the second extraction phase, if one is used, because very little lignin is removed in the later stages of the bleaching process.
Chlorine Chemistry with Elemental Chlorine |
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Lignin |
+ |
Cl 2 |
=> |
Chlorinated organic compounds |
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Chlorine Chemistry with Chlorine Dioxide |
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Lignin |
+ |
ClO 2 |
=> |
Oxidized Lignin |
+ |
HOCl <=> Cl 2 |
Elemental chlorine and chlorine dioxide react differently with lignin. Elemental chlorine breaks up the lignin molecule by adding chlorine to the lignin.
Chlorine dioxide transfers an oxygen to the lignin to break up the rings. Hypochlorous acid is also formed. It can react directly with the aromatic rings on the lignin or be converted to elemental chlorine.
Thus, the only way to be sure that chlorinated organic compounds including dioxins are not formed in the bleaching process is to eliminate all chlorine-based bleaching chemicals.
Transition metals present in wood react with ozone and hydrogen peroxide; thus, they are removed before these chemicals are applied to the pulp.
A Acid wash
Q Chelating agent