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Pulping and Bleaching PSE 476 Lecture #3 Mechanical Pulp Bleaching Lecture #3 Mechanical Pulp Bleaching.

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Presentation on theme: "Pulping and Bleaching PSE 476 Lecture #3 Mechanical Pulp Bleaching Lecture #3 Mechanical Pulp Bleaching."— Presentation transcript:

1 Pulping and Bleaching PSE 476 Lecture #3 Mechanical Pulp Bleaching Lecture #3 Mechanical Pulp Bleaching

2 A genda Mechanical pulp bleaching overview Reductive bleaching »Chemistry »Effect of variables Oxidative bleaching »Chemistry »Effect of variables Mechanical pulp bleaching overview Reductive bleaching »Chemistry »Effect of variables Oxidative bleaching »Chemistry »Effect of variables

3 Mechanical Pulp Bleaching After the refining process, the resulting fibers do not have the brightness required for most paper applications. Bleaching is therefore required. Goal of mechanical pulp bleaching: »Brighten the pulp with little yield loss. There are two approaches to this problem: »Reducing the colored structures. »Oxidizing the colored structures. After the refining process, the resulting fibers do not have the brightness required for most paper applications. Bleaching is therefore required. Goal of mechanical pulp bleaching: »Brighten the pulp with little yield loss. There are two approaches to this problem: »Reducing the colored structures. »Oxidizing the colored structures.

4 Example of Chromophores Found in Mechanical Pulp

5 Mechanical Pulp Bleaching Reducing Bleaching Chemistry Reduction of chromophores (colored species) to leucochromophores (uncolored species). Sodium dithionite (hydrosulfite Na 2 S 2 O 4 ): most commonly used chemical. Breaks down (dissociates) into reactive species: -Sulfur dioxide radical ion SO Sulfur Dioxide SO 2 -Sulfur Dioxide dianion (sulfoxylate) SO 2 2- Dithionite is sold as a stabilized powder or produced on site from sodium borohydride and sodium bisulfite. Reduction of chromophores (colored species) to leucochromophores (uncolored species). Sodium dithionite (hydrosulfite Na 2 S 2 O 4 ): most commonly used chemical. Breaks down (dissociates) into reactive species: -Sulfur dioxide radical ion SO Sulfur Dioxide SO 2 -Sulfur Dioxide dianion (sulfoxylate) SO 2 2- Dithionite is sold as a stabilized powder or produced on site from sodium borohydride and sodium bisulfite. Text

6 Lignin Preserving Bleaching Reducing The object is the reduction of chromophores, ie unsaturated structures like the quinone shown below. The last step is photoyellowing

7 Mechanical Pulp Bleaching Brightness Gain Using Hydrosulfite Reducing bleaching not as efficient as oxidative (H 2 O 2 ). Factors influencing brightness »Amount of bleach »Temperature »Time »pH »Pulp consistency »Chelating agents Color reversion a big issue. Wood species important. Source: Pulp and Paper Manufacture, Volume 2 Mechanical Pulping, page 229

8 Mechanical Pulp Bleaching Hydrosulfite: Effect of Variables

9 Mechanical Pulp Bleaching Oxidative Bleaching Chemistry Peroxide oxidizes chromophores to uncolored species. Reactive species: hydroperoxy anion HO 2 - Reactions very pH dependent. » At pH 10.5, only 10% of H 2 O 2 is the hydroperoxy anion. »At higher pH’s, there is more of the anion but also more decomposition of the peroxide to oxygen and water. »NaOH and sodium silicate are used to control pH. Silicates are added to stabilize peroxides. MgSO 4 and chelating agents added to slow the metal induced decomposition of H 2 O 2. Peroxide oxidizes chromophores to uncolored species. Reactive species: hydroperoxy anion HO 2 - Reactions very pH dependent. » At pH 10.5, only 10% of H 2 O 2 is the hydroperoxy anion. »At higher pH’s, there is more of the anion but also more decomposition of the peroxide to oxygen and water. »NaOH and sodium silicate are used to control pH. Silicates are added to stabilize peroxides. MgSO 4 and chelating agents added to slow the metal induced decomposition of H 2 O 2.

10 Lignin Preserving Bleaching Oxidative Unlike reductive bleaching, the colored compounds are destroyed so they cannot reform (ring cleavage reactions). Hydrogen (mostly) or sodium peroxide are used. Active species: perhydroxyl (or hydroperoxy) anion HO 2 - (pH 9-11) »Reacts with carbonyl structures. Important to stabilize metals to lower radical formation. »Magnesium silicates or chelating agents are added. Unlike reductive bleaching, the colored compounds are destroyed so they cannot reform (ring cleavage reactions). Hydrogen (mostly) or sodium peroxide are used. Active species: perhydroxyl (or hydroperoxy) anion HO 2 - (pH 9-11) »Reacts with carbonyl structures. Important to stabilize metals to lower radical formation. »Magnesium silicates or chelating agents are added.

11 Mechanical Pulp Bleaching Brightness Gain Using Peroxide Higher brightness reached than with hydrosulfite. Factors influencing brightness »Amount of bleach »Temperature »Time »pH »Pulp consistency »Silicates/Chelating agents Color reversion a big issue. Wood species important.

12 Mechanical Pulp Bleaching Peroxide: Effect of Variables

13 Photoyellowing This is the process your newspaper goes through on the front porch in the sunshine. Yellowing initiated by lignin chromophores adsorbing UV light ( nm). »Oxygen essential to process. Radical are formed (both organic and oxygen radicals). Lignin is degraded,  -O-4 linkages broken, methoxyl groups lost. Reaction products of these radical processes include carbonyls, quinones, acids, and aldehydes. »First 2 of this list can be very colored. This is the process your newspaper goes through on the front porch in the sunshine. Yellowing initiated by lignin chromophores adsorbing UV light ( nm). »Oxygen essential to process. Radical are formed (both organic and oxygen radicals). Lignin is degraded,  -O-4 linkages broken, methoxyl groups lost. Reaction products of these radical processes include carbonyls, quinones, acids, and aldehydes. »First 2 of this list can be very colored.


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