Pulping and bleaching pse 476
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Pulping and Bleaching PSE 476. Lecture #3 Mechanical Pulp Bleaching. A genda. Mechanical pulp bleaching overview Reductive bleaching Chemistry Effect of variables Oxidative bleaching Chemistry Effect of variables. Mechanical Pulp Bleaching.

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Pulping and Bleaching PSE 476

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Pulping and bleaching pse 476

Pulping and BleachingPSE 476

Lecture #3

Mechanical Pulp Bleaching


A genda

Agenda

  • Mechanical pulp bleaching overview

  • Reductive bleaching

    • Chemistry

    • Effect of variables

  • Oxidative bleaching

    • Chemistry

    • Effect of variables


Mechanical pulp bleaching

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.


Example of chromophores found in mechanical pulp

Example of Chromophores Found in Mechanical Pulp


Mechanical pulp bleaching reducing bleaching chemistry

Mechanical Pulp BleachingReducing Bleaching Chemistry

  • Reduction of chromophores (colored species) to leucochromophores (uncolored species).

  • Sodium dithionite (hydrosulfite Na2S2O4): most commonly used chemical. Breaks down (dissociates) into reactive species:

    • Sulfur dioxide radical ion SO2-.

    • Sulfur Dioxide SO2

    • Sulfur Dioxide dianion (sulfoxylate) SO22-

  • Dithionite is sold as a stabilized powder or produced on site from sodium borohydride and sodium bisulfite.

  • Text


    Lignin preserving bleaching reducing

    Lignin Preserving BleachingReducing

    • The object is the reduction of chromophores, ie unsaturated structures like the quinone shown below.

    The last step

    is photoyellowing


    Mechanical pulp bleaching brightness gain using hydrosulfite

    Mechanical Pulp Bleaching Brightness Gain Using Hydrosulfite

    • Reducing bleaching not as efficient as oxidative (H2O2).

    • 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


    Mechanical pulp bleaching hydrosulfite effect of variables

    Mechanical Pulp Bleaching Hydrosulfite: Effect of Variables


    Mechanical pulp bleaching oxidative bleaching chemistry

    Mechanical Pulp BleachingOxidative Bleaching Chemistry

    • Peroxide oxidizes chromophores to uncolored species.

    • Reactive species: hydroperoxy anion HO2-

    • Reactions very pH dependent.

      • At pH 10.5, only 10% of H2O2 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.

    • MgSO4 and chelating agents added to slow the metal induced decomposition of H2O2.


    Lignin preserving bleaching oxidative

    Lignin Preserving BleachingOxidative

    • 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 HO2- (pH 9-11)

      • Reacts with carbonyl structures.

    • Important to stabilize metals to lower radical formation.

      • Magnesium silicates or chelating agents are added.


    Mechanical pulp bleaching brightness gain using peroxide

    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.


    Mechanical pulp bleaching peroxide effect of variables

    Mechanical Pulp Bleaching Peroxide: Effect of Variables


    Photoyellowing

    Photoyellowing

    • This is the process your newspaper goes through on the front porch in the sunshine.

    • Yellowing initiated by lignin chromophores adsorbing UV light (300-400nm).

      • Oxygen essential to process.

    • Radical are formed (both organic and oxygen radicals).

    • Lignin is degraded, b-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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