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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 BleachingPSE 476

Lecture #3

Mechanical Pulp Bleaching


Agenda

  • Mechanical pulp bleaching overview

  • Reductive bleaching

    • Chemistry

    • Effect of variables

  • Oxidative bleaching

    • Chemistry

    • Effect of variables


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


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 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

    • 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 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 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

    • 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


    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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