Pulping and Bleaching PSE 476

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Pulping and BleachingPSE 476

• Lecture 3
• Mechanical Pulp Bleaching

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Agenda

• Mechanical pulp bleaching overview
• Reductive bleaching
• Chemistry
• Effect of variables
• Oxidative bleaching
• Chemistry
• Effect of variables

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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.

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Example of Chromophores Found in Mechanical Pulp
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Mechanical Pulp Bleaching Reducing 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
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Lignin Preserving BleachingReducing

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

The last step is photoyellowing
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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
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Mechanical Pulp Bleaching Hydrosulfite Effect
of Variables
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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 pHs, 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.

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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.

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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.

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Mechanical Pulp Bleaching Peroxide Effect of
Variables
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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.