Pulping and Bleaching PSE 476

1. Pulping and BleachingPSE 476 Lecture #3 Mechanical Pulp Bleaching

2. Agenda • 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.

4. Example of Chromophores Found in Mechanical Pulp

5. 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

6. Lignin Preserving BleachingReducing • 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 (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

8. Mechanical Pulp Bleaching Hydrosulfite: Effect of Variables

9. 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.

10. 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.

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 (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.