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

2. Agenda Mechanical pulp bleaching overview Reductive bleaching
Chemistry
Effect of variables
Oxidative bleaching

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.
The brightness of high-yield pulps is close to that of the wood raw material. The wood substrate is always colored, the colour varies between wood species. Poplar-48, brightness, %, aspen 70.
Bleaching is not based on the removal of lignin (low yield pulp) but bases on the destruction of some of the colored groups in lignin. By elimination of chromophores it is possible to reduce light absorption of lignin and thus increase pulp brightness.
Contrary to chemical pulps, mechanical are not always bleached to max brigh. (saving money).

4. Example of Chromophores Found in Mechanical Pulp
Chromophores: that absorb light and make lignin colored. Coniferyl aldehyde and alpha-carbonyl groups and various quinones are the most common chromphores.
Storage of the wood material decreases brightness.
One-year storage can reduce the brightness by 7% ISO.
Bark very important 3% bark, ISO reduced by 10%.
During the pulping process, at elevated T, we can have some condensation of lignin and the metal ions can initiate darkening reactions.

5. 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.
Shown below are examples of the types of reactions which occur under reducing bleaching conditions. The upper pathways shows the reduction of an ortho quinone structure to a catechol group. The ortho quinone structure is extremely highly colored while the catechol structure is colorless. The second half of the pathways shows the catechol being oxidized back to the colored quinone structure. This is part of the photoyellowing process; what happens to your newspaper on the front porch on a sunny day. The second reaction pathways shows a conjugated carbonyl structure (very colored) being reduced to an alcohol structure (not as colored).
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
Sodium dithonite, white crystals, mill will buy lignin or solids or produced on-site.
The mechanism of reaction is no completely known, however it is:
-the reduction of quinoidic groups of hydroquinones
-reduction of alpha-carbonyl groups
-reduction of colored Fe3+ to less colored Fe2+

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.
Dose: 0.5-1%, 1% it is a ceiling, which is 20lb per ton of pulp,
Max brightness improved 10% ISO
Source: Pulp and Paper Manufacture, Volume 2 Mechanical Pulping, page 229

8. Mechanical Pulp Bleaching Hydrosulfite: Effect of Variables
pH: zinc 4-6pH
Sodium
The bleaching time: not critical, most reaction occurs in the first 10-20minutes. Longer time-to utilize fully the bleaching agent.
Temperature, increased T increased bleaching reaction, but brightness reversion. Above 75C, drops slightly
Higher pH darker pulp, lower dithionite will decompose, lower than 4, immediate decomposition
Consistency: low consistency, to avoid air. Oxygen will decompose dithionite, oxygen will also re-oxidize pulp (darken). New equipment MC mixers, you can do it at 8-12% consistency

9. Mechanical Pulp Bleaching Oxidative 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.
Hydrogen peroxide, colorless liquid, delivered to the pulp and paper mill 35-75% solution.
Mechanism through perhydroxyl or hydroperoxy anion,

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 HO2- (pH 9-11)
Reacts with carbonyl structures.
Important to stabilize metals to lower radical formation.
Magnesium silicates or chelating agents are added.
Magnesium silicate added to stabilize peroxide
Transition metals from the equipment will waste hydrogen peroxide, loss of perxydroxyl ion
How it works:
HP oxidizes p and o quinone groups into colorless structures and decompose confieryl aldehyde and conjugated =.

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
Dose up to 4%, max 15-20% ISO, specific effect decrease in increasing dose. (not the straight line)
Remove all the metals before the reaction, destroy the perhydroxyl, remove with chelating agents EDTA, ethylenediaminetetracetic acid, or DTPA, diethylenetetraaminepentaacetic acid
Sodium silicate increases brightness, more better but if is too much it will form buildups in closed water loops , 3% common,
silicate stabilize peroxide, act as a buffer, form complexes together with the transition metals so prevent the decomposition.
Consistency: higher consistency improves bleaching,
Faster reaction,
Higher final brightness.
Why?
More chemicals in the mixture,
Less metal in water
Increasing up to 35-40% we can see the improvements, it is common to use up to 30% equipment related: high consistency dewatering systems were not available
Higher tem, faster reaction but not only bleaching reaction but also decomposition of peroxide reaction. More than 30 minutes at high T, darkening pulp, low temp it is slower but darkening reaction is also slow. Less alkali at higher temp.
Increase temperature, shorter reaction time
At the end of bleaching, pulps’ pH is reduced to 5-6 pH to avoid darkening reaction.
How do we do bleaching:
Bleaching tower: we need to remove water first, mix chemicals. Tower bleaching provides a proper retention time, dilution at the bottom and acidification of the bleached pulp.
Usually 15-20% consistency.
Refiner bleaching.
If the very high brightness is not necessary we can use r. bleaching. TMP peroxide introduce in the refiner. Short retention time, max increase 10%ISO. Benefit: no need for the tower, can improve strength of pulp and reduce E

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, 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.