Pulping and BleachingPSE 476

Lecture #3Mechanical Pulp Bleaching

Lecture #3Mechanical Pulp Bleaching

Agenda

• Mechanical pulp bleaching overview• Reductive bleaching

» Chemistry» Effect of variables

• Oxidative bleaching» Chemistry» Effect of variables

• 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.

• 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

O

O

Coniferaldehyde 10-20% Adsorbed Light

o-Quinines 30 - 65%Adsorbed Light

p-Quinone Methide

o-Quinone Methide

OH

OCH3

CH

HC

CHO

O

OCH3

CH

OH

OCH3

O

OCH3C

O

H3CO

CH CH

OCH3

OH

p, p' Stilbene Quinone

O

OCH3

O

p-Quinones

O

O

Fe

Chelated Metals

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

Lignin Preserving BleachingReducing

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

Na2S2O4 + 4HO- 2Na2SO3 + 2H2O + 2e-

Na2S2O4

O

O

OH

OHLight

O2

O

O

Colored Not colored Colored

The last stepis 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.

• 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.

0

2

4

6

8

10

12

0 8 16 24 32 40 48

Pounds of Bleach per Ton of Pulp

Bri

ghtn

ess In

crea

se (%

MgO

)

0

2

4

6

8

10

12

0 8 16 24 32 40 48

Pounds of Bleach per Ton of Pulp

Bri

ghtn

ess In

crea

se (%

MgO

)

Source: Pulp and Paper Manufacture, Volume 2 Mechanical Pulping, page 229

Mechanical Pulp Bleaching Hydrosulfite: Effect of Variables

Factor Ranges Optimum Results

Addition Rate 5-20 lb/ton NA 7-10 brightness points at standardrange

Temperature >30 C 60C At 30C no brightness obtained,higher temps than 60 C morebrightness and more brightnessreversion

Retention Time 20-60 min VariableDependent

Retention time dependent upon othervariables: temp, charge

pH 4.5-6.5 4.5-5.5 Lower pH results in faster bleaching.

Consistency 3-5% ? 3-5% does not affect brightness.Need to find results on newer highconsistency studies

Chelating Agent 2-5 lb/tonpulp

? Various agents can be used toeliminate the effect of metals

Factor Ranges Optimum Results

Addition Rate 5-20 lb/ton NA 7-10 brightness points at standardrange

Temperature >30 C 60C At 30C no brightness obtained,higher temps than 60 C morebrightness and more brightnessreversion

Retention Time 20-60 min VariableDependent

Retention time dependent upon othervariables: temp, charge

pH 4.5-6.5 4.5-5.5 Lower pH results in faster bleaching.

Consistency 3-5% ? 3-5% does not affect brightness.Need to find results on newer highconsistency studies

Chelating Agent 2-5 lb/tonpulp

? Various agents can be used toeliminate the effect of metals

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.

• 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.

O

O

OOH

O

O

O(-)

OH

(-)

OO

O

OH

(-)

CO2HCO2

(-)

Mechanical Pulp Bleaching Brightness Gain Using Peroxide

0

4

8

12

16

0 20 40 60

lb peroxide/ton pulp

Bri

ghtn

ess

0

4

8

12

16

0 20 40 60

lb peroxide/ton pulp

Bri

ghtn

ess

• 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.

• 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

Factor Range Results

Bleach charge 1 to 2% on pulp Amount charged effects amount of brightness.Also depends on how much degrades

pH 9-12 Too low pH = very slow bleaching, too high pH= degradation of peroxides. Peak point

Temperature 40-70 C Higher temp increases rate. Most of the pulpbrightening in first portion of reaction

Consistency 20-30 The higher the consistency, the better thebrightening

Time 10-60 min Rate dependent on the temperature

Silicates 1-5% wt% Increased silicates increases brightness. From 1to 5% silicates = 3 point brightness gain

Chelating agentsMgSO4 7H2O

0.25%0.05%

DPTA, etc. slows the degradation of peroxideEpsom salts stabilizes peroxide during storage

Factor Range Results

Bleach charge 1 to 2% on pulp Amount charged effects amount of brightness.Also depends on how much degrades

pH 9-12 Too low pH = very slow bleaching, too high pH= degradation of peroxides. Peak point

Temperature 40-70 C Higher temp increases rate. Most of the pulpbrightening in first portion of reaction

Consistency 20-30 The higher the consistency, the better thebrightening

Time 10-60 min Rate dependent on the temperature

Silicates 1-5% wt% Increased silicates increases brightness. From 1to 5% silicates = 3 point brightness gain

Chelating agentsMgSO4 7H2O

0.25%0.05%

DPTA, etc. slows the degradation of peroxideEpsom salts stabilizes peroxide during storage

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, -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.

• 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, -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.