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Pulping and BleachingPSE 476/Chem E 471

Lecture #17Introduction to Bleaching

Lecture #17Introduction to Bleaching

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Agenda

• Brightness• General Bleaching Principles

» Chemistry» Process» Chemicals

- Description- Advantages/Disadvantages

• Brightness• General Bleaching Principles

» Chemistry» Process» Chemicals

- Description- Advantages/Disadvantages

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Why Bleach?

• Improve brightness.• Improve brightness stability.• Clean up pulp (impurities).

» Wood based (bark, resins, sand, shives).» Process based (carbon specs, rust, rubber).» External sources based (plastics, grease,

ash).

• Increase capacity of paper to accept printing.

• Improve brightness.• Improve brightness stability.• Clean up pulp (impurities).

» Wood based (bark, resins, sand, shives).» Process based (carbon specs, rust, rubber).» External sources based (plastics, grease,

ash).

• Increase capacity of paper to accept printing.

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The purpose of bleaching

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Bleach plant

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Brightness Determination (1)

•Light is scattered by fibers at air/fiber interfaces•Light is adsorbed by certain chemicals in the fibers (lignin)

•Light is scattered by fibers at air/fiber interfaces•Light is adsorbed by certain chemicals in the fibers (lignin)

Light

Transmitted

Reflected

Absorbed

Light shinning on a sheet of paper is either transmitted, adsorbed, or reflected.

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Brightness Determination (2)

• Brightness is measurement of how much light is reflected from a sheet of paper.

• Whiteness does not mean brightness.» Whiteness is a physical phenomena related to

how the eye views the paper.» A very white looking piece of paper may not

have high brightness.» Example: blue dye added to a yellow tinged

sheet of paper will give a white sheet of paper with low brightness.

• Brightness is measurement of how much light is reflected from a sheet of paper.

• Whiteness does not mean brightness.» Whiteness is a physical phenomena related to

how the eye views the paper.» A very white looking piece of paper may not

have high brightness.» Example: blue dye added to a yellow tinged

sheet of paper will give a white sheet of paper with low brightness.

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Brightness Determination (3)

• Brightness determination method:» Light reflectance measured and compared to light

reflectance from MgO.» MgO assumed to reflect 100% light.» Brightness is reported as % of MgO reflectance

(85 brightness is equivalent to 85% of MgO).• Variables:

» Angle of light: Light is applied to sheet at 45° angle.

» Wavelength: 457 nm (blue light most sensitive).• Pine kraft

» Unbleached-ISO 23-28%» Semi bleached-ISO 60-80%» Bleached-ISO 88-91%

• Brightness determination method:» Light reflectance measured and compared to light

reflectance from MgO.» MgO assumed to reflect 100% light.» Brightness is reported as % of MgO reflectance

(85 brightness is equivalent to 85% of MgO).• Variables:

» Angle of light: Light is applied to sheet at 45° angle.

» Wavelength: 457 nm (blue light most sensitive).• Pine kraft

» Unbleached-ISO 23-28%» Semi bleached-ISO 60-80%» Bleached-ISO 88-91%

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General Principles

• Two types of bleaching:» Lignin removing: chemical pulps.» Lignin retaining: mechanical pulps.

• Bleaching is used because at a certain point in the pulping process, carbohydrate degradation becoming greater than lignin removal.» Bleaching chemicals are more selective for lignin.» Bleaching chemicals much more expensive than

pulping chemicals so they are not used in pulping.

• Two types of bleaching:» Lignin removing: chemical pulps.» Lignin retaining: mechanical pulps.

• Bleaching is used because at a certain point in the pulping process, carbohydrate degradation becoming greater than lignin removal.» Bleaching chemicals are more selective for lignin.» Bleaching chemicals much more expensive than

pulping chemicals so they are not used in pulping.

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General Principles:Chemistry

• Pulping» Pulping typically involves cleavage of ether

linkages and some substitution (sulfonation).

• Bleaching» Bleaching involves attacks on aromatic rings,

olefinic structures, and carbonyl groups.» Substitution reactions play a big role.

• Pulping» Pulping typically involves cleavage of ether

linkages and some substitution (sulfonation).

• Bleaching» Bleaching involves attacks on aromatic rings,

olefinic structures, and carbonyl groups.» Substitution reactions play a big role.

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Multiple stages of bleaching

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General Principles:Process

• Bleaching uses a combination of chemicals in series.» One chemical alone will not remove residual lignin.» Each step reacts with material modified in previous step.

• Bleaching uses a combination of chemicals in series.» One chemical alone will not remove residual lignin.» Each step reacts with material modified in previous step.

Unbleached

PulpBleachedPulp

EO D EO D

NaOHO2

ClO2NaOH

O2

ClO2

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Washing

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General Principles:Chemicals (1)

Chemical Formula Stage Manufacture Form Applied

Chlorine Cl2 C By electrolysis ofNaCl

PressurizedGas

Hypochlorite NaOCl H By reaction of Cl2

with NaOH (on site)Solution:~40g/l as Cl2

Chlorinedioxide

ClO2 D On-site by reductionof chlorate (ClO3)

Solution:~10g/l

Oxygen O2 O Separation from air.Shipped or generatedon site

PressurizedGas

Chemical Formula Stage Manufacture Form Applied

Chlorine Cl2 C By electrolysis ofNaCl

PressurizedGas

Hypochlorite NaOCl H By reaction of Cl2

with NaOH (on site)Solution:~40g/l as Cl2

Chlorinedioxide

ClO2 D On-site by reductionof chlorate (ClO3)

Solution:~10g/l

Oxygen O2 O Separation from air.Shipped or generatedon site

PressurizedGas

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General Principles:Chemicals (2)

Chemical Formula Stage Manufacture Form Applied

Hydrogenperoxide

H2O2 P By reduction ofoxygen: shipped as50% solution

Solution 2-5%

Ozone O3 Z On site by silentelectric discharge inO2

PressurizedGas (12% O3

in O2)

Hydrosulfite Na2S2O4 Y Reduction of bisulfite Solution

Enzymes - X Fermentation Solution

SodiumHydroxide

NaOH E By electrolysis ofNaCl; 50% solution

Solution~10%

Chemical Formula Stage Manufacture Form Applied

Hydrogenperoxide

H2O2 P By reduction ofoxygen: shipped as50% solution

Solution 2-5%

Ozone O3 Z On site by silentelectric discharge inO2

PressurizedGas (12% O3

in O2)

Hydrosulfite Na2S2O4 Y Reduction of bisulfite Solution

Enzymes - X Fermentation Solution

SodiumHydroxide

NaOH E By electrolysis ofNaCl; 50% solution

Solution~10%

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General Principles:Chemicals (3a)

Chemical Function Advantages Disadvantages

Cl2 Oxidize and chlorinate lignin

Effective, economical, good particle removal

Banned in USA because of environmental problems

NaOCl Oxidize, decolorize, solubilize lignin

Easy to make and use, low cost

Can cause strength loss, CHCl3 formation Banned in USA

ClO2 1. Oxidize, decolorize, solubilize lignin

2. Can protect cellulose

Achieves high brightness without strength loss. Good particle bleaching

Made on site, some chlorinated organics, highly corrosive

Chemical Function Advantages Disadvantages

Cl2 Oxidize and chlorinate lignin

Effective, economical, good particle removal

Banned in USA because of environmental problems

NaOCl Oxidize, decolorize, solubilize lignin

Easy to make and use, low cost

Can cause strength loss, CHCl3 formation Banned in USA

ClO2 1. Oxidize, decolorize, solubilize lignin

2. Can protect cellulose

Achieves high brightness without strength loss. Good particle bleaching

Made on site, some chlorinated organics, highly corrosive

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General Principles:Chemicals (3b)

Chemical Function Advantages Disadvantages

O2 Oxidize andsolubilize lignin

Low chemicalcost, Cl freeeffluent

Large amount ofequipment, somestrength loss

H2O2 Oxidize anddecolorize lignin

Easy to use, lowcapital cost

High chemical cost,Poor particle bleachingStrength losses

O3 Oxidize, decolorize,solubilize lignin

Effective,chlorine freeeffluent

Must make on site,Poor particle bleachingStrength losses

Chemical Function Advantages Disadvantages

O2 Oxidize andsolubilize lignin

Low chemicalcost, Cl freeeffluent

Large amount ofequipment, somestrength loss

H2O2 Oxidize anddecolorize lignin

Easy to use, lowcapital cost

High chemical cost,Poor particle bleachingStrength losses

O3 Oxidize, decolorize,solubilize lignin

Effective,chlorine freeeffluent

Must make on site,Poor particle bleachingStrength losses

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General Principles:Chemicals (3c)

Chemical Function Advantages Disadvantages

Na2S2O4 Reduce anddecolorize highyield pulps

Easy to use, lowcapital costs

Decomposes rapidly,low brightness gain

Enzymes

(Xylanase)

Catalyze xylanremoval, aid inlignin removal

Easy to use, lowcapital costs

Limited effectiveness,high cost

NaOH Hydrolyzechlorolignin andsolubilize lignin

Effective andeconomical

Darkens pulp

Chemical Function Advantages Disadvantages

Na2S2O4 Reduce anddecolorize highyield pulps

Easy to use, lowcapital costs

Decomposes rapidly,low brightness gain

Enzymes

(Xylanase)

Catalyze xylanremoval, aid inlignin removal

Easy to use, lowcapital costs

Limited effectiveness,high cost

NaOH Hydrolyzechlorolignin andsolubilize lignin

Effective andeconomical

Darkens pulp

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Groups of bleaching chemicals (2)

• Bleaching chemicals can be divided into three groups according to their function:

• 1 Group» The chlorine (Cl2), ozone (O3) reacts with all

aromatic lignin units (phenolic groups and their =bonds)

• 2 Group» The chlorine dioxide(ClO2) and oxygen (O2) reacts in

general with lignin structures that have free phenolic hydroxyl groups

• 3 Group» The hypochlorite (H) and hydrogen peroxide (H2O2)

reacts only with certain functional groups, for example carbonyl groups

• Bleaching chemicals can be divided into three groups according to their function:

• 1 Group» The chlorine (Cl2), ozone (O3) reacts with all

aromatic lignin units (phenolic groups and their =bonds)

• 2 Group» The chlorine dioxide(ClO2) and oxygen (O2) reacts in

general with lignin structures that have free phenolic hydroxyl groups

• 3 Group» The hypochlorite (H) and hydrogen peroxide (H2O2)

reacts only with certain functional groups, for example carbonyl groups

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• Bleaching chemicals are used primarily as oxidants, to break down residual lignin and to increase its solubility.

• Mode of operation:» Electrophiles (oxidative reactions, low pH,

involve cations)» Nucelophiles (reductive reactions, high pH,

anions)» Radicals

• Bleaching chemicals are used primarily as oxidants, to break down residual lignin and to increase its solubility.

• Mode of operation:» Electrophiles (oxidative reactions, low pH,

involve cations)» Nucelophiles (reductive reactions, high pH,

anions)» Radicals

Bleaching reactions

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Bleaching Generalities

• It is important to note that when bleaching with a specific reagent, it will be converted into a number of different reactive species which will react with lignin and carbohydrates differently. A simple example is when chlorine gas is added to water; both hypochlorous acid and/or hypochorite is formed depending on the pH.

• It is important to note that when bleaching with a specific reagent, it will be converted into a number of different reactive species which will react with lignin and carbohydrates differently. A simple example is when chlorine gas is added to water; both hypochlorous acid and/or hypochorite is formed depending on the pH.

Cl2 + H2O H+ + Cl - + HOCl pk = 3.4

HOCl H+ + ClO - pk = 7.5