1 pulping and bleaching pse 476/chem e 471 lecture #17 introduction to bleaching lecture #17...
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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
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