PSE 476: Lecture 18 1

Pulping and BleachingPSE 476/Chem E 471

Lecture #18Bleaching Fundamentals and

Bleaching Sequences

Lecture #18Bleaching Fundamentals and

Bleaching Sequences

PSE 476: Lecture 18 2

Bleaching Sequences Agenda

• Bleaching chemistry fundamentals» Cationic mechanisms» Radical mechanisms» Anionic mechanisms

• Bleaching chemicals nomenclature rules

• Typical Sequences• Practice

• Bleaching chemistry fundamentals» Cationic mechanisms» Radical mechanisms» Anionic mechanisms

• Bleaching chemicals nomenclature rules

• Typical Sequences• Practice

PSE 476: Lecture 18 3

Bleaching: Basic Chemical Principles

• Bleaching reactions can be divided into anionic, cationic (?) and radical reactions.

• There are a multitude of different reactions that occur during bleaching.

• We are going to cover the chemistry in very generic terms.

• Bleaching reactions can be divided into anionic, cationic (?) and radical reactions.

• There are a multitude of different reactions that occur during bleaching.

• We are going to cover the chemistry in very generic terms.

PSE 476: Lecture 18 4

Bleaching Mechanisms

Cationic Radical Anionic

Acidic Acidic or Basic Basic

Cl+.

, O3, OH+ ClO2

., O2

., Cl

.,

HO., HO2

.

ClO2

-, HO2

-

Electrophiles Nucleophiles

Aromatic and Olefinic Carbonyl and Conjugated

Carbonyl

PSE 476: Lecture 18 5

Cationic Reactions

• Cationic reactions occur under acidic conditions.• Reactive species:

» Chlorine: Cl+

» Peroxy acids: OH+

» Ozone: O3

• How can these species be cations???????» Electrophilic substitution reaction:

C

CCl Cl

-

C Cl

Cl -+

PSE 476: Lecture 18 6

Cationic Reactions: Generalities (2)

The most important reaction is electophillic substitution onto the aromatic ring.» There can be multiple

substitutions.» Substitution at #1 ring

position can later cause side chain elimination.

» Substitution at #3 or #4 position can lead to the formation of an ortho quinone.

» There is no ring cleavage.

The most important reaction is electophillic substitution onto the aromatic ring.» There can be multiple

substitutions.» Substitution at #1 ring

position can later cause side chain elimination.

» Substitution at #3 or #4 position can lead to the formation of an ortho quinone.

» There is no ring cleavage.

OCH3

OR

OCH3

OR

+ Cl+Cl+

OCH3

OR

OCH3

O

Cl

R

O

O

Cl

Cl

Cl

Cl

Cl

PSE 476: Lecture 18 7

Radical Reactions: Generalities

• Nobody adds free radicals to a bleaching reaction; they are formed from other additives.

• Examples:» Chlorine: Cl• (chlorine radical)» Oxygen: -O2• (superoxide radical)

HO• (hydroxyl radical)

• Many free radicals react very rapidly with lignin so they are good except that they degrade carbohydrates very rapidly so they are also bad.

• Nobody adds free radicals to a bleaching reaction; they are formed from other additives.

• Examples:» Chlorine: Cl• (chlorine radical)» Oxygen: -O2• (superoxide radical)

HO• (hydroxyl radical)

• Many free radicals react very rapidly with lignin so they are good except that they degrade carbohydrates very rapidly so they are also bad.

PSE 476: Lecture 18 8

Radical Reactions: Generalities (2)

• Pathways are pH dependent.• Free radicals couple with aromatic rings.

» This and further mechanisms affected by free versus etherified phenolic hydroxyls.

» Results in generation of free radical structures.

• These structures can undergo additional substitutions and oxidation reactions. » Generation of ortho quinones.» Ring opening reactions.» Ring substitution by bleaching agent.» Some side chain cleavage.

• Carbohydrates will react and are degraded.

PSE 476: Lecture 18 9

Radical Reactions: Generalities

Ring Cleavage Ortho Quinones

Ring SubstitutionSide Chain Cleavage

O

OCO2HCO2H

OCH3

O

X

R

O

OCH3

(.)

R=Aryl

R=H

O

OCH3

O

PSE 476: Lecture 18 10

Anionic Reactions:Generalities

• These are alkaline bleaching reactions.• ClO -, HOO – added reagents

» Other species generated: . O2-, . O-, etc.

» These chemicals are involved in nucleophilic attacks.

• Target of attacks: carbonyls and conjugated carbonyls.

• Products of reactions:» Ring opening: Dicarboxylic acids» Side chain cleavage

PSE 476: Lecture 18 11

Anionic Reactions: Generalities

O

OCH3

CH

CH

HC O

O

OCH3

CH

CH

HC O-

XO

O

OCH3

CHXO

HC O

O- CH

O

OCH3

HC O

OCH

CH

O

OCH3

CHO

X = OH, Cl

O

O

OOO

X

(-)+ OX

(-)

CO2HCO2

(-)

X = OH, Cl, ClO

oxirane

PSE 476: Lecture 18 12

Bleaching Sequences Nomenclature Rules (1)

• Nomenclature rules found in TAPPI Information Sheet TIS 0606-12.

• Bleaching sequences are described by capital letters which describe the chemicals used.» C = chlorine, H = hypochlorite, D = chlorine

dioxide, O = oxygen (with NaOH), P = peroxide, Z = ozone, Y = hydrosulfite, X = enzymes, E = NaOH, Q = chelants, Paa = peracetic acid

• Letter represents each chemical followed by a washing step.» CED = Cl2/wash/NaOH extraction/wash/

ClO2/wash

• Nomenclature rules found in TAPPI Information Sheet TIS 0606-12.

• Bleaching sequences are described by capital letters which describe the chemicals used.» C = chlorine, H = hypochlorite, D = chlorine

dioxide, O = oxygen (with NaOH), P = peroxide, Z = ozone, Y = hydrosulfite, X = enzymes, E = NaOH, Q = chelants, Paa = peracetic acid

• Letter represents each chemical followed by a washing step.» CED = Cl2/wash/NaOH extraction/wash/

ClO2/wash

PSE 476: Lecture 18 13

Bleaching Sequences Nomenclature Rules (2)

• If washing is omitted, a parenthesis is added around the steps.» C(EO)D = Cl2 /wash/NaOH/Oxygen/wash/ClO2/wash

• If chemicals are added simultaneously, the letters are placed in parenthesis separated by a + sign. C(E+O)D = Cl2 /wash/NaOH+Oxygen/wash/ClO2/wash» People will also list this as CE/OD

• Sometimes small amounts of chemicals are added to fortify a step. The additives are typically listed as subscripts: EO or EP or EOP

• Subscripts are also used to designate differences in conditions (ie temp, pH, etc): D0, D1, D2

• If washing is omitted, a parenthesis is added around the steps.» C(EO)D = Cl2 /wash/NaOH/Oxygen/wash/ClO2/wash

• If chemicals are added simultaneously, the letters are placed in parenthesis separated by a + sign. C(E+O)D = Cl2 /wash/NaOH+Oxygen/wash/ClO2/wash» People will also list this as CE/OD

• Sometimes small amounts of chemicals are added to fortify a step. The additives are typically listed as subscripts: EO or EP or EOP

• Subscripts are also used to designate differences in conditions (ie temp, pH, etc): D0, D1, D2

PSE 476: Lecture 18 14

Differences in Chlorine Dioxide Stages

Condition D0 D1 D2

Final pH 1.5-2.5 3.5-5 3.5 - 5

Temp °C 40°C - 60°C

55°C - 75°C

60°C - 85°C

Consistency

10-15% 10-15% 10-15%

Time 30-80min 2-4 hours 2-4 hours

Charge 1-2 times kappa #

4-6 times kappa #

4-6 times kappa #

PSE 476: Lecture 18 15

Bleaching Sequences Typical Bleaching Sequence (1990)

Stage Chemicals Charge (kg/ton pulp)

Temp. ( C)

Time (min)

O O2 + NaOH 15 + 20 100 60

D+C ClO2 + Cl2 12 + 18 70 45

EO NaOH + O2 15 + 5 70 120

D ClO2 17 70 180

EP NaOH + H2O2 8 + 3 70 90

D ClO2 5 70 180

PSE 476: Lecture 18 16

Bleaching Sequences Traditional Sequences

Sequence Comments CEH Classical sequence for semi-bleached

pulp (C+D)EODED Market pulp sequence of 1980’s

DEODED ECF for kraft pulp

OZEOD First sequence using ozone in North America

YP Two stage for mechanical pulp

Sequence Comments CEH Classical sequence for semi-bleached

pulp (C+D)EODED Market pulp sequence of 1980’s

DEODED ECF for kraft pulp

OZEOD First sequence using ozone in North America

YP Two stage for mechanical pulp

PSE 476: Lecture 18 17

Bleaching Sequences Some Current (2001) Sequences

Sequence Comments OD0EOPD1E(p)D2 Don't know why (p)????

OD0EOPD1D2

OD0EOPD1P

Oq(OP)(ZE)DD Z-ECF Bleaching

(Z(EO))DnD Dn designates the addition of NaOH to bring up the pH

Oq(OP)(ZQ)(PO) TCF Bleaching

Sequence Comments OD0EOPD1E(p)D2 Don't know why (p)????

OD0EOPD1D2

OD0EOPD1P

Oq(OP)(ZE)DD Z-ECF Bleaching

(Z(EO))DnD Dn designates the addition of NaOH to bring up the pH

Oq(OP)(ZQ)(PO) TCF Bleaching

PSE 476: Lecture 18 18

Elemental Chlorine Free

• ECF pulping signifies that there is no elemental chlorine (Cl2) used in the bleaching sequence.» Most often oxygen or ClO2 is used for lignin

removal.» Brightening is most often accomplished by

using DED.» Peroxide is used mainly to reinforce oxygen

or in the extraction stages.

• ECF pulping signifies that there is no elemental chlorine (Cl2) used in the bleaching sequence.» Most often oxygen or ClO2 is used for lignin

removal.» Brightening is most often accomplished by

using DED.» Peroxide is used mainly to reinforce oxygen

or in the extraction stages.

PSE 476: Lecture 18 19

Total Chlorine Free

• TCF means that there is no chlorine of any kind used in the bleaching process.

• There has been significant effort made in this direction for the following reasons:» There is no possibility of the generation of AOX (AOX

= Absorbable Organically Bound Halogens).» Chlorine free bleach plant effluent can be burned

thus allowing a pulp mill to have basically no effluent.• The problem to date with TCF is that it has not

been possible to obtain desired brightness without significant strength loss.

• TCF means that there is no chlorine of any kind used in the bleaching process.

• There has been significant effort made in this direction for the following reasons:» There is no possibility of the generation of AOX (AOX

= Absorbable Organically Bound Halogens).» Chlorine free bleach plant effluent can be burned

thus allowing a pulp mill to have basically no effluent.• The problem to date with TCF is that it has not

been possible to obtain desired brightness without significant strength loss.

PSE 476: Lecture 18 20

Bleaching Sequences Name the Sequence (1)

1. O2/wash/Cl2 & ClO2/wash/NaOH then H2O2/wash /ClO2

2. Cl2/wash/NaOH/wash/NaOCl

3. Cl2 & ClO2

/wash/NaOH/wash/NaOCl/wash/ClO2

/wash/NaOH/wash/ClO2

4. O2/wash/ClO2 /Cl2 /wash/NaOH then O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2

5. O2/wash/EDTA/wash/NaOH then H2O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2

1. O2/wash/Cl2 & ClO2/wash/NaOH then H2O2/wash /ClO2

2. Cl2/wash/NaOH/wash/NaOCl

3. Cl2 & ClO2

/wash/NaOH/wash/NaOCl/wash/ClO2

/wash/NaOH/wash/ClO2

4. O2/wash/ClO2 /Cl2 /wash/NaOH then O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2

5. O2/wash/EDTA/wash/NaOH then H2O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2

PSE 476: Lecture 18 21

Bleaching Sequences Name the Sequence (2)

• Can you name this sequence?• Hint: Oxygen bleaching (O) is typically carried

out in only the first phase. This allows the effluent to be sent to the recovery system (no possibility of Cl compounds)

• Can you name this sequence?• Hint: Oxygen bleaching (O) is typically carried

out in only the first phase. This allows the effluent to be sent to the recovery system (no possibility of Cl compounds)

PSE 476: Lecture 18 22

Bleaching Sequences Agenda

• Bleaching chemistry fundamentals» Cationic mechanisms» Radical mechanisms» Anionic mechanisms

• Bleaching chemicals nomenclature rules

• Typical Sequences• Practice

• Bleaching chemistry fundamentals» Cationic mechanisms» Radical mechanisms» Anionic mechanisms

• Bleaching chemicals nomenclature rules

• Typical Sequences• Practice