Pulping and Bleaching
PSE 476/Chem E 471
Lecture #18
Bleaching Fundamentals and
Bleaching Sequences
PSE 476: Lecture 18
1
Bleaching Sequences
Agenda
• Bleaching chemistry fundamentals
» Cationic mechanisms
» Radical mechanisms
» Anionic mechanisms
• Bleaching chemicals nomenclature rules
• Typical Sequences
• Practice
PSE 476: Lecture 18
2
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.
PSE 476: Lecture 18
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Bleaching Mechanisms
Cationic
Radical
Anionic
Acidic
Acidic or Basic
Basic
+.
Cl , O3, OH+
.
.
ClO2 , O2
.
, Cl ,
-
-
ClO2 , HO2
.
HO , HO2.
Electrophiles
Aromatic and Olefinic
PSE 476: Lecture 18
Nucleophiles
Carbonyl and
Conjugated
Carbonyl
4
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

Cl
-
+
Cl
C
C
PSE 476: Lecture 18
Cl
-
Cl
5
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.
+ Cl
+
Cl
+
OCH3
OCH3
O
O
R
R
Cl
Cl
Cl
Cl
OCH3
OCH3
Cl
O
R
PSE 476: Lecture 18
Cl
O
O
O
R
6
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.
PSE 476: Lecture 18
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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
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Radical Reactions: Generalities
Ring Cleavage
Ortho Quinones
CO2H
CO2H
O
.
O
R=Aryl
OCH3
O
( )
O
R=H
X
OCH3
OCH3
O
O
R
Side Chain Cleavage
Ring Substitution
PSE 476: Lecture 18
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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
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Anionic Reactions:
Generalities
-
HC O
HC O
CH
CH
HC O
CH
O
CH
CH
XO CH
OCH3
OCH3
O
HC O
XO
O
CH
CH
OCH3
O
CHO
OCH3
OCH3
O
O
O
oxirane
X = OH, Cl
(-)
+ OX
(-)
O
O X
O
O
O
(-)
CO2
CO2H
X = OH, Cl, ClO
PSE 476: Lecture 18
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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
PSE 476: Lecture 18
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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
PSE 476: Lecture 18
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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
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Bleaching Sequences
Typical Bleaching Sequence (1990)
Stage
Chemicals
Charge
Temp.
(kg/ton pulp) ( C)
15 + 20
100
O
O2 + NaOH
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
Time
(min)
60
15
Bleaching Sequences
Traditional Sequences
Sequence
CEH
(C+D)EODED
Comments
Classical sequence for semi-bleached
pulp
Market pulp sequence of 1980’s
DEODED
ECF for kraft pulp
OZEOD
First sequence using ozone in North
America
Two stage for mechanical pulp
YP
PSE 476: Lecture 18
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Bleaching Sequences
Some Current (2001) Sequences
Sequence
OD0EOPD1E(p)D2
Comments
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
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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.
PSE 476: Lecture 18
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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.
PSE 476: Lecture 18
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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
PSE 476: Lecture 18
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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)
PSE 476: Lecture 18
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Bleaching Sequences
Agenda
• Bleaching chemistry fundamentals
» Cationic mechanisms
» Radical mechanisms
» Anionic mechanisms
• Bleaching chemicals nomenclature rules
• Typical Sequences
• Practice
PSE 476: Lecture 18
22