1. Pulping and Bleaching PSE 476/Chem E 471
Lecture #18
Bleaching 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
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.
PSE 476: Lecture 18

4. Bleaching Mechanisms
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:
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.
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.
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
Side Chain Cleavage
Ring Substitution
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
oxirane
PSE 476: Lecture 18

12. Bleaching Sequences Nomenclature Rules (1)
Nomenclature rules found in TAPPI Information Sheet TIS
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
PSE 476: Lecture 18

14. Differences in Chlorine Dioxide Stages
Condition D0 D1 D2
Final pH
3.5-5
Temp °C
40°C - 60°C
55°C - 75°C
60°C - 85°C
Consistency
10-15%
Time
30-80min
2-4 hours
Charge
1-2 times kappa #
4-6 times kappa #
PSE 476: Lecture 18

15. Bleaching Sequences Typical Bleaching Sequence (1990)
PSE 476: Lecture 18

16. Bleaching Sequences Traditional Sequences
PSE 476: Lecture 18

17. Bleaching Sequences Some Current (2001) Sequences
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.
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.
PSE 476: Lecture 18

20. Bleaching Sequences Name the Sequence (1)
O2/wash/Cl2 & ClO2/wash/NaOH then H2O2/wash /ClO2
Cl2/wash/NaOH/wash/NaOCl
Cl2 & ClO2 /wash/NaOH/wash/NaOCl/wash/ClO2 /wash/NaOH/wash/ClO2
O2/wash/ClO2 /Cl2 /wash/NaOH then O2/wash/ClO2 /wash/NaOH then H2O2/wash/ClO2
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)
PSE 476: Lecture 18

22. Bleaching Sequences Agenda
Bleaching chemistry fundamentals
Cationic mechanisms
Radical mechanisms
Anionic mechanisms
Bleaching chemicals nomenclature rules
Typical Sequences
Practice
PSE 476: Lecture 18