1. Pulping and Bleaching PSE 476/Chem E 471
Lecture #20
Bleaching: Ozone and H2O2

2. Ozone/Hydrogen Peroxide Bleaching: Agenda
Advantages/Disadvantages
Lignin & Carbohydrate Reactions
Effect of Process Variables
Hydrogen Peroxide

3. Ozone Advantages/Disadvantages
Powerful oxidizer
No chlorinated hydrocarbons generated
Disadvantages
Very low water solubility ( °C)
Toxic at low levels but readily detectable at very low levels
Less selective than Cl2 or ClO2

4. Ozone Conditions
Ozone generated from oxygen or air through an ozone generator (electric discharge).
Ozone applied with air or oxygen (4-14%).
Ozone is used as a delignifier to replace Cl2 and ClO2.
Does not achieve the same level of delignification.
Requires additional other stages.

5. Ozone

6. Ozone Bleaching Lignin Reactions
Methanol is a VOC which is monitored
coming off the bleaching sequence
Ozone functions as a electrophile
under acidic conditions
Aliphatic double bonds are cleaved.
Aromatic ring is cleaved forming muconic acids.
Hydroxyl groups are introduced into the ring
Radical oxygen species -O2•, HO• generated.
Source: Wood Chemistry, Sjostrom page 188

7. Ozone Bleaching Carbohydrate Reactions
Ozone is 106 times more reactive towards lignin than towards carbohydrates.
Unfortunately, the reactions products of ozone (superoxide, hydroxyl radical) are very reactive towards carbohydrates.
Therefore, ozone treatments are limited because of cellulose degradation.
Reactions are the same as described under oxygen bleaching conditions.

8. Ozone Bleaching Conditions (1)
Ozone Charge
Low solubility of O3 means that the rate determining step in oxidation is the accessibility of fiber to gas.
Rate is increased by increasing concentration of O3 in gas as well as increasing the pressure of the system.
Consistency
Low (3.5%): Low viscosity allows good mixing. Diffusion rate determining step.
Medium: Vigorous mixing required.
High (>25%) Pulp “fluffed” to achieve gas contact. Very little free water.

9. Ozone Bleaching Conditions (2)
Temperature/Time
Low temperature improves selectivity by reducing ozone degradation to radicals.
25-35°C for high/medium consistency.
As low as 0°C for low consistency.
Time: Very short (4 minutes ?). pH
Most efficient at a pH of approximately 2.

10. H2O2 Bleaching Conditions
Alkaline Conditions
For brightening, HO2- is the reactive species: requires alkaline conditions (>pH 9).
Chelants
Chelating agents used prior to bleaching to remove metals that decompose H2O2 to radical species.
Three methods of use
Alone for brightening, the end of sequence.
With oxygen for lignin removal ( %)-booster, beginning of the sequence.
With NaOH in extraction stage.

11. H2O2 Bleaching Lignin Reactions
Delignification
H2O2 does not degrade (remove) lignin.
At elevated temperatures (90°C), H2O2 is degraded to superoxide and hydroxide radicals which degrade the lignin.
Ring opening reactions.
Brightening
Lower temperature reaction to eliminate radical formation.
HO2- reacts mainly with carbonyl compounds.

12. H2O2 Bleaching Carbohydrate Reactions
Under delignification conditions (90°C), H2O2 less selective than Cl2, ClO2, and oxygen.
Reduction in pulp viscosity (strength).
Two major degradation pathways.
Glycosidic cleavage by radicals (OH•).
Peeling induced through oxidation.

13. H2O2 Bleaching Conditions (1)
Charge
Higher levels of H2O2 and NaOH increase delignification but also cellulose degradation.
Consistency
10% typical although increases to 25% improve delignification.
Temperature
Delignification: Typical temperature of 90°C although temperatures as high as 120°C work.
Brightening: °C.

14. H2O2 Bleaching Conditions (2)
Time: typically °C
pH: 9-12, typically >10.5
Metals
Removal of metals using chelants (EDTA) is important to reduce cellulose loss.
Even though radical formation is necessary for lignin removal, selectivity is better if the generation of radicals is slow.
Silicates do not work, magnesium sulfate does provide some protection.