Pulping and Bleaching PSE 476/Chem E 471

Lecture #20 Bleaching: Ozone and H 2 O 2

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Ozone/Hydrogen Peroxide Bleaching: Agenda

• Ozone » Advantages/Disadvantages » Lignin & Carbohydrate Reactions » Effect of Process Variables • Hydrogen Peroxide » Advantages/Disadvantages » Lignin & Carbohydrate Reactions » Effect of Process Variables 2

Ozone Advantages/Disadvantages

• Advantages: » Powerful oxidizer » No chlorinated hydrocarbons generated • Disadvantages » Very low water solubility (0.98 mg/liter @ 25°C) » Toxic at low levels but readily detectable at very low levels » Less selective than Cl 2 or ClO 2 3

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 Cl 2 and ClO 2.

» Does not achieve the same level of delignification.

» Requires additional other stages.

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Ozone

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Ozone Bleaching Lignin Reactions

O C CHO O OCH 3 C HC O 3 O 3 O OCH 3 O 3 C HC OH O OCH 3 C HC O O H 2 O O OCH 3 C HC

Methanol is a VOC which is monitored coming off the bleaching sequence

O O O OH + CH 3 OH • 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 -O 2 •, HO• generated.

Source: Wood Chemistry, Sjostrom page 188 6

Ozone Bleaching Carbohydrate Reactions

• Ozone is 10 6 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.

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Ozone Bleaching Conditions (1)

• Ozone Charge » Low solubility of O 3 means that the rate determining step in oxidation is the accessibility of fiber to gas.

» Rate is increased by increasing concentration of O 3 as well as increasing the pressure of the system.

in gas • 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.

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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.

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H

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O

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Bleaching Conditions

• Alkaline Conditions » For brightening, HO 2 is the reactive species: requires alkaline conditions (>pH 9).

• Chelants » Chelating agents used prior to bleaching to remove metals that decompose H 2 O 2 to radical species.

• Three methods of use » Alone for brightening, the end of sequence.

» With oxygen for lignin removal (0.2-0.5%)-booster, beginning of the sequence.

» With NaOH in extraction stage.

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H

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O

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Bleaching Lignin Reactions

• Delignification » H 2 O 2 does not degrade (remove) lignin.

» At elevated temperatures (90°C), H 2 O 2 is degraded to superoxide and hydroxide radicals which degrade the lignin.

- Ring opening reactions.

• Brightening » Lower temperature reaction to eliminate radical formation.

» HO 2 reacts mainly with carbonyl compounds.

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H

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O

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Bleaching Carbohydrate Reactions

• Under delignification conditions (90°C), H 2 O 2 less selective than Cl 2 , ClO 2 , and oxygen.

» Reduction in pulp viscosity (strength).

• Two major degradation pathways.

» Glycosidic cleavage by radicals (OH•). » Peeling induced through oxidation.

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H

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O

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Bleaching Conditions (1)

• Charge » Higher levels of H 2 O 2 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: 50-70°C.

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H

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O

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Bleaching Conditions (2)

• Time: typically 120 minutes @ 90 ° 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.

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