Chemistry of Chlorine Dioxide Pulp Bleaching

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Bleaching History • Bleach - blecan (Anglo Saxon) – to fade • First dates back to ancient Gauls: sunlight on vegetable fibers moistened with alkaline solution from wood or vegetable ash • Process: alkali treatment, exposure on grassy meadows to sun, washing, repeat, final treatment with lactic acid from sour milk – Became known as “grass bleaching” – Perfected around Haarlem, Holland – Material generally of linen fibers [email protected]

Bleaching History • 1756 – Francis Home (Scotland) – Discovered that by substituting dilute sulfuric acid for lactic acid in last step, operating time is reduced. – Still is a grass bleaching operation and not used yet for paper – “White” paper made from sorted white rags

• 1774 – Karl Wilhelm Scheele (Swedish Chemist) – discovered chlorine and pioneered use as a bleaching chemical on vegetable fibers

• Berthollet – French chemist – discovered chlorine could be absorbed in solution of caustic potash and resulting solution had efficient bleaching action with less degrading effect on the finished goods (product) [email protected]

Bleaching History • Thomas Henry – English – extended use of bleaching solution to paper (rag)

• 1798 – Charles Tennant – Scotland – formulated calcium hypochlorite by reaction of chlorine gas with milk of lime

• 1799 – Charles Tennant – patent on production of bleaching powder by action of chlorine on slaked lime – became world’s most dominant bleaching agent [email protected]

Bleaching History • 1920’s: Continuous operating bleaching introduced by Thorne in multi-stage bleaching – for purification of pulp from demand for large tonnages of nitrocellulose during WWI – 2 stage hypochlorite then added alkaline extraction in between hypochlorite stages

• Use of Chlorine dioxide investigated from 1920 – 1940, put into production 1940’s [email protected]

Chlorine Dioxide History • 1946 – 1980’s used as a later bleaching stage, not for delignification – CEHDED, CEDED

• Late 1980’s realized that ClO2 and Cl2 used together had a higher delignification efficiency than Cl2 alone • Environmental regulations dictated the switch from elemental chlorine (Cl2 & HOCl) to ClO2 and other TCF methods [email protected]

Chlorine Dioxide • Molecular Weight: 67.45 • Boiling Point: 11°C • Yellow green to orange gas, with a sharp pungent odor • Water soluble, 10 g/L • Oxidant • Density: 2.4 x’s water • Decomposes to Cl2 and O2 with noise, heat, flame, and minor pressure wave [email protected]

Formation of Chlorine Dioxide • Reduction of chlorate in acidic medium: – ClO3- + 2 H+ + e- Æ ClO2 + H2O

• Oxidation of chlorite ion: – ClO2- Æ ClO2 + e-

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Reductive Chemistry • Reducing agents are used to make the e– – – –

SO2 + 2 H2O Æ SO42- + 4 H+ + 2 eCH3OH + H2O Æ HCOOH + 4 H+ + 4 eCl- Æ ½ Cl2 + eH2O2 Æ O2 + 2 H+ + 2 e-

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Making Chlorine Dioxide • 2 ClO3- + SO2 Æ 2 ClO2 + SO42• 4 ClO3- + CH3OH + 4 H+ Æ 4 ClO2 + HCOOH + 3 H2O • ClO3- + Cl- + 2 H+ Æ ClO2 + Cl2 + H2O • 2 ClO3- + H2O2 + 2 H+ Æ 2 ClO2 + O2 + 2 H2O • ClO3- + 6 H+ + 6e- Æ Cl- + 3 H2O [email protected]

Chlorine Dioxide Chemical properties: One of several known oxides of chlorine. Chlorine dioxide is a powerful oxidizing agent - an electron receiver. This means that the chlorine dioxide molecule is in constant search for an additional electron. → Disinfection The destruction of pathogenic and other kinds of microorganisms by physical or chemical means

When a bacterial cell comes into contact with chlorine dioxide it donates an electron from its cell wall, thereby creating a breach in the cell wall through which cell contents pass in an attempt to bring the concentrations on either side of the cell membrane to equilibrium. The cell dies through lysis.

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Chlorine Dioxide How long has chlorine dioxide been used? Chlorine dioxide has found widespread use since the early 1950s in the treatment of drinking water and swimming pools.

Today, chlorine dioxide is used by many large cities in Europe, such as (1956) Brussels, Zurich, Düsseldorf, Toulouse and Vienna, to sanitize the drinking water supply.

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Chlorine Dioxide Chlorine dioxide has many applications: •

Food industry Fruit and vegetable washing Meat and poultry disinfection Sanitizing food process equipment

•

Medical “Tristel” sterilizing solutions for medical instruments Air disinfection and building decontamination. (2001 anthrax attacks, US)

•

Personal hygiene Mouthwashes (~0.003%) Toothpastes Contact lens cleaners

•

Other industry Cooling systems and towers in the control of Legionella.(Gram negative bacterium) [email protected]

Bleaching Pulp with Chlorine Dioxide Advantages ¾High brightness and brightness stability ¾Excellent for shive and dirt removal - the best ¾Highly selective - little degradation of pulp ¾Less organic chlorine than Cl2 and ClO¾Radical scavenger

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Bleaching Pulp with Chlorine Dioxide Disadvantages ¾ Highly explosive – hence generate on-site ¾ Highly corrosive - need titanium equipment high capital cost ¾ Expensive ¾ Toxic - handle with care ¾ AOX ¾ Chlorate formation ¾ 26-40% loss in oxidation power [email protected]

ClO2 Delignification Process Conditions • Total Chemical Charge: 0.15 – 0.25 kappa factor • Chlorine dioxide charge: 25 – 100% of the total • Temperature: 30 -60° C • Total Time: 20 – 60 minutes • End pH: 1.5 – 3 • Consistency: 3 – 4% [email protected]

Chlorine Dioxide Puffs • Puff – deompostion of chlorine dioxide – 2 ClO2 Æ Cl2 + 2 O2 + heat – Low speed wave of reaction (< 1m/s) • Explosion: > 300 m/s

– Generators designed for up to 200 mm Hg

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AOX, kg/admt

AOX vs. ClO2 Substitution 8 7 6 5 4 3 2 1 0

Kappa Unbl 30 EO 23.5 EOP 20.5 O 16.6

0

20

40

60

80

100

ClO2 Substitution, % [email protected]

Liebergott

Substitution of Chlorine Dioxide • AOX Generation – AOX (kg/t) = 0.1 (Cl2, kg/t active chemical) – AOX (kg/t) = 0.1(1/2.63)(.526)(ClO2, kg/t active chemical) 0.02(ClO2, kg/t act chemical)

• On an equal weight basis ClO2 is 2.63 times as as reactive as Cl2 Cl2 + 2e- = 2Cl⇒ 71/2 = 35.5 ClO2 + 5e- = Cl⇒ 67.5/5 = 13.5 35.5 / 13.5 = 2.63 [email protected]

Chloroform, kg/odmt

Chloroform – CHCl3 % ClO2 sub

0.35 0.3

100% 71% 46% 28% 0%

0.25 0.2 0.15 0.1 0.05 0 0

0.1

0.2

Chlorine Factor [email protected]

0.3

0.4

Effect of Chlorine Multiple on Dioxin Formation

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Substitution of Chlorine Dioxide for Chlorine Old Bleaching Sequences: (CD)E1D1E2D2 O (CD)E1D1E2D2 ECF Sequences: D0E1D1E2D2

O D0E1D1E2D2

ECF = Elemental Chlorine-Free [email protected]

Chlorine Dioxide

O Cl O

O Cl O

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O Cl O

Chlorine Dioxide • Chemistry ClO2 + e- ⇒ ClO2ClO2- + 3H+ + 2e- ⇒ HClO + H2O HClO + H+ + 2e- ⇒ Cl- + H2O ClO2 + 4H+ + 5e- ⇒ Cl- + 2H2O Equivalent Weight: ClO2 = 67.5/5 = 13.5 Cl2 = 71/2 = 35.5 [email protected]

Basic Cation Radical Mechanism of Chlorine Dioxide R

OCH3 R1

O ClO2

R = Alkyl or R2

R

R1 = H, Alkyl or Aryl

ClO2

CT - Complex

OCH3 R1

π-Complex

O + H+ - HClO2

R

R

OCH3 O

R

R1 = H

R

OCH3

OCH3 + OH

R1 = Alkyl or aryl

R1

O

Phenolic structures

R

OCH3 R1

O+

OCH3

R1

O

+

Non-phenolic structures

Brage, Ericksson and Gierer, Holzforschung, 45(1):23 (1991) [email protected]

Basic Phenolic Compound Reactions CH3

OCH3 O CH3

CH3

CH3

ClO2

-

ClO2

OClO

CH3

H -

OClO

+ H2O - HOCl

H3C

COOCH3

OCH3

OCH3

O

O

O

- HClO2

CH3

- HOCl

Bicreosol

CH3O

OCH3 O

O O

CH3

CH2

CH3

O CH3O

OCH3

OCH3 O

OCH3

ClO2

O

- HClO2

O

Dimers and polymers

OCH3

O

+ H2O - HOCl

H3C

COOCH3

OCH3

O + H2O

OH

CH2OH

OClO-

+ H2O

O - CH OH 3

+ HClO2

H2C OClO-

CH3

CH3

OH

OCH3

CH3

OCH3 OH

OCH3 OH

- HOCl

CHO

O CO2CH3 O

OCH3

CO2H OH

Brage, Ericksson and Gierer, Holzforschung, 45(1):23 (1991) [email protected]

Basic Non-Phenolic Compound Reactions CH3 CH3 ClO2

CH3

ClO2

ClO2 OCH3

+

OCH3

+

OCH3

OCH3

+ OCH3

OCH3

CH3

CH3

H

CH3

CH3

H OCH3

CH3 H OClO

OCH3

ClO2 -

OClO

ClO2

CH3 H -

OCH3

OCH3 +

OCH3

+

+ H2O - HOCl - CH3OH

CH3

O

H

CO2CH3

CH3O ClO2

+

+

OCH3 + H2O - CH3OH - HOCl - H+

CH3

CH3

CH3O

OCH3 OClO+ H2O - CH3OH - HOCl - H+

CH3

O OCH3 OCH3

OCH3

CH3

OCH3

+ H2O - HClO2 - H+

+

CH3

OClO

OCH3

O HO

OCH3

+ OCH3

ClO2

-

+

OCH3

+ OCH3

+ ClO2

O OCH3

CO2CH3 CO2CH3

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Brage, Ericksson and Gierer, Holzforschung, 45(1):23 (1991)

ClO2 Oxidation of Methylveratrylalcohol Effect of pH on Rate of Reaction Percent Compound I

100 80

pH 2 pH 4

60

pH6 pH 8

40 20 0 0

50 100 Reaction Time (min.) [email protected]

150

Effect of pH on the Reaction of ClO2 with Methylveratrylalcohol (MVA) HO

CH3

MVA OCH3 OCH3

O H3C

O

O OCH3 H3C

OCH3

OCH3

O

OCH3

3

5

HO

CH

CH3

HO

O O

6 CH

CH3

Cl

Cl Cl

Gunnarsson and Ljunggren, Acta Chem. Scand., 50: 442 (1996)

OCH3 OCH3

8 [email protected]

OCH3

OCH3 OCH3

9

OCH3

10

Effect of End pH in a D1 Stage on Brightness and Chlorite and Chlorate Formation

Brightness, %

Rapson, H., and C.B Anderson, Tappi, 61 (10):97 (1978) 86

84 82

1.8 1.6

80

1.4

78 76 74 72

1.2 1.0 0.8 0.6 0.4 0.2

70 68 66 2

3

5

7

8

End pH in the D1 stage [email protected]

10

ClO2ClO3ClO3- + ClO2Brightness

Chlorate and/or chlorite, % of available chlorine on pulp

Effect of pH on ClO3- and ClO2- formation in ClO2 prebleaching of oxygen delignified kraft pulp

Concentration (mM)

5.0 4.0 Chlorite

3.0

Chlorate 2.0 1.0 0.0 0

2

4

6

8

10

12

End pH

Kappa no. 10.7, kappa factor 0.20, pulp consistency, 3.5%, 60 oC, 60 min [email protected] Wang, L. J. and B. H. Yoon, Paper presented at the International symposium on Cellulose and Lignocellulosics Chemistry 2000, Dec. 16-18, 2000, Kunming, China

Effect of End pH in a D1 Stage on Brightness and Chlorite and Chlorate Formation

Brightness, %

Rapson, H., and C.B Anderson, Tappi, 61 (10):97 (1978) 86

84 82

1.8 1.6

80

1.4

78 76 74 72

1.2 1.0 0.8 0.6 0.4 0.2

70 68 66 2

3

5

7

8

End pH in the D1 stage [email protected]

10

ClO2ClO3ClO3- + ClO2Brightness

Chlorate and/or chlorite, % of available chlorine on pulp

Chlorate Forming Reactions

ClO3-

ClO2 + Free Radical + H2O -

(1) -

2ClO2 + HO

-

HClO2 + ClO2

HClO2 + ClO3

(2)

ClO3

(3)

HOCl +

2ClO2 + HOCl + H2O

2ClO3- + HCl + 2H+

(4)

ClO2- + HOCl + H2O*

*ClO3- + HCl + H2O

(5)

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Dissociation Constants of Hypochlorous and Chlorous acids C lO 2 - + H +

H C lO 2 p K a ~ 2 .3

K1

K2

Cl2 + H2 O

ClO - + H+

H + Cl + ClOH pK2 ~ 7.5 pK1 ~ 1.8

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Disproportionation of Chlorous Acid -

HClO2 + ClO2

Slow

-

HOCl + ClO3 2

-

(3)

-

-d(ClO2 )/dt = k1(HClO2) + k2(ClO2 )(HClO2)

pKa~2.3 ClO2- + H+

HClO2 -

+

HClO2 + Cl + H

Fast

2HOCl

Kieffer andGordon, Inorganic Chem., 7(2):239 (1968) Hong and Rapson, Canadian J. Chem., 46:2053 (1968) [email protected]

(7)

Competitive Reactions of Hypochlorous Acid ClO2- + HOCl

-HO-

O Cl Cl O

ClO2-

2ClO2 + Cl-

(6)

ClO3- + HCl + H+

(5)

H2O

Oxidized Lignin + Cl-/ 2ClLignin + ClOH/Cl2 Organic-Cl + H2O/ [email protected]

Summary • In D0, the increase in chlorate and bleaching efficiency levels off at end pH below 3.4, whereas AOX continues to increase with decreasing pH. • In D0, the phenolic hydroxyl content of lignin in pulp has little effect on either chlorate formation or bleaching efficiency.

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Summary • The phenolic lignin structures have demonstrated enhanced reactivity with chlorine dioxide over that of the non-phenolic units. • The initial stage of ClO2 delignification is believed to be the abstraction of an electron from the phenolate anion followed by further degradation caused by additional equivalents of chlorine dioxide. • Lactones, muconic acid esters, maleic acid, oxiranes and quinoid structures are the dominant oxidation products along with significant levels of methanol. • Chlorinated organics are produced during ClO2 bleaching, primarily due to the in situ formation of hypochlorous acid. [email protected]

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