discovered chlorine and pioneered use as a bleaching chemical on vegetable
fibers ... discovered chlorine could be absorbed in solution of caustic potash.
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
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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)
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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
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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
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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
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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
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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
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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)
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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
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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%
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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, %
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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
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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
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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
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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
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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)
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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)
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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.)
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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
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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
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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
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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
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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)
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(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/
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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.
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