clo2 bleaching

CHLORINE DIOXIDE BLEACHING

Presented by:

Scott CarmichaelSenior Process Engineer

Eka Chemicals

PURPOSE OF CHEMICAL BLEACHING

• Selectively Remove Lignin

• Brighten Pulp• Remove Contaminants• Remove Extractives

MAJOR WOOD COMPONENTSAT DIFFERENT STAGES OF

DELIGNIFICATION

30

40

50

60

70

80

90

100

0 20 40 60 80 100Lignin Removed, %

Yiel

d, %

(On

Woo

d)

Lignin Content

Hemicellulose Content

Pure Cellulose Content

Total Pulp Yield

Extractives

Bulk Delignification

MacLeod, M., PAPRICAN, TAPPI Kraft Pulping

FOUR STAGE BLEACH LINE

`

D0Eop D1 D2

BLEACHING OF PULP

• Sequence Development - Chemical Pulps• Pre-bleaching to remove residual lignin

– Fragmentation from the cellulose portion in an oxidation/substitution step

– Solution of fragments for removal in washing• Final bleaching or brightening by oxidation of coloring

material– Chemical selectivity important because of low lignin to

cellulose ratio

BLEACHING AGENTS

Chlorine SodiumHypochlorite

ChlorineDioxide

Oxygen HydrogenPeroxide

Ozone

Stage Design. C H D O P ZChem. Form. Cl2 NaOCl ClO2 O2 H2O2 O3Delig. Eff. E E E P/F F GBright. Eff. P E E P E GSelectivity G F/P E F E PShive Reduct. E E E F F PEnvir. Impact P P G/E G/E G/E G/ETotal Cost I I Ex I M/Ex VE

E = Excellent G = Good F = Fair P = PoorI = Inexpensive M = Moderate Ex = Expensive VE = Very Expensive

Developed from TAPPI: Pulp Bleaching, Principles and Practice

COMMON BLEACHING TERMS

• Kappa number (Kp#) measure of lignin content– Lignin~(Kp#)*0.15

• Permanganate number (K#) measure of lignin content– K# ~ (Kp#)*0.67

• Active chlorine (aCl) term used to relate chlorine based bleaching compounds– aCl = (2.63* ClO2) + Cl2

• Kappa Factor = (aCl as % on pulp)/ (kappa number)• Oxidative Equivalents (OXE) term used to convert bleaching

compounds. – OXE is the quantity of substance that receives one mole of

electrons when a substance is reduced.

OXE FOR BLEACHING CHEMICALS

Chemical OXE/kg aCl

Cl2 28.20 1.0ClO2 74.12 2.63O2 125.00 4.43H2O2 58.79 2.09O3 125.00 4.43

FACTORS EFFECTING ClO2 PERFORMANCE

• Active Chlorine Distribution

• pH

• Charge

• Retention Time / Temperature

• Chemical Variants

– mixing, lignin, carryover, consistency

MULTI-STAGE BLEACHING

72

76

80

84

88

92

96

0.2 0.6 1 1.4 1.8

Chlorine Dioxide, %

Brig

htne

ss %

ISO

Do-EoD1D2

ClO2 DISTRIBUTION EFFECT ON TOTAL USAGE

Sequence: D (EO) D, 14 Kappa (9.4 K#)Reference: EC bleaching model

35

37

39

41

43

45

35 45 55 65 75 85

% of total ClO2 in first D

Tota

l ClO

2 lb/

T

D1 VERSUS D1+ D2 ClO2 DEMAND FOR BRIGHTENING

10

15

20

25

30

35

40

45

0 25 50 75 100 125

Tota

l ClO

2 C

onsu

med

In D

1 and

D2 St

ages

kg

eq.C

l2/t

pulp

92 ISO90 ISO88 ISO87 ISO

% ClO2 Consumed in the D1 Stage

Reference: Axegard, et al., “E2 Stage Improves the Reactivity of Pulp Towards Chlorine Dioxide”

D0 STAGE OPERATIONS

EFFECT OF pH IN D0 STAGE(SW)

6

9

12

15

18

21

1 2 3 4 5pH

D 0 K

appa

Num

ber

KF = 0. 10

KF = 0. 17

KF = 0. 23

KF = 0. 17 DE

20

25

30

35

40

45

1 2 3 4 5pH

D 0 IS

O B

right

ness

, %

KF = 0. 23

KF = 0. 17

KF = 0. 10

Reference: Basta, et al., “New and Improved Possibilities in D100 Bleaching,”Proceedings of 1992 TAPPI Pulping Conference

DELIGNIFICATION EFFICIENCY

23456789

101112

0.1 0.15 0.2 0.25 0.3Kappa Factor

Eop

Kapp

a #

SWHW

KAPPA NUMBER VS TIME

Minutes

Source: A Mathematical Model for Chlorine Dioxide Delignification; Savoie, Martin and Tessier, Patrick, TAPPI Journal June 1997

EFFECT OF COD CARRYOVER (SW - O2 Delig.)

1.6

2.0

2.4

2.8

3.2

3.6

0.17 0.19 0.21 0.23 0.25 0.27 0.29KF

E K

appa 10 kg/t COD

15 kg/t COD20 kg/t COD

BLEACH PLANT OPERATIONSD0 STAGE SUMMARY

• D0 Stage pH optimum at 2.5 - 3.2• Temperature 120 - 130 F, depending on time• Allow for COD carryover in charge• D0 Kappa Factor 0.18 - 0.22 for Economic &

Environmental• Residual not necessary• Charge approx. 50 - 60 % of total active chlorine for D0

• Brightness / Kappa relationship difference for Hardwood

D1 STAGE OPERATIONS

EFFECT OF TERMINAL pH ON D1 STAGE

68

70

72

74

76

78

80

82

84

86

2 3 4 5 6 7 8 9

pH at End of D1

Brig

htne

ss a

t End

of D

1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

%A

ctiv

e C

hlor

ine

Brightness

Chlorate + Chlorite

Chlorite

Chlorate

EFFECT OF pH AND ClO2 CHARGEON D1 STAGE BRIGHTNESS

78

80

82

84

86

88

1.5 2.5 3.5 4.5 5.5pH D1

Brig

htne

ss D

1 %

ISO

O2 - Delignified ECF Softwood Kraft, Kappa # 18 (K# 11.3)Reference: Basta, et al., “Reducing of AOX Levels,”

Proceedings of 1991 Appita Annual General Conference

K# E1 = 2.28% ISO E1 = 63.5Viscosity E1 = 31.8 cPs

K# E1 = 2.28% ISO E1 = 63.5Viscosity E1 = 31.8 cPs

23 lb/T D (as ClO2)

15 lb/T D (as ClO2)

8 lb/T D (as ClO2)

EFFECT OF END pH IN D1 STAGE ONSHIVES IN PULP AFTER THE D1 STAGE

02468

10121416182022

3.0 3.5 4.0 4.5 5.0 5.5 6.0

D1 Tower Terminal pH

D1 W

ashe

r Shi

ve C

ount

Reference: Pulp Bleaching - Principles and Practice; C. Dence & D. Reeve


• Maintain residual 0.02-0.05 gpl for brightness

• Operate pH 3.5-4.5

• Lower pH leads to improved shive bleaching but also strength and viscosity loss

• Temperature range 160-175º F

• Charge approx 30-40% of total active chlorine

EFFECT OF RESIDUAL ANDTEMPERATURE ON BRIGHTNESS

74

76

78

80

82

84

86

1 2 0 1 5 0 1 8 0 2 1 0 2 4 0 2 7 0 3 00

Time (min)

Brig

htne

ss, %

ISO

40 lb/T ClO2

30 lb/T ClO2

20 lb/T ClO2

Residual ClO2 No residual

D1 EFFECT ON D2 BRIGHTNESS CEILING

30

40

50

60

70

80

90

100

0 0.25 0.5 0.75 1 1.25 1.5

D1 Chlorine Dioxide, % on Pulp

Brig

htne

ss, %

GE

Rapson, W.H., “Bleaching of Pulp”. Tappi Monograph

D1

D2

D2 ClO2 = 0.5% on pulp


• Operate pH 3.5 - 5.0

• Temperature range 155 - 175 ºF

• Maintain residual at 0.02 - 0.05 gpl

• Residual important for brightness stability

• Important to stay away from brightness ceiling

QUESTIONS?

clo2 bleaching

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