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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
BLEACH PLANT OPERATIONSD1 STAGE SUMMARY
• 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
BLEACH PLANT OPERATIONSD2 STAGE SUMMARY
• 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?