CHLORINE DIOXIDE BLEACHING
Presented by:
Scott Carmichael
Senior Process Engineer
Eka Chemicals
PURPOSE OF CHEMICAL BLEACHING
• Selectively Remove
Lignin
• Brighten Pulp
• Remove Contaminants
• Remove Extractives
MAJOR WOOD COMPONENTS
AT DIFFERENT STAGES OF
DELIGNIFICATION
100
Extractives
90
Yield, % (On Wood)
Total Pulp Yield
80
70
Lignin Content
60
Bulk Delignification
50
Hemicellulose Content
40
Pure Cellulose Content
30
0
20
40
60
Lignin Removed, %
MacLeod, M., PAPRICAN, TAPPI Kraft Pulping
80
100
FOUR STAGE BLEACH LINE
D0
`
Eop
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
Stage Design.
Chem. Form.
Delig. Eff.
Bright. Eff.
Selectivity
Shive Reduct.
Envir. Impact
Total Cost
E = Excellent
I = Inexpensive
C
Cl2
E
P
G
E
P
I
Sodium
Hypochlorite
H
NaOCl
E
E
F/P
E
P
I
Chlorine
Dioxide
D
ClO2
E
E
E
E
G/E
Ex
G = Good
F = Fair
P = Poor
M = Moderate
Ex = Expensive
Oxygen Hydrogen
Peroxide
O
P
O2
H2O2
P/F
F
P
E
F
E
F
F
G/E
G/E
I
M/Ex
Ozone
VE = Very Expensive
Developed from TAPPI: Pulp Bleaching, Principles and Practice
Z
O3
G
G
P
P
G/E
VE
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
ClO2
O2
H2O2
O3
28.20
74.12
125.00
58.79
125.00
1.0
2.63
4.43
2.09
4.43
FACTORS EFFECTING
ClO2 PERFORMANCE
• Active Chlorine Distribution
• pH
• Charge
• Retention Time / Temperature
• Chemical Variants
– mixing, lignin, carryover, consistency
MULTI-STAGE BLEACHING
96
Brightness %ISO
92
88
84
80
Do-Eo
D1
D2
76
72
0.2
0.6
1
Chlorine Dioxide, %
1.4
1.8
ClO2 DISTRIBUTION EFFECT ON
TOTAL USAGE
Total ClO2 lb/T
45
43
41
39
37
35
35
45
55
65
75
% of total ClO2 in first D
Sequence: D (EO) D, 14 Kappa (9.4 K#)
Reference: EC bleaching model
85
D1 VERSUS D1+ D2 ClO2 DEMAND FOR
BRIGHTENING
45
92 ISO
90 ISO
88 ISO
87 ISO
Total ClO2 Consumed In D1 and D2 Stages
kg eq.Cl2/t pulp
40
35
30
25
20
15
10
0
25
50
75
100
125
% 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)
21
45
KF = 0. 23
40
KF = 0. 10
D0 ISO Brightness, %
D0 Kappa N umber
18
15
KF = 0. 17
12
KF = 0. 23
9
35
KF = 0. 17
30
KF = 0. 10
25
KF = 0. 17 DE
6
20
1
2
3
pH
4
5
1
2
3
4
pH
Reference: Basta, et al., “New and Improved Possibilities in D100 Bleaching,”
Proceedings of 1992 TAPPI Pulping Conference
5
Eop Kappa #
DELIGNIFICATION EFFICIENCY
12
11
10
9
8
7
6
5
4
3
2
SW
HW
0.1
0.15
0.2
Kappa Factor
0.25
0.3
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.)
3.6
E Kappa
3.2
2.8
10 kg/t COD
15 kg/t COD
20 kg/t COD
2.4
2.0
1.6
0.17
0.19
0.21
0.23
KF
0.25
0.27
0.29
BLEACH PLANT OPERATIONS
D0 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
86
1.8
84
1.6
82
1.4
80
1.2
Chlorate + Chlorite
78
1.0
Chlorite
76
0.8
74
0.6
Chlorate
72
0.4
70
0.2
68
0.0
2
3
4
5
6
pH at End of D1
7
8
9
%Active Chlorine
Brightness at End of D1
Brightness
EFFECT OF pH AND ClO2 CHARGE
ON D1 STAGE BRIGHTNESS
88
K#
K#E1
E1==2.28
2.28
%%ISO
ISOE1
E1==63.5
63.5
Viscosity
E1
Viscosity E1==31.8
31.8cPs
cPs
Brightness D1 % ISO
23 lb/T D (as ClO2)
86
84
15 lb/T D (as ClO2)
82
80
8 lb/T D (as ClO2)
78
1.5
2.5
3.5
4.5
pH D1
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
5.5
EFFECT OF END pH IN D1 STAGE ON
SHIVES IN PULP AFTER THE D1 STAGE
22
D1 Washer Shive Count
20
18
16
14
12
10
8
6
4
2
0
3.0
3.5
4.0
4.5
5.0
D1 Tower Terminal pH
Reference: Pulp Bleaching - Principles and Practice; C. Dence & D. Reeve
5.5
6.0
BLEACH PLANT OPERATIONS
D1 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 AND
TEMPERATURE ON BRIGHTNESS
86
84
Brightness, % ISO
No residual
Residual ClO2
40 lb/T ClO2
82
30 lb/T ClO2
80
78
76
74
20 lb/T ClO2
120
150
180
210
Time (min)
240
270
3 00
D1 EFFECT ON D2 BRIGHTNESS CEILING
Brightness, %GE
100
D2
90
80
D1
70
60
50
40
D2 ClO2 = 0.5% on pulp
30
0
0.25
0.5
0.75
1
D1 Chlorine Dioxide, % on Pulp
Rapson, W.H., “Bleaching of Pulp”. Tappi Monograph
1.25
1.5
BLEACH PLANT OPERATIONS
D2 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?