ECF Bleaching of Eucalyptus
Kraft Pulps
Tapani Vuorinen
Aalto University
Department of Forest Products Technology
[email protected]
Presentation at 5th ICEP
International Colloquium
on Eucalyptus Pulp
Porto Seguro, Brazil
May 9-12, 2011
About our pulp bleaching research
• Eucalyptus pulp bleaching
– Aalto University: Anna-Stiina Jääskeläinen, Katri Toikka, Jose
Medina; Andritz: Christian Järnefelt; Universidad de la
República: Leonardo Clavijo, Maria Noel Cabrera
• Chemical reaction mechanisms and kinetics
– Aalto University: Anna-Stiina Jääskeläinen, Zhen Zhou,
Immanuel Adorjan, Tuula Lehtimaa; INPG Pagora: Gerard
Mortha; BOKU: Antje Potthast, Paul Kosma; VTT: Tarja
Tamminen, Tiina Liitiä, Taina Ohra-aho…
• Virtual pulp bleaching model
 Aalto University: Ville Tarvo, Susanna Kuitunen, Tuula
Lehtimaa, Juhani Aittamaa, Ville Alopaeus
Virtual pulp bleaching model
• Phenomena based simulation
– Fiber wall model; physical structure, chemical
composition, Donnan phenomenon…
– Heat and mass transfer
– Elementary chemical reactions; kinetics,
equilibria…
• Links to unit processes
• Links to existing technical characteristics
• Ability to carry out bleaching by computation
Eucalyptus pulp ECF bleaching
• Past: D-E-D-D
– > 30 kg active Cl per ton
• Today: A/D-EOP-D-P, DHot-EOP-D-P
– 20-25 kg active Cl per ton = 110-140 mmol ClO2/kg
• Best today: A-EOP-D-P
– < 15 kg active Cl per ton = < 85 mmol ClO2/kg
• Future: Our target
– < 10 kg active Cl per ton = < 55 mmol ClO2/kg
Need to understand bleaching chemistry in
details
Recent relevant publications
 Ville Tarvo (2010), Modeling chlorine dioxide bleaching
of chemical pulp, Doctoral Dissertation, Aalto University
(http://lib.tkk.fi/Diss/2010/isbn9789526031927/).
 Tuula Lehtimaa (2010), Reactions of chlorine (III) and
their kinetics in the chlorine dioxide bleaching of kraft
pulps, Doctoral Dissertation, Aalto University
(http://lib.tkk.fi/Diss/2010/isbn9789526032030/)
 Leonardo Clavijo (2010), Optimization of bleaching
sequences A(EOP)DD and A(EOP)DP, Master’s Thesis,
Universidad de la República.
 Ville Tarvo et al. (2010), A model for chlorine dioxide
delignification of chemical pulp, J. Wood Chem. Technol.
30: 230-268.
Characteristics of oxygen delignified
eucalyptus kraft pulps
• Kappa number 10-12
• Hexenuronic acid content 50-70 mmol/kg
• Lignin content < 1 %
– Equal to monomer content of < 50 mmol/kg
– Phenolic lignin
– Non-phenolic lignin
Primary oxidation reactions of ClO2
• ClO2 reacts very slowly with non-phenolic lignin
structures (e.g. benzylic structures)
• Chlorine dioxide reacts easily with phenolic lignin
(k = 103 M-1s-1), especially in its ionized phenolate
form (k = 109 M-1s-1)
– Each phenol consumes 2 equivalents ClO2
– Each phenol produces equivalently ClO2- and HOCl
– Phenols are oxidized through phenoxy radicals to quinones
or muconic acid derivatives (preferred)
– Stoichiometric reaction applied in quantification of phenols
– ArOH + 2ClO2 a OxLig + HOCl + HClO2
Hypochlorous acid and chlorine
• Hypochlorous acid is in equilibrium with hypochlorite
and chlorine
• Conversion of HOCl to Cl2 is relatively slow at low
acidity
• HOCl + H3O+ + Cl-  Cl2 + 2H2O
– HOCl + H3O+ + Cl- a Cl2 + 2H2O
• HOCl + H2O  ClO- + H3O+
k = 5.104 M-2s-1
pKa ~ 7.5
Secondary reactions of HClO2 (ClO2-)
• HClO2 (pKa = 2) reacts rapidly with hypochlorous acid
– Cl2O2 is formed
– Cl2O2 reacts very fast with ClO2- producing either ClO2 and
chloride or HOCl and chlorate
• HClO2 + HOCl  Cl2O2 + H2O
• ClO2- + Cl2  Cl2O2 + HO-
k = 4.102 M-1s-1
k = 9.103 M-1s-1
• Cl2O2 + ClO2- a 2ClO2 + Cl• Cl2O2 + ClO2- + H2O a ClO3- + 2HOCl
Secondary reactions of HClO2 (ClO2-)
• HClO2 oxidizes aldehydes to carboxylic acids
• Stoichiometric reaction applied in quantification of
aldehydes
• Formaldehyde has been described as an bleaching
booster (HOCl formation)
• HClO2 + RCHO  RCH(OH)OClO
• RCH(OH)OClO a RCO2H + HOCl
d[HClO2]/dt = -k[HClO2][RCHO]
Oxidation rates of various aldehydes
with HClO2
k (25°C) (M-1s-1)
Ea (kJ/mol)
Formaldehyde
11.0  0.8
54  5
Unhydrated formaldehyde
1.2  104
-
Vanillin
0.59  0.02
44  4
Veratraldehyde
1.00  0.03
42  2
Benzaldehyde
5. 6  0.3
26  3
3.39 ( 0.08)  10-3
63  2
104  2
32  1
Glycolaldehyde
39.0  1.9
45  3
Unhydrated glycolaldehyde
4.5  102
-
5-formyl-2-furancarboxylic acid
5.2 0.2
37  2
Reactant
Glucose
Free aldehydes of glucose1
From: T. Lehtimaa et al., Ind. Eng. Chem. Res. 49 (2010) 2688-2693
Secondary reactions of HOCl/Cl2
• Cl2O2 formation with HClO2
• Electrophilic addition on ethylenic structures (e.g. HexA,
formation of unstable AOX1)
• Electrophilic aromatic substitution (formation of stable
AOX and OX)
• Electrophilic addition on alcohols or acids and their
subsequent oxidation
• Nucleophilic addition on carbonyls and their subsequent
oxidation
1Unstable
AOX is easily removed through SN2 reaction with strongly
nucleophilic SO32-
Derivatives of Cl2 (HOCl) in chlorine
dioxide bleaching
• General reaction (nucleophilic substitution on chlorine):
Nu- + Cld+-Cld-  Nu-Cl + Cl-
Nucleophile
Product
Name
H2O/HOClOClO2RCO2H
ROH
ArOH
HOCl
Cl2O
Cl2O2
RCO2Cl
ROCl
ArOCl
Hypochlorous acid
Chlorine monoxide
Dichlorine dioxide
Acyl hypochlorite
Alkyl hypochlorite
Aryl hypochlorite
Oxidation by HOCl - step 1
O
1a)
O
K1a
+
R
+
R
HOCl
OCl
OH
O
1b)
OH
K1b
+
R
R
HOCl
H
1c)
OH
+
HOCl
Step 1 Formation of organic hypochlorites
H
OCl
K1c
R
H2O
R
OCl
+
H2O
Oxidation by HOCl – step 2
O
H
O
2a)
O
k2a
H
R
Cl
O
+
H
CO2
+ HCl
R
R1
O
H
R2
O
Cl
2b)
2c)
R1
+
R1
H
Cl
R2
Step 2 Cleavage of organic hypochlorites
H
+
R2
H
+
HCl
O
k2c
O
O
O
k2b
R1
R2
HCl
Reaction rates of HOCl with various
compounds at 25 oC
Substance
Products
k (M-1s-1)
HClO2
Cl2O2
400
HexA (monomer)
Complex mixture
400
Cl-
Cl2
50 (pH 3)
3,4-Dimethoxytoluene
Not analyzed
40
4-Methylguaiacol
Not analyzed
40
Acetic acid
Acetyl hypochlorite
4.5
Oxalic acid
CO2
2
ClO2-
ClO3- and Cl-
1.6
Formic acid
CO2
0.2
2-Furoic acid
Not analyzed
0.2
Glycolic acid
CO2 and HCHO
0.04
Chlorination of phenols by HOCl
• Phenolate ion is chlorinated rapidly by HOCl although
each added chlorine atom decreases the reactivity
ArO- + HOCl a ClArO-, k = 103 M-1s-1
ClArO- + HOCl a Cl2ArO-, k = 102 M-1s-1
Cl2ArO- + HOCl a Cl3ArO-, k = 101 M-1s-1
• Undissociated phenol is chlorinated relatively slowly
ArOH + HOCl a ClArOH, k = 100 M-1s-1
Summary on inorganic chlorine
species in chlorine dioxide bleaching
Species
Oxidation number
Chlorate (ClO3-)
Chlorine dioxide (ClO2)
Chlorous acid (HClO2, pKa 2.0)
Chlorite (ClO2-)
Dichlorine dioxide (Cl2O2)
Hypochlorous acid (HOCl, pKa 7.5)
Hypochlorite (ClO-)
Chlorine monoxide (Cl2O)
Chlorine (Cl2)
Chloride (Cl-)
+5
+4
+3
+3
+2
+1
+1
+1
0
-1
Reactivity
Radical
Nucleophile
Nucleophile
Electrophile
Electrophile
Nucleophile
Electrophile
Electrophile
-
General reaction scheme of chlorine
dioxide bleaching
Cl2O2.ClO2-
ClO2
Cl-
ClO3-
HOCl + HClO2
RCHO…
ClO2-
Cl2O2
Main reactions in D-stage
• Chlorine dioxide oxidizes phenolic lignin units; chlorous
and hypochlorous acids are formed
– ArOH + 2ClO2 a OxLig + HOCl + HClO2
• Chlorous and hypochlorous acids react with each other;
either chlorine dioxide and chloride or chlorate and
hypochlorous acid are formed through Cl2O2
– HOCl + 2HClO2 a 2ClO2 + Cl– HOCl + 2HClO2 a ClO3- + 2HOCl
• Hypochlorous and chlorous acids oxidize HexA; chloride
is formed
– HexA + HOCl + HClO2 a OxHexA + 2Cl– HexA + 3HOCl a OxHexA + 3Cl-
Stoichiometry of overall reactions
• 2ArOH + HexA + 4ClO2 a 2OxLig + OxHexA + ClO3- + 3Cl• ArOH + HexA + 2ClO2 a OxLig + OxHexA + 2Cl• 6ArOH + HexA + 6ClO2 a 6OxLig + OxHexA + 6Cl1:2 molar ratio
30
1:4 molar ratio
25
20
1:6 molar ratio
15
Chlorate (mmol/kg)
10
5
HexA reduction
(mmol/kg)
0
0
20
40
60
80
100
Chlorine dioxide (mmol/kg)
120
Stoichiometry in A-EOP-D bleaching of eucalyptus kraft pulp
Data from: Leonardo Clavijo, Master’s Thesis, 2010
Stoichiometry of overall reactions
• 2ArOH + HexA + 4ClO2 a 2OxLig + OxHexA + ClO3- + 3Cl• ArOH + HexA + 2ClO2 a OxLig + OxHexA + 2Cl• 6ArOH + HexA + 6ClO2 a 6OxLig + OxHexA + 5Cl0,25
0,2
D0
Dhot
A/D0
D1
A-EOP-D
0,15
0,1
0,05
0
Chlorate formation (mol/mol ClO2)
Chlorate formation in ECF bleaching of eucalyptus kraft pulps
Data from: Jose Medina, Master’s Thesis, TKK, 2007
Stoichiometry of overall reactions
• 2ArOH + HexA + 4ClO2 a 2OxLig + OxHexA + ClO3- + 3Cl• ArOH + HexA + 2ClO2 a OxLig + OxHexA + 2Cl• 6ArOH + HexA + 6ClO2 a 6OxLig + OxHexA + 6Cl120
100
80
60
40
20
0
After 1 min
After 30 min
ClO2
Oxidation of
consumption
HexA
Chlorate
formation
Unreacted
chlorite
Chloride
formation
ECF bleaching (D0) of birch kraft pulp (values in mmol/kg)
Data from: T. Lehtimaa et al., J. Wood Chem. Technol. 30 (2010) 1-18
How to evaluate bleaching sequences
• Lignin and HexA contents by UVRR spectroscopy
– Normalization relative to cellulose (1100 cm-1)
– Lignin (aromatic) band at 1600 cm-1
– HexA (ethylenic) band at 1650 cm-1
• Kappa number (total content of lignin + HexA)
• Chlorate formation
– Measure of unselectivity
• Final brightness
• Final brightness stability
– Mainly affected by HexA, lignin and carbonyl contents
• AOX formation
UV Raman spectra of ECF bleached
(DEDD) eucalyptus kraft pulps
DEDD, 25 kg aCl / t pulp
4
HexA
D0 kappa factor 0,19
D0 kappa factor 0,15
D0 kappa factor 0,11
Count
3
Lignin
2
Cellulose
1
0
1900
1800
1700
1600
1500
1400
1300
Raman Shift, 1/cm
1200
1100
1000
900
UV Raman spectra of ECF bleached
(DEDD) eucalyptus kraft pulps
DEDD, 35 kg aCl / t pulp
4
D0 kappa factor 0,19
D0 kappa factor 0,15
D0 kappa factor 0,11
Count
3
2
HexA
Lignin
Cellulose
1
0
1900
1800
1700
1600
1500
1400
1300
Raman Shift, 1/cm
1200
1100
1000
900
UVRR spectra of eucalyptus kraft pulps
in A/D-EOP-D-P bleaching sequence
12
O2 delig pulp
(A/D)_rs
10
EP_rs
D_rs
Intensity (a.u.)
8
P_rs
6
4
2
0
500
700
900
1100
1300
Raman shift
1500
1700
1900
(cm-1)
From: Leonardo Clavijo, Master’s Thesis, 2010
UVRR spectra of eucalyptus kraft pulps
in A-EOP-D-P bleaching sequence
12
O2 delig. pulp
A_120n
10
EOP_120n
D1_12014n
Intensity (a.u.)
8
P_12014n
6
4
2
0
500
700
900
1100
1300
Raman shift (cm-1)
1500
1700
1900
From: Leonardo Clavijo, Master’s Thesis, 2010
UVRR spectra of DEDD, ZDED and
A/DED bleached eucalyptus kraft pulps
4
D0, kf 0.19
D0, kf 0.15
D0, kf 0.11
Z/D, kf ~0.19
Z/D, kf ~0.15
A/D, kf 0.11, t = 2 min
3
Count
H
2
L
C
1
0
1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900
Raman Shift, 1/cm
Effects of A-stage delay and active Cl
charge on eucalyptus pulp (A-EP-D)
D1
120/12
D1
120/16
D1
120/20
D1
60/12
D1
60/16
D1
60/20
D1
45/12
D1
45/16
D1
45/20
3.1
2.9
4.5
2.9
2.9
3.0
2.8
3.0
2.9
ClO2 consumed
(kgactCl/BDT)
12.0
16.0
20.0
12.0
16.0
20.0
12.0
16.0
18.6
Kappa Number
1.8
0.9
0.9
2.2
1.4
1.0
3.7
2.4
1.7
ISO Brightness (%)
73.0
81.8
84.4
75.8
78.4
80.7
72.4
76.4
78.7
Viscosity (mL/g)
662
676
747
712
719
733
796
776
784
Viscosity Drop (%)1
31
30
23
26
25
24
18
20
19
TOC (g/BDT):
1851
2276
3016
2162
2670
2885
1947
3211
3353
ClO3- content
(g/BDT)
937
2501
2821
1009
1385
2370
746
1501
2385
AOX content
(g/BDT)
21.4
33.0
45.4
34.8
38.4
33.3
26.2
31.9
35.0
Yield (%)
98.9
97.9
99.7
99.8
99.7
99.8
98.4
99.9
99.0
Final pH:
Brightness stability of ECF bleached
eucalyptus kraft pulps
• Most important contributors:
– Residual lignin content
– Residual HexA content
– Carbonyl group content (overbleaching)
Oxidation of bleached (A/D-E-D-P) eucalyptus
pulp with HOCl
pH = 5
50 °C
-5
(3.0 0.30)10
80
0,8
65 °C
(8.0 0.51)10
0,6
70
-5
Ref.kraft
0,4
80 °C
(2.5 0.32)10
0,2
-4
0,0
0
15
30
45
60
Reaction time (min)
CO content, umol/g
ln[HOCl]
1,0
60
50 C
65 C
80 C
hemi.extr.
50
40
Hemi.extrcted
30
20
10
0
0,00
0,02
0,04
0,06
0,08
0,10
0,12
0,14
0,16
Active Cl2 consumption, g/l
From: Z. Zhou et al., Holzforschung 65 (2011) 289-294
Effect of carbonyl content on brightness
reversion of ECF bleached eucalyptus pulp
reference pulp (xylan 12%)
hemicellulose-reduced pulp (xylan 7%)
PC, 48h ageing
16
14
12
10
8
a Brightness reversion is only caused
by carbonyl groups!
6
4
Original pulp
2
0
0
10
20
30
40
50
60
70
80
90
Carbonyl content, mol/g
From: Z. Zhou et al., Holzforschung 65 (2011) 289-294
AOX and OX formation by HOCl
• The overall effect of various reactions of HOCl on AOX
formation is complex to understand
• Generally following observations can be made
– Less AOX is formed when less active Cl is used
– Smaller usage of active Cl does not necessarily mean lower
OX formation
– At higher pH less unstable AOX is formed
– pH does not affect so much stable AOX and OX formation
Effect of final pH on AOX formation in D0
stage of eucalyptus kraft pulp bleaching
Cumulative AOX and final OX formation
in eucalyptus pulp bleaching
Prestage
D0-EP-DND
Act. Cl
(kg/t)
29.8
AOX
OX
kg/t (mmol/kg) kg/t (mmol/kg)
0.63 (18)
0.16 (5)
DHT-EP-DND
26.4
0.42 (12)
0.08 (2)
A/D-EP-DND
26.2
0.28 (8)
0.13 (4)
A-EP-DND
19.5
0.10 (3)
0.11 (3)
Data from: Jose Medina, Master’s Thesis, TKK, 2007
Future outlook
• In principle it is still possible to decrease ClO2
use by 50 % compared to the best current ECF
bleaching sequences (< 15 kg active Cl per ton)
• This requires the following targets to be reached
– Eliminate chlorate formation (average 20 %
conversion): potential for 30 % increase in efficiency
– Block the secondary reactions of HexA: potential to
save 2-5 kg active Cl per ton of pulp depending on
the prior removal of HexA (A-stage)
– Prevent formation of stable AOX: potential to save 12 kg active Cl per ton of pulp
Acknowledgements
• Financial support:
 TEKES (Finnish Funding Agency for Technology and
Innovation)
 Forestcluster Ltd (Strategic Centre for
Science, Technology and Innovation)
 Andritz, Botnia, Kemira, Stora Enso, UPM
[email protected]