Peroxide Bleaching of Eucalyptus CTMP
Using Mg(OH)2 as the Alkali Source
Z. He; Y. Ni
University of New Brunswick, Canada
Conventional Peroxide Bleaching of Mechanical Pulp
• Use NaOH as the alkali source
• High alkalinity, especially in the initial phase
• Peroxide decomposition and darkening
reactions
• High COD load of the effluent
• High anionic trash production
• Decreased light scattering coefficient
• Decreased pulp yield
Mg(OH)2: An Alternative Alkali for
Peroxide Bleaching
• Mild alkalinity
• Gradual release of HO- due to low
solubility
• One unit weight of Mg(OH)2 equals to
1.4 NaOH in the capacity of accepting
protons.
Mg(OH)2 - PM Bleaching
Process
Mg(OH)2
H2O2
PM MC
#1
MC
#2
DTPA/Na2S2O4
“QY”
PMg(OH)2 Versus PNaOH
PMg(OH)2 PNaOH
Lower pH Higher pH
Silicate decreased or eliminated Silicate needed
Longer time required Less sensitive to time
Less dissolved organics More dissolved organics
Lower COD Higher COD
Less anionic trash More anionic trash
Objectives
• To compare the Mg(OH)2-based and NaOH-
based peroxide bleaching of a eucalyptus
CTMP, in terms of
– Pulp brightness
– COD
– Anionic trash
– Pulp yield
Experimental
• Eucalyptus CTMP from a Brazilian mill
• Chelation: 0.2% DTPA (as 100%), 3%
consistency, pH 6.0, 70C, 30 minutes.
• Bleaching conditions:
– 2.5%, 4.0%, 6.2% H2O2;
– 1.5%-6.0% NaOH, or 0.5%-2.5% Mg(OH)2;
– 16% (MC), 30% (HC) pulp consistency;
– 80-90 C, 2-6 hours.
Medium-Consistency (MC) System
Effect of Peroxide and Alkali
Charge on Brightness (MC System)
Other bleaching conditions: 2.6% silicate, 0.13%
Epsom salt (NaOH-based process), 16% pulp
consistency, 80C, 3 hours
70
72
74
76
78
80
82
84
86
0 2 4 6 8
NaOH charge (%)
Bri
gh
tness (
% I
SO
)
2.5% peroxide
4.0% peroxide
6.2% peroxide
60
62
64
66
68
70
72
74
76
78
80
0 0.5 1 1.5 2 2.5 3
Mg(OH)2 charge (%)B
rig
htn
ess (
% I
SO
)
2.5% peroxide
4.0% peroxide
6.2% peroxide
Eucalyptus CTMP vs. Maple CTMP:
Mg(OH)2-based Peroxide Bleaching
64
66
68
70
72
74
76
78
80
2.5% peroxide,
1.0% Mg(OH)2
4.0% peroxide,
1.7% Mg(OH)2
6.2% peroxide,
2.5% Mg(OH)2
Bri
gh
tne
ss
(%
IS
O)
Eucalyptus CTMP
Maple CTMP
• The eucalyptus CTMP is less bleachable in the Mg(OH)2-
based process, compared with the maple CTMP.
Why Is the Eucalyptus CTMP Less
Bleachable in the Mg(OH)2-based Process?
• Wood properties
– Extractives
– Lignin
• Pulping process
– Chemical pretreatment (alkali dosage)
Effect of Acetone Extraction
70
72
74
76
78
80
82
84
86
Bri
gh
tne
ss
(%
IS
O)
4 6.2
Peroxide dosage (%)
Mg Process
Na Process
70
72
74
76
78
80
82
84
86
Bri
gh
tness (
% I
SO
)
4 6.2
Peroxide dosage (%)
Mg Process
Na Process
Without acetone extraction With acetone extraction
Other bleaching conditions: 2.6% silicate, 0.13% Epsom
salt (Na process), 16% pulp consistency, 80C, 3 hours (6
hours for the Mg process).
Alkaline Pre-treatment on Peroxide
Bleaching of A TMP
NaOH
%
Mg(OH)2
%
Silicate
%
DTPA
(40%) %
End
pH
Residual
H2O2, %
Bright.
% ISO
Without alkali pre-treatment
1.85 0 2.0 0 7.97 1.24 71.5
1.85 0 0 0.4 7.65 1.24 71.6
0 1.0 0 0.4 7.81 1.70 71.2
With alkali pre-treatment
1.0 0 2.0 0 9.20 1.43 67.6
1.0 0 0 0.4 9.39 1.22 67.3
0 1.0 0 0.4 8.90 1.83 62.7
Conductivity of Bleach Filtrate
(MC System)
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8
NaOH charge (%)
filt
rate
co
nd
ucti
vit
y (
ms/c
m)
2.5% peroxide
4.0% peroxide
6.2% peroxide
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.5 1 1.5 2 2.5 3
Mg(OH)2 charge (%)F
iltr
ate
co
nd
ucti
vit
y (
ms/c
m)
2.5% peroxide
4.0% peroxide
6.2% peroxide
Other bleaching conditions: 2.6% silicate, 0.13%
Epsom salt (NaOH-based process), 16% pulp
consistency, 80C, 3 hours
Anionic Trash Formation
(MC System)
0
20
40
60
80
100
0 1 2 3 4 5 6 7
NaOH charge (%)
An
ion
ic t
rash
fo
rmed
(m
eq
/kg
)
2.50% peroxide
4.00% peroxide
6.20% peroxide
0
5
10
15
20
25
0 0.5 1 1.5 2 2.5 3
Mg(OH)2 charge (%)A
nio
nic
tra
sh
fo
rmati
on
(m
eq
/kg
)
2.5% peroxide
4.0% peroxide
6.2% peroxide
Other bleaching conditions: 2.6% silicate, 0.13%
Epsom salt (NaOH-based process), 16% pulp
consistency, 80C, 3 hours
Summary
• In the MC system
– The Na process was effective
– The Mg process gave ~ 5 units lower
brightness than the Na process at the same
peroxide dosage.
– But the Mg process produced less anionic
trash and conductivity
High-Consistency (HC) System
Effect of Temperature and Time on
Brightness (HC System)
NaOH-based Process Mg(OH)2-based process
• Other bleaching conditions: 2.6% silicate, 0.13% Epsom salt
(NaOH-based process), 30% pulp consistency.
72
74
76
78
80
82
84
86
2.5% Peroxide,
2.25% NaOH
4.0% peroxide, 3.0%
NaOH
6.2% peroxide, 4.0%
NaOH
Bri
gh
tne
ss
(%
IS
O)
70.0
72.0
74.0
76.0
78.0
80.0
82.0
84.0
2.5 peroxide,
0.75% Mg(OH)2
4.0% peroxide,
1.0% Mg(OH)2
6.2% peroxide,
1.5% Mg(OH)2
Bri
gh
tness (
% I
SO
)
80 degree C, 3 hrs
80 degree C, 6 hrs
90 degree C, 6 hrs
80C, 2 hrs
Comparison of the Bleaching Performance
at Optimized Conditions (HC)
Process NaOH-based (2.6% silicate, 0.13% Epsom
salt, 80 C, 3 hrs)
Mg(OH)2-based (2.6% silicate, 90 C, 6 hrs)
H2O2, % 2.5 4.0 6.2 8.0 2.5 4.0 6.2 8.0
NaOH, % 2.0 2.5 4.0 4.0 0 0 0 0
Mg(OH)2, % 0 0 0 0 0.75 1.0 1.5 2.0
Resid. H2O2, % 0.60 1.08 1.53 2.30 0.67 1.53 2.05 2.97
Bright., % ISO 78.5 82.9 85.8 86.9 76.9 80.8 83.2 85.0
For a very high-brightness CTMP
• Peroxide bleaching first
• Followed by the addition of OBA/FWA
Addition of Fluorescent Whitening Agent to
Pulps from the Mg(OH)2-based Process
FWA (Tinopal UP), % 0 0.2 0.4 0.8
Brightness, % ISO 83.2 85.5 87.2 87.8
Flu. comp., % ISO 0 2.76 4.00 5.17
CIE whiteness, % 59.0 66.9 71.5 72.9
L* 97.1 97.2 97.3 97.3
a* -2.12 -1.51 -1.21 -1.04
b* 7.41 5.71 4.77 4.46
Other bleaching conditions: 6.2 % H2O2, 2.6% silicate, 30% pulp
consistency, 90C, 6 hours.
Total Anionic Trash Formation in
the HC System
0
20
40
60
80
100
76 78 80 82 84 86 88
Brightness of bleached pulp (% ISO)
To
tal
an
ion
ict
trash
fo
rmati
on
(m
eq
/gkg
)
Na-based
Mg-based
Cationic demand of filtrate measured at pH 7.0
Sources of Anionic Trash
• Polygalacturonic acids (pectic acids):
polymer with uronic type of carboxylic
groups (pKa= ~3.4).
• Oxidized lignin: macromolecule with
lignin-bound carboxylic groups (pKa=
~5.4).
• Colloidal particles of extractive: aliphatic
type of carboxylic groups (pKa= ~5.5-
6.4).
De-methylation and Dissolution of
Pectins
O
COOCH3
O
OH
O
OH
*
*
n
O
COO-
O
OH
O
OH
*
*
n
Anionic trash
DissolutionDe-esterification
Pectic acids
pKa= ~3.4
Pectin
Lignin Oxidation and Dissolution
O
O
C
HC O Lignin
COOHCOOH
C
HC O Lignin
Oxidation
C
OCH3
OH
CH O Lignin
COD&
Anionic Trash
Dissolution
Oxidation
Dissolution
COD
pKa= ~5.4
Extractives
• Dissolved and dispersed extractives – Example: resin acids, fatty acids
COOH COO-
Resin acids
pKa= ~5.5-6.4
Anionic Trash Measured at Various pH
• The measured amount of anionic trash is pH-dependent.
• But the cationic demand of pectic acids is pH-independent
from 4.5 to 7.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
3 4 5 6 7 8 9
pH
Cati
on
ic d
em
an
d (
meq
/l)
Sodium PolyGalA
Filtrate (alkaline
treatment)
Filtrate (peroxide
bleaching)
Estimation of the Contribution of Pectic Acids,
Lignin and Extractives to the Total Anionic Trash
The increase of cationic demand with pH is due to: • Extractives
•Oxidized lignin
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
3 4 5 6 7 8 9
pH of the DCS sample for CD titration
Ca
tio
nic
de
ma
nd
(m
eq
/l)
Alkaline treatment
4% H2O2
lignin
Extractives
Pectic Acids
Hemicellulose Type of Anionic Trash in
Peroxide Bleaching of the Eucalyptus CTMP
0
10
20
30
40
50
60
2 2.5 3 3.5 4 4.5
Pulp yield loss during bleaching (%)
Hem
icell
ulo
se t
yp
e o
f an
ion
ic t
rash
(meq
/kg
)
Na-based
Mg-based
Measured as the cationic demand of
filtrate measured at pH 4.5
Lignin and Extractives Types of Anionic Trash in
Peroxide Bleaching of the Eucalyptus CTMP
0
10
20
30
40
50
2 2.5 3 3.5 4 4.5 5
Pulp yield loss (%)
lig
nin
an
d e
xtr
acti
ves t
yp
es o
f
an
ion
ic t
rash
(m
eq
/kg
) Na-based
Mg-based
Difference of filtrate cationic demand measured
between pH 7.0 and 4.5
Comparison of Pulp Yield Loss at
Various Brightness Levels
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
76 78 80 82 84 86 88
Brightness (% ISO)
Pu
lp y
ield
lo
ss (
%)
Na-based process
Mg-based process
• Lower yield loss for the Mg- based process.
Correlation of Pulp Yield Loss
and COD
y = 0.0408x + 1.3516
R2 = 0.9951
0
1
2
3
4
5
0 20 40 60 80 100
COD of the bleaching effluent (kg/t)
Pu
lp y
ield
lo
ss (
%)
• The same correlation for both the NaOH-
based and Mg(OH)2-based processes
Dissolved Solid (HC system)
0
0.2
0.4
0.6
0.8
1
1.2
2.5% peroxide 4.0% peroxide 6.2% peroxide
Dis
so
lved
so
lid
in
fil
trate
(g
/L) Na-Based
Mg-based
Filtrate Conductivity (HC System)
0
0.2
0.4
0.6
0.8
1
2.5 4.0 6.2 8.0
Peroxide charge (%)
Fil
trate
co
nd
ucti
vit
y (
ms/c
m)
Na-based
Mg-based
Summary • Under the optimized conditions, the Mg process
in the HC system gives 1-2 units lower brightness than the Na process at the same peroxide dosages.
• The Mg process requires higher temperature and longer retention time to reach the brightness targets.
• At a similar brightness, the Mg process has – 50-80% less anionic trash
– 40-50% less COD,
– 20-30% less loss of pulp yield
– 50-60% less dissolved solid
– 50-60% less conductivity
Conclusions
• The Mg(OH)2-based process has been
successfully implemented in a number of mills
in N.A.
• In comparison with the NaOH-based process,
the Mg(OH)2-based process offers:
– Lower bleaching cost
– Similar or slightly lower brightness
– Higher light scattering coefficient
– Higher pulp yield
– Lower COD
– Less anionic trash
Acknowledgements
• Dr. Paulo Pavan, Melhoramentos Papéis
• Mark Wajer, Martin Marietta Magnesia
Specialties