BLEACHING
The use of magnesium hydroxide as a cost
effective cellulose protector in the pressurized
alkaline peroxide (Eop) bleaching stage
BY A. THAKORE, J. OEI, B. RINGROSE, A. GIBSON AND M. WAJER
Abstract: Magnesium ion is an effective protector of cellulose against degradation in the pressurized alkaline peroxide (Eop) bleaching stage. Laboratory experiments and mill scale trials at the
Western Pulp Squamish mill have successfully shown that application of magnesium hydroxide in
the Eop stage provides a cost effective partial replacement of caustic soda and protection of cellulose from alkali degradation as compared to magnesium sulfate. There is no adverse impact on final
pulp brightness or viscosity.
HE SQUAMISH PULP MILL, located 40 km
north of Vancouver, British Columbia,
Canada produces 800 Adt/day of fully bleached softwood kraft pulp. The
bleaching sequence, as illustrated in
Fig. 1, is [DE]EopD. The first two stages of the
sequence are carried out in an Ahlstrom Kamyr
displacement-bleaching tower containing four
diffusion washers. The third stage is a Kvaerner
pressurized Eop reactor with a single diffuser
washer, followed by a D displacement tower,
which bleaches pulp to 89% ISO brightness.
Magnesium sulfate (MgSO4) is used in the
pressurized Eop stage of kraft pulp bleaching to
protect cellulose from degradation and viscosity
loss. In 2002, we found a technical paper by Gibson and Wajer [2] that suggested the use of magnesium hydroxide slurry as an effective cost saving substitute for magnesium sulfate in the
alkaline peroxide bleaching stage. We decided to
T
B. RINGROSE
Western Pulp,
Squamish Operation
Squamish, BC
46
❘❘❘ 106:5 (2005)
A. THAKORE
Western Pulp,
Squamish Operation
Squamish, BC
[email protected]
J. OEI
Western Pulp,
Squamish Operation
Squamish, BC
A. GIBSON
Martin Marietta Magnesia
Specialties, LLC
Baltimore, MD
M. WAJER
Martin Marietta Magnesia
Specialties, LLC
Baltimore, MD
further investigate this possibility as a part of our
bleaching cost optimization program.
Magnesium hydroxide [Mg(OH)2] is often
used as an alkali for pH adjustment of acidic
wastes, precipitation of heavy metals from industrial wastewaters and biological treatment of
organic-enriched waste streams. It has replaced
caustic soda in many environmental applications,
as it provides superior pH buffering and safe handling properties.
The chemistry of magnesium hydroxide in the
Eop stage as a provider of hydroxyl and magnesium ions for bleaching and cellulose protection
is simple. Unlike caustic soda and magnesium sulfate, magnesium hydroxide has low solubility in
water with a Ksp of 5.66 x 10–12. This solubility value is valid when magnesium hydroxide is in equilibrium with water. However, in a complex mixture and at the prevailing conditions in a
pressurized Eop stage, the solubility of Mg(OH)2
increases rapidly as shown by the following reactions [1,2]:
Mg(OH)2 (s) Mg+2 + 2 OH–
(1)
2 OH– + 2 HOOH 2 H2O + 2 OOH–
(2)
Magnesium hydroxide dissociates into magnesium and hydroxyl ions as per reaction 1. The
hydroxyl ions are now available for activation of
peroxide to perhydroxyl ions as per reaction 2.
The perhydroxyl ions are primarily responsible
for delignification and oxidation reactions. The
consumption of the hydroxyl ion by equation 2
causes the first reaction to shift continuously to
the right until all of the magnesium hydroxide is
consumed.
Similar to magnesium sulfate, magnesium
hydroxide functions as a cellulose protector by
capturing and precipitating transition metal ions
as metal hydroxides. When magnesium sulfate is
employed, it readily dissociates and reacts with
OH– from caustic soda to form magnesium
hydroxide:
MgSO4 Mg+2 + SO4–2
(3)
Mg+2 + 2 OH– Mg(OH)2
(4)
T 121 Pulp & Paper Canada
BLEACHING
TABLE I. Solubility products of metal
hydroxides.
Metal Hydroxide
Fe (OH)3
Cu(OH)2
Fe(OH)2
Mn(OH)2
Mg(OH)2
FIG. 1. DEEopD bleaching sequence at Squamish.
Transition metals such as iron and
manganese co-precipitate with magnesium hydroxide to prevent cellulose
degradation. Adding magnesium hydroxide instead of magnesium sulfate in the
pressurized Eop stage eliminates equation
3 above.
Thus, magnesium hydroxide dissociates not only as a result of the reactions
in equations 1 and 2, but also as a result
of its reaction with metal ions with lower
Ksp values. As shown in the following reaction, a metal ion (denoted as M+x) is
exchanged with the magnesium ion from
equation 4 to form insoluble metal
hydroxide.
M+x + 2 OH– M(OH)x (s) + Mg+2
(5)
Metal hydroxides that are less soluble
than magnesium hydroxide precipitate
out as a result of the reaction between
metal ions present in the bleach system
and hydroxyl ions derived from magnesium hydroxide (see equation 5). Table I
lists the solubility products (Ksp) for various metal hydroxides [3,5,6]:
As shown in Table I, common transition metals found in pulp, such as iron,
copper and manganese, will be captured
by the addition of magnesium hydroxide
in the pressurized Eop stage for cellulose
protection.
LABORATORY EXPERIMENTS
To determine the feasibility of using
Mg(OH)2 at the Squamish pulp mill, an
independent lab was hired to carry out
bleaching studies. Table II shows the
bleaching conditions used in the experiments. An industrial grade of Mg(OH)2
slurry (61% solids, 98.8% Mg(OH)2 (dry
basis), 700 ppm Fe and 100 ppm Mn) was
employed in the laboratory study.
The first set of laboratory bleaching
Pulp & Paper Canada T 122
studies used Sq-K Grade brown stock pulp
(80% hemlock/ 20% cedar) from our
mill to examine the impact of replacing
MgSO4 with Mg(OH)2 in the Eop stage of
the DEEopD bleaching sequence. The
kappa number of the washed brown stock
was 28.2. Due to the limitation of the laboratory facility, bleaching of the Control
and the Substitution runs had to be done
on two different days.
The second and third set of studies
used partially bleached mill pulp from the
E stage instead of brown stock pulp. This
change allowed the laboratory to carry
out the Control and Substitution experiments simultaneously, thus eliminating
the possibility of experimental errors and
the impact of aging.
Substitution of MgSO4 in the Eop Stage
In the first laboratory bleaching study,
three different scenarios were examined:
a) Substitution of MgSO4 with Mg(OH)2
based on an equivalent chemical weight
replacement of Mg+2.
b) Direct weight to weight substitution of
MgSO4 with Mg(OH)2.
c) Twenty-five percent substitution of
Eop caustic soda with Mg(OH)2 based on
OH–.
In the second and third studies,
MgSO4 was substituted with Mg(OH)2 on
a direct weight to weight basis.
Note: In all cases, duplicate bleaching
runs of Control and final D stages were
done.
LABORATORY RESULTS
As shown in Tables III through V, substituting MgSO4 with an equivalent chemical
weight, a direct weight to weight of
Mg(OH)2, as well as substituting 25% of
NaOH on an OH- basis with the equivalent amount of Mg(OH)2, did not
adversely impact the final brightness.
There was a minor drop in viscosity in the
Ksp
2.8
2.2
4.9
2.0
5.6
×
×
×
×
×
10–39
10–20
10–17
10–13
10–12
first bleaching trial, but this was not
observed in either the second and third
trial or the full mill scale trial.
Although brightness in the Eop stage
was slightly lower for the Mg(OH)2 runs,
equivalent final brightness was achieved
in the D1 stage without having to increase
the chlorine dioxide charge. In addition,
Eop kappa numbers in each of the
Mg(OH)2 runs were maintained. The D1
pulp viscosity values were lower in the
second and third study due to lower than
normal mill E pulp kappa on the days
samples were taken.
MILL TRIAL
Based on the positive lab bleaching results
and potential economic benefit, Fig. 2,
mill management decided there was
enough justification to proceed with a
one week mill scale trial.
Figure 2 plots the predicted total cost
of NaOH, MgSO4 and Mg(OH)2 in the
Eop stage relative to the Control case,
where no Mg(OH)2 is used. It shows that
the maximum cost savings is achieved by
replacing MgSO4 with the chemically
equivalent weight of Mg(OH)2 [i.e. 0.48
kg Mg(OH)2/kg MgSO4] and reducing
NaOH by the amount of OH- contributed
by Mg(OH)2. On an equivalent chemical
weight basis, 1 kg of Mg(OH)2 replaces
2.08 kg of MgSO4 and 1.37 kg of NaOH.
Since 1 kg of Mg(OH)2 currently costs
more than 1.37 kg of NaOH at the time of
this writing, neither 25% substitution of
NaOH with Mg(OH)2 [i.e. at 2.36 kg/t
Mg(OH)2] nor direct weight to weight
substitution [i.e. at 1.0 kg/t Mg(OH)2]
maximizes cost savings.
Mill Trial Preparation
The Mg(OH)2 supplier provided the mill
with 1,040-liter totes, an air-operated
transfer pump, transfer tubing to the process, a day tank equipped with an agitator
and a variable speed progressive cavity
feed pump. The test equipment was skidmounted to allow for easy set up near the
process application point (i.e. Eop stage).
The mill was responsible for providing
flush water, compressed air and 120 VAC,
60 Hz, 30 amp service.
Application and Testing
The mill trial period, which was planned
to last for a week from November 18 - 25,
was extended to November 28 because of
mill operational problems unrelated to
Mg(OH)2, which necessitated an inter106:5 (2005) ❘❘❘
47
BLEACHING
TABLE II. Bleach operating condition at 10% consistency.
Stage
D0
Chemical
% on pulp
0.20
2.2% C1O2
2.3% NaOH
1.3% NaOH
0.7% H2O2
0.8% O2
0.1% MgSO4
or various
Mg(OH)2
1.3% C1O2
E
Eop
D1
Pressure
kPa
Temp
°C
Time
min
End
pH
68
15
2.2
80
40
11.3
92
30 + 120
11.3
70
70
3.9
375
(30 min)
0
(120 min)
TABLE III. First bleaching study. (Lab bleached pulp E kappa: 5.3-5.5).
Chemical
per Adt
Eop Stage
Brightness Visc. Kappa
% ISO mPa•s
1.0 kg
MgSO4
(Control)
0.48 kg
Mg(OH)2
(eq. chem. wt.)
1.0 kg
Mg(OH)2
(direct wt.)
2.36 kg
Mg(OH)2
(25% NaOH sub.)
D1 Stage
Brightness Visc.
% ISO
mPa•s
65.4
21.1
3.5
85.8
20.5
63.4
19.8
3.4
85.4
19.5
62.8
19.8
3.5
85.2
19.5
61.7
19.8
3.5
85.5
19.8
TABLE IV. Second bleaching study. (Soft stock — mill E
pulp kappa: 3.8; typical range 4.3-5.0).
Chemical
per Adt
Eop Stage
Brightness Visc. Kappa
% ISO mPa•s
1.0 kg
MgSO4
71.9
(Control)
1.0 kg
Mg(OH)2
71.1
(direct wt.)
2.36 kg
Mg(OH)2
69.8
(25% NaOH sub.)
D1 Stage
Brightness Visc.
% ISO
mPa•s
13.5
1.9
89.5
11.9
12.7
1.9
89.4
11.9
13.1
2.0
89.5
12.1
TABLE V. Third bleaching study. (Soft stock — mill E pulp
kappa: 4.4; typical range 4.3-5.0).
Chemical
per Adt
Eop Stage
Brightness Visc. Kappa
% ISO mPa•s
1.0 kg
MgSO4
(Control)
1.0 kg
Mg(OH)2
(direct wt.)
48
❘❘❘ 106:5 (2005)
D1 Stage
Brightness Visc.
% ISO
mPa•s
67.3
15.9
2.5
88.4
14.2
66.3
15.7
2.6
88.2
14.3
mittent bleach plant shutdown for the first three days.
The production rates, operating conditions and chemical
applications were held as steady as possible and a 4-hour
On/Off run of MgSO4 vs. Mg(OH)2 was made. Any abnormalities such as swings of digester kappa and black liquor carryover,
if above their normal range, were noted. Bleach plant operating conditions were recorded and logged by the operator as
normal.
The following variables, some of which are routine tests,
were tested by a mill lab technician to evaluate the effectiveness
of Mg(OH)2 vs. MgSO4:
1. Eop filtrate pH - every half hour
2. Eop filtrate Residual NaOH - every half hour
3. Machine pulp Brightness - every hour (routine)
4. Machine pulp Viscosity - every 56 bales (routine)
Initially, the transfer of magnesium hydroxide slurry and the
application pumping rate adjustments were done by the supplier’s engineer but later handled by the bleach operator’s assistant once he was fully trained.
Mill Trial Results
The first three days of trial data were discarded due to mill
operation interruption and a Kamyr digester problem. Subsequently, Mg(OH)2 was added on an On/Off basis, alternating
with MgSO4 every four hours. The Mg(OH)2 slurry was added
in the same location as MgSO4. During the last three days of the
trial, Mg(OH)2 was added on a continuous basis.
1. No observable change (i.e. decrease) of final pulp brightness
and viscosity was observed when an equivalent chemical weight
of Mg+2 and direct weight to weight from 1.0 kg/Adt of MgSO4
were substituted with Mg(OH)2, as shown in Fig. 3.
2. Eop filtrate residual NaOH and pH tests showed that complete dissociation of Mg(OH)2, as described by equations 1 and
2, transpired under the pressurized Eop stage conditions. Theoretical calculations of expected OH– ions formed from the
measured Mg(OH)2 feed rate matched that calculated from Eop
filtrate flows, residual NaOH and pH measurements, Fig. 4.
MG(OH)2 BULK STORAGE CONSIDERATION
Because Mg(OH)2 has certain properties and comes as a 61%
solids slurry, certain requirements have to be satisfied to ensure
dependable storage and application:
- Freeze protection for temperatures below 5°C.
- Periodic agitation (not constant) for long term bulk storage to
maintain suspension stability.
- A positive displacement type pump (i.e. progressive cavity)
that can deliver a low feed rate up to 1.0 l/min.
- A minimum pipe velocity above 0.3 ft/sec to assure reliable
flow to avoid solids settling or blockage; for our mill requirement, a 3/8” or 1/2” diameter tubing would be required.
- An automatic flush system of the feed line leading to the process to prevent blockage.
The conversion of our MgSO4 storage to accommodate
Mg(OH)2 was completed in mid-May. Prior to this conversion we
added 0.48kg/ADt Mg(OH)2 to the process using the skid
described earlier in the mill trial section. The results of the first
25 days of operation confirm close to 0.66kg/ADt NaOH savings,
and virtually no impact on pulp brightness or viscosity. Subsequent data after the full scale conversion confirms similar savings
without any significant impact on pulp brightness or viscosity.
CONCLUSIONS
1. Mg(OH)2 is a viable economic replacement for MgSO4 for
protecting pulp viscosity in the mill’s pressurized Eop stage. No
adverse impact on final brightness and viscosity was noticed
during the week long mill trial.
2. For optimum cost savings, Eop caustic soda can be reduced
by the amount of OH– contributed by adding Mg(OH)2 on an
equivalent chemical weight basis to MgSO4. In this case, 0.48 kg
of Mg(OH)2 replaces 1.0 kg of MgSO4 + 0.67 kg of NaOH.
3. When the cost of 1 kg of Mg(OH)2 is less than the cost of
T 123 Pulp & Paper Canada
BLEACHING
FIG. 2. Relative cost of substituting MgSO4 with Mg(OH)2
in the Eop stage.
FIG. 3. Impact of MgSO4 and Mg(OH)2 on final pulp brightness and viscosity.
This paper was previously presented at the PAPTAC Pacific
Coast Branch Mini-Conference in Parksville, British Columbia in
April 2004, PACWEST Conference in Jasper, Alberta in May
2004 as well as the PAPTAC Bleaching Committee Meeting in
Fredericton, New Brunswick in June 2004.
LITERATURE
FIG. 4. Impact of MgSO4 and Mg(OH)2 on Eop diffuser filtrate pH and residual NaOH.
1.37 kg of NaOH, up to 25% substitution of NaOH with
Mg(OH)2 on an OH– basis is economically viable. However,
since other studies have shown that excessive Mg+2 addition may
lead to a lower pulp brightness, increasing addition rate of
Mg(OH)2 will require close monitoring.
4. As a result of favorable laboratory and mill trial data, the
Squamish pulp mill has converted the existing MgSO4 system to
Mg(OH)2 in order to benefit from the significant cost savings.
ACKNOWLEDGEMENTS
The authors would like to thank mill management for their
unwavering support, from the initial proposal of this project to
the authorization of the mill trial. We also would like to acknowledge the fine work of Econotech Lab, who carried out the laboratory bleaching studies and offered valuable suggestions and
interpretation of the results; Paul Schmidtchen of Martin Marietta, who assisted with the mill trial; Andrew Crane our Project
Engineering group leader who managed the installation of the
system and our mill technician, K. Adams, who assisted in the
residual caustic tests and other related tests.
Pulp & Paper Canada T 124
1. GIBSON, A., WAJER, M., SCHMIDTCHEN, P. The Use of CellGuard™OP Magnesium Hydroxide Slurry as a Cellulose Protector in Oxygen Delignification. Proceedings from the 90th Annual PAPTAC Conference, Montreal, Quebec, B-1409
(2004).
2. GIBSON, A., WAJER, M. The Use of Magnesium Hydroxide as an Alkali and Cellulose Protector in Chemical Pulp Bleaching. Proceedings from the 2002
PACWEST Conference, Jasper, Alberta, 4B4 (2002).
3. LIDE, D.R. Handbook of Chemistry and Physics - 82nd Edition. CRC Press, Boca
Raton. 8.117-8.120 (2001).
4. DEAN, J.A. Lange’s Handbook of Chemistry - 15th Edition. McGraw Hill, New York.
8.9-8.12 (1999).
5. DENCE, C.W., REEVE, D.W. Pulp Bleaching: Principles and Practice. TAPPI Press,
Atlanta. 224 (1996).
6. SEIDELL, A. Solubilities of Inorganic and Metal Organic Compounds. D.Van Nostrand
Company, Inc., New York . 982 (1940).
Résumé: L’ion magnésium empêche efficacement la dégradation
de la cellulose à l’étape de blanchiment au peroxyde alcalin (Eop) sous
pression. Les essais en laboratoire et en usine à l’usine Western Pulp
de Squamish ont démontré que l’application d’hydroxyde de magnésium à l’étape Eop permet de remplacer économiquement la soude
caustique et protège davantage la cellulose contre la dégradation alcaline que le sulfate de magnésium. Il n’existe aucun effet négatif sur la
blancheur ou la viscosité de la pâte finale.
Reference: THAKORE, A., OEI, J., RINGROSE, B., GIBSON, A.,
WAJER, M. The use of magnesium hydroxide as a cost effective cellulose
protector in the pressurized alkaline peroxide (Eop) bleaching stage.
Pulp & Paper Canada 106(5): T121-124 (May, 2005). Paper presented at
the 2004 PACWEST Conference in Jasper, AB, May 19-22, 2004. Not to
be reproduced without permission of PAPTAC. Manuscript received
June 02, 2004. Revised manuscript approved for publication by the
Review Panel on July 28, 2004.
Keywords: MAGNESIUM HYDROXIDE, PEROXIDE BLEACHING,
PROTECTANTS, CELLULOSE, COST EFFECTIVENESS.
106:5 (2005) ❘❘❘
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