Magnesium-based alkalis for hydrogen peroxide bleaching of

T256 bleaching
Magnesium-based alkalis
for hydrogen peroxide bleaching
of mechanical pulps
By D. F. Wong, J. A. Schmidt and C. Heitner
Abstract: We have evaluated magnesium oxide and magnesium hydroxide alkalis for hydrogen
peroxide bleaching of softwood thermomechanical pulp. A maximum ISO brightness of 78% was
achieved with either MgO or Mg(OH)2. The magnesia charge needed to reach a specific brightness target is smaller (10 kg/t or less) than when using NaOH, offering potential savings in chemical costs. The magnesium-based alkalis produced pulps with higher yield and bulk, and effluents
with lower BOD, dissolved solids, and cationic demand than conventional peroxide bleaching.
ECHANICAL PULPS are being used more
and more in value-added printing and
writing papers. The high brightness
demanded by these grades requires
hydrogen peroxide bleaching.
In peroxide bleaching, an alkali is required to
convert hydrogen peroxide to the hydroperoxyl
anion (Equation 1), which is the reactive bleaching species:
M
HOOH + OH៬ HOO– + H2O
D. F. WONG,
Paprican
Pointe-Claire,
QC
J. A. SCHMIDT,
Paprican
Pointe-Claire,
QC
C. HEITNER,
Paprican
Pointe-Claire,
QC
(1)
Conventionally, the alkali is sodium hydroxide. Due to its strong alkalinity, NaOH also
hydrolyses and dissolves hemicelluloses and
lignin, decreasing pulp yield (1) and increasing
the amount of dissolved and colloidal substances
(DCS), and subsequently increasing BOD and
COD in the bleach effluent (2-4). Excess alkalinity can also lead to alkali-induced darkening of
the pulp, limiting the brightness ceiling (5).
In the late 1980’s, Soteland et al. (6-9) proposed using magnesium-based alkalis to replace
sodium hydroxide in peroxide bleaching of
mechanical
pulps.
Subsequently,
other
researchers (10-17) continued to explore the use
of divalent alkalis for peroxide bleaching of
mechanical pulps. Successful mill trials have been
conducted in Australian mills using MgO in
either complete or partial replacement of the
caustic and silicate (10, 18). Recently, two trials
have been reported in North American TMP
mills, one involving refiner peroxide bleaching
(19, 20). There have also been several patent
applications (21) and patents issued (22-26), for
the use of MgO or Mg(OH)2 in various forms,
either alone or in partial replacement (22, 23) of
NaOH for peroxide bleaching of pulps. Recently,
magnesium oxide and a wetting agent successfully replaced NaOH in the deinking of recycled
newsprint in a mill trial (27).
The main advantage of magnesia-based
bleaching is the quality of the bleach effluent.
The low solubility of the magnesium bases yields
a lower bleaching pH, and therefore less dissolution of hemicelluloses and lignin. BOD and COD
reductions of 30-50% as compared to peroxide
bleaching using NaOH alkali have been reported
68 • 107:12 (2006) • PULP & PAPER CANADA
(6, 11-13, 17). Effluent treatment should therefore be reduced, and have been documented
(19). The lower amount of DCS should also lower the cationic demand in the bleach filtrate and
thereby reduce wasteful consumption of cationic
polymers in the paper machine wet end.
Despite the number of reports in the recent
literature, implementation of magnesia-based
peroxide bleaching has been slow. This could be
due to several reasons: slightly lower maximum
brightness gains when using magnesia, and the
higher unit cost ($/kg) of the magnesia alkalis.
Recently, driven by high energy costs, the price
of NaOH has risen sharply. This has made magnesia alkalis more attractive, since magnesia pricing is less sensitive to fluctuations in energy costs.
Here, we present a thorough examination of
magnesia-based alkalis used in hydrogen peroxide bleaching of eastern Canadian softwood TMP.
Building on the existing knowledge, our intention is to further develop the understanding of
the magnesia alkali process, and overcome the
current limitations at high brightness levels.
EXPERIMENTAL
Softwood TMP (100% Balsam fir, CSF = 100 mL)
was obtained from an eastern Canadian mill. The
pulp was collected at ~30% consistency from the
bleach press just prior to bleach chemical addition. Prior to bleaching, the pulps were treated
with 0.2% diethylenetriaminepentaacetic acid
(DTPA) on pulp at 1% consistency, 60°C for 60
minutes. Metal analysis showed that essentially all
of the manganese was removed in the chelation
(<1ppm remaining).
Laboratory grade magnesium oxide was
obtained from Fisher Scientific. Laboratory
grades of magnesium hydroxide were obtained
from Fisher Scientific and Fluka Chemie. Martin
Marietta Magnesia Specialties supplied the industrial grades of magnesium oxide and magnesium
hydroxide.
Pulp bleaching was carried out in polyethylene
bags immersed in a heated water bath. Bleaching
chemicals and an appropriate volume of dilution
water to give desired bleach consistency were
mixed together in a beaker, and then added to
the pulp. For the conventional bleaching 0.05%
bleaching
FIG. 1. Brightness response contour plot for hydrogen peroxide bleaching of softwood TMP using NaOH as alkali.
Each line represents a constant brightness level (indicated
by the values on the graph) for the given combination of
peroxide and NaOH charge. Bleaching conditions: 10% consistency, 70°C, 180 minutes. Additional bleach chemicals:
2% Na2SiO3, 0.2% DTPA, 0.05% MgSO4.
FIG. 2. Brightness response contour plot for hydrogen peroxide bleaching of softwood TMP using laboratory grade
MgO as alkali. Each line represents a constant brightness
level (indicated by the values on the graph) for the given
combination of peroxide and MgO charge. Bleaching conditions: 10% consistency, 70°C, 240 minutes.
TABLE I. Comparison of undiluted bleach effluent quality for peroxide bleaching of softwood TMP using NaOH and laboratory grade MgO alkali.
Bleaching Temp. (°C)
Alkali
Alkali charge (% on pulp)
Time (min)
Brightness (% ISO)
Residual H2O2 (% of applied)
BOD (mg/L)
COD (mg/L)
Dissolved Solids (g/L)
Cationic demand (meq/L)
NaOH
60
MgO
NaOH
70
MgO
MgO
NaOH
80
MgO
5
180
76.8
35
4320
9800
15.5
12.59
4
180
76.4
40
1720
4917
6.63
3.93
4
180
77.1
38
4576
9507
18.9
11.19
4
180
77.1
45
3428
6010
14.0
3.73
2
180
76.5
54
2912
5010
12.5
4.35
4
120
75.4
29
4270
10100
15.1
13.68
4
120
75.2
39
1720
5087
6.78
4.04
Bleaching with 4% H2O2 at 10% consistency. Bleaching with NaOH alkali required 2% Na2SiO3 (100% basis), 0.2% DTPA and 0.05% MgSO4 in the
bleach liquor. Initial ISO brightness: 59.8%.
MgSO4, 0.2% DTPA and 2% sodium silicate (100% basis) were added to the dilution water, in that order, followed by
NaOH, then peroxide. For the bleaching
experiments with magnesia alkali, magnesia, sodium silicate and/or chelant, if
used, were first added to the dilution
water followed lastly by the peroxide. The
pulp and bleach liquor were blended
together using a Hobart mixer, and then
transferred to the polyethylene bag. Initial
pH, final pH and residual peroxide were
measured. At the end of the bleach reaction, the pulp was dispersed in deionized
water to 5% consistency and neutralized
with sodium metabisulphite to pH 6 to
6.5. The pulp was filtered with a 150-mesh
screen and fines were recovered by passing the filtrate through the pulp pad.
Optical and physical properties of the
pulps were measured.
The bleach filtrate was analyzed for
BOD, COD, TOC and dissolved solids. To
prevent interference in the BOD and
COD analyses, more bisulphite was carefully added until all the peroxide was
destroyed, as determined by peroxide
indicator strips. The filtrate was then
purged with air for 30 minutes to destroy
excess bisulphite. The filtrate was passed
through a 1.2 µm Millipore filter to
remove any remaining fines and other
suspended materials.
Cationic demand of the bleach effluent
was measured by polyelectrolyte titration.
RESULTS AND DISCUSSION
Conventional Hydrogen Peroxide Bleaching
Conventional hydrogen peroxide bleaching (NaOH as alkali) was carried out to
provide a reference benchmark for the
pulp optical and physical properties. Silicate and a chelating agent are also added
to stabilize the peroxide. The silicate also
contributes some alkalinity and buffering
(28) of the bleach liquor pH.
The bleach response of softwood TMP
was determined by a 4-factor 3-level statistical experimental design. The parameters examined in the design are summarized below:
Parameter
H2O2 charge
NaOH charge
Time
Temperature
Range
2 – 6% on pulp
2 – 8% on pulp
60 - 180 minutes
60 – 95°C
The following conditions were kept constant in all the experiments: Na2SiO3
charge: 2%, DTPA charge: 0.2%, MgSO4
charge: 0.05%, bleaching consistency: 10%.
The conventional bleaching results are
shown in the contour plot in Figure 1. In
this plot, each line represents the combinations of caustic charge and peroxide
charge that will give a fixed brightness.
The optimized chemical charges for the
maximum ISO brightness of 80% were
6% peroxide and 6% NaOH on oven-dry
pulp basis.
Peroxide Bleaching with Magnesium
Oxide as Alkali
Laboratory Grade Magnesium Oxide
The solubility of magnesium oxide in
water is only 0.086g/L at 60°C and
0.062g/L at 80°C (29). Because of its low
solubility, MgO exhibits buffer-like
behaviour maintaining the pulp slurry at a
constant pH of approximately 10 throughout the bleach reaction. In comparison,
the peroxide/NaOH system, starts with
high initial pH (>11) which falls throughout the reaction due to consumption of
the NaOH and the formation of acidic
groups from hydrolysis of esters.
We initially used a laboratory grade
magnesium oxide in a four parameter,
three level statistical experimental design
to determine the optimum response. The
parameters of the statistical design experiments are summarized below:
PULP & PAPER CANADA • 107:12 (2006) •
69
T257
T258 bleaching
FIG. 4. Brightness response contour plot for hydrogen peroxide bleaching of softwood TMP using a high purity industrial grade of Mg(OH)2. Each line represents a constant
brightness level for the given combination of peroxide and
Mg(OH)2 charge. Bleaching conditions: 10% consistency,
70°C, 240 minutes.
FIG. 3. Brightness response as a function of bleaching consistency for hydrogen peroxide bleaching of softwood TMP
using industrial grades of MgO as alkali. Bleaching conditions: 6% H2O2, 6% MgO, 70°C, 240 minutes.
◆ MagChem 35, ■ MagChem 50M
TABLE II. Comparison of pulp physical and optical properties for peroxide bleaching of softwood TMP using NaOH and laboratory grade MgO alkali.
H2O2 (% on pulp)
Alkali
Alkali charge (% on pulp)
Time (min)
Brightness Achieved (% ISO)
Residual H2O2 (% of applied)
Bulk (cm3/g)
Tensile (N.m/g)
TEA Index (mJ/g)
Tear Index (mNm2/g)
Scott bond (J/m2)
Gurley Air (s/100mL)
Unbleached
none
NaOH
4
MgO
NaOH
5
MgO
NaOH
6
MgO
0
57.7
2.85
41.8
667.7
7.71
150.0
58.9
4
180
75.8
24
2.33
50.7
900.0
7.58
216.6
122.0
4
240
75.1
28
2.47
42.3
726.8
8.05
154.5
80.44
5
180
77.9
26
2.29
52.5
1004.7
7.50
187.4
134.4
5
240
77.7
32
2.54
40.2
674.4
7.94
176.0
67.24
6
180
79.1
18
2.16
56.6
1058.0
7.15
209.6
156.0
6
240
78.6
31
2.56
40.5
712.9
8.42
155.8
74.6
Bleaching at 10% consistency, 70°C. Bleaching with NaOH also included 2% Na2SiO3 (100% basis), 0.2% DTPA and 0.05% MgSO4 in the bleach
liquor. Initial ISO brightness: 57.7%.
Parameter
H2O2 charge
MgO charge
Time
Temperature
Range
2 – 6% on pulp
2 – 6% on pulp
60 - 240 minutes
60 – 95°C
The brightness response model is summarized in the contour plot in Figure 2. In
contrast to conventional bleaching, the
final brightness depends mainly on the
hydrogen peroxide charge and is almost
independent of the MgO charge for all but
the very highest brightness. This can be
attributed to the buffering effect of MgO,
which maintains a relatively constant pH
throughout the bleaching reaction.
The bleach response curves in Figure 2
suggest that even less than 2% MgO could
be used for all but the highest brightness.
Charges greater than 2% MgO was required
only for brightness exceeding 79%. Thus,
the maximum ISO brightness of 80.0% was
achieved with 6% peroxide and 6% MgO.
In Tables I and II, the bleach effluent
properties and pulp properties are compared at the same final pulp brightness.
The BOD and COD levels of the
bleach filtrate were as much as 50% lower,
and cationic demand was two-thirds lower,
when MgO was the alkali (Table I).
Tensile strength, TEA index and Scott
bond of handsheets made from pulps
bleached with MgO as alkali were lower,
while bulk was higher than those made
from pulps bleached with NaOH. This
can be attributed to the lower alkalinity
of the magnesia bleach liquor resulting
in less fibre swelling and weaker interfibre bonding. The tensile strength of
the pulp bleached with MgO alkali was
essentially unchanged from that of the
unbleached pulp.
Industrial Grade Magnesium Oxide
We also evaluated peroxide bleaching
with several industrial grades of magnesium oxide. The grades differed in the
surface area and/or particle size. Two
products, MagChem 50M and HSA 10,
were micronized forms of MgO having a
mean particle size of 1 micron, whereas
the standard products had a particle size
of 3 – 5 microns. A larger surface area
gives a more reactive grade and faster formation of alkalinity, but the rate of hydration of magnesium oxide to magnesium
hydroxide (Equation 2), a less soluble and
less reactive species (30), is also increased.
MgO + H2O៬ Mg(OH)2
70 • 107:12 (2006) • PULP & PAPER CANADA
(2)
All of the industrial grades showed
significantly higher levels of iron and
manganese than the laboratory grade
(Table III).
The highest ISO brightness obtained
in bleaching with the industrial grades was
only 76%, several points lower than that
obtained with laboratory grade magnesium oxide. The higher transition metal
content of the commercial grades of MgO
is probably an important factor contributing to the lower brightness ceilings. At 6%
MgO charge, the iron content could be
increased by as much as 60 ppm, and the
manganese content by up 9 ppm based on
oven-dry pulp. When combined with the
native metal content of the pulp, these
levels are high enough to cause manganese-catalyzed decomposition of the
peroxide and formation of coloured ironlignin complexes.
Effect of MgO Charge, Bleaching Consistency
and Stabilizers on Bleach Response
Working with the product MagChem 35,
we tried to improve the performance of
industrial grades of magnesium oxide.
MgO charge, bleaching consistency, and
the effect of chelating agents and peroxide
stabilizers added to the bleach liquor were
bleaching
FIG. 5. Brightness response as a function of bleaching consistency for hydrogen peroxide bleaching of softwood TMP
using industrial grades of Mg(OH)2 as alkali. Bleaching conditions: 6% H2O2, 6% Mg(OH)2, 70°C, 240 minutes.
◆ Cellguard OP, ■ MH 10M
examined. Bleaching was carried out at
three fixed charges of MgO, 6%, 2% and
1% on pulp. Consistency was varied
between 5% and 20%. The effect of metal
sequestering agents such as DTPA, or
diethylenetriaminepenta (methylenephosphonic acid) (DTPMPA), as well as a sodium silicate was examined. The results at
two bleaching consistencies are summarized in Table IV.
Surprisingly, at high MgO charge,
increasing the bleaching consistency from
10% to 20% significantly decreased the
brightness gains in all cases, except when
silicate was added to the bleach liquor. In
contrast, at low MgO charge (1% on pulp),
increasing the bleaching consistency had
either no effect or increased the brightness.
The addition of DTPA provided a moderate improvement in the bleach response
as compared to the unchelated system in
all cases. DTPMPA, on the other hand, did
not improve the bleach response at high
MgO charge and actually decreased the
bleach response at high consistency. A possible explanation can be observed in the
residual peroxide levels. Adding DTPMPA
actually reduced the residual peroxide levels as compared to the unstabilized system.
However, at low MgO charge, DTPMPA
was beneficial, increasing both the brightness response as well as the residual peroxide. These results suggest that at elevated levels of transition metals, the DTPMPA
may be forming metal-ligand complexes
that actually decompose peroxide.
The best results were obtained with
the addition of 0.65% (on pulp basis)
sodium silicate to the bleach liquor.
Residual peroxide levels were significantly higher in the presence of silicate. The
silicate may be stabilising peroxide by
precipitating out the manganese as manganese silicates, thereby inhibiting its catalytic activity (31).
Figure 3 summarises the bleach
response as a function of bleach consis-
FIG. 6. Brightness response as a function of bleaching consistency and added stabilizer for hydrogen peroxide bleaching of softwood TMP using Cellguard OP Mg(OH)2 as alkali.
Bleaching conditions: 6% H2O2, 1% Mg(OH)2, 70°C, 240
minutes.
TABLE III. Metals content of various industrial grades of MgO.
MgO sample
Surface Area
(m2/g)
Cu
(ppm)
Fe
(ppm)
Mn
(ppm)
Mg
(%)
Lab Grade
MagChem 35
MagChem 40
MagChem 50
MagChem50M
HSA 10
NA
30
45
65
65
160
2.6
0.6
1.4
1.5
1.3
1.3
108
1011
903
886
841
953
1.6
148
136
137
136
134
52.1
58.3
57.6
57.4
57.0
55.2
tency for MagChem 35 and MagChem 50M.
The somewhat better response of
MagChem 50M may be a consequence of
its smaller average particle diameter
(1µm), which increases its rate of dissolution and reactivity.
Peroxide Bleaching with Magnesium
Hydroxide as Alkali
Magnesium hydroxide is an intermediate
in the industrial production of magnesium oxide from brine. The solubility of
Mg(OH)2 (0.009 g/L at 18°C and 0.04
g/L at 100°C (29)) is even lower than that
of MgO. Both industrial as well as laboratory grades were evaluated. Two of the
three industrial grades of magnesium
hydroxide (Cellguard OP and FloMag
MHM) were supplied as a 60% solids suspension in water. The third sample (MH
10M) was a micronized form supplied as a
dry powder. The laboratory grades,
obtained from chemical suppliers, were
all in the powder form. Transition metal
content of the various Mg(OH)2 products
are summarized in Table V.
High-Purity Grades of Magnesium Hydroxide
We used FloMag MHM as representative
of a high purity grade to benchmark the
optimum brightness response of
Mg(OH)2 alkali in peroxide bleaching of
softwood TMP. A four-factor statistical
experimental design was used; the parameters are summarized below:
Parameter
H2O2 charge
Mg(OH)2 charge
Time
Temperature
Range
2 – 6% on pulp
2 – 6% on pulp
60 - 240 minutes
60 – 95°C
The model predicted a maximum ISO
brightness of 76.5% at bleach chemical
dosages of 6% peroxide and 6% Mg(OH)2.
This maximum brightness was several
points lower than that achieved with either
NaOH or MgO alkali. The lower pH of the
bleach liquor (pH ~ 9 – 9.5) may have been
a contributing factor. The results are summarized in the contour plot in Figure 4.
Industrial Grades of Magnesium Hydroxide
The bleach response of an industrial
grade of magnesium hydroxide, Cellguard
OP was compared. This product is more
cost competitive with NaOH, but contains
higher levels of detrimental transition
metals (Table V).
In the absence of peroxide stabilizers,
the brightness results were significantly
lower than that obtained with the high
purity grades, due to metal catalyzed peroxide decomposition.
Effect of Mg(OH)2 Charge, Bleaching Consistency, and Stabilizer on Bleach Response
To try to improve the bleach response,
various bleaching parameters were manipulated. The impact of bleaching consistency, Mg(OH)2 charge, and bleach stabi-
PULP & PAPER CANADA • 107:12 (2006) •
71
T259
T260 bleaching
TABLE IV. Bleaching with MagChem 35: Effect of MgO charge, bleaching consistency and peroxide stabilizers on final brightness and residual peroxide. Bleaching conditions: 6% H2O2, 70°C, 240 minutes. Initial brightness: 57.9% ISO.
MgO Charge: (%)
6
Consistency: (%)
10
Stabilizer
% ISO % Residual % ISO
None
0.2% DTPA
0.65% Silicate
0.2% DTPMPA
76.1
76.7
75.6
76.0
34.6
32.6
35.1
9.2
1
20
% Residual
71.4
72.9
75.9
68.7
lizer addition on bleach response are
summarized in Table VI.
Like the MgO system, the maximum
brightness gains were again obtained at
high bleaching consistency using a low
Mg(OH)2 charge in combination with
sodium silicate. Unlike the MgO results,
adding DTPMPA yielded significant
improvements in brightness gain.
In the presence of silicate, residual peroxide levels were significantly higher with
Mg(OH)2 as compared to bleaching with
either MgO or NaOH alkali. At low (1%)
Mg(OH)2 charge, high peroxide residuals
were obtained, likely the result of the lower pH of the bleach liquor. This offers
potential savings in bleach chemical costs
if the residual peroxide can be recycled.
Figures 5 & 6 summarize the effects of
consistency and peroxide stabilizers on
bleach response.
We also examined the impact of supplementing the alkalinity of the bleach
liquor with a small amount of NaOH on
the bleach response. The results are summarized in Table VII. At the high
Mg(OH)2 charge, the addition of NaOH
provided no further benefits but at the
low Mg(OH)2 charge, the NaOH augmented the ISO brightness gain by about
0.6 points if either silicate or DTPMPA was
present. The optimal supplemental
charge of NaOH is 1% on pulp.
Bleach Effluent Quality
We compared the bleach effluent quality
for pulp bleached with either Mg(OH)2 +
silicate, or NaOH + silicate to target ISO
brightness of 78%. The bleaching conditions used, and the effluent properties are
summarized in Table VIII.
A higher Mg(OH)2 charge did not
increase final brightness but increased the
dissolution of pulp components into the
effluent; however the levels were still significantly lower than conventional bleaching. The results in Table VIII show that
Mg(OH)2 with silicate can produce a
cleaner bleach effluent with approximately 50% less effluent BOD, COD and TOC,
and significantly less cationic demand
than NaOH/silicate in bleaching to equivalent brightness.
CONCLUSIONS
Published work on using industrial grades
of magnesia to replace NaOH in peroxide
bleaching of softwood TMP reported
2.7
3
7.8
0.7
10
% ISO % Residual
74.2
75.5
76.8
76.5
% ISO
20
% Residual
75.0
75.7
78.3
76.2
12.5
14.5
54.2
18.8
21.4
40.8
65.8
51
2
20
% ISO % Residual
71.3
–
79.6
72.9
2
31.5
2.7
TABLE V. Metals content of various grades of Mg(OH)2.
Mg(OH)2 Grade
Lab Grades
Industrial Grades
Mg(OH)2 sample
Cu (ppm)
Fisher
Fluka Chemie
FloMag MHM
Cellguard OP
MH 10M
2.3
<1
1.2
<1
1.5
maximum ISO brightness of 76%. We
have been able to increase this brightness
limit to over 78%. This was achieved by
bleaching at 20% consistency and including sodium silicate or DTPMPA in the
bleach liquor. The sodium silicate charge
was one-third of that used in conventional bleaching. The optimum charges of the
magnesium alkalis were lower than that of
NaOH, offering potential savings in
bleach chemical costs.
The strength properties of the pulp
bleached with the magnesium-based alkalis are lower compared to pulps bleached
using NaOH as alkali. However, relative to
the unbleached pulp, the magnesia alkali
has no detrimental effect on the pulp
strength properties. The lack of strength
development during peroxide bleaching
with the magnesia alkalis may be compensated by increasing the specific energy
applied during refining to increase fibre
development.
The magnesia alkalis yielded significantly cleaner bleach effluents than conventional bleaching with NaOH. The lower COD of the bleach effluent implied a
higher yield after bleaching. He et al.
have reported a 1% higher bleached yield
when using magnesium hydroxide (32).
The lower cationic demand which accompanies the reduced amount of DCS
should lower expenditure on wet-end
chemicals required to neutralize anionic
trash. Alternatively, existing demands
could be met with reduced washing.
The decision to choose either MgO or
Mg(OH)2 for bleaching depends on several factors. Similar brightness can be
obtained with either alkali, and both offer
the benefit of a cleaner bleach effluent.
MgO is manufactured by the calcination
of Mg(OH)2 slurry, and is therefore more
costly on a mass basis. However, the cost
advantage of Mg(OH)2 can be nullified by
the higher cost of transporting a slurry
containing 40% water. While Mg(OH)2
slurry is easier to handle onsite and
72 • 107:12 (2006) • PULP & PAPER CANADA
Fe (ppm) Mn (ppm)
154
25.2
80
665
776
24.4
2.1
4.5
98.5
103
Mg (%)
39.2
40.1
37.9
39.5
40.6
requires less costly capital equipment
than MgO powder, there are two potential
problems to be aware of: settling of the
slurry if it is not mixed periodically, and
freezing of the water phase during the
winter months in northern climates.
ACKNOWLEDGEMENTS
The authors thank David Giampaolo for
technical assistance with the laboratory
work, and Martin Marietta Magnesia Specialties for financial assistance.
REFERENCES
1. HOLMBOM, B., EKMAN, R., SJOHOLM, R., ECKERMAN, C. and THORNTON, J., “Chemical Changes
in Peroxide Bleaching of Mechanical Pulps”, Papier,
45(10A): V16-V22, (1991).
2. BRÄUER, P., KAPPEL, J. and HOLLER, M.,
“Anionic trash in mechanical pulping systems”, Pulp
Paper Can., 102(4): 44-48, (2001).
3. THORNTON, J., ECKERMAN, C. and EKMAN, R.,
“Effects of Peroxide Bleaching of Spruce TMP on Dissolved and Colloidal Organic Substances”, Proceedings:
6th International Symposium on Wood and Pulping
Chemistry, Melbourne, Australia, 1: 571-577, (1991).
4. THORNTON, J., EKMAN, R., HOLMBOM, B. and
EKERMAN, C., “Release of potential “anionic trash”
in peroxide bleaching of mechanical pulp”, Paperi Ja
Puu, 75(6): 426-430, (1993).
5. KUTNEY, G. W. and EVANS, T. D., “Peroxide
bleaching of mechanical pulps. Part 1. Alkali darkening —the effect of caustic soda”, Svensk Papperstidn.,
88(6): R78-R82, (1985).
6. ABADIE-MAUMERT, F. A. and SOTELAND, N.,
“Bleaching of Mg-Bisulphite Pulps in One stage Using
Peroxide and Magnesium Oxide”, Proceedings: 1985
Internationa1 Pulp Bleaching Conference, Quebec
City, Canada, 99-103, (1985).
7. SOTELAND, N. and OMHOLT, I., “Magnesium
BCTMP”, Proceedings: TAPPI Pulping Conference,
Orlando, Florida, 2: 987-996, (1991).
8. SOTELAND, N., MAUMERT, F. A. A. and
ARNEVIK, T. A., “Use of MgO or CaO as the Only
Alkaline Source in Peroxide Bleaching of High Yield
Pulps”, Proceedings: International Pulp Bleaching
Conference, Orlando, Florida, 231-236, (1988).
9. ARNEVIK, T. A. and SOTELAND, N., “Peroxide
Bleaching of Mechanical Pulps at High Consistency”,
Proceedings: International Mechanical Pulping Conference, Helsinki, Finland, 201-212, (1989).
10. MAUGHAN, S. E., BEDDOE, J., COX, R. E. and BANHAM, P. W., “Use of Magnesium Oxide as an Alkali for
Refiner Brightening of Pinus Radiata TMP”, Proceedings:
46th Appita Annual General Conference, Launceston,
Australia, 123-131, (1992).
11. NYSTRÖM, M., PYKÄLÄINEN, J. and LEHTO, J.,
bleaching
TABLE VI. Bleaching with Cellguard OP: Effect of Mg(OH)2 charge, bleaching consistency and peroxide stabilizers on final
brightness and residual peroxide. Bleaching conditions: 6% H2O2, 70°C, 240 minutes. Initial brightness: 58.0% ISO.
Mg(OH)2 Charge: (%)
6
Consistency: (%)
10
Stabilizer
%
%
%
ISO Residual ISO
None
0.2% DTPA
0.65% Silicate
0.2% DTPMPA
73.3
73.7
75.6
74.6
23.4
30.5
56
47.3
2
20
1
10
20
10
20
%
Residual
%
ISO
%
Residual
%
ISO
%
Residual
%
ISO
%
Residual
%
ISO
%
Residual
5.6
6.8
32.8
21.5
72.6
73.1
74.1
74.3
29.2
42.5
60.5
66.6
72.9
74.5
77.9
77.3
10.2
15.9
39.4
43.7
72.6
74.0
75.0
75.7
44.3
56
67.7
75.8
75.3
77.1
77.7
77.9
36.2
53.1
56.9
68.3
73.3
74.0
78.5
76.0
TABLE VII. Effect of adding NaOH to bleach liquor on brightness and residual peroxide. Bleaching conditions: 6% H2O2, 20%
consistency, 70°C, 240 minutes.
Mg(OH)2 Charge (%):
NaOH Charge (%):
0.5
Stabilizer
%
%
ISO Residual
None
0.65% silicate
0.2% DTPMPA
72.5
77.7
5
28.7
6
1.0
1.5
1
1.0
0.5
1.5
%
ISO
%
Residual
%
ISO
%
Residual
%
ISO
%
Residual
%
ISO
%
Residual
%
ISO
%
Residual
72.0
78.3
3.6
28.2
72.6
78.2
3.9
29.1
75.3
78.1
78.4
14.3
46.7
55.3
73
78.4
78.4
8.2
40.5
46.9
72.9
78.5
78
6.4
37.6
38
“Peroxide bleaching of mechanical pulp using different types of alkali”, Paperi ja Puu,
75(6): 419-425, (1993).
12. DIONNE, P. Y., SECCOMBE, R., VROMEM, M. R. and CROWE, R. W., “The
Use of Soda Ash and Magnesium Oxide as Alkali Sources for the Hydrogen Peroxide Bleaching of Mechanical Pulp”, Proceedings: 18th International Mechanical
Pulping Conference, Oslo, Norway, 403-408, (1993).
13. VROMEN, M. R. and CROWE, R., “Magnesium oxide-an alkali substitute for
sodium hydroxide in the peroxide bleaching of TMP pine, CSSC eucalypt and
bisulphite pine”, Proceedings: 47th Appita Annual General Conference, 1: 167171, (1993).
14. KÜNZEL, U., STRITTMATTER, G. and BERTOLOTTI, H., “Smoothing the
way”, Paper, 218(4): 30-33, (1993).
15. GRIFFITHS, P. and ABBOT, J., “Magnesium oxide as a base for peroxide
bleaching of radiata pine TMP”, Appita, 47(1): 50-54, (1994).
16. MAHAGAONKAR, M. and ABBOT, J., “Peroxide bleaching of radiata pine
TMP and euclyptus regnans cold caustic soda pulps with sodium hydroxide and magnesium oxide”, Appita, 48(1): 40-44, (1995).
17. SUESS, H. U., DEL GROSSO, M., SCHMIDT, K. and HOPF, B., “Options for
Bleaching Mechanical Pulp with a Lower COD Load”, Proceedings: 55th Appita
Annual General Conference, Hobart, Australia, 419-425, (2001).
18. VINCENT, A. H. D., RIZZON, E. and ZOOEFF, G., “Magnesium oxide driven
peroxide bleaching, an economical and environmentally viable process”, Proceedings: 51st Appita Annual General Conference, Melbourne, Australia, 2: 411148, (1997).
19. LI, Z., COURT, G., BELLIVEAU, R., CROWELL, M., MURPHY, R., GIBSON,
A., WAGER, M., BRANCH, B. and NI, Y., “Using Magnesium Hydroxide
(Mg(OH)2) as the Alkali Source in Peroxide Bleaching at Irving Paper”, Pulp Paper
Can., 106(6): 24-28, (2005).
20. PARRISH, A., HARRISON, R., MCCARTHY, G., GIBSON, A. and WAGER, M.,
“TMP Refiner Bleaching with Magnesium Hydroxide and Hydrogen Peroxide”,
Proceedings: PAPTAC 92nd Annual Meeting, Montreal, A: A203 - A208, (2006).
21. VINCENT, A. H. and MCLEAN, I. A., Orica Australia Pty. Ltd., “Bleaching Process”, Australia 2,278,399, (2000).
22. DUBREUX, B., Atochem, “Process for a two stage peroxide bleaching of pulp”,
France 4,734,161, (1988).
23. BERTOLOTTI, H. and KÜNZEL, U., Haindl Papier GmbH, “Verfahren zur
oxidativen Bleiche von Faserstoffen für die Papiererzeugung”, Germany DE 41
11574 A1, (1992).
24. HETZLER, B. H., EADIE, D. T. and TURNBULL, J. K., MacMillan Bloedel
Limited, “Method of brightening mechanical pulp using silicate-free peroxide
bleaching”, Canada 5,223,091, (1993).
25. KÜRZENDER, S., “Verfahren zur Helligkeitssteigerung bei Bleiche von
Holzstoffen mit Wasserstoffperoxid”, Germany DE 44 00954 A1, (1995).
26. VINCENT, A. H. and MCLEAN, I. A., Orica Australia Pty. Ltd, “Process for peroxide bleaching of pulp using MgO particles”, Australia 6,056,853, (2000).
27. STACK, K., CLOW, M., KIRK, M. and MAUGHAN, S., “Use of wetting agents to
improve flotation deinking with magnesium oxide”, Appita, 54(5): 465-468, (2001).
28. FAIRBANK, M. G., COLODETTE, J. L., ALI, T., MCLELLAN, F. and WHITING, P., “The Role of Silicate in Peroxide Brightening of Mechanical Pulp: 4. The
role of Silicate as a Buffer During Peroxide Brightening”, J. Pulp Paper Sci., 15(4):
J132-J135, (1989).
29. WEAST, R. C., “CRC Handbook of Chemistry and Physics”, (1987-88).
30. SMITHSON, G. L. and BAKHSKI, N. N., “The Kinetics and Mechanism of the
Hydration of Magnesium Oxide in a Batch Reactor”, Can. J. Chem. Eng., 47(508513, (1969).
31. LIDÉN, J. and ÖHMAN, L. O., “On the Prevention of Fe- and Mn-Catalyzed
TABLE VIII. Bleaching Conditions and Bleach Effluent Analysis for Peroxide Bleaching Using Different Alkali.
Alkali
Peroxide charge (%)
Alkali charge (%)
Silicate charge (%)
DTPA charge (%)
MgSO4 charge (%)
Time (min)
Temperature (°C)
Brightness (%ISO)
Residual H2O2 (%)
BOD (kg/tonne)
COD (kg/tonne)
TOC (kg/tonne)
Cationic Demand (meq/L)
NaOH
Mg(OH)2
Mg(OH)2
6
4%
2
0.2
0.05
180
70
79.8
34
28.0
74.1
17.1
5.38
6
1%
0.65
0
0
240
70
77.7
59
13.8
28.3
9.5
0.81
6
6%
0.65
0
0
240
70
77.9
30
20.9
42.1
13.3
1.76
H2O2 Decomposition Under Bleaching Conditions”, J. Pulp Paper Sci., 24(9): 269275, (1998).
32. HE, Z., WEKESA, M. and NI, Y., “Pulp properties and effluent characteristics
from the Mg(OH)2-based peroxide bleaching process”, Tappi J., 3(12): 27-31, (2004).
Résumé: Nous avons utilisé des alcalis à base d’oxyde de magnésium et d’hydroxyde magnésium pour le blanchiment au peroxyde d’hydrogène de pâtes thermomécaniques de résineux. Le degré de
blancheur ISO maximal atteint a été de 78 % avec le MgO ou le
Mg(OH)2. La charge de magnésium requise pour réaliser le degré de
blancheur visé est inférieure (10 kg/t ou moins) à la charge de NaOH
requise, ce qui devrait permettre de réduire le coût des produits chimiques. Le rendement et le bouffant étaient plus élevés avec l’alcali à
base de magnésium, tandis que la DBO, les matières solides dissoutes,
et la demande cationique des effluents étaient inférieures à celles du
blanchiment classique au peroxyde.
Reference: D. WONG, J. SCHMIDT, C. HEITNER. Magnesia-Based
Alkalis for Peroxide Bleaching of Mechanical Pulps. Pulp & Paper Canada 107(12): T256-261 (December, 2006). Paper presented at the 91st
Annual Meeting in Montreal, QC, Canada, February 7-10, 2005. Not to
be reproduced without permission of PAPTAC. Manuscript received
November 11, 2004. Revised manuscript approved for publication by the
Review Panel June 7, 2005.
Keywords: MECHANICAL PULPS, HYDROGEN PEROXIDE,
BLEACHING, ALKALIS, MAGNESIUM OXIDE, MAGNESIUM
HYDROXIDE, BRIGHTNESS
PULP & PAPER CANADA • 107:12 (2006) •
73
T261

Download Report