PEROXIDE BLEACHING
A comparative study of Mg(OH)2-based and
NaOH-based peroxide bleaching of TMP: Anionic
trash formation and its impact on filler retention
By Z. He, M. Wekesa and Y. Ni
Abstract: The Mg(OH)2-based peroxide bleaching process produced significantly less anionic
trash than conventional NaOH-based process, which is due to the dissolution of less hemicellulose
and oxidized lignin in the former. The formation of less anionic trash during the Mg(OH)2-based
process resulted in less carryover of anionic trash to the paper machine, consequently, the bleached
pulp from the Mg(OH)2-based process offered significantly higher first-pass filler retention than
from the NaOH-based process at a given retention polymer charge.
is the dominant
process for brightening thermomechanical pulp (TMP), and it is usually
conducted under alkaline conditions,
using sodium hydroxide as the alkali
source. However, the high alkalinity of sodium
hydroxide leads to extensive dissolution of organic substances during peroxide bleaching, resulting in significant decreases in bleached pulp yield
[1] and COD load in the effluent [1-7], as well as
the formation of anionic trash [8-10]. Anionic
trash can interfere with the papermaking process,
for example, forming polyelectrolyte complexes
with papermaking polymers, which then reduces
their adsorption on fibres.
Alternative alkali sources have been under
investigation in order to search for a suitable substitute for sodium hydroxide to reduce the problems related to its use in peroxide bleaching of
mechanical pulp without adverse effects on the
optical and strength properties of bleached pulp.
A number of studies have shown that magnesium
hydroxide (Mg(OH)2) is a promising weak alkali
for this purpose [1-7].
In a previous study [1], it has been shown that
the lower COD from the Mg(OH)2-based process
is mainly due to the formation of less acetic acid,
methanol and dissolved lignin. Dissolved and oxidized lignin accounts for a major part of anionic
trash generated in peroxide bleaching [10]. Other important components of anionic trash
released from peroxide bleaching of mechanical
pulps include anionic hemicellulose, fatty acids
and resin acids [12].
The objective of this study is to obtain further
understanding on how anionic trash generation
is affected by substituting Mg(OH)2 for NaOH as
the alkali source in peroxide bleaching of TMP
pulp. It was carried out by determining the contribution of dissolved hemicellulose, oxidized
lignin, and resin and fatty acids to the anionic
trash. In addition, the anionic trash carryover in
the pressed pulp and its impact on filler retention
were evaluated in a laboratory system.
P
EROXIDE BLEACHING
EXPERIMENTAL
Material: A mill-chelated softwood TMP sample
(mainly spruce) was obtained from a mill in East-
Pulp & Paper Canada T 55
ern Canada. The pulp sample had a consistency
of 11% with an original brightness of 58.2 %, and
was stored in a cold room until its use. Chemicals
were reagent grades purchased from Aldrich
except sodium silicate (40%, National Silicate),
magnesium hydroxide (61% slurry, Martin Marietta Magnesia Specialties), cationic polyacrylamide (Percol 292), bentonite (Hydroco 0), precipitated calcium carbonate (Albacar HO,
Specialty Mineral Inc.) and clay filler (Imery
Kaolin), all of which were industrial grades.
Methods: Peroxide bleaching experiments were
conducted in plastic bags using the conditions
outlined in Table [1]. To make the bleach liquor
for the NaOH-based peroxide bleaching process,
the chemicals were mixed in a beaker by the following order: water, sodium silicate, sodium
hydroxide and hydrogen peroxide. The prepared
bleach liquor was then added to the pulp, which
was pre-heated to 70°C, and good mixing was provided by kneading. The plastic bag was sealed and
placed into a water bath for the desired retention
time. When running the Mg(OH)2-based peroxide bleaching process, Mg(OH)2 and DTPA were
mixed with the preheated pulp before the addition of peroxide (The PM process [13]). The rest
of the procedure was exactly the same as that of
the NaOH-based process.
At the completion of the bleaching time, the
pulp sample was cooled down to room temperature and subjected to the procedure outlined in
Fig. 1. The filtrate was recycled once to go
through the fibre mat to collect the fines, and
then filtered further through a Whatman medium filter paper to remove the residual fines.
The filler retention response of the bleached
pulp was determined using a dynamic drainage
jar (DDJ) with a 100-mesh screen [14]. A dualretention system [cationic polyacrylamide
(CPAM) and bentonite] was used to retain the
filler, with various CPAM charges and a fixed bentonite charge of 0.1%.
Anionic trash measurement was done at two pH
levels: 4.5 and 7.0. The filtrate sample was adjusted
to the desired pH and then tested for the cationic
demand [10]. For the lignin analysis, the filtrate
sample (pH adjusted to 7.0 ± 0.1) were filtered
through a 0.22-␮m membrane filter and tested for
Z. HE,
University of New
Brunswick,
Fredericton, NB
M. WEKESA,
University of New
Brunswick,
Fredericton, NB
Y. NI,
University of New Brunswick,
Fredericton, NB
[email protected]
107:3 (2006) ❘❘❘
29
PEROXIDE BLEACHING
UV absorbance at 280 nm on a Spectronic
1001 Plus spectrophotometer (Milton
Roy). The lignin content was calculated
using an absorptivity of 21.9 L g–1cm–1 for
spruce milled wood lignin [15].
RESULTS, DISCUSSION
Anionic Trash Generation in Peroxide
Bleaching: The amount of anionic trash is
measured as the cationic demand of the
dissolved and colloidal substances in the
water phase (filtrate) [8- 0]. From the
results in Fig. 2, it is observed that the
Mg(OH)2-based bleaching process produces significantly less anionic trash than
the NaOH-based process, and this is true
for the results at both pH 4.5 and pH 7.0.
Furthermore, the amount of anionic trash
released from the Mg(OH)2-based process does not change appreciably with the
increasing chemical charges. In contrast,
for the NaOH-based process, the formation of anionic trash increases significantly with the chemical charges.
We then made further efforts to estimate the contribution of anionic hemicellulose, oxidized lignin and resin and
fatty acids to the total anionic trash formation from both the Mg(OH)2-based
and NaOH-based peroxide bleaching processes. Their contribution to anionic trash
can be estimated by determining the
cationic demand of the anionic trash at
both pH 4.5 and pH 7.0 based on the difference of pKa of their carboxylic groups
[10]. At pH 4.5, only the uronic type of
carboxylic groups of anionic hemicellulose, such as polygalacturonic acids, can
effectively dissociate into anionic groups;
at pH 7.0, all the three types of carboxylic
groups (uronic, lignin, resin and fatty
acids) can dissociate into anionic groups
[10,16,17]. Therefore, the anionic trash
determined at pH 4.5 is mainly due to
anionic hemicellulose. The difference of
the cationic demand between pH 7.0 and
pH 4.5 is attributed to oxidized lignin and
resin and fatty acids. The results in Fig. 2
show that for the Mg(OH)2-based process,
the anionic trash at pH 4.5 was notably
lower than for the NaOH-based process,
indicating that less anionic hemicellulose
was dissolved from the Mg(OH)2-based
process. The difference of anionic trash
tested between the two pH levels was quite
small for the Mg(OH)2-based process,
suggesting that the contribution of oxidized lignin and resin and fatty acids to
anionic trash is very little. However, for
the NaOH-based process, the oxidized
lignin and resin and fatty acids contributed much more to the total anionic trash,
as indicated by the much greater difference of anionic trash tested between the
two pH levels. The total extractive content
found in the filtrate was generally low for
both the Mg(OH)2-based and the NaOHbased processes [1], suggesting that the
contribution of resin and fatty acids to
anionic trash was relatively small. There-
30
❘❘❘ 107:3 (2006)
Table I. Characteristics of secondary filtrate samples.
Bleaching Process
Filtrate label*
C
Mg(OH)2 - H2O2
D
E
Cationic demand at pH 4.5 (␮eq/g pulp)
H2O2 charge 1.0%
5.56
5.57
2.0%
5.56
5.61
3.0%
5.54
5.91
Cationic demand at pH 7.0 (␮eq/g pulp)
H2O2 charge 1.0%
6.66
6.48
2.0%
6.64
6.79
3.0%
6.79
7.29
Lignin (%, on pulp)
H2O2 charge 1.0%
0.22
0.26
2.0%
0.24
0.31
3.0%
0.25
0.31
NaOH - H2O2
D
E
F
C
F
5.95
5.98
6.14
5.78
5.90
6.28
8.48
9.87
11.6
9.69
11.4
13.1
9.19
10.6
11.9
9.18
10.8
11.9
6.91
6.98
7.21
6.99
7.30
7.51
10.7
12.5
13.9
11.8
14.2
16.3
10.9
13.3
14.5
11.0
13.2
15.1
0.16
0.19
0.17
0.22
0.23
0.23
0.26
0.28
0.33
0.38
0.37
0.46
0.20
0.21
0.24
0.27
0.28
0.32
Refer to Figure 1 for definition of C, D, E and F.
FIG. 1. Experimental procedures for collecting the filtrate samples.
FIG. 2. Anionic trash generated from
the Mg(OH)2-based and NaOH-based
peroxide bleaching process. The
amount of anionic trash was measured at two pH levels: 4.5 and 7.0.
fore, the decreased dissolution of polygalacturonic acids and oxidized lignin are
mainly responsible for the observed lower
anionic trash from the Mg(OH)2-based
bleaching process.
The dissolved lignin was quantified by
the UV absorbance at the wavelength of
280 nm, and the results were summarized
in Fig. 3. Although lignan and other ligninlike substances also have absorbance at 280
nm, their amounts found in bleached TMP
suspension are generally small compared
with the amount of lignin-related substances [11]. The results in Fig. 3 show that
there was a significant difference of lignin
FIG. 3. Comparison of the lignin dissolution between Mg(OH)2-based and
NaOH-based processes.
dissolution between the two bleaching processes. The lignin dissolved from the
Mg(OH)2-based process was probably also
less oxidized than that from the NaOHbased process [10]
Effect of Mg2+ Ions on Anionic Trash
Release: As discussed in the previous section, a significant decrease of anionic trash
is achieved by replacing NaOH with
Mg(OH)2 as the alkali source for peroxide
bleaching of TMP, and this is largely
attributed to the weak alkalinity of magnesium hydroxide. However, the presence of
Mg2+ ions may also contribute to the
observed decrease of anionic trash, since
T 56 Pulp & Paper Canada
PEROXIDE BLEACHING
FIG. 4. The effect of MgSO4 addition on the amount of
anionic trash in he pulp suspension from the NaOH-based
peroxide bleaching process (at 1.0% H2O2, 1.5% NaOH). The
conductivity of the pulp suspension was adjusted to 1.0
ms/cm with NaCl in all cases.
FIG. 5. The effect of Mg2+ on the anionic charge density of
sodium polygalactoronate (170 mg/L, pH 4.5). The conductivity of the polygalactoronate solution was adjusted to
1,000 ␮s with NaCl in all cases.
FIG. 6. Effect of acidification on the amount of anionic trash
tested at pH 7.0 (Refer to Fig. 1 for details on filtrates A and B.)
FIG. 7. First-pass retention of the PCC filler at various CPAM
charges (0.1% bentonite, 20% PCC, 0.5% pulp consistency).
the Mg2+ ions can bind to the polygalacturonic acids, oxidized lignin and resin acids,
neutralizing their anionic charge. To verify
this hypothesis, various amount of MgSO4
was added to the pulp suspension (at 1%
pulp consistency) from the NaOH-based
process. The conductivity of the suspension
was adjusted to 1.0 ms/cm with one mol/L
NaCl solution to provide similar ionic
strength in all the cases. The filtrate was
then separated from the mixture and tested
for anionic trash. As Fig. 4 shows, the anionic trash decreased with increasing addition
of MgSO4. At 2% charge of MgSO4 (equivalent to 1.0% Mg(OH)2 charge in the
Mg(OH)2-based process), the anionic trash
decreased by 15% due to the addition of
Mg2+ ions at pH 7.0. In another set of experiments, sodium polygalacturate was used as
the anionic trash model to demonstrate the
effect of Mg2+ ions on anionic trash. The
results in Fig. 5 showed that the cationic
demand of polygalacturonate decreased linearly with increasing MgSO4 charge. The
Pulp & Paper Canada T 57
conductivity of the polygalacturonate solution was adjusted to the same level in all cases. Therefore, it can be concluded that Mg2+
ions are partly responsible for the observed
reduction of anionic trash in the Mg(OH)2based peroxide bleaching, and the effect of
Mg2+ was estimated to be about 15% of the
total reduction of anionic trash.
Carryover of Anionic Trash and its Effect
on Filler Retention: In industry, a press is
usually placed after the peroxide bleaching
to remove anionic trash from pulp slurry,
and it could be an option whether or not
acidification is practiced prior to the press.
To simulate the difference, we studied the
effect of acidification of the pulp slurry
before pressing on the removal of anionic
trash. The results are shown in Fig. 6. Filtrates A and B resulted from thickening
the bleached pulp, with and without acidification beforehand, respectively. It shows
that the anionic trash from the Mg(OH)2based process is much less than that from
the NaOH-based process whether or not
an acidification was performed, which is in
agreement with earlier studies [2,5,6]. As
Fig. 6 shows, different responses to acidification treatment were observed for the two
processes. For the NaOH-based process,
the acidification step before pressing significantly decreased the anionic trash in
the filtrate, and it can be explained by the
deposition of anionic trash onto fibres due
to acidification. However, for the
Mg(OH)2-based bleaching process, the
amount of anionic trash in the press filtrate
was not affected. This is probably due to
the weak alkalinity of the Mg(OH)2-based
bleaching process, and that the amount of
dissolved organics is small and their flocculation/deposition on pulp fibres is not
so sensitive to the pH change.
The anionic trash in the pressed pulp
carries to the papermaking process. To
simulate the traditional acidic and neutral
papermaking processes, the pressed pulp
was diluted to 1% pulp consistency at pH
of either 4.5 or 7.0. The detailed experi107:3 (2006) ❘❘❘
31
PEROXIDE BLEACHING
FIG. 8. First-pass retention of the clay
filler at various CPAM charges (0.1%
bentonite, 20% PCC, 0.5% pulp consistency, pH 7.0).
mental design was given in Fig. 1. The
obtained filtrates (C, D, E, F) represent
the carryover of anionic trash to the paper
machine under different circumstances.
As Table I shows, the anionic trash in the
pressed pulp from the Mg(OH)2-based
process was substantially lower in all the
cases than the NaOH-based process. For
the Mg(OH)2-based peroxide bleaching
process, the acidification before pressing
decreased slightly the anionic trash in the
pressed pulp, and it was true for both the
acidic and the neutral papermaking systems (filtrates C versus E, D versus F).
For the NaOH-based peroxide bleaching process, the acidification before pressing reduced anionic trash slightly in the
acidic system (pH 4.5). However, in the
neutral pH system, the acidified/pressed
pulp contained slightly more anionic
trash than the non-acidified/pressed
pulp. Generally, the acidification step did
not change significantly the anionic trash
in the pressed pulp, which is in agreement
with a previous study [18].
We made further effort in comparing
the wet-end behaviour of the bleached
pulp from the Mg(OH)2-based and the
NaOH-based processes. Figure 7 presents
the first-pass retention of precipitated calcium carbonate (PCC) filler with the
retention aid system of cationic polyacrylamide (CPAM) and bentonite. At a given
charge of CPAM, the bleached pulp from
the Mg(OH)2-based process had much
higher filler retention than from the
NaOH-based process. Vincent et al. [6]
also found that using the MgO/DTPA
peroxide bleaching process the average
retention chemical requirement for the
bleached CTMP could be reduced significantly in comparison with the NaOH/silicate peroxide bleaching process.
Although anionic trash in the pressed
pulp is usually the key factor in affecting
the performance of retention chemicals
[19,20], the charge density of fibres may
also have an effect on filler retention. To
eliminate the effect of anionic trash, the
bleached pulp samples were washed and
then, the retention experiments with PCC
fillers were repeated under identical conditions. As Fig. 7 shows, similar filler
32
❘❘❘ 107:3 (2006)
retention was achieved with the washed
pulp samples bleached by the two processes, which supports the conclusion that
the anionic trash, not the charge density
of the bleached pulp fibres, is the key in
determining the performance of retention chemicals.
Similar results were obtained for the
clay filler, although a much higher CPAM
dosage was required to reach the same
retention degree as that for the PCC filler,
Fig. 8. To achieve a 50% clay retention
rate, about 0.22% CPAM was needed for
the pulp from the NaOH-based process
while only 0.12% CPAM is needed for the
pulp from the Mg(OH)2-based process.
CONCLUSION
In the peroxide bleaching of the TMP
pulp, the anionic trash generated from the
Mg(OH)2-based process is about 50 to
60% lower than that from the NaOHbased process. The formation of less
anionic trash during bleaching leads to a
decreased carryover of anionic trash in the
pressed pulp to the paper machine. Consequently, the bleached pulp from the
Mg(OH)2-based process offered significantly higher first-pass filler retention than
from the conventional NaOH-based process at a given retention polymer charge.
The reduced anionic trash formation
achieved by the Mg(OH)2 based process is
due to the dissolution of less pectic acids
and oxidized lignin as a result of the lower alkalinity of the system. The presence
of the magnesium divalent cations (Mg2+)
also contributes to the observed reduction
of anionic trash released from the
Mg(OH)2-based process. The Mg2+ ions
in the system can be adsorbed by the polygalacturonic acids and lignin, neutralizing
the anionic charges.
LITERATURE
1. HE, Z., WEKESA, M., Ni, Y. Pulp Properties and
Effluent Characteristics from the Mg(OH)2-Based
Peroxide Bleaching Process. Tappi J.3(12):1 (2004).
2. LI, Z., COURT, G.D., CROWELL, M., GIBSON, A.,
WAJER, M. NI, Y. Using Magnesium Hydroxide
(Mg(OH)2) as the Alkali Source During Peroxide Bleaching at Irving Paper. Pulp Paper Can. 106(6):24 (June 2005)
3. ZHANG, J.X., NI, Y., ZHOU, Y., JOLIETTE, D. Using
Magnesium Hydroxide (Mg(OH)2) as the Alkali Source
During Peroxide Bleaching at Irving Paper. Proc., 90th
PAPTAC Annual Meeting, Montreal (Jan. 2004).
4. JOHNSON, D.A., PARK, S., GENCO, J.M., GIBSON,
A., WAJER, M. BRANCH, B. Hydrogen Peroxide Bleaching of TMP Pulps Using Mg(OH)2. Proc., 2002 TAPPI
Fall Conference & Trade Fair, Atlanta, U.S. (2004).
5. NYSTRÖM, M., PYKÅLÅINEN, J., LEHTO, J. Peroxide Bleaching of Mechanical Pulp Using Different
Types of Alkali. Paperi Ja Puu 75(6):419-425 (1993).
6. VINCENT, A.H.D., RIZZON, E., ZOOEFF, G. Magnesium Oxide Driven Peroxide Bleaching, An Economical and Environmentally Viable Process. Proc.,
Appita 51st Annual General Conference. Paper no.
3A41:411-418, Melbourne (1997).
7. SUESS, J.U., GROSSO, M., SCHMIDT, K., HOPF, B.
Options for Bleaching Mechanical Pulp with a Lower
COD Load. Proc., Appita Conference, Melbourne, 419425 (2001).
8. THORNTON, J., EKMAN, R., HOLMBOM, B.,
ECKERMAN, C. Release of Potential ‘“Anionic Trash”
in Peroxide Bleaching of Mechanical Pulp. Paperi Ja
Puu, 75(6):426-431 (1993).
9. BRAUER, P., KAPPEL, J., HOLLER, M. Anionic
Trash in Mechanical Pulping System. Pulp Paper Can.
102(4):44-48 (April 2001).
10. HE, Z., NI, Y., ZHANG, E. Further Understanding
of Anionic Trash Formation During Peroxide Bleaching of Mechanical Pulp. J. Wood Chem. And Technol.,24(2):153 (2004).
11.GULLICHSEN, J., PAULAPURO, H. (series editors), SUNDHOLM, J. (book editor). Papermaking Science and Technology. Book 5. Mechanical Pulping.
Helsinki: Fapet Oy (1999).
12. SUNDBERG, A., PRANOVICH, A., HOLMBOM, B
Distribution of Anionic Groups in TMP Suspensions.
J. Wood Chem. Tech., 20(l):71-93 (2000).
13. NI, Y., LI, Z., COURT, G., BELLIVEAU, R., CROWELL, M. Improving Peroxide Bleaching of Mechanical
Pulps by the PM Process. Pulp Paper Can. 104(12):7881 (2003).
14. PELTON, R., ALLEN, L.H., NUGENT, H. Measuring Fines Retention of Newsprint Pulps. Pulp Paper
Can. 80 (12): T425-T429 (December 1979).
15. LIN, S.Y., DENCE, C.W. Method in Lignin Chemistry.
Berlin: Springer-Verlag, 46 (1992).
16. LAINE, J., STENIUS, P., BUCHERT, J. Spectroscopic and Potentiometric Studies of Kraft Pulp
Fibers. Proc., 48th Appita Annual General Conference, Melbourne, Paper No. 1B33: 109-115 (1994).
17. SJÖSTRÖM, E. The Origin of Charge on Cellulosic Fibers. Nordic Pulp and Paper Research Journal
4(2):90-93 (1989).
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Factors Affecting the Effectiveness of Some Retention
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Résumé: Le procédé de blanchiment au peroxyde à base de Mg(OH)2 a produit beaucoup
moins de déchets anioniques comparativement au procédé classique à base de NaOH, en raison
de la dissolution d’une quantité moindre de lignine oxydée et d’hémicellulose dans le formeur. La
formation moins importante de déchets anioniques pendant le procédé à base de Mg(OH)2 a permis de réduire les déchets anioniques transportés vers la machine à papier et, ainsi, la pâte
blanchie du procédé à base de Mg(OH)2 a offert une rétention beaucoup plus importante des
charges lors du premier passage que celle du procédé à base de NaOH à une charge de polymère
de rétention donnée.
Reference: HE, Z., WEKESA, M., NI, Y. A comparative study of Mg(OH)2-based and NaOH-
based peroxide bleaching of TMP: Anionic trash formation and its impact on filler retention. Pulp
& Paper Canada 106(3): T55-59 (March, 2006). Paper presented at the 91st Annual Meeting in
Montreal, QC, on February 7 to 10, 2005. Not to be reproduced without permission of PAPTAC.
Manuscript received on November 08, 2004. Revised manuscript approved for publication by the
Review Panel on August 2, 2005.
Keywords: THERMOMECHANICAL PULPS, PEROXIDE BLEACHING, MAGNESIUM
HYDROXIDE, SODIUM HYDROXIDE, SOFTWOODS, ANIONIC COMPOUNDS, IMPURITIES,
RETENTION, FILLERS.
T 58 Pulp & Paper Canada