BLEACHING
Winner of the
2005
DOUGLAS
ATACK
Award
Using magnesium hydroxide (Mg(OH)2)
as the alkali source in peroxide
bleaching at Irving paper
By Z. Li, G. Court, R. Belliveau, M. Crowell, R. Murphy, A. Gibson, M. Wajer, B. Branch and Y. Ni
Abstract: Mg(OH)2 was used to replace sodium hydroxide and sodium silicate in a TMP peroxide bleaching process. Brightness gain with Mg(OH)2 as the sole alkali source is comparable to the
caustic/silicate based process. The Mg(OH)2 process produced less anionic trash in the pulp and
lower effluent COD content. The sequential addition of chemicals with hydrogen peroxide last was
superior to the conventional peroxide bleaching process. Improved economics and performance
have been proven in the mill application.
sodium hydroxide
(NaOH)-based peroxide bleaching
process is effective in increasing pulp
brightness for mechanical pulps.
However, as a strong base, sodium
hydroxide can cause the dissolution of carbohydrates, particularly hemi-cellulose, into the
T
HE CONVENTIONAL
Z. LI,
Irving Paper Ltd
Saint John, NB
G. COURT,
Irving Paper Ltd
Saint John, NB
R. BELLIVEAU,
Irving Paper Ltd
Saint John, NB
M. CROWELL,
Irving Paper Ltd
Saint John, NB
R. MURPHY,
Irving Paper Ltd
Saint John, NB
A. GIBSON,
Martin Marietta Magnesia
Specialities LLC
Baltimore, MD
A. WAJER,
Martin Marietta Magnesia
Specialities LLC
Baltimore, MD
B. BRANCH,
Neill and Gunter
Fredericton, NB
Y. NI,
Limerick Pulp and Paper
Research and Education Centre,
University of New Brunswick
Fredericton, NB
24
❘❘❘ 106:6 (2005)
bleach effluent and is responsible for the
increased COD and BOD load to the secondary
treatment system. Also, the high alkalinity of the
process leads to the formation of a significant
amount of anionic trash, which is carried onto
the wet-end of the paper machine. This can have
negative consequences on papermaking operations, such as increased polymer/additive cost,
reduced drainage and decreased product quality.
In recognition of the drawback associated with
the NaOH-based peroxide process, peroxide
bleaching using weak alkali sources, such as magnesium hydroxide (Mg(OH)2), has received
much attention recently. Griffiths and Abbot [1]
studied the use of magnesium oxide as the alkali
source in a peroxide bleaching of pine TMP, and
found that brightness gain is inferior to that of
NaOH based peroxide process (about 2 units less
at the same peroxide charge). Soteland et al. [2]
observed that the size and shape of magnesium
oxide particles have a significant impact on the
bleaching results and under optimum condition,
the magnesium oxide-based peroxide process
produced a bleached pulp with about 1.5 units
lower in brightness than the NaOH-based process
under otherwise identical conditions. Recently,
Seuss et al. [3] reported that the application of
magnesium hydroxide as an alkali source to
bleach mechanical pulps is very attractive. The
COD is decreased by 30-40% and the sodium silicate charge can be decreased significantly. Johnson et al. [4] recently concluded many advantages associated with using Mg(OH)2 as the alkali
for bleaching mechanical pulps. In addition to
the open literature, there are a number of
patents or patent applications on using magnesium oxide or magnesium hydroxide as the alkali source for bleaching of mechanical pulps [5,6].
Irving Paper has made significant progress in
decreasing the production cost in its peroxide
bleaching stage [7,8], and the effort continues.
Most recently, it tested the magnesium hydroxidebased peroxide process at the mill. In this paper,
we will report the laboratory and the mill results.
EXPERIMENTAL
Material
Hydrogen peroxide (30% H2O2), magnesium sulfate (MgSO4.7H2O), sodium hydroxide (NaOH)
T 125 Pulp & Paper Canada
BLEACHING
FIG. 1. Peroxide decomposition under different Mg(OH)2
systems (1 g/L Mg(OH)2, 1.6 g/l H2O2, 60°C.
and manganese sulfate monohydrate (MnSO4.H2O) were supplied by Fischer Scientific. Sodium silicate used throughout the
study was a clear industrial solution, its Na2O/SiO2 molar ratio is
3.1. Diethylenetriaamine pentaacetic acid sodium salt (DTPA)
was 50% industrial solution. An industrial-grade Mg(OH)2slurry
(61.2% Mg(OH)2, the transition metal contents are 1.9 ppm Cu,
797 ppm Fe, and 113 ppm Mn) was supplied by Martin Marietta
Magnesia Specialties. The mill chelated TMP with 2.6 ppm Cu,
71.2 ppm Mn, 16.8 ppm Fe, an initial brightness of 55.8% ISO,
was received from Irving Paper. Bleached sulfite pulp, used for
decomposition trials for Figures 2 - 4, was supplied by a mill in
Eastern Canada at 17.7% pulp consistency (1.6 ppm Cu, 25.2
ppm Fe and 1.4 ppm Mn).
Peroxide Stabilization Study
It was carried out with continuous stirring in capped polyethylene containers supported in a constant water bath at 60°C. The
initial concentration of hydrogen peroxide was 1.6 g/L. Whenever pulp was needed its consistency was 4%. The total volume
of the solution used was 250mL. Unless otherwise stated, the
procedure for mixing of solutions simulated the cascade bleach
liquor preparation method practiced in industry, that is the stabilizer (DTPA, Na2SiO3) the alkali source, and H2O2 were mixed
together and then the mixture was charged to pulp to which
Mn2+ had been added.
The extent of the reaction was monitored by withdrawing
5mL samples at specified times, and the residual hydrogen peroxide was determined by following PAPTAC standard testing
method, J.16P.
Conventional Peroxide Bleaching
The bleach liquors were prepared by adding, in order, the
required amounts of magnesium sulfate, sodium silicate or
DTPA (if needed), sodium hydroxide or Mg(OH)2, and hydrogen peroxide solution to a beaker containing deionized water.
Then, the mixed solution was added to the pulp in a polyethylene bag and the pulp slurry kneaded for two minutes. After the
required retention time, a liquid sample was taken to determine
the ending pH and the residual peroxide. The pulp slurry was
diluted to 2% consistency, acidified and washed with deionized
water.
The PM Process [8]
DTPA and Mg(OH)2 were added to a pulp suspension in
a polyethylene bag. After a thorough mixing, the bag, with its
contents, was placed into a water bath at 60°C for one minute.
Then, the required amount of hydrogen peroxide was added
to the bag. Again, a thorough mixing was provided and the
bleaching was allowed to continue for 5 hours at 60°C. The subsequent procedures were the same as those in the conventional
P process.
Pulp & Paper Canada T 126
FIG. 2. Decreasing Mn-induced peroxide decomposition in
Mg(OH)2 system (4% pulp consistency, 1 g/L Mg(OH)2, 3
g/L Na2SiO3, 0.5 g/L DTPA, 3 ppm Mn2+, 1.6 gL H2O2, 60°C).
FIG. 3. Peroxide Stabilization with Silicate in Mg(OH)2 Based process and NaPH-based process (4% pulp consistency, 3 ppm Mn2+, 1 g/L Mg(OH)2, 1 g/L NaOH, 0.5 g/L
MgSO4, 1.6 g/L H2O2, 60°C, 90 min).
RESULTS AND DISCUSSION
H2O2 Stability in the Mg(OH)2 System
We first studied the H2O2 stability with an industrial grade
Mg(OH)2 since it contains some transition metals, (see Experimental for details), under typical bleaching conditions.
Figure 1 shows that in Run 1, with industrial-grade Mg(OH)2
slurry, some peroxide decomposition occurred. This must be
caused by the impurities in the system, since no peroxide
decomposition was observed in Run 2, with analytical grade
magnesium hydroxide.
Na2SiO3 or DTPA was then tried as the stabilizer to decrease
the peroxide decomposition in the industrial-grade Mg(OH)2
system. From the results in Figure 2 which show that in Run 1,
Na2SiO3 was used as the stabilizer at a typical concentration for
commercial peroxide bleaching at medium consistency, one can
observe that no peroxide decomposition occurred. In Run 2,
DTPA was used as the stabilizer at 0.5% concentration, on pulp
at 10% pulp consistency, one can find that the peroxide stability was similar to that in Run 1. In Run 3, no stabilizer was used
and the peroxide decomposition was extensive. Hence, Na2SiO3
and DTPA are good stabilizers to minimize the peroxide
decomposition in such a system.
Figure 3 compares Mg(OH)2 and NaOH systems when using
Na2SiO3 to decrease the peroxide decomposition. One can
observe that silicate is much more effective in decreasing the
peroxide decomposition for the Mg(OH)2 solution than for the
NaOH solution. At a silicate charge of 1.5 g/L there is almost no
hydrogen peroxide decomposition under the conditions studied, as indicated in Run 1 for the Mg(OH)2 process. This is significantly different from the sodium hydroxide based process,
which needs much higher silicate to have satisfactory stabilization under otherwise the same conditions. This difference must
be attributed to the fact that the Mg(OH)2 system has a mild
alkalinity in comparison to NaOH.
106:6 (2005) ❘❘❘
25
BLEACHING
TABLE I. Comparison of the NaOH-based and the Mg(OH)2based peroxide processes. (Mill chelated TMP, 2% H2O22,
5 hrs, 10% pulp consistency, 70°C).
Process NaOH-based
Process
(3% Na2SiO3)
FIG. 4. Peroxide Stabilization with DTPA in Mg(OH)2-based
and NaOH-based-processes (4% pulp consistency, 3 ppm
Mn2+, 1 g/L NaOH, 1 g/L H2O2, 60°C, 90 min).
NaOH or
Mg(OH)2 (%)
End pH
Residual
peroxide (%)
Brightness
(% ISO)
Mg(OH)2-based process
(0.1% DTPA)
P
PM
1.6
0.75
1.0
1.5
0.75
1.0
1.5.
7.84
0.70
6.85
1.22
7.14
0.95
7.92
0.28
6.80
1.38
7.12 7.91
1.04 0.49
70.4
69.0
70.0
69.6
69.8
70.7 70.6
TABLE II. Pulp strength properties from NaOH-based and
Mg(OH)2 based processes (~70% ISO brightness).
Parameter
Tensile index (N•m/g)
Tear (mN•m2/g)
Burst (kPa•m2/g)
Bulk (cm3/g)
NaOH proces (P) Mg(OH)2 process (PM)
32.9
7.41
1.77
2.52
32.6
7.34
1.72
2.58
reach the same degree of peroxide stabilization. The decrease in
residual peroxide at a very high DTPA charge for both
Mg(OH)2- and NaOH-based peroxide processes can be
explained by the peroxide consumption in reaction with DTPA.
The reaction of peroxide with tertiary amines is well documented in literature [10].
In summary, the presence of contaminants in the industrialgrade Mg(OH)2 system induces hydrogen peroxide decomposition, however, it can effectively be reduced with conventional stabilizers, such as Na2SiO3 and DTPA. In comparison with the
NaOH-based process, the Mg(OH)2-based process needs substantially lower amounts of stabilizer to reach the same degree
of peroxide stabilization.
FIG. 5. Comparison of COD in the effluent between the
NaOH-based and the Mg(OH)2-based processes.
The implication from the above results is that for the
Mg(OH)2-based process, the silicate charge (and/or other stabilizers) to achieve effective bleaching would be much less than
that in the conventional NaOH process. This is beneficial for
reduced bleaching cost and reduced formation of anionic trash.
Silicate is an important source of anionic trash during the peroxide bleaching process.
Figure 4 presents the results regarding the use of DTPA as a
stabilizer to decrease peroxide decomposition in both
Mg(OH)2- and NaOH-based peroxide processes. For the
Mg(OH)2-based peroxide process the peroxide stability increases with increase in DTPA charge up to 0.5g/L, thereafter an
increase in DTPA results in lower residual peroxide. For the
NaOH-based process the same trend is observed however, a
higher DTPA charge (2 g/L) is needed to attain the maximum
stability. This is in agreement with the study of Qiu [9] who also
observed that at a higher pH more DTPA charge is needed to
26
❘❘❘ 106:6 (2005)
Laboratory Trials
A comparison of NaOH-based and Mg(OH)2-based peroxide
processes is shown in Table 1. For the Mg(OH)2 system, three
Mg(OH)2 dosages were tested and the PM process [8] was investigated. The results show that the optimum Mg(OH)2 dosage is
1% and the conventional mode (the P process), the Mg(OH)2based peroxide process gives a slightly lower brightness than the
NaOH-based process. The PM process (DTPA and Mg(OH)2
added to pulp slurry first, and 1 min later, H2O2 added), results
in a higher brightness and a higher H2O2 residue. This confirms
that the PM process is applicable to the Mg(OH)2-based process,
and improves the performance of peroxide in bleaching
mechanical pulps.
Due to a low alkalinity associated with the Mg(OH)2-based
peroxide process, pulp fibers may not swell to the same extent,
which may affect fiber to fiber bonding. This could result in
decreased strength properties of the bleached pulp. A comparison of strength properties of the two bleached pulps is shown in
Table 2. The difference in tensile, tear and burst are very small.
There is also a small increase in bulk (from 2.52 to 2.59 cm3/g).
This is quite different from the results obtained from other
pulps, such as maple CTMP [11]. A possible explanation is that
the freeness of the Irving TMP pulp (spruce/a small amount of
fir) is about 70 mL, significantly lower than the BCTMP maple
grade (about 350 mL). The pulp fibers of Irving TMP are well
developed, and the change in alkalinity from the NaOH-based
process to the Mg(OH)2-based process would have a negligible
effect on fiber development for the Irving TMP.
The COD in the bleach process effluent and the anionic
T 127 Pulp & Paper Canada
BLEACHING
FIG. 6. Comparison of anionic trashes in the bleachedpulp
between the NaOH-based and the Mg(OH)2-based processes.
FIG. 7. Flowsheets of NaOH-based and Mg(OH)2 based peroxide processes at Irving paper.
TABLE III. Mill trial results.
Parameter
H2O2 Tensile Tear Caliper Porosity Brightness
(%)
(% ISO)
NaOH-based 1.55
process
1.55
Mg(OH)2
-based process
FIG. 8. Cationic demand of bleached TMP.
trash in the bleached pulp are of significant interest from a practical point of view. The comparisons of COD and anionic trash
in the effluent, between the NaOH-based and the Mg(OH)2based processes, are found in Figures 5 and 6, respectively. The
Mg(OH)2-based process produces about 20% less COD than the
NaOH-based process. The decrease in anionic trash in the
bleached pulp is more dramatic, a 40% reduction is noted in
Figure 6 for the Mg(OH)2-based process. Both are related to the
much lower alkalinity offered by the Mg(OH)2-based process.
Mill Trials
Based on the positive results from the laboratory study, a mill trial was conducted at Irving Paper in December 2002. The following two cases were tested:
• The NaOH based process (control run), see Figure 7, 1.55%
H2O2, 1.0% NaOH, 3.0% silicate (as solution), 60°C.
• The Mg(OH)2 based peroxide process (Figure 7), 1.51%
H2O2, 1.0% Mg(OH)2, no silicate, 60ßC.
In the control run , H2O2 was added to the pulp slurry via the
MC pump #2, while caustic soda and silicate were added via the
T-mixer, about one minute downstream from the #2 pump (top
Figure 7). For the Mg(OH)2-based process, Mg(OH)2 slurry was
added via the #2 pump and H2O2 via the T-mixer.
The results from the mill trial are summarized in Table 3.
The strength properties (tensile, tear) and brightness results
were similar between the two processes. It should be noted that
the bleaching chemical cost of the Mg(OH)2-based process is
less than that of the NaOH-based process. In addition, the
cationic demand of bleached pulp from the Mg(OH)2-based
process was about 15 to 20% lower than that from the NaOHbased pulp This translates to significant saving in the cationic
polymers requirement of the paper machine wet-end operation.
In view of the potential benefits of the Mg(OH)2-based pro-
Pulp & Paper Canada T 128
34.8 285.8
50.9
28.0
67.9
35.0 284.6
49.9
28.8
67.5
cess, Irving Paper decided to proceed with an extended trial in
early 2003. Since it was decided to use DTPA as a peroxide stabilizer rather than silicate, the silicate tank was converted to a
Mg(OH)2 supply tank. An existing recirculation system was modified to providing sufficient agitation to the Mg(OH)2 slurry. A
mixing eductor was installed onto the existing silicate tank recirculation line. The extended trial started in the summer of 2003.
Figure 8 shows the cationic demand of the bleached pulp, which
is significantly lower for the Mg(OH)2-based peroxide process.
The benefits experienced by the mill include:
- decreased COD (about 40%) load to the effluent treatment system
- decreased bleaching chemical cost of 8%
- no significant changes in strength and optical properties
- decreased cationic demand (20 - 30%).
The Mg(OH)2 system has operated well since the July, 2003
start-up. One change was to replace the silicate control valve
with a metering pump for delivering the Mg(OH)2 slurry. It was
also noted that the bleaching time management is critical for
achieving target brightness for the Mg(OH)2-based process due
to its slower bleaching reaction kinetics in comparison with that
of the NaOH-based process. Also, the pH of the pulp slurry to
the paper machine has to be controlled to a lower level to
ensure no residual peroxide carryover.
CONCLUSION
The Mg(OH)2-based peroxide process is effective in improving
the bleaching process for mechanical pulp. It decreases bleaching costs while producing bleached TMP with similar optical and
strength properties. The PM process makes a significant contribution in this regard. The process has been successfully trialed
at the Irving Paper bleach plant. Benefits have been seen
throughout the mill, including, i) a lower bleaching cost, ii)
decreased COD in the bleaching effluent and iii) decreased
anionic trash. Since September 2003, the mill has achieved a significant reduction in the overall manufacturing cost.
REFERENCES
1. GRIFFITHS, P. and ABBOT, J., “Magnesium Oxide as a Base for Peroxide
Bleaching of Radiata Pine TMP”, Appita J. 47(1):50-54 (1994).
106:6 (2005) ❘❘❘
27
BLEACHING
2. SOTELAND, N. ABERDIE MAUMERT, F.A. and T
ARNERIK, T.A., “Use of MgO and CaO as the Only
Source in Peroxide Bleaching of High Yield Pulps”,
Proceedings, International Pulp Bleaching Conference, 231-236 (1988).
3. SEUSS, H.U., DEL GRASSO, M., SCHMIDT, K. and
HOPT, B., “Options for Bleaching Mechanical Pulp
with Lower COD Load”, Proceedings, Appita Conference, 419-422 (2001).
4. JOHNSON, D., PARK, S., GENCO, J., GIBSON, A.,
WAJER, M. and BRANCH, B., “Hydrogen Peroxide
Bleaching of TMP Pulps Using Mg(OH)2”, TAPPI Fall
Conference and Trade Fair, (2002).
5. VINCENT, A.H. and ALEXANDER, M.I., Canadian
Patent Application 2, 278, 399.
6. WAJER, M.T., GIBSON, A.R., GENCO, J., JOHNSON, D.A. and BRANCH, B. US Patent Application
Publication No. US 2001/0050153 A1
7. NI, Y., COURT, G., LI, Z., MOSHER, M. TUDOR,
M. and BURTT, M., “Improving Peroxide Bleaching of
Mechanical Pulps by an Enhanced Chelation Process”,
Pulp and Paper Canada, 100(10):51 (1999).
8. NI, Y., LI, Z., COURT, G., BELLIVEAU, R. and
CROWELL, M., “Improving Peroxide Bleaching of
Mechanical Pulps by the PM Process”, Pulp and Paper
Canada, In press
9. QIU, Z., NI, Y. AND YANG, S., “Using DTPA to
Decrease the Manganese-Induced Peroxide Decomposition”, J. Wood Chemistry and Technology, 23(1):1
28
❘❘❘ 106:6 (2005)
(2003)
10. CHALLIS, B.C. and BUTLER, A.R., “Substitution
at Amino Nitrogen” in: The Chemistry of Amino
Group, p277-348, Patai, S. (Editor), Interscience,
London, (1968).
11. ZHANG, X.Z., NI, Y., ZHOU, Y. and JOLIETTE,
D., “Mg(OH)2-Based Peroxide Process for a CTMP
Hardwood Pulp”, Proceedings, Paptac Annual Meeting, Montreal, January 2004
Résumé: Du Mg(OH)2 a été employé pour remplacer l’hydroxyde de sodium et le silicate de
sodium pour le blanchiment au peroxyde d’une PTM. Le gain de blancheur avec le Mg(OH)2
comme seule source d’alcali est comparable à celui du procédé caustique-silicate. Le procédé au
Mg(OH)2 a produit une pâte ayant moins de déchets anioniques et des effluents présentant une
plus faible teneur en DCO. L’ajout successif de produits chimiques, avec le peroxyde d’hydrogène
en dernier lieu, a donné un meilleur résultat que le blanchiment au peroxyde classique. L’amélioration des coûts et de la performance ont été confirmés lors de l’application en usine.
Reference: Z. LI, G. COURT, R. BELLIVEAU, M. CROWELL, R. MURPHY, A. GIBSON, M.
WAJER, B. BRANCH, Y. NI. Using Magnesium Hydroxide (Mg(OH)2 as the Alkali Source During
Peroxide Bleaching at Irving Paper. Pulp & Paper Canada 106(6): T125-129 (June, 2006) Paper presented at the 90th Annual Meeting in Montreal, QC, Canada, Jan 26,2029. Not to be reproduced
without permission of PAPTAC. Manuscript received January 18, 2003. Revised manuscript
approved for publication by the Review Panel on July 28, 2004.
Keywords: MAGNESIUM HYDROXIDE, THEMOMECHANICAL PULPS, PEROXIDE
BLEACHING, ALKALI, POLLUTION CONTROL
T 129 Pulp & Paper Canada