mechanical pulps
Partition of soluble and precipitated oxalate and its
implication on decreasing oxalate-related scaling
during peroxide bleaching of mechanical pulps
By L. Yu and Y. Ni
Abstract: We determined the effect of various process parameters (hydrogen peroxide charge,
caustic soda charge, and bleaching consistency) on the partition between soluble and precipitated
oxalate. At a higher peroxide charge or caustic soda charge, more oxalate is in the soluble form.
Decreasing the pulp consistency can increase the fraction of soluble oxalate. Also, a partial replacement of NaOH with Mg(OH)2 as the alkali source during peroxide bleaching can increase the
amount of soluble oxalate, thus effectively decreasing oxalate-related scaling.
T
HE FORMATION OF CALCIUM OXALATE
scale continues to be a severe problem in many bleach plants [1,2,3].
During peroxide bleaching of
mechanical pulps, particularly for the
production of high-brightness bleached mechanical pulps, a significant amount of oxalate is
formed, which leads to calcium oxalate-related
scaling in mills’ processing equipment [3].
Oxalate in pulp slurry can be present in both
soluble and precipitated forms. The precipitated
oxalate can (i) be with colloids and/or fines in
bleaching filtrate, (ii) deposit onto pulp fibers.
The presence of the precipitated oxalate was
observed in recent studies on both a bleached
aspen mechanical pulp and a bleached CTMP
maple pulp [4,5]. The soluble fraction of oxalate
contains mainly oxalate ions. It may also contain
CaC2O4 nuclei and small CaC2O4 crystals which
are too small to be separated in the 0.45 µm
(micrometer) membrane; the latter, however,
accounts for a small fraction.
The partition between the soluble and precipitated oxalates is an important issue at pulp mills.
Increased soluble oxalate in bleaching filtrate will
lead to less CaC2O4 related scaling in process
equipment, as well as less precipitated oxalate in
pulp fibers. While the negative effect of scaling is
well known, the presence of the precipitated
oxalate in pulp fibers could cause problems in
the subsequent operations, such as lower sizing
degree [6].
The partition between the soluble and precipitated oxalates is strongly dependent on process
conditions, including [7,8]:
* the solubility of CaC2O4,
* ion supersaturation,
* the amount of nucleation sites,
* precipitation rate,
* contacting time.
Peroxide bleaching is an ion supersaturated
system, with sufficient nucleation sites; its residence time is long enough to have adequate contact time. In addition, the precipitation rate is
high in the process [8]. Therefore, one can conclude that the solubility of CaC2O4 is a key parameter in the oxalate partition.
The CaC2O4 solubility is often affected by tem-
perature, ionic strength, pH, and dissolved organic substances. The change in the solubility of
CaC2O4 as a function of temperature and ionic
strength was found to be rather small, under typical conditions interesting to the pulp and paper
industry [8,9]. The concentration of dissolved
organic substances and the pH are the key factors
affecting the CaC2O4 solubility in pulp slurry, and
consequently affecting the oxalate partition.
In the present paper, we studied the effects of
various factors on the oxalate partition during
peroxide bleaching, including hydrogen peroxide charge, caustic soda charge, and bleaching
consistency. The underlying mechanism accounting for some observations was investigated by
determining the CaC2O4 solubility of a model system containing lignin and polygalacturonic acid
(PGa). We also examined what effect replacing a
portion of the NaOH with Mg(OH)2 as the alkali
source would have on the partition between soluble and precipitated oxalate, and its implication
on decreasing oxalate-related scaling.
EXPERIMENTAL
Samples
CTMP maple pulp was received from a mill in
Quebec.
Chelation
Prior to peroxide bleaching, a chelation stage
with DTPA was carried out at 70°C and 3% pulp
consistency for 30 min. The pulp was acidified to
pH 6-6.5 by the addition of H2SO4 before 0.5%
DTPA was charged. After the chelation stage, the
pulp was washed thoroughly and its brightness
was 42.9% ISO.
Peroxide Bleaching
Hydrogen peroxide bleaching was performed in
sealed polyethylene bags at 16% consistency (or
other specific consistencies) and 80°C for 3
hours. The bleaching chemicals were composed
of H2O2 (0.2-6.2%), NaOH (1-7%), silicate
(2.6%), and MgSO4 (0.13%). When Mg(OH)2
was needed, it was directly added into the pulp
slurry before other chemicals. At the completion
of peroxide bleaching, samples were taken for
final pH, pulp brightness and chemical oxygen
L. YU
Limerick Pulp and Paper Centre
University of New Brunswick
Fredericton, NB
Y. NI
Limerick Pulp and Paper Centre
University of New Brunswick
Fredericton, NB
[email protected]
PULP & PAPER CANADA • 108:1 (2007) •
39
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mechanical pulps
demand (COD), which were carried out in accordance with the
PAPTAC standard methods.
The Solubility Determination of Model Systems
The solubility of a model solution of bleaching filtrate was studied by observing the spontaneous precipitation from supersaturated solutions [9]. The experiments were carried out at room
temperature for 18 hours in pure aqueous solutions containing
lignin sulfonates or carbohydrate (sodium polygalacturonic
acid). The pH of solutions was adjusted to a target, and the equilibrium pH was determined after. Samples were immediately filtered through a 0.45µm pore size filter.
Analytical Procedure
The oxalate analysis was as follows: for the total oxalates (sum of
soluble and precipitated oxalates), we treated pulps and bleaching filtrates with cationic exchange resin (5% based on the
weight of the treatment solution) at room temperature for 2
hours, according to a procedure established earlier [10]. For the
soluble oxalates, the sample was obtained from filtering bleaching filtrate through a 0.45µm filter. The oxalate concentration
was analyzed in an IC (Ion Chromatographic) system. The precipitated oxalates were then calculated as the difference
between the total and soluble oxalates. A Dionex DX-300 IC System, together with a suppressed conductivity detector, was used
to determine the oxalate concentration. The conditions were:
AS4A-SC analytical column, AG4A-SC guard column, 1.8 mM
Na2CO3 and 1.7 mM NaHCO3 as the eluent at 2.0 ml/min.
RESULTS AND DISCUSSION
Effect of Hydrogen Peroxide Charge on Oxalate Partition
Figure 1 shows the amount of soluble and precipitated oxalate
in bleached pulp slurry as a function of hydrogen peroxide
charge. Two distinct phases of the oxalate partition in the pulp
slurry can be identified: both the soluble and precipitated
oxalate increased when the H2O2 charge was raised from 1 to
4%; then they leveled off when the H2O2 charge was further
increased from 4 to 6.2%.
The increase in soluble oxalate in the first phase of Fig. 1
indicates an increase in the CaC2O4 solubility in the bleaching
filtrate. As mentioned in the introduction above, pH and dissolved organic substance (measured as COD) are the key factors
affecting the CaC2O4 solubility. Since the pH of the bleaching
filtrate in Fig. 1 was similar, the change in the CaC2O4 solubility
is mainly due to the change in COD.
Included in Fig. 1 are also the results for COD of the filtrate
as a function of the H2O2 charge. As expected, the COD increases with the H2O2 charge, and the increase in the CaC2O4 solubility in the first phase can be explained by the increase in dissolved organic substance. This is in agreement with the results
of Fiskari et al [11] and Ulmgren et al [8,12]. For example,
Fiskari et al found that a higher TOC in the bleaching filtrate
from a P stage of a hardwood pulp TCF sequence resulted in
more soluble oxalate in their system.
We offer the following explanation to account for the effect
of dissolved organics on the CaC2O4 solubility [12]: the
increased interaction between organic substance and calcium
oxalate leads to more CaC2O4 crystals with disordered lattice,
which are active and unstable, and thus an increased solubility.
This hypothesis is supported by the experimental observation
that low molecular weight dissolved substances with high anionic charges are particularly effective for crystal distortion [1,13].
In the second phase, as the COD of the bleaching filtrate
increased further from 12.4 to 15.6 g/L, its effect on the CaC2O4
solubility is negligible. Ulmgren et al [12] also reported that at
a higher COD level, a further increase in organic substance had
only slight effect on the CaC2O4 solubility. This indicates that
the effect of COD has reached a plateau at this high level. A possible explanation would be that the properties of dissolved
organic substances had changed at this relatively high COD val-
40 • 108:1 (2007) • PULP & PAPER CANADA
FIG. 1. Amount of soluble and precipitated oxalate as a
function of H2O2 charge.
ue. For example, the molecular weight of organic substance
might increase with the rise in COD [14]. These organic substances with higher molecular weight would have less anionic
sites than those with lower molecular weight. The anionic sites
are responsible for crystal distortion.
Solubility of Model Systems
Since the bleaching filtrate contains dissolved organics, we
determined the effect of lignin and carbohydrate (polygalacturonic acid as a model) on the CaC2O4 solubility. The observed
solubility (Ls) of CaC2O4 of this solution was calculated as
Ls = [Ca2+]total dissolved[C2O42–]total dissolved.
For a pure water system without any organic substance present, log Ls –7.8 (pH 4.5-7.5, 20°C, Ca2+/C2O42– = 1.3) was
determined. The result was checked against those from the literature: log Ls –7.7 (pH 5-10, 30°C, Ca2+/C2O42– = 1) by Ulmgren et al [9] and log Ls –7.6 (for CaC2O4 monohydrate, at
25-50°C) by Tomazic et al [15]. The observed solubility here is
higher than log Ksp of CaC2O4 (–-8.5) because of the different
ionic strength, I, (I 2.25 (10–4 mol/L, versus 0.0266 mol/L for
the present system).
The effect of dissolved organics on the CaC2O4 solubility was
also confirmed in the model system. In Fig. 2, the observed
CaC2O4 solubility was higher when lignin and PGa were present.
A higher lignin/PGa concentration resulted in a higher solubility.
As shown in Fig. 2, the solubility is greater with a lower pH in
the acidic regime for all systems, mainly due to the formation of
oxalic acid. On the other hand, the solubility also increased with
increasing pH when the pH was above 6. Ulmgren et al studied
the effect of pH on the solubility of CaC2O4 in several bleach
plant filtrates (D1, Q, Z) [8]. They found that when pH was
raised from 5 to 12, the CaC2O4 solubility remained almost constant, which is different from that in Fig. 2. This difference is due
to different experimental conditions. In their case, the ionic
strength remained the same at different pH values, while in Fig.
2 the ionic strength increased at a higher pH due to the higher
Na+ and –OH concentrations. According to another study of
Ulmgren et al [16], without controlling of ionic strength, the
lowest dissolved oxalate was found when the pH was between 5
and 8, and this is consistent with the results in Fig. 2.
Effect of Caustic Soda Charge on Oxalate Partition
Figure 3 shows the amount of the soluble and precipitated
oxalate as a function of the NaOH charge. Included are also the
COD results and the pH of filtrates at the end of bleaching (as
bracket on the COD curve). An increase in the caustic soda
mechanical pulps
FIG. 2. The effect of dissolved organic substances on the
observed solubility of CaC2O4.
FIG. 3. Amount of soluble and precipitated oxalate at varioius NaOH dosages.
FIG. 4. The effect of pH on the oxalate parition.
FIG. 5. The effect of bleaching consistency on the oxalate
formation and parition.
charge results in the increased pH and
COD, thus affecting the oxalate partition.
As shown in Fig. 3, the soluble oxalate
increased with the increased NaOH
charge, while the precipitated oxalate was
at its highest level at about 4% NaOH
charge (its end pH about 8), and then
decreased significantly. According to the
study of the model solution, at a pH of
higher than 6, the CaC2O4 solubility
increased with pH. Thus, the increased
solubility at higher than 4% NaOH
charge, as seen in Fig. 3, was due to the
synergetic effect of a higher pH and more
dissolved organic substances.
We further studied the oxalate partition by adjusting the pH of the bleached
pulp slurry and the results are shown in
Fig. 4. The pH at the completion of
bleaching under the specified conditions
was 8.7, which was then adjusted to 3.2 or
10.3, respectively. In both cases, the
amount of the precipitated oxalate
decreased, indicating that some of the
precipitated oxalate was dissolved. The
practical implication is that the calcium
oxalate deposit onto pulp fibers could be
largely dissolved by adjusting the system to
be more acidic or alkaline. For example, a
kraft pulp mill reported a better washing
of oxalate from pulp at a higher pH [17].
Effects of Consistency on Oxalate Partition
Figure 5 shows the effect of bleaching
consistency on the formation and parti-
tion of oxalate. The amount of total
oxalate formed increased slightly when
the consistency was raised from 5 to 16%.
This can be explained by the higher reactant concentrations at higher bleaching
consistency, resulting in faster kinetics, as
well as more total oxalate formation.
As shown in Fig. 5, there is more soluble oxalate in filtrate at a lower pulp consistency than at a higher consistency. Certainly this is because more water is present
at the lower consistency.
Usually, the CaC2O4 scaling is difficult
to remove. However, our results from Fig.
5 show that it could be readily re-dissolved.
Krasowski et al also found that effective
washing can reduce oxalate-related scaling
[18]. They removed 50% of the total
PULP & PAPER CANADA • 108:1 (2007) •
41
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mechanical pulps
oxalate in bleached pulp slurry from Cstage by filtering with deionized water
[18]. The difference could be explained
by the different crystal structures between
newly formed CaC2O4 and the oxalaterelated scaling. If it is in the form of dihydrate or very fine monohydrate crystals
with a disordered lattice, both of which are
unstable [12], it can be re-dissolved readily. The dihydrate can be converted to stable monohydrate at equilibrium [13]. The
CaC2O4 scaling is mostly in the form of
monohydrate [3], which is derived from
dihydrate when sufficient time is given.
Partial Replacement of NaOH with
Mg(OH)2
Table I gives the experimental conditions
and bleaching results [22], and Fig. 6
shows the oxalate formation and the partition between the precipitated and soluble oxalate. The Mg(OH)2 replacement
ratio is defined as the mole equivalent of
Mg(OH)2 on a total alkali of 4.2% NaOH.
The amount of the newly-formed
oxalate for bleaching experiments with 050% Mg(OH)2 replacement was found to
be at the same level of 1.85±0.09 g/kg
pulp, Fig. 6. This can be explained by the
similar
peroxide
consumption
(4.2±0.1%), Table I. Our earlier results
[19] showed a close relationship between
the amount of oxalate formed and the
peroxide consumption. The magnesium
hydroxide slurry used in the present study
contains impurities, including transition
metals. When more Mg(OH)2 was
charged, more transition metals were
introduced into the system. This explains
the higher peroxide consumption and
consequently the higher oxalate formation at 75% and 100% replacement. Other researchers also reported that a higher
Mg(OH)2 charge led to increased peroxide consumption [20, 21].
Figure 6 further shows that when the
Mg(OH)2 replacement increased from
0% to 50%, the precipitated oxalate
decreased significantly. In fact, in the
range of 30-50% Mg(OH)2 replacement,
the amount of precipitated oxalate was
negligible. Therefore, one can conclude
that at a 30-50% replacement of NaOH
with Mg(OH)2, the calcium oxalate scaling is essentially inhibited, which is the
basis of our recommendation to the
industry to decrease the oxalate-related
scaling during the manufacturing of
bleached mechanical pulps [22]. The
explanation for the above results is as follows: magnesium ions, not calcium, preferentially interact with oxalate under the
conditions of peroxide bleaching, and
magnesium oxalate has a higher solubility
[21,23,24]. Figure 6 also shows an
increase in the precipitated oxalate at a
Mg(OH)2 substitution of 75% and 100%,
which may be due to: i) the increased
oxalate formation, or ii) more calcium
introduced into the system by the very
42 • 108:1 (2007) • PULP & PAPER CANADA
TABLE I. Results for partial replacement of NaOH with Mg(OH)2.
Mg(OH)2
Repl ratio
(%)
0
10
15
20
25
30
40
50
75
100
16% consistency, 80°C, 3 hours, 6.2% H2O, 2.6% silicate
NaOH
Mg (OH)2
H2O2
Brightness
Init. pH
charge
charge
cons.
(%)
(%)
(%)
(% ISO)
4.20
3.78
3.57
3.36
3.15
2.94
2.52
2.10
1.05
0
0
0.31
0.46
0.61
0.77
0.92
1.22
1.53
2.30
3.06
4.2
4.2
4.1
4.1
4.2
4.3
4.3
4.3
4.8
4.8
high charge of Mg(OH)2.
Another benefit of the partial substitution of Mg(OH)2 for NaOH during peroxide bleaching of mechanical pulps is a
higher brightness gain, as shown in Table
I. In a range of 15-40% Mg(OH)2 replacement ratio, the brightness of the bleached
pulp was higher than the control (0%
Mg(OH)2 replacement ratio). This is in
agreement with earlier results [21]
obtained in a two-stage peroxide process.
The improved brightness gain when
part of the NaOH was replaced by
Mg(OH)2 may be partly explained by the
decreased alkaline darkening in the peroxide process. It is known that alkaline
darkening occurs during peroxide bleaching and has negative effects on the bleaching efficiency and the brightness ceiling
of mechanical pulps [25-29]. Results [29]
have shown that the higher the pH, the
more the brightness loss during alkaline
darkening, also not all of the newlyformed chromophores during alkaline
darkening are reactive towards peroxide.
When Mg(OH)2 partially substituted for
NaOH, the pH profile was lower than the
control, Table I. As a result, the alkaline
darkening would be decreased, which
would be then partially responsible for
the higher brightness.
It is noted that at the 100% Mg(OH)2
replacement ratio, the brightness of the
resulting pulp was lower than the control,
which is in agreement with our earlier
results for a very high brightness CTMP
hardwood pulp [21,29]. On the other
hand, for pulp grades of about 70%ISO,
the Mg(OH)2-based peroxide process
offered a similar brightness gain to the
NaOH-based peroxide process [30].
CONCLUSIONS
• The formation of oxalate inevitably
occurs during peroxide bleaching of
mechanical pulp. It was found that the
hydrogen peroxide and caustic soda
charges can significantly affect the partition between the soluble and precipitated
oxalate. These results can be explained by
the change in the CaC2O4 solubility. The
80.2
80.2
80.9
81.2
81.0
80.8
81.2
80.2
80.4
78.4
10.7
10.5
10.5
10.2
9.9
10.2
9.6
9.7
9.7
8.6
End pH
8.3
8.1
8.2
8.2
8.0
8.0
8.0
8.1
8.1
8.0
pH and the amount of dissolved organic
substance present in the bleaching filtrate
are the two main parameters affecting the
CaC2O4 solubility.
• Lowering the bleaching consistency will
lead to more soluble oxalate in the
bleaching filtrate.
• A partial replacement of NaOH with
Mg(OH)2 is effective in decreasing the
amount of the precipitated calcium
oxalate during peroxide bleaching. At a
30-50% replacement ratio, the formation
of precipitated oxalate is negligible. The
chemistry is that magnesium ions, not calcium, preferentially interact with oxalate
under the conditions of peroxide bleaching and the formed magnesium oxalate
has high solubility. Also, a higher brightness is achieved at a partial substitution of
Mg(OH)2.
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Résumé: Nous avons déterminé l’effet de divers paramètres de fabrication (charge de peroxyde d’hydrogène, charge de soude caustique, uniformité du blanchiment) sur la séparation de l’oxalate soluble et de l’oxalate précipité. Lorsque la charge de peroxyde ou de soude caustique est
plus élevée, une plus grande partie de l’oxalate est sous forme soluble. Réduire la concentration
de la pâte peut faire augmenter la fraction d’oxalate soluble. Le remplacement partiel du NaOH
par du Mg(OH)2 comme source d’alcali pendant le blanchiment au peroxyde peut aussi faire augmenter la quantité d’oxalate soluble, ce qui permet de réduire de façon efficace l’entartrage
attribuable à l’oxalate.
Reference: YU, L., NI, Y. Partition of soluble and precipitated oxalate and its implication on
decreasing oxalate-related scaling during peroxide bleaching of mechanical pulps. Pulp & Paper
Canada 108(1): T6-10 (January, 2006). Paper presented at the 92nd Annual Meeting in Montreal,
QC, February 6-10, 2006. Not to be reproduced without permission of PAPTAC. Manuscript
received December 01, 2005. Revised manuscript approved for publication by the Review Panel
June 29, 2006.
Keywords: OXALATES, SCALING, PEROXIDE BLEACHING, MECHANICAL PULPS, PRECIPITATES, PROCESS VARIABLES
PULP & PAPER CANADA • 108:1 (2007) •
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