M E C H A N I C A L PA P E R S
Calcium ion removal by a
synthetic zeolite in the manufacture
of mechanical grade papers
By J.-B. Thibodeau, B. Chabot and C. Daneault
Abstract: The removal of free calcium ions from paper machine white-water by a synthetic zeolite (type 4A) was examined. We have studied the effect of various parameters and operating conditions. Results showed that zeolite 4A is very efficient at removing free calcium ions from whitewater. Conductivity, reaction time, temperature, fibre content and wood extractives concentration
have no significant detrimental effect on calcium ion removal by the zeolite.
ALCIUM CARBONATE fillers (PCC and
GCC) are still not widely used in
wood-containing papers in North
America. Two limitations are associated with their use. First, the high alkalinity of the calcium carbonate slurry reduces the
brightness of mechanical pulp [1]. Second, the
pH strongly influences the solubility of calcium
carbonate filler [2,3]. Paper machines traditionally operate under acidic pH. Under these conditions, calcium carbonate is attacked by acids
releasing free calcium ions and carbon dioxide in
white-water systems [4]. Free calcium ions can
then react with dissolved and colloidal substances
(DCS), promoting the formation of deposits that
can affect paper machine operation and reduce
paper quality [5].
Inhibition of calcium carbonate dissolution
has been studied for several years. Compton et al
[6,7] determined the effect of specific chemicals
at inhibiting calcium carbonate dissolution. An
acid tolerant precipitated calcium carbonate (ATPCC) has recently been developed for use as a
filler in wood-containing paper manufacturing
[8]. This technology is based on a buffer effect
between a conjugated base (sodium hexametaphosphate) and a weak acid (phosphoric
acid). Addition of carbon dioxide gas to the pulp
slurry has also been tried to retard dissolution of
calcium carbonate filler [9,10]. Both technologies are currently used in some mechanical paper
mills in Europe, but their acceptance is still limited in North America.
White-water management around the paper
machine and TMP pulp mill must also be considered when dealing with calcium carbonate fillers.
Large volumes of excess white-water from the
paper machine are used at the TMP plant for
dilution purposes and refiners operations. Acidic
conditions prevailing at the TMP plant will completely dissolve calcium carbonate filler carried
over with dilution white-water. This problem can
be limited by using a properly operating disk filter to recover solid calcium carbonate filler as
much as possible. Otherwise, calcium carbonate
filler will be transferred into the cloudy filtrate, to
the paper machine white-water chest, and eventually to the TMP plant. Complete separation of
white-water systems for the paper machine and
C
J.-B. THIBODEAU
Centre de recherche
en pâtes et papiers, UQTR,
Trois-Rivières, QC
B. CHABOT
Centre de recherche
en pâtes et papiers, UQTR,
Trois-Rivières, QC
[email protected]
C. DANEAULT
Centre de recherche
en pâtes et papiers, UQTR,
Trois-Rivières, QC
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❘❘❘ 106:3 (2005)
the pulp mill could also be considered.
Zeolites are three-dimensional networks of silicate and aluminate tetrahedra linked together by
shared oxygen atoms. Their structure is a framework that encloses cavities occupied by cations
weakly held to the structure to compensate for
the charge imbalance created by the substitution
of Al3+ for Si4+ in the tetrahedral structure. They
have pore structures with interconnecting channels large enough for ions to enter in the cavities
without disrupting the network. The type 4A zeolite is a sodium-form zeolite, used successfully in
the laundry detergents industry as a builder to
improve detergent efficiency in hard water due to
its great capacity for calcium-ion exchange [11].
Zeolites are also used for softening drinking
water [12]. Studies have also shown the effectiveness of zeolites to remove heavy metal ions from
wastewaters [13-16]. We have recently studied the
potentials of zeolite 4A in the manufacture of
supercalendered papers [17]. This work was
mainly focusing on the use of zeolites as a substitute to traditional fillers and their potential
effects on retention, drainage and sheet properties. However, preliminary results in simulated
white-water systems have shown high calcium-ion
exchange ability.
The objective of this paper is, thus, to further
investigate the effect of several parameters on the
calcium-ion exchange capacity of a synthetic zeolite (type 4A) under papermaking conditions.
EXPERIMENTAL
Materials: The zeolite (Valfor 100) was supplied
by PQ Corporation. It is a sodium aluminosilicate
hydrated type Na-A zeolite. The nominal pore
size diameter is 4 Angstroms (Type 4A). According to the supplier, the ion exchange and calcium
exchange capacities are 7 meq/g anhydrous zeolite and 270-300 mg CaCO3/g zeolite (anhydrous
basis), respectively [17]. Table I shows other
important properties of the zeolite studied [18].
A commercial calcium carbonate (GCC) slurry
was supplied by Omya. The solid content was 65%.
Unbleached thermomechanical pulp (TMP)
was sampled at an Eastern Canadian paper mill.
Pulp properties are shown in Table II.
White-waters were prepared by diluting an
appropriate amount of TMP pulp to a consisten-
T 71 Pulp & Paper Canada
M E C H A N I C A L PA P E R S
Table I. Properties of zeolite 4a.
Solids Active Composition (Dehydrated, mol%) Molar ratio
(%)
(%)
Al
Si
Na
Ca,K
Al:Si
77.9
100
14.4
13.1
17.2
0.6
1.1
Table II. Unbleached TMP pulp properties.
CSF
(mL)
Ash
(%)
Fines
(%)
Fibre length*
(mm)
Brightness
(%)
88
1.41
59.8
0.34
63.0
*Average
Table III. Disk filter white-water samples properties.
Sample Consistency Ash CaCO3
(%)
(%)
(mg/L)
Cloudy Filtrate
Clear Filtrate
Rich WW
4.57
3.06
7.45
150
160
130
0.053
0.037
0.253
FIG. 2. Effect of conductivity on the calcium-ion exchange
ability of zeolite 4A in white-water.
cy of 0.2% with tap water to simulate
paper machine white-water.
Dissolved and colloidal substances
(DCS) were extracted from black spruce
chips by steaming for 20 minutes in a
pressurized vessel. After steaming, the
condensate containing wood extractives
was collected. For several experiments, a
known amount of extractives was added to
the white-water sample to determine the
effect of DCS on calcium ion removal by
the zeolite.
White-water samples from a paper
machine disk filter were also collected at
the same paper mill. Table III shows their
properties. The zeolite efficiency was
determined directly in those samples.
Calcium ion Removal by Zeolite: Figure
1 shows the experimental set-up for calcium ion removal by the zeolite 4A.
Experiments were carried out at 50°C to
simulate a typical papermaking temperature. Several experiments were also
done at 25°C to study the effect of temperature. The following sequence was
used for all experiments. First, a preheated white-water sample (500 mL) was
Pulp & Paper Canada T 72
FIG. 1. Experimental set-up for calcium ion removal by zeolite.
FIG. 3. Effect of temperature on the calcium-ion exchange
ability of zeolite 4A in white-water.
poured in a beaker. The beaker was then
fitted in the temperature-controlled
bath. The stirrer was set at 500 rpm to
ensure proper agitation. Then, a predetermined amount of GCC was added
to the system. The amount was determined to set the concentration at 200
ppm (or 1,000 ppm in few trials) (as
CaCO3) in the beaker. The slurry was
then adjusted to pH 5 using diluted
hydrochloric acid to completely dissolve
GCC filler into free calcium ions.
Then, zeolite 4A was added to the slurry and mixing was allowed for 5 minutes at
500 rpm. Zeolite dosages were varied from
0 to 1 g/L (up to 5 g/L in several experiments). The pH, conductivity and temperature were recorded after all steps. After
the mixing stage, a sample of the slurry
was filtered on a 0.8-µm microporous
membrane filter. The filtrate was saved for
free calcium ion concentration determination by EDTA titration using hydroxynaphtol blue indicator. Calcium ion concentrations were reported as CaCO3.
Experiments were repeated three times to
determine the experimental error.
RESULTS, DISCUSSION
Effects of Conductivity, Temperature, and
pH: Figure 2 shows the effect of conductivity on calcium-ion removal by the zeolite
4A. Conductivity was varied by adding
sodium chloride in the white-water system.
Free calcium ion concentration is
decreasing with zeolite dosage at all conductivity level studied. It is clear that the
residual free calcium ion concentration is
dependent on zeolite dosages. This is
attributed to the ion exchange capacity of
zeolite 4A. However, the zeolite efficiency
is slightly reduced as conductivity is
increased. The curve at high conductivity
(2,630 µS/cm) is slightly offset compared
to others curves. This can be attributed to
the higher starting calcium carbonate
concentration used in this series of experiments. The effect of conductivity on zeolite exchange ability can be attributed to
the higher osmotic pressure developed in
the water phase when sodium ion concentration is increased. This higher
osmotic pressure reduces the ability of
sodium ion to get out of the zeolite cage,
106:3 (2005) ❘❘❘
43
M E C H A N I C A L PA P E R S
FIG. 4. Effect of suspension pH on the calcium-ion
exchange ability of zeolite 4A in white-water.
FIG. 5. Effect of contact time on calcium-zeolite stability.
Experiments were carried out without pH control.
FIG. 6. Effect of contact time on calcium-zeolite stability for a white-water system.
Experiments were carried out at pH 5.
thus reducing the over-all calcium ion
uptake. Therefore, the ability of the zeolite 4A will be reduced at high conductivity levels although the efficiency is still
very high.
Figure 3 shows the effect of temperature on the zeolite performance. Increasing the temperature from 25°C to 50°C
resulted in a slight improvement of the
zeolite performance at removing free calcium ions from the white-water system.
The gap between the two curves increases
with zeolite dosage up to 0.5 g/L, and
then should remain constant within the
experimental error. Brouillette et al have
recently shown that an increase in the
temperature promotes dissociation of the
surface hydroxyl groups of the zeolite surface resulting in a higher anionic charge
[19]. A higher temperature also increases
calcium ion dehydration resulting in a
smaller ion size. Therefore, calcium ions
can more easily enter inside the zeolite
structure improving the over-all calcium
ion uptake by the zeolite 4A.
The effect of suspension pH on calcium uptake was also determined, Fig. 4.
Two important phenomena must be considered. First, the zeolite is removing the
free calcium ion in the white-water system
at all pH studied. Second, the concentration of free calcium ion in the slurry is
lower at higher pH as it is clearly demon-
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❘❘❘ 106:3 (2005)
strated when no zeolite is added in the system. The large variation of the free calcium content is due to the solubility of GCC
filler, which is strongly dependent on the
suspension pH [1-3]. As the pH is
decreased to lower pH values (from 7 to
4), the higher the concentration of free
calcium ions available for exchange by the
zeolite. Although the efficiency of zeolite
seems to be lower at neutral pH, lower
zeolite dosages will be required to reduce
free calcium ion concentration under 100
mg/L which is considered acceptable levels in white-water systems.
The properties of the calcium-zeolite
aggregates could also be affected by the
white-water suspension pH. Brouillette et
al have shown that the zeolite is attacked
at lower pH values (in the range of 3 to 4)
[19]. This was shown to induce a certain
dissolution of the zeolite framework.
Therefore, it was necessary to evaluate the
effect of contact time under acidic conditions to determine if already exchanged
calcium ions could be released in the system after long contact time under these
acidic detrimental conditions.
Figures 5 and 6 show the effect of contact time on calcium ion uptake, pH and
conductivity of the white-water suspensions. The zeolite dosage was adjusted at
0.5 g/L in both experiments. Figure 5
shows that the suspension pH was
increased from 5 to 7.6 with contact time,
while free calcium content remained constant at about 65 mg/L. This increase of
the pH can be attributed to the adsorption of hydroxyl ions on the negative sites
of the zeolite during the experiment and
to a partial hydrolysis of the zeolite structure [20].
We did the same experiment but the
pH was kept at 5 (the initial starting value) using a pH controller (see Fig. 6). In
this case, the free calcium ion concentration slightly increased with time from
about 95 mg/L to 105 mg/L, indicating
that free calcium ions were released in the
white-water during the experiment.
Although the amount released was very
low, this showed that acidic conditions did
not destroy the zeolite structure during
contact time (up to one hour). Therefore,
it is believed that paper machine whitewater containing calcium-zeolite particles
could be used safely at the TMP mill without any significant calcium ion loss during
pulp processing.
Effect of Contaminants in the White-Water
System: Experiments were also performed
to determine if contaminants carried over
with process white-water could affect the
efficiency of zeolite 4A at removing free
calcium ions. Figure 7 shows the effect of
added dissolved and colloidal substances
(DCS) in the system on calcium ion
removal by the zeolite 4A. DCS were
added directly to the white-water system.
Results indicate that DCS added to the
system have no significant effect on the
performance of the zeolite studied.
To further investigate the possible
detrimental effect of DCS on the efficiency of zeolite 4A, we also carried out experiments using paper machine white-water
samples. Samples were collected from a
disk filter (rich, cloudy, clear filtrates).
Results are shown in Fig. 8. No significant
detrimental effects were observed. This
indicates that the zeolite 4A is not affected by DCS and fines contents. These also
show that our proposed process is very
robust and can be applied anywhere in
the paper machine water loops to control
calcium ion concentration.
T 73 Pulp & Paper Canada
M E C H A N I C A L PA P E R S
FIG. 7. Effect of dissolved and colloidal substances concentration on the calcium-ion exchange ability of zeolite 4A.
CONCLUSIONS
• This study revealed that the zeolite 4A
has a high calcium-ion exchange capacity
in white-water systems prepared under
various conditions.
• Conductivity had no significant detrimental effect on calcium ion uptake.
• Higher temperature slightly improved
the zeolite performance at all dosages
studied.
• The efficiency of the zeolite was dependent on the calcium ion concentration,
which is also strongly related to the suspension pH.
• Addition of dissolved and colloidal substances (DCS) had no effect on zeolite
efficiency.
• Trials using paper machine white-waters
sampled from a disk filter also showed no
detrimental effect of calcium ion uptake.
• The zeolite 4A can, thus, be used anywhere in the paper machine white-water
loops to control the calcium ion level.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the
Natural Sciences and Engineering
Research Council of Canada (NSERC) for
their financial support. Thanks are also
offered to Omya Inc. (GCC), PQ Corporation (Zeolite 4A) and Kruger Trois-Rivières (pulps) for providing samples
required for this work.
LITERATURE
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FIG. 8. Effect of white-water quality on the calcium-ion
exchange ability of zeolite 4A.
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Résumé: Nous avons étudié la capacité d’extraction des ions calcium contenus dans une eau
blanche de machine à papier par une zéolite synthétique (type 4A). Nous avons évalué l’impact
de plusieurs paramètres et conditions d’opérations. Les résultats démontrent que la zéolite 4A est
très efficace pour éliminer les ions calcium libres contenus dans l’eau blanche. La conductivité, le
temps de réaction, la température, le contenu de fibres et de matières extractibles du bois n’ont
pas d’effet néfaste sur l’efficacité de la zéolite.
Reference: THIBODEAU, J.-B., CHABOT, B., DANEAULT, C. Calcium ion removal by a synthetic zeolite in the manufacture of mechanical grade papers. Pulp & Paper Canada 106(3): T7174 (March, 2005). Paper presented at the 90th Annual Meeting in Montreal, QC, on Jan 26, 2004.
Not to be reproduced without permission of PAPTAC. Manuscript received on December 12, 2003.
Revised manuscript approved for publication by the Review Panel on May 28, 2004.
Keywords: CALCIUM, IONS, REMOVAL, ZEOLITES, WHITE WATER, MECHANICAL
PAPERS.
106:3 (2005) ❘❘❘
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