Anais do 45º Congresso Brasileiro de Cerâmica
0801901
30 de maio a 2 de junho de 2001 - Florianópolis – SC
SELECTION OF DISPERSANTS FOR
HIGH-ALUMINA ZERO-CEMENT REFRACTORY CASTABLES
A.R. Studart, V.C. Pandolfelli
Via Washington Luís, km 235, C.P. 676, CEP 13565-905, São Carlos-SP
E-mail: pars@iris.ufscar.br ou vicpando@power.ufscar.br
Universidade Federal de São Carlos, DEMa
ABSTRACT
The efficient dispersion of fine particles has been an essential requirement for
developing refractory castables that can be easily installed and that exhibit superior
performance at high temperatures. In addition to the optimization of rheological
behavior, the dispersion of particles also enables a reduction of the admixing water
content, resulting in castables with enhanced mechanical properties. Phosphate and
polyacrylate salts are the additives most extensively used to disperse refractory
castables through the well-known electrosteric stabilizing mechanism. Nevertheless,
more efficient dispersants have been required in order to further reduce the admixing
water content and, thus, facilitate and speed-up the castable de-watering process. The
main objective of this paper is to establish some criteria for the appropriate selection of
dispersants for high-alumina refractory castables. Results have shown that the
molecule length, the charge density and the adsorption ability are the main features to
be considered when selecting the dispersant.
Key words: alumina, particles, refractory castable, dispersion, flowability.
INTRODUCTION
Several recent studies have shown the importance of the dispersion of matrix
fine particles on the development of high-performance refractory castables
(1-4).
The
appropriate dispersion of particles enables the production of highly fluid castables with
minimal water additions, rendering refractory materials with superior mechanical
properties before and after firing.
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Currently, efforts have been focused on attempts to further reduce the castable
water content in order to facilitate the refractory de-watering process
(5).
This requires
the use of more effective dispersants that could decrease the water content, keeping
castable high flowability.
Polyacrylate and phosphate salts have been the main additives used to
disperse high-alumina castables. The adsorption of such molecules on the surface of
particles gives rise to an electrical double layer and a steric layer that prevents
agglomeration through the well-known electrosteric mechanism. In order to accomplish
particle dispersion the thickness of these layers must be sufficiently high to overcome
the action of the attractive van der Walls forces. The particle zeta potential after
dispersant adsorption, the liquid ionic strength and the dispersant molecular length are
the main factors that control the dispersion state of particles, since they determine the
thickness of the steric and electric double layers.
The main objective of this paper is to use these surface chemistry parameters
as guidelines to properly select dispersants for high-alumina refractory castables. This
would enable a reduction of the castable water content for a given flowability level,
aiding the de-watering process and improving the castable mechanical properties.
EXPERIMENTAL PROCEDURE
(a) Dispersants
Sodium polyacrylate (BASF S.A., Brazil) of varying molecular weight (1200,
2500, 5100, 8000 and 15000 g/mol) were utilized in this work to investigate the effect
of the dispersant molecular length on the rheological behavior of concentrated
alumina suspensions. Gallic acid (Mallinckrodt, USA), citric acid
(6)
(Labsynth, Brazil)
and diammonium citrate (Fluka Chemie AG, Switzerland) were also evaluated.
Electroacoustic measurements (DT-1200, Dispersion technology, Inc., U.S.A)
were performed to assess the zeta potential of particles after dispersant adsorption.
Such tests were accomplished in dilute suspensions (2 vol%) prepared by adding the
powder and chemicals into distilled water, followed by ultrasonication for 15 min. The
suspension pH was adjusted with small additions of HCl and NaOH, 1 – 2 N
solutions.
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The thickness () of the steric layer formed around particles after dispersant
adsorption (adlayer) was estimated from the polymer molecular weight (MW) in the
case of polyacrylates, employing the equation suggested by Lewis et al. (7):
  0.06M W
12
.
(A)
For citrate ions, this thickness was assumed to be of approximately 0.5 nm, as
predicted by Leong et al. based on experimental results (8).
(b) Alumina suspensions
A highly pure alumina (Ceralox HPA-0.5, 8.9 m2/g, d50 ~ 0.5 m, Condea
Vista, U.S.A.) was utilized to evaluate the dispersing behavior of the additives under
very stable conditions in terms of ionic strength and particle size distribution.
Suspensions prepared with this alumina were first ball-milled for 24 hours in order to
promote particle de-agglomeration and, subsequently, degassed for 5 minutes (100
mbar) before rheological measurements proceeded.
The rheological behavior of highly pure alumina suspensions prepared with
different dispersants was analyzed under steady-stress conditions (CS-50 rheometer,
Bohlin Instruments, UK), by applying a shear sweep between 0.06 and 500 s-1 in a total
of 40 steps. Due to the high solid loading of the suspensions evaluated (50 – 58 vol%),
the vane tool was utilized in all measurements as a means to prevent wall slipping.
(c) High-alumina castables
High-alumina castables were prepared using white fused alumina (EK8R,
Alcoa, Brazil) as aggregates (~ 75 wt%) and the calcined aluminas A-3000 FL (~ 15
wt%) and A-1000 SG (~ 10 wt%) as the matrix constituents (Alcoa Chemicals, USA).
The particle size distribution of such castables was adjusted to a theoretical curve
based on Andreasen packing model (q = 0.21)
(1,2),
in order to obtain potentially self-
flow compositions.
An optimum mixing procedure was adopted to prepare castables, as
described in a recent article by the authors
(9).
The free-flow test adapted from the
ASTM 860 standard (1) was performed to provide an index of castable flowability.
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RESULTS AND DISCUSSION
(a) Effect of adlayer thickness
The steric layer (adlayer) thickness is known to display a dual effect on the
rheological behavior of suspensions. At low solid loading, an increase in the adlayer
thickness is expected to have a positive effect on the suspension rheology, as it
prevents particle agglomeration through a pronounced steric mechanism. On the
other hand, excessively thick steric layers usually cause an increase in the
suspension effective concentration that can be high enough to abruptly increase the
viscosity of suspensions with high solids content
(10).
Furthermore, the long-chain
molecules necessary to provide thick steric layers may lead to a substantial increase
in suspension viscosity, when they are not adsorbed on the particle surface and
remain in the liquid medium (10).
This ambiguous aspect of the adlayer thickness has motivated the evaluation
of its effect on the rheological behavior of concentrated suspensions, in order to
investigate whether there is an optimum thickness range for the efficient dispersion of
refractory castables.
Such evaluation was accomplished by first determining the effect of the
dispersant content on the viscosity of 50 vol% alumina suspensions. Figure 1
presents the results obtained for suspensions containing diammonium citrate and
sodium polyacrylates with different molecular weight.
No evident correlation can be established between the polyacrylate molecular
weight and their optimum dispersing content in terms of mg/m 2. However, Table I
reveals that a correlation exists when the optimum content is calculated based on the
number of molecules per surface area (mol/m2). It is observed that the amount of
polyacrylate molecules necessary to minimize the suspension viscosity increases
with the decrease of the dispersant molecular weight. Similar results were obtained
by Leong et al.
(11),
who found that superior contents of dispersant (in mol/m 2) are
required to shift zirconia isoelectric point (IEP), when the polyacrylate molecular
weight is increased.
A more impure alumina obtained from the Bayer process (A-1000 SG, 8.9
m2/g, d50 ~ 0.5 m, Alcoa Chemicals, USA) was also used in this study to assess the
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behavior of dispersants under the more unstable conditions normally encountered in
refractory applications. In this case, the suspension were neither ball-milled nor
degassed in order to reproduce the dispersing conditions of refractory castables.
Ceralox HPA-0.5
Dispersants:
50 vol% suspensions
-1
Apparent viscosity at 50 s (mPas)
10000
Diammonium citrate
1000
1200
2500
5100
100
Sodium
Polyacrylate
(g/mol)
8000
15000
10
0.1
0.2
0.3
0.4
0.5
0.6
Dispersant content (mg/m2)
Figure 1: Apparent viscosity of 50 vol% alumina suspensions (Ceralox HPA-0.5) for
different additions of diammonium citrate and sodium polyacrylates of varying
molecular weight.
Table I: Adlayer thickness (), isoelectric point shift (IEP) and ratio IEP/
expected (or obtained) after addition of distinct dispersants at their optimum contents
into alumina suspensions (Ceralox HPA-0.5).
Sodium polyacrylate (g/mol)
Diammonium
Gallic
citrate
1200
2500
5100
8000
15000
acid
0.26
0.34
0.31
0.30
0.46
0.50
-
(1.150)
(0.283)
(0.124)
(0.059)
(0.057)
(0.033)
 (nm)
0.5 – 0.8
2.1
3.0
4.3
5.4
7.3
0.5 – 0.8 
IEP
4.6
4.0
4.0
3.9
5.2
5.2
4.2 
IEP/ (nm-1)
5.6 – 9.3
1.9
1.3
0.9
1.0
0.7
5.3 – 8.4
Optimum content
in mg/m2
(mol/m2)

expected range.

data obtained from reference (13).
Anais do 45º Congresso Brasileiro de Cerâmica
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30 de maio a 2 de junho de 2001 - Florianópolis – SC
Suspensions prepared with Bayer-process alumina required dispersant
contents slightly lower than those presented in Table I to achieve minimum viscosity.
This was attributed to the fact that these slurries were not ball-milled before
measurements, which might have resulted in the presence of agglomerates in these
suspensions. Except for this slight difference in the optimum dispersant content, the
rheological behavior of these suspensions was in general very similar to that
obtained for slurries containing highly pure alumina.
Electroacoustic measurements revealed that the addition of optimum contents
of ammonium citrate and sodium polyacrylate (Table I) gives rise to zeta potential
values as high as 45 - 60 mV in the pH range of castables (8-10).
Such optimum contents were utilized for the preparation of alumina
suspensions at higher solid loadings, in order to compare their rheological properties
at the best dispersing conditions. At these conditions, the dispersants are assumed
to have covered most of (or even all) the surface area of particles.
Figure 2 shows the viscosity results obtained for highly concentrated
10000
Sodium polyacrylate
Range I
-1
Apparent viscosity at 50 s (mPa.s)
suspensions (50 – 58 vol%) as a function of the dispersant adlayer thickness.
Solid
loading
(vol%):
Range II
1000
50
55
100
58
Suspensions with
optimum
dispersant content
Diammonium
citrate
10
0
1
2
3
4
5
6
7
8
Adlayer thickness (nm)
Figure 2: Effect of the dispersant adlayer thickness on the apparent viscosity of
highly concentrated alumina suspensions (Ceralox HPA-0.5) dispersed either with
ammonium citrate or sodium polyacrylates of varying molecular weight.
In the case of the polyacrylate series, two different ranges of adlayer thickness
() may be distinguished in terms of the effects observed on the suspension viscosity.
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In the lower  range (~ 2 – 4.5 nm), the reduction of the adlayer thickness results in a
gradual increase in viscosity, due probably to the decrease of the steric contribution
to dispersion. On the other hand, in the higher  range (~ 4.5 – 7.5 nm), a viscosity
increase is observed as the adlayer becomes thicker. This may be attributed to the
increase of the suspension effective concentration, as well as the occurrence of
several other viscosity-increasing effects usually imparted by non-adsorbed longchain molecules (bridging, depletion, entanglement)
(10,12).
As a result of these distinct  ranges, an intermediate adlayer thickness of
approximately 4 nm seems to be the most appropriate for reducing the viscosity of
concentrated suspensions (50 - 58 vol%) containing sodium polyacrylate as
dispersant.
Viscosity results obtained for suspensions containing optimum amount of
diammonium citrate are also presented in Figure 2. Surprisingly, the addition of
diammonium citrate resulted in suspensions with quite low viscosity, in spite of the
very thin adlayer formed around particles in this case.
A more detailed examination of these apparently contradictory results gives
evidence to a major feature that molecules must display to work as effective
dispersants for highly concentrated suspensions.
Table I compares the shift in the alumina isoelectric point (IEP) obtained after
adsorption of different dispersants. This parameter may be considered as an
indicative of the molecule ability to incorporate electrical charges onto the particle
surface. One can notice that the IEP shift accomplished with citrate ions is slightly
higher than that obtained with low-molecular-weight polyacrylates ( 5100 g/mol).
Such superior IEP shift is expected to compensate the thin layer of citrate ions
formed around particles, enhancing the magnitude of electrostatic repulsive forces in
detriment of the steric contribution to dispersion.
Although this special feature of citrate ions may not lead to significant changes
in the rheological behavior of less concentrated suspensions ( 50 %vol), it has a
major impact at higher solid loadings (> 50 vol%), where the presence of thicker
adlayers tend to increase the suspension viscosity (Figure 2). This becomes evident
when the efficiency of these dispersants is evaluated under very high solid loading
conditions in high-alumina castables, as shown in Figure 3. It is observed that
castables containing sodium polyacrylate with optimum molecular weight (5400
Anais do 45º Congresso Brasileiro de Cerâmica
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g/mol) displayed free-flow values inferior than 10%, whereas those prepared with
citric acid achieved flowability 8 times higher, with values in the self-flow range (80 110%). Figure 3 also points out that the flowability of castables dispersed with citric
acid is still markedly higher than that of compositions containing sodium polyacrylate
even when prepared with lower water content.
100
Gallic acid (14 vol% water)
Free-flow (%)
80
Citric acid
(15 vol% water)
Citric acid
(14 vol% water)
60
40
Sodium polyacrylate - 5400 g/mol
(15 vol% water)
20
0
0.1
0.2
0.3
0.4
0.5
Dispersant content (mg/m2)
Figure 3: Flowability of castables as a function of the content of different dispersants
for distinct water additions.
The effectiveness of citrate ions as charge generators on the surface of
particles may be attributed to its higher number of dissociable sites per chain length
(~ 0.80 sites/Å) in comparison to that of polyacrylate ions (~ 0.38 sites/ Å). It is also
worthy mentioning that the short length of citrate chains impedes these molecules to
display the spatial configurations (tails, loops and trains) normally expected from
polyacrylate chains on particle surface. Therefore, one may suppose that citrate
molecules tend to pack better on the surface of particles, since they are not able to
develop opened conformations as in the case of polyacrylates molecules. This might
be another factor that could contribute for the high efficiency of citrate ions as surface
modifiers.
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(b) Short-chain highly-charged dispersants
It becomes clear from the above considerations that the ratio IEP/ must be
maximized if an effective dispersant for highly concentrated suspensions (or
castables) is aimed. This parameter takes into account two of the main factors that
control the rheological properties of suspensions, namely zeta potential and the
adlayer thickness. However, the ionic strength has also a major influence on these
properties and, thus, must also be considered when selecting a dispersant.
Dispersants may increase the ionic strength of suspensions if a significant
portion of their molecules is not adsorbed on the surface and remains in the liquid
medium. This can be avoided by choosing dispersants that adsorb to a large extent
over the pH range of interest.
Hidber et al.
(13)
have shown that the adsorption behavior of organic short-
chain molecules on alumina surface is determined by the dissociation constants (pKa
values) of the molecule functional groups. Maximum adsorption is observed in the pH
range that corresponds to the pKa values of the dissociable groups.
The adsorption behavior of citrate and polyacrylate molecules on alumina
surface displays similar pH dependence, as both dispersants possess several
carboxylic groups (-COOH) along their molecular chain (Figure 4). The typically low
pKa values of these groups (i.e., 5.82, 4.50 and 3.48 for citric acid three (-COOH)
groups) lead these molecules to adsorb more prominently in the acid pH range, as
illustrated in Figure 5 by data after Hidber et al. (13). Therefore, a significant amount of
citrate and polyacrylate ions are expected to remain in the liquid medium and
increase its ionic strength at the usual pH range of high-alumina castables.
~5Å
HOOC
HO
COOH
~ 2.6 Å
COOH
COOH
OH
CH CH2
COOH
n
OH
Citric acid
Polyacrylic
acid
COOH
OH
2,3,4-Trihydroxybenzoic acid
HO
OH
OH
Gallic acid
Figure 4: Molecular structure of the dispersants evaluated.
Anais do 45º Congresso Brasileiro de Cerâmica
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In contrast to the (-COOH) groups, hydroxyl groups usually display
significantly higher pKa values (typically > 9)
(13)
and, thus, seem to be more suitable
for dispersing high-alumina castables. Figure 5 shows that a benzoic acid containing
three (-OH) side groups (2,3,4-trihydroxybenzoic acid) adsorbs in a large extent (>
90%) in the pH range of alumina castables, which would result in minimal increase of
Adsorbed content / Added content (%)
the liquid ionic strength.
100
Usual pH range
of highhigh-alumina
castables
80
60
Citric acid
40
2,3,4-Trihydroxybenzoic acid
20
4
5
6
7
8
9
10
pH
Figure 5: Adsorption behavior of citric acid and 2,3,4-trihydroxybenzoic acid on
alumina surface as a function of pH (data obtained by Hidber et al. (13)).
The efficiency of molecules with hydroxyl groups on dispersing alumina
castables was assessed in this work by evaluating the dispersing ability of gallic acid.
Gallic acid is the name commonly used for the 3,4,5-trihydroxybenzoic acid, whose
structure is illustrated in Figure 4. The chain length and IEP shift obtained with gallic
acid are comparable to those of citric acid, which enables this compound to be a
potential dispersant for highly concentrated suspensions. The absence of (-OH)
groups in the ortho position of the benzene ring (Figure 4) is expected to further aid
the adsorption of gallic acid on alumina in comparison to that of the 2,3,4trihydroxybenzoic acid, since this steric configuration prevents the occurrence of
unfavorable electrostatic repulsion between the carboxylate ion and the alumina
surface (13).
Anais do 45º Congresso Brasileiro de Cerâmica
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Figure 3 shows that castables prepared with gallic acid can achieve higher
flowability levels than that of compositions containing citric acid for the same water
content, confirming the superior efficiency of gallic acid as a dispersant for alumina
castables. This indicates that the use of gallic acid enables further reductions of the
castable water content, which aids the refractory de-watering process.
Stability maps of 58vol% alumina suspensions dispersed with citric acid and
gallic acids were built in this study, following the procedure described elsewhere by
the authors (2) (Figure 6). It can be noticed that the superior adsorption ability of gallic
acid at alkaline pH conditions results in matrix suspensions displaying inferior
viscosity in the pH range of high-alumina castables, which explains the flowability
results presented in Figure 3.
0.35
(a)
Citric acid
0.30
0.25
Dispersant content (mg/m2)
0.35
0.20
Viscosity
(mPa.s)
0.30
0.15
Castable
initial pH
0.25
0.10
0.20
0.35
180
210
6
7
8
9
10
11
12
250
300
0.15
0.30
0.10
0.25
400
pH inicial do
concreto
6
7
8
9
0.15
11
12
pH
(b)
Gallic acid
0.20
10
Castable
initial pH
0.10
6
7
8
9
10
11
12
pH
Figure 6: Stability maps of 58 vol% alumina suspensions dispersed with (a) citric and
(b) gallic acids. These maps show iso-lines that delineate the dispersant content–pH
combinations necessary to provide suspensions with a given viscosity level.
Anais do 45º Congresso Brasileiro de Cerâmica
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CONCLUSIONS
This paper showed that the appropriate selection of dispersants can
significantly reduce the castable water content for a given flowability level and,
therefore, may be considered as a crucial step for speeding-up the refractory dewatering process.
It was observed that the use of long-chain molecules could markedly
deteriorate castable flowability properties. Therefore, efficient dispersants would be
those that associate minimum chain length with maximum surface modifying ability
(high IEP/ ratio). These features are most likely encountered in molecules
presenting a high number of dissociable groups per chain length.
The nature of the dissociable groups is another major factor that influences
the dispersant efficiency, since it controls the molecule adsorption behavior as a
function of pH. The aim in this case should be to choose dissociable groups that lead
to maximum adsorption over the pH of castables, minimizing the dispersant negative
effect on the liquid ionic strength. This can be accomplished by selecting dispersants
containing functional groups with dissociation constants (pKa values) similar to the
pH values expected for high-alumina refractory castables.
ACKNOWLEDGEMENTS
The authors would like to acknowledge FAPESP and Alcoa / Brazil for the financial support to
this work.
REFERENCES
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Anais do 45º Congresso Brasileiro de Cerâmica
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