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Proceedings of
XV BALKAN MINERAL PROCESSING CONGRESS
Sozopol, Bulgaria, June 12 – 16, 2013
VOLUME I
Edited by
Ivan Nishkov, Irena Grigorova, Dimitar Mochev
2013
Proceedings of the XV Balkan Mineral Processing Congress, Sozopol, Bulgaria, June 12 – 16, 2013
ELECTROKINETIC AND RHEOLOGICAL STUDIES ON CERAMIC KAOLINS DISPERSED WITH
DIFFERENT ELECTROLYTES
Emin Caner Nalbant1, Hayrünnisa Dinçer Ateşok1, Mustafa Salih Eygi2 and Gündüz Ateşok1
1
2
Istanbul Technical Uni., Mining Faculty, Mineral Proc. Dept., Maslak-Istanbul, Turkey
Tales Engineering, K.S.S. 9.Blok No:4, Adıyaman, Turkey
ABSTRACT. This study covers the investigations based on the electrokinetic and rheological properties to explain the interactions between ceramic grade kaolin mineral
and different electrolytes. For this purpose, two kaolin samples of ceramic grade and three electrolytes (a sodium silicate and two anionic polyelectrolyte dispersants) were
selected. In the electrokinetic studies, zero point of charge (zpc) values of both kaolin samples could not be observed in the presence of all electrolytes. Natural surface
charges of Kaolin A and Kaolin B in water were found as negative and retained in between -12 mV, -7,5 mV. Similarly it is retained as also negative while the amount of
the dispersant increases. The type of dispersant and its concentration that provide the minimum value of viscosity for suspensions prepared each kaolin samples varies
with the type of dispersant and the properties of the suspension. The yield stress values of Kaolin A and Kaolin B suspensions prepared each dispersants was found below
and above 0,1 N/m2 respectively. This critical value explains that Kaolin B have higher plasticity than Kaolin A.
Keywords: Kaolin, Polyelectrolyte, Sodium silicate, Rheology
There are plenty of papers in the scientific literature concerning the
rheological characterization of ceramic clay suspensions in
hydrocarbon media (polyelectrolyte) have been investigated (Sjöberg
et. all, 1999; Eygi and Atesok, 2008a and 2008b; Guldberg-Petersen
and Bergström, 2000; Marco et. all, 2004).
Thus, the purpose of this work is to investigate the effects of
sodium silicate and polyelectrolyte dispersants on high solid contained
kaolin suspensions. For this reason, two types of ceramic kaolin
samples were chosen and electro kinetic and rheological behavior of
this kaolin samples were studied systematically, depending on the
type and amount of dispersants.
INTRODUCTION
Kaolin is often used in the ceramics, paper coatings, water-based
paints and inks and as an additive in plastics. The unit cell of the
kaolin lattice has the composition [Si2Al2O5(OH)4]. It’s crystals consist
of alternating layers of silica tetrahedra and alumina octrahedra, and
each particle consists of a stack of about 50 sheets of twin-layers,
held together with hydrogen bonds. The particles are plate-like and
the aspect ratio (particle diameter/particle thickness) is about 5–15
(Solomon and Hawthorne, 1983; Sjöberg et. al, 1999). There is a
significant difference in the chemical composition of the edges and
the basal planes of the kaolin particles (Brady et. al, 1996). There are
often crystal imperfections in the kaolin crystals, with ionic
substitutions of Al for Si or Mg for Al, which result in an overall deficit
of positive charge. This substitution thereby gives rise to a net anionic
charge of the basal surfaces of the particles. However, it has been
shown that the negative charge of the basal surfaces varies slightly
with pH, the origin of this pH-dependent charge could be due to the
presence of some amphoteric silanol groups on the basal surfaces
(Ferris and Jepson, 1975). At the edges of the kaolin particles, the
octahedral alumina and tetrahedral silica sheets are broken, exposing
aluminol and silanol groups which can yield positive charges in acidic
solution and negative charges in alkaline solution. The point of zero
charge of the edges is around pH 7 (Herrington et. al, 1992).
Whilst the basal load of kaolin particles is in general negative (-) at
all pH values, the edge loads convert to positive (+) load at acidic pH
and negative (-) load at basic pH. In this case, to provide a tagging
[(-) loading] (provide a dispersion) of the edges and the surfaces at
the same time is only possible at basic pH (Zaman et.al, 2002). Some
of alkaline silicates and carbonates such as sodium silicate (Na2SiO3)
or calcium silicate (CaSiO3), and sodium carbonate (Na2CO3) or
calcium carbonate (CaCO3) are used as dispersant to provide the
fluidity of both ceramic clays and suspensions. Among these common
dispersants, sodium silicate (Na2SiO3) is known to show good
performance on dispersing clays in ceramic processing. On the other
hand, anionic polyelectrolytes such as polyacrylates and
polyphospates are also used in some case to regulate suspension
viscosity. The dispersing mechanism of sodium silicate in the claywater system is known as electrostatic, but polyelectrolytes
accomplish their dispersing action through a concerted mechanism:
(i) by increasing the overall negative surface charge, being adsorbed
as an anion especially at the edges of the clay mineral particles; (ii)
by complexing the dissolved flocculant alkaline earth cations and
replacing them by its cations, therefore increasing the thickness of
the electrical double layer (Andreola et al., 2004 and 2006). However,
many points concerning the details of this dispersion mechanism still
remain obscure and need to be further investigated. As these
chemicals are of anionic character and have variable pH, they set the
importance of the system pH to a secondary level. The interaction
system herein, as brought out by several researchers, is explained by
the steric adsorption of the free anionic ends of the polymer to the
(+) loaded parts of the kaolin particles (Leese and Maguire, 1996;
Sjöberg et.al., 1999; Brezina and Thomas, 2000; Guldberg and
Bergstöm, 2000; Papo et.al., 2002; Marco et.al., 2004;). As at this
interaction, called as the electrosteric effect, the steric held particles
by the free anionic ends of the polymer chains do not get close to
each other, the system viscosity decreases.
Material and Method Materials
In the experiments, ceramic grade high purity kaolins, coded as
Kaolin A (low plasticity) and Kaolin B (high plasticity) obtained from
Eczacıbaşı Esan Endüstriyel Hammaddeler San. ve Tic. A.Ş. (Turkey)
were used. 85.42% of Kaolin A had particle size less than 20µ and
41.59% less than 2.7µ; whereas, 85.74 % of Kaolin B had particle
size less than 20µ and 66.02% less than 2.7µ. The chemical
compositions of both clays are given in Table 1.
Tabl. 1 Chemical analysis of Kaolin A and Kaolin B.
Chemical Content, %
SiO2
Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O
Kaolin A 47,41 36,23
Kaolin B 54,13 29,11
0,41
1,61
0,62 0,3 0,21
1,2 0,31 0,55
0,2
0,2
LOI
1,12 13,39
1,51 11,21
LOI: Lost of Ignition
Also, two commercially available anionic polyelectrolytes and a
sodium silicate were used as dispersants in order to study their
effects on electrokinetic and reological behaviors on kaolin. These
dispersants
are
coded
as
Dolapix
G6,
Dolapix
SPC7
(Zschimmer&Schwarz, Germany) and EgeNat 2102 (Ege Kimya A.Ş,
Turkey) by the suppliers respectively.
This experimental study involves two steps as electrokinetic studies
which is investigating surface characteristic features of kaolins
belonging to the dispersant type and their amount and rheological
studies which investigates viscosity in high solid content kaolin
suspensions belonging to the dispersant type and amount.
Methods
In the experiments involving electrokinetic studies were
investigated as the change in the electrical charge of the kaolin
particles surface (zeta potential) by dispersant type and its amount.
The methods applied at these works, are given below:
When preparing samples for zeta potential measuring, 0-1%
dispersant containing in 80 ml pure water solutions have prepared in
100 ml glass beaker firstly. Then, Kaolin A and Kaolin B were added
for obtaining 1% solid loading solutions. After that, dispersant and
solid loaded suspensions were mixed 5 min in magnetic disperser.
Finally, these suspensions were centrifuged in 5.000 rpm for 15 min
in centrifugal labor equipment to separate clearly solid and liquid
502
Proceedings of the XV Balkan Mineral Processing Congress, Sozopol, Bulgaria, June 12 – 16, 2013
values anticipate that increase in the specific conductance is sourcing
from increase of dispersant concentration unlike releasing ions from
kaolins structure. Since both kaolins have low cation changing
capacity in water. Therefore this results support either the trustability
of the obtained zeta potential values or the increase of zeta potential
is sourcing from the adsorption of dispersant to the surface of the
kaolins.
phases. After separating, decent samples have taken from suspension
by small sampler in order to measure zeta potentials.
In the experiments involving rheological studies were investigated
as a function of the type and amount of dispersant. With this purpose
in mind, various suspensions of 60% solid content were prepared.
Depending on experimental conditions, dispersants were added
during the preparation of suspensions, which amounted to between
0.05% and 0.5% based on solid material (wt/wt). The viscosity
measurements of suspensions were done using a Brookfield “R/S plus
Coaxial Cylinder Rheometer” at 26 s-1 shear rate. The optimization
experiments previously carried out indicated that the best
measurements would be at this shear rate. In all measurements the
CC48 DIN spindle of the device was used.
Rheological studies
The results of rheological studies are given in Figure 2 and 3 based
on the type of dispersant. As is observed in Figure 2 (A and B),
dispersant concentrations up to a specific optimum value lower the
viscosities of suspensions at low shear rates, but beyond this specific
optimum concentration the viscosity values no longer decrease, but
either stay constant or begin to increase.
The optimum dispersant concentration which produces the best
dispersion condition by maximum dispersion of kaolin particles, thus
resulting in a minimum value for viscosity, is different for the two
ceramic kaolins (in Kaolin A suspensions 0.1% (wt/wt) with Dolapix
G6, 0.15% (wt/wt) with Dolapix SPC7 and 0.2% (wt/wt) with EgeNat
2102; and in Kaolin B suspensions 0.3%, 0.3% and 0.2 % (wt/wt)
with respective dispersants).
This is resulted from the differences of dispersing mechanism
between sodium silicate and polyelectrolytes. Dispersion with sodium
silicate (here in EgeNat 2102) in both kaolin suspensions occurs
through the combination of the silicate ions (SiO3-2) with the divalent
flocking cations such as calcium (Ca2+) and magnesium (Mg2+) in the
suspension, forming insoluble calcium or magnesium silicates. Sodium
ions (Na+) then replace the divalent cations in the newly opened
structure of high-clay content system, creating the electrostatic
repulsive forces that cause dispersion (Eygi and Ateşok, 2008a).
However, when polyvalent exchangeable cations are present in the
suspension, dispersion is not considered complete unless these
cations are displaced and retired as insoluble exchange reaction salts.
On the other hand, Dolapix G6 and Dolapix SPC7 (polyelectrolytes)
provide more homogenous dispersion of the clay particles in the
suspension than that of EgeNat 2102. They adsorb on positively
charged surfaces of kaolin samples with their anionic groups, thus
particles come closer and move together due to polymeric bridges
(Kohut, 1992; Eygi and Ateşok, 2008b). Therefore, the adsorbed
groups of polyelectrolytes provide a steric barrier between the
particles which keep them well dispersed (Sjöberj et. all, 1999;
Zaman et al., 2002; Guldberg-Petersen and Bergstöm, 2000; Marco
and et al., 2004).
Results
Electrokinetic studies
The changes in the electrical charge of the Kaolin A and Kaolin B
particle surfaces (zeta potential) by dispersant type and its amount
are given in Figure 1 (A and B) respectively.
Dispersant Concentration, %
0
0,2
0,4
0,6
0,8
1
0
-10
Zeta Potential ( ), mV
-20
-30
-40
-50
Kaolin A
-60
Dolapix G6
Dolapix SPC 7
Egenat 2102
-70
-80
Dispersant Concentration, %
0
0,2
0,4
0,6
10000
0,8
1
Kaolin A
Shear rate: 26 s -1
Appearent Vis c os ity , mPa.s -1
.
0
-10
Zeta Potential ( ), mV
-20
-30
-40
0
Kaolin B
-70
100
10
-50
-60
Dolapix G6
Dolapix SPC 7
Egenat 2102
1000
0,1
0,2
0,3
0,4
0,5
0,6
Dispersant Concentration, %
Dolapix G6
Dolapix SPC 7
Egenat 2102
100000
Kaolin B
Shear rate: 26 s -1
Dolapix G6
Dolapix SPC 7
Egenat 2102
.
-80
Appearent Viscosity, mPa.s-1
10000
Fig. 1 Effect of dispersants on zeta potential of Kaolin A and
Kaolin B.
Observing from the related figures, natural surface charges of
Kaolin A and Kaolin B in water are negative and retained in between 12 mV, -7,5 mV. Similarly it is retained as also negative and the
absolute value of the particle surface charges of the both kaolin
samples increase while the amount of the dispersant increases.
As a matter of fact in this study, the highest surface charge
obtained in Kaolin A (-73 mV) by Dolapix SPC7 dispersant (%1), and
the highest surface charge obtained in Kaolin B (-70 mV) by Dolapix
G6 dispersant (%1). This results show that the best dispersion
conditions obtained in Kaolin A is with all type of dispersants and for
Kaolin B is only with Dolapix G6. On the other hand specific
conductance values that are 50-500 mhos/cm, do not make any
abnormal increase with or without dispersant usage. These normal
1000
100
10
0
0,1
0,2
0,3
0,4
0,5
0,6
Dispersant Concentration, %
Fig. 2 Effect of dispersants on apparent viscosities of Kaolin A
and Kaolin B suspensions prepared with 60% solids.
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Proceedings of the XV Balkan Mineral Processing Congress, Sozopol, Bulgaria, June 12 – 16, 2013
CONCLUSİONS
Figure 3 represents variation of the shear rate-shear stress
rheograms of both kaolin samples prepared with %60 solid loading
with different dispersants that provides the minimum viscosity and
maximum dispersible conditions. The evaluation of that figures
indicates that in all suspensions, independent of the type of
dispersant present, the shear stress values increase in a non-linear
manner (slightly convex) with increasing shear rates.
The results indicate that all kaolin suspensions prepared with all
dispersants used display a non-Newtonian fluid behavior. All
suspensions prepared with dispersants showed the Bingham plastic
flow behavior with dilatant character. From the rheograms, the main
differences can be seen in the yield stress values. The yield stress
values of Kaolin A and Kaolin B suspensions prepared each
dispersants was found below and above 0,1 N/m2 respectively. This
critical value explains the differences of plasticity of both kaolin
samples. It explains that Kaolin B have higher plasticity than Kaolin A.
The following conclusions have been reached upon evaluation of
results of experiments designed to investigate the effects of different
dispersants on the electrokinetic behaviour and rheology of ceramic
clay suspensions:
-
-
1000
Kaolin A
100
Dolapix G6
Dolapix SPC 7
Egenat 2102
Shear Stress (τ), N/m
2
.
-
10
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0,1
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10
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-1
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with different dispersants.
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Proceedings of the XV Balkan Mineral Processing Congress, Sozopol, Bulgaria, June 12 – 16, 2013
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