STUDIES OF KAOLIN RHEOLOGY
Carla N. Barbatoa,c*; Marcio Nelea; José C. Pintob and Silvia C. A. Françac
a
b
Universidade Federal do Rio de Janeiro, Escola de Química, tel: 55(21)38657359
Universidade Federal do Rio de Janeiro, Programa de Engenharia Química da COPPE,
tel:55(21)25628304
c
Centro de Tecnologia Mineral – CETEM, Cidade Universitária, Av. Pedro Calmon, 900. CEP
21941-908 – Rio de Janeiro – RJ Brasil, tel: 55(21)25627717
ABSTRACT
The rheological behavior of kaolin suspensions depends on a group of
variables: particle size distribution, aggregation, surface charge and morphology of
particle, pH, solids and chemicals (dispersant) concentration, and mineralogical and
chemical impurities in the clay.
Particles of kaolin are small enough to be unaffected by gravity. The
interparticle interactions and their behavior are controlled by a range of attractive
(van der Walls) and repulsive (electrostatic repulsion) forces.
The aim of the present work was to evaluate the influence of solid
concentration, deflocculant concentration (sodium hexametaphosphate) and pH on
the viscosity of kaolin suspensions.
The techniques used to particle characterization were: X-ray fluorescence, Xray diffraction, scanning electron microscopy, particle size distribution (Malvern) and
zeta potential.
The rheological tests were run out in Haake Rheo Stress Rheometer and
coaxial sensor (ZN-20 DIN). The tests were carried out at 25 ºC, based on the
flowing steps: (1) a linear increasing of shear rate from 0 to 1,000 s-1 for 600 s, the
maximum shear rate (1,000 s-1) kept for 10 s, (3) a linear decreasing of shear rate
from maximum shear rate to 0 s-1 for other 600 s. The variables investigated were:
solids concentration (50 to 70% w/w), dispersant concentration (6 to 8 kg/t), pH (7 to
10), and shear rate (0 a 1,000 s-1).
The kaolin suspensions, under the experimental conditions used at the present
work, presented a thixotropic behavior. Solids concentration had the biggest
influence over the pulp viscosity. The kaolin suspensions with 50 – 60% solids
presented low viscosity, while the viscosities of the pulps with 70% solids were
higher.
Key- words: kaolin, viscosity, surface chemistry, rheology.
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INTRODUCTION
The term kaolin refers to a rock that contains the mineral kaolinite and also the
product resulting from its beneficiation. It is a clay mineral, with low iron content and
white color. The theoretical chemical composition (% in weight) is: 39.55 Al2O3, 46.54
SiO2 and 13.96 H2O, however, small variations can be observed in this composition
(Silva, 2007).
The morphology of kaolinite is pseudo – hexagonal and it consists in alternating
layers of silica tetrahedral and alumina octahedral. Each kaolinite particle consists of
a stack of about 50 sheets of twin layers, held with hydrogen bonds (Sjösberg et al.,
1999).
The superficial charge of kaolinite results of two types: permanent and no
permanent. The permanent charges are consequence of the imperfections in the
kaolinite crystals, with ionic substitutions of Al+3 for Si+2 or Al+3 for Mg+2, which results
in negative charge in the face. The no permanent charges are consequence to the
broken octahedral alumina and tetrahedral silica sheets placed, at the edges and
base of the kaolinite particles, exposing aluminol and silanol groups which can yield
positive charges in acidic solution and negative charges in alkaline solution (Stjöberg
et al., 1999 and Tombácz e Szekeres, 2006).
Figure 1 shows the schematic representation of crystalline structure of kaolinite:
the lateral and the base of kaolinite are influenced strongly by the pH, and the base is
less reactive than the lateral.
Figure 1 – Schematic representation of crystalline structure of kaolinite
(Tombácz e Szekeres, 2006).
Permanent Charges
pH Dependent Charges
The kaolinite, in aqueous solution, forms silanol and aluminol complex. The
silanol groups, in consequence of the deprotonation reaction, forming a complexed
anion what contributes to increasing the negative surface charge. The aluminol
groups are amphoteric, playing protonation reaction in low pH forming a complexed
cationic and deprotonation reaction in high pH giving complexed anion. The
protonation and deprotonation reaction can be observed below (Tombácz e
Szekeres, 2006).
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AlOH + H+ → AlOH+2 (protonation)
AlOH + OH- → AlO- + H2O (deprotonation)
SiOH + OH- → SiO- + H2O (deprotonation)
Particles of kaolin are small enough to be unaffected by gravity. A range of
attractive (van der Walls) and (electrostatic repulsion) forces controls the interparticle interactions and their behavior (Cunha, 2004). The objective of studying the
rheological properties of ore pulps is the optimization of the shear stress necessary
to pump these pulps (Nasser and James, 2008). The volumetric solids fractions,
particle size distribution, particle shape, flocculation and electroviscous effects can
affect the suspensions rheology (Ortega et al., 1997).
The aim of the present work was to evaluate the influence of solid
concentration, deflocculant concentration (sodium hexametaphosphate) and pH on
the viscosity of kaolin suspensions.
METHODOLOGY
The raw material studied was a sample of kaolin from Borborema – Seridó
region (Northeast of Brazil), which the chemical composition (% in weight) is: 39.06
Al2O3, 0.418 Fe2O3, 0.7 K2O, 0.063 P2O5, 46.91 SiO2 and 12.85 loss on ignition. The
average particle size is d50 = 9,267 µm (laser particle-size analysis).
The measurements of zeta potential (Zeta Probe-Colloidal Dynamics) were run
in the pH range 2.5 to 12. The suspensions were prepared with distilled water and
4% (w/w) of kaolin. The pH was adjusted with diluted solutions of NaOH and HCl.
The zeta potentials of kaolin suspensions were determined in absence and presence
of sodium hexametaphosphate.
Viscosity values were obtained in a Rheo Stress Haake Rheometer (RS1),
equipped with coaxial cylinder measuring sensor (Z-20 DIN) in isothermal conditions
at 25 ºC. Viscosity was measured under the same conditions to all the sample using
three cycle: a shear rate increase from 0 to 1,000 s-1 in 600 s, followed by 10 s at a
shear rate of 1,000 s-1 and a shear rate decrease from 1,000 to 0 s-1.
The suspensions were prepared in the following conditions: deflocculant
(sodium hexametaphosphate) concentration - 6 to 8 kg/t, solids concentration - 50 to
70% (w/w) and pH - 7 to 10. The designed experiments are presented in the Table 1.
The experiments were run in a random sequence and the viscosity was measured
immediately after slurry preparation.
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Table 1: Experimental conditions according to the planning of complete
factorial experiment with star plan and central point.
Tests
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Solids Concentration
(%)
50
50
50
50
70
70
70
70
70
50
60
60
60
60
Deflocculant
Concentration (kg/t)
6
6
8
8
6
6
8
8
7
7
6
8
7
7
pH
7
10
7
10
7
10
7
10
8.5
8.5
8.5
8.5
7
10
RESULTS AND DISCUSSIONS
Zeta Potential
Figure 2 reports the variation of zeta potential with the increase of pH and
concentration of sodium hexametaphosphate. This behavior has been observed by
Cunha, (2004), Andreola et al. (2006) and it can be attributed to progressive
deprotonation of surface edge sites and the adsorption of the sodium
hexametaphosphate on kaolin.
The isoelectric point of the curve that represents the zeta potential of the
suspensions with 4% of solids and no defocculant is about at pH 1.7-2.5. This result
is in accordance to the literature (Tombácz and Szekeres, 2006).
In all cases with deflocculant, the zeta potential are negative and no apparent
isoelectric point was detected. This can be understood since the basal surfaces of
plate-like kaolinite, which represent the most significant part of the overall surface of
the particles, present a negative charge within the whole pH range (Andreola et al.,
2006).
The zeta potential becomes increasingly negative as pH increases. This
behavior can be attributed to the deprotonation of aluminol group on the edge of
kaolinite giving a complexed anion what contributes to increasing the negative
surface charge and, consequently, to the repulsive forces among particles.
The sodium hexametaphosphate (NaPO3)6 used as deflocculant in the kaolin
slurries is chemisorbed on the edge of kaolinite. The anions (PO3-) interact with the
exposed cations of Al+3, giving complexed anions. The chemisorption produces a
surface excess of negative charges and therefore an increase of the repulsion forces
among the particles, as a consequence, the zeta potential of the clay particles
increases (Andreola et al., 2004).
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Figure 2 – Variation of zeta potential with pH (4% solid and different
sodium hexamataphosphate concentrations).
0
0
2
4
6
8
10
12
14
pH
-5
Zeta Potential (mV)
-10
-15
-20
-25
-30
-35
-40
no deflocculant
4kg/t
6kg/t
10kg/t
8kg/t
Rheological Studies
Based on the experimental conditions run in this work, the kaolin suspensions
performed thixotropic characteristics. In this type of flow, the maximum microstructure
is seen when alignment and spatial distribution are random (in three dimensions) and
entanglement density is at maximum. Both conditions result in the greatest viscous
(and usually elastic) response. On the other hand, minimum microstructure occurs
when the maximum alignment with particle spatial distribution is asymmetrical in the
flow direction, or there are a minimum number of entanglements or associations – all
these leading to minimum viscous and elastic response (Barnes, 1997).
Rheology of Kaolin Slurries without Deflocculant
The slurry of kaolin without deflocculant was prepared with 50% of solids. It was
not possible to obtain more concentrated slurry without deflocculant due to the high
viscosity in these pulps.
Figure 3 reports the decreasing of the viscosity with the increasing of shear rate
that is the characteristics of thixotropic flow. According to Frank, shear rates smaller
than 10 s-1 are considered low, shear rates between 10 to 100 s-1 are considered
medium and those larger than 1,000 s-1 are considered high. At low shear rate, the
variation of viscosity is larger than medium shear rate and at higher shear rate the
viscosity values tend to stabilization.
The decreasing of the viscosity with increasing of shear rate can be attributed
to the destruction of the structures three-dimensional and the structural components
align themselves in the direction of flow. In this case, the molecules of water, that
were hold in the three-dimensional structures, were released from these structures,
as consequence, the viscosity of slurries decreased. The attractive edge-face forces
in the kaolinite particles originate the three-dimensional structures, because in the
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natural pH (≈ 4,3) the slurry of kaolin presented negative charge on the face and
positive charge on the edge (Cunha, 2004).
Figure 3 – Viscosity versus shear rate of kaolin slurry with 50% of solids
and without deflocculant.
4000
3500
Viscosity (mPa.s)
3000
2500
2000
1500
1000
500
0
0
200
400
600
800
1000
-1
Shear Rate (s )
Rheology of Kaolin
Concentration and pH
Slurry
with
Flocculant,
Variation
of
Solids
Table 2 shows the values of viscosity at 200, 400, 600, 800 and 1,000 s-1. The
experimental data showed that the viscosity deviates strongly from the linear
behavior and solids concentration has the strongest influence on the viscosity of
suspensions, followed by shear stress, deflocculant concentration and pH.
The kaolin pulps used in the tests 1, 2, 3, 4 and 10, whose solid concentration
was 50% (w/w), were characterized by a change in the thixotropic behavior to
rheopectic when the shear rate was higher than 600 s-1. This behavior can be
attributed to the agglomeration of particles, at higher shear rate, because the water
flows more quickly than the particles.
To a solid concentration increasing from 50 to 70% (w/w) the viscosity
increased approximately 10 times. This change in the viscosity happened due to the
decreasing of the layer of water among the particles, giving larger attrition and
interaction among them.
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Table 2 – Viscosity of suspensions at 200, 400, 600, 800 e 1,000 s-1.
Viscosity (mPa.s)
Tests
-1
-1
-1
-1
-1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
τ = 200 s
τ = 400 s
τ = 600 s
τ = 800 s
τ = 1000 s
12.21
8.857
13
8.683
1,708
912.4
1231
882.8
1346
12.96
51.9
48.9
46.03
63.43
11.19
8.581
11.37
8.081
1,368
789.5
911.6
724.2
1068
11.06
46.69
43.23
40.57
53.96
10.97
8.838
10.86
8.263
1,002
627.4
676.8
579.5
830.1
10.48
44.97
40.67
38.61
49.53
11.97
9.227
10.77
8.784
772.3
509.5
542.4
475
672.5
10.29
42.82
38.39
36.73
46.04
11.03
9.67
10.76
9.51
647.7
437.42
469.69
403.56
567.65
10.31
40.45
36.09
34.75
42.5
In the slurries with 70% of solids, there were more possibilities to formation
agglomerates of particles because the interactions forces are more intense in these
slurries.
The agglomerated particles are larger than colloidal particles, therefore, the
effect of Brownian motion in slurries with particle agglomerations is less than in
slurries with colloidal particles. Thus, flocculated suspensions settle more quickly
than the dispersed suspensions. Besides, the agglomerated particles hold water
molecules in their internal structures. This water doesn't contribute with the flow of
the particles, as consequence the ratio between solid and liquid increases and the
viscosity of flocculated suspensions is higher than dispersed suspensions (Ortega et
al., 1997).
Kalolinte layer have amphoteric properties. The face or surface carrys a
permanent negatives charge and depending on the pH, there is a positive or negative
charge on the edges. Kaolinite platelets can associate in edge-edge (E-E), edge-face
(E-F) configurations. The easy formation of the different types of association depends
on the balance of electrostatic interactions (attractive or repulsive), which are
controlled by the chemistry of the dispersions, and the attractive van der Waals
forces between the particles.
Figure 4 predicts kaolin edge-face, face and edge interaction forces scaled
relative to the maximum attractive edge-face force, and multiplied by the effective
interaction area to simulate the rheological response. Hamaker constants of 8.5 x 1021
, 1.8 x 10-20 and 1.3 x 10-20 were used in calculation of face-face, edge-edge and
edge-face interaction forces, respectively. An inter-particle separation of h0 = 20 Å
was used in all calculations (Johnson et al., 1998).
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Figure 4 – The predicted kaolin edge-face, face-face and edge-edge
interaction forces (Johnson et al., 1998).
1.2
edege/face
H/Hmax
0.8
0.4
edege/edge
0.0
-0.4
face/face
-0.8
3
4
5
6
7
8
9
10
11
pH
At lowest pH, most particles are anticipated to interact in an edge-face manner.
However, interactions between faces will also be significant due to large ratio of the
face to edge surface area. Face-face association has previously been noted to drive
formation of lamellar structure in kaolin suspensions. As the pH rises, the attractive
face-face forces are expected to diminish in magnitude and the particles will be reorientated in order to allow further edge-face association to occur. It will be expected
an increasing in the inter-particle attraction.
The maximum in the edge-face interaction force occurs at pH 6. Above this pH
the edge-face attractive force diminishes and repulsion between the kaolin faces
increases, so a fast diminishing in the interaction (attractive) forces is expected.
Edge-edge association is predicted to increase, but will not represent a significant
force. At high pH, both the edge-face and edge-edge interaction forces approach to
zero. Under these circumstances, a completely dispersed suspension will result
(Johnson et al., 1998 e Nasser e James, 2006).
The viscosity of kaolin pulps decreased with the increasing of pH. This behavior
can be attributed to the reaction of deprotonation of aluminol group on edge of
kaolinite giving a complexed anion what contributes to the increase of the negative
charge, as showed the zeta potential (Figure 2) and consequently of the repulsive
forces among the particles and the best orientation of particle (both the edge-face
and edge-edge interaction forces approach zero). The effect of pH was stronger in
the kaolin pulps with 70% of solids.
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The viscosity of kaolin pulps decreased with the increase of deflocculant
concentration. This behavior can be attributed to chemisorption of sodium
hexametaphosphate on the edge of kaolinite (Al+3 - positive site). The chemisorption
produces an excess of negative charges in the particle surface and, increases the
repulsion forces among the particles, as showed by the values of zeta potential to the
clay particles (Figure 2). The effect of deflocculant concentration was stronger in the
kaolin pulps with 70% of solids. Cunha (2004) and Andreola et al. (2006) reported the
same results in their researches.
CONCLUSIONS
When the solid concentration changed from 50 to 70%, the viscosity of the
kaolin pulp increased, approximately, 10 times, so solid concentration is the most
important variable that influences the viscosity of suspensions. It can be attributed to
the decreasing of the layer of water among the particles, giving larger attrition and
interaction among them.
The increasing of deflocculant concentration and pH contributed to the
decreasing of viscosity of kaolin pulps. Theses effects were stronger in the
suspensions with 70% of solids, where the interaction forces is more intense than in
kaolin pulps with 50% of solid concentration.
The variation of deflocculant (sodium hexametaphosphate) concentration and
pH induced the production of an excess of negative charges in the particle surface
and, as a consequence, an increasing of the repulsion forces between them, as
showed by the values of zeta potential.
To obtain low viscosity of kaolin pulp is necessary: low solid concentration
(50%), high pH (10) and high deflocculant concentration (8 kg/t).
ACKNOWLEDGMENTS
The authors are grateful to CAPES (Brazilian Research Funding Agency) for the
financial support and to CETEM and EQ/UFRJ for the laboratorial facilities.
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