Clay Minerals, (2008) 43, 449–457
Sedimentation characteristics of two
commercial bentonites in aqueous
suspensions
S. AKTHER, J. HWANG
AND
H. LEE*
Division of Earth and Environmental Science Systems, Pusan National University, Busan 609-735, Korea
(Received 10 April 2007; revised 31 March 2008)
A B S T R A C T : The sedimentation characteristics of two commercial bentonites, Tixoton
(organically treated) and Montigel-F (untreated), were investigated using a 3% w/v clay suspension
at different concentrations (1, 3.5 and 10%) of NaCl and pH values (2, 7 and 12). Settling rates, floc
diameters and sediment volumes were derived from changes in light transmittance using a Turbiscan
Ma 2000 instrument.
Both bentonite suspensions were unstable (flocculated) in NaCl solutions. The settling rate
increased with increasing concentration of NaCl and was directly related to floc diameter. The
sediment volume reduced with increasing NaCl concentrations, a result of greater double layer
compression caused by increased ionic strength. At comparable salt concentrations, the organicallytreated bentonite (Tixoton) settled at a much slower rate and had a greater sedimentation volume.
The suspensions of both organically-treated and untreated bentonites were stable (dispersed) above
pH 7 and unstable in acidic conditions. The settling rate for Tixoton under acid conditions was much
smaller than that for the Montigel-F. Differences in sedimentation characteristics between the two
bentonite samples are probably due to the presence of an anionic polymer (carboxymethyl cellulose:
CMC) in Tixoton.
The viscosity of the bentonite suspensions was also studied. The viscosity of the clay suspension is
closely related to clay dispersivity in solution. The CMC was highly effective in increasing the
viscosity of the bentonite suspensions, but only under neutral and alkaline conditions.
KEYWORDS: bentonite, suspension, dispersion, flocculation, settling rate, sediment volume, NaCl, pH, drilling
fluid, viscosity.
Bentonite is mainly composed of montmorillonite
and is widely used in various industrial products
and processes such as paints, ceramics, pesticides,
pharmaceuticals, cosmetics, cement and drilling
fluids to modify the rheology and control the
stability of the systems. Providing adequate particle
dispersion in such applications is important.
The rheological properties of bentonite-water
systems are not yet fully understood. The complex
* E-mail: [email protected]
DOI: 10.1180/claymin.2008.043.3.09
behaviour is due to the anisometric clay particles
exposing different crystal faces which vary both in
charge and the magnitude of the surface potential
(Gungor, 2000). Recently, many studies have been
performed on the interaction of organic-inorganic
compounds with the bentonite surface by means of
rheological measurements. Some examples of these
organic-inorganic compounds are sodium chloride
(Akther et al., 2007), salts, gypsum, polyacrylamide, polyacrylic and sodium carboxymethyl
cellulose (CMC) (Erdogan & Demirci, 1996),
linear alkyl benzene-sulphonate and distearly
dimethyl ammonium chloride (Gungor, 2000),
# 2008 The Mineralogical Society
450
S. Akther et al.
TABLE 1. Mineralogical composition (wt.%) of the samples.
Sample name
Tixoton
Montigel-F
Montmorillonite
Quartz
Feldspar
Mica
Cristobalite
89.1
86.8
1.0
0.9
8.1
10.2
0.3
1.5
2.1
polyacrylamide (Heller & Keren, 2002; Hwang &
Dixon, 2000), electrolytes, polyvinyl pyrolidone and
linear alkyl benzene-sulphonate (Ece et al., 1999),
or tamarind gum and polyanionic cellulose (Mahto
& Sharma, 2004). The addition of polymers to a
bentonite suspension can act as a dispersant or as a
flocculant depending upon the polymer characteristics such as molecular weight, molecular structure,
concentration, electrical charge, degree of adsorption, conformation in the adsorbed layer, solvent,
and dielectric properties (Sato & Ruch, 1980).
Although the flocculation behaviour of bentonite
suspensions in different solutions has been determined, few studies have compared the effects of
organic substances on the settling behaviour of the
same bentonite suspensions under different electrolyte concentrations. This information is useful for
clay and modified-clay mineral processing.
Therefore, the present study was performed to
investigate the effects of an organic compound
(CMC) on the sedimentation characteristics of
bentonite suspensions and viscosity under different
concentrations of NaCl and at different pH values.
MATERIALS AND METHODS
Two commercial bentonites, Tixoton and MontigelF, were used for this study. The samples were
obtained from Süd-chemie Korea Co. Ltd. and used
without any further treatment. The dominant
mineral in these samples is Na-montmorillonite.
The bentonites were activated during the manufacturing process using Na2CO3. X-ray diffraction
(XRD) patterns of powder samples were obtained
with a Rigaku Geigerflex 2301 diffractometer using
Cu-Ka radiation at 30 kV and 15 mA using a
Ni-filter. The scan speed was 2º2y/min for range
2 40º. To investigate the mineralogical properties
of the bentonite, samples were heated at 500ºC for
2 h and ethylene glycol tests were carried out. The
mineral compositions were quantified by Siroquant
version 2.5. The mineral compositions of the
samples are indicated in Table 1.
According to the manufacturer’s information,
Tixoton contains 0.2% carboxymethyl cellulose
(CMC) with 4.1% Na2CO3 and 0.1% Mg(OH)2.
Thus, we considered Tixoton bentonite as an
organically-treated sample. Montigel-F, Na-activated bentonite, was untreated with polymers. The
physicochemical properties of the samples obtained
from Süd-Chemie Korea Co. Ltd are listed in
Table 2. The experiments were conducted with clay
suspensions made with 3 g of bentonite and 100 ml
of solution at different concentrations (1, 3.5 and
10%) of NaCl and at different pH values (2, 7 and
12). After the addition of dry bentonite powder to
solution, the sample was agitated via magnetic
stirring for 30 min to allow dispersion. After
30 min of rapid mixing, the suspensions were
stored for 8 h 15 min to achieve homogeneity.
And after a further 15 min of rapid mixing, 8 ml of
the suspension was extracted and measured for
settling rate and sediment volume.
The settling rate and sediment volume were
measured using a liquid dispersion optical characterization instrument (Turbiscan MA 2000). The
samples (~4.71 cm3) were carefully added into a
cylindrical glass cell. The reading head scans across
the height of the sample over time. It reports the
percentage of light either transmitted through the
sample (180º from the incident light, transmission
sensor) or back-scattered (45º from the incident
radiation, backscattering detector) from the sample.
TABLE 2. The physicochemical properties of the
samples obtained from Süd-Chemie Korea Co. Ltd.
Properties
Moisture contents (wt.%)
Outer sp. gravity (g/l)
pH
Swelling volume (ml/2g)
Particle size (residues on
0.075 mm sieve; wt.%)
Tixoton
Montigel-F
9.50
1.02
10.10
23.00
15.50
8.90
1.02
10.00
15.00
15.80
Sedimentation characteristics of commercial bentonites
The transmission percent was used to estimate the
settling rate of the samples.
Solution pH was adjusted with 0.5 M H2SO4 and
NaOH solution. The solution pH values of 2, 7 and
12 were selected because positive charges can only
develop in a protonation reaction of Al-OH sites at
edges below pH ~6.5, and deprotonation of the
Si-OH rather than Al-OH sites takes place with
increasing solution pH, resulting in negative
charges at edges (Tombacz & Szekeres, 2004).
The solutions of three different salinities (1, 3.5 and
10%) were prepared using NaCl.
The viscosity of homogeneous suspensions was
measured using a Brookfield digital (Model
RVTD-1) viscometer. For the purpose of this
study, we used spindle No. 1 and rotational speed
100 s 1.
RESULTS AND DISCUSSION
XRD analysis
The qualitative XRD analysis of bulk samples
indicates that the mineralogy is dominated by
montmorillonite, with quartz, feldspar, mica and
cristobalite minerals occurring as minor components
(Table 1). The XRD patterns of Montigel-F and
Tixoton are presented in Fig. 1, and the XRD
patterns of the samples peak at ~15 Å, corresponding to montmorillonite. The half width (d001)
of the samples was 1.55º2y for the Tixoton sample
and 0.95º2y for the Montigel-F sample, which
indicated that the montmorillonite particles of the
451
Tixoton sample are smaller than those of the
Montigel-F sample. After treatment with ethylene
glycol, the samples display a sharp d001 peak (Fig. 2)
which shifted towards 17 Å, behaviour typical of
montmorillonite. After heating at 500ºC (2 h), the
peak shifted back to 10 Å, and this confirmed the
presence of montmorillonite (Fig. 2). The XRD and
heat-treatment analyses of the samples suggested
that these bentonites mainly consist of montmorillonite, and showed no significant changes in
basal spacing after treatment with CMC. This is
probably due to the anionic nature of the CMC that
cannot be adsorbed onto the clay surface or in the
interlayer because of charge repulsion (Hwang &
Dixon, 2000). If polymer had adsorbed in the
interlayer space, it would have caused a change in
basal spacing.
Sedimentation behaviour analysis
The experimental solutions were classed as
‘neutral’, ‘salt’, ‘acid’ and ‘alkaline’ and the
samples classified as ‘organically treated’ and
‘untreated’. NaCl was chosen due to its iondissolving effects on organically-treated clay
relative to untreated clay. Acid and alkaline
solutions were used to identify the effects of
protonation and deprotonation reactions on the
clay samples. The CMC polymer-bearing clay was
used to collect data on the stability and settling
behaviours of organically-treated bentonite in
different solutions. The difference in behaviour of
these two bentonites may be correlated with their
FIG. 1. XRD patterns of Tixoton and Montigel-F (M: montmorillonite, C: cristobalite, Q: quartz, F: feldspar).
452
S. Akther et al.
FIG. 2. XRD patterns of the sample: (a) treated with ethylene glycol; and (b) after heating at 500ºC.
physicochemical properties (Table 2), as well as
montmorillonite contents (Table 1).
The results of the light-transmittance analysis of
Tixoton (organically-treated) and Montigel-F
(untreated) samples in neutral solution are given
in Fig. 3. No transmittance was observed for the
Montigel-F sample because the untreated sample
had large negative charges on the particle surfaces,
resulting in stable dispersion. The organicallytreated sample did not show any detectable settling
pattern until 40 min had elapsed. Before that time
the settling curves of the organically-treated sample
had 0% transmittance, which indicates that the clay
particles were well dispersed in the solution due to
strong charge repulsion. After 40 min, however, the
organically-treated sample showed a small change
in transmittance (<0.22%). This change was
FIG. 3. Variations in light transmittance showing the
sedimentation behaviour of bentonite suspensions
under neutral conditions.
probably due to a weak material separation which
occurs in the top portion of the suspension. We
observed that the top portion (~0.01 cm3 from
theoretical calculations) of the solution was clear
and had separated from the clay suspension. The
change might also have arisen due to the gel-like
dispersion. From a rheological point of view, this
behaviour can be defined as ‘thixotropy’ (Günister
et al., 2004; Güven, 1992).
The settling curves of Tixoton and Montigel-F
samples in the presence of NaCl are given in Fig. 4.
For this experiment, we used 1, 3.5 and 10% NaCl.
According to Abend & Lagaly (2000) and Luckham
& Rossi (1999), at small salt concentrations the
interaction is attractive between edges (+) and face
( ); at larger salt concentrations, it is attractive
between the faces too. In NaCl solutions, all the
bentonite suspensions settled with very similar
patterns (Fig. 4) but the organically-treated sample
showed slower settling curves than the untreated
sample. The average settling rates and diameters of
settled particles (flocs) from bentonite suspensions
were directly calculated from Turbiscan MA 2000
and are listed on Table 3. The settling rate tended to
decrease with increasing salt concentration and was
directly related with average floc diameter. The
settling rates increased with increasing floc diameter.
This relationship was clearly observed with untreated
bentonite in NaCl solutions. The variations in settling
rates and floc diameters with salt concentrations were
significant with untreated bentonite, but were
insignificant in the organically-treated samples. The
organically-treated bentonite showed a significantly
453
Sedimentation characteristics of commercial bentonites
FIG. 4. Variations in light transmittance showing the sedimentation behaviour of bentonite suspensions under
different salt concentrations.
smaller settling rate than the untreated bentonite in
all salt concentrations. This experimental result
suggests that the organically-treated bentonite may
be effectively applied as a better drilling fluid for salt
environments by controlling the CMC concentration
to retard the settling of bentonite suspension.
The volume of settled particles (sediment
volumes, Table 3) decreased as the salt concentration increased. This is probably due to more double
layer compression in solutions of greater salt
concentration. In NaCl solutions, the Tixoton
sample gave larger sediment volumes than
untreated Montigel-F (Table 3). This may be due
to the anionic CMC of Tixoton trapped in double
layers. It is considered that the trapped anionic
CMC results in greater particle-particle distances in
coagulated bentonite with charge repulsion. Another
possible reason for the increased sediment volume
is that other factors such as greater montmorillonite
contents and/or the greater swelling ability of the
Tixoton compared to that of Montigel-F (Tables 1
and 2) affected sediment volume.
The solution pH significantly affects the settling
of the bentonite suspensions. Clay minerals exhibit
two types of electric charges: permanent charges on
interlayer surface and variable charges on broken
edges. The permanent charges are structural charges
originating from isomorphic substitutions in the
octahedral and tetrahedral sheets and do not vary
with pH. The surfaces of broken edges have surface
TABLE 3. Sedimentation properties of bentonite suspensions.
Solutions
Distilled
water
1% salt
3.5% salt
10% salt
pH 2
pH 12
– Organically-treated bentonite (Tixoton) –
Average floc
Average setSediment vol.
diameter
tling rate
at 60 min
(mm)
(mm/min)
(ml)
— Untreated bentonite (Montigel-F) —
Average floc
Average setSediment vol.
diameter
tling rate
at 60 min
(mm)
(mm/min)
(ml)
0.00
0.00
0.00
0.00
0.00
0.00
1.82
1.05
0.43
1.85
0.00
0.25
0.26
0.22
0.11
0.00
2.26
2.35
2.34
1.54
0.00
1.02
0.56
0.26
1.45
0.00
1.06
1.00
0.84
0.98
0.00
4.86
5.39
5.71
3.28
0.00
454
S. Akther et al.
groups (Al-OH and Si-OH) that can be protonated or
deprotonated depending on the solution pH and can
have variable charges. The pH of the aqueous
solution can affect these two types of charges. The
high affinity of H+ ions can neutralize the permanent
negative charges and can reduce and/or prevent
formation of a dominant electric double layer on
surfaces. Chemical species (H+ and OH ) can be
provided to the surface protolytic reactions on edge
sites in which the pH-dependent hidden electric
double layer forms (Tombacz & Szekeres, 2004).
The settling curves of Tixoton and Montigel-F
samples in the presence of acid are given in
Fig. 5. At low pH, the samples showed a different
pattern in the settling curves. The untreated
bentonite (Montigel-F) suspension showed settling
with a clear solid-liquid interface within 15 min,
whereas the organically-treated bentonite (Tixoton)
suspension settled at a much slower rate (Fig. 5).
Since amphoteric sites are conditionally charged,
positive charges can develop at the edges by
attracting H+ ions transferred from the aqueous
phase (pH 2). For this reason, in acidic conditions,
the attraction between negatively-charged surfaces
and positively-charged edges resulted in rapid
sedimentation of the Montigel-F. It was shown that
edge-to-face contacts leading to card-house structures are only formed in acidic media and result in
flocculation (Permien & Lagaly, 1995). For Tixoton,
however, the CMC anions are attached on
positively-charged clay particles and prevent the
edge-face contacts of the card-house flocculation
(Ece et al., 1999). For this reason, slower settling
occurred. The CMC molecules could be tightly
bound to edges of clay particles, forming bridges
FIG. 5. Variations in light transmittance showing the
sedimentation behaviour of bentonite suspensions
under acid conditions.
between particles. As a result, the clay particles
bridged by CMC were finally flocculated by
increasing the sediment volume. Another possible
cause of increased sediment volume is that the
greater content of montmorillonite and/or greater
swelling capacity of Tixoton (Tables 1 and 2) led to
a greater sediment volume.
The broken edges of bentonite particles become
negatively-charged in alkaline solutions (Tombacz
& Szekeres, 2004). Therefore, the electrostatic
repulsion between the same charges on basal
surfaces and edges resulted in stable dispersion of
treated samples, as shown in Fig. 6. Organicallytreated bentonite maintained stable dispersion for
up to 40 min, but began to show small changes in
transmittance after that time (Fig. 6). This is
probably due to a weak material separation in the
upper-most suspension as observed in neutral
solution. For this reason, no detectable settling
rate and sediment volume were calculated from
transmittance of the treated sample (Table 3). In
conclusion, both untreated and treated bentonite
suspensions can maintain good dispersion states in
alkaline and neutral solutions.
Viscosity analysis
Viscosity of bentonite suspensions is an important factor when determining the effectiveness of
the bentonite in drilling fluids, paper coatings,
detergents, paint, pharmaceuticals and other industrial products. For oil drilling purposes, a drilling
fluid of at least 15 mPa s viscosity is needed to
clean the hole of rock fragments produced by the
drilling process.
FIG. 6. Variations in light transmittance showing the
sedimentation behaviour of bentonite suspensions
under alkaline conditions.
455
Sedimentation characteristics of commercial bentonites
The flow behaviour of any system is described in
terms of the relationship between the shear stress t
and the shear rate g_ .
The shear rate is defined as the change in shear
strain per unit time, and the shear stress as the
tangential force applied per unit area. The ratio of
shear stress t to shear rate g_ is called viscosity Z:
Z = t/g_
Hence, Z is a measure of a fluid’s resistance to
flow (Heller & Keren, 2002; Luckham & Rossi,
1999).
In general, the flow behaviour of clay suspensions depends on clay concentration, particle size,
shape and the strength of the interactions among
clay particles (Brandenburg & Lagaly, 1988; Chen
et al., 1990; Keren, 1988). When there is little or no
interaction between the clay particles in an aqueous
suspension, the flow of the suspension is Newtonian
in behaviour (the shear stress is proportional to the
shear rate). Conversely, when the clay particles
interact, the flow of the suspension is nonNewtonian (viscosity varies with shear rate)
(Heller & Keren, 2002; Luckham & Rossi, 1999).
Clay suspensions containing polymers behave as
pseudoplastic flows (Luckham & Rossi, 1999),
which may be described by the power-law/
Ostwald model (Luckham & Rossi, 1999;
Meunier, 2005):
t = Kg_ n
Other models have been considered in describing
the rheological behaviour of clay suspensions, such
as the Herschel-Bulkley equation:
t = t0 + Kg_ n
where K is a measure of the consistency of the fluid
and n the flow behaviour index, which is a measure
of the decrease of effective viscosity with shear rate
(Meunier, 2005). In light of this information,
Newtonian behaviour is expected from Montigel-F
in solutions with pH values 57 and non-Newtonian
behaviour is expected from Montigel-F in solutions
with a pH of 2. Consequently, pseudoplastic, nonNewtonian and Newtonian behaviours are expected
from Tixoton. By using anionic polymer, Heller &
Keren (2002) observed Newtonian, non-Newtonian
and pseudoplastic behaviours in clay suspensions.
The viscosity measurement of bentonite suspension revealed that the CMC-treated sample generally
showed greater viscosity than the untreated sample
in all solution-testing conditions. Caenn & Chillingar
(1996) and Heller & Keren (2002) indicated that
CMC is a viscosity modifier that can increase
viscosity of the clay suspension. However, the
effectiveness of CMC as a viscosity modifier
differs with various solution conditions (Table 4).
The viscosity of both untreated and CMC-treated
bentonite suspensions increased with increasing
solution pH (Table 4). Variation in the viscosity
of the clay suspension with solution pH is closely
related to the changes in the surface charge and
double-layer structure with pH of the solution
(Akther et al., 2007; Luckham & Rossi, 1999;
Permien & Lagaly, 1994; Brandenburg & Lagaly,
1988). In a suspension under the predominance of
the repulsive electrostatic forces particles tend to
take up positions as far from each other as possible
(Güven, 1992). This leads to a regular arrangement
of the particles and contributes to viscosity increase.
This effect was observed in high pH conditions,
especially in the CMC-treated bentonite suspension
(Table 4). Tixoton showed an appreciably larger
viscosity increase under neutral and alkaline
conditions. This result indicates that the anionic
CMC is highly effective in increasing the viscosity
of bentonite suspension at pH 57.
The viscosity of both untreated and CMC-treated
bentonite suspensions strongly decreased with
increasing NaCl concentrations (Table 4). This can
be explained by the electroviscous effect. Change in
a liquid’s viscosity induced by a strong electrostatic
field is referred to as the electroviscous effect
(Abend & Lagaly, 2000; Güven, 1992). Upon the
addition of NaCl, the double layers are compressed
and the range of double layer repulsion is reduced.
This causes a decrease of viscosity. Luckham &
Rossi (1999) also explained that a reduction in
viscosity is caused by edge-to-face (EF) type
associations in a solution of a very small NaCl
concentration owing to the opposite charge attracTABLE 4. Viscosity of the samples (mPa s).
Suspension
solutions
Distilled water
1% salt
3.5% salt
10% salt
pH 2
pH 12
Organically-treated
Tixoton
Untreated
Montigel-F
20.50
7.30
4.80
3.60
7.20
22.00
6.00
3.90
3.00
2.80
3.90
7.30
456
S. Akther et al.
tion. In this case the viscosity tends to decrease
further due to face-to-face (FF) associations as salt
concentrations increase. Our experimental result also
showed that the viscosity of the CMC-treated sample
was not significantly increased compared with the
untreated sample (Table 4). Even in 1% NaCl
solution, the CMC-treated bentonite did not have
appreciably high viscosity. This indicates that the
effectiveness of CMC as a viscosity modifier was
not significant in NaCl solutions.
From our experimental analyses for sedimentation characteristics and viscosity of bentonite
suspensions, it can be concluded that the viscosity
of clay suspension is closely related to clay
dispersivity in solution, and that a polymer is also
not very effective in increasing viscosity if the same
polymer is not very effective in increasing the clay
dispersivity.
CONCLUSIONS
In this study we investigated the sedimentation
characteristics of two commercial bentonites
according to the properties of solutions grouped as
‘neutral’, ‘salt’, ‘acid’ and ‘alkaline’. We can
conclude the following from our observations and
measurements.
(1) The XRD analyses of the samples suggested
that these bentonites mainly consist of montmorillonite and showed no significant changes in
basal spacing with treatment of organic substance
(CMC). This fact indicates that anionic CMC was
not intercalated into the interlayer space of
organically-treated bentonite.
(2) A light-scattering method was very effective
in analysing the sedimentation characteristics of the
clay suspensions by obtaining settling rate, floc
diameter and the sedimentation volume. The
sediment behaviours varied greatly with the solution
properties.
(3) In NaCl solutions, all bentonite suspensions
were unstable (flocculated). The settling rate
increased with increasing concentrations of NaCl.
The settling rate is directly related to average floc
diameter. The settling rates also increased with
increasing floc diameter, although the sedimentation
volume decreased with increasing NaCl concentrations. This is due to more double-layer compression
caused by increased ionic strength.
(4) At comparable salt concentrations, the settling
rate for CMC-treated Tixoton was appreciably
smaller than for Montigel-F. Tixoton showed
gre a t e r s e di m e nt vol um e tha n u nt re a t e d
Montigel-F. This is may be due to the anionic
CMC of Tixoton being trapped in double layers.
The trapped anionic CMC results in greater
particle-particle distance in flocculated bentonite
with charge repulsion.
(5) The dispersion and flocculation behaviours
were also strongly controlled by pH. The suspensions of both organically-treated and untreated
bentonites were stable (well dispersed) at pH 57
and unstable (flocculated) in acidic conditions. The
settling rate for organically-treated bentonite under
acid conditions was much smaller than for
Montigel-F.
(6) The viscosity of the clay suspension was
closely related with clay dispersivity in solution.
The CMC was very effective in increasing the
viscosity of bentonite-based drilling fluids only
under neutral and alkaline conditions. In acidic and
salt solutions, however, the CMC seemed to be
ineffective in obtaining an acceptable viscosity.
ACKNOWLEDGMENTS
We gratefully acknowledge the support from BK 21
Coastal Environmental Systems School (CESS). We
also thank Dr Robert D. Cody of Iowa State University,
for his advice and careful corrections which improved
this manuscript.
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