Thermostability of montmorillonitic clays

Overseas Foundry CHINA FOUNDRY
Vol.11 No.3 May 2014
Thermostability of montmorillonitic clays
*Petr Jelínek1, Stanisław M. Dobosz2, Jaroslav Beňo1, and Katarzyna Major-Gabryś2
1. VSB – Technical University of Ostrava, Faculty of Metallurgy and Material Engineering, Department of Metallurgy and Foundry Engineering,
Czech Republic;
2. AGH University of Science and Technology, Faculty of Foundry Engineering, Department of Moulding Materials, Mould Technology and
Foundry of Non-ferrous Metals, Al. Mickiewicza 30, 30-059 Krakow, Poland
Abstract: Bentonite is one of the most widespread used clays connected with various applications. In the
case of foundry technology, bentonite is primarily used as a binder for mold manufacture. Thermal stability
of bentonites is a natural property of clay minerals and it depends on the genesis, source and chemical
composition of the clay. This property is also closely connected to bentonite structure. According to DTA
analysis if only one peak of dehydroxylation is observed (about 600 ºC), the cis- isomerism of bentonite is
expected, while two peaks of de-hydroxylation (about 550 and 850 ºC) are expected in the trans- one. In this
overview, the bentonite structure, the water – bentonite interaction and the swelling behavior of bentonite in
connection with the general technological properties of bentonite molding mixture are summarized. Further,
various types of methods for determination of bentonite thermostability are discussed, including instrumental
analytical methods as well as methods that employ evaluation of various technological properties of bentonite
binders and/or bentonite molding mixtures.
Key words: bentonite; thermostability; binder; clay
CLC numbers: TG221+.1
Document code: A
B
entonite as a natural and abundant soil is widely
used in a large range of industrial applications.
The surface reactivity of the bentonite determines
their suitability for use in ceramics, cosmetics, nanocomposites, environmental protection, waste water
treatment, nuclear waste deposits or manufacture
of foundry molds and cores. The thermostability of
the bentonite, connected with the temperature of the
clay de-hydroxylation, is one of the most important
properties of this kind of foundry binder. The bentonite
behavior under high temperatures (thermostability) is
mainly connected to its structure and genesis.
There are many criteria and many divisions of core
and moulding sands. Dobosz, et al. [1] divides moulding
sands into four generations, depending on the type of
* Petr Jelínek
Male, born in 1937, Ph. D, Professor. He obtained his CSc. (Ph.D.) in 1972
and was named as a professor in 1990 for Faculty of Metallurgy and Material
Engineering (FMME) of VŠB - Technical University of Ostrava. He also obtained
Dr.h.c. in 2003 for Technical University of Košice, Slovakia. During the 1990 1996 he was the Dean of FMME. During the last 10 years he was serviced as
manager of many projects, mainly concerning various moulding sands and their
relation to casting´s quality. He is a member of Czech Foundry Society (CFS), and
a member of Specialist Committee of CFS for moulding materials.
Corresponding author: Katarzyna Major-Gabryś
E-mail: [email protected]
Received: 2013-09-16; Accepted: 2014-02-28
Article ID: 1672-6421(2014)03-201-07
binding materials:
- 1st generation molding sands - with clay as the
binding material;
- 2nd generation molding sands - with adhesive glue
as the binding material;
- 3rd generation molding sands - without binding
materials, known also as molding sands bonded by
physical factors;
- 4th generation molding sands - bonded with
biotechnological factors.
Further development of the technology for making
the core and molding sands is strictly limited by
rigorous requirements connected to protection of the
natural environment. Many processes, even though
proved effective in production, must be replaced with
more ecologically friendly solutions [1].
This paper is devoted to ecological 1st generation
molding sands with bentonites as clays.
As the most frequent criteria used in evaluating
the foundry bentonites quality the following ones are
given [2, 3]:
- Montmorillonite (MM) content
- Carbonates and Fe contents
- Thermostability
- Binding property (in compression and in wet
tensile strength).
In connection with introducing the green sand
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CHINA FOUNDRY Overseas
Vol.11 No.3 May 2014
system (GSS) the binder thermostability is coming forward
compared to other physical properties such as residual
strengths (formation of lumpiness), dispersion, etc.
Bentonites represent the montmorillonitic clay as a rule
with higher concentration of the montmorillonite mineral
than 70% to 75%, which means that it contains up to 30% of
other materials, alumino-silicates above all, feldspars, and
carbonates. Individual bentonite localities also differ with their
genesis [4]. Often the bentonites are not different in chemical
composition in principle, but their behavior is quite different.
Nowadays the required properties are achieved by mixing the
bentonites from different localities.
The thermostability of bentonite is connected with the
temperature of the clay de-hydroxylation, i.e. with liberating
the OH- groups from the octahedral network in the form of H2O
(g). The residual oxygen remains in the structure. At the same
time, the binding properties are gradually lost and the “burnout bentonite” is formed. The whole process is endothermic
and it can be well monitored with thermal analysis (DTA).
The thermostability and its testing methods have been
studied by Jelínek et al. [5-7]. In bentonites of mean quality, the
endothermic reactions are present in the temperature region of
450 to 550 °C. Bentonites with high thermostability (loss of
crystallic water, dehydroxylation, occurs in range from 700 up
to 750 °C) have often even two peaks at 500 ºC and 700 °C.
This state is explained as follows:
- Substitution of ions in montmorillonite octahedrons and
tetrahedrons (influence of Fe) [8]
- Defects in the lattice (vacancies)
- Difference of the size of montmorillonite particles [9].
The influence of Fe on de-hydroxylation is not yet
unambiguously explained. Low concentration of Fe (<8% [8]) results
in a low de-hydroxylation temperature; Fe rich bentonites
have higher de-hydroxylation temperatures [10] . On the
contrary, Grefhorst [11] and Kaplun [12] give an opinion that with
increasing Fe2O3 content, the de-hydroxylation temperature is
falling (in the range of 2% to14%) and in the presence of black
coal, the fall is even more intense.
In dependence of the dispersion ability, the bentonites are
divided into “hard” and “soft” ones. A considerable part of
bentonites with high thermostability is just the “hard” one that
require a long processing time in case of GSS.
In a simplified way the bentonites’ formation is divided into
two groups: (1) The bentonites of a volcanic origin from the
Cretaceous and Jurassic times, being formed by weathering
of volcanic tuffs in an alkaline medium under the influence of
pressures and temperatures. Bentonites of the volcanic origin
have a characteristic endothermic peak under 700 °C. (2) The
bentonites of a mineral origin. They have their peak under
500 to 600 °C [13, 14]. Getting acquainted with montmorillonite
structure, the water – clay interaction and the regularities
of swelling of clays are helpful in finding answers to some
questions of daily interest to all the manufacturers of bentonite
binders, but the foundry practice also.
1 Structure of montmorillonite
Smectites, alumino-silicates, among which the
montmorillonite (bentonites) are also included, have a typical
triple-layer structure marked 2:1 [two outer layers form a
network of tetrahedrons and between them a network of
octahedrons (Fig. 1)].
Tetrahedral
Na
Na
+
Na
+
OH
Octahedral
+
OH
Na
Na
d(001)
OH
+
eet
al sh et
e
hedr
tetra edral sh eet
octah edral sh
h
tetra
+
+
nH2O
Na
Na+
Na+
nH
O
nH2O
2
+
+
Na+
Na
Na
Tetrahedral
Interlayer
Tetrahedral
Fig. 1: Schematic of montmorillonite structure [14]
Generally the tetrahedrons can be described as [TO 4] mwhere the T represents the Si4+ and the Al3+, Fe3+ also, while
the octahedrons can be described as [MA6]n- where M can be
Al3+, Fe3+, Fe2+, Mg2+, Ca2+, Li+, and the A represents anions
such as O2- and OH-, and F- (Fig. 2).
In the tetrahedral networks not only [SiO4]4- can be present,
but also the [AlO4]5- or [FeO4]5- being formed by substitution.
Because of their different sizes in the network strain occurs.
Substitution of Al for Si does not exceed 50% and it is not
quite random. In smectites the tetrahedral network is always
202
Fig. 2: Schematic of octahedron (left) and tetrahedron
(right). Central cations are marked with a full ring,
anions with empty ones [15]
Overseas Foundry CHINA FOUNDRY
Vol.11 No.3 May 2014
closely connected with the network of octahedrons through the
plane of apical oxygen (Fig. 2).
The second constructional elements of a smectite are the
octahedral networks [MA6]n-. They not only share the peaks,
but also half of the edges (Fig. 3). The extent of substitutions in
the octahedrons is more extensive than that in the tetrahedrons.
The substitution of Al3+ for Fe3+ or of two cations Al3+ for three
Mg2+ can be complete.
Fig. 3: Octahedral network in atomic (left) and polyhedral (right) representation. Octahedral cations
are marked with full rings and anions with empty ones [15]
(b)
M1
M2
M1
M3
M2
M3
Fig. 4: Marking of positions of central cations of M1, M2
and M3 octahedrons in the octahedral network
according to occupation of positions of octahedral
anions: (a) Networks with 4 O2- atoms and 2 OH- (Cl- or
F-) groups in every octahedron; (b) Networks with 2 O2atoms and 4 OH- groups in every octahedron. The rings
mark the positions occupied by OH- groups [16]
If the triplet of neighboring octahedrons (M1, M2, M3) is
considered, then the octahedron around the M1 has the OHgroups in opposed peaks. The arrangement is as follows: one
of them belongs to the upper triplet and the second one to the
lower triplet of anions. The position of the OH- in case of the
octahedron around the M1 is designated as trans- and in case
of the M1 and M2 as cis- (Fig. 5).
This is a typical configuration of e.g. smectites. Then
Fig. 4 shows a configuration of so-called 1:1 layers typical
for kaolinites (4 OH - for 2 O-). Some authors have stated
that the montmorillonites with the cis- isomerism have dehydroxylation in the range of 650 to 700 °C [18], and that
ones with growing trans- content have it in the range of
500 to 550 °C [19, 20]. However, as shown in Fig. 4, we have
nothing to do with a mixture of both isomers.
2-
O
OHMetals (AI, Mg, Fe)
Fig. 5: Cis- and trans- positions of hydroxylic
ions in octahedron [17]
The probability of hydrogen jumping in the nearest OHgroup and thus forming H2O depends on the distance of the
neighboring OH- groups [21]. The shorter the distance is, the
lower thermal energy is needed for the liberation of both OHgroups.
The importance and differentiation of the effects of cis- and
trans- isomers was shown by Wolters [22] by combining the
DTA methods and a mass spectrometer (MS) (Figs. 6 and 7).
For the cis- isomer the DTA corresponds to one peak (at 678
°C), and for the trans- two peaks (at 555 and 855 °C) [17].
In di-octahedral networks that have one of three octahedrons
vacant and the remaining two occupied by Al3+, a non-uniform
force field is formed which predetermines to a considerable
extent the level of the octahedral network deformation (Fig.
8). The vacant octahedron is always the biggest one. Figure 8
0
DTA ( V)
(a)
cis-ocahedron
cis-octahedron
trans-octanhedron
trans-octahedron
-60
MS (A*1010)
The arrangement of octahedrons in the network forms the
upper and the lower plane of anions. The distance between
the planes is the height of the octahedral network. The central
positions of octahedrons can be occupied by different or the
same cations or they can remain free (vacant). Chemical
analyses of natural montmorillonitic clays show that the
mono-octahedral networks (in every triplet of neighboring
octahedrons there is the same central cation) occur seldom.
In octahedral networks besides O2- the OH- groups are also
present (Fig. 4.).
1.5
925℃
exo
871℃
678℃
147℃
146℃
680℃
0.5
200
400
600
800
1000
Fig. 6: DTA (a) and MS (b) analysis of smectite
with cis- isomerism [16]
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CHINA FOUNDRY Overseas
Vol.11 No.3 May 2014
DTA ( V)
0
exo
855℃
555℃
-60
105
151℃
MS (A*1010)
148℃
1.0
0.5
o
556℃ 646℃
200
400
600
800
(Ⅱc)
M1
M3
(Ⅱa)
M2
M3
(Ⅱb)
M3
M3
(Ⅱd)
M1
M2
M2
M2
M1
M3
(Ⅲa)
M1
(Ⅲb)
M1
Fig. 9: Electrons orbitals in a water molecule (water
dipole) around oxygen core = O and hydrogen
cores = H. The water molecule is electrically neutral
but charges of their components are irregularly
distributed as a result of different speeds of electrons
in differently long orbits [21]
M1
M3
(Ⅲc)
M2
M2
M1
M3
M2
M1
where: e is electron charge, μ is dipole moment of the water
molecule, and R is radius of the cation + radius of the adsorbed
water dipole.
It follows from equation 1 that the hydration force of
cations is indirectly proportional to the square of the cation
radius. This relationship is evident from Table 1. The highest
value of hydration value was found for the cation Li+ (120) as
compared with the Cs+ cation (13).
Table 1: Influence of size of interlayer cation on
hydration value [23]
M3
M2
Fig. 8: Types of octahedral networks according to relative
moisture of individual octahedrons around M1, M2
and M3 positions [16]
shows that the most frequent is the configuration (II.a) and that
above all the chemical composition of montmorillonitic clays
has considerable influence on the deformations of octahedral
networks.
2 Water - clay interaction
Smectites characteristically have considerable sorption
capacity that is influenced by the initial content of molecular
water. If it remains in the inter-layer then the clay rehydrates
easily. If exposed to higher temperatures the clay dehydrates
more quickly but its rehydration and sorption abilities are
considerably reduced.
Water leaks during de-hydroxylation (2OH → H2O + O) and
one oxygen remains in the structure. Molecules of adsorbed
water can be imagined as dipole molecules or as molecules of
tetrahedral arrangement. They suggest the structure of ice (Fig. 9).
The water molecules are adsorbed by positive corners
of H2O tetrahedrons facing towards oxygen surfaces of the
clay mineral. Negative corners of tetrahedrons of the H 2O
molecules are attracted by positive charges of inter-layer
cations. The hydration value of monovalent alkaline cations
grows with their hydration forces (P) according to Eq. 1 [23]:
-em
(1)
P=—
[erg]
R2
204
-
-
1000
Fig. 7: DTA (a) and MS (b) analysis of smectite
with the trans- isomerism [16]
(Ⅰ)
H
+
H
+
889℃
Li+
Na+
K+
Rb+
Cs+
Radius of the cation [Å]
0.78
0.98
1.33
1.49
1.63
Hydration value *)
120
66
17
14
13
If the montmorillonite contains the Na+ cation only, then
the water molecules attracted by the positive hydrogen
shell of the dipole towards the surface of the clay material
predominantly change the orientation on the hydrogen
bond (O – H). On the contrary, the Ca2+ in montmorillonite
inter-layers are bound more strongly as the rearrangement
axis between H2O and Ca2+ is changed to a stronger bond
of metal – oxygen (Ca 2+ – O). For higher dispersion of
Na + – montmorillonite in comparison with Ca 2+ –, the
decomposition of the clay packets (Fig. 10) and swelling of
clays are connected with it also.
3 Swelling of clays
Swelling of clays has two phases as follows:
(a) Internal crystalline swelling (intra-crystalline swelling)
(b) External swelling (osmotic)
During internal crystalline swelling, the structural layers
are gradually moving away (by expansion), but this process is
discontinuous (Fig. 11).
A configuration of water molecules similar to the ice
structure is achieved in the case of Ca2+ – montmorillonite
only after the sorption of the fourth layer of molecules in the
inter-layer space. Na+ – montmorillonite adsorbs more than 2
Overseas Foundry CHINA FOUNDRY
Vol.11 No.3 May 2014
20 μm
(a) Particles [24]
(b) Packet of particles [25]
Fig. 10: Dispersion of Ca2+, Na+ packets of montmorillonite
Na
Ca
Linear expansion (%)
70
60
50
40
30
20
14
10
0
0.3
montmorillonite surface. That is why the osmotic swelling of
the Na+ – montmorillonite is extraordinarily strong and it is
especially intensive when clay – water is already in the plastic
state. Crystalline swelling is continuously changed to osmotic
swelling but the expansion during water adsorption in the case
of Ca2+ – montmorillonite runs much more quickly than in case
of Na+ – montmorillonite.
Unfortunately, no close dependence between the swelling
degree of the montmorillonite (bentonite) and the binding
property has been confirmed (Fig. 12).
Green compression strength (N·cm-2)
80
12
0.4 0.5 0.6 0.7 0.8 0.9 1.0
Relative moisture
Fig. 11: Linear expansion (%) in dependence on relative
moisture (p/po). Na+ –, Ca2+ – of montmorillonite
from Wyoming [23]
times the amount of water under the influence of continuing
osmotic swelling and it enlarges its volume more than 4 times
in comparison with the Ca2+ – montmorillonite.
Osmotic swelling is based on the existence of a great
difference between the concentration of ions adsorbed to the
surface of the clay mineral (denser aggregation of cations) and
the concentration of the same ion in neighboring pore water
(thinner dispersion of cations).
The forces of attraction between two basal planes of
the neighboring triple layer are the rather weak van der
Waals forces and electrostatic forces. In the case of Na + –
montmorillonites, the osmotic swelling can even lead to the
total separation of structural layers (Fig. 10). The voluminous
growth of aggregates in the case of Ca2+ – montmorillonites
during water sorption is caused above all by intra-crystalline
swelling. Substantially higher voluminous growth of
aggregates of the Na+ – montmorillonite is caused largely by
the osmotic swelling. The Na+ cation supports the sorption
of thicker and denser layers of orientated water on the total
10
8
6
4
2
0
10
20
30
40
Swelling volume [mL·(2g)-1]
50
60
Fig. 12: Relationship of swelling volume and green
compression strength of bentonites [11]
On the contrary, there exists a close correlation between the
bentonite swelling property and wet tensile strength (Fig. 13).
Osmotic swelling of the Na+ – montmorillonite is especially
intensive in comparison with the Ca2+ – montmorillonite as the
clay – water system (in the condensation zone, WCZ) is in the
plastic state (“ions are flowing”). That is why the “natrified”
(soda-activated) bentonites are also of higher stability (higher
strength) when over moistened.
4 Methods to determine
thermostability of foundry
bentonites
Thermal stability, a natural property of clay minerals, depends
205
Wet tensile strength (KPa)
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CHINA FOUNDRY Overseas
Vol.11 No.3 May 2014
2.8
5 Conclusions
2.4
(a) The substance of thermostability is de-hydroxylation of
montmorillonite, loss of water liberated from the octahedral
network (2OH → H2O + O). The thermal energy needed for
de-hydroxylation depends on the hydrogen jumping in the
nearest OH- group, i.e. on the distance of both OH- groups.
If the montmorillonite is of the configuration of 2 OH- per
4 O2- octahedrons, then one OH- group belongs to the upper
and the second to the lower triplet of anions of the octahedral
network, and they can hold to one another the cis- or transpositions. But as a matter of fact, the question is the mixture
of both isomers. Above all the genesis of the montmorillonitic
clay determines which position is a predominant one. One
peak of de-hydroxylation (about 600 °C) is expected in the
cis- isomerism, two peaks of de-hydroxylation (about 550 and
850 °C) are expected in the trans- one. The second important
factors are deformations of the octahedral network and the
formation of vacancies. The third factor is the occupation of
the montmorillonite inter-basal planes with ions.
(b) Bentonites are distinguished by considerable sorption
capacity of water. It turns out that the rehydration and sorption
abilities are influenced by the initial content of molecular water
(an important condition for moisture of the return GSS).
(c) A no less important property is the swelling property of
bentonites that is a result of the water – clay interaction and
it leads to the dispersion of the clay packets typical for Na+
– montmorillonites (intra-crystalline and osmotic swelling)
while the Ca2+ – montmorillonite (intra-crystalline swelling)
is keeping the packets cohesion. Soda-activation admittedly
does not lead to substantial changes of binding properties but,
besides the growth of thermostability, it significantly influences
mechanical properties during over-moistening (the growth of
WCZ). Some experience in practice is leading to mixing of
both bentonite kinds (Ca+Na – bentonites) for the purpose of
the moisture stabilization.
(d) Practical results of measurements of thermostability
of foundry bentonites have proved the importance of the ion
exchange (Soda-activation). Besides the thermostability, the
dispersion ability and the binding property are also important.
(e) Nature has played a decisive role in the thermostability of
montmorillonites; therefore, to obtain the required properties
the mixing of bentonites from different localities is a possible
solution.
2.0
1.6
1.2
0.8
0.4
0
2
4
6
8
10 12 14
Swelling volume (mL)
16
18
20
22
Fig. 13: Dependence between swelling property of
bentonites and wet tensile strength [26]
on the genesis, source and chemical composition of the clay.
In practice there are many methods usually used for the
determination of the bentonite thermal stability. One group of
methods are based on evaluation of the mechanical strength
of the green sand system (compression strength, splitting
strength, wet tensile strength), after preheating the bentonite
to a defined temperature [27, 28]. The annealing temperature is
usually derived from DTA/TG analysis and it is selected to be
close to bentonite temperature of de-hydroxylation. Generally,
the range of annealing temperature is 500 to 650 °C with
different firing time (0 to 3 h) [11, 12, 27, and 28].
A very interesting way to consider the thermal stability
of bentonite is the evaluation of so-called temperature of
half strength (Halbwertemperatur; °C/50%) [27] . Thermal
stability is evaluated as a decrease in green compression
strength, splitting strength and wet tensile strength. Kvaša [28]
suggests a brand new technique. Bentonite mixture with 5%
preheated binder (550 °C/1 h) is prepared, and then a mixture
with the same composition, but with only pre-dried binder
is prepared. Further the content of active bentonite by MB
test (MB = methylene-blue test) is processed, and then the
thermal stability is evaluated as a ratio of MB consumption in
percentages.
Another way of evaluating the thermal stability was
described by Jelínek et al. [6-7] , which is based on the
reversal hydration of a preheated bentonite (up to 800 °C).
The preheated bentonites are subjected to the process of
rehydration under laboratory conditions (20 °C, 80% relative
humidity) for a period of 50 h and after this experiment, the
so-called coefficient of reversible hydration (R) is calculated
using eq. 2.
(2)
where d mR is loss of weight after re-hydration, d mD is
dehydration (in % of original sample).
Thermal stability of clay binders (bentonites) should also be
evaluated by using instrumental analytical methods, especially
DTA/TG analysis and/or X-ray diffraction analysis.
All tasks elaborated above are also important in the context
of re-bonded molding sands with bentonites [29, 30].
206
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