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Thermostability of montmorillonitic clays

* Petr JelínekMale, 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-GabryśE-mail: katmg@agh.edu.pl

Received: 2013-09-16; Accepted: 2014-02-28

*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

Bentonite 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, nano-composites, 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

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 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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Fig. 1: Schematic of montmorillonite structure [14]

Fig. 2: Schematic of octahedron (left) and tetrahedron (right). Central cations are marked with a full ring, anions with empty ones [15]

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 “burn-out 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 montmorilloniteS m e c t i t e s , a l u m i n o - s i l i c a t e s , a m o n g w h i c h t h e 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)].

Generally the tetrahedrons can be described as [TO4]m-

where 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

tetrahedral sheet

octahedral sheet

tetrahedral sheet

Na+

nH O2 Na+

Na+

nH O2 Na+

Na+

nH O2

Na+

Na+

d (00

1)

OHNa+

Na+

Na+

Na+

OHOH

Tetrahedral

Tetrahedral

Tetrahedral

Octahedral

Interlayer

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Fig. 3: Octahedral network in atomic (left) and polyhedral (right) representation. Octahedral cations are marked with full rings and anions with empty ones [15]

Fig. 5: Cis- and trans- positions of hydroxylic ions in octahedron [17]

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,

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.).

Fig. 6: DTA (a) and MS (b) analysis of smectite with cis- isomerism [16]

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 O2- atoms and 4 OH- groups in every octahedron. The rings mark the positions occupied by OH- groups [16]

M1

M2 M3

M1

M2 M3

(a) (b)

If the triplet of neighboring octahedrons (M1, M2, M3) is considered, then the octahedron around the M1 has the OH-

groups 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 de-hydroxylation 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.

0

-60

1 5.

0 5.

200 400 600 800 1000

147℃

146℃

680℃

871℃

678℃

925℃exo

DTA

(V)

MS

(A*1

0)

10

The probability of hydrogen jumping in the nearest OH- group 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 OH- groups.

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

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.

trans-octanhedron cis ocahedron-

O2-

OH-

Metals AI Mg Fe( , , )

trans-octahedron cis-octahedron

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Fig. 8: Types of octahedral networks according to relative moisture of individual octahedrons around M1, M2 and M3 positions [16]

P = — [erg]-emR2

Table 1: Influence of size of interlayer cation on hydration value [23]

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

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]

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).

H

105

o

H+

+

- -

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 interactionSmectites 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 H2O 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]:

(1)

Fig. 7: DTA (a) and MS (b) analysis of smectite with the trans- isomerism [16]

0

-60

1 0.

0 5.

200 400 600 800 1000

151℃

148℃

646℃

855℃

555℃

889℃exo

556℃

DTA

(V)�

MS

(A*1

0)

10

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 (Ca2+ – O). For higher dispersion of Na+ – montmorillonite in comparison with Ca2+ –, the decomposition of the clay packets (Fig. 10) and swelling of clays are connected with it also.

3 Swelling of claysSwelling 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

M1

M2M3

M1

M2M3

M1

M2M3 M3 M2

M1

M1

M3M2

M1

M3 M2

M1

M2M3

M1

M2M3 M2M3

(Ⅰ)

(Ⅱa)

(Ⅱb)

(Ⅱc)

(Ⅱd)

(Ⅲa)

(Ⅲb)

(Ⅲc)

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Fig. 10: Dispersion of Ca2+, Na+ packets of montmorillonite

Fig. 11: Linear expansion (%) in dependence on relative moisture (p/po). Na+ –, Ca2+ – of montmorillonite from Wyoming [23]

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).

(a) Particles [24] (b) Packet of particles [25]

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

NaCa

Line

arex

pans

ion

()

%

0 3. 0 4. 0 5. 0 6. 0 7. 0 8. 0 9. 1.00

10

20

30

50

40

60

70

80

Relative moisture

Fig. 12: Relationship of swelling volume and green compression strength of bentonites [11]

2

6

8

10

12

14

4

0 2010 30 40 50 60Swelling volume mL 2g) ][ -1·(

Gre

enco

mpr

essi

onst

reng

th(N

·cm

)-2

20 μm

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

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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 dmR is loss of weight after re-hydration, dmD 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].

5 Conclusions(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 trans- positions. 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.

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Wet

tens

ilest

reng

th(

)K

Pa

Swelling volume mL)(

2 4.

2 0.

16

1 2.

0 8.

0 4.0 2 4 6 8 10 12 14 18 20

1 6.

2 8.

22

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