Powder Technology 189 (2009) 2–5
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Powder Technology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Short communication
Synthesis of nano-layered vermiculite of low density by thermal treatment
Y. El Mouzdahir a, A. Elmchaouri a,⁎, R. Mahboub a, A. Gil b, S.A. Korili b
a
b
Université Hassan II Mohammedia, Faculté des Sciences et Techniques, Laboratoire d'Electrochimie et Chimie Physique, BP 146, 20650 Mohammedia, Morocco
Department of Applied Chemistry, Los Acebos Building, Public University of Navarre, Campus of Arrosadia, E-31006 Pamplona, Spain
A R T I C L E
I N F O
Article history:
Received 5 December 2007
Received in revised form 4 June 2008
Accepted 21 June 2008
Available online 27 June 2008
Keywords:
Exfoliation
Thermal treatment
Vermiculite
A B S T R A C T
The present work consists in the study of the modification of a nano-layered vermiculite by thermal treatment up
to 900 °C. Changes in the structure and texture after thermal treatment were used for evaluation of dehydration
properties of the studied material. The dehydration properties of the clay are strongly affected by the crystal
structure.
The Differential Thermal Analysis (DTA) allows the determination of the specific temperatures at which phase
modifications take place, principally the ones attributed to the removal of the interlayer water molecules and the
formation of a series of less hydrated phases. Structural and textural studies were carried out using Scanning
Electron Microscopy (SEM) and X-ray Diffraction (XRD) analysis. The SEM micrographs reveal structural changes
of the sample, such as exfoliation phenomena and contraction of the vermiculite, related to the heating
temperature. These observations are confirmed by the XRD patterns, which demonstrate that the d-spacing of the
first basal diffraction varies depending on the applied heating temperature, this showing several states of
dehydration. As a complementary characterization, porosity analysis by Hg-porosimetry has also been carried out.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Vermiculite is a mica-type mineral used for insulation, in composite
cements, in horticulture and as a substitute for asbestos. It has been
exploited widely over the past 50 years or more and because of the low
particle size it is used as coating, lightweight additive, etc [1]. This
natural silicate mineral is usually formed by the hydrothermal alteration
of mica minerals such as biotite and phlogopite [2,3]. Although its
dimensions vary from microscopic particles of clay mineral to lustrous
brown sheets up to half a meter in size, the particle diameter is usually in
the range of 1 mm to 1 cm. Most vermiculites when heated quickly to
above 230 °C lose their interlayer water and this results in the flakes
exfoliating to form concertina-shaped granules. Being lightweight and
resistant to thermal decomposition, this exfoliated vermiculite is
valuable as an insulation material and filler, among its many other
uses [4–7]. Various methods have been proposed for delaminating and
reducing the particle size of vermiculites, such as sonication [8],
mechanical treatment and chemical process using hydrogen peroxide
[9]. These methods have been used to prepare nanometric vermiculite
particles [10,11]. Sonication produces delamination in the [00l] direction
and breaking of layers in the other crystallographic directions, while the
crystalline character is retained [11]. Muromtsev et al. [12] found that in
⁎ Corresponding author.
E-mail address: elmchaouri@hotmail.com (A. Elmchaouri).
0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.powtec.2008.06.013
the reaction between the vermiculite and a 30 % hydrogen peroxide
solution, the exfoliation is related to the separation of silicate layers with
oxygen formed by the decomposition of peroxide and also to the
disruption of the equilibrium between the layers and the interlayer
cations, due to vigorous release of hydroxide groups from the structure.
Upon heating quickly at elevated temperatures, the vermiculite
exfoliates and the bulk volume increases 8–12 times [13]. The expansion
is related to the separation of the layers due to the sudden release of
water; the highest expansion was shown by samples containing mica or
mica-vermiculite, which at lower temperatures produced thermal
effects compared to pure vermiculites [14,15]. The thickness of a single
layer of this silicate material is actually of nanometer size: it is known
that the platelets of a vermiculite type layered clay mineral can be
exfoliated to single layers that are 1 nm thick [16].
The objective of this work is to study the effect of thermal treatment
up to 900 °C on the structure and texture of Palabora vermiculite. Data
obtained by DTA, XRD, SEM and mercury porosimetry were used with
the aim of revealing the exfoliation mechanism.
1.1. Experimental
The vermiculite samples used in this work were supplied by Palabora
Mining Co., South Africa, in particles having an average size of 0.7×0.5 cm
and a thickness between 0.1 and 0.3 cm. The chemical composition was
determined by X-ray fluorescence analysis on a Philips PW 1480
spectrometer and the results are given in Table 1. The cation exchange
Y. El Mouzdahir et al. / Powder Technology 189 (2009) 2–5
3
Table 1
Chemical composition of the starting vermiculite
Oxides
SiO2
Al2O3
Fe2O3
MgO
K2O
P2O5
TiO2
Cr2O3
L.O.I
wt.%
47.05
10.73
5.02
26.91
2.5
0.3
0.6
0.01
11.81
capacity of the starting vermiculite was determined as described by Bain
and Smith [17] and was found to be 293 meq/100 g. The BET specific
surface area and the density were given by the supplier and were 5.3 m2/g
and 0.08 g/cm3, respectively.
The following structural formula was established from the raw
vermiculite chemical analysis:
ðSi3:26 Al0:74 ÞIV ðMg2:79 FeðIIIÞ0:14 K0:07 ÞVI Mg0:3 O10 ðOHÞ2 d 4:5H2 O
Mineralogical composition of the raw sample was determined by
XRD analysis on a Rigaku-Geiger Flex diffractometer, using Cu Kα
radiation. DTA experiments were performed in air flow on a Setaram
M4 thermal microanalyzer, by heating the samples from 25 to 900 °C
at a rate of 10 °C/min. SEM observations were carried out on a Philips
CM 200 microscope. The mercury porosimetry measurements were
performed by a Quantachrome type PoreMaster 60 porosimeter.
1.2. Results and discussion
The DTA curve of starting material is presented in Fig. 1, with four
peaks which indicate the thermal changes of the vermiculite.
The first endothermic peak corresponds to a large water loss at
about 110 °C, which is attributed to moisture present in the sample.
Another endothermic process takes place at 780 °C where dehydroxylation is substantially complete. The exothermic reaction at 740 °C is
probably due to the destruction of the silicate structure accompanying
the loss of the last hydroxyl water. An exothermic process at about
830 °C could be attributed to a re-crystallization as mullite. This
process distinguishes vermiculite from smectite for which the
reaction takes place at temperatures higher than 920 °C [18,19].
The XRD of the raw material shows the peaks characteristic of
vermiculite (Fig. 2a). Several peaks at 6.22°, 7.13° and 7.42°, indicate the
presence of various hydrated interlayer cations such as Mg2+and K+. A
large peak at 3.52° corresponds to some interstratification, while a peak
at 18.36° is due to a low amount of octahedral iron in this sample, found
by chemical analysis (Table 1). The thermal treatment of vermiculite
leads to the removal of the interlayer water molecules and to formation
of series of less hydrated phases [19–21]. In a study of the dehydration
and the rehydration of a Mg-vermiculite by thermoanalysis and in situ
XRD, the results confirmed the existence of a number of definite states of
hydration [22]. In this work, dehydration started at 25 °C from the 6.22°
state and a critical temperature was reached at 300 °C corresponding to a
basal spacing of d = 14.10 Å (6.27 ) (Fig. 2b). Further loss of water
Fig. 2. XRD patterns of vermiculite. a) Raw sample, b) treated at 300 °C, c) treated at
600 °C and d) treated at 900 °C.
Fig. 1. DTA curve of raw vermiculite.
molecules leads to the formation of the 8.89° phase at 600 °C.
Transformation of the 8.89° phase to the dehydration structure 9.07°
has been fully completed at 900 °C. The initial stage of dehydration may
be explained by removal of water molecules not in immediate contact
with the cation such as Mg in the normally hydrated structure [19]. The
end of this stage coincides with the displacement of the 6.22° phase to
4
Y. El Mouzdahir et al. / Powder Technology 189 (2009) 2–5
Fig. 3. SEM micrographs of vermiculite flake. a) Raw sample; b) heated to 300 °C; c) heated to 600 °C; d) heated to 900 °C.
6.27°. According to Weiss et al. [23] dehydration of the 6.16° phase leads
to the formation of the 6.43° unstable phase at 100 °C, followed by the
development of the intermediate 7.69° phase at 150 °C and the nonhydrated 9.86° talc-like phase when temperature increases to 550 °C
and 700 °C. Similar results are obtained with the materials of the present
study, where a 9.07° talc-like phase is identified after heating at 900 °C.
On the other hand, dehydroxylation of vermiculite after heating at
temperature higher than 300 °C causes a significant and irreversible
change also in morphology of individual flakes (particles). Indeed, the
initial stages of dehydroxylation are attended by a contraction of the
lattice with the development of diffraction characteristics like those of
biotite [24]. Also, interstratification was indicated, in the X-ray pattern of
the raw vermiculite, by a broad and weak reflection at about 3.52° and
remained after calcination at 600 °C.
SEM observations have showed that an abrupt heating of vermiculite
to about 300 °C induced exfoliation of the mineral, see Fig. 3. The
exfoliation does not take place upon heating to about 100 °C, a fact from
which it may be concluded that all interlayer water must be previously
removed. After heating up to 900 °C, not only exfoliation of flakes is
observed, but also a deformation of their surface due to spontaneous
escaping of the interlayer molecule water. This observation can also be
related with the possible formation of mullite at high temperature.
According to the results of the mercury porosimetry, presented in
Table 2, total porosity increased with increasing heating temperature up
to 600 °C, while it decreased for the sample heated at 900 °C, which is in
accordance of SEM images. A reason for this behaviour may be the
dealumination of clay sheets, with the extracted framework Al atoms
accumulating in the micropores and/or at the external surface of the
crystallites as extra-framework Al [25]. Another plausible explanation is
the formation of mullite which takes place upon treatment at high
temperatures (see Fig. 1). Mullite is in principle a more condensed phase
than the exfoliated vermiculite and its formation may lead to a decrease
in the pore volume.
The samples treated at high temperatures could be expected to show
considerable variation depending on lattice substitutions and exchangeable ions. No information is available concerning the expansion–
contraction of vermiculite as it is heated to elevated temperatures. The
temperature of fusion of the mineral and its vitrification range was
found to be 1330 °C.
2. Summary and conclusions
Table 2
Hg-porosimetry results of raw and treated vermiculite
Pore volume (cm3/g)
Raw sample
Treated at 300 °C
Treated at 600 °C
Treated at 900 °C
1.05
1.10
1.29
1.06
Thermal treatment of vermiculite was used in order to study the
exfoliation characteristics of this expansible clay sample as a result of the
heating temperature. The expansion was related to the separation of the
layers due to the water molecules release, which provokes significant
and irreversible changes in the morphology of individual particles. The
dehydration of vermiculite at temperature ranging from 300 °C to 600 °C
Y. El Mouzdahir et al. / Powder Technology 189 (2009) 2–5
caused the exfoliation of the clay flakes and the formation of a low bulk
density material of potential economic importance.
Acknowledgements
This work was supported by the CNRST-Morocco and the AECI-Spain
(A/2825/05) and (A/6525/06). S.A.K. acknowledges the financial support
from the Spanish Ministry of Education and Science through the Ramony-Cajal program.
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