Powder Technology 189 (2009) 2–5 Contents lists available at ScienceDirect 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. References [1] J. Addison, Vermiculite: a review of the mineralogy and health effects of vermiculite exploitation, Reg. Toxicol. Pharm. 21 (1995) 397. [2] G.W. Brindley, G. Brown, Crystal structure of clay minerals and their X-ray identification, Mineralogical Society Monograph 5 (1980). [3] P.W. Harben, Industrial Minerals Geology and World Deposits, Metal Bulletin Plc, London, 1990. [4] J.R. Hindman, Vermiculite, Industrial Minerals and Rocks, SME, 1994, pp. 1103–1111. [5] J. Martins, R. 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