Thermal behaviour of ground and unground acid leached vermiculite L. A. Perez-Maqueda • C. Maqueda • J. L. Perez-Rodriguez • J. Subrt • Z. Cerny • V. Balek Received: 29 December 2010 / Accepted: 9 March 2011 / Published online: 30 March 2011 Ó Akade ´miai Kiado ´, Budapest, Hungary 2011 Abstract Acid leaching of vermiculite is an interesting procedure to prepare high surface area porous silica. Thermal behaviour of unground and ground vermiculite leached with HCl solutions has been studied by TG, DTA, ETA and high temperature XRD. Important differences have been observed in the thermal behaviour of unground and ground vermiculite after the acid treatments. Thus, for the acid-treated unground vermiculite, dehydrated ver- miculite, enstatite and cristobalite were formed during the heating, while for the acid-treated ground vermiculite only iron oxides and cristobalite phases were observed. Struc- tural modifications due to acid treatment were responsible for changes in the transport properties determined by ETA for the vermiculite samples. Keywords Vermiculite  Grinding  Acid leaching  DTA  TG  EGA  Emanation thermal analysis Introduction Vermiculite is a layered mineral of practical importance as a raw material for technical ceramics, as sorbent for contaminants of the environment, as thermal insulator, etc. [1]. For many applications of natural vermiculite the particle size of the starting material has to be in the nanometer, submicron and micron size. Mechanical treatments like ultrasound and wet or dry grinding have been proposed for delaminating and reducing the particle size of the clay minerals. The mechanical treatment causes modifications of the clays such as in their surface area, structure, morphology, porosity and reactivity [2–9]. For vermiculite, it has been observed that ultrasound causes a considerable particle size decrease, reaching nanometer sizes, while keeping the initial structure [10]. Besides, grinding causes a decrease in particle size, as well as an intense structural degradation of vermic- ulite, resulting in a loss of the lamellar shape and in a pro- gressive amorphization, accompanied with the formation of hard agglomerates by cold-welding [11]. Selective leaching by acid has been used to prepare porous silica from various clay minerals, including ver- miculite [12–15]. The presence of iron in the vermiculite structure influences the value of the surface area obtained after the acid treatment [16]. Grinding of clay minerals significantly influences their leaching behaviour [17]. Ground vermiculite shows a considerable particle size reduction with increasing grinding times, which induces structure degradation. The increased surface area, increased porosity and structural defects produced by grinding of vermiculite influences its leaching behaviour [12]. A very high surface area, i.e. 720 m 2 g -1 , for the product prepared by acid leaching of the ground vermicu- lite has been recently reported [12]. The prepared sample contains amorphous phase (silica) and akaganeite (b-Fe- OOH). The presence of iron in the product obtained after acid treatment is important in the preparation of amorphous silica with a high surface area, which made possible to enlarge the field of the material application. L. A. Perez-Maqueda (&)  J. L. Perez-Rodriguez Instituto de Ciencia de Materiales de Sevilla (CSIC), Americo Vespucio 49, 41092 Sevilla, Spain e-mail: maqueda@cica.es C. Maqueda Instituto de Recursos Naturales y Agrobiologı ´a (CSIC), Apdo. 1052, 41080 Sevilla, Spain J. Subrt  Z. Cerny  V. Balek Institute of Inorganic Chemistry AV CR, v.v.i., 250 68, Husinec- Rez, Czech Republic 123 J Therm Anal Calorim (2012) 107:431–438 DOI 10.1007/s10973-011-1480-2