Elaboration and Characterization of Mullite Refractory Products from Moroccan Andalusite
Enrichment of schist mined in Morocco resulted in an extract essentially made of andalusite, quartz and muscovite, which was investigated with the aim to synthesize mullite refractory materials. X-ray diffraction analysis was used to assess phase transformations upon heating at different temperatures. Mullite formation starts as early as 1200℃ and seems almost total at 1450℃. This process correlates with a weak shrinkage (3.5% at 1450℃) and the formation of an important glass phase. Green compacts obtained by unidirectional pressing the extract powder were heated under air in the temperature range 1200℃ - 1600℃. Chemical and mechanical properties of these materials were investigated and correlated with their microstructure.
Thermal Behaviour and Structural Transformations of Andalusite-Rich Aluminous Nodules from Morocco
Mullite is the only stable phase of the Al2O3-SiO2 system. It is credited with a high technological interest in the ceramic refractories domain due to its excellent thermal properties, chemical inertia and mechanical behaviour at high temperature. Mullite can be synthesized by different ways, and also by the thermal transformation of natural aluminosilicates, namely the andalusite. A physico-chemical characterization of scattered andalusite crystals has been achieved The thermal behaviour and the different phase transformations have been investigated for temperatures up to 1450 °C. Introduction The salient properties of refractory materials include the capacity to withstand high temperatures and stresses, to remain unreactive and inert when exposed to severe environments. In the industry, a particular silica-alumina compound, the mullite (3Al2O3,2SiO2), is often used. Due to its rarety and because of its excellent intrinsic properties, several methods of synthesis of the mullite have been developed. The synthesis can be achieved from the Al2O3 and SiO2 oxides (ceramic and sol-gel route)[1,2] or by calcination at high temperature of some natural hydrate minerals (kaolinite Al2(Si2O5)(OH)4) [3] or anhydrous minerals (andalusite 3(Al2O3,SiO2)) [4,5]. It is on that point that we shall focus our contribution herein. Indeed, the andalusite is unstable at high temperature and decomposes into mullite and amorphous silica according to the following reaction : 3 Al2O3.SiO2 3Al2O3.2SiO2 + SiO2 Geological setting The andalusite deposit under concern is localized in the region of Sidi Bou Athmane (province of Kalaât Sraghna), in close connection with the Jebilet Palaeozoic massif in the Moroccan western Meseta (Fig. 1). The andalusite appears as nodules associated with biotite-rich schists (Fig. 2) that form a contact metamorphism halo issued from the formation of granite-like rocks [6]. Figure 1 . Geographical and geological locations of the andalusite deposit in the Jebilet Palaeozoic massif. MC : Central Massif; J : Jebilet ; R : Rehamna . In black: the granitic massifs ; M : Ment ; O : Oulmes ; Z : Zaers ; S : Sebt Brikine. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 1815-1818 doi:10.4028/www.scientific.net/KEM.264-268.1815 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,18:14:10)
Dense mullite from attrition milled kyanite and α-alumina
Effects of attrition milling on the decomposition of kyanite (Al2SiO5) and its reaction with α-alumina (Al2O3) to form stoichiometric mullite (3Al2O3·2SiO2) are reported. Kyanite-alumina mixes were attrition milled for times from 1 h to 12 h. With increasing milling time, the kyanite decomposition was accelerated and secondary mullite formation from the reaction between the rejected silica and the added alumina was enhanced. Milling reduced the decomposition expansion from +15.0% to +0.1% and the final contraction or densification from +2.5% to −13.7%. During firing complete mullite formation occurred after firing at only 1623 K/1h (1350C/1 h) to produce a dense mullite ceramic with a fine grain size (~1 μm).