Microporous castable refractories in the Al2O3-MgO-CaO system for thermal insulation above 1200ºC

Abstract

Porous ceramics comprehend a wide range of applications that involve the generation, exchange and maintenance of heat at high temperatures, as in steelmaking, petrochemical and cement industries. Such good performance is explained by the combination the low thermal conductivity of the porous structures with high chemical refractoriness and chemical inertia of ceramic materials. For applications as thermal insulators working at temperature ranges above 1000°C for long periods, there are two characteristics of these materials that require further development: a) their low thermomechanical resistance (due to the high porosity) and difficult maintenance of porosity and low thermal conductivity levels for long periods. The combination of aluminum oxide (Al2O3) and aluminum hydroxide (Al(OH)3) can minimize those aspects because, whereas Al2O3 forms a high-strength dense matrix, Al(OH)3 domains generate micropore-rich regions. Therefore, the final structure shows above 70% porosity levels even after sintering at high temperatures (1500ºC), with compressive strength higher than 40 MPa and very low thermal conductivity (0.1 W / (m.K) at 1200ºC). For comparison purposes, conventional high-alumina insulation bricks show 3-4 W / (m.K) thermal conductivity levels at 800°C and 5-10 MPa compressive strength. The present Project will explore two aspects. Initially, it aims at improvements in the maintenance of porosity and thermal insulation capacity at high temperature using ceramic matrices and porogenic agents of higher resistance to densification during sintering. Magnesium aluminate or spinel (MgAl2O4) and calcium hexaluminate (CaAl12O19 or CA6) are the main candidates for the application, once they combine intrinsic difficulty in producing dense structures and high refractoriness (respectively, 2122ºC and 1833ºC). Solid ceramic solutions, as those compounds, show a more intense grain growth at high temperatures than densification due to their complex structure and crystals of highly asymmetric growth habit. Moreover, when they are generated in situ from Al2O3 and CaO or MgO, their formation is an expansive process, because of their lower density, which hampers densification. The candidate raw materials are: Al2O3 and pre-formed MgAl2O4 and CaAl12O19 for dense matrices, Al(OH)3, Mg(OH)2, MgCO3, Mg6Al2(CO3)(OH)16.4H2O, CaCO3 and CaOH)2 as porogenic agents and calcium aluminate cement and hydratable alumina as hydraulic binders. The second aspect concerns the use of aqueous suspension-based processing techniques for the production of monolithic porous structures. In comparison with pressing-based methods, the use of hydraulic binder for suspension molding enables a faster installation of high-volume structures and many design options. However, challenges related to the preparation of stable co-dispersions of high flowability and workability involve balance of Zeta potential levels and signals, interactions between ions, counterions and dispersants, and sensitivity to pH and temperature of the medium. Several dispersants based on poly(carboxylate-ether), poly(ethyleneglycol) and poly(ammonium acrylate) will be applied and rheological characterization techniques will be used to improve processing. After the physical, thermal and morphological characterization of the porous structures produced, the results will be compared with those of traditional insulators.