Compositions: ceramic – Ceramic compositions – Refractory
Patent
1997-11-28
1999-01-12
Marcheschi, Michael
Compositions: ceramic
Ceramic compositions
Refractory
501127, 501128, 501129, 501130, 501153, C04B 3566, C04B 3563
Patent
active
058589007
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to refractory monolithic formulations comprising activated alumina and little or substantially no calcium aluminate cement.
Monolithic refractories have been used for many years in the steel industry and elsewhere. Generally they are produced using a blend of coarse refractory aggregate such as fused alumina, tabular alumina or calcined bauxite grains of graded size, a fine alumina, and a relatively small proportion of calcium aluminate cement. A fine silica is also commonly present in the blend, and this will react to form mullite, which can provide fired strength.
The blend is mixed with water to form a concrete-like slurry, cast in shape and/or position and allowed to cure and dry. Subsequently the refractory is fired.
Calcium aluminate cement provides sufficient green strength after curing (green strength) and after drying (dried strength) for the refractory to survive before the firing process.
To avoid the development of excess porosity during drying and firing, and to develop maximum density, the very minimum of water should be employed, and this can be achieved by careful size grading of the coarse refractory aggregate grains and other components so as to optimise particle packing. Ideally only enough water should be added for stoichiometric hydration of the calcium aluminate cement, but in practice a small additional amount may be needed to ensure good flow of the castable composition. This additional water is largely controlled by particle size distribution of the fine fraction of the castable blend, e.g. the calcium aluminate cement, fine alumina and fine silica.
However, in the presence of fine silica, calcium aluminate cement can react at the temperature of use to form phases with a low melting point, thereby leading to a reduction in hot strength. As a consequence, in recent formulations the calcium aluminate cement component has been reduced, the resulting blends being known as low cement castables (LCC) (about 5-10 weight percent calcium aluminate cement) or ultra-low cement castables (ULCC) (about 2 weight percent calcium aluminate cement).
The thermal dehydration (250.degree.-1150.degree. C.) of aluminium hydroxides results in the formation of activated alumina (Alumina Chemicals --Science and Technology Handbook, The American Ceramic Society, 1990, pp 93-108). Activated aluminas are produced from aluminium hydroxides by controlled heating to eliminate most of the water of constitution. Their crystal structure is chi, eta, gamma, and rho alumina in the low temperature range (250.degree.-900.degree. C.) and delta kappa and theta alumina in the high temperature range (900.degree.-1100.degree. C.). The various activated alumina types may be distinguished by X-ray diffraction, IR and .sup.27 Al NMR techniques (".sup.27 Al Nuclear Magnetic Resonance Spectroscopy Investigation of Thermal Transformation Sequences of Alumina Hydrates", R C T Slade et al, J Mater Chem, 1991, I(4), 563-568).
It is known that activated aluminas can be used to bind refractory formulations. U.S. Pat. No. 3,953,563 filed by Advanced Mineral Research discloses that activated alumina can be used as a binding agent in a refractory brick formulation where there is sufficient moisture in the refractory raw materials to convert the activated alumina to boehmite during the firing operation. The patent states that activated alumina, i.e. transition aluminas, contains different alumina phases, such as gamma alumina, theta alumina, and often some corundum, alpha alumina.
Patents by Nihon Tokoshurozai Kabushiki Kaisha company in Japan (British Patent Specification No. 2,024,195 and U.S. Pat. No. 4,331,773) refer to the use of rho alumina as a binder in a no cement castable (NCC) formulation, the U.S. specification also referring to a formulation containing glass powder. The body of the patent states that the rho alumina contains 60% rho alumina and some chi alumina and others. A paper by Yasuo Hongo entitled "Rho Alumina Bonded Castable Refractories", which describes the experimental work on a commer
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Azizian Farid
Wills Kevin John
Alcan International Limited
Marcheschi Michael
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