Abstract: The invention relates to an abrasive grain containing electro-fused alumina, formed of crystals of hexagonal structure less than 100 &mgr;m in size, preferably less than 30 &mgr;m, and further preferably less than 5 &mgr;m, having a density that is greater than 97%, preferably 98%, of the theoretical density of alumina and a Knoop hardness of more than 2000.

Abstract: The subject of the invention is ceramic fibers for the reinforcement of refractory materials, being constituted by sintered bauxite fibers with a length/diameter greater than 10. Preferably, they have a cylindrical or prismatic shape truncated by two surfaces, for example, substantially planar surfaces perpendicular to the axis of the prism. They enable the mechanical strength and the lifetime of the refractory products to be significantly increased.

Abstract: The invention relates to an abrasive grain based on electromelted alumina consisting of crystals with a hexagonal structure, of size less than 100 &mgr;m, and preferably less than 30 &mgr;m, and more preferably less than 5 &mgr;m, with density higher than 97%, preferably 98%, of the theoretical density of alumina, and a Knoop hardness higher than 2000. The invention also relates to a method for making abrasive grains based on alumina consisting in melting alumina, casting it at a constant flow rate less than 80 kg/mn, and cooling it by dispersing the melted alumina in fine droplets to obtain articles with size less than 1 mm. The dispersion is preferably performed by ultrasound-assisted spraying, at a frequency ranging between 15 and 50 MHz. The inventive grains are used in particular for making grindstones.

Abstract: Alumina-based abrasive grains having a skin constituted by a mixture of finely dispersed aluminum hydrate and sodium silicate, and a process for the production of such grains consisting of (a) preparing a gel by mixing a solution of sodium silicate and a colloidal solution of aluminum hydrate in acid medium, (b) coating the abrasive grains with the gel in an amount of 0.1 to 5 percent weight, and (c) subjecting the coated grains to a drying treatment at a temperature of between 100 and 500.degree. C. The grains according to the invention are used for the manufacture of cloths and abrasive paper and also grinding wheels.

Abstract: The invention relates to a process for the preparation, by nitriding, of a refractory powder containing more than 40% of silicon nitride from spent contact masses issuing from chlorosilane synthesis. The process involves mixing with the copper-free spent mass, silicon or a silicon-rich alloy and optionally a refractory compound to adjust the non-oxidised silicon content to a predetermined value >30%, heating the mixture in a nitrogen atmosphere to trigger the exothermic nitriding reaction, then reducing the product obtained to powder. The refractory powder thus obtained can be mixed with an organic binder to produce taphole blocking masses for blast furnaces or electric furnaces for pyrometallurgy.