GB776435A - Method of manufacture of aluminous abrasives - Google Patents

Abstract

Alumina abrasives containing not less than 95 per cent alumina, Al2O3, are prepared by melting alumina or aluminous substances containing a high proportion of alumina in an electric furnace in which the current is conducted by a thin layer of fused material in the liquid state supported on a surface of solidified material, the liquid layer being maintained in electrical contact with the furnace electrodes by fusion of added material, material from the bottom of the liquid layer being caused to cool and solidify and the depth of the liquid layer being kept substantially constant by relative movement vertically between the electrodes and the bottom of the furnace. The alumina employed may be mixed with small amounts of oxides of iron, vanadium, beryllium, manganese, molybdenum, zirconium, titanium, boron and silicon and also chromium oxide or carbide, and carbon. The latter may be provided by the use of graphite or carbon electrodes. The electric furnace employed may comprise an insulated shell having a vertically displaceable floor and adjustably mounted electrodes which may be vertical or inclined to the horizontal. In operation the furnace floor is raised to its uppermost position and the furnace charged with alumina, the electrodes are adjusted to be in contact with the charge and close together and the current therebetween is initiated by loose pieces of coke or graphite which conduct the current until the mass commences to fuse, when the conducting layer of liquid is formed. Further quantities of charge are then added which cover the electrodes, and as the area of the molten conductive layer grows, the distance between the electrodes is increased. The floor of the furnace is then lowered and the fused material at the bottom of the charge begins to cool and solidify thereby building up a solid mass from the floor of the furnace. The thickness of the liquid layer which conducts the current may be from a half to one and a half inches. If desired the furnace may be rotated slowly to produce a fused mass of circular configuration, and the movements of the electrodes and the floor of the furnace may be controlled automatically. According to an example a 100 kw. furnace with a 3 phase transformer giving an e.m.f. of 110 was employed, the full load of the current being 500 amps. The load was maintained substantially constant by withdrawing the electrodes from each other as the operation commenced, to a maximum separation of 25 to 30 inches. Under these conditions the consumption may be as low as 1,200 kwh. per ton of abrasive produced. Specific abrasive compositions referred to contain (a) 1 1/2 to 3 per cent. chromium oxide, (b) 0.01 to 1 per cent. of zirconium oxide, and (c) 0.01 to 1 per cent. of chromium oxide with 0.01 to 1 per cent. of titanium oxide. The alumina abrasive may be employed in the manufacture of grinding wheels or segments or abrasive coated articles or powders. Centreless, internal, external and form grinding wheels of vitrified bond may contain above 53 per cent. by volume of abrasive per wheel; for centreless wheels and for external and internal grinding at least 57 per cent is suitable; for form grinding as much as 61 per cent. volume abrasive per wheel may be employed.