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CARBORUNDUM
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CARBORUNDUM , a silicide of See also:carbon formed by the See also:action of carbon on See also:sand (See also:silica) at high temperatures, which on See also:account of its See also:great hardness is an important abrasive, and also has possible applications in the metallurgy of See also:iron and See also:steel. Its name was derived from carbon and See also:corundum (a See also:form of alumina), from a mistaken view as to its See also:composition. It was first obtained accidentally in 1891 by Acheson in See also:America, when he was experimenting with the electric See also:furnace in the See also:hope of producing artificial diamonds. The experiments were followed up in an incandescence furnace, which on a larger See also:scale is now employed for the See also:industrial manufacture of the product. A full description of the See also:process has been given in the Journ. See also:Soc. Chem. See also:Industry, 1897, vol. xvi. p. 863. The furnace is rectangular, about 16 ft. See also:long and 5 ft. wide by 5 ft. high, with massive See also:brick end walls 2 ft. thick, through which are built the carbon poles, consisting of bundles of 6o parallel 3-in. carbon rods, each 3 ft. in length, with a See also:copper See also:rod let into the See also:outer end to connect it with a copper cap, which in turn is connected with one of the terminals of the generating See also:dynamo. The spaces between the carbons of the electrode are packed tightly with See also:graphite. In preparing the furnace for use, transverse iron screens are placed temporarily across each end, the space between these and the end walls being rammed with See also:fine See also:coke, and that in the interior is filled to the level of the centre of the carbon poles with the See also:charge, consisting of 34 parts of coke, with 54 of sand, 10 of sawdust and 2 of See also:salt. A See also:longitudinal See also:trench is then formed in the See also:middle, and in this is arranged a cylindrical See also:pile of fragments of coke about z in. or more in See also:diameter, so that they form a core, about 21 in. in diameter, connecting the carbon poles in the end walls. Temporary See also:side walls are then built up, the iron screens are removed, and a further quantity of charge is heaped up about 3 ft. above the See also:top of the furnace. An alternating current of about 1700 amperes at 190 volts is now switched on; as the See also:mass becomes heated by the passage of the current the resistance diminishes, and the current is regulated until after about 2 See also:hours or less from starting it is maintained See also:constant at about 6000 amperes and 125 volts. Carbon monoxide is given off and See also:burns freely around the sides and top of the furnace, tinged yellow after a See also:time by the See also:sodium in the salt mixed with the charge. Meanwhile a shrinkage takes See also:place, which is made See also:good by the addition of a further quantity of charge until the operation is See also:complete, usually in about 36 hours from the commencement. The current is then switched off, and the side walls, after cooling for a See also:day, are taken down, the comparatively unaltered charge from the top is removed, and the products are carefully extracted. These consist of the inner carbon core, which at the temperature of the furnace will have been for the most See also:part converted into graphite, then a thin See also:black crust of graphite mixed with carborundum, next a layer of nearly pure crystallized carborundum about a See also:foot in thickness, then See also:grey amorphous See also:carbide of siliconmixed with increasing proportions of unaltered charge, and lastly, on the outside, the portion of the charge which had never reached the temperature necessary for reaction, and which is altered only by the intrusion of salt from the inner part of the furnace. See also:Special precautions are taken in making and breaking the intense current here used (amounting at the end to about 750 kilowatts, or l000 E.H.P.), a See also:water-regulator consisting of removable iron plates dipped in salt water being used for the purpose. In such a furnace as that above described the charge weighs about 14 tons, the yield of carborundum is about 3 tons, and the See also:expenditure of See also:energy about 3.9 kilowatt-hours (5.2 H.P.-hours) per See also:pound of finished product. The carborundum thus produced is crystalline, greenish, bluish or brownish in See also:colour, sometimes opaque, but often translucent, resisting the action of even the strongest acids, and the action of See also:air or of See also:sulphur at high temperatures. The crude product can therefore be treated with hot sulphuric See also:acid to purify it. In hardness it nearly equals the See also:diamond, and it is used for See also:tool-grinding in the form of vitrified wheels (mixed with powdered See also:porcelain and iron, pressed into shape and fired in a See also:kiln). Carborundum See also:paper, made like See also:emery paper, is now largely used in place of See also:garnet paper in See also:American See also:shoe factories, and finds a See also:market in other directions. The amorphous carbide, which was at first a See also:waste product, has been tried, it is reported, with success as a lining for steel furnaces, as it is said not to be affected by iron or iron See also:oxide at a See also: Additional information and CommentsThere are no comments yet for this article.
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