CORUNDUM, ARTIFICIAL. Artificial corundum (H. 9+; G. 3.95) is known by various trade-names such as alundum, aloxite, lionite, borolon and oxaluma. Probably the most important advance in the abrasive field was the development of artificial corundum or manufactured aluminous abrasive, which is essentially crystalline alumina (A1,03). After small scale experi ments by Henri Moissan and others, the early development of artificial corundum can be followed through patents granted to the following inventors : Ivan Werlein of France in 1894; Franz Hasslacher of Germany in 1896; C. B. Jacobs of New Jersey, U.S.A., in 19oo; and C. M. Hall of Niagara Falls, N.Y., U.S.A., in i9o1.
Artificial corundum is made by melting calcined bauxite (q.v.), with coke and iron borings, in an electric arc furnace at a tem perature of about 2,2oo° C. Hanging carbon or graphite electrodes are used to carry the electric current into the melt. The power used is about sookw. at oo volts, alternating current. Most of the furnaces are operated on a periodic basis and consist of a removable conical steel shell on a flat carbon lined car.
The shell is cooled and protected from the molten material by a spray of water which runs down the outside. The func tion of the coke in the mixture is to reduce or deoxidize the major portion of the impurities to the elementary form. The carbon is eliminated as carbon monoxide (CO) which burns to carbon dioxide (COO as it leaves the furnace. The reduced metals unite with the added iron to form a magnetic alloy called ferro silicon, which settles to the bottom of the melt, leaving the alumina in a purified condition. By close control of the mixture and furnace operation the composition of the alumina product is maintained as follows: alumina 95%, silica i•5%, iron oxide o.5% and titania 3%. In about 24 hours, when the furnace is full of melted material, the electrodes are withdrawn. A few hours later the shell can be lifted off and the 5-ton ingot set aside to cool. As the alumina solidifies it crystallizes into a solid structure of crystals of irregular shape. After cooling the material for at least a full week the ingot is broken up and cleaned. Some abrasive manufacturers make for special uses an artificial corun dum containing over 99% alumina by melting chemically purified alumina.
The lump abrasive is passed through jaw crushers, then through a series of rolls or some other type of crushing equipment until it has been reduced to the desired grain sizes. The grains are run over powerful electromagnetic machines which remove any of the metallic alloy left in the abrasive, and are then usually roasted ip a rotary cylinder of the cement kiln type in order to prepare better the grains for the various types of bonding. The product is then washed, dried and graded into upwards of 24 grain sizes ranging from 8 to 24o meshes per inch and even finer. This sift ing operation is done on a series of flat screens of silk or wire of the desired mesh agitated by eccentrics. The wire screens are ordinarily used for the coarser sizes only.
Artificial corundum is commonly reddish brown and translu cent, but the special pure varieties are white to pink. In addition to being very hard it is the strongest abrasive with the single exception of steel shot. It fractures in such a way that new and sharp cutting points are formed when the original points are dulled through use. These properties adapt it particularly to the working of materials of high tensile strength, such as the various steels, which is the largest and most important use for abrasives. The grains are used extensively in all kinds of grinding and polish ing wheels, coated paper, cloth and disks, and to a less extent as loose grains. The specific applications of these are far too numer ous to catalogue completely, but include automatic and hand grinding operations such as snagging steel castings, surface grind ing, cylindrical grinding, internal grinding and honing, tool and cutter-grinding, polishing articles such as ploughs and cutlery, and in preparing automobile bodies for painting. The loose grains are used to polish plate-glass.
See F. B. Jacobs, The Abrasive Handbook (1928) . (R. MAcD.)