CARBORUNDUM, a trade name for silicon carbide (SiC). This compound is produced by heating sand and carbon together in the electric furnace. It is characterized by extreme hardness, and its principal use is for abrasive purposes, as a substitute for corundum and emery. It was discovered in 1891 by E. G. Acheson, and is now manufactured in large quantities in the great electric laboratories at Niagara Falls. The operation is carried on in a furnace in which the bed and ends are per manent, and the side walls temporary, being made up of loose firebrick for each charge treated. The furnace is 16 feet long and S feet wide. The electrodes enter the interior through the ends. They consist of clusters of carbon rods, interspersed with copper connec tions. The material used consists of 542 parts of sand, mixed with 9.9 parts of sawdust and 1.7 parts of common salt — the last acting as a flux. The amount of each charge is about 30,000 pounds, and this weight includes 342 parts of coke broken into pieces about the size of pea coal. A part of the charge is spread on the bed of the furnace so as not to touch the electrodes but up to their level. In the centre of this is built a core of coke connecting the electrodes. The remainder of the charge is then heaped upon the core, the walls being built up to a height of five feet, and the heap between them reaching eight feet. The current used is at the beginning of the process 165 volts and 1,700 amperes. Later this is reduced to 125 volts but increased to 6,000 amperes. The run continues for 36 hours, during which 1,000 horsepower is expended. The furnace is then allowed to cool for two days, when the walls are torn down and the contents removed. The
coke core has been graphitized by the heat, and outside of this is a layer a foot thick of gra phitic carbon. Next to this layer is the cry:v. tallized carborundum, amounting to about 6,700 pounds, and in addition • there is about 5,000 pounds of amorphous silicon carbide. The carborundum is broken up in a crusher and the crushed material digested with ml phuric acid for three days at a temperature of 212° F. It is then washed, and after being kiln-dried it is graded by screens to the sev eral degrees of fineness in which it is sold. Carborundum in powdered form is placed on the market in considerable quantities as car bide of silicon for the introduction of silicon into iron, the material being very readily dis solved by the fused metal. Since carborundum is infusible and is only oxidizable at extremely high temperatures in a large amount of free oxygen, it follows that the temperatures orcfi narily generated for smelting ores and metals are much below its point of destruction. Finely powdered carborundum is made up into a highly refractory paste with fire clay, lime and sodium silicate, which is applied by means of a .brush or otherwise to bricks which are intended to be used for building a furnace, or the bricks are actually immersed in the viscous liquid for a certain time. If the furnace has already been built, the paste can be painted on the sur faces exposed to the fire. It is stated that a coating one-twelfth of an inch thick will pro tect the bricks from the attack of the highest temperature that is ever produced by combus tion methods in ordinary work.