CHEMISTRY; ELECTROCHEMICAL INDUSTRIES.
Commercial Furnaces.— The carborundum furnace employs a continuous core of coke be tween the electrodes. This core of broken coke constitutes the resistance, and a high tempera ture is obtainable. The material for making the carborundum is finely divided coke, sand and sawdust. These being packed around the core, the current is turned on at about 200 volts pressure, and when the furnace heats this may be reduced to less than half the voltage. The coke core changes to graphite and the mixture into crystalline silicon carbide in from 30 to 36 hours. The great Acheson carborundum fur naces at Niagara Falls are over 15 feet long, and 42 feet cross section. For electrodes car bon rods two feet long and three inches diameter are grouped in bundles of 60. The resisting core of coke is nine feet long and two thick. About 1,000 horse power are required to run such a furnace, and the product is about one-quarter pound of carborundum per kilowatt hour.
There is a considerable by-product of graphite, and the principle of this furnace is made use of in the manufacture of graphite, for which there is an increasing demand.
For the manufacture of calcium carbide, furnaces with carbon electrodes are employed, or sometimes one of the electrodes is of metal. One electrode may be formed of a mixture of broken coke and lime in a crucible, while a car bon or a set of connected carbon rods may constitute the other electrode. Alternating cur rent is usually employed. An arc being formed, the lime begins to give off oxygen, which com bines with some of the carbon, forming carbon monoxide, which gasifies and passes away, leav ing the pure calcium to unite with the remain ing carbon as calcium carbide. A temperature of about 2,000° C. is desirable for the process, though calcium carbide can be formed at a considerably lower heat. The King furnace is described under ELECTROCHEMICAL INDUS