ELECTRIC FURNACES. These furnaces are devices for localizing the heat of an elec tric circuit and utilizing it. In the usual tech nical use of the term it signifies a device or receptacle in which a comparatively high tem perature is developed for the purpose of effect ing a chemical reaction or producing a change of state in the substance to be treated, such, for instance, as the reduction of an ore, the forma tion or disruption of a compound, or the fusion or volatilization of a metal or compound. tric furnaces comprise means for developing the necessary heat at the point or points desired, and for subjecting the material to be treated, technically known as the gcharge,z' to the influ ence of this heat. The several types or classes of electric furnace will be briefly described ac cording to the principles employed. The heat development in any given portion of a circuit is proportionate to the resistance offered to the passage of the current; hence those portions of the circuit external to the furnace proper are always composed of metals which conduct the current well, and generally of copper or alumi num, whereas the resistance of those portions of the circuit in which the heat is to be localized is relatively high. These latter portions of the circuit may consist of gases, in which case an arc is formed and the localization of the heat is extreme; of substances of high melting and boiling points, in a state of fusion, when an electrolytic effect, to be hereinafter more fully referred to, usually supervenes; or of solids, such as platinum and other difficultly fusible metals, carbon, graphite and carbonaceous mix tures, or such bodies as the oxides of the alka line earths which become conductive when heated. These three classes of furnace, wherein the heat is localized in a gas, a liquid and a solid, respectively, may be conveniently desig nated by the terms arc furnace, electrolytic furnace and incandescent furnace, although as will appear it is not always easy to apply one or another of these names to the actual construc tions. Although electrically developed heat is relatively costly, the fact that it can be locally applied, within the interior of the charge if so desired, is an important advantage, and the utilization of the heat is often so complete that its use represents a real economy. The heat lost is that which is carried from the furnace by the escaping products of the reaction, and that which is conveyed by radiation, convection or conduction, from the walls, the electrodes and other exposed portions of the structure. Inasmuch as the exposed surfaces of a furnace are roughly proportionate to the square of its dimensions, whereas its capacity varies as the cube, it is evident that, other things being equal, the larger the furnace the less will be the percentage of total heat which is lost and the greater will be the efficiency. This indicates the employment of large units. It is always pos sible to reduce the expenditure of electrical energy by making use of heat otherwise gen erated, such heat being employed for raising the temperature of the charge previous to its intro duction into the electrically heated zone, or for heating the exposed surfaces of the furnace structure in order to check conduction from within. Furnaces in which chemical reactions are conducted, as, for instance, those in which calcium carbide is produced, often yield gaseous products which are not only themselves very highly heated, but are capable, by combustion, of further heat development. It has frequently been proposed to utilize this heat by conducting such gases through or around the incoming charge or by burning them in flues surrounding the furnace, but the greater complexity of the structure and the difficulty of purifying the large volume of dust-laden gas constitute prac tical difficulties of a serious nature.
The Arc Furnace.— When an electric arc is formed in air between carbon terminals there is observed to be a definite limit to the length of arc which can be maintained with a given current strength; furthermore this limit, which at first increases almost in proportion to the current strength, increases very slowly as the current density reaches higher values. The
maximum length of the arc is therefore limited. The temperature of the carbon terminals may attain 3,500° C., at about which point, under atmospheric pressure, carbon volatilizes. The temperature of the incandescent gases of the arc is perhaps a thousand degrees higher. It follows that the arc furnace, in its simplest form, is adapted particularly for subjecting small charges to extremely high temperatures, and its value for experimental work is apparent. For use upon a commercial scale it is generally necessary so to distribute the heat from the arc that a comparatively large body of the charge may be acted upon at a given time. This result may be accomplished by establishing a plurality of arcs in adjacent portions of the charge, by exposing the charge to the heat radiated from one or several arcs not in contact with it, by causing the arc to move relatively to the charge, or by moving the charge through or past the arc. The temperature of that portion of a charge which is in immediate contact with an arc may be practically that of the arc itself and is uncontrolled; the operations for which this method of procedure is suitable and economical are relatively few. The high temperatures which the electric furnace is capable of produc ing have opened a new field to chemistry, but in order to insure the formation, in theoretical quantity and in a state of purity, of many com pounds, a careful regulation of the temperature is essential; for the highest attainable tempera tures are capable not only of giving rise to new combinations but also of breaking them down, resolving them into similar bodies or even into their elements. A single instance of the import ance of heat regulation may be offered: If a mixture of sand and coke be cautiously heated in an electric resistance furnace a partial reduc tion of the sand occurs, and a product contain ing silicon oxygen and carbon and known as •spoxicon* is formed; at a somewhat higher temperature the reduction is complete and there results an amorphous body having the essential composition of carbid of silicon and technically called stuff)); at somewhat higher tem perature ranges this amorphous body passes into the crystalline carbid of silicon acarborun dum,D a compound approximating in hardness the diamond itself ; and at still higher tempera tures, approximating those of the arc, this car borundum is broken down, its silicon escapes as a vapor, and its carbon remains in the form of graphite. The effect of high temperatures upon reactions is twofold: in the first place the velocity of the reaction is increased, so that chemical changes which at lower temperatures occur slowly or not at all take place rapidly or even with explosive violence; in the second place new conditions of equilibrium are estab lished, and the chemical elements, entering into that combination which, under the circum stances, is the most stable, sometimes give rise to compounds not before known. To produce a given result, however, it is usually necessary to work within definite temperature limits, and since the heat of the arc cannot well be con trolled, it is necessary to govern the tempera ture of the charge by limiting the duration of its exposure to this heat. As above pointed out, this may be accomplished by moving the arc through or near the charge or by moving the charge through or past the arc, the duration of contact being so adjusted to the quantity of charge and to its specific heat as to bring it to the desired temperature. As a rule, however, the arc as a source of heat is both wasteful and inefficient.