Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/878

 842 FURNACE height called the flue bridge D. A short diagonal flue or up-take E conveys the current of spent flame to the chimney &quot; FIG. 4. Longitudinal section of Reverberatory Furnace. F which is of square section diminishing by steps at two or three different heights, and provided at the top with FIG. 5. Reverberatory Furnace (horizontal section). a covering plate or damper G, which may be raised or lowered by a chain reaching to the ground, and serves for FIG. C. Reverberatory Furnace (elevation at Hue end). regulating the speed of the exhaust gases, and thereby the draught of air through the fire. Where several furnaces are connected with the same chimney stack, the damper takes the form of a sliding plate in the mouth of the con necting flue, so that the draught in one may be modified without affecting the others. The fire bridge is partially protected against the intense heat of the body of flame issuing through the fire arch by a passage to which the air has free access. The material to be melted is intro duced into the furnace from the hoppers HH through the charging holes in the roof. When melted the products separate on the bed (which is made of closely packed sand or other infusible substances), according to their density; the lighter earthy matters forming an upper layer of slag are drawn out by the slag hole K at the flue end into an iron waggon or bogie, while the metal subsides to the bot tom of the bed, and at the termination of the operation is run out by the tap hole L into moulds or granulated into water. The opposite opening M is the working door, through which the tool for stirring the charge is introduced. It is covered by a plate suspended to a lever, similar to that seen in the end elevation (fig. 6) in front of the slag hole. According to the purposes to which they are applied, reverberatory furnaces may be classed into two groups, namely, fusion or melting furnaces, and calcining or wasting furnaces, also called calciners. The former have a very extended application in many branches of industry, being used by both founders and smelters in the fusion of metals ; in the concentration of poor metallic compounds by fusion intoregulus; in the reduction of lead and tin ores; for refining copper and silver ; for making malleable iron by the puddling processes and welding ; and for the manufacture of carbonate from sulphate of sodium in chemical works, &c. Calcining furnaces have a less extended application, being chiefly employed in the conversion of metallic sulphides into oxides by continued exposure to the action of air at a temperature far below that of fusion, or into chlorides by roasting with chloride of sodium. As some of these sub stances (for example, sulphide of lead and copper pyrites) are readily fusible when first heated, but become more re fractory as part of the sulphur is dissipated and oxygen takes its place, it is important that the heat should be very carefully regulated at first, otherwise the mass may become clotted or fritted together, and the oxidizing effect of the air soon ceases unless the fritted masses be broken small again. This is generally done by making the bed of the furnace very long in proportion to its breadth and to the fire grate area, which may be the more easily done as a not inconsiderable amount of heat is given out during the oxida tion of the ore, such increased length being often obtained by placing two or even three working beds one above the other, and allowing the flame to pass over them in order from below upwards. Such calciners are used especially in roasting zinc blende into oxide of zinc, and in the con version of sulphides of copper into chlorides in the wet ex traction process, In some processes of lead smelting, where the minerals treated contain sand, the long calciner is pro vided with a melting bottom close to the fire-place, so that the desulphurized ore leaves the furnace as a glassy slag or silicate, which is subsequently reduced to the metallic state by fusion with fluxes in blast furnaces. Muffle Furnaces. A third class of furnaces are so ar ranged that the work is done by indirect heating; that is, the material under treatment, whether subjected to calcina tion, fusion, or any other process, is not brought in contact either with fuel or flame, but is raised to the proper tem perature by exposure in a chamber heated externally by the products of combustion. These are known as muffle or chamber furnaces; and by supposing the crucibles or retorts to represent similar chambers of only temporary duration, tho ordinary pot melting air furnaces, and those for the reduc tion of zinc ores or the manufacture of coal gas, may be in-