The mechanical hearth-furnaces are hearth-furnaces with mechanical devices for raking or stirring the charge. They have circular hearths, rotating under fixed rakes; or fixed hearths, either circular or rectangular, and movable rakes.
The cylindrical roasting-furnaces are cast-iron cylinders. lined with fire-brick,' through which the flame draws from a stationary fire-box at one end to the flue and dust-chamber at the other. The charge is stirred, so that all its parts are subjected to the action of the flame, by the rotation of the cylinder. The Bruckner, Douglas, White, and liowell-White furnaces are types of this class.
The cost of roasting varies with the character of the ore, the kind of furnace, and the cost of fuel and labor. The lead-smelters at Denver, Col., roast ore in reverberatory furnaces at an average cost of $2 per ton. With a mechanical hearth-furnace at the Ilaile mine, North Carolina, pyrites concentrates are roasted preparatory to chlorination at a cost of *2.621 per ton. Under favorable circumstances, pyrites concentrates have been roasted in the 'West, even where labor and fuel is high, for as low as $1 per ton.
KILNS,—The ordinary type of roasting-kil? is too well known to require description. They are, obviously, used in roasting coarsely broken ()ITS only. A modification of the com mon kiln which is in general use for calcining iron-ores may be termed shaft-kilns, working upon the same principle as shaft-furnaces—m. e., the ore hieing desulphurized while descending through a rising current of flames, but, as in the kilns, the ore is in coarse lumps and is made to descend slowly rather than in a shower of fine ore as in the shaft-furnaces. The Gjers kiln and the Davis-Colby roaster are furnaces of this class.
The (ijpr8 kit n. extensively used in calcining iron-ores, is a circular shaft-furnace built of fire-brick eased with malleable iron plates. The bottom of the brick-work rests in a cast-iron ring. mid the whole is supported by cast-iron pillars about 21 ft. high, leaving a clear space between the bottom of the kiln and the floor. The latter is covered by iron plates, in the center of which is fixed it east-iron cone 8 ft. in diameter at the base and S ft. high, extend
ing up within the shaft. Around the lowest tier of plates incasing the kiln are openings which ire usually closed by doors, but which serve foradmission of air or tools in case the ore becomes sintered. The ore, mixed with a 'limper proportion of coal, is fed into the furnace at the top, which is surrounded by a gallery for the workmen. The roasted ore descending, is caused by the interior cone to pass outward at the bottom of the furnace. These furnaces are usually 33 ft. in height ; at the base they are 18 n. in diameter, widening to 24 ft. 10 ft. higher up. The upper purl of the kiln is cylindrical, and 24 ft. in diameter. A kiln of this size has a capacity of about 8,000 cu. ft., and calcines about 115 tons of iron-ore per 24 hours, the con sumption of mai amounting to 1 ton for 23 tons of ore.
Thr Davix-Colby Ore-Boimler, which is also much used for desul ph uri zing iron-ores, consists of a circular hollow shaft with walls about 2 ft. in thickness, in which are located lire melte: fed with gas. \Odell gas may be taken from any source—gas-producers, nal mat wells. or t he waste pipes of blast-furnaces. The gas.-Inains enter dues built in the masonry directly over the time arches, and the guts is drawn through o tenings left in the top or hot torn into t he arches, where it takes air and is consumed—the resulting flames being drawn directly into and through the ore. In the center of the kiln there is a smaller hollow shaft, starling from the bottom and extending up through the entire portion of the kiln and terminating in time draft-stack being, in fact, the draft-stack itself, In the walls of this central shaft, and opposite the fire arches, are a series of openings through which the products of combustion are drawn directly into the stack and discharged so that the heat from the burning gases is across a nar row body of ore instead of up through the overlying mass, and the liberated sulphur allowed to pass off directly. There may be any number of rows of fire arches, and below each of these is a row of openings for admission of air.