The White Roasting-Furnace (Fig. 4) consists of a long cast-iron revolving cylinder inclined toward the fire end, and fed at the upper end with crushed pulp from stamp batteries or other pulverizer. The cylinder is made in sections to facilitate transportation. It is supported on four wheels or rings resting on truck-wheels and guided in a central position by rollers in upright frames, and revolved by friction of truck-wheels through gears and pulleys. The angle of inclination is changeable. The cylinder is lined with fire-brick throughout, and projecting bricks raise portions of the pulp and drop it through the flames, assisting the process. Salt for chloridizing is added before the pulp enters the cylinder. The advantages claimed for this furnace are that it is continuous in its operation, discharging its product regularly into a pit at the lower end, and this roasted pulp need be withdrawn only as required; also that it submits the ore to a gradually increasing tem perature. which is the true theory of perfect roasting. By changing the inclination, the ore can be retained to a longer or shorter period as necessary. The furnace is commonly made in three sizes, :1.5 follows: 40 in. by 24 ft., capacity 15 to 20 tons; 52 in. by 27 ft., capacity 20 to 30 tons ; GO in. by 27 ft., capacity 30 to 45 tons.
The Howell- White Roasting-Furnace is designed and works upon file same principle as the White, but has an auxiliary fireplace at the flue end, through the flames of which the dust from the roasting ore is drawn, and that would otherwise pass off unoxidized or michloridized is thereby roasted. The larger part of the cylinder at the fire end is lined with fire-brick. leaving the met-al on the smaller portion exposed, as the greatest heat takes effect at the fire end. Cast-iron spirally arranged shelves assist in raising and showering the pulp through the flames. This furnace is fed in somewhat the same manner as the White, and is made in the same sizes, its capacity also being about the same.
Hofmann's Roasting-Furnace is an improved revolving cylinder furnace. with a fire place and flue at each end. The flues are between the fireplace and cylinder, descending to the dust-chambers, which are connected with the main flue. The arrangement is alike on both sides. By means of dampers the current of the air and gases can he made to pass through the furnace in either direction. The object of this double fireplace arrangement is to enable the operator to expose the charge of ore to a uniform temperature. The fire is kept first on one place, with closed dampers on the same side, while the flue connection on the opposite side is open. After a few hours a fire is built in the other fire-box, and the position of the dampers is reversed. By changing the fire once or twice during roasting, both halves of the charge are exposed to the required temperature, without overheating one portion of the charge, thus, it is claimed. producing a higher and more uniform chlorination and diminishing the formation of balls. This furnace is especially suitable for ores which either require a very low roasting temperature or a very high one. By closing one of the large dampers near the main flue and opening the damper of the descending flue and corresponding plug-door, a current of live air can be made to enter the furnace together with the flame, thus assisting the combustion of the fire-gases and the oxidization of the ore. It is apparent
that this arrangement permits the construction of cylinders of larger capacity than it is prac tical for furnaces with only one fireplace.
The Douglas Roastiny-Furnace is a revolving cylindrical furnace with a fixed flue within the cylinder. The ore to be roasted is charged within the annular space between the outer shell and the central flue, through which the flames draw, as in the Brfickner, White, and other furnaces of this class. This arrangement constitutes a revolving muffle, in fact, and it is claimed, makes a more efficient oxidizing furnace, as in the ordinary cylinder the flames, coming in direct contact with the ore, have a reducing action for a time after each firing. This evil effect is felt more in the cylinders, which are closed front end to end. than in the ordinary reverberatory furnace, which is furnished with a large number of side by each of which more or less air enters to maintain oxidation. lit the Douglas furnace the admission of air to the roasting ore is regulated by a register at the discharge end. The loss of heat by its transmission through the walls of the •flue is trifling. The degree of heat required, even at the fireplace end of the cylinder, is small, and but very little of this escapes into the chimney after its passage through a flue of 30 ft. or so in length.
The central flue may be constructed of cast-iron pipe. supported by spiders, and the ore be agitated by shelves, as in the ordinary cylinder, but a square or triangular tile-flue, supported by heavy tiles built into the lining, is preferable. If the tiles be of good material and well locked together in the cylinder, the flue and its supporting shelves can not work loose or fall to pieces. Such a cylinder is converted into three or four muffles, and the ore is agitated by a gentle rolling motion, which, it is claimed, is preferable to the pounding action to which the particles are exposed when dropped from shelves, and which case-hardens them during the plastic state through which most ores pass in the early stage of roasting. Another advantage claimed for the flue consists in reducing the current of air in contact with the ore, and therefore the amount of 41tist carried into the dnst-ehamber. In order to burn the com bustion gases, amid supply the necessary surplus of oxygen to the ore, the amount, of air and gas striking the ore in the ordinary cylinder is necessarily much greater and the current niore rapid than that which is admitted to the roasting, compartments only of the tlue-cylinder.
An ore liable to sinter, such as galena, or matte rich in lead, as well as the higher grades of capper matte, can not safely be roasted in the confined inaccessible space of the cylinder; but all other ores and products can be calcined in Ibis furnace.
Works for Reference; !toasting Gold and ,Shyer Ores, by Guido Kfistel, 1880; Lcachitig (;07(1 and ,S'ileer Ores, by C.11. Aaron, 1881; Metallurgy of Silver, Gold, and Mercury in the (Vied Slates, by T. Egleston, vol. i, 1887, vol. ii, 1890; Metallurgy of Gold. by Manuel Eissler, 1891; Metaltartly of Silver, by Manuel Eissler, 188!) ; Modern American Methods of ('opper-Smating, E. I), I 'eters, .1 r.. 1891 ; The Li.erri«lion of Silver Oreswith fly posalphile Solutions, 1M88; Chlorhliz tog, ltoasliwg, imrf Lixiviation )'wirers, by George .1, Hoclovell, Engineering rend sfllufnq Journal, February 4, 1888, et seg.