:MECHANICAL HEARTIT-FURNACES.—The Rotary-Pan Furnace (Fig. 2) used at the Haile mine, North Carolina, for roasting fine pyrites for chlorination, is a combination of the rever beratory furnace with the mechanical hearth-furnace. It is a reverberatory furnace with step-hearths and a circular rotating hearth at the fire-box end. The charge is fed at the fine end and gradually worked forward by hand to the circular hearth, where the roasting is fin ished. Thies and Phillips (Trans. A. I. M. E., xis, 601) give the capacity of this furnace, roasting pyrites concentrates, as ai tons per 24 hours. A double-hearth reverberatory furnace with 400 sq. ft. hearth area, at the same place and with the same ore, desulphurizes tons per 24 hours. Each furnace consumes cord of wood per ton of roasted ore, and requires the labor of 4 men, which is not very good practice compared with what is done with single hearth reverberatory furnaces in the West.
The Spence Automatic Desulphurizinq Furnace consists of a series of hearths placed one above another. with a mechanical device for raking and stirring the charge on each. Each hearth has an opening at alternate ends, through which the charge drops to the next hearth below. On each hearth there is a rake of nearly the same width as the hearth, which is moved backward and forward from end to end of the furnace by a rod working through a stuffing-box at one end. The ends of the rods outside the furnace are supported by a rack or carriage which travels on a railway. The necessary supply of air is admitted through adjust able ports below the lowest hearth. The number of hearths varies from three to seven, according to the character of the ore to he roasted. Connected with the furnace is a pair of 7 x 10 engines, which run at 40 revolutions per minute, and quietly and positively operate by means of geared wheels the rods to which the toothed rakes in the furnace are attached. The charge is raked at intervals of about five minutes, and in the mean while the rakes are pulled to the back end of the furnace and the driving-engines are stopped. Connected with the furnace there is also a sinall auxiliary engine, which runs constantly, and by suitable mechan ism puts the large engines and rakes in operation at the proper times. The ore is fed into a hopper on the top of the furnace, and is gradually admitted to the latter through a port which is opened and closed by the movement of the rakes. Falling on to the uppermost hearth it is gradually worked along until it drops through the hole to the next hearth below, when it is worked backward, dropping on the third hearth. and so on. From the lowest hearth it is discharged into a bin or cars, through a port which is also opened and closed by the movement of the rakes. When the rakes have finished the forward stroke the engines reverse automatically, and the rack returns to position and stops until the auxiliary engine pots the driving-engines in operation for another cycle. This furnace was especially designed for roasting fine pyrites for the manufacture of sulphuric acid, and has given exoellent results in that work, fine ore with from 40 to 47 per cent sulphur having been desulphurized to 1' to 2.5 per cent sulphur, at the rate of from to 10 tons per 24 hours. in roasting pyrites
for sulphoric-acid manufacture no extraneous fire is used, the itself burning freely on the lower shelves. In roasting fine auriferous pyrites down to + or per cent sulphur preparatory to chlorination, a fire-box connected with the lowest shelf is used with the fur nace. At the Trcadwell mill. Douglas Island, Alaska, six Spence furnaces were used for desnIphurizing pyrites concentrates for chlorination, with the result that slightly more than 3 tons per 24 hours were roasted "dead," with an expenditure of cord of wood per ton of ore. Two inert per shift attended to six furnaces.
The O'llara Rua-sling-Furnace (Fig. 3) is a mechanical reverberatory furnace made with two separate hearths, one for desulphurizing and the other for chloridizing the ore, both processes being performed nut one operation. :\ ttached to an endless chain at proper dis tances apart are iron frames formed into a triangular shape: on these frames are a munber of plows or hoes set at an angle. One set turn the ore toward the center, the next set inert it in an opposite direction toward the walls. These plows move through the ore every minute and expose a new surface of ore to the flames and gases. The space between the roof and hearth of each compartment is quite small. so as to confine the heat close to the ore. The operation of this furnace is as follows: The ore is fed continually into a hopper, through which it then falls on the upper hearth. The plows, actuated by the endless chain, stir the ore over and over on the 'hearth and move it gradually to the ma.ning, where it falls to the lower hearth, As the ore is passed along in the upper compartment it is thoroughly desul phurized by the heat furnished by the fires, as described, and by the combustion of the sul phur in the on-. This action is assisted by the oxygen in line supply as admitted at intervals through the sides of the furnace by the openings. For a chloridizing roasting salt is mixed with the on. as it is fed into the hopper, and becomes Ila.roughly intermingled with it by the stirring action of the plows. When the ore fulls through the opening and on to the lower hearth fall breaks spongy lumps or masses that may have been formed. and the ore is again stirred over and over. :Ind moved along through the flame and gases over the lower hearth by the nelion of the plows toward the discharge-opening. The ore has grad ually more and more heated in its passage through the upper hearth, and by the linie the extra heat is required as stated it mines immediately in front of the Annie Pecs which have during the whole process furnished the heal. Ordinarily the ore will he from five to ten hours in passing Ihroligh the fortutec, according to its di:minter. Only one man is required to attend the tires, no other attention being necessary, as the ore May he led to the furnace by meehanical 1111.anS, and discharged from the furnace into a ear, conveyer, or elevator. This furnace is also used with excellent results for oxidizing roasting.