FURNACES, ROASTING. Roasting-fiirnaces are either oxidizing or chloridizing, according as the purpose for which they are used is to convert the metals in the ores treated to oxides or chlorides. There are six kinds of roasting-furnaces in common use, viz.: kilns, muffle-furnaces, reverberatory furnaces (Fortschaufelungsofen), shaft-furnaces, mechanical hearth-furnaces, and cylindrical furnaces.
Reverberatory furnaces, which are most commonly used for calcining fine ores, consist of a long brick hearth. with a low roof, and a series of small doors on one or both sides. At one end of the hearth is a fire-box, and at the other a flue connecting with the chimney, a dust chamber usually being interposed. The fine ore to be roasted is fed in through a hole in the roof at the flue cad, and is gradually worked forward toward the fire-box end by men using long rabbles through the doors in the sides. The flames from the fire-box draw over the ore toward the flue, the low roof throwing them down on to the ore. The roasted ore is pulled out of the furnace through the doors next to the fire-box. Reverberatory furnaces are fre quently built with two hearths, and sometimes three o• four. placed one above the other, the flames drawing successively through each. The object of this arrangement is obviously to increase the length of the hearth, and its utility is determined by the character of the ore to be roasted. The length of the hearth, according to Dr. E. D. Peters. Jr.. is limited chiefly by the capacity of the ore to generate heat during its oxidation, the immediate influence of the fireplace being seldom capable of maintaining the requisite temperature upon a hearth over 16 ft. in length without resorting to the use of a forced blast, or of a draft so powerful as greatly to increase the loss in dust, as well as the coimmption of fuel. An ore carrying, less than 10 per cent sulphur will not furnish sufficient heat to warrant the addition of a second hearth to the first 16 ft.; an increase to 13 per cent will be sufficient, however, to heat a sec
ond hearth, while a 20 per cent sulphur-ore will work rapidly in a three-hearth furnace. The addition of a fourth hearth is rendered justifiable by the increase of the average sulphur con tents to 25 per cent. As there seems to be almost no limit to the extent of surface over which the requisite temperature may he obtained in the calcination of highly sulphureted ores, much longer furnaces have been used, 120 ft. being the extreme inside limit. The width of the furnace should be as great as is compatible for convenient manipulation. Experience has shown 16 ft., inside measurement. to be the extreme limit. The capacity of a large reverber atory furnace varies from 6 to 16 tons per 24 hours, depending upon the character of the ore. The cost of calcining ranges from *.1.25 per ton upward.
In the shaft-furnaces the material to be roasted is allowed to fall as a shower of dust through a Shaft that is traversed from bottom to tep by the flames from a lateral fireplace. In one class of shaft-furnaces the dust falls freely; in others there are obstacles in the way. The welhknown Stetefeldt furnace is the most successful furnace of the open-shaft class, and the GerstenhOfer and may be taken as types of the latter class. The Stetefehlt furnace is generally used for ehloridizing roasting, but experiments have shown that it mav be also an efficient oxidizing furnace, although it has not yet come into practical use for that purpose. The capacity of the Stetefeldt furnace. according to 31r. C. A. Stetefeldt, is from 33 to 80 tons per 24 hours. If the ore is so base that 75 or SO per cent of it is in the form of sulphurets, 35 tons is the maximum limit for really good work. In most cases, how ever, where the ores contain only a moderate percentage of sulphurets, a large furnace will easily handle from 60 to SO tons per 24 hours, but the latter figure is probably the economical limit.