BLAST). For the reduction of zinc and quicksilver ores retort-furnaces are employed.
The shaft-furnaces used in lead and copper smelting are known as furnaces if their horizontal section is circular, or Rasehette furnaces if it is rectangular furnaces are now very little used. Although the circular form possesses certain advantages, experience has shown that the ordinary blast used in lead and copper smelting, seldom exceeding 1 lb. per sq. in., can not well penetrate to the center of a charge in a furnace of greater diameter than 50 in. The size, and consequently the capacity, of a Pilz furnace are therefore limited.
The general construction of the Raschette furnace, which is used in lead-smelting, is shown in Fig. 1. The crucible, resting upon a solid foundation, is built of fire-brick and lined with fire-clay, the whole being surrounded by a curb of thor oughly braced wrought-iron plates. Upon the brick-work within the curb is placed the water-jacket, which is make of wrought-iron boiler-plate in four parts, two side and two end pieces. It is so constructed that no seams appear next to the fire, and all four parts are hound by wrought iron forgings, which can be quickly unfastened when nec essary. Cast-iron spouts are riveted to the jackets for overflow and feed water. Dand-holes are also provided for cleaning out sediment, and in the side jackets are openings for the tuyeres, the number of which vary with the size of the furnace. Four iron columns, resting upon the founda tion of the furnace, support the brick-work above the water jacket, the brick-work resting upon a deck-frame of wrought-iron 1-beams, united by wrought-iron plates and bolts. Angle-iron corner - binders hold the brick - work against all cracking. The slags rout is shown at the end of the furnace, the lead-well at the side, and the charging door just above the upper floor.
The furnace is surrounded with a large pipe of galvan ized iron, called the bustle-pipe, to receive the air-blast from the main blast-pipe and distribute it to the tuyeres. The bustle pipe is connected with the tuyOres by flexible pipes, usually made of canvas. The tuyeres are short, con ical iron pipes pointing into the furnace, passing through the water-jacket. The outer ends of the tuyeres can be opened, so that a rod may be inserted to clear them of slag if they should become thus clogged. The furnace shown in Fig. 1 is equipped with the Devereux adjustable tuyiTes. These consist of a loose iron sleeve, cast with a central bore at a considerable angle, and capable of being quickly re volved by the hand to point the blast up or down at any angle between the extremes. The ttlyre rests in the tu yere-hole funned by a bronze-metal tube in the water-jacket, and is thus cooled.
The average size of the Raschette furnaces used at Den ver, Col., where the practice of lead-smelting has been car ried to a higher degree of perfection than anywhere else in the world, is 33 in. wide and 100 in. long. The average amount of ore smelted in these fur naces is 40 tons per 24 hours. The largest furnace in use is 00 in. wide and 120 in. long, the water-cooled tuy(lres protruding 6 in. on either side. The capacity of this furnace is 80 tons per 24 hours. The average cost of smelting in Denver is $4.75 per ton. excluding the cost of roasting, which amounts to about (42 per ton. I am indebted to Prof. H. 0. Hofman, of the Massachusetts Institute of Technology, for the foregoing figures.
The general construction of the Rasehette furnace used for smelting copper-ores is similar to that used for lead„the main point of difference being the crucible. For the reduction of oxidized ores furnaces with an interior crucible are long, used. Fig. 2 shows a furnace of this type. 33 in. wide at the tuyin.es and 66 in. long, designed by Carl Ilenrieh for the Detroit Copper Company's smelting-works at Morenci, Arizona. It consists of a lower and an upper water-jacket of wrought iron, the lower one supported from the cast-iron bottom plate and short columns, the upper one resting upon four long columns by means of cast-iron lugs or brackets. Between the lower jacket and bottom plate is a wrought-iron curb, con fining the metal crucible, which is formed with fire-clay. Above the upper jacket is a. short sheet-iron casing, extending to the charging floor and lined with one thickness of fire-brick. and containing the outlet for connection to dust-chamber. A floor-plate of cast iron is pro vided with inside hoppers. The stack is of telescope pattern, the stationary part being pro vided with roof-plate and umbrella, while the movable part is provided with balance-weights, that permit pushing it up out of the way when the furnace is in operation, and allows it to be quickly lowered when blowing out. Two water-jackets are introduced to provide a water cooled surface from crucible to the top, in order to do away with brick almost entirely. Both upper and lower jackets are made in four sections (two side and two end pieces). There arc fourteen tuyOres, five in each lower side jacket and two in each lower cad jacket. Two distinct sets of water-pipes are provided for water supply and discharge. A galvanized bustle-pipe, surrounds the furnace, and connection to tuytre el bows and nozzles is made by canvas hose. The tuyiire elbows or nozzles arc provided with a ball-end, which makes a universally adjustable joint in the tuy6re. which is made to suit it.