The internal shape and general construction of a modern Anieriean blast-furnace are shown by Fig. 1. As will be seen. the structure re sembles in shape the common glass lamp-chim ney. The eylindrical portion at the bottom is called the hearth, the bellying portion next above is the hoshes, and the conical portion above forms the stack. The general construction is a masonry lining within a steel shell. To keep the hearth cool it is surrounded by a water jacket. Air is supplied by Dimes drawing from a circular blast-main. At the top there is a funnel-shaped charging hopper closed by a coni cal door and suitable exits for the furnace gases. These gases pass down through metal pipes to the stoves for heating the blast. Blast pressure is supplied by blowing engines Willa draw in cold air and discharge it through the hot-blast stove, whence it passes into the blast•mains and then through the tuyeres into the furnace. The fur nace is charged by means of a ear carrying a suspended skip, which is hauled up an incline to the top of the furnace and then discharged into the charging hopper. While the foregoing description applies to a particular furnace, it will serve in a general way for all blast-fur naces.
In operation a blast-furnace is charged at tho top with approximately alternate layers of ore, limestone. and fuel, usually coke. At the hot tom of the furnace is introduced a current of hot air through the tuyeres. The materials in troduced into the furnace, therefore, form two currents moving in opposite directions, one •ur rent being composed of hot, guises and the other of solid substances. The chemical react ions which take place between these two currents re stilt in the production of molten iron, molten slag, and gasps. These reactions are of very complex nature, and can only be indicated in a general way. When the highly heated blast enters the furnace the carbon of the fuel burns with the oxygen of the air to form carbon di oxide; this, at the high temperature prevailing in the hearth, is almost immediately dissociated, and the liberated oxygen combines with more corium to produce carbon monoxide. Carbon monoxide, being a powerful reducing agent, takes oxygen from the ore to produce carbon dioxide. When the iron ore is charged into the furnace it at. first suffers no chemical change, but gradually absorbs heat until at a temperature of about 200° C. it begins slowly to lose oxygen. As the temperature rises and the materials descend in the furnace the reduction becomes more rapid until at G00° C. it is very rapid. At this tem perature also the limestone decomposes. forming quicklime and liberating earbon dioxide, part of which takes up carbon from the fuel-produeing carbon monoxide. When the charge has passed through about 30 feet of the stack it has been deoxidized. and consists of lumps of spongy iron,
side by side with pieces of coke and quicklime. The descent of the charge continues for 30 or 40 feet without much change. until a tempera ture sufficient for the formation of slag has been readied. when the silica and other bases combine with the lime to produce slag. The charge then melts and runs down into the hearth, and collects below the level of the tnyeres in two layers, one of molten iron at the bottom and the other of molten slag on top.
The next step is to tap the furnace and draw off, first the molten slag. and then the molten metal. Generally the slag is run to waste. hut sometimes it is preserved and submitted to treatment which Is its utilization. Some of these uses are road metal, railway ballast, slag bricks, slag wool, and hydraulic cement. Bricks are made by casting the molten slag in molds. To produce slag wool the molten slag is blown by a jet of steam. which produces small globules, to each of which is attached a long thin filament. (For the utilization of slag in cement-making, see Slag, whatever use may be made of it, is only an incidental product: the essential thing is to secure the molten iron in suitable form for use. To do this the molten iron is run into molds, which produce east ingots or bars called pigs.
Formerly the casting was performed entirely in sand molds formed in t he east ing floor: hut at present the largest and best equipped blast furnace plants employ easting machines. In casting in sand molds, these molds of trough shape are formed thickly over the casting floor: the iron from the furnace flows into a large main channel, thence into side channels, and thence into the individual molds, until a part or all of them are filled with molten metal. This is allowed to solidify, and then the pigs are removed and the molds prepared for another set of pigs. In pig-casting machines two endless chains carry a series of pressed-steel molds, which travel at slow speed past a trough ter minating in two mouths. The molten iron from the furnace is tapped into a ladle, which is car ried by a ladle-ear into position to discharge into the double-mouthed trough. As each mold passes it is filled with molten iron. The end less chain of molds as it moves along descends under a tank of water, where the molten metal cools and hardens. The cooling is continued by sprays of water as the molds ascend from the water and move on toward the tail sheaves, where the pigs are discharged onto cars. The casting machine of one of the most recent Ameri can blast-furnace plants has a chain travel of 20 feet per minute, the molds are spaced 1 foot apart on the chains, and the machine delivers 20 pigs per minute averaging in weight 110 pounds each.