ry, and the blast was supplied by bellows operated by water-power. In 1713 Abraham Darby, of Coalbrook Dale, in Shropshire, revived the use of coke fuel for blast-furnaces, and after much labor succeeded about 1740 in making a success of it. The use of coke now spread rapidly; and this, with the development of the steam-engine for blowing purposes and for supplying power to mills and forges, gave an enormous impetus to iron manufacture.
No further improvement in blast-furnace practice occurred until 1828, when J. B. Neilson, an Englishman, brought forward a proposition to heat the air for the blast. The greatly increased output which resulted from the use of the hot blast set on foot a series of improvements in blast-furnace construction. These improvements took the form of increased dimensions and capacity of the furnace and of the use of an increased number of tuyeres; the stove for heating the blast air was improved, more efficient and powerful blowing engines were employed, apparatus for hoisting the ores, fluxes, and fuel, and for charging them into the furnace were introduced, and a great variety of minor improvements were made. Until 1880 British furnaces led the world in size and output; but about this time American iron-makers began to take the lead in these respects, and have maintained it ever since. A few figures selected at random will show the progress of growth in blast-furnace dimensions, capacity, and output: 1855 to 1861, height 30 feet, capacity 2000 cubic feet, output 200 tons weekly; 1882 to 1885, height 70 feet, capacity 8200 cubic feet, output 800 tons weekly; 1892 to 1895, height 90 feet, capacity 18,200 cubic feet, output 2500 tons weekly; 1900, height 100 feet, capacity 24,000 cubic feet, output 600 tons every 24 hours. At this point it may be noted that the first iron-furnace in America was a bloomery erected in Virginia in 1619, and the first blast-furnace with a forced blast was built about 1714 in the same State. Shortly after the Revolutionary War numbers of charcoal-furnaces were working. From this time on the growth of iron and steel manufacture in America was rapid, until in 1890 the United States took first place among the iron-working nations of the world.
The internal shape and general construction of a modern American blast-furnace are shown by Fig. 1. As will be seen, the structure resembles in shape the common glass lamp-chimney. The cylindrical portion at the bottom is called the hearth, the bellying portion next above is the boshes, 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 tuyeres drawing from a circular blast-main. At the top there is a funnel-shaped charging hopper closed by a conical 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 which 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 furnace is charged by means of a car 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-furnaces.
In operation a blast-furnace is charged at the top with approximately alternate layers of ore, limestone, and fuel, usually coke. At the bottom of the furnace is introduced a current of hot air through the tuyeres. The materials introduced into the furnace, therefore, form two currents moving in opposite directions, one current being composed of hot gases and the other of solid substances. The chemical reactions which take place between these two currents result in the production of molten iron, molten slag, and gases. 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 dioxide; this, at the high temperature prevailing in the hearth, is almost immediately dissociated, and the liberated oxygen combines with more carbon 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 600° C. it is very rapid. At this temperature also the limestone decomposes, forming quicklime and liberating carbon dioxide, part of which takes up carbon from the fuel-producing 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 temperature sufficient for the formation of slag has been reached, 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 tuyeres 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, but sometimes it is preserved and submitted to treatment which permits 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 Cement.) 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 the casting floor; but at present the largest and best equipped blast-furnace plants employ casting 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
