The. oell and cone at D is for the purpose of closing the mouth of the furnace so as to save the " waste gases," chiefly carbonic oxide, which are allowed to escape in open-mouthed furnaces (see BLAST-FURNACE). These are conveyed away by pipes frcm openings just under the cone at D, and are turned to raise steam, heat the blast, etc.
The operation of smelting is thus performed: The roacted ore, coal, and lime (flux) are either hoisted, or, if the nature of the ground permits, mo7ed-alono' a platform or gang way to the gallery near the top of the furnace, and fed into it at intervals through the openings in the side, when the mouth is open, or by lowering the cone D, when it is closed. We may here state that the furnace is kept continually burning except when under repair. '1 he materials are of course raised to a very high heat, and gradually fuse into a softened mass. The clay of the ironstone then unites with the lime to form a coarse glass or slag; the oxide of iron at the same time gives up its oxygen to the fuel, and allows the metal itself to collect on the hearth at the bottom of the furnace, united with from 3 to 5 per cent of carbon, which it takes from the fuel, forming the variety called cast-iron. Every•12, and sometimes every 8 hours, the metal is run off from the furnace, by means of a tap-hole at the bottom of the hearth, into rows of parallel molds, called pigs, which are formed in sand, hence the name " pig-iron." The slag which floats on the melted iron is run off by an opening at the top of the hearth. If the furnace is 4. irking well, the slag should be of a light-gray color; a dark-brown orblack color shows that too much iron is passing into it.
The quantity of materials necessary to yield a ton of pig-iron may be taken roundly as follows: 2 tons of calcined ironstone; 21 tons of coal, of which about 8 cwts. are required for the blowing-engine and hot-air pipes; and from 12 to 16 cwts. of broken limestone. The proportions, however, vary in different districts according to the nature of the fuel and ore. The weekly produce of a single blast-furnace varies extremely—from under 100 to more than 500 tons in some of the larger furnaces.
Different districts classify their pig-irons in slightly different ways, but, as a rule, No. 1 td No. 4 are known as gray iron. No. 1 is largest and brightest in the grain, brings the highest price, and is best adapted for fine castings. Nos. 2, 3, and 4 become succes sively less in the grain, of a duller luster, and lighter in color, but up to No. 3 are known
as foundry pigs. After No. 4 the metal ceases to be gray, and though higher numbers -are sometimes employed, the other qualities are more usually known as forge, mottled, ;and white pig-irons. Gray iron has its carbon partly in the chemically combined, but chiefly in the uncombined or graphitic state, and requires a higher temperature to melt it than white iron, though very fluid when melted. White iron has its carbon wholly in the combined state, and is chiefly available for conversion into malleable iron. Hematite pig-iron suitable for making Bessemer steel has an exceptionally high value.
The hot-blast process which has been described above was introduced in 1830 by Mr. James B. Neilson of Glasgow, and has been productive of very remarkable. effects on the iron trade. The whole invention consists in simply the air blown into the furnace, and yet the saving of fuel by this is about one-half, and the production of iron, since it came into use, has enormously increased. The " cold-blast " is still, however, to a limited extent employed, and produces the strongest iron, though necessarily at a much higher cost. The difference in quality appears to be caused by the greater heat in the case.of the hot blast facilitating the passage of impurities into the iron.
Of date years much attention has been given to plans for saving fuel in the blast furnace. Previous to the introduction of the hot-blast as much as 8 tons of coal, as coke, were consumed for every ton made. Even when this is reduced to under 3 tons of raw coal per ton of pig-iron, fully three-fourths of all the heat produced is still wasted in open-mouthed furnaces. The method of saving the waste gases by clos ing the mouth of the furnace, as shown in fig. 1, now generally adopted when coke is used, is.attended with so much economy, that, in the Cleveland district alone, 600,000 tons of coal per annum are saved by adopting it. There being a difficulty in closing the mouth of the furnace when raw coal is used, Ferric of Monkland, a short time ago, patented a self-coking blast-furnace, by which, among other advantages, the gases can be saved. It has now been in use for sonic years in Scotland, and profices a ton of pig-iron with 34.instead of 53 cwts. of coal previously required. Raising the temperature of the blast to from 900° to 1000° F. has also been attended with a saving, and so likewise, in sonic districts, has an addition to the height of the furnace.