The blast-furnace on a grand scale is worthy of admiration, and is one of the most magnificent spectacles offered by our industry ; and, while simple in appearance, its successful and economical operation demands, perhaps, more experience and intelligence than any other pursuit of man, if we consider the progressive development of the principles involved and the improvements desirable and perhaps attainable.
The proper mixture of the ores, coal, and flux, depends on the quality of the materials used. If the ore is rich and pure, and the coal free from slate, bone, and sulphur, only a comparatively small amount of flux is required. But if the ores are refractory and silicious and the coal indifferent, a larger amount of both coal and flux is necessary.
With a burden of magnetic and hematitic ores, yielding an average of fifty per cent. of metallic iron, two tons of ordinary anthracite and one and a half of flux or lime stone are required in the generality of our anthracite furnaces. In a few instances from two tons and a quarter to two tons and a half of coal are used ; but there are furnaces on the IIudson in which less than one ton and a half of anthracite is used to the ton of pig produced, with about the same proportions of ore.
We have assisted in the management of a furnace using hematitic ores alone, but from various localities and with a small percentage of manganiferous hematites, in which one ton and a quarter of anthracite produced a ton of pig iron as an average of six months' operation.
The secret of this difference in the amount of fuel used does not lie in the character of the ore or the purity of the coal alone, but in the construction and operation of the furnace, as well as in the items of coal and ore. An admixture of ores even of one class, though from several localities, reduces the quantity of flux and economizes the cost of reduction ; but a small quantity of manganese or manganiferous ores not only saves flux and fuel, but improves the make of iron.
A careful selection of the coal used is also an important item. The best coal used in our anthracite furnaces contains ten per cent. of silicious matter, and much of it not less than fifteen per cent. of incombustible impurities. The bone and slate not only will not burn, but they require an equal amount of carbon to reduce them to a fluid state, and the same equivalent of lime to flux them. Therefore ten per cent. of impurity in
coal adds thirty per cent, to the quantity of coal and lime required. It would be better to take the time necessary to examine the coal and separate it from the bone and slate, as far as possible, than to feed it to the furnace as it comes from the mines.
The larger the amount of earthy or silicious matter the coal, ores, and limestones contain, the greater will be the amount of flux and fuel required.
In regard to the construction of the furnace there are several important objects to be aimed at. The hearth should be deep enough and large enough to hold the metal for twelve hours, or a fixed period, beneath a protecting cover of fluid cinder, and below the direct influence of the blast ; but it should not be so deep as to allow the " melting zone" to fall below the tuyer on running out the metal at " casting." Nor should the size of the hearth be so great as to prevent a blast of five pounds' pressure per square inch from reaching beyond its centre. In order to meet all these requirements, an oblong hearth is most desirable ; and, to allow of the prepared burden coming freely and regularly to the melting zone, the lower part of the boshes should also have an oblong shape to conform to the style of the hearth.
The body of the furnace should be as large and capacious as possible to be in uni formity with the bosh and the flues. It does not seem desirable that its dimensions should increase above the boshes, because it is supposed that the expansion of the materials only reaches the maximum at that point, and the regular descent of the charges and ascension of the gases depend on the open condition of the mass and its freedom from all liability to jam from expansion or from contraction of area of the body of the fur nace. The greatest expansion takes place between the throat and the flues, but the heat does not increase in proportion below that point until the bushes are reached; and the expansion cannot be great below the flues. Yet for the free descent of the ores it is supposed that the body should increase slightly in diameter from the flues to the top of the boshes.