Blast Furnace
a structure built of refractory material in which metallic ores are smelted in contact with fuel and flux, the combustion of the fuel being accelerated by air under pressure. The materials are fed in at the top of the furnace, and after the ores are reduced, the metal, or in some cases the matte, and the resulting slag are tapped in a molten state at or near the bottom, as a rule the slags, being of less specific gravity than the metal, float upon it.
A typical vertical section of a blast furnace consists of a cylindrical or rectangular hearth or crucible, into which the air is admitted, under pressure, through tuyeres. On this hearth is superposed an inverted frustum of a cone forming the boshes, and on this inverted cone a right frustum of a cone, forming the shaft, is superposed. The shafts are inclosed by shells of sheet steel or by crinolines formed of bands and beams, and carried on columns. The boshes are usually secured by bands and the crucibles by sheet metal jackets. The materials are charged into the shaft so that layers of fuel alternate with layers of ore and flux, the taper of the shaft being sufficient to permit of expansion as the materials are heated, and facilitate their delivery to the hopper formed by the boshes, where reduction of the ores takes place. The reduced ore, meeting the burning fuel near the tuyeres, is melted, and the liquid slag and metal drop into the hearth or crucible (the cinder or slag floating on the liquid metal), from which they are tapped out from time to time. By heating the blast before it enters the tuyeres combustion is accelerated, and the furnaces produce increased quantities of metal with reduced fuel consumption per unit of product.
As a rule, blast furnaces smelting other ores than those of iron have the top of the furnace stack open, while, in those producing iron, the top is usually sealed by a bell closing against a hopper, to distribute the stock in the wide throat of the furnace and to control the gases which are the result of the smelting operation, so as to employ the calorific value of these gases for heating the blast or for generating steam in boilers to operate machinery. The practicability of using these gases in engines, where the gas, in also been demonstrated. The blast is heated in hot blast stoves, generally cylinders from 14 to 25 feet in diameter and from 50 to 115 feet high, filled with checker work of fire brick. These stoves are placed in series; the gas being admitted to and burned in a stove raises the temperature of the masonry, when the gas is shut off and the blast forced through the highly heated checkers. By alternating a series of stoves on gas or blast, at intervals of one or two hours, uniform temperature is maintained.
The blast, after passing through the hot blast stoves, is conveyed in iron or The blast, after passing through the steel conduits, lined with fire brick, to tuyeres, set in the walls of the crucible.
These tuyeres are formed of an inner and outer shell with closed ends, water cirouter shell with closed ends, water cir- culating between the two shells. The tuyeres are mostly made of bronze or copper and are set in larger tuyere blocks (also water cooled) of iron or bronze.
Nozzles connect the lined air conduits to the tuyeres. The cooling water remodern blast furnace quired by amounts to millions of gallons daily. A of from 3,000 to 3,500 horse power for large furnace requires a boiler equipment of from 3,000 to 3,500 horse power for chinery, electric plant, etc. a There were on Jan, 1, 1920, 262 blast furnaces in operation in the United chinery, electric plant, etc. furnaces in operation in the United oval or globular r form form, like that of a bird), an order of echinoderms, called oval or globular form, like that of a in Palæozoic rocks.