BLAST FURNACE, a mechanical struc ture built of refractory material in which metallic ores are smelted in contact with fuel and flux, the combustion of the fuel being ac celerated 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. The sizes of blast furnaces vary from a few feet to over 100 feet in height, a horizontal section through the structure showing either circular or rec tangular interiors, the circular form being adopted for the larger sizes, while those of smaller height are often made rectangular to permit of introducing a number of tuyeres with air nozzles into a narrow hearth.
A vertical section of a modern American blast furnace shows at the lower part, the hearth or crucible of the shape desired, 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 above these the shaft of the furnace ascends in the form of a right cone. The shafts are enclosed 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 and metal jackets, the jackets being made strong enough to resist the expansion of the brick work. Modern hearth practice tends toward cast steel or iron hearth jackets with pipe-cooling coils cast integral with the sections of the jacket. When jackets are extended about four feet below the hearth line and banded together strongly, and thor oughly water-cooled, the danger of 'break outs') is very slight. 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 de livery to the hopper formed by the boshes, where the fusing or reduction of 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 and drying 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.
The large blast furnaces smelt ores of iron or manganese, or of iron and manganese, and are from 40 to over 100 feet in height, a cross section at the top of the boshes showing a circle from 10 feet to 23 feet in diameter. The blast is heated to 1,200°, and sometimes to 1,500°, or 2,000° F., and is forced into the crucibles or hearth through a set of tuyeres, at pressures from 8 to 18 and, at times, exceeding 20 pounds per square inch. The blast furnaces smelting silver or copper ores seldom exceed 30 feet in height, the horizontal section being rectangular, and the blast pressure but a fraction of a pound. A large modern blast furnace will produce from 400 to 800 tons of pig iron daily, requiring from 1,200 to 2,500 tons of ore, fuel and flux. The cost for construction and equipment of one of these modern furnaces, with its necessary rail road tracks, storage room and bins for receiv ing the raw material, the mechanism for elevat ing it to the top of the stack with sufficient blowing engines, boilers, hot blast stoves, etc., ranges from $500,000 to over $1,000,000.
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 use of these gases for firing boilers and also for expansion in gas engines for power purposes is increasing. 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, large furnaces having as many as four stoves. The gas being admitted to and burned in a stove raises the temperature of the masonry, after which the gas is shut off and the blast forced through the highly heated checkers. By ri alternating a series of stoves on gas or blast, at intervals of one or two hours, a nearly uniform temperature is maintained.