The reduction of the iron ores is brought about by a reaction  with carbon from the fuel used to smelt them, at the high temperature made to prevail in the container of the material — the furnace. The advantages of the blast furnace lie in the recovery of practically all the iron from the ore", the delivery of both iron and slag in liquid form, and the desulphurization of the metal, and in the cheapness of the method.

Construction of the Blast  Furnace

General. — The changing industrial conditions have brought about radical changes in the construction of the modern blast furnace. A blast furnace is now designed so that a mixture of ore, fuel, and flux in proper proportions can be charged through a specially constructed opening in the top of a cylindrical-shaped furnace and top-gas and flue dust withdrawn for recovery, treatment and use. At the same time heated air from stoves, which obtain their heat from the combustion of this top gas, is blown in near the bottom through opening called tuyeres, forming carbon monoxide, which passes up through the openings in the charge to reduce the iron ore. The process is a continuous one except for the periodic removal of the impurities, in the form of slag, and of the metal, through large openings in the crucible of the furnace.

In present-day practice the production of a ton of pig-iron involves the use of about 2 tons of iron ore, 1 ton of fuel, 4 tons of air, and 0.35 ton of flux and produces, besides the 1 ton of pig-iron, 6 tons of gas, 0.6 ton of slag, and 75 to 375 pounds of flue dust.

While the central equipment in a blast-furnace plant is the furnace with its auxiliary apparatus for hoisting the materials to the top and handling the slag and molten metal, a considerable investment, and much space must be provided for auxiliary apparatus.

The modern 1000-ton blast furnace is circular in cross section, 90 to 100 feet high and lined with fire brick, held in a close-fitting steel shell which is divided vertically, for convenience, into three main parts. The bottom, cylindrical in form and some 10 to 12 feet deep, is termed the hearth or the crucible; the second section, in the form of an inverted frustum of a cone, 9 to 12 feet in height, is called the bosh; and resting on the bosh, and extending up for a distance of about 70 feet, is the stack. Finally, the whole is capped by a furnace top, providing means for the introduction of the charge and the withdrawal of the gas and the flue dust.

Foundations. — The foundations of a modern blast furnace must carry a total load of nearly 4000 tons. In view of this immense weight and the fact that a small amount of settling will crack the lining or throw the skip and the charging devices out of line, the stability of this foundation is of great importance. No generalization can be given as to size and depth; they will vary with the conditions of soil, (sand, clay, or rock) on which the furnace is built. On a proper bed, the foundation is built up with several feet of concrete, on top of which is laid common brick of good quality and strength except directly beneath the hearth and walls of the furnace, where fire brick is used.

Hearth or Crucible. — This portion of the furnace which contains the molten metal and slag is constructed of fire brick of the best qual­ity, usually 60 inches or more in thickness and reinforced by means of strong bands and protected in places with water-cooled plates. The bottom is given a cup-shaped cross section. In the large furnaces the hearth will be 22 to 28 feet in diameter and 11 feet in depth. In view of the fact that the 7 to 10 feet of slag and metal within the crucible creates a considerable pressure on this hearth (namely about 30 pounds per square inch) and because of the high temperature prevailing there these refractory bricks must be reinforced by heavy metal jackets made of riveted steel or plates of segmental iron castings that are closely fitted and bolted together. They are always water cooled: those of cast iron by internal circulating systems and those of steel by coolers set between the brick work of the jackets.

Tapping Hole and Cinder Notch. — The tapping hole or cinder notch is an opening about 6 to 8 inches on the inside, through which the molten metal may be drawn from the crucible. The outside dimensions are somewhat larger in order to permit the insertion of the tapping tools, and at this point the hearth jacket is usually protected by a water-cooled "dam plate". There is usually only one cinder notch, located about 6 feet from the floor of the hearth and 4 or 5 feet above the tapping hole, generally placed 45 degrees from this opening if there are two notches, or 90 degrees if there is only one.

Tuyeres. — Tuyeres, 10 to 16 in number, through which heated air is blown into the furnace, are distributed symmetrically about the upper circumference of the hearth just below the bosh. Indirectly, they also determine the height to which the slag in the furnace may rise above the slag notch; in most furnaces this is about 3 feet, allowing for the collection of 50 to 75 tons of slag. They are protected by tuyere coolers, similar to the cinder coolers. The tuyere which projects into the furnace is usually made of copper or bronze, 4 to 7 inches internal diameter, and like the cooler, water-cooled to prevent corrosion and distortion. The tuyere stock connects with the nozzle of the gooseneck, to which it is clamped by means of keys, and that in turn meets, at right angles, the neck of the bustle pipe. The bustle pipe, about 4 feet in outside diameter, and lined with fire-clay brick, encircles the furnace just below the mantle and distributes hot air to the tuyeres. This, as well as all connections down to the blow, is lined with fire brick. The life of a tuyere will vary from a few days to 10 months, depending upon furnace conditions and irregularities, raw materials, and conditions of water.

Bosh. — The bosh is that part of the furnace, at the mantle level, just above the upper limits of the crucible, where the furnace attains its greatest diameter. Actually, it is the weakest part and must be very carefully constructed and maintained. Starting at the top of the hearth, the brickwork, which is normally about 27 inches in thickness, is stepped outward, externally, about 6 inches for each 12 inches of vertical rise. Each of these step-outs is supported by means of heavy steel "bosh bands". Between these bosh bands, or in large furnaces each pair of bosh bands, are inserted cooling plates called bosh plates. These bosh plates are in horizontal rows about 2 feet vertically apart, the plates in each row being about 2 feet apart and the plates in different rows staggered vertically so as to avoid making weak joints in the brickwork. The bosh plates, which are of course water-cooled, serve to protect the brickwork, because this point, being opposite the zone of fusion in the furnace, is likely to be corroded very rapidly by the hot slag. At the upper limit of the bosh is the furnace. Upon it rests the weight of the 'stack and this in turn is supported by a series of cast iron or fabricated steel columns which rest on the main furnace foundation.

Shaft or Stack. — This comprises that part of the furnace located above the bosh but below the bells, and, for convenience, is divided into three almost equal parts, called the upper, middle, and lower inwalls. The blast furnaces are lined with thick walls up to 5 feet in thickness. The brickwork forming the hearth, bosh, and inwalls is referred to as the lining; and obviously, because of the very different thermal and mechanical conditions existing at these different levels, each requires a different type of brick. Fire-clay brick is universally used and great care must be exercised because the life of the furnace depends in a large measure upon the lining.

Bell and Hopper. — In olden times the top of the blast furnace was "by day in a pillar of cloud... by night in a pillar of fire...» for no attempt was made to collect the gas. Today every effort is made in top construction to equalize the distribution of the stock, to ' eliminate or compensate for irregularities in operation, and to hold at a minimum the dust carried over with the gas, and the gas lost to the atmosphere during charging. To collect the gas economically and easily an arrangement known as the bell and hopper was put into use since 1850. Considerable gas was lost during the charging period, so later this was improved by introducing a second bell. Essentially this consists of a lower large bell and just above it a smaller bell and hopper, thus providing a gas-tight space between the two. The raw materials of the charge are first dropped into the upper hopper, whence it may fall into the lower one if the small bell is lowered. When suf­ficient charge has collected over the larger bell for charging into the furnace, the larger bell is lowered, permitting   the   charge to fall into the furnace without the escape of much gas. These bells are usually made of cast steel with a slope of 45 to 55 degrees, sufficient to permit the charge to slide off readily.

Stoves. — A blast-furnace stove is a chamber where the blast or air for the blast-furnace is heated. A blast-furnace stove has the shape of a cylinder of boiler plate about 22 feet in diameter by 100 feet in height filled with a checkerwork of the fire-brick. This checkerwork is heated by the combustion of blast-furnace gas and subsequently gives up a part of its heat to air from the atmosphere, passed through it in the reverse direction. Common practice is to provide four stoves for each blast-furnace, on the basis that, as the number of stoves increases: 1) the velocity of the gas, and hence the rate of heat transfer falls off; 2) the heat losses through radiation increase; and 3) leakage increases.

Uses of Blast-Furnace Gas. — The burning of blast-furnace gas in stoves for the regenerating heating of air for the blast-furnace is the oldest and, metallurgically, the most important of uses. It is universally performed with unmixed blast-furnace gas and normally requires for a blast temperature 730° C not over 25 per cent of the total gas generated by the blast-furnace.

Great Lakes Steel Corporation Blows-in Largest Blast Furnaces

A new blast furnace went into production of iron for steelmaking June 9, 1955 at the Great Lakes Steel Corporation, Detroit, Mich., division of National Steel Corporation.

The blast furnace "A" has a hearth diameter of 30 ft., 3 in.

The furnace has been undergoing warm-up, trial and adjustment since being "blown-in" June 5. Forty tons of initial iron were cast the next night. Subsequent casts of progressively increased amounts have been drawn, and six production casts averaging 150 tons each were made June 9. Production at full-rated capacity of 50,000 tons per month is expected to be attained within 10 days. Engineers predict that, as operating experience develops, the furnace may average over 60,000 net tons per month.

The new furnace rises 252 ft. above the yard level. It rests on concrete pads 71 ft, in diameter and 16 ft. thick, which, in turn, rest upon steel piling extending 84 ft. into the ground to bed rock. Firedbrick of various dimensions equivalent to 2,844,694 of the standard 9-in. size were used to line the furnace, its three hot-blast stoves and other auxiliaries. Total weight of the mammoth iron-maker when fully charged is 12,813 tons.

The new facility replaces a 20,000-tons-per-month furnace that was dismantled last year after. 19 years service.

The new Furnace "A" was designed and built with every possible attention to elimination of air and stream pollution in its operation.

Before vapors are released to the atmosphere, they are passed successively through a mechanical dust catcher, a washing system and units to electrostatic precipitators which hold dust and other solids until they are removed for re-use and for disposal. Water discharge lines are connected to the plant's system for removal of objectionable matter from water before being emptied into the river.

Electrostatic precipitators which comprise one of the furnace's anti-pollution devices contain 34,000 sq. ft. of electrode plates for colliding dust or material particles. These plates are installed in the gas streams which carry such particles. The particles pass through an electrical field which gives them a negative electrical charge. The collecting plates are charged positively, and thus draw the particles bearing the opposite electrical charge to them. The particles then are held by the plates until they can be removed for re-use or for disposal. An additional step to avoid air pollution is provided for in the gas bleeder system. This is the system that automatically relieves pressures should they get too high.

While in most blast furnaces with high pressure operation, any excess pressure that may develop is relieved by an automatic discharge of hot, abrasive gases, blast furnace "A" is designed to relieve the pressure through the discharge of gases which already have undergone cleaning processes.

Bunkerage facilities for furnace "A" include 16 ore and scrap bins, eight coke bins, two nut-coke bins and six stone bins. These provide coke for a minimum of 7 1/2 hours operation with provision for auxiliary supply of 2 1/2 hours additional, and ore for a minimum of 20 hours operation.

The actual dimensions of the new "A" blast furnace are;

Hearth Diameter       —    30 ft. — 3 in.

Bosh Diameter         —    33 ft. — 3 in.

Stockline Diameter    —    23 ft. — 0 in.

Bell Diameter           —    16 ft. — 6 in.

Crucible Process

High grade tool steels and some alloy steels are still made by the crucible process, although the electric furnace is now capable of making steel equal in quality to crucible steel. In the crucible process, wrought iron, or good scrap, together with a small amount of high purity pig-iron, ferro manganese, the necessary alloying metals, and slagging materials are placed in a clay or clay-graphite crucible, covered with an old crucible bottom and melted in a gas-or-coke-fired furnace. After the charge is entirely molten, with sufficient time allowed for the gases and impurities to rise to the surface, the crucible is withdrawn, the slag removed with a cold iron bar, and the resulting fifty or one hundred pounds of steel poured into a small ingot which is subsequently forged to the desired shape. The crucible process differs from other steel-making in that little or no refining is included; the purity of the metal depends almost entirely upon the purity of the materials charged. The chief advantage of the process is that it removes most of the impurities, including oxygen and entangled particles.