BLAST FURNACE. The discovery that metallic iron can be reduced from the natural iron mineral with the aid of carbon and heat is lost in the mists of antiquity. In the middle ages iron was made in furnaces of relatively small size, in which pure ore was reduced to a pasty mass by means of charcoal, and this was subsequently hammered to get rid of the impurities. About the 15th century the type of furnace was altered by the addition of an upper portion, or stack, in which primary preparation of the materials took place. This revision made possible the melt ing of the metallic product, so freeing it from earthly impurities and giving a material, pig-iron, altogether different from the forged metal formerly made. Thus the blast furnace first came into use.
Up to the 18th century charcoal only had been used as the furnace fuel, but the depletion of the forests began to give cause for grave alarm. The difficulty was overcome by employing coke in place of charcoal, and the former is now used almost exclu sively. The exact date of the introduction of coke is uncertain, as also is the name of the pioneer who first used it, but there is no doubt that Darby had established coke-smelting of iron in England on a commercial scale as early as 1709. Progress since then has been continuous, but slow, until the dawn of the 20th century. During the subsequent period the increase in number of furnaces and in the producing capacity of the units has been rapid. The outputs of the six principal iron producing countries and the world total are given below : 1924 1925 tons tons United States ... . 31,406,000 36,700,566 Germany . . . . . . 7,707,000 9,927,00o France . . . 7,569,000 8,336,00o British Isles 7,307,000 6,262,000 Belgium . . . . . . 2,798,000 2,502,000 Luxembourg . . . . . 2,123,000 2,325,000 World total . 66,690,000 75,081,000 Theory of the Process.—Iron ores are distributed widely in nature in various forms, the principal being the oxides, ferrous oxide (Fe0), the magnetic or black oxide and ferric oxide The bulk of the ore used is ferric oxide which is composed of two atoms of iron combined with three atoms of oxygen. Means therefore have to be provided to abstract the oxygen, thus leaving the metallic iron. The reducing agent is carbon, used under suitable temperature conditions. These conditions are readily ensured because the combination of carbon and oxygen, the ordinary process of combustion, results in the liberation of ample heat. Iron oxide, however, is never found in a pure state in nature. The ore always contains a proportion of gangue composed of silica, lime, alumina, magnesia, etc., which have to be removed in the process. At sufficiently high tempera tures lime and silica combine along with the other earthy mate rials, and form a liquid slag which is lighter than the liquid iron. The slag therefore separates from the metal and floats on the top.
Pig-iron is by no means pure iron, the latter representing only about 91.o% of the total. With the iron are combined varying amounts of carbon, silicon, manganese, sulphur, and phosphorus, each constituent having an important effect on the quality of the pig-iron. In order that the product should be the quality required it is evident that the mass of material, or "burden" as it is called technically, from which the pig-iron is made must contain, in addition to iron, the requisite proportions of the other con stituents mentioned. There is charged into the furnace therefore, (a) The burden which includes the ironstone, fluxes for forming the slag, and any other additions, e.g., manganese ore or phos phoric rock, needed to give the proper manganese and phosphorus contents; (b) The fuel, which is generally coke, though charcoal is still employed for making special quality iron ; (c) The air blast, which is injected above the hearth of the furnace to burn the fuel and maintain a sufficiently high temperature to render both metal and slag freely molten.
The section of a modern furnace is shown in fig. i, the parts of the interior known as the stack, bosh and hearth respectively being indicated. The solid materials are charged into the top of the furnace whilst the air blast is blown through tuyeres into the hearth where the highest temperature (about 3,200° F) is generated. The oxygen of the air combines with the carbon of the coke, momentarily forming 'carbon dioxide, but as the hearth is full of incandescent coke, a reverse reaction rapidly takes place, the carbon dioxide being converted into carbon monoxide which is a powerful reducing agent. The hot gases composed of the carbon monoxide and the nitrogen of the air, along with some hydrogen derived from the dissociation of the moisture carried in the air, pass upwards through the column of solid materials, their temperature being progressively reduced as a result of direct contact and chemical reaction, until they leave the top at about 460° F. The constituents of the burden pass through the furnace in 14 to 18 hours. When charged they immediately come in contact with the hot gases and any moisture is quickly driven off. As they slowly descend the stack, the temperature increases and the carbon monoxide in the gases combines with the oxygen of the iron oxide to form carbon dioxide, leaving metallic iron in a finely divided form, or as iron sponge. The non-ferrous materials are mixed intimately with the reduced iron, and separation does not take place until near the top of the bosh, where both the iron and the slag begin to melt. The coke passes through the furnace with little change, except a constant increase in temperature, until it reaches the tuyere zone, where intense combustion takes place with the oxygen of the air blast. The bosh and hearth however are filled with coke and, as the slag and iron melt, the liquids trickle down until they reach the well formed in the bottom of the hearth, where they slowly accumulate and separate, the slag layer being on the top. The iron and slag are tapped from the furnace through separate tap boiler at intervals timed in accordance with the ca pacity of the furnace. The difference in the nature of the slag as compared with iron is apparent to experienced operators who skim the slag from the liquid stream into ladles placed to receive it. A large volume of gases, sufficiently rich in combustibles to be of value, is produced during the process. The combustible gases are the carbon monoxide and hydrogen, but the major volume is com posed of the non-combustibles, nitrogen and carbon dioxide.
Neilson in 1829 discovered that increasing the temperature of the air blast before entry into the furnace resulted in a marked fuel economy, and air blast temperatures up to 1,800° F are now employed. Cowper stoves are used for heating the air blast, and each unit is composed of a mass of brickwork, arranged as chequer work, contained in a steel casing. The temperature of the brick work is raised to the required intensity by the combustion of blast furnace gases. The gases are then shut off and the air blast allowed to pass through the stove on its way to the furnace. Two or more stoves are therefore required in order that the process may be continuous. The gases leaving the furnace are passed through primary dust-catchers and ultimately through towers where they are washed with water before use in the Cowper stoves. The remainder of the gases are utilized in raising steam, or as direct fuel in blast furnace gas-driven engines. The tribution of the available gases is approximately as follows:— Cowper stoves . . . ..... = 27% Power for air blast . . . . . . . = 15 Wastage . . . ..... = 3 Available for other purposes ..... = 55 I00% The diagram in fig. 2 shows a skeleton arrangement of a blast furnace plant and the data for typical practice are indicated.