HOW THE IRON IS EXTRACTED FROM THE ORE.
The second process is to smelt the ore, which operation is performed in " blast furnaces." The blast furnace is one which receives its supply of air for combustion, &c., in the shape of a strong blast from a pair, or more, of blowing engines. Our figure will give an idea of the form, construction, and arrangement of a common blast furnace, with its throat A, its body B, its boshes C, its hearth E, its crucible AE, its tympstone G, its tyrnpplate H, its dam stone I, &c. The fuel and ore are introduced through the chimney or tunnel K, and fall into the body B, gradually and freely descending to the boshes C. Below C the furnace contracts, so that the solid substances are checked in their descent, till they reach the top of the hearth at A. Into the hearth, AH, a blast is introduced, as seen in the figure. The blast pipes are seen in the plan below, where a a a are three tuyeres; the fourth side, 6, being occupied by the tympstone and tymp-plate, with the dam stone and dam plate immediately below them. The part where the molten metal accumulates, near H, is called the crucible. The metal is kept in the crucible by the tap hole, which is a hole at the bottom of the crucible and dam plate, being well filled and rammed with binding sand. Between the tymp and dam stones is an opening, A (above the tap hole), through which the slag, a melted earthy substance lying on the top of the metal, is allowed to flow out. To keep the tuyeres and tymp-plate cool, pipes pass inside them, or else they are made hollow, so that water can be circulated within them. Opposite the tap hole is a channel to convey the iron to the moulds, which consist of long lines called " sows," from which run short parallel lines, about 3 feet to 4 feet long, called "pigs." -Hence iron, as it comes from the blast furnace, is usually named " pig iron." In our figure three tuyeres are shown, which is the usual number, but sometimes more are employed. To put a furnace " in blast," or to " blow it in," takes a considerable timefrom ten to fifteen daysas it must not be heated too rapidly for fear of injury from expansion.
This entails great expense, so that when once at work it V T ' is not " blown out" for six or seven years, or until repairs are absolutely required, pig iron falls in price, or the supply exceeds the demand. The charge depends upon the size of the furnace and the nature of the ironstone. Generally, furnaces are about the same sizefrom 50 to 65 feet highbut in the Cleveland district, where coke is used which will bear a greater pressure, they are built larger, from 75 to 85, or even 100 feet high. The charge for a blast furnace consists of the ore, fuel in the shape of coke, and flux, which vary according to the kind of ore employedsome ore containing a greater per centage of metal, and some containing no clay, and so requires but little flux. But one condition is uniform, the furnace must be kept full. A rough estimate is the following for clay ironstone: Two tons of calcined ironstone, Two tons of coal (8 cwt. for blowing), 12 to 16 cwt. of broken limestone, produce 1 ton of pig iron. The furnace is fed from the top, as shown in the figure, from three to six hundredweight of iron ore, according to its richness, is thrown on to the top of the burning fuel, with about one-third of the same weight of broken limestone for the flux; on to this is thrown from 4 to 6 cwt. of coke. This charge gradually descends, taking about two days and nights to pass from the top to "the bottom. It is repeated every half hour, and run off or tapped every twelve hours. The chemical action is this:
The oxygen has to be separated from the oxide of iron, and the iron alone left; the heat causes the silica and alumina of the clay, or the clay in the ironstone, to combine with the lime, forming a glassy slag. The clay of the ironstone has a greater affinity for the lime than for the iron. As the blast passes through the burning fuel it takes from the oxide of iron its oxygen, passing out as carbonic oxide, and thus compels it to yield up the whole of the metal iron. The iron, as it melts in the furnace, combines with the carbon of the fuel, and becomes cast iron.
Although the process involved in refining purifies the iron to a great extent, it is not yet wrought iron; it has yet to pass the puddler's reverberatory furnace, of which the accompanying figure shows a sectional elevation above and ground plan below. It is generally built of Stourbridge fire-clay bricks, and bound round with iron. B is the fireplace, the fuel, generally coal, being introduced at C; while A is the hearth, F the flue, H and E the bridges. The iron is introduced at D, before which opening the workman stands, and moves the balls of iron about over the hearth A. The working opening D is covered by a door which can be raised by a chain and lever, while in the door itself is an aperture, through which the workman introduces his iron tools, called the paddle and rabble. At the top of the flue is a damper, by which the workmen regulate the draught. From E to G- is an inclined plane, like the rest of the fumace made of fire-brick, down which the slag runs, the bridge at E being only about 2 1/2 inches high.
The slag is discharged at the Floss hole G, near which a small fire is kept burning, to keep the slag sufficiently fluid to allow it to pass freely away. There is another floss hole immediately beneath the working door. The bottom of the health A is generally made of cast iron plates, and covered -with " Bulldog," which consists of cinders, either from the refinery, or from previous puddlings; over this is placed the "fettling," a paste made from oxide of iron. A charge consists of about 4 or 5 cwt. of pig and refined iron, which is broken into pieces and placed in heaps round the furnace. The puddler has to stir this up with melted oxide of iron, so that every portion of the metal shall come in contact with it and the air; the air being thrown on to it by the shape of the furnace. The puddler has to regulate his fire and draught, so that the iron shall not liquefy too rapidly; hence he moves it about, and continually exposes fresh surfaces to the heat. In a short time the whole metal becomes a pasty mass, and the skill of the puddler is now employed to keep the metal at such a consistency that it will mix with the oxide of iron on its surface. As the process is continued, the metal swells up or boils, throwing out carbonic oxide gas, thus passing the carbon out of the iron; after, this the iron "comes to nature," or the process is complete. When the iron comes to nature it works heavier, and sticks together, the cinder or slag passing from it in a liquid state. The iron is next worked into balls or blooms of about 60 or 80 pounds each, and then placed in the hottest part of the furnace to soften, so that the slag can be pressed out with the paddle, and the whole raised to a welding heat, when it is tit to be taken from the furnace to the squeezers or shingling hammer. In puddling, a ton of iron requires a ton of coal. The effect of the whole is to remove the carbon, silicon, phosphorus, sulphur, &c., on precisely the same principle as involved in refining.