CAST IRON 5. Manufacture. The amount of pig-iron obtained from a ton of ore depends on the rich ness of the ore—that is, on the amount of pure iron that it contains. The richness of ore varies considerably, the percentage being low in some of the lean ores of New Jersey, and high in the rich ores of the Lake Superior Region. The latter contain about sixty to sixty-five per cent of iron when the usual ten or twelve per cent of water is driven off.
In order to make cast iron, ore, together with about two-thirds to three-fourths of its weight in coke and a certain proportion of limestone, is dumped in the top of a blast furnace. This fur nace should be of the general form and dimen sions of Fig. 2. It should be from 65 to 100 feet high. Eighty feet, with an angle of the boshes of .givarerrum Pig. 2. Section of Blast Furnace.
75 degrees, has been proved about the best size. A wide difference in results is obtained from ores of different percentages of richness and in different districts. American ores are unsur passed in quality, and a comparison of working conditions in blast-furnace practice shows also that American methods are unequaled in general efficiency.
The fire is in the lower part of the furnace; and as the charge which was dumped in the stack descends to where the fire is burning, changes take place. The charge finally reaches the flames, which are fed by the coke, and they in turn melt the iron in the ore. Many of the impurities in the ore and in the coke—such as phosphorus and sulphur—combine with the limestone, which by this time has been burned to lime, and form what is known as a slag. The iron, being the heaviest of the materials in the furnace, sinks to the bottom, and the slag floats on top of it. At certain periods when the iron gets high enough in the hearth, the slag is drawn off from the top, and the iron is drawn out through an opening at the bottom. Fresh charges of ore, fuel, and limestone (which is called a flux) are continually dumped in at the top, so as to keep the stack full.
In order to obtain the intense heat necessary, air under considerable pressure is blown in at the bottom of the stack through pipes called tuyeres (pronounced tweyers). This air fur
nishes means for increasing the draft. It passes up through the furnace to the throat of the fur nace, and then, with other gases, passes out through the down-runner pipe which leads to the hot-blast stoves.
Fig. 3. Interior of Hot-Blast Stove when Blast is Going through /t.
These stoves are huge, hollow iron stacks built in pairs and filled with a checkerwork of fine brick. See Fig. 3. The burning gases from the furnaces are led into one of these, and allowed to pass upward to the top, burning all the time. This heats up the bricks to a white heat. The burning gases are then shut off from this stove, and are turned into the other one of the pair; and the air which was going through the other stove, and which is used for the blast for the blast furnace, is now turned into the stove which has just been heated. The air gets heated by going through this stove, and then it goes into the blast furnace. In this manner, one stove is being heated by the waste gases from the furnace while the other one is heating the blast.
The heating of the blast effects a great economy of fuel in the blast furnace. By these stoves, the air can be heated up to 1,000° F. to 1,200° F. Plate 2 shows blast furnaces and hot blast stoves. These stoves are sometimes called after the names of their inventors, as "Couper" or "Kennedy-Couper." If the waste gases from the furnaces are more than sufficient to run the stoves, they are conducted to boilers and burned there instead of using coal, or they may be used in the operation of gas engines furnishing power to run electrical generators or other machinery.
6. Casting. After the iron has been tapped from the furnace, it is cast into pigs, either by being run into sand moulds which form "sand cast" pigs, or by being run into huge ladles and poured into a machine which casts the pigs. In the latter case the pigs are called "machine cast" pigs.
In case it is not desirable to cast into pigs, the contents of the ladle are poured into a huge cylindrical furnace which is on rollers. This