Page:The New International Encyclopædia 1st ed. v. 10.djvu/879

IRON AND STEEL. main channel, thence into side channels, and thence into the individual molds, until a part or all of them are filled with molten metal. This is allowed to solidify, and then the pigs are removed and the molds prepared for another set of pigs. In pig-casting machines two endless chains carry a series of pressed-steel molds, which travel at slow speed past a trough terminating in two mouths. The molten iron from the furnace is tapped into a ladle, which is carried by a ladle-car into position to discharge into the double-mouthed trough. As each mold passes it is filled with molten iron. The endless chain of molds as it moves along descends under a tank of water, where the molten metal cools and hardens. The cooling is continued by sprays of water as the molds ascend from the water and move on toward the tail sheaves, where the pigs are discharged onto cars. The casting machine of one of the most recent American blast-furnace plants has a chain travel of 20 feet per minute, the molds are spaced 1 foot apart on the chains, and the machine delivers 20 pigs per minute averaging in weight 110 pounds each.

In the preceding description of the blast-furnace process of producing cast iron the attempt has been made to avoid confusion by subordinating the accessory details of furnace equipment and operation to the main narrative. Some of these matters deserve further mention. The essential accessories to the blast-furnace in the nature of a working plant are the blowing engine, the hot-blast stove, and the material hoist. The older and less-used form of blowing engine is the beam engine, the steam-cylinder being connected to one end of the beam, and the air or blowing cylinder to the other. The present form of blowing engine is either a vertical or a horizontal direct-acting engine, the former being the more common type. Very often the steam end of the engine is compound. Until very recently steam blowing engines have been used almost exclusively, but in Europe considerable progress has been made within the last few years in the adoption of (q.v.) operated by blast-furnace gas. In America a separate engine is generally provided for each furnace, but in Europe one engine commonly supplies air to two or more furnaces. The blast pressure provided by blowing engines in actual working varies from 5 pounds to 14 pounds per square inch, American iron-makers favoring the higher pressure.

The first form of hot-blast stoves were all heated by solid fuel. About 1860 E. A. Cowper. an Englishman, applied the regenerative method of Siemens to heating the blast with waste furnace gases. The Cowper stove, or modifications of it, are now extensively used. Two stoves are required for each furnace. Briefly described, each stove consists of an outer shell of steel plates lined with fire-brick to form a vertical cylinder with a dome-shaped top. A circular flue extends from the bottom to the top of the stove, and the remainder of the space is filled with special-shaped fire-bricks which form a honeycombed or cellular filling. Hot gases from the furnace are discharged into the bottom of the vertical flue, where they meet the air-inlet and combination takes place. The burning gases pass up the flue and thence down through the cellular filling to an outlet leading into the

chimney. In passing through the cellular filling the gases heat the brick to a red heat. When this is accomplished the gases are shut off by means of suitable valves, and by opening other valves the cold blast from the blowing engine is passed through the stove in the direction opposite to that taken by the heating gases, and thence through the tuyeres into the furnace, being highly heated in the passage. Meanwhile the gases diverted from the first stove are being passed through the second stove and are heating it; that is, one stove is absorbing heat while the other is heating the blast. Sometimes a third stove is supplied to be used in case of accident to one of the others. The Whitwell stove is another popular form. It is a cylindrical sheet-steel, fire-brick lined structure like the Cowper stove; but the interior construction consists of a series of vertical fire-brick passages through which the hot gases and blast-air pass alternately up and down, but in opposite directions. The operation of the Whitwell stoves is similar to that of the Cowper, one receiving heat from the furnace gases while the other is heating the blast. Several combinations and modifications of the Cowper and Whitwell stoves are in use.

The material hoist is a necessary adjunct to the modern blast-furnace, where hundreds of tons of coke, ore, and limestone have to be raised daily 100 feet or more to the charging hopper. These hoists are operated by hydraulic, pneumatic, or steam power, and in general consist of an elevator cage or corresponding apparatus on which the material cars may be raised and lowered. In the furnace illustrated the cars are hauled up and down an inclined plane. These hoists are arranged to work as nearly automatically as possible. Among the other appliances necessary to the operation of a modern blast-furnace are pumping engines for supplying cool water to the hearth-jacket and to the cooling rings and for other purposes; dust-catchers for separating the dust from the furnace gases previous to their utilization in the hot-blast stove, or for other purposes; the tuyeres: apparatus for tapping the furnace; ladles of various kinds, etc.

A few words may now be said regarding the fuel and fluxes used in blast-furnace smelting. As has been said already, charcoal was the fuel originally employed. At present but few charcoal furnaces are in operation, but the iron produced from them is of the best quality, and is much used for special purposes. Coke is the fuel most extensively used, and its manufacture is fully described in the article. Next to coke, coal is the most commonly used fuel. Attempts have been made in a few instances to use gaseous fuel, but they have met with little success. The flux universally used in smelting iron is limestone. The purpose and action of this flux has been explained briefly in describing the chemical reactions which take place in the blast-furnace.

The product of the blast-furnace, as already said, is cast iron. As this is the preliminary form of practically all iron for making both wrought iron and steel, its properties require a brief description. Cast iron consists of metallic iron combined with at least 1½ per cent. of carbon and smaller percentages of silicon, sulphur, phosphorus, and manganese; the elements other than metallic iron being about