Pig-Iron Production

The basic materials used for the manufacture of pig iron are iron ore, coke, and limestone. The coke is burned as a fuel to heat the furnace; as it burns, the coke gives off carbon monoxide, which combines with the iron oxides in the ore, reducing them to metallic iron. This is the basic chemical reaction in the blast furnace; it has the equation: Fe2O3 + 3CO = 3CO2+ 2Fe. The limestone in the furnace charge is used as an additional source of carbon monoxide and as a “flux” to combine with the infusible silica present in the ore to form fusible calcium silicate. Without the limestone, iron silicate would be formed, with a resulting loss of metallic iron. Calcium silicate plus other impurities form a slag that floats on top of the molten metal at the bottom of the furnace. Ordinary pig iron as produced by blast furnaces contains iron, about 92 percent; carbon, 3 or 4 percent; silicon, 0.5 to 3 percent; manganese, 0.25 to 2.5 percent; phosphorus, 0.04 to 2 percent; and a trace of sulfur. A typical blast furnace consists of a cylindrical steel shell lined with a refractory, which is any nonmetallic substance such as firebrick. The shell is tapered at the top and at the bottom and is widest at a point about one-quarter of the distance from the bottom. The lower portion of the furnace, called the bosh, is equipped with several tubular openings or tuyeres through which the air blast is forced. Near the bottom of the bosh is a hole through which the molten pig iron flows when the furnace is tapped, and above this hole, but below the tuyeres, is another hole for draining the slag. The top of the furnace, which is about 27 m in height, contains vents for the escaping gases, and a pair of round hoppers closed with bell-shaped valves through which the charge is introduced into the furnace. The materials are brought up to the hoppers in small dump cars or skips that are hauled up an inclined external skip hoist. The raw material to be fed into the furnace is divided into a number of small charges that are introduced into the furnace at 10- to 15-min intervals. Slag is drawn off from the top of the melt about once every 2 hr, and the iron itself is drawn off or tapped about five times a day.

The air used to supply the blast in a blast furnace is preheated to temperatures between approximately 540° and 870° C. The heating is performed in stoves, cylinders containing networks of firebrick. The bricks in the stoves are heated for several hours by burning blast-furnace gas, the waste gases from the top of the furnace. Then the flame is turned off and the air for the blast is blown through the stove. The weight of air used in the operation of a blast furnace exceeds the total weight of the other raw materials employed. An important development in blast furnace technology, the pressurizing of furnaces, was introduced after World War II. By “throttling” the flow of gas from the furnace vents, the pressure within the furnace may be built up to 1.7 atm or more. The pressurizing technique makes possible better combustion of the coke and higher output of pig iron. The output of many blast furnaces can be increased 25 percent by pressurizing. Experimental installations have also shown that enriching the air blast with oxygen can increase the output of blast furnaces.

The process of tapping consists of knocking out a clay plug from the iron hole near the bottom of the bosh and allowing the molten metal to flow into a clay-lined runner and then into a large, brick-lined metal container, which may be either a ladle or a rail car capable of holding as much as 100 tons of metal. Any slag that may flow from the furnace with the metal is skimmed off before it reaches the container. The container of molten pig iron is then transported to the steelmaking shop.

Modern-day blast furnaces are operated in conjunction with basic oxygen furnaces and sometimes the older open-hearth furnaces as part of a single steel-producing plant. In such plants the molten pig iron is used to charge the steel furnaces. The molten metal from several blast furnaces may be mixed in a large ladle before it is converted to steel, to minimize any irregularities in the composition of the individual melts.

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