BLAST FURNACES Having followed thus far the mining, transport and preparation of ore, attention should next be given to the process of smelting it into metal. In the present state of the art, this is done in a blast furnace, and the metallic product is a high carbon alloy of iron called pig iron and containing more or less silicon and man ganese. Since an article is devoted to the construction and opera tion of the blast furnace (q.v.) these matters will be summarized only briefly here; more attention however will be given to that en tire assemblage of auxiliary equipment (see fig. I) which together goes to form the blast furnace department of a modern steel mak ing plant. A blast furnace and its top works often rise 200 ft. above the ground level. It is round in cross section, bound with steel plates, and lined with a thick wall of special fire brick. Pierc ing the very top of the shell are two side openings leading into brick lined pipes—downcomers—for venting the furnace gases. The top is closed by a double bell, which in effect is an air-lock for introducing charges of ore, coke and limestone, while permit ting no gas to escape. Charging is continued at such a rate that the furnace shaft is always full. About 8 ft. above the furnace base is a series of io or 12 water-cooled nozzles called tuyeres, equally spaced around the circumference, through which hot air is blown into the interior. The coke in the mixed charge burns in this air, producing a region of intense heat sufficient to melt the metallic iron descending to that level from above, and superheat it so much that when it trickles to the hearth it collects at the bottom of the shaft in a pool which remains molten for hours. Five or six times a day this iron is drawn off or cast by opening a tap-hole at the very bottom.
Much study has been given to the chemical reactions which reduce the ore to iron. They can only be presented here in highly simplified form. At the tuyeres the heat is so intense that coke burns to carbon monoxide, thus: 2C + 02 = 2C0 It is this gas which reduces the ore, and at a fairly low tempera ture, i.e., near the top, by a series of reactions which may be sim plified as Fe203 3C0 = 2Fe 3CO2 The secret of rapid furnace driving to produce big tonnages is to use such ore, or prepared sinter, that the gases may permeate it readily and completely. Pure iron is relatively infusible—melting point 1,53o° C; when alloyed with 4.3% carbon its melting point is about 1,13o° C. The freshly reduced iron in the blast furnace picks up this necessary carbon to convert it into the more fusible pig iron, either by physical contact with the hot carbon, or by the gas reaction 3Fe+ 2C0 =
a reaction which is the basis of the case-hardening phenomena. By no means all the CO is oxidized to
by the furnace reactions. A typical analysis of the gases from the furnace top is Carbon monoxide (CO) . . . • 4-5% by volume Carbon dioxide (CO2) . . . . 12.0 Hydrogen (H2) - • • . . I • 25 (from moisture in rich enough in the combustible gases CO,
and CH4 to be val uable as a fuel. Leakage must be carefully avoided, for the mon oxide content makes it highly lethal.
Slag Formation.—Impurities in the ore and the coke ash must also be disposed of. They consist of earthy substances like clay. If they contain a preponderance of a single oxide like silica
they cannot even be melted unless sufficient limestone is present to flux them—that is to say, convert them into an easily fusible glass or lava-like substance called slag (q.v.) or cinder. The formation temperature of slag depends upon the relative proportion of basic oxides (lime CaO, or magnesia Mg0) and acid oxides (silica
or alumina
which are present in the furnace charge. The operator therefore chooses his ores and fluxes with the aim of producing a slag which works well at some tem perature, which he does not measure, but which he knows will produce the kind of pig iron which is demanded by his customers. Like the iron, slag melts in the hot region above the tuyeres and trickles down, forming a pool floating on the iron. It is tapped or flushed through a cinder notch located between the tuyere line and the furnace bottom; usually just before the iron is tapped.
A medium-size furnace making 600 tons of pig iron a day will consume 1,16o tons of ore, 58o tons of coke, 290 tons of limestone, and 2,370 tons of air blast. It produces in addition to the iron, 330 tons of slag, 7o tons of dust and 3,400 tons of gas. It requires 17,000 tons of water to keep the furnace parts cool and to wash the dust from the gas. Therefore equip ment must be provided to handle expeditiously no less than 26,000 tons of material each 24 hours. Such auxiliaries are so large and so numerous that the furnace itself is frequently inconspicuous, despite its huge bulk (fig. I). In America, much space is ordinarily given to stock piles. Ore that comes from such storage, or direct from the mine, is dumped into bins alongside the furnace. Other compartments hold coke and flux. On a track underneath operates a scale car, which stops under appropriate gates and the attendant draws off a specified amount. When the proper quantities have been assembled, the car proceeds to the skip pit, and drops its load into a car which in turn is hoisted up the inclined bridge and dumped into a hopper at the very top of the furnace.
Furnaces a century ago were usually built on sloping ground, and the ore and charcoal wheeled off the hill top and dumped into the furnace. Later, when the value of the gases was dis covered, a single bell was used to close the furnace top, and as the tonnage requirements increased, ore, coke and flux were shovelled from stock piles into carts, which were then pushed to the furnace, hoisted on platform elevators, and dumped by hand.
Material handling by machinery is essential for furnaces making more than a hundred tons of iron a day.
In view of the congested sur roundings, the pig iron and slag tapped from a big furnace are led by clay-lined troughs or runners to the edge of the founda tion where they flow into ladle-cars standing below, and are hauled away while still liquid. If the furnace is serving a steel making plant, the iron may be taken direct to a steel-making furnace. More usually it is poured into a mixer that is in effect a storage reservoir, sometimes of 1,500 tons' capacity. A mixer is usually shaped like a huge barrel, made of strong steel plate and lined with fire brick it may be rolled on its foundation by suitable trunnions so the opening may at one time receive metal from a ladle-car above, or later discharge it into another for transfer to the steel furnace. The bad effect on steel-making processes of sudden variations in composition of the pig iron, so-called off casts, is minimized by dilution in a pool of reserve metal. Furthermore, some of the harmful sulphur may be elimi nated from high manganese pig iron in the mixer, if the tempera ture is properly controlled. Iron which cannot be converted promptly must be cast into pigs. Much of this work is done by machinery. A pig casting machine is an endless chain to which is attached a series of moulds. A ladle-car of iron pours slowly as this conveyor moves underneath. After the iron is chilled, the conveyor passes through a long water tank for more rapid cooling. When the chain moves over the tail pulley the moulds turn upside down and the now solid pig iron drops into a railroad car. Some furnaces specializing on foundry iron, and of rather limited tonnages, run their iron direct from the tap hole into pig moulds formed in the sandy floor of the furnace house (see PIG IRON). Slag may be hauled to a nearby bank, dumped and wasted. In some localities, the slag may be marketed for railway ballast, filling material, or concrete aggregate. It would then be run molten into a water sump, the stream being broken up by a strong jet of water. Granulated in this manner, it can be reclaimed by grab-bucket and crane and shipped. Since the slag is essentially a calcium aluminium silicate, large tonnages are utilized as raw material in the manufacture of Portland cement.
As noted above, gases from the furnace top are good fuels, but they bear too much dust to be used without cleaning. Even when burned under boilers it is found that dust will fuse to the water tubes, reducing the effi ciency of heat transfer and increasing the cost of maintenance to a prohibitive figure. For use in internal combustion engines, the cleaning must be even more complete. Much of the coarser ore and coke particles swept out of the furnace with the gas remains in the dust catchers, one or two cylindrical tanks placed in series just alongside the furnace proper. Essentially they are enlarge ments of the downcomer pipes; the velocity of the gas and its carrying power are reduced in passing through them. A change in direction of travel also aids in catching solids, which as they accumulate are drawn off from the hopper-like bottom. Usually the reclaimed dust is mixed with fine ore, sintered, and resmelted. Mixed blast furnace and coke oven gas is a good fuel for heat ing cold steel. Formerly it was usual to send hot gas from the dust catchers direct to the stoves, but lately furnace managers have been convinced that this rough gas contains entirely too much dust for this purpose, and the efficiency of the heat transfer suffers therefrom. Consequently further cleaning, either by scrubbing towers, by electrostatic precipitation, or by filtering through bags is now commonly undertaken. Electrostatic precipi tation has the advantage that it handles gas without cooling it and may recover valuable amounts of potash for fertilizer. In this process the slightly humidified gas is passed through vertical pipes ; in the centre of each is a wire charged at high voltage. In the electrical field so formed solid particles drift toward the grounded pipe walls, adhering there until dislodged by hammering.
Fig. 1 shows a scrubber, of which two would be required, each about So ft. high by 12 ft. diameter. By means of spray nozzles and screens the ascending gas is intimately mixed in these towers with fine particles of water. The dust content, which on entering is about 3.5 grains per cu. ft. is washed out except for about 0.2 grain. A powerful fan is necessary to draw the gas through the scrubber and to give it the necessary delivery pressure ; water jets are introduced through the shell at several places so the fan acts as a further cleaner, delivering gas to stoves or boilers with about 0.05 grain of dust per cu. ft. Frequently all the gas is burned in this way, utilizing the steam so generated to drive blowing engines or electric generators. When it is desired to use the heat more efficiently, internal combustion prime movers may be installed (see GAS ENGINES) whereupon an even cleaner gas is required. Dust may be reduced to less than o•oi grain per cu. ft. by a rotary washer, which is a combination turbo-blower and cleaner. The water used for these cleaning purposes may be again used after passing it through a cooling' tower and settling basin.