63. Shaping and Adjusting Processes.—Besides these extraction and purification processes there are those of adjustment and shaping. The adjusting processes adjust either the ultimate composition, e.g. carburizing wrought iron by long heating in contact with charcoal (cementation), or the proximate composition or constitution, as in the hardening, tempering and annealing of steel already described (§§ 28, 29), or both, as in the process of making malleable cast iron (§ 31). The shaping processes include the mechanical ones, such as rolling, forging and wire-drawing, and the remelting ones such as the crucible process of melting wrought iron or steel in crucibles and casting it in ingots for the manufacture of the best kinds of tool steel. Indeed, the remelting of cast iron to make grey iron castings belongs here. This classification, though it helps to give a general idea of the subject, yet like most of its kind cannot be applied rigidly. Thus the crucible process in its American form both carburizes and remelts, and the open hearth process is often used rather for remelting than for purifying.
64. The iron blast furnace, a crude but very efficient piece of apparatus, is an enormous shaft usually about 80 ft. high and 20 ft. wide at its widest part. It is at all times full from top to bottom, somewhat as sketched in figs. 7 and 8, of a solid column of lumps of fuel, ore and limestone, which are charged through a hopper at the top, and descend slowly as the lower end of the column is eaten off through the burning away of its coke by means of very hot air or “blast” blown through holes or “tuyeres” near the bottom or “hearth,” and through the melting away, by the heat thus generated, both of the iron itself which has been deoxidized in its descent, and of the other minerals of the ore, called the “gangue,” which unite with the lime of the limestone and the ash of the fuel to form a complex molten silicate called the “cinder” or “slag.”
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| Fig. 7.—Section of Duquesne Blast Furnace. | |
GG, Flanges on the ore bucket; HH, Fixed flanges on the top of the furnace; J, Counterweighted false bell; K, Main bell; O, Tuyere; | P, Cinder notch; RR′, Water cooled boxes; S, Blast pipe; T, Cable for allowing conical bottom of bucket to drop. |
| Fig. 8.—Lower Part of the Blast Furnace. |
* The ore and lime actually exist here in powder. They are shown in lump form because of the difficulty of presenting to the eye their powdered state.
| Fig. 9.—Method of transferring charge from bucket to main charging bell, without permitting escape of furnace gas (lettering as in fig. 7). |
Interpenetrating this descending column of solid ore, limestone and coke, there is an upward rushing column of hot gases, the atmospheric nitrogen of the blast from the tuyeres, and the carbonic oxide from the combustion of the coke by that blast. The upward ascent of the column of gases is as swift as the descent of the solid charge is slow. The former occupies but a very few seconds, the latter from 12 to 15 hours.
In the upper part of the furnace the carbonic oxide deoxidizes the iron oxide of the ore by such reactions as xCO + FeOx = Fe + xCO2. Part of the resultant carbonic acid is again deoxidized to carbonic oxide by the surrounding fuel, CO2 + C = 2CO, and the carbonic oxide thus formed deoxidizes more iron oxide, &c. As indicated in fig. 7, before the iron ore has descended very far it has given up nearly the whole of its oxygen, and thus lost its power of oxidizing the rising carbonic oxide, so that from here down the atmosphere of the furnace consists essentially of carbonic oxide and nitrogen.
But the transfer of heat from the rising gases to the sinking solids, which has been going on in the upper part of the furnace, continues as the solid column gradually sinks downward to the hearth, till at the “fusion level” (A in fig. 7) the solid matter has become so hot that the now deoxidized iron melts, as does the slag as fast as it is formed by the union of its three constituents, the gangue, the lime resulting from the decomposition of the limestone and the ash of the fuel. Hence from this level down the only solid matter is the coke, in lumps which are burning rapidly and hence shrinking, while between them the molten iron and slag trickle, somewhat as sketched in fig. 8, to collect in the hearth in two layers as distinct as water and oil, the iron below, the slag above.