This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
Hot and Cold Blast.
The air for the hot blast is heated by passing it through the above mentioned iron pipes around which the gases play ; or, where the brick chambers are used, the gases are turned into the chambers alternately, that is, one after another until the brick linings of one set of chambers are highly heated ; the gases are then turned into the alternate set of chambers and the air to be used in the hot blast is admitted to the first set and becomes quickly heated to a temperature of from 900° to 1500° F. by contact with the hot bricks. As these cool the process is reversed, the alternate chambers being now used. These chambers are about 17 feet diameter, 60 feet high cylinders of plate-iron, made air-tight and lined with fire-brick. The interior being lined with a mass of intersecting flues of firebrick. The balance of the hot gases which do not pass to these chambers are used to heat the boilers, which supply the necessary steam-power for the hoisting machinery, forced blasts, etc.
The process of smelting ores into pig-irons is, then, roughly this: The ore, flux and fuel are charged into the furnace from the top, in alternate layers at stated periods. A fierce fire is kept going and supplied with the necessary air (either hot or cold) for combustion at the bottom by means of a forced draught. As the above layers descend in the furnace they change their nature. The fuel gives out carbonic oxide which reduces the iron. The latter gradually separates from its impurities and combines with more or less carbon from the fuel. The ashes of the fuel and impurities of the ores combine with the flux (the melted limestone), and when all reaches the bottom we have the pure melted iron (with more or less carbon) at the very bottom or hearth; over this, in the crucible, float the melted flux and combined impurities; above come layers of less perfect iron, flux, partly-consumed fuel, and so on to the top. Before drawing off the melted iron at the Hot Blast.
Description of Process, bottom, the impurities and flux, known as the "slag," immediately-above the melted iron, are first drawn off.1
To be more technical in the above description, we should say that the ores of iron, whether oxides, carbonates or hydrates, are reduced either by their preparatory roasting or during their early passage down the top of the furnace shaft to the state of oxide of iron (ferric oxide).
The ensuing reaction in the furnace is, therefore, for all practical purposes, the reduction of this ferric oxide (Fe2 O3) when red hot by the action of carbonic oxide (C O) produced by the incomplete combustion of the fuel farther down the furnace. The iron gives up its oxygen to the carbonic oxide leaving metallic iron (which then takes up with some carbon) and carbonic dioxide (C O2) which passes away in the waste gases. It should be noted here that pure metallic iron is infusible at the temperature obtainable in the blast-furnace. Its combination with carbon, however, to the extent of from 2 per cent to 5 per cent renders it easily fusible, and constitutes the pig or cast iron. Were it not for this fact the blast-furnace would be impracticable, as can be readily imagined.
The mission of the limestone or other fluxes, is mainly, when melted, to effect a more ready fusion or separation of the earthy impurities or "gangue" in the ore and to take up the ashy remnants of the fuel. It is found that the earthy bases are more fusible to an extraordinary degree when they are present together in numbers. Further, the addition of lime takes care of the silica present in the ores, which otherwise would unite with the iron, funning silicates of iron, which, though fusible, are difficult of reduction, and further prevent to a certain extent the taking up of carbon by the reduced iron, thus entailing a waste in two ways. The ordinary gangue or matrix of iron ore itself is clayey (argillaceous) or quartzose (silicious). The addition of lime or limestone (or dolomite) results in the formation of a "slag" which is readily fusible at the existing temperature. This slag, which when cold somewhat resembles bottle-glass, is much lighter than the molten iron, and as it collects above it, is drawn off just before casting from the surface of the melted iron in the health through openings placed at the proper level, just below the crucible.
Use of Flux.
1 This Blag forms the basis of the "mineral wool," largely used for various purposes.
To undertake to enumerate all of the brands of pig-iron used in casting would be an endless task. A few, however, may be here mentioned.
Amongst those principally used in the New York market are:
All Scotch irons : used as softeners, in connection with scrap-iron or lower grades of American pig-iron for cheap and inferior castings.
English Bessemers : are soft and strong and are used in place of best Scotch iron.
Manhattan. (New York),
All American brands.1 The Manhattan is very fine; Secaucus and Castle are very strong; Thomas and Glendon are very popular and their Nos. 1 and 2 largely used for strong and good castings. The last three on the list are weak and soft cinder irons and are unfit for architectural uses, being used principally for stove-plates and pipe making.
Low Moor, (Virginia),
All pig-irons are graded in three kinds, namely, Mill iron, Foundry iron and Bessemer 2 iron. Each of these is again subdivided into the following six grades:
No. 1, No. 2, No. 3, Grey Forge, Mottled, White.
1 For makers' names and addresses, see "The Directory of the Iron and Steel Works of the United States," published by the American Iron and Steel Association, 261 S. 4th Street, Philadelphia.
-Any foundry iron which is sufficiently low in phosphorus (not over 0,1 per cent) and silicon can be used in the Bessemer process.
"No. 1" is the best and strongest, "No. 2" the next best, and so on to the "White," which is the poorest quality. Grey irons contain more graphitic carbon and are softer and more fusible than white irons, which contain more combined carbon, and are much harder and more brittle.
If the pig-iron on fracture is dark grey with spots it is soft and will run freely into the mould, making a good casting but not a strong one. Black specks, if present, mean carbon. If the carbon in the iron is chemically combined, it will show white metal, with no specks, on fracture, in which case the iron is very hard and brittle and will not flow easily into the mould, but will make a very strong casting.