Slag heap

“ In the furnaces the molten iron, because of its greater specific gravity, settles to the bottom, while the slag floats to the top. The slag, by the way, is not, as I had supposed, altogether worthless, but is used for railroad ballast and in the manufacture of cement.
The molten iron drawn from the blast furnaces runs in glittering rivulets (which, at a distance of twenty or thirty feet, burn the face and the eyes) , into ladle cars which are like a string of devils' soup bowls, mounted on railroad trucks ready to be hauled away by a locomotive and served at a banquet in hell. ”

“ The next step is to tap the furnace and draw off, first the molten slag, and then the molten metal. Generally the slag is run to waste, but sometimes it is preserved and submitted to treatment which permits its utilization. Some of these uses are road metal, railway ballast, slag bricks, slag wool, and hydraulic cement. Bricks are made by casting the molten slag in molds. To produce slag wool the molten slag is blown by a jet of steam, which produces small globules, to each of which is attached a long thin filament. ”

“ The disorders to which blast furnaces are liable, have a tendency always to produce white cast iron. The colour of the slag or scoriæ is the surest test of these derangements, as it indicates the quality of the products. If the furnace is yielding an iron proper for casting into moulds, the slag has an uniform vitrification, and is slightly translucid. When the dose of ore is increased in order to obtain a gray pig iron, fit for fabrication into bars, the slag is opaque, dull, and of a greenish-yellow tint, with blue enamelled zones. ”

“ Smelting of the drawn slag, on the slag-mill hearth.—The black slag of the reverberatory [758] furnace is broken by hammers into small pieces, and mixed in proper proportions with the coal cinders that fall through the grate of the reverberatory fire. The leaden matts that float on the surface of the bath, and the dust deposited in the chimney, are added, along with some poor ore containing a gangue of fluor spar and limestone, which had been put aside during the mechanical preparation. With such a mixture, the slag-hearth, already described, figs. ”

“ The earthenware pot is now again found to be about half full, and its contents are poured off, allowed to cool, and put up in jars.
Utilization of Blast-Furnace Slag.—Within a few years great progress has been made in the utilization of blast-furnace slag, and that material is now applied in many ways with great advantage. Thus, slag "sand" is employed for making concrete, building-bricks, mortar, and cement; slag "shingle" for concrete, also for roadways; slag "wool" for covering steam-boilers and pipes, ice-houses, etc., also for filtering-purposes ”

“ The slag is so much lighter than iron that when the ore is melted the slag floats on top just as oil floats on water, and can be drained out of the furnace through a higher opening than that through which the iron flows. The slag tap is open most of the time, but the iron tap is opened only once in about six hours. It is a magnificent sight when a furnace is "tapped" and the stream of iron drawn off. ”

“ The reason why the slag is not heated effectively is that the heat is developed only in the layer of metal itself, by its resistance to the induced current, and hence the only heat which the slag receives is that supplied to its lower surface by the metal, while its upper side is constantly radiating heat away towards the relatively cool roof above. ”

“ Relays of boys keep up a continual blast, and, when the basin for the reception of the metal is nearly full, the charging of the furnace is discontinued, and it is blown out. Through an aperture at the bottom the greater part of the slag is withdrawn, and the temperature of the furnace not being sufficient to reduce the metal to the fluid state, it is mixed up with a quantity of impurities, and broken, when still warm, into small pieces. ”

“ When all the slag has been drawn off, there is a moment's pause, and then a new cascade begins. The steel is beginning to flow, not in a daffodil stream like the slag, but in a cascade of exquisite turquoise blue, melting away at the sides into iridescent opal. Sometimes a great cloud of steam from the pit below passes across the mouth of the crucible, and then the torrent of molten steel takes on all the colours of the rainbow, and the great shed, with its alert, swiftly moving figures, is suffused with a radiance of unearthly beauty. ”

“ This is cast iron; you see pigs of it piled up on the wharves in seaports, the outside incrusted with the sand in which it was run, and looking as rough, some of it, as the cinders of a smith's forge. It is highly charged with carbon, coarse, hard, and brittle; can neither be filed, welded, nor worked, under the hammer; is more or less filled with slag and other impurities, and fit only, when melted again and purified, to be cast into pots, pans, stoves, wheels, and various articles. It is now melted two or three times more, and slightly hammered, to beat off some of the slag. ”

“ In building modern reverberatories, the foundation for the hearth is constructed of solid masonry or brickwork, or as at Anaconda, of a solid bed of slag, some 24 inches in depth, run in from an adjacent furnace. The I-beams used for carrying the bracing are erected in a surrounding trench, and a further quantity of slag (4 feet thick by 2 feet deep) is run in, thus yielding a perfectly rigid and impervious foundation (Fig. 26) . On the top of this slag-foundation is built a layer, 12 inches thick, of silica bricks, and upon this, the actual working bottom of the furnace is constructed. ”

“ The slag-pot is a conical vessel, with a rounded apex, poised, base uppermost, on four little legs; when it is full, an iron frame work of a cart runs up, seizes it on opposite sides as though with two hands, and wheels it, glowing and fuming, out where a mole of slag is pushing itself over into the white gravel of the gulch, and where it is deposited red and crackling among heaps of like cones, some fading into the ashy hues of spent heat, some black and shining like inverted crucibles of polished iron. ”

“ Slag felt, a preparation of slag wool, which is a material produced by blowing off waste steam into the slag of iron furnaces, is also used for this purpose. In the case of the slag felt the process is as follows: On the under side of the joists, fillets are nailed to wooden blocks 1 in. thick, and to these fillets the lathing for the plaster ceiling is affixed. ”

“ Their composition must, however, be various, depending upon the nature of the ore from which they are produced; in all cases they are formed from the earthy matter contained in the ore and coal, which the metallic oxides convert into a glass.
In working a slag hearth, the workman's attention is principally required to supply grey slag and fuel as it is melted down and consumed, to keep the nozzle of the bellows clear, and to guard against the metallic lead running along with the slag, into the pit of water. ”

“ Slag as a Building-Material.—"What shall be done with the slag?" is always a very urgent question for the proprietors of iron-works. Many are the plans which have been proposed for the utilization of this waste material, but, if we are to judge of their value by the amount of slag utilized, it must be confessed that they do not help to answer the iron-manufacturers' question. And yet many of the processes for the conversion of slag into a material for building would seem to promise very fair results. ”

“ 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. ”

“ Indeed, this local cooling aggravates the frothing. A cold lump of ore chills the slag immediately around it, just where its oxygen, reacting on the carbon of the metal, generates carbonic oxide; the slag becomes cool, viscous, and hence easily made to froth, just where the froth-causing gas is evolved. ”

“ The stones are completely glazed, and according to the different kinds of slag used are tinted of different colours. This operation is also employed advantageously with tiles, pipes, and earthenware.
If, in the preparation of fire-proof bricks, a certain proportion of mixture of clay and granulated slag be added to the mixture, very hard and durable fire bricks are obtained. These have been tested in a brass furnace, and experiments are being tried as to their applicability to building puddling furnaces. This granulated slag may also be advantageously used for manure. ”

“ But it is to the earthy impurities here given off that we especially wish to direct attention. This slag, as it is termed, when drawn from the furnace, is run into oblong sand-moulds, from which, when cold, it is taken outside to the 'slag-bank,' as it is called—'slag-mountain,' we prefer saying; and an ugly mountain it is!—where it is broken into small pieces, examined to see whether it still contains metal, and if not, is left to accumulate. It consists essentially of silicon, oxygen, iron; or, to speak more correctly, it is a silicate of the protoxide of iron. ”

“ It is filled up to a considerable height with clay to protect it from the hot fire inside, and the pressure of air is applied through those pipes, one or both of them, as may be necessary. When the fire is extinguished or removed, the debris or slag that comes from the metal, and the ashes and cinders from the material used for fuel, can be taken out through the trap at the bottom. ”

“ While the worthless slag is doing so much sputtering and making so lively a show of itself, the silver and lead have quietly sunk to the bottom as fast as the heat liberated them from the mass of the boiling ore, and now come oozing up from a small exit far below the slag-spout, into a well at the side of the furnace. As fast as needful, this liquid ‘bullion’ is ladled out and poured into iron moulds, where it remains until it cools into solid ‘pigs’ or bars of lead weighing about fifty pounds each, and carrying about two per cent. ”

“ The trapping device is an important and essential feature in connection with the modern practice of rapid and continuous running, the principle being to arrange a sufficient height of molten material at the outer side of the slag opening to overcome the inside blast pressure, and thus prevent the escape of blast with its attendant inconveniences and danger. ”

“ Sixteen million tons of refuse! What can be done with it? In some places, land has been bought or hired to provide space for the ugly heaps, and many attempts have been made to lessen the accumulation by finding uses for the slag. It has been made into blocks and bricks for paving; into slabs, pipes, brackets, and friezes; into cement; into sand for fertilising purposes; and while in the molten condition, has been blown into a substance resembling cotton-wool. But some of these attempts have failed, and not one has sufficed to diminish the heaps of slag. ”

“ Since the character and composition of the slag vary in accordance with these conditions, depending largely upon the temperature in the tuyere zone, the furnace works most rapidly and satisfactorily when slags of high formation temperature are being produced. These can only be formed if much iron is being oxidised, because iron is the chief fuel in the process. ”

“ A vitreous material resembling wool, made by subjecting slag, when in a molten condition, to a strong blast. It was originally made in Wales in 1840, but the process was found injurious to the workmen, and so the manufacture was abandoned until about thirty years later, when it was taken up in Germany. As now made, a jet of steam is made to strike upon a flowing mass of molten slag as it falls from the spout into the slag-carriers. The steam scatters the slag into globules, and as each globule leaves the molten stream it turns into a fine thread. ”

“ This process is used with ores containing much phosphorus, the removal of which is necessary in steel-manufacture. The “converters” which hold the molten iron are lined with lime and magnesia, and the impurities of the iron form a “slag” with these materials. For a long time the slag was regarded as a waste product, but ultimately it was found that, by grinding it very finely, it had distinct agricultural value, and now its use is universal. Basic slag is of various grades, containing 12 to 20% of phosphoric acid, which is believed to exist in the form of a tetracalcic phosphate. ”

“ The thick fluid glass, called black slag, after reaching the cast-iron bottom of the furnace, having cooled and thickened a little, does not sink further, but is made to issue through a small taphole, and flow over the cinders placed in the pan, running into the pit filled with water in a continued stream. By falling while hot into cold water, the black slag is granulated, and, as small particles of lead may be carried over with it, through inattention on the part of the workman, or otherwise, the granulated slags are carefully washed at most smelting mills before being thrown away. ”

“ Trouble may arise occasionally by leakages occurring in the jackets or spouts, but by the modern methods of sectional construction and by the devices for time-saving in making the necessary connections, working is usually not seriously interfered with for a very long period. Even for the removal or replacement of a slag spout, the slag-hole is plugged, and the repair is completed within an hour and a-half, by which time slag is again running freely over the replaced slag spout. ”