The disposal of the enormous output of slag or scoria from blast furnaces has always been one of the serious difficulties of the iron trade. Taking an average of all the districts in England, for each ton of iron made, 25 cwt. of slag is produced; and from the official returns of 1879 of the iron smelted, no less than 8,000,000 tons of slag were produced. The space occupied by this mass, when loosely tipped, is something like 170,000,000 cub. ft., whilst the bulk of the iron occupies only J of the same space. There is, however, this great difference between iron and its .refuse, that, whilst the former is diffused and finds its way into every corner of the world, the latter is left behind at the smelting-works, absorbing something like 250,000/. annually in its disposal, and destroying hundreds of acres of valuable agricultural land. While we produce such enormous quantities of iron, so long will these heaps go on accumulating; and there is little chance that existing masses will ever be turned into a marketable product.
At the same time, blast-furnace slag possesses many valuable properties, which may in certain localities be converted into things useful to the arts and sciences, and at considerable profit.
Of other slags produced in metallurgical operations-such as in the smelting of copper, lead, zinc, and tin ores-no use is made; but there arc also slags, or cinders, produced in the manufacture of wrought iron, some of which are re-smelted, after which no great bulk of refuse is left.
Blast-furnace slag, as it flows from the furnace when making foundry iron, is usually of a grey colour, of much the same consistency as molten glass, a substance, in many points, it greatly resembles, particularly when the more siliceous ores are being smelted. It is very fluid, and has a temperature considerably above the melting-point of cast iron; in proof of which, if a piece of cold cast iron be placed in a block or wagon of fresh molten slag, it readily melts. At this high temperature it contains a large quantity of gas, a considerable portion of which is thrown off or exuded as the slag cools down or becomes set. So much is this the case, that a large "block' or "ball," technically so termed, will often burst, an hour or two after being run, from the accumulation of this gas in the inside.
The bursting of these balls at the ironworks is of constant occurrence, and a source of danger, caused by the liquid slag and the outside shell dropping after the bail has burst. This is partially overcome by making the workmen knock a hole through the top crust before leaving the furnaces. Again, the least derangement in working the furnace is quite sufficient to alter the nature of the slag, and often, within 1/2 hour, will the slag be changed from grey to a perfect black. Such a colour usually indicates imperfect smelting, and the slag will be found to contain a larger proportion of iron than it should do.
For many years the only known use for blast-furnace slag was in road-making, and for this purpose it is still largely employed. In Northamptonshire, and in certain districts of Yorkshire, the whole of the slag produced is sold at a considerable profit. These, however, are local exceptions. Large quantities of slag are used in the works on the breakwater at the Tees mouth, something like 500,000 tons annually. A similar class of work is carried on at Barrow-in-Furness, from the slag produced at the hematite furnaces in that town; but in consequence of the large amount of lime contained in this slag, much greater care has to be taken in its selection. The slag used at the Tees breakwater is chiefly taken away upon bogies, in blocks weighing 3 1/2 tons each. The slag is run into these blocks upon the waggons at the furnaces, a case or box being placed upon the bogie for this purpose. When the slag is sufficiently "set" this case is removed, and the waggon, with the block upon it, is taken a distance of about 6 miles to the breakwater. A large quantity is also tipped upon a platform on the riverside, in such a position that the tide completely covers it; it is then wheeled into hopper barges.
Fowler and Wood devised c plan for shipping the bogies with the hot balls into barges, and towing them down the river for discharging; each bargc is constructed to carry 40 begies and is about 220 tons burden. The leading of these barges at all states of the tide has necessitated the construction of special machinery by Appleby Bros., Southwark.
The next stage in slag utilization is the endeavour which has at various times been made to run the liquid slag as it flows in a stream from the furnace, into moulds; or, in other words, making slag castings. Such an idea at first sight would seem natural enough. Here, it may be said, is a material flowing to waste, in a liquid state, capable of being run into moulds, and of taking impressions almost equal to those of cast-iron. The castings also, when successfully made, are exceedingly durable, and even beautiful to look at. So alluring has been the idea of casting, that during the last 50 years the Patent Office has recorded almost annually the attempts of some inventor impressed with the notion that he could treat this treacherous fluid successfully. To describe these various schemes, or to give even an outline of them, would occupy too much space, but the following remarks will afford a general idea of the difficulties.
The high temperature at which the slag leaves the furnace has been before noticed-namely, about 3000° F. (1727° C), but, when it is brought into contact with anything cold, in the shape of a mould, it readily parts with its heat, and, in so doing, suddenly contracts. The surface contracting, becomes filled with fine cracks or flaws; so much is this the case that, if allowed to assume entire consolidation in the moulds, these cracks will be found to penetrate completely through the casting, and upon exposure to the air the casting falls to pieces. This is the more vexing as, when slag is run into a large mass - say into a pit of sand 8 or 10 ft. deep, and containing 30 to 40 tons - there is such an enormous amount of heat accumulated that it becomes self-annealing, the outside of the mass is kept at a high temperature, and, if allowed to remain until cool, not a flaw will be found, and the slag becomes so tough and hard that it may be quarried- in the same way as granite or whinstone, and used for street-paving.