By the solution of their carbon, these carbon-brick are, of course, destroyed, and this is'the reason that many bottoms of blast-furnaces, as well as the lower portions of the hearth, con structed of carbon-brick, are rapidly dissolved. Hearth-bottoms constructed of carbon-brick have shown the least durability, whilst the same material, when used for the portions of the walls of the hearth and of the boshes which do not constantly come in contact with the liquid iron, is apparently quite durable. However, since carbon-brick have been in general use for a short time only, experiences in regard to the latter subject are not sufficient to allow of a final judgment. Formerly, when blast-furnaces were worked more slowly, only the refractory brick-work of the hearth was dissolved, but in consequence of the constantly increasing demands made on the blast-furnaces by working with the introduction of more and hotter blast, the dissolution first extended to the boshes and lately even to the refractory brick-work of the stack.
Cooling with water being the only means known at present for preserving the brick-work of blast-furnaces, not only the hearth, but also the boshes, and in recent times, the stack as well are cooled.
It is not necessary to discuss here the cause of the great wear of certain portions of the stack of a blast-furnace in which ferromanganese is produced, the production of the latter being a limited one. As causes of the rapid wear of the brick-work of the stacks of blast-furnaces in general, from a height of sev eral meters to a few millimeters towards the exterior, must be mentioned : 1. Abrasion by the downward passage of the charge.
2. The action of constituents of blast-furnace gases, for in stance, cyanogen and its salts.
3. Melting off by common salt, which is contained in coke.
4. Cracking by separations of carbon and carbonic acid caused by iron particles which are formed from iron disulphide in the refractory brick.
Of the above-mentioned causes, either one or all may act upon the brick of the stack of a blast-furnace.
Ad. 1. With the present improvements, it is not difficult to produce refractory brick which will resist abrasion by the down ward passage of the charge. Hence, this cause need only in exceptional cases be taken into consideration for explaining the rapid wear of the brick-work of blast-furnace stacks.
Ad. 2. It is a well-known fact that blast-furnace gases contain much cyanogen. The formation of it in the blast-furnace is much faciliated by the occurrence of nitrogen together with carbon in coke. By coking Westphalia coals in coke-ovens or gas-retorts, 31 to 36 per cent. of their nitrogen remains, accord ing to Dr. Knoblauch, in the coke ; 1.5 to 2 per cent, of the nitrogen passes over as cyanogen, whilst I to 3 per cent, of it is found in the tar, and To to 14 per cent, of it in the ammonia.
One cubic meter (1.308 cubic yards) of the top-gas of a modern blast-furnace contained 1.97 to 6.6 grammes (30.4 to 101.83 grains) of cyanogen, whilst the gases from the melting zone were still richer in it.* One ton of coke yielding about 4733 cubic meters of gas, the amount of cyanogen produced from it in the blast-lurnace would be from 8.48 to 31.23 kilo grammes (18.65 to 68.7 lbs.). Hence a blast-furnace with a daily consumption of but r00 tons of coke would produce 848 to 3.123 kilogrammes (1865.6 to 6870.6 lbs.) of cyanogen.
These immense, and therefore improbable, quantities of cyanagon, in connection with alkalies or alkaline earths, and perhaps also with volatile metals, would suffice to explain the rapid wear of all parts of the refractory brick above the melt ing zone of the blast furnace. In the refractory clays, which are products of the decomposition of feldspathic minerals, alka lies generally occur.
Whether the cyanogen in the blast-furnace gases is capable of withdrawing, at the temperatures prevailing in the stack of a blast furnace, alkalies from the refractory brick, and thus to decompose them, would have to be determined by experiments. It is, however, an established fact that parts of the cast iron of the apparatus for collecting the waste gases as well as of the iron used as a support of the top of the furnace are frequently pletely corroded by the constituents of the blast-furnace gases.
It has not yet been determined whether cyanogen in the blast-furnace gases is capable of withdrawing alkalies and earths from the materials of the charge, and in the form of cyanide of potash or volatile cyanide exerting a destructive effect upon the refractory brick. Large quantities of cyanide of potash are dissolved from the walls of blast-furnaces by the cooling water, and fused salts of cyanogen also drip frequently from the joints of the walls.
Ad. 3. As is well-known, the coal-measures contain springs strongly impregnated with common salt, and hence the latter is frequently found also in the coal. According to experiments made in 1884, a charge of 6,00o kilogrammes (13,200 lbs.) of coal of a coke-oven contained 1.8 kilogrammes (3.96 lbs.) of common salt ; another charge of the same quantity of coal, 3 kilogrames (6.6 lbs.), and one as much as 22 kilogrammes (48.4 lbs.). Common salt is volatile ; it partially evaporates in coking the coal, and in coke ovens, worked without gaining the by-products, it, together with the hot products of combustion of the gases—hence at a white heat—comes in contact with the hot refractory brick, and, by the silica of the latter, is decom posed to sodium and chlorine.