In the construction of the blast furnace the conditions to be met are diverse and to some extent conflicting. The efforts to meet them have been almost innumerable, and the most varied types of construction have been used for almost all portions of the furnace. An outline of the requirements to be met will assist in explaining the types of construction used.

Resistance To Temperature

Resistance to temperature is necessary to some extent throughout the furnace, but particularly in the hearth and bosh. The temperatures in these regions in the days of cold-blast furnaces were moderate, probably around 2200° to 2400° F., and were resisted with some success even by natural refractories such as sandstone, which was sometimes used for the bottom and for the lining of the hearth and bosh of furnaces, even in coke practice, within little more than a generation.

But with the development of the furnace in the direction of greater output, and the more accurate control of the quality of the metal produced, the requirements have steadily grown more severe. Temperatures in the hearth and bosh in coke practice now probably range from 2500° to 2800°, these being the temperatures of the iron and slag as they come from the furnace, but it is almost certain that even higher temperatures exist around the tuyere zone. Probably 3000° is not far from the working temperature prevailing in that zone in many furnaces. This approaches the temperature at which all . materials lost a great portion of their resistance. If they do not melt they soften and are easily deformed or abraded.

Resistance To Chemical Action

The lining of the blast furnace is probably exposed to more diverse chemical conditions than that of any other apparatus in industrial use. Iron itself, when molten, particularly before it is completely carbonized, is exceedingly active chemically and will dissolve or react with almost anything with which it comes in contact, especially substances in their elementary condition.

In the early days when sandstone was used as a refractory, slags used in coke practice were relatively neutral - that is, they were what would now be called very acid. The requirements were then not nearly so severe in the matter of desulphurization, and as a consequence not much more lime was added than enough to make a fusible slag. But as the demand for a purer product has grown, more and more base has been added to the slag until now its chemical activity in most cases is intense. Few materials with which it comes in contact can long resist its solvent action unless protected by water-cooling and so kept below the temperature at which they can be attacked.

As we go further up in the furnace we come to the zone in which the cyanides resolve themselves into different chemical compounds of great chemical activity in consequence of their composition and their nascent condition. We have here also an atmosphere consisting almost wholly of nitrogen and carbon monoxide nearly at a white heat initially, which gradually declines as the top of the furnace is approached. The reducing power of carbon monoxide under these conditions is enormous and is reinforced by the presence of incandescent carbon itself in the bosh of the furnace. Therefore, any metallic oxide in the lining which is subject to reduction by either of these agencies would be immediately reduced with consequent destruction of any refractory of which it composed a part. It is probable that many mysterious cases of bad results with firebrick in the early days of their manufacture were caused by the presence in the brick of iron, which being reduced by the action of the furnace caused disintegration of the brick and the early destruction of the lining.

The fume in blast-furnace gas which gives the white color to the waste gas from a hot working furnace contains an appreciable quantity of soda and potash; these are liable to be precipitated by the decreasing temperature toward the top of the furnace, and to have a highly solvent action on the silica of the lining when this happens.

When zinc and lead, particularly the former, are present in the charge, even in almost infinitesimal quantities, they are vaporized and have a variety of actions on the lining which is almost incredible to one who has not torn out linings subjected to these conditions, as I have many times done. As an instance of the peculiar actions which this substance has, it may be illuminating to describe what I once saw in the bosh of a furnace blown out for repairs to the upper lining, the bosh being good and left intact.

The furnace was blown until the stock had been burnt out below the level of the tuyeres, and crawling in through a cooler opening as soon as the little material remaining in the hearth could be cooled down sufficiently, I was astonished to see what looked like little stalagmites, yellowish white in color, varying in length from two inches to six or more, and about a quarter of an inch in diameter at the base, standing up all over the bosh wall. When touched they fell to pieces and it was with great difficulty that some specimens were taken out whole. They were hollow, with walls only about as thick as wrapping paper, and very light and flocculent, hardly more than dust. Careful examination showed that they were the oxide of zinc which had been contained in the brickwork of the bosh, presumably in some sort of state of solution, until after the pressure of the blast had disappeared. The oxide had then exuded, or more properly perhaps I should say "sublimed" out of the brick into the solid condition, and, being carried upward by the air let in at the tuyeres to cool off the furnace, it formed these little stalagmites.

In tearing down the brick outer walls of the older types of furnaces I have found large lumps of crystals of silicate of zinc in cavities in the lining at a distance of four or five feet from the nearest point to the inside of the furnace.

I have seen bricks taken out of the old lining apparently perfect in size and shape but somewhat dark in color. When lifted these were found to be two or three times the weight of ordinary bricks and if measured carefully would be found to be half an inch larger in each dimension than the original brick from which they had grown. Theses were saturated with zinc oxide, which had swelled them as a whole without the least sign of cracking, the bricks being firm and, if anything, stronger than in their natural condition.

These instances are mentioned only to show the vast variety of chemical actions which the lining of the furnace must resist.