Fundamental Conditions - Definition of Blast-Furnace -Classification by Fuels Used - Essentials of Plant.

Iron is one of the most common elements in nature. Geologists state that it comprises about 4.4 per cent. of the earth's crust and that there is strong proof that the whole interior of the earth below a depth of a few miles is composed of iron. The vast segregations which have gone on throughout geological time have resulted in concentration of iron to all degrees from the lowest up to 72 per cent. The concentration never exceeds this degree (Fe3O4) for the reason that iron is unlike most other of the useful metals in one important particular. It is seldom or never found native on the earth, except in the form of meteorites which have fallen from space.

This is in consequence of the great affinity of iron for oxygen, with which it will unite under almost all conditions of temperature and chemical environment found on the earth. This brings us to the fundamental fact in the metallurgy of iron which distinguishes it from all the other useful metals.

Its only ores are its oxides. The sulphides are probably the most important ores of the other useful metals but sulphide of iron is not regarded primarily as an ore of iron but only as an ore of sulphur, partly because its sulphur value is greater than its iron value and partly because the blast furnace cannot produce good iron from material containing much sulphur.

When the sulphur is burnt off for conversion to sulphuric acid the iron takes up oxygen to replace the sulphur and becomes an oxide. These artificial oxides then become iron ores under certain conditions and with certain limitations which will be discussed later. But the use of iron sulphide as iron ore either by burning the sulphur off to waste, or directly in the furnace, as is so commonly done with the sulphides of other useful metals, is entirely unknown in the metallurgy of iron.

It is true that carbonates are important sources of iron in Europe, but their carbonic acid is not strongly held and before they are charged into the furnace it is driven off by an oxidizing roast which converts the resulting ferrous oxide FeO to ferric oxide Fe2O3. This reaction takes place so easily that it even occurs slowly under ordinary atmospheric conditions, but with the slight modification in that case that the ferric oxide takes up water of combination and becomes limonite.

More than 90 per cent. of the world's production of pig iron is derived from ores which occur in nature as oxides and the remainder comes from the roasted carbonates and the ferric oxide or "pyrite cinder" derived from the sulphides.

From the point of view of the blast-furnace, therefore, it is correct to say that the only ores of iron are the oxides.

The removal of the final traces of oxygen from iron is so difficult that it can commonly only be done with an expensive gas like hydrogen or methane or by solid incandescent carbon and as the latter is commercially so much more available we may say that industrially iron is reduced only with solid carbonaceous fuel.

Fundamentally then the metallurgy of iron consists of the reduction of the oxides through the use of solid carbonaceous fuel. This operation is carried on to the extent of a few hundred tons per year in the Catalan forge and to the extent of some sixty or seventy million tons in the blast-furnace, which, therefore, constitutes the foundation of the iron industry.

The blast-furnace is a shaft furnace, the "high-furnace" of France and Germany, and takes its English name from the fact that the air to support combustion must be forced into it under pressure because of the resistance to the passage of the combustion gases offered by its high shaft filled with material. It became, therefore, a "blast" furnace as opposed to a draft furnace, which obtains its air through chimney draft.

The blast-furnace is historically the outgrowth of an attempt to utilize the waste heat of a "low furnace" or Catalan forge so as to reduce the fuel required for the latter and is probably the first industrial application of what is called the counter-current principle.

This may be defined as a method of transmitting the properties of one mass of matter to another by causing the two masses to travel past one another in opposite directions in intimate contact. This process leads naturally also to continuous as opposed to intermittent operation, one of the great desiderata of industrial processes.

This practice of superposing a shaft on a hearth has so long and so universally prevailed in the iron industry that it has caused us to overlook one of the most important facts concerning the blast furnace and one to which we shall have occasion to revert many times; namely that it is an apparatus in which we have merged two distinct processes. First, the reduction of iron oxide to metallic iron. Second, the melting of the reduced material.

The deep fire necessary for melting the "iron sponge" or reduced iron in a reducing atmosphere (without which it would instantly become oxidized again) was not unlike a gas producer in its construc-tion and operation, and consequently discharged a hot gas rich in CO.

The utilization of this heat was the object sought by those early unknown pioneers who added the shaft above the low hearth or Catalan forge, increased the blast pressure to suit, and so transformed it to the blast-furnace.

The fuel economy desired was obtained to such an extent that the low hearth declined to a position of insignificance, although the change brought many other changes not contemplated by the pioneers, some of which were not to their liking.