The reason for the importance of this requirement is that next to carbon these two elements exert the most important influence of any of the common constituents in steel, both being destructive of quality if present to an extent greater than one-tenth of one per cent. If present in the pig iron of which steel is made they cannot be eliminated at all when certain processes are used, and in the others only with increased difficulty and expense. In foundry irons phosphorus is not objectionable within certain limits, but sulphur is almost as much so as in steel, in all ordinary practice.

The blast-furnace may be so operated as to remove the sulphur in the charge but only with increased cost for fuel and flux and with decreased output and in some cases with reduced quality.

For these reasons sulphur in fuel is always bad and progressively worse as its amount increases, although its removal is possible.

This is not the case with phosphorus, virtually all of which present in the charge goes into the pig iron, in normal operation.

Phosphorus in the fuel, on the other hand, may not be objectionable in any given quantity for certain iron, but if objectionable is prohibitive of the use of that material because its removal in the blast-furnace is impossible.

Porosity

The importance of this quality of the fuel comes from the fact that the velocity of combustion of carbon in air is not infinite but limited. Only a limited amount of carbon per unit of surface exposed can combine with oxygen, no matter how favorable the conditions, and in order to secure the rapid combustion necessary to produce high output from a given size of furnace, much surface must be exposed by the fuel.

This cannot be done by reducing its size and so increasing the ratio of surface to volume and mass, except to a very limited degree for reasons already explained, and high combustion velocities can, therefore, only be obtained with porous fuels.

The porosity also has a direct bearing on the volume of a given weight of fuel, and this also will be shown subsequently to have an important influence on the operation of the blast-furnace.

Physical Strength

This is important because without it, no matter how large the lumps of fuel may be initially, they are broken by the superincumbent weight of the charge column as the coke descends into the furnace and we have that condition of fineness of fuel which is so objectionable.

On account of the fundamental effects on the operation of the furnace and the quality of the iron caused by these chemical and physical variations in their fuel, furnaces and the irons they produce are commonly and quite properly classified by the fuels used. These are in the order of their importance as follows: Coke, charcoal, anthracite, raw coal.

The Magnitude And Components Of The Plant

The iron blastfurnace is now probably the largest industrial apparatus in use, not only in size of plant, but in the quantity of raw materials consumed, and output. Furnaces have often been built which produced 600 tons of iron per day, but modern practice tends to slightly smaller units, furnaces now being built only for an output of about 500 to 550 tons per day; but this is maintained as an average output for months and years together.

To produce a ton of iron under favorable conditions requires about 2 tons of ore, 1/2 ton of flux, 1 ton of fuel and about 4 tons of blast; greater quantities are required when the conditions are less favorable. With each ton of iron is also produced about 5 1/2 tons of waste gases and about 1/2 ton of slag, the latter, as well as the iron, being handled in the liquid condition. For a 500-ton furnace we have, therefore, to handle every twenty-four hours about 1700 tons of solids, 750 tons of white-hot liquid and 4800 tons of hot gases (blast and waste gases together).

To do this on an industrial scale and with a labor cost amounting in good practice to less than 50 cents per ton, a great plant is necessary. Such a plant must contain the following parts: First: The shaft-furnace itself, through which the entire mass of material just mentioned passes, the solids passing down to the liquid condition at the bottom and the gases upward through them. Second: Means for handling the raw materials, ore, fuel, flux, proportioning them properly and delivering them in certain accurate and positive relations into the top of the furnace. Third: Means for raising the atmospheric air to a pressure sufficient to force it through the furnace in the quantity desired. Fourth: Means for heating this supply of air uniformly and regularly to the temperature desired. Fifth: Means for handling the molten iron on its discharge from the furnace and turning it into a commercially salable product. Sixth: Means for removing the slag from the vicinity of the furnace. Seventh: Means for removing the gases from the top of the furnace and (as these gases are a valuable fuel) for leading them to those portions of the plant where they can supply the energy for compressing the blast and heating it. There is an eighth requirement not embodied in our fundamental definition, but which has become an absolute necessity notwithstanding; a supply of water for cooling certain parts of the furnace and preventing the intense heat inside from destroying the structure.