Truran, an eminent English founder and engineer, argues strongly that the best form of furnace is that of a gradual increase in diameter from the hearth to the throat. His experience entitles his opinions to respect; but we have not heard that his views have been practically demonstrated, though experiments were in progress over ten years ago by Mr. Coleman, of the North Lebanon furnaces, who erected a stack on Truran's plan. It did not, however, produce successful results ; but we are informed the difficulty was not in the stack or plan, but in some minor details of blast and machinery.
The benefits arising from a large area in the upper part of the boshes and the body of the furnace are several. It admits of the ready escape of the products of combus tion by affording space for their ascent, and an open condition of the burden to admit a free passage.
The longer the mass of ores are exposed to the escaping heat and carbon, the longer will they be subject to torrefaction, oxidization, and carbonization, and, consequently, the better prepared on arriving at the point of fusion to be reduced or separated from their earthy impurities.
Truran says there should be no escape or waste of carbon ; that it should be entirely taken up by the ores in the body of the furnace, and that the gases which now escape or are used under the boilers and in the hot-blast oven really constitute "waste heat," and could be put to a better purpose in preparing the ore than in generating steam or heating the blast.
It seems probable that such a consummation would produce economical results; but the difficulty lies in the application and mechanical construction of the furnace. The common theory is that the escaping carbon unites with the oxides of the ores and forms carbonic oxide. It does not appear, however, that the carbon of combustion unites with the oxides of the ores for their expulsion. It may expel the oxide by taking its place, thus forming a carbide of iron. If it necessarily unites with the oxide in order to deoxidize the ore, we cannot understand why the quantity of oxygen should be diminished by the process, since the carbonic acid of combustion is changed to carbonic oxide in ascending through the furnace.
In the economy of combustion the fixed carbon of coal is not consumed or destroyed, but reduced from a solid to a vapor or gas, in combination with the oxygen of the blast, forming carbonic acid, which consists of one equivalent of carbon and two of oxygen. In passing up the furnace, through the coals, this carbonic acid loses part of its oxygen and becomes carbonic oxide. It is, therefore, impossible that the vapors of combustion should affect the deoxidization, since the volume of oxygen is reduced instead of in creased in ascending through the furnace. The coal, therefore, which comes in contact
with the ores must absorb the oxide of the latter, while the ores absorb part of the carbon of the former, and both are thus improved thereby. The coal is better prepared for combustion, and the ores for reduction.
We do not pretend to be experts in the science of chemistry, and our arguments may be fallacious; but such appears to us to be the operations going on in the body of the furnace.
The carbon of the coal, expanded by heat to many hundred times its original bulk, and the entire amount of air thrown into the furnace by the blast, also vastly expanded, must escape in vapor; but neither the one nor the other is destroyed. They return again to the earth and the air through the chemical agencies of nature, which operate constantly. Heat rarefies them, but cold condenses them.
It would appear, therefore, that the greater the mass of coal and ore through which the "waste heat" of furnaces is diffused, and the longer they remain in contact, the greater will be the quantity of oxide extracted, and the greater the quantity of carbon absorbed by the ore. But it is not probable that the vapor of carbon—arising from the combustion of coal before the blast—should carbonize the ores or unite with their oxides in our present blast-furnaces, since carbon, in combination with oxygen or hydrogen, can be reduced from a vapor to a solid only by intense cold or great pressure, or both combined. The vapor of combustion in ascending through the furnace changes from carbonic acid to carbonic oxide, and must lose a portion of its oxygen in the pro cess. If it took up the oxides of the ores, this could not happen, since the volume of oxygen would be increased; but, on the contrary, we find it decreased.
Combustion does not take place above the zone of fusion, or beyond the direct in fluence of the blast, in smelting-furnaces, and this region cannot extend much above the hearth. The product of this zone is carbonic acid gas, which extinguishes combustion; therefore neither the carbon of the coal nor the oxide of the ore can be consumed or vaporized when in contact with this gas; and, since it arises immediately from the zone of fusion, these processes cannot go on except when under the direct influence of the blast. Therefore the ores which are not deoxidized by the coal must be deoxidized at the expense of the burning carbon at the point of fusion.