The development of the retort oven, partic ularly with the saving of by-products, has made possible the coking of coal that is practically non-coking in the ordinary beehive oven. At the same time, it has reduced the cost of cok ing considerably, depending upon the nature of the coal and the market value of the by-prod ucts at the ovens.
With fuel, as with ores, it is important to eliminate earthy impurities before the smelt ing operation. It is therefore found advanta geous to wash coal that is high in sulphur and ash.
By-product coke lacks the silvery color of beehive coke and is not quite as efficient in the blast furnace, pound for pound of the carbon contents. It is also frequently high in moisture, due to faulty methods of quenching. But since the advent of higher blast temperatures these disadvantages are largely overcome and the use of this coke increases.
In order of efficiency in the blast furnace, charcoal comes first, next anthracite, beehive coke, retort coke.
Small furnaces operate with lower fuel con sumption on anthracite than on coke, and it is always necessary to lighten the burden when changing such furnaces from anthracite to'coke fuel. Other things being equal, the fuel with high combined carbon is more efficient in the blast furnace than one of lower carbon content. For example, the best coke from the Pocahon tas region is more efficient than the best Con nellsville; the former, while a soft coke, has from 5 to 7 per cent of ash, while the latter, although hard and silvery, has from 10 to 12 per cent of ash.
Fluxes.— Purity is here a desideratum as well as in ores and fuels, but the only way it can be obtained is by choosing as pure a de posit as possible, then strip off the overlying earth carefully and, in quarrying, throw out stratas or dykes of silicious material.
It is important that the flux for the furnace be crushed to conform with the average of the stock, which ordinarily means broken to pass a three-inch ring.
Both dolomite and calcite are used as flux; the latter is a more active desulphurizing agent but does not make as fluid a slag, and this lack of fluidity offsets to a large extent its greater affinity for sulphur. The greater fusibility of dolomite slag increases the opportunities of calcium present to combine with the sulphur, and hence, as a rule, one flux is as efficient as the other, as a purifying agent in the blast furnace process.
Throughout the South dolomite is used when i basic pig is desired and calcite when foundry iron is sought. In other words, dolomite is found to give low silicon and low sulphur, while calcite gives higher silicon in the pig.
Blast Heating.— Each furnace should- be equipped with blowing engines capable of deliv ering the full quota of air at 30 pounds pressure, if necessary, and provided with governors to give a constant speed without regard to the. pressure of the blast. This practice has been one important cause of the very large produc tion obtained for American blast furnaces, and has been adopted by some of the English iron masters after remodeling their plants.
The volume of the blast required for a mod ern stack is 85 cubic feet per minute for each ton of pig made per 24 hours, and weighs nearly one and one-half times all the solid materials charged into the furnace, hence any variations in the quantity or temperature of this blast acts quickly upon the smelting process going on in the furnace.
In the desire to return as much heat as pos sible to the furnace, the use of the iron-pipe stove, where the maximum temperature of the blast is limited to 950° F., has been superseded by the firebrick stove, where the temperature is only limited by the refractory quality of the firebrick lining. The iron-pipe stoves have the advantage of maintaining a nearly constant temperature of the blast so long as there is gas enough to fully supply the burners in the stove setting, but has the disadvantage of cool ing off very rapidly when blast is taken off the furnace for any purpose. Firebrick stoves drop in temperature from 50° to 250° F. from the beginning to the end of an hour's blow, the usual period, but hold their heat when closed up tightly during a shut-down of the furnace for a short period.
It is a good practice to maintain the temper ature of the stoves 200° hotter than the blast going into the furnace as a reserve to be called upon should the furnace turn cold. With blast at constant volume and temperature there is still another variable which needs controlling, and that is the humidity of this blast.