Furnace gas is made much more efficient under boilers if cleaned before use, and, if go ing to internal-combustion engines, a thorough cleaning is absolutely necessary.
For use under boilers, the cleaning may be effected by the wet dust catcher of the contact type where the gas is repeatedly directed against a surface of water kept clean by circulation.
Such dust catchers remove 95 per cent of the solid matter in the gas, absorbing less than 1 per cent of moisture and reducing the temper ature about 5 per cent.
Dry Blast.— No furnace plant to-day is complete without some means of regulating the amount of moisture admitted into the furnace in the blast, and the most satisfactory way to do this is to reduce the amount to the lowest possible minimum. This is accomplished by refrigeration of the air admitted to the air cylinders of the blowing engines, a process pat ented by James Gayley and accomplishing greater economical results than was estimated possible in that direction.
In the Gayley process the air is passed in contact with cold pipes to relieve it of moisture by freezing. The dry air thus made is then blown through the tuyeres. When it is re membered that in a large modern blast furnace, such as those used in the Pittsburgh district, about 40,000 cubic feet of air per minute is required to maintain the tremendous combus tion, it is easy to understand that on days when the humidity is high several hundred gallons of water per hour will be carried into the fur nace if undried air is supplied. This water has to be steamed and decomposed, imposing a burden of wasted energy on the furnace. The critical point of temperature in the blast furnace is about 2750° F. If damp air is sup plied at a temperature of 1500°, large fuel con sumption is needed to maintain temperature above the critical point at which the iron is fused. With perfectly dry air at 2000° F. the demand upon the coke fuel is markedly les sened. Not only is fuel saved, but a higher temperature is so easily made that tendency to slow down is removed and production in creased. There is a wider working margin of temperature. Hearth temperature with ordi
nary hot blast is about 1600 B. T. U. per pound of coke; with 50 per cent of oxygen it rises to about 2400 B. T. U., so that with 50 per cent oxygen blast an ore that would use 2,100 pounds of coke is fused with 1,400 pounds.
Iron Ores.— Such ores are smelted when containing from 40 per cent and upwards of iron, and, in case of calcareous ores, even lower grade material has been treated profit ably, but the costs of manufacture increase very rapidly as the yield of metallic iron drops in the mixture. It is, therefore, important to give attention to the preparation of the ore before smelting, with the view of removing objectionable elements. This concentration fre quently removes considerable phosphorus which is practically unaffected by the smelting process, and occasionally reduces the sulphur, which is always a difficult and expensive element to remove.
In concentration, it is usually necessary to crush fine, and, as the ore grains decrease in size below what would stay on a 60-mesh sieve, the difficulty of smelting increases; hence two methods are used to agglomerate this fine prod uct and thus render it • more easily and eco nomically smelted.
One is briquetting either with or without binder; in the latter case, it is necessary to burn the product in a continuous furnace. The other is nodulizing, that is, agglomerating by use of the rotary kiln. This latter process practically removes all the sulphur that occasions any trouble in the smelting, breaking up sulphates as well as sulphides. Ores carrying 6 per cent of sulphur contain, after such treatment, less than 3-10 of 1 per cent. Some ores, principally the soft brown hematites, found quite abun dantly in the Southern States, are best concen trated by washing.
Fuels.— For the blast furnace, coke (q.v.) is to-day the most commonly used, on account of the wide distribution of coking coals, but anthracite is still used largely by the plants within easy radius of the anthracite fields of Pennsylvania and charcoal is still used where timber is abundant.