The humidity of the atmosphere may vary from nine grains in the summer to as low as one grain in the winter, and between these extremes the humidity varies widely and rapidly during even a few hours of any day. Just the importance of this variable to furnace operation was never demonstrated until Mr. James Gayley constructed at the Isabella fur naces in the year 1904 his desicating apparatus to furnish dry air for that plant. This trial showed that with blast at less than two grains of moisture per cubic foot, a saving of 20 per cent of the fuel required per ton of coke was effected.
Gas.—The waste gases issuing from the furnace consist principally of nitrogen, carbonic oxide (CO), carbonic acid (CO,) and water in the form of steam., The ratio CO and CO, indicates the char acter of the combustion taking place in the fur nace hearth, the larger percentage of CO,, the better the combustion and the lower the fuel consumption. A good average ratio for the United States is CO-2 and that is (2 to 1). In rare instances it has been as low as PA to 1, but with a hot furnace making foundry or high silicon pig, it may reach 4 to 1, or in speigle manufacture from 10 to 15 to 1, depending on the mixture being smelted.
To obtain the maximum economy in iron smelting, every effort is made to utilize the heat units escaping in the waste gases. This is ac complished in two ways. One portion is used in heating the blast, as already described, and the remainder is burned for power, either under water-tube boilers, for the generation of steam, or in the cylinder of gas engines, about 65 per cent of the total waste gas produced being usu ally available for this purpose, and the balance, 35 per cent, going into the stoves for heating the blast.
The modern furnace is a large producer of power in excess of its own requirements, es pecially when the gas is utilized in gas-driven engines. Such engines may furnish the blast required and electrical energy for distribution about the furnaces, providing also an excess for sale or distribution elsewhere, amounting to 800 H.P. per ton of pig per hour.
Cinder or This by-product in the manufacture of pig iron is a silicate of the oxides of the metals not reduced in the process. Various attempts have been made to utilize this material, and it is quite extensively used for road-making and for railroad ballast. For this purpose it is frequently run when hot onto an endless chain of cast-iron pans and dis charged, broken and chilled, in cars for distri bution. This method has the advantage of mak
ing the surface of the slag vitreous and thus impervious to water.
The most remunerative use for furnace slag of certain composition is in the manufacture of slag cement. For this purpose the slag must not be over 4 per cent in magnesia and from 12 to 14 per cent in alumina. Two kinds of slag cement are manufactured, the ordinary Puzzo lan, made direct from the slag without reburu ing, and slag Portland cement, made by clinker in g the slag in a rotary kiln and then grinding. When slag is intended for cement purposes it is granulated, that is, run while hot into water, which breaks it up in the form of sand. Such material is also useful to replace sand in mak ing concrete.
The quantity of slag made per ton of pig produced varies from 600 pounds per gross ton of pig to 3,000 pounds and over. The slag has an important bearing on the quality of the pig made and is one of the great purifying agents of the blast furnace. The greater part of the furnace slag produced, however, is unutilized and is usually tapped into iron cars, called ladles, and hauled in the fluid state to the bank, where it is poured out.
Modern practice seeks to reduce the loss of iron in slag, which is considerable, consisting in part of metal chemically held in the slag as a leso oxide, and fine shots of metallic iron suspended in the slag.
Iron This metal is a crude car bide of iron, containing about 94 per cent of metallic iron, from 325 per cent to 3.75 per cent of carbon and graphite, silicon varying usually from Y2 per cent to 4 per cent, and sulphur usually from .01 per cent to .10 per cent, while the phosphorus in Bessemer pig is less than .10 per cent and in low phosphorus pig down to .03 per cent, and in basic from .10 per cent to 3 or 4 per cent, depending on the ores used. In the manufacture of pig iron, it is possible to vary the percentage of carbon somewhat and the proportions of carbon to graphite.
It is also possible to control the sulphur, and the silicon, but the phosphorus must be con trolled solely by the choice of the materials charged. This choice also influences the for mation of other elements under discussion, but in case of phosphorus, it is the only means for effecting such control or regulation.