Page:The American Cyclopædia (1879) Volume IX.djvu/412

 398 IRON MANUFACTURE considerable oxide of iron. When this is the case, it is called a scouring cinder. The fol- lowing analyses show the composition of sev- eral varieties of blast-furnace slags : ELEMENTS. 1 2 8 4 5 Silica 83-48 15-18 32 -(-2 0-76 1-62 7-44 2-22 1-92 0-15 43-07 14-85 28-92 2-58 1-87 6-87 1-90 1-84 27-65 24-09 40-00 0-72 0-85 8-55 1-95 1-45 0-26 42-17 18-59 88-02 1-28 0-27 8-81 0-64 61-06 6-88 19-81 8-29 2-68 T-12 Sulphide of calcium Alkalies Total 100-54 100-85 100-62 99-28 99-29 Nos. 1 and 2 are from raw coal, used at Dow- lais, Wales, the first making gray, the second white iron; 3, coke, at Clarence, England, making gray iron from Cleveland ores ; 4, an- thracite, at Boonton, N. J., making gray forge iron ; 5, charcoal iron, at Gosberg, Sweden. Slags are classified, according to the ratio which the oxygen of the silica bears to the oxygen of the bases, as tri-, bi-, mono-, and subsilicates. According to Bodemann, the most fusible sili- cate of lime and alumina is a bisilicato of the following composition : silica 56 per cent., lime 30, alumina 14. Blast-furnace slag is almost entirely a waste product, and one that is very difficult to dispose of. Acres of valu- able land are often sacrificed as a dumping ground for it. Many attempts have been made to utilize it, and with considerable success. Building brick, paving stone, hydraulic cement, sand for mortar, &c., have been successfully made; but no regular manufacture has yet been introduced that can work up even a mi- nute fraction of the ever-increasing slag pro- duction of the world. WROUGHT IRON is either made directly from the ore or from pig iron. In the former case the process is one of reduc- tion of the iron from its oxide ; in the latter it is one of oxidation of the carbon, silicon, &c., of the pig iron. Although the iron pro- duced in low furnaces and bloomaries is usually of great purity, yet owing to the small produc- tion and the waste of iron, and the necessity of rich ores and charcoal, the direct process has almost entirely disappeared from civilized coun- tries, as it is not able to compete commercially with the indirect production of wrought iron from pig. (See BLOOMART.) The conversion of pig into wrought iron is effected either in a hearth similar to a bloomary or in a reverber- atory furnace. The nature of the process is the same in both cases, and consists in the oxidation of the silicon, manganese, carbon, phosphorus, sulphur, &c., in the pig iron, by the oxygen of the air, and also by that of oxide of iron. The latter is generally added as such, but is always formed in the process itself. The oxygen in the solid form is the most active, since it can be intimately incor- porated with the iron, while the oxygen of the air merely acts on the surface of contact. The order in which the foreign substances in the pig iron are removed is that in which they are named above ; or, more especially, all these substances are oxidized coincidently, but the rate of oxidation is in the order given. The iron is also promptly oxidized, and forms with the silica resulting from the oxidation of the silicon a basic iron silicate, or cinder, which reacts on the carbon and silicon remaining, converting them into oxides, while an equiva- lent amount of metallic iron is reduced. Iron thus serves as a carrier of oxygen to the non- metallic elements. This interchange of ele- ments continues until the iron is nearly or quite decarburized. Manganese, if present in the pig iron, may replace the iron in the cin- der, but it does not act in the same manner as a carrier of oxygen. The hearth process, now almost exclusively confined to the moun- tainous region of southern Europe, Sweden, and South Wales, was previous to the latter part of the last century the method universally employed for the conversion of pig iron. The process is a very simple one. The iron is melt- ed with charcoal and exposed while molten to the direct action of a blast of air from one or two inclined tuyeres in the side of the hearth. The iron gradually loses carbon, silicon, phos- phorus, &c., and is converted into a pasty lump or bloom which is hammered into slabs. The process in its essential features is the same everywhere, but owing to slight variations in construction of hearth or in the details of manipulations it has received a great variety of names. In Sweden there are three methods employed, the Walloon, the Franche-Comt6, and the Lancashire. The latter, which is the one most generally used and most economical of fuel, will be briefly described. The hearth is quadrangular, and formed of cast-iron plates. The tuyere side is slightly inclined inward, the opposite side and the back are inclined out- ward, and the front is vertical. The bottom plate is cooled by running water. The blast is supplied by one or two tuyeres at a tempera- ture of 210 to 390 F., and at a pressure of 1 to 1^ Ib. per square inch. The waste heat from the hearth is used to heat the blast and also the pig iron previous to charging. The manipulation consists in piling up the already heated pig iron to the amount of 200 to 250 Ibs. on the burning charcoal, and melting down. As the molten metal drops past the blast it is partially oxidized, and the oxide thus formed, together with the rich basic cinder remaining from the previous opera- tions, act with the blast in decarburizing the iron. The imperfectly refined iron, which sinks to the bottom, is broken up with an iron bar and brought repeatedly before the tuyere, until the iron is rendered thoroughly soft and malleable. Finally, a high heat is given, and the iron is collected in a mass or bloom in the bottom of the hearth. It is then taken out and forged under a trip-hammer to a prismatic shape. The process lasts about 1 J to