Page:The American Cyclopædia (1879) Volume IX.djvu/409

IRON MANUFACTURE 395 to the studies of Bell in England, Tunner in Austria, Akerman in Sweden, and Gruner in France. What follows contains the principal results of these investigations, especially those of I. Lowthian Bell, which have been the most extensive and the most fruitful of valuable conclusions. The mutual reactions of carbon, carbonic acid, carbonic oxide, metallic iron, and oxide of iron are expressed in the fol- lowing formulas : xFe + y'CO, = Fe,O s + yCO xFe + yCO = Fe f O,+yC FeA i + CO =FeA+0 = 200 These reactions show that carbonic oxide not only abstracts oxygen from oxide of iron, but also imparts oxygen both to metallic iron and to its lower oxide ; that carbonic acid, which results from the reduction of iron by carbon or carbonic oxide, may also oxidize metallic iron ; and that carbonic acid is capable of ta- king up a second atom of carbon. The nature of the reaction in any given instance is de- pendent mainly on the temperature, and also on the relative proportions of the gases. The breaking up of carbonic oxide and the depo- sition of its carbon was discovered by Bell. The conditions governing this remarkable re- action have been studied by him and by Gruner. The following are the results of Gruner's ex- periments on this point : On passing carbonic oxide over a fragment of iron ore at a tempera- ture of 300 to 400 C. (572 to 752 F.), the latter is gradually reduced, the reduction pro- gressing from the surface to the interior of the mass. When metallic iron is formed on the surface, the ore cracks and expands, and becomes covered with a fine deposit of carbon. This carbon deposition diminishes as the re- duction of the ore proceeds ; and were it pos- sible to effect complete reduction in this way, it would finally entirely cease. Pure carbonic oxide is not decomposed by metallic iron at 300-400 0. ; but when the carbonic oxide is mixed with carbonic acid, deposition of carbon takes place. . The amount of the CO, must not, however, exceed 1 volume to 2 volumes of CO. This deposited carbon is not pure, but contains 5 to 7 per cent, of metallic iron, and also some oxide of iron, mainly magnetic. The forma- tion of this deposit of ferruginous carbon is the result of the mutual reaction of two mo- lecules of carbonic oxide, whereby 2CO = COj + C ; but it is absolutely necessary that there should be present simultaneously metallic iron and ferrous oxide, the former to fix the carbon ' and the latter to hold for an instant the oxygen, losing it again by the action of carbonic oxide. The following formulas express the reactions : 3FeO + CO,; and soon indefinitely, provided that the reducing action of carbonic oxide is tempered by a certain amount of carbonic acid. If the temperature is raised to a red heat the deposition ceases, and the carbon already de- posited unites with the remaining oxide of the ore. It is probable that the deposited carbon plays an important part in the final reduction of the ore in the blast furnace. The dissocia- tion of carbonic oxide (200 = CO 2 + C) is ac- companied with a development of heat, every unit of carbon deposited corresponding to an evolution of 3,134 heat units. The tempera- ture at which the iron ore begins to lose oxy- gen in the blast furnace depends on the mo- lecular constitution of the ore and the relative amount of carbonic oxide in the gas. Bell has found that the temperature of incipient reduc- tion of oxide of iron by pure carbonic oxide varies from 141 0. (285 F.) to 208 C. (407 F.), according to the nature of the oxide or ore. The temperature at which carbonic acid begins to oxidize metallic spongy iron was found by him to be about the temperature of melting zinc, 417 C. (782 F.). In both cases the energy of the action is promoted by increasing the temperature, but the oxidizing action of the carbonic acid increases in a greater ratio than the reducing action of the carbonic oxide. The point of equilibrium of the two gases toward metallic spongy iron at different tem- peratures was found to be : Low red heat 1 60 vols. of CO. for each 100 vols. of CO. Full " 47 " " " Approaching white- ness 11 " " The point of equilibrium of a mixture of car- bonic oxide and carbonic acid toward oxide of iron depends likewise on the temperature, and also on the molecular structure of the oxide. At a red heat a mixture of 100 volumes of car- bonic oxide and 600 of carbonic acid is nearly neutral to calcined Cleveland ore, while at 417 C. the point of equilibrium is found in a mix- ture containing 100 volumes of carbonic oxide to 50 of carbonic acid. Again, a mixture of equal volumes of carbonic acid and carbonic oxide at a temperature of 417 C. is found to reduce Eixenerz spathic ore actively. The tem- perature at which carbon (coke) begins to de- compose carbonic acid is, according to Bell, 410 C. (770 F.). The composition of blast-furnace gases at different levels of the furnace has been investigated by a number of observers. The following analyses are by Bell and Tunner : Wear Furnace, 80 ft. high. Capacity, 1T,500 cul. ft. Consumption per ton of iron, 28-5 cwts. of coke and 12-8 cwts. limestone. Ore, roasted carbonate. (Bell.) BY VOLUME. BT WEIGHT. K. CO. CO,. N. CO. CO,. 60-57 29-99 9-44 56-9 28-2 14-9 16J ft. from top 6.V46 31-66 2-88 64-8 80-8 4-4 26 ' 67-09 28-54 4-87 65-5 27-8 6-T 89 " " 65-76 81-98 2-26 64-9 81-6 8-5 62J " " " 64-48 84-66 0-91 64-5 84-1 1-4 66 6V05 184-64 0-81 64-9 84-6 0-5 70J " " " 65-22 84-78 66-2 84-8 o-o 76i " " " st the tuyeres 62-76 86-59 1-65 62-7 84-7 2-6