Page:The American Cyclopædia (1879) Volume IX.djvu/411

IRON MANUFACTURE 397 seven hours for complete reduction, while an- other ore requires fourteen hours. If it is now desired to smelt the refractory ore so as to have a production equal to that afforded by the first ore, it is necessary either to give it four- teen hours' exposure or to increase the rapidity of reduction. The first of these conditions is accomplished by doubling the height of the furnace, and the second by increasing the tem- perature through the use of more fuel. In the latter case there is no more heat utilized, in spite of the greater amount of fuel, than when smelting the easily reducible ores, presuming that they have approximately the same compo- sition. It is merely the rate of reduction that is increased ; and the excess of heat passes off in the escaping gases. Of these two methods of smelting refractory ores, the latter was the one adopted until a comparatively recent peri- od, when increased height of furnace was found to give the same result. As an instance may be given a Scotch furnace 53 ft. high and using 40 cwts. of coke per ton of iron produced from black-band. By adding 18 ft. to the height the amount of fuel was reduced to 28 cwts. Akerman in Sweden was the first to suggest what is probably the principal cause of the economy in using hot blast. The heat which is produced by the combustion of the fuel in the furnace is contained in the carbonic oxide formed and in the accompanying nitrogen, while the heat that is conveyed by the blast is not attended with the development of any gas- eous products, and does not therefore increase the bulk of the gases in the furnace. Now, as the temperature of the gases is inversely as their bulk, it follows that the temperature of the furnace must be higher when using hot blast, and the rate of reduction corresponding- ly rapid. Further, the rapidity of the upward current will be diminished, and the more thor- ough will be the reducing action of the car- bonic oxide. It has been shown that increas- ing the height of the furnace beyond a certain limit only serves to raise the zone of reduction, and does not cause further saving of fuel. The theoretical limit of temperature of the blast is attained when the amount of fuel consumed in the furnace is so far replaced by the heat in the blast, that the carbonic oxide formed is just sufficient to do the work of reduction. This point has never been reached in practice ; but the significant circumstance has been no- ted, that the rate of saving for a given number of degrees decreases as the temperature of the blast is raised. From the above it will be evi- dent that the ultimate practical economy of t fuel attainable in blast furnaces depends on a number of conditions. In the Cleveland dis- trict, England, where the furnaces have attain- ed colossal dimensions and the blast is heated to over 1000 F., the lowest consumption of coke per ton of No. 3 pig (see IKON) is about 21 cwts. ; while at the Wrbna furnace in Aus- tria, which is but 36 ft. high, and where the temperature of blast is 752 F., the consump- tion of charcoal is but 13-20 cwts. per ton of iron. The daily production of furnaces is de- pendent on the same conditions as determine the consumption of fuel, and also on the rate of driving of the furnace, i. e., the amount of blast in a given time. The extremes are small charcoal furnaces yielding but 4 to 5 tons per day, and large furnaces yielding 80 tons per day. The absolute amount of heat produced in the blast furnace, the amount absorbed in work done, and the amount lost by radiation and in the gases, have been calculated by a number of authorities. The following is Bell's estimate expressed in cwt. heat units per ton of iron produced : HEAT PRODUCTION. Oxidation of carbon 81,886 units. Contributed by blast 11,919 " HEAT ABSORPTION Evaporation of water in coke 81 2 units. Reduction of iron 38,108 ' Carbon impregnation 1,440 " Expulsion of CO, from limestone 5,054 ". Decomposition of this CO, 5,243 " " water in blast 2,720 " Phosphorus, sulphur, and silicon re- duced 4,174 " Fusion of pig iron 6.600 " "slag 16,720 " 93,455 75,876 HEAT LOSS. Transmission through walls of furnace. 8,658 units. Carried off in tuyere water 1,818 " " " gases 8,860 " Expansion of blast, loss from hearth, &c. 8,743 18,079 98,455 Occasionally ores occur which contain the proper proportion of earthy matters to form a fusible slag (self-fluxing ores). When this is not the case, the substances in deficiency must be added ; and this may often be advantage- ously accomplished by mixing ores of different characters. In the large majority of cases, limestone is added as flux, since most ores contain silica and alumina, which with the lime form a fusible slag. It is a matter of great importance that the composition of the ores and fluxes should be accurately determined, in order that a slag (cinder) may be formed of the desired fusibility. Blast-furnace slags are usually double silicates of alumina and lime, in which the latter is often partially replaced by magnesia, oxide of manganese, and (when the reduction is incomplete) by protoxide of iron. The fusibility increases with the amount of silica, up to about 60 per cent, of the latter, and decreases with the amount of lime. Basic slags are white and stony in character, and re- quire a very high temperature for fusion. The conditions in the furnace producing such a slag are therefore favorable to the complete reduction of the ore and the formation of a highly carburetted siliconized iron. Basic slags also take up sulphur in considerable quantities. White iron is generally accompanied with a more acid cinder, which sometimes contains