Page:The American Cyclopædia (1879) Volume IX.djvu/410

 396 IRON MANUFACTURE Wrbna Furnace, 86 ft. Ugh. Capacity, 1,200 cub. ft. Consumption, 14 cu-ti. of charcoal per ton of iron. (Tunner.) LEVEIS. BY TOLFME. BY WEIGHT. H. N. CO. co a. H. N. CO. CO S. At mouth 18 ft. from top 36i" 28 " " " 82 " " " 84i 5-61 13-89 8-93 3-45 2-91 1-54 56-8224-40 60-47 -'4-21 56-71 25-92 66-48 27-95 56-G2;28-42 57-.V.'f(l" 13-67 11-98 13-44 12-12 12-06 2-94 0-39 1-01 0-27 0-24 0-20 0-11 54-90 68-88 54-57 55-00 54-84 57-85 28-81 25-82 24-87 26-28 27-82 87-93 20-90 19-79 20-29 18-50 18-14 4-61 The hydrogen in the gases from the Wear fur- nace was not determined ; it is never present in large quantity, and plays no important part in the blast-furnace process. Hydrocarbons and cyanogen are also sometimes present in small quantity. The former are abundant in the upper part of the furnace, when raw bitu- minous coal is used. It will be noticed in the above analyses that the escaping gases still contain a considerable amount of carbonic oxide. An ideal furnace process would be one in which the carbonic pxide was completely utilized, and only carbonic acid escaped from the furnace. From the experiments of Bell, quoted above, it is evident that such a condi- tion is practically impossible ; yet the relative amount of carbonic oxide is a measure of the economy of working. Formerly the gases were allowed to burn at the mouth of the fur- nace ; now they are invariably utilized, either wholly or in part, to heat the blast, raise steam for the blowing engines, or roast ores. The gases likewise possess a certain amount of sensible heat, from which also the economy of working may be judged. The ultimate practi- cal economy of fuel that can be attained in the furnace is reached when the gases contain such a relatively small amount of carbonic oxide that they are no longer capable of reducing the ore at the temperature at which they leave the furnace. In the year 1829 there were used in Scotland about eight tons of coal in the form of coke to produce one ton of pig iron ; at the present time in Cleveland the consumption of coal is but 33 cwts. per ton of iron. This great economy of fuel has been reached by increas- ing the dimensions of the furnace, by com- plete utilization of the carbonic oxide in the escaping gases, and by the use of heated blast. Increasing the height and diameter of the fur- nace prolongs the contact of the charge with the hot reducing gases, whereby the sensible heat of the latter is more completely trans- ferred to the descending materials, and the re- ducing power of the carbonic oxide is more thoroughly utilized. Widening the throat and mouth of the furnace has the same effect, by decreasing the rapidity of the gaseous current. It would at first sight appear that the dimen- sions of the furnace could be so far increased as to intercept the total amount of sensible heat. This is found not to be the case in practice. Bell has shown by experiment that the temperature of the gases at the mouth of a furnace having a height of 80 ft. and a capacity of 12,000 cub. ft. is not materially higher than that of gases from furnaces 103J ft. high with a capacity of 33,000 cub. ft. The cause of this has been shown by Bell to be, that there is a constant source of heat production in the upper part of the furnace, caused by the re- duction of the ore and the deposition of car- bon ; and increasing the height of the furnace merely serves to raise this zone of reduction. From the reduction of the ore by carbonic oxide, the heat development is very small, as a given weight of oxygen uniting with iron or carbonic oxide produces in either case nearly the same amount of heat. According to Du- long, one litre of oxygen gas gives 6,216 units of heat when it combines with iron, and 0,260 when it combines with carbonic oxide. But the heat development by the dissociation of carbonic oxide, as described above, is 3,134 heat units for each unit of carbon deposited. The utilization of the carbonic oxide in the gases for heating blast or boilers directly ef- fects a saving of a corresponding amount of coal. The saving of fuel consequent on heat- ing the blast is very marked. The cause of this saving was for a long time sought in vain. On the first introduction of the hot blast in Scotland it was found that by burning 5 cwts. of coal to heat the blast to 450 F. there was effected a saving of 47 cwts. of coal in the fur- nace ; and by burning 8 cwts. of coal, to heat the blast to 612 F., the saving was 83 cwts. or 69 per cent. Experience has shown that the economy of fuel attained by the use of hot blast depends on the height of the furnace and on the reducibility of the ore. The higher the furnace and the more susceptible the ore to the reducing action of carbonic oxide, the smaller is the saving of fuel effected. The enormous saving shown in the Scotch furnaces was due to the fact that both conditions were favorable to the use of hot blast, viz. : the fur- nace was low and the ores were refractory. The economy of fuel by hot blast in modern high blast furnaces does not often exceed 10 to 12 cwts. of coke per ton of pig iron. The ideal working of the blast furnace is as follows: A certain amount of fuel is burned before the tuyeres, and generates enough heat to melt the reduced iron and the cinder. The carbonic oxide, which is the end result of this combus- tion, ascends through the ore, which it reduces to metallic iron, and passes out of the furnace as carbonic acid. It is found, however, that the minimum amount of fuel that will melt the iron and cinder does not, under the con- ditions that obtain in the blast furnace, supply sufficient carbonic oxide to do the work of re- duction. It has further been found that the rapidity of the reduction of the ore depends on its molecular constitution, or the proportion that carbonic oxide bears to carbonic acid in the gases, and on the temperature. Let it be supposed that in a blast furnace working with a given quantity of fuel, a certain ore requires