at the temperature of 300° to 400° the action is not intense. There is only a small quantity produced in variable proportions of Fe2O3, Fe2O3 + FeO, and FeO; that is to say, of the peroxide, the magnetic oxide, and the protoxide of iron, and these oxides are never accompanied by a deposit of carbon. 7. The formation of ferruginous carbon is the result of a sort of dissociation of CO. 2CO is transformed to CO2 + C, but this reaction never takes place directly. That it may take place there must be simultaneous presence of metallic iron and protoxide of iron-the metallic iron to fix the carbon, the protoxide to retain for a moment the oxygen. But this passing reoxidation of the protoxide, which resists its final reduction by this very reaction, can only be produced if the reducing action of the CO be partially tempered down by CO2. This, I repeat, is the sine qua non condition of the deposit of carbon. This double reaction is expressed by the formulas 3FeO + CO Fe30 + C (this carbon having been united to the iron), and Fe3O4 + CO 3FeO + CO, = and so on indefinitely, provided that the CO is always tempered in reductive action by a certain portion of CO2. In one word, CO pure is not dissociated by iron absolutely free from an oxidized element. In the same way, CO2, if it acts alone on iron, does not deposit ferruginous carbon; and lastly, the two gases united, provided CO be in excess, produce an abundance of ferruginous carbon by the simultaneous action on metallic iron at the low temperature of 300° to 400°. 8. Spathore iron, or the protoxide of iron (FeO), is rapidly transformed into magnetic oxide under the action of CO2 without any deposit of carbon, whilst CO in the same circumstances rapidly deposits large quantities of ferruginous carbon. 9. If we raise the temperature to a lively red, in those experiments which give ferruginous carbon, the deposit immediately ceases, and, moreover, the carbon already deposited is burned again so long as there is present a sufficient quantity of oxide unreduced. Thus the reactions are in this respect quite different at high temperatures from what they are at temperatures of 300° to 400°. 10. In reference to the theory of the blast furnace, it may be remarked that the carbon must deposit itself on the ores in the upper regions of the furnace, and that this carbon dust must facilitate the ulterior reduction of the ores, and that of CO2, in the middle regions of the furnace. At all events, in consequence of this reaction, the carbon deposited will be burned a second time in the zone of fusion. 11. The dissociation of CO takes place with a development of caloric. For each unit of carbon deposited there is a disengagement of 3134 calories. To the résumé of what goes before, I add two observations. The dissociation of CO does not take place when a mixture of equal volumes of CO and CO2 are made to act on the oxide of iron, that is when m = 1.581. According to M. Debray's experiments, only FeO is formed in this case, whilst metallic iron is necessary to determine the deposit of carbon dust. If, on the other hand, we have 2 vols. CO to 1 vol. CO2, which corresponds to m = 79, then the carbon commences to deposit, for then also metallic iron begins to appear, provided of course that the current of gases be sufficiently rapid to carry off, without delay, the CO2 formed. My second observation is relative to the disengagement of 3134 calories, due to the deposit of ferruginous carbon. This is the essential point of the phenomenon in question, as regards the temperature of the gases and the limit of height of blast furnaces. The caloric produced by the combustion of 2 C to 2 CO is, as we know, 2 × 2473 = 4946 calories. On the other hand, when the half of 2 C is transformed into CO2, the caloric developed is 8080 calories: therefore the dissociation of 2 CO into C+ CO2 unquestionably sets free 8080 4946 = 3134 calories. We may therefore conclude that when the escaping gases have a temperature of 300° to 400°, and such a compo sition that the ratio m = CO2 0-80, there will be not only partial reduction of the ores, but also impregnation and deposit of carbon dust, with a notable disengagement of caloric, from which there results a sort of stationary condition without any further lowering of temperature of the gases, notwithstanding greater height given to furnace. But what actually takes place is this: In the upper part of the furnaces, two different reactions take place at the same time. On the one hand, ore is reduced by CO giving CO2 without sensible change of temperature. On the other hand, 2 CO is transformed to C + CO2, with a production of 3136 calories. The carbon C resulting from this dissociation is restored to the charge in the shape of carbon dust. It descends with the charge, and then reforms CO in the lower hotter regions. The CO thus reproduced ascends again, and is dissociated in its turn into (C + 1⁄2 CO2). And the same reaction is renewed indefinitely, and thus the sum of the reactions is just as if the 2 CO had been directly transformed into 2 CO2 by the oxygen of the ores; that is, as if the reduction took place, as assumed in the ideal working, by the action of CO without consumption of solid carbon, and consequently without sensible variation of temperature. The dissociation of the CO brings the ordinary good working nearly up to the ideal working, and consequently whatever favors this dissociation will reduce the consumption of fuel; and hence we may conclude increase of height of furnaces would lead to a gradual diminution of consumption, the extreme limit of which would be the ideal working of the furnace. But, in fact, this could not be realized. In the first place, the height of the furnaces is limited by the increasing resistance of the charges to the passage of the blast. Again, although the sum total of the caloric disengaged depends solely on the value of the ratio m for a given consumption, its distribution will be chiefly regulated by the manner of production of CO2. When CO2 comes from the dissociation of 2 CO, there is production of caloric in the upper region of the furnace, whilst there would be absorption in the middle region where the carbon desposited is transformed into CO by the oxygen of the ore. It is true that there is only a displacement of caloric, but the gases thus reheated in the upper part of the furnace will, from want of time, yield less. caloric to the cold elements of the charges than if their temperature had been raised in the middle regions by the simple oxidation of CO to CO2 by the ores. Raising the height of furnaces tends to a sort of constant calorific state by favoring the dissociation of CO, but after this there can be no sensible advantage. I shall only add one more observation. All that I have said applies only to blast furnaces in which calcined anhydrous ores alone are charged, as in Cleveland. Elsewhere, in the districts of France where hydrated oxides are used, for example, and in districts also where incompletely calcined spathic ores are used, the tunnel-head is much cooler. Thus I have quoted from M. Tunner's memoir the example of Eisenerz, where the metallic iron, and with it the dissociation of CO, does not begin to declare itself till 23 feet below the top. But in furnaces of small height, the charges would get into the highly heated zones in which this reaction ceases again very soon after. Any raising of the height of furnaces, in such cases, might enlarge the zone in which dissociation would take place, and thus lead finally to a certain economy by rendering the gases richer in CO2. But it is evident, for the reasons above given, that there is here again a limit at which real economy becomes insignificant.-En résumé— After a certain height has been attained, variable with the ores and the section of the furnaces, there is no longer any advantage in enlarging their capacity. § 25. Influence of highly heated blast.-Let us now, in the second place, endeavor to appreciate the influence of variations of the temperature of the blast on the working of the furnace. Let us, for this purpose, compare the two furnaces of Consett, cited in our synoptical table. On The dimensions and the section differ very little; the charges are identical; the pig-iron yielded is the same. the other hand, the temperature of the blast is 454°.5 in the one, and 718° in the other. The difference in the results |