Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/330

 314 IRON amount that would be requisite could perfect combustion be effected. If, however, the issuing gases be so burnt in heating the blast that more heat is brought into the furnace than is carried out by the waste gases, the excess is virtually obtained by more perfect combustion, though not actually so burnt inside the furnace; whilst, if the gases are also employed to raise steam for the blowing engines and lifts, &c., the fuel thus saved virtually is equi valent to a diminution in the blast furnace consumption ; for, were perfect combustion obtainable in the furnace, extra fuel would have to be burnt outside for these purposes. These remarks apply a fortiori to furnaces in which coal is employed as fuel instead of coke or charcoal. The heat of combus tion of average coal (after allowing for ash and supposing it to be burnt to carbon dioxide and water vapour] may be taken as about 8300 (see 10) ; hence to afford sufficient heat for the requirements of a furnace smelting average Cleveland ironstone, viz., 3850, only mS = 464 parts of coal would be requisite, could complete combustion be ensured, or 9| cwts. per ton of pig (assuming the sensible heat carried out by the waste gases and brought in by the blast to be equal). The actual con sumption in furnaces using raw coal is, however, several times this amount, 30 cwts. being a low estimate in such cases, whilst 40 and even 50 cwts. of coal per ton of pig made is not an infrequent consumption : thus even with Feme s self-coking furnace ( 12), which reduced the consumption of coal from 52 - 5 to 33 5 cwts. per ton of pig, the consumption was upwards of three times the theo retical amount ; with anthracite-consuming furnaces, such as those used in America, the consumption of fuel varies from 25 cwts. per ton of pig in the largest and best constructed furnaces to 40 cwts. or so in the older and smaller furnaces, the consumption being as a rule, however, somewhat less than that of more bituminous raw coal in the English open-topped furnaces. The reason for the extra fuel consumption in raw eoal furnaces is simply that the nature of the chemical reactions taking place in the upper part of the furnace, especially the action of heat alone upon the coal, necessarily causes the evolution of much free hydrogen, carburetted hydrogen, and carbon oxide, which escape unburnt, thus prevent ing the consumption of the fuel to the maximum advantage : where the gases are collected and burnt, this loss would not be material were it not that ordinarily the heat obtainable from the gases is far in excess of that requisite to raise steam for blowing and lifting the burden to the furnace top, &c. These reasons are also to a great extent operative with anthracite as compared with coke. On the other hand, the smaller weight of charcoal ordinarily requisite to smelt a given ore is partly due to the more ready action of carbon dioxide on charcoal than on coke forming carbon oxide, so that virtually the ore is partly reduced by the carbon of the charcoal (this being converted into carbon oxide, which deoxidizes the ore) to a greater extent with charcoal than with coke ; i.e., the charcoal is more completely oxidized, and the ore is more deoxidized at the top of the furnace and less at the bottom than is the case (casteris paribus) with coke, so that a smaller weight of charcoal ultimately performs the same work as a larger quantity of coke. The larger amount of alkalies in charcqal, producing more cyanides, probably also aids in the more rapid reduction relatively to the weight of fuel used. In a prize essay, Professor Habets has given formulae for calculat ing the value of a given weight of iron ore of given composition, the price of the pig iron made from it, and the quantities of ore and limestone requisite to produce a unit of weight of pig, &c. (see abstract in Journ. I. and S. Inst., 1877, 225), and has also arranged formulae for calculating the amount of fuel that ought to be required for the smelting of such ores, &c., assuming that the duty actually performed by the fuel is 48 per cent, of the possible maximum amount. In these calculations slightly different values are taken for certain of the heat requirements from those given above; thus for the reduction of pig iron (containing carbon, silicon, &c. ) the total heat consumption is taken as 1984, the amounts assumed by Bell, Crossley, and Tiinner as above described being respectively 1931, 1871, and 1670 ; that carried out by the molten pig is taken at 260 for cold working, 270 for medium, and 285 for hot, Bell s figure (and Vathaire s) being 330, whilst Tiinner takes 340 from Rinman s observations ; and the loss by radiation (presumably in cluding the tuyere water) is taken as 400 (Bell = 349 deduced by the present writer from a round general average result by difference; Crossley =- 359; Tiinner = 192, the furnace being a much smaller one in this latter case), and so on throughout ; but on the whole Habets s formulae are based on much the same valuations as those above cited. The instances given above, however, indicate that the results obtainable with one class of ores, fuels, &c. , are only applicable to another class with considerable latitude of variation, and that it is im practicable to fix a hard and fast line as the limit of economy of fuel universally applicable. Where, however, the fuel is burnt differ ently (to less advantage, for instance, so that, instead of one part of carbon giving 48 of the total heat production as &quot;duty, it only gives say 40), the formulas of Habets will still be applicable, only requiring the application of a coefficient (^|^=l - 2 in the case supposed). Temperature of Blast Furnace at Various Levels. Many observations of the rate of increase of temperature from the mouth of the furnace have been made by Tiinner, Ebelmen, and Lowthian Bell, When fresh materials have been just introduced cool into the furnace, of course they intercept heat from the escaping gases, acting precisely like the brick work stacks of a Siemens regenerative furnace ; this effect, however, cannot be carried so far as to prevent the escaping gases from passing out at an average temperature which, if not elevated, is at least sensible, the actual temperature varying with the conditions obtaining. Since heat is gene rated by the reduction of ferric oxide by carbon oxide, more heat being evolved by the oxidation of the carbon than is absorbed in the reduction of the iron oxide, roughly in the ratio of 3 to 2, it results that there is always a source of heat in the upper part of the furnace ; and, unless fresh materials can be supplied sufficiently rapidly to keep the escaping gases always at a given low temperature by their direct cooling effect, the temperature must rise by the reduction of the ore. A condition of equilibrium as to temperature is consequently finally arrived at when the sums of the generations of heat by chemical action at each and every particular level, and of the absorptions of heat by direct communication to the fresh charges added from time to time, balance one another; when this condition of things is arrived at the temperatures of the escaping gas, and of the substances generally at each level, become constant, or rather would do so were the fresh materials added continuously instead of intermittently, and were the action of the furnace absolutely uniform. The circumstances which regulate the most advantageous way in which fuel can bs burnt, i.e., the economy of fuel in the furnace, consequently regulate the temperature of the escaping gases, which accordingly is variable with the quantity of fuel burnt per unit of iron smelted, with the size and shape of the furnace, the character of the ore employed, &c. Under particular conditions, especially when a large mass of fresh materials has been added, the escap ing gases may be so cool that the aqueous vapour present is condensed into mist, whilst the hand may be placed in the gases without being burnt ; ordinarily, however, the temperature at the mouth averages 200 or 300 C., and with small furnaces and difficultly reducible ore requiring a large fuel consumption may be much higher. One great effect of increasing the height of furnaces smelting clay ironstone (e.g., Cleveland ore) is the reduction of the amount of fuel requisite owing to the cooling influence exerted upon the temperature of the escaping gases which pass off, there by leaving in the furnace heat which otherwise would have to be provided by burning more fuel. Of the numerous particular determinations that have been made of the tem peratures at different levels in different cases, the following may be cited as examples : Wrlna Furnace (Eisenerz); height 3 feet; using soft charcoal with a burden of spathic ore, cast iron scrap, and grauicacke-schist (as flux), in the proportion 0/383, 8, and 20 respectively (Tiinner and Ricliter). Distance from top in Feet. Temperature. Distance from top in Feet. Temperature.

320 21 840 C. 7 340 24 910 11 350 25-5 950 15 640 29 1150 17 (580 34 1450 i The temperature values were deduced by introducing alloys of known melting points, and noticing -which were fused.