Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/331

IRON 315 Clarence Furnaces; using hard coke and calcined Cleveland iron stone with limestone as flux (Lou thian Bdl). Distance from top in Feet. Forty-eight foot Furnace. Eighty foot Furnace. (exit gases). 4-25 9-7-) 15 50 21-25 2C-75 452 C. Not visibly red hot. Dull red heat. Bright red heat. Verv bright red heaf. Do. do. 332 C. Not visibly red hot. Do. do. Dull red heat. Bright red heat. Very bright red heat. A paper on the "Thermic Curves of Blast Furnaces," discussing a number of results of this class, and the conclusions to be drawn from them, by H. M. Howe, is given in the Trans. Amcr. Inst. Mining Engineers, 1876 ; see also Iron, vol. x. p. 326 sq. 21. Conditions Regulating Economy of Fuel and Cost of Production. It is evident from the data given in the pre ceding section that, when the amount of cinder formed is less, a smaller quantity of fuel will do the work of iron smelting, the amount of heat absorption for the variable requirements being lessened the less limestone is causticized and the less hot cinder flows out of the furnace. Evidently also, the hotter the blast and the cooler the issuing gases, the less fuel must be burnt to generate the particular amount of heat requisite for a given furnace working under given conditions as to quality of ore, <fcc.; whilst the more carbon is burnt to carbon dioxide and the less to carbon oxide the less total carbon is requisite for the same end. As regards the possibility of diminishing fuel con sumption by the use of richer ores and less flux, this is largely a matter of local circumstances ; when there is a choice possible between two deposits of ore, one richer than the other, and each equally good in other respects, the cost of smelting the richer ore will evidently be somewhat less than that of working the other ; as regards the other circumstances, it is by no means a matter of indifference in reference to cost of production whether the more econo mical conditions be attended to or not ; thus, in the case of the 48 and 80 foot furnaces referred to in the last sec tion, the difference between 22 3 and 28 8 cwts. of coke consumed per ton of iron involves a very considerable difference in cost of production. It is calculated that the increased size of furnaces used in England at the present day, as compared with those in use some thirty or forty years ago, represents an average saving of fuel equivalent to about 50 per cent, of the weight of the pig iron made in coal, i.e., a saving of 10 cwts. of coal per ton of iron. The saving effected by the use of waste gases for heating the blast, although less than this, still represents an enor mous aggregate: in the Cleveland district alone the saving in coal from this cause is calculated to amount to upwards of 1,000,000 tons annually. To some little extent the amount of fuel consumed is variable with the rate at which the furnace is driven ; that is, if a given quality of pig is produced when the furnace is making 300 tons per week with a given coke consumption (the fuel being burnt under the most favourable conditions practicable), and the furnace be then driven at a higher speed so as to make 400 tons, either a little more coke must be added relatively to the burden, or else there will be a tendency to make a less strongly marked crystalline pig ; instead of producing the coarsest crystallized iron (No. 1 grey pig), this quality will sink to the less largely crystalline numbers (Nos. 2, 3, or 4), or even to mottled or white iron. If, however, the fuel is not_ burnt to the maximum possible advantage in the first instance, increasing the rate of production may under certain circumstances not only not cause an increase in the fuel consump tion, but may even slightly diminish it, by lessening the amount of heat lost by radiation, &c., relatively to the metal produced. Everything remaining the same, decreasing the fuel relatively to the burden decreases the quality of the iron run, and vice versa with Cleveland ironstone an extra consumption of half a cwt. or less of coke per ton of pig suffices to raise the quality of iron run one. number in the scale, i.e., from No. 4 to 3, from 3 to 2, or from 2 to 1. According to Crossley (loc. tit.} about 2 cwts. of coke per ton of pig corresponds to the difference between running white iron and grey Bessemer pig, the ore smelted being Askam haematite and Fisher lied Bay ore. Tiinner states (loc. cit.} that at Neuberg, with a blast at 200, from 15 4 to 15 6, cwts. of charcoal were used per ton of white iron, but 23 to 24 cwts. per ton of grey iron, making a difference of some 8 cwts. per ton, or four times the amount given by Crossley ; at Heft, with a blast at 200, 12 6 to 14 cwts. of charcoal were fonnerly required per ton of white iron, and 20 cwts. and upwards per ton of grey iron, making a difference of more than 6 cwts. per ton, or upwards of three times Crossley s figure. Although a variation in the amount of fuel burnt under constant conditions denotes a correlative variation in the tempera ture of the hearth (No. 1 iron being actually hotter as it Hows from a furnace in regular work than No. 4, for instance), yet the influence thereby produced on the chemical composition is, accord ing to Lowthian Bell s experiments, not marked, at any rate so far as Cleveland pig is concerned, the differences between various specimens of different numbers not being greater than those between various specimens of the same numbers, the difference in the crystallinity of the pig being in fact more probably ascrib- able to the circumstance that the hotter the iron the longer is the time taken to solidify, and the more completely is the exfoliation of the graphite and the crystallization upon it of the remaining iron, &c., effected, than to differences of actual constitution. On the other hand, it is a matter of usual opinion, if not of invariable experience, that highly crystalline grey Bessemer pigs are richer in silicon than others, and that this is due to the higher temperature of the hearth facilitating reduction of silicon ; again, it is usually considered that white irons are apt to contain more sulphur than grey irons made from the same ore ; this circumstance, however, is possibly rather due to differences in the average composition of ore smelted and amount of flux added than simply to variations in amount of fuel consumed ; thus diminishing the amount of lime added as a flux below a certain point often increases the sulphur in the pig, so that upwards of 1 per cent, may be present, when with more lime the quantity would be only one or two tenths per cent., the iron being white in the first case and grey when more lime is employed. As regards the temperature of the blast, the fact that heating the blast enabled iron to be made with much less fuel consumption than was requisite with cold blast was recognized immediately after Neilson s discovery or inven tion ; but it soon became also manifest that a deteriorating effect was often produced upon the iron smelted from certain kinds of ore by the use of the hot blast, due to the higher temperature attained in the hearth facilitating the reduction of phosphorus, sulphur, and especially of silicon. The advocates of the very highly heated blast obtainable by means of the Whitwell stove consider that, as the heat is obtained from the waste gases at a comparatively nominal cost, the more heat can be thus introduced into the furnace the better, the saving in fuel being directly proportionate to the increase in blast temperature ; it would seem from Bell s observations, however, that the advantages thus obtainable are not always quite so great as appears at first sight to be the case. If less fuel be burnt in the furnace, a less absolute weight of blast per given amount of pig run will be requisite, and consequently a higher propor tionate temperature must be given to the blast to enable it to introduce the heat equivalent to the fuel thus saved ; for each successive diminution of fuel to the extent say of 1 per cent, an increase in temperature of blast will be requi site, the rate of increase not being constant, but accelerating at a rapid rate ; so that finally an increase in blast tem perature of 100 from 1100 to 1200 only represents about half the amount of heat introduced into the furnace that would be introduced by raising the blast from 300 to 400. The advantage of highly heating the blast then is not directly proportionate to the temperature attained as re gards saving of fuel, but something less ; according to Bell, practical experience shows that with certain ores, e.g., Cleveland ironstone, the effect of highly superheating the blast to temperatures much above 500 is not to render any considerable saving of coke practicable, but mainly only to raise the temperature of the issuing waste gases. Other iron masters, however, dissent from this view, and consider that by the use of a blast heated to 700 and upwards by a Whitwell stove, instead of to 450 or 500 by the older iron stoves, a distinct saving in the amount of coke requisite to produce a ton of pig iron from Cleveland ironstone is