Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/313

IKON 297 would be were less carbon dioxide introduced in the form of flux, since the total amount of dioxide in the escaping gases is limited (see 20) ; accordingly, a further saving in fuel might be expected to accrue by calcining the limestone previously, when large amounts of limestone flux are employed. The two sources of saving jointly sometimes considerably exceed the fuel expenditure in the limekiln during the process of burning the lime ; thus at Ougree, near Liege, comparative trials lasting over some three and a half years indi cated not only increased production but also a notable saving in fuel when lime was used. Two similar furnaces gave the following results per unit of iron run (A), the results (B) being obtained with the same furnace throughout : Furnace with Quicklime. Furnace with Raw Limestone. Saving. (A) Coke consumed (15) Do. do 1-465 1-4725 1-605 1-C20 0140 0-1475 Analogous results, but not so strongly marked, were obtained by Eck at the Royal Smelting Works, Upper Silesia : Furnacewith Quicklime. Furnace with Haw Limestone. Saving. 1st Furnace Coke per unit of iron ... 2d Do. ilo. do. 2-230 2-1825 2-280 2"2725 0-050 0-090 1st Furnace Increase of maku l>y using quicklime, 3-3 per cent. 2d Do. do. do. 24 From these results as well as others described by Percy (Metal 3 lurgy), and by G rimer (Annalcs des Mines [vi.], xx. 525) and others, it would seem that, with certain ores at any rate, a distinct practical advantage attends the use of quicklime instead of raw limestone. On the other hand, Lowthian Bell regards the advantage with Cleveland ores as at least doubtful, his own experiments un mistakably indicating "that the expense of calcining the limestone was unaccompanied by any advantage whatever in the operation" (Journ. I. and S. Institute, 1875, 406) ; similar results have also been observed by others. As regards the quantity of flux requisite to produce good results, no general statements can be made, the proportion being highly variable with circumstances, and especially with the nature of the ore. Thus, with Cleveland ironstone containing after calcination some 40 per cent, of iron, about 11 cvts. of limestone are usually requisite per ton of pig iron, or about 22 per cent, of the weight of "mine" used. Much larger quantities have been employed at various Continental works using poor ores with much earthy matter, up to 20 cwt. and more per ton of iron ; on the other hand, some of the Swedish ores require no flux at all, and Lake Superior ores often do not require more than some 2 cwts. of limestone per ton. When it is practicable to mix aluminous, calcareous, and silicious ores together, the amount of flux otherwise requisite may be largely reduced ; but the conditions governing the amount and nature of flux to be iised are too variable to be briefly generalized. 12. Conslmdiori of Blast Furnaces. Intermediately between the comparatively open hearths of the Catalan forge and analogous early arrangements fur the direct pro duction of iron from its ores ( 29) and the completely closed- in blast furnaces of gigantic dimensions in use at the present day, may be classed the smaller closed-in blast furnaces used amongst various nations, the products of which were either something approaching to malleable iron, more or less carbonized and imperfectly fused, a fused or semi-fused steel, or a completely melted more highly car bonized cast iron, according to circumstances. Of this intermediate class of furnace the " Stiickofen," or high bloomery furnace, formerly considerably used on the Con tinent, may be taken as a type. By increasing the amount of fuel relatively to the ore smelted, a completely fluxed cast iron resulted, run out as in the ordinary modern blast furnace through a tapping hole; with less fuel, i.e., increased " burden," the product approximated more to the pasty mass produced in the Catalan forge, being extracted as a ball through a much larger opening in the hearth than was necessary for simple tapping ; in this latter mode of working the cinder was usually allowed to escape pari passu with its formation so as not to allow the mass of reduced metal to be covered and protected from the oxidizing action of the blast, otherwise a too highly carbonized metal resulted. Essentially the Stiickofen was a brickwork tower of some 10 to 15 feet in height, the inner cavity being shaped like two truncated cones placed base to base ; in short differing from the ordinary blast furnaces for producing cast iron in little but dimensions. As far back as 1841 these appliances were stated by Karsten to have been entirely abandoned ia Carinthia, Carniola, and Styria (where formerly they were largely employed), on account of their large consumption of fuel ; at that period they were still in use to a small extent in Hungary and near Henneberg in Germany. The "Osmund" furnace, formerly in use in Sweden for converting bog iron ores into malleable iron, was essentially a Catalan forge with the sides built up to a height of several feet so as to constitute a small blast furnace entirely closed in save at the top. The modern blast furnaces for pig iron production in use in different districts vary considerably in the details of their construction. The changes that have been intro duced during the last half century are mainly in the direc tion of increased size, which up to a certain point has been found advantageous so far as the consumption of fuel is concerned, at any rate with certain classes of ore. Thus about 1830 the largest furnaces in use in Great Britain were usually but little upwards of 40 feet in height, with a capacity of 4000 to 5000 cubic feet, and were often much smaller; about 1864 Vaughan of Middlesborough built a much larger one, 75 feet high; at the present day furnaces of 80, 90, and even upwards of 100 feet in height and of 20,000 to 40,000 cubic feet capacity are in use in certain localities, some of the largest being those at Ferryhill and Ormesby in the Middlesborough district, furnaces of 103.1 feet in height and 33,000 cubic feet capacity, and of 90 feet in height and 40,000 cubic feet capacity, having been built at these places respectively. The researches of Lowthian Bell on blast furnnces smelting Cleveland iron stone, and the practical experience of iron smelters using this ore alone, or mixed with haematite, coincide in indicating that, whilst a considerable saving in fuel consumed (several cwt.) per ton of iron of given quality made at a given rate of working from a given class of ore and flux accompanies the increase in dimensions from 40 or 50 feet in height and 5000 or 6000 cubic feet capacity up to about 80 feet in height and some 12,000 cubic feet capacity, the economical effect of in creasing the dimensions beyond these limits is not marked, although, according to Bell, a further increase in capacity up to some 15,000 or 16,000 cubic feet (without increase of height beyond 80 feet) appears on the whole to tend to increase of regularity during working. On the other hand, when a very hard coke (such as that specially made in Durham and Northumberland for the purpose) is not obtainable as fuel, or when very friable ores are employed, the extra weight of material in a very high furnace so crushes and pulverizes the ore and fuel as seriously to interfere with the working; so that with charcoal or soft anthracite or other coal as fuel, or with ores which either at first or when partially reduced have but little coherence, the disadvantages of increasing the size of the furnace beyond certain limits outweigh the benefit derived from the saving of fuel. Accordingly the 80 feet furnaces and upwards of the Cleveland district are but little used out of England ; the furnaces employed in the North Lanca shire and "Whitehaven districts, where haematite mainly is smelted, are usually 60 to 70 feet in height, and those in use in the United States for similar ores rarely exceed 55 to 65 or 70 feet in height, with a maximum width of 15 or 16 feet at the boshes, and are often only from 40 to 50 feet in height ; whilst furnaces very much smaller than these are in use both in America and on the Continent where charcoal iron is made ; thus at Vordernberg in Styria furnaces of only some 28 feet in height, 6 feet greatest in ternal diameter, and 450 cubic feet capacity were in use a few years ago and probably are still, whilst in various places XIII. 38