Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/328

312 IRON Carbon oxide 25 20 percent. Carbon dioxide 17-30 ,, ,, Hydrogen 0-10 Nitrogen 57-40 ,, and that per unit weight of pig made the amount of coke used was 1-116, of which 0-096 was ash and moisture, leaving 1 020 of actual carbon, whilst the limestone and mine (calcined Cleveland ore) con tained 082 carbon and 219 oxygen in the form of carbon dioxide, the iron being contained wholly as ferric oxide . The pig contained 3 per cent, of carbon, so that 030 of the total carbon entering the furnace did not escape in the gases ; consequently the weight that did escape was 1 020 + 082 -0 -030 = 1 072, whence the gases (leaving the hydrogen out of consideration) were made up of the following amounts : Containing Carbon. Containing Oxygen. 3-965 1-741 0-746 0-995 1-195 0-326 0-869 901 1-072 1-864 The coke, however, contained a small amount of moisture (some 2 5 per cent.), which would escape as aqueous vapour in the gases ; this, together with the hydrogen, would make the weight of the escaping gases a little more, about 6 93. The blast introduced con taining 3 965 parts of nitrogen (which must have been associated with 1 1S5 of oxygen forming air) must consequently have weighed 5 150, or making allowance for the moisture contained in it aoout 5 20 parts ; the total oxygen introduced into the furnace, therefore, must have been 1 185 in the air of the blast and 219 from the car bon dioxide of the flux ; whilst, as the pig contained about 95 per cent, of iron, f x 95 = 407 is the corresponding oxygen, making a total of 1185 + 0-219 + 0-407 = 1 811, to which must be added the oxygen in the moisture of the blast, and that contained in the silica and other impurities reduced in the pig iron, which consequently will give a total very near to the 1 864 parts calculated from the composition of the gases, and consequently to some extent checking the accuracy of the determination. The total development of heat inside the furnace is then as follows. The carbon contained in the pig, being produced by the reduction of carbon oxide, is formed in virtue of a reaction which absorbs heat ; the total fuel added being in the first instance burnt to carbon oxide will develop 1 020 x 2400 = 2448 ; of this a certain portion is further converted into carbon dioxide in the upper part of the furnace, the amount so converted containing of carbon 326 -0 082 = 244, and consequently giving a further heat evolution of 244 x (80002400) = 1366 1 (the 082 being originally contained in the flux as carbon dioxide) ; whence the total heat evolution is 2448 + 1366 = 3814. The heat brought in by the blast is 5 -20 x 485 x 237 = 597, 485 being the average temperature of the blast and 237 the specific heat of air ; whilst that taken out of the furnace by the waste gases is 6 93 x 332 x 0-24 = 553, 332 being their average temperature, and 24 their average specific heat ; hence finally the total amount of heat used up in doing the work of the furnace, and lost by radiation, &c., is 3814 + 597 -553 = 3858. In a similar way in a number of other instances the quantities of heat used up in the various furnace require ments were measured, with the general result that, with large furnaces of the construction employed in the Cleveland and Durham districts (48 to 80 feet in height), smelting calcined Cleveland ironstone either alone or mixed with haematite, a certain amount of heat is required for purposes practically constant and not varying with the richness of the ore and the amount of flux consequently requisite : whilst another portion of the heat is used up for purposes which are much more variable. Including the heat absorbed in the reduction from the gases of the carbon in the pig, the comparatively constant requirements of such furnaces are close to 2600, so that in the instance taken for example above, the non-constant requirements amounted to about 1258, the two sets being made up as follows : 1 Lowthian Bell calculates the total carbon in the carbon dioxide of the gases as being formed from carbon oxide, considering that the carbon dioxide of the flux is converted into carbon oxide by the hot carbon of the fuel pari passu with its formation ; and hence introduces an additional item into the table of furnace requirements, viz., the heat absorbed in this conversion, and an additional complication into the calculation owing to the subtraction of the carbon of the fuel thus supposed to be consumed in the upper part of the furnace from that burnt by the blast at the bottom. Approximate Comparatively constant heat requirements. nverugo amour.t Heat absorption during reduction of iron from ferric oxide 1650 ,, ,, carbon from carbon oxide 72 ,, phosphorus, silicon, and sulphur 209 Heat carried away by water used to cool tuyeres 91 ,, molten pig 330 ,, radiation and conduction 248 2600 More variable heat requirements." Heat absorption during causticizing limestone flux 250 ,, decomposition of moisture of blast 142 ,, ,, evaporation of water in coke 16 Heat carried away by molten slag (one part of slag carrying away 550) . units of heat) f b 1258 3858 Similar results were obtained in several other analogous series of observations, the variable heat requirements differ ing somewhat in some instances on account of the use of poorer or richer ores, different amounts of flux, &c. ; for the details of the methods used in the determination of the various items of furnace requirements see Bell s Chemical Phenomena of Iron Smelting, and also the earlier experi ments of Vathaire (Etude sur les Hants Fourneaux). Various publications of Griiner, and also an essay by Habets (abstracted in the Journ. I. and S. Inst., 1877, 225) may also be consulted with advantage; the numerical values of certain of the items deduced by Griiner from his observa tions differ somewhat from those of Vathaire and Bell, but not to any very material extent. The general result of these observations is that, with the large English furnaces used in smelting Cleveland ironstone with coke and lime stone, about 3850 is the amount of heat required on an average per unit of weight of pig iron made for the various furnace requirements, and that in even the most economi cally working furnaces the quantity of carbon contained in the issuing gases in the form of carbon dioxide does not exceed one-third of the total quantity of carbon, and rarely exceeds 30 per cent, of that amount, the remainder passing out as carbon oxide. Some of the carbon that does escape as dioxide, about one-fourth, is originally introduced as carbon dioxide contained in the flux (in the illustrative example, given above, 082 was contained in the flux out of 326 escaping as dioxide), so that only about three-fourths is derived from the fuel ; whence it results that even under the most favourable conditions, rarely met with in practice with this class of ores, not more than x |, or 25 per cent, of the carbon of the fuel is ultimately burnt to dioxide, the remaining 75 per cent, being burnt to carbon oxide, thus giving an effective heat development of 25 x 8000 + 075 x 2400 = 3800 instead of 8000 ; i.e., the " duty " actually performed by the fuel is only , or 47 5 percent, of the possible maximum amount ; so that if the amounts of heat brought in by the hot blast, and carried out in the waste gases, are approximately equal (which is the case when ordi nary cast iron stoves are used, delivering blast at tempera tures near 450), it finally results that, to produce the heat requisite for the various furnace requirements (amounting on an average to 3850), an amount of carbon must be burnt equal to ff or 1 013 times the weight of the pig iron made. Taking average coke to contain 7 per cent, of moisture and ash, and, consequently 93 per cent, of carbon, this represents 1 o 9 1 , j 3 = 1 OSO parts of coke per unit weight of pig iron, or 2 If cwts. per ton as the minimum prac ticable consumption of fuel attainable with calcined Cleve land ironstone, under the condition that the blast brings in as much heat as the waste gases carry out. Tt does not by any means follow, however, that only this minimum practicable consumption of coke will be requisite in any given fur nace. ^Vhen the dimensions are not such as to cause the most economical employment of the fuel, a considerably larger quantity of fuel may be requisite to enable the whole of the iron in the ore 2 Exclusive of that clue (according to Bell s mode of calculation) to absorption of heat by the carbon dioxide of the flux in being converted into carbon oxide.