guided by the commercial aspects of the question. The fuel used in heating the blast is the gas which escapes from the furnace, which in many, indeed in most, cases would be wasted; but may at the best be valued as small coal, which in the North of England can be had for threepence per cwt. If then by burning two cwts. of coal worth sixpence, or, still better, by burning furnace gas costing nothing, three cwts. of coke, worth it may be two shillings, could be saved, a great gain would arise from such a change. The question raised a few pages back was the extent to which the fuel, actually burnt in the furnace, could be lessened by a corresponding amount of heat being communicated to the blast. It would be difficult perhaps to predict how far this substitution could be carried, in face of the principles which have just been explained; but the real extent is of less consequence, because there is one circumstance which will, in all probability, prove an effectual barrier to much more being done than what has already been achieved. In the table recently given, page 89, the temperatures of the blast in burning different quantities of carbon were as follows: Units of carbon burnt Temperature, degree F. Increase over preceding quantity... ... 18 17 16 15 869 1,236 1,652 2,152 367° 416° 500° The nature of the rise of temperature in the air, renders it impossible, as may be easily understood, to continue the increase further; because no apparatus could be made to stand the wear inseparable from a temperature of 2,152° F., this being close to the fusing point of cast iron. If we adopt 1,600° to 1,700° F. as a practicable point to be attained, the equivalent consumption in coke (using about 12 units of limestone for smelting Cleveland stone), works out to the figures given below: Units of carbon per 20 units of pig burnt with blast at 1,652° F.... 1600 1.44 Carbon in iron *60 18.04 Add for impurities to 1804 carbon to bring it to coke, say 71 per cent. 1.35 19.39 It must not be supposed that this weight of coke is unchangeable, as it is susceptible of some trifling modification arising from differences in the richness of the ore or of the quantity of flux required. With this qualification 19 cwts. of coke may, in my opinion, be accepted as the possible limit with which a ton of Cleveland foundry iron will be produced, using air at a temperature of nearly 1,700° F. If so we have a gain of one cwt. of fuel or thereabouts, as compared with the case when the blast is only heated to 1,000° F. In connection with the use of superheated air, there are certain considerations, beyond those referred to, which may ultimately lead to its large extension. The difficulty of forcing a large volume of blast through a very lofty column of solid material is well understood by the practical smelter. This difficulty has been notably augmented by the use of the higher furnaces now almost universally adopted in the North of England. The resistance to the passage of the blast did not prove however to be greater than could be dealt with, at the time, by a trifling addition to the pressure applied by the blowing engines. Of late years the demand for coke for iron works has been so great, in the quarter referred to, that the Durham collieries have been compelled to use inferior seams of coal, to mix with the best in their ovens. This change has not been followed by any material alteration in the purity of the coke, but it has, it is thought, considerably altered its ability to support the heavy load to be carried in a furnace 75 or 80 feet in height. In some cases, as in the anthracite furnaces of the United States, increased resistance to the entry of the blast is overcome by a mere addition to the pressure; which I found occasionally to be as high as 12 lbs. and even more on the square inch. It may be that similar treatment might prove efficacious in the North of England; although the limited trials which have been made in this direction have not been very encouraging. The inconvenience, attending such a state of things as that just mentioned, no doubt gives rise to irregularities in the descent of the materials, and in their uniform permeation by and exposure to the reducing gases. This is only met by an increased consumption of coke, which aggravates the evil by calling for a corresponding addition to the quantity of air required for its combustion. These remarks point to the expediency of diminishing the height of the column, through which the blast has to be forced; and this, we have already seen, can be accomplished by increasing the temperature of the air. It is almost superfluous, however, to repeat that the quantity of carbon present in the furnace, must always be sufficient to maintain a reducing action in the gas produced by the oxidation of the fuel at the tuyeres. It is only therefore when the consumption of coke notably exceeds the limit supposed to be essential for this object, that the use of superheated air can be expected to be beneficial, to any great extent, in saving fuel. The use of superheated air will be referred to again in a future Section, which will be reserved for the subject of the fuel employed in the blast furnace. SECTION VII. ON THE QUANTITY AND QUALITY OF THE FUEL REQUIRED IN THE BLAST FURNACE, USING AIR OF DIFFERENT TEMPERATURES. IN Sections V. and VI. I have endeavoured to give a concise account of those chemical and physical laws, which are concerned in the reduction of the ore and the fusion of the products in the blast furnace. In the remaining Sections devoted to the smelting process, these laws, at the risk of some repetition, will be examined with greater minuteness, and at the same time some further explanation will be given of the nature of the experimental researches undertaken during the course of my investigations. Some confusion may arise in considering the subject of this Section, by overlooking the very different conditions which often attend the smelting of iron from different kinds of ore. It is not meant to restrict the observation just made to mere richness of metal contained in the mineral, a circumstance to which undue importance may easily be given, for reasons shortly to be explained. We have already seen that the economy effected by the hot blast was by no means of an uniform character, nor did the advantages derived from the enlargement of the furnaces, when smelting Cleveland and some other ironstones, always attend a similar alteration, when applied to those engaged on ores of a different kind. In these last mentioned cases, the difference in the results was due to the greater or less facility with which the ore in use parted with its oxygen. But if two minerals are identical in their facility of reduction, but one is poorer in iron than the other, the additional fuel required is that which is needed for melting the additional quantity of slag, and for providing for some minor sources of heat absorption. The reducing power of the gases must always be maintained, and is regulated therefore by the actual quantity of iron to be reduced. Pig metal however, as is well known, is not only impure iron, but the impurity it contains varies in amount according to its origin or its mode of manufacture. Such foreign matters as silicon, phosphorus, and some others, are, like the iron itself, reduced from a state of chemical combination, for which reduction, heat and carbon are indispensable: hence it may well happen. that similar quantities of pig may require somewhat varying measures of heat, etc., and therefore of fuel, for their treatment in the blast furnace. The desirability of knowing the actual quantity of heat, necessary for producing a given quantity of pig iron, is obvious; and thanks to scientific research, we have at our command such information, of a sufficiently exact nature for our present purpose. The following table is an estimate of the number of calories, or heat units, supposed to be required for smelting Cleveland iron of average composition, including certain sources of waste which may be regarded as inseparable from the operation. The quantity of iron taken as a basis of calculation is 20 kilogrammes, which permits of a ready comparison with 20 cwts., the familiar weight to the mind of an English manufacturer : Expansion of blast, escape into foundations, etc., supposed 3,389 8,789 79,100 Carried off in escaping gases from a modern furnace 7,900 87,000 |