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SECTION VIII.

ON THE SOLID PRODUCTS OF THE BLAST FURNACE.

Or the substances, which the title of the present heading is intended to include, the pig iron is of course the most important. The slag, or cinder as it is usually styled, next demands consideration, because it constitutes the principal index of the performance of the furnace itself. A third body which, for our present object, may be classed among the solid products, is the vast volume of white smoke which is seen leaving most furnaces, and which is particularly conspicuous in the treatment of Cleveland ironstone and of some others.

Cast Iron requires the presence of only two elements for its formation, the metal itself and carbon: that is, a compound consisting exclusively of these two substances may be formed, which would be fusible at a moderate temperature. It is strictly true, however, to say that pig iron, as run from the blast furnace, always contains other matter, which may to some extent be regarded as foreign to its essential composition. This impurity, as it may be considered, often includes several different elements, and as Cleveland iron will serve as an illustration of this complex composition, five analyses of this make are subjoined :

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Against iron containing, as in the examples just given, 3 to 4 per cent. of substances which are not essential to its composition, some of which, if not removed, are for certain purposes, absolutely injurious, may be set the analysis of other varieties of pig, in which some of the above-named elements are either wanting or are found in greatly diminished quantities. The following are examples of analyses taken from Dr. Percy's work-a fact which may be accepted as a guarantee for their correctness.

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It is now above 50 years since Neilson introduced the use of hot air in the blast furnace; and it is a somewhat remarkable fact that even to-day there is an absence of a complete and systematically conducted course of experiments, to prove that cold blast iron is really, as is pretended, superior in point of strength and quality generally to that made with hot blast. On the face of it, there are some obvious reasons why hot blast iron should be purer and therefore stronger, looking at the nature of the impurities, than that made with cold blast. All mineral fuel contains phosphorus and sulphur; in that of one of the most renowned works in Great Britain the former varies from 025 to 110 per cent., and the sulphur exists to something like 5 per cent. If we assume that the consumption of fuel is reduced from 40 cwts. to 30 cwts. per ton of iron, we secure a corresponding reduction in the quantity of phosphorus in the iron; and the sulphur, a portion of which at all events finds its way into the metal, is also reduced in quantity.

There is a kind of vague idea among the advocates of cold blast iron, that the alleged higher temperature of a hot blast furnace affects, in some way they have never explained, the quality of the iron it produces. It is stated by Sir W. Fairbairn and others that the

repeated application of heat to solid iron, cast or wrought, does deteriorate its quality; probably owing, I imagine, to a new arrangement of its molecules, to a change in chemical composition, or to an occlusion of oxygen or other gaseous matter. But there does not exist any proof that any such effect takes place, or could take place, in the blast furnace, even were the temperature higher when it is blown with hot air than with cold. But there is really no substantial ground for pretending that the hearth of a cold blown furnace is not quite as hot, taking it as whole, as that of one receiving heated air. The quantity of heat evolved in the two, so far as combustion of fuel is concerned, is actually higher in the cold than in the hot blast furnace. Neglecting the ash in both cases, we have the following figures, representing the carbon used per 20 units of iron

Cold blast furnace, taking the air

even at the freezing point of water }

}

Calories.

40 units of carbon × 2,400 = 96,000

Hot blast furnace, receiving its 30 units of carbon × 2,400 = 72,000 air at about 900° F. (482° C.) Calories in blast = 16,500

88,5001

Besides this, if we are to believe that the quality (or greyness) of the metal depends on the actual temperature to which it is exposed in the blast furnace, what becomes of the pretended differences of temperature between the use of hot and cold air, both running the same quality of iron?

Again, let us consider the probable effect of an increase of heat on the behaviour of the elements found in combination with iron in its form of pig.

Reasons have already been adduced for believing that an elevation of temperature strengthens the affinity of sulphur for lime or calcium, at all events less is found in iron run from a hot working furnace and I am not aware that there is any information to show that any possible differences of heat can affect the action of phosphorus in its behaviour towards iron during the process of smelting. With regard to calcium

These figures are those which set forth the heat evolved in smelting iron in furnaces of small dimensions. The carbon burnt in large furnaces is very much less, and would illustrate the argument in a still more striking way.

and the other metals, it is not improbable that an increase of temperature might be accompanied by a larger amount of these in the pig; but I know of no instances where any ill effects have been connected with their presence.

In the hope of throwing some light on this question, advantage was taken during the blowing out of one of the Clarence furnaces, to reduce the burden and use the blast cold. The iron was white and had the composition given below. Alongside this analysis is placed an example of white iron made with hot air:

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From these figures it will be seen that the most notable difference is in the silicon, but that most of the other substances are in less quantities in the cold than in the hot blast iron. At the same time the short duration of the experiment does not afford sufficient data to speak with much confidence on the relative quality of the metal. On puddling the pig so made, the malleable iron showed no kind of superiority over that smelted with hot air.

Viewing the question as one of the application of pig iron to castings, the late Sir William Fairbairn, F.R.S., published some experiments undertaken for the purpose of determining the kind of metal to be used in the construction of the High Level Bridge at Newcastle-onTyne.

This competent authority gives in a table the following results :

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A second table contains the following:

Breaking weight applied as above. Mean ultimate deflection-inches.

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The irons ultimately selected for the Newcastle Bridge were :—

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Bars from the above mixture, tested as before, supported 705 lbs. before they broke, and deflected 89 inches, being superior in strength and power of deflection to Ridsdale, a Northumberland iron, which gave the best results when examined separately.

It would be hazardous for any one to pretend from the above figures, taken as a whole, that cold blast iron is really superior for castings to hot blast; for the information as given above does not settle the question either one way or the other. Ridsdale hot blast may be better, as the trials would indicate it to be, than any of the brands with which it is compared, not because it is hot blast, but on account of the minerals used in its manufacture. Nothing short of the two systems being applied, over a period of time, to precisely the same minerals can satisfactorily settle this question. In the list compiled by Fairbairn there are 3 cases given of the same make of iron smelted with cold and hot air, but it is not positively stated that there were no differences made in the ore, fuel, and limestone, conjointly with the change of blast.

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