Principles of the Manufacture of Iron and Steel

The heat produced by the actual combustion of the coke is necessarily affected by the amount of impurity it contains. This is very variable and uncertain; but as the difference will not exceed 2 per cent. in any case, this cause of disturbance is not, generally speaking, taken into the account.

(d) The quantity of heat which escapes unutilized in the gases.— This is estimated by observations on their temperatures; and the composition and weight for a given quantity of iron made being known, it is easy to calculate the amount of heat thus carried off. Care must of course be taken to extend the observations over a sufficient period of time to secure an average representation of the whole; because, as has been explained in Section IX., page 180 et seq., the temperature varies very considerably according to circumstances.

Beyond any irregularity in the composition of the coke, there are many other circumstances which render it impossible to claim perfect correctness in the estimates of the performance of a blast furnace.

The ironstone may be, and occasionally is, imperfectly calcined. This mineral, as it occurs in the Cleveland hills contains about 22 per cent. of carbonic acid or 6 per cent. of carbon. If we assume 65 units of raw stone to be consumed for each 20 units of iron we have 39 units of carbon in this weight of ore. Supposing 5 per cent. of this carbon to remain unexpelled we may have 19 of a unit of this element derived from this source instead of being contributed by the coke. The retention of this carbon as carbonic acid would, were it combined with the iron, mean that a corresponding quantity of the metal remains in the form of protoxide, instead of being peroxidized as happens in roasting the ore. The ore for each 20 units of pig iron might therefore afford 26 units less oxygen than is estimated for in my calculations.

The unexpelled carbonic acid however may be partly or possibly wholly retained by lime and magnesia which earths constitute about 12 per cent. of Cleveland raw ironstone. We should then have the relation between carbon and oxygen as they exist in the imperfectly calcined ore disturbed from this cause.

The coke itself may contain oxygen, generally in very minute

quantities; but I have recently met with a case in which 24 per cent. of this substance was found in a quantity furnished by a mine in the county of Durham.

Again the air blown into the furnace is constantly altering in the quantity of moisture it contains. Kämtz of Halle found over a series of years that the mean quantity of water in the atmosphere at that place varied from 61 to 86-2 per cent. of the quantity required for complete saturation. Taking 100 units of air as the consumption for 20 units of iron we should have 34 units of water entering the furnace were 61 per cent. of what is required for complete saturation and 48 units of water were it 86.2 per cent., or nearly one-half more of the first-named quantity.

Now when it is remembered that a large Cleveland furnace may contain at one time 1,000 tons of material, to smelt which about 1,000 tons of air is required it is easy to conceive the nature of the obstacles which may prevent perfect accuracy in any calculation founded on the process itself.

This difficulty of a correct appreciation of the various factors which have to serve as a basis of calculation makes us rarely able to produce an exact correspondence between the quantity of heat estimated to have been produced and that really required. In all cases, the coke consumed appears to give a little more heat than is actually needed, according to the figures taken for the rate of absorption. In the nine examples, contained in the two tables, the, average discordance between the two sides of the account is a trifle under 5 per cent.; but in each case the weight of coke itself has been taken as the basis of computing the consumption of fuel for the process.

The calculations which follow apportion to each function of the furnace its own equivalent of coke, together with the heat in the blast-the latter reduced to an equivalent of coke, so as to give at a glance the actual value of the hot blast in each case. These functions are divided into three sections, as follows:

Class I.-The elements requiring heat which depend upon the quantity of slag to be melted, the weight of limestone requiring dissociation, the quantity of water to be evaporated from the materials, and the dissociation of the moisture in the blast.

Class II. Those requirements of a furnace which comprise reduction and fusion of iron, dissociation of carbonic oxide, reduction of metalloids absorbed by the pig, loss of heat through walls of furnace, and heat carried off by tuyere water in the case of hot blast furnaces.

Class III.-Heat leaving the furnace in the escaping gases.

The first example, A, Table I., represents an ideal case of smelting Cleveland stone with cold blast, the particulars of which are sufficiently close to the truth to enable any one to compare the sources which make up the saving effected by the use of hot blast.

In this case, the mode of dividing the items of heat appropriation is set forth at length; but to save space the details are omitted except in two other examples, the elements being estimated according to the yield of the minerals and the quantity of flux required.

The work of the cold blast furnace-one of old construction, say of 6,000 cubic feet capacity-is estimated on a consumption of 45 cwts. of coke, requiring as much as 25 cwts. of limestone, with ironstone of the same richness as the other examples of Cleveland iron mentioned in the table-say 42 per cent. or 47.6 cwts. of calcined stone per ton of metal.

The three items for this furnace are thus calculated.1

Class I.-Elements of heat absorption:

370

3 (C) ×

3,200

1.16 ×

Decomposition of water in blast

...

Expulsion of carbonic acid from minerals 25 (CaCO ̧) ×

Decomposition of carbonic acid in do.

Evaporation of water in coke

The elements of absorption have been formerly (Chem. Phen., p. 163) divided into variables, constants and absorption by gases. The variables were subject to great fluctuations and are given under Class I. The constants were less liable to change but not being absolutely constant are viewed under Class II.

The large consumption of coke necessitates a great increase in the weight of air required for its combustion as compared with a furnace using hot blast: hence the excessive absorption of heat under this item.