THEORY OF THE BLAST FURNACE,

577

blast-pipes, which are connected with powerful blowing machines for supplying air under a pressure of from 2 lb. to 3 lb. upon the inch. A steady and most intense heat is thus uniformly maintained. At the lowest point of the furnace is the tap-hole, for drawing off the melted metal at suitable intervals, and which, except at such times, is closed with sand and clay: K, K, are galleries, which allow the workmen free access to the tuyères and lower portion of the furnace, the base of which is kept dry and well drained by the arched channels, M. Above the crucible the furnace suddenly widens, forming the boshes, D; the lining, c, is formed of firebricks, which are continued up to the throat, A, of the furnace : the whole is cased in solid masonry, E, E, and supported by iron bands. When working regularly, such a furnace is charged through the opening, в, near the top, at intervals, first with coal, and then with a suitable mixture of roasted ore and of a limestone flux broken into small fragments. As the fuel burns away, and the materials sink down gradually, fresh layers of fuel, and of ore, are added; so that the furnace becomes filled with alternate layers of each.

The principal substances which are acted upon in such a furnace are the following :

1st, the oxygen contained in the air of the blast; 2nd, the roasted ore, consisting of oxide of iron, silica in the shape of sand or quartz, clay or silicate of aluminum, and a little magnesia and oxide of manganese; 3rd, coal or coke,-composed chiefly of carbon, with a small proportion of hydrogen; and 4th, carbonate of calcium, which in the heat of the furnace soon becomes quicklime.

(745) Theory of the Blast Furnace. The chemical changes may be traced as follows, beginning at the bottom of the furnace :-The oxygen contained in the air of the blast, as soon as it comes into contact with the fuel in the crucible, combines with the carbon and forms carbonic anhydride, attended with a combustion of intense activity. The blast is thus soon deprived of all its free oxygen; nearly the whole of the nitrogen escapes unchanged, but the carbonic anhydride, in its passage over the ignited fuel, is decomposed; each atom of the anhydride combines with an additional atom of carbon, and becomes converted into carbonic oxide; for each volume of carbonic anhydride 2 volumes of carbonic oxide are produced. This formation of carbonic oxide is attended with a large absorption of heat, so that the temperature of the furnace, above the crucible, becomes rapidly reduced,

578

GASEOUS PRODUCTS OF THE BLAST FURNACE.

and a quantity of highly combustible gas is thus formed.* This carbonic oxide becomes mingled with carburetted hydrogen and free hydrogen, which are derived from the fuel contained in the upper part of the charge, as it gradually descends towards the focus of intense heat below. A proportion of the gases which escape from the opening at the top of the furnace, varying from 35 to 40 per cent., is combustible; the remainder consists principally of nitrogen, with a small amount of carbonic anhydride. The ore having been rendered porous by the previous roasting, is easily penetrated by these ascending gases, by contact with which the iron becomes reduced in the upper

* Bunsen and Play fair, in their examination of the gases produced in a hot-blast furnace at Alfreton, found that a considerable amount of cyanide of potassium was formed in the hotter portions of the furnace (British Association Reports, 1845, p. 182): part of the nitrogen, derived probably both from the blast and from the coal, had therefore entered into combination with carbon, and had united with the potassium contained in small quantities in the ore and in the ashes of the coal.

The furnace in which these experiments were made was 40 feet deep from the top of the charge to the hearthstone, and was charged every twenty minutes with 420 lb. of calcined clay ironstone, containing about 60 per cent. of oxide of iron, 390 lb. of coal, and 170 lb. of limestone: each charge yielded 140 lb. of pig-iron. The blast was under a pressure of 6.75 inches of mercury, and had a temperature of 626°.

These chemists state that at a depth of 2 feet from the tuyère, or 34 feet from the top of the furnace, the gases which they collected contained 134 per cent. of cyanogen. The following table furnishes a summary of the results which they obtained ::

Analysis of Gases from a Hot-blast Furnace.

The process of coking, which is effected in the upper part of the furnace, did not appear to be complete until the charge had reached a depth of 24 feet, but was most active at a depth of 14 feet; the principal reduction of the ore seemed to take place just below the point at which the coking was completed: the maximum heat of this furnace occurring between about 3 and 4 feet above the tuyère, or 33 feet from the top.

In a furnace fed with charcoal, Bunsen found the reduction of the ore to commence nearer the throat of the furnace, for in this case no absorption of heat occurred similar to that occasioned by the process of coking the coal, which takes place in the upper part of the hot-blast furnace. The body of a charcoal furnace consequently does not require to be so high as that of a furnace in which coal is used. Similar experiments by Ebelmen lead to conclusions substantially the same.

THEORY OF THE BLAST FURNACE-SLAGS.

579

part of the boshes, where the heat is comparatively moderate. By degrees the reduced metal, mixed with the earthy matter of the ore, sinks down to the hotter region. Here the earthy matters melt and become vitrified; whilst the iron, in a minutely divided state, being brought into contact with the carbon of the fuel, combines with it and forms the fusible compound well known as cast iron. This carbide of iron melts, sinks down below the tuyères through the lighter vitrified slags, and is protected by them from the further action of oxygen. The bulk of the slag is 5 or 6 times as great as that of the iron produced it floats above the melted metal, and is allowed to flow over continually at the opening left for the purpose; whilst the iron is run off at intervals of 12 or 24 hours, by withdrawing the stopping of clay and sand from the tap-hole at the bottom.

The furnace slags constitute an imperfect species of glass, which is sometimes more or less distinctly crystalline, and which varies in colour with its composition, being grey, blue, green, brown, or black. They consist principally of silicates of calcium, magnesium, and aluminum, with generally a small proportion of silicates of manganese and iron. In the formation of these slags the siliceous matters of the ore react upon the earthy bases, lime, magnesia, and alumina, and really neutralize them.

The general composition of these slags may be scen from the subjoined analyses: I. A slag obtained from Merthyr Tydvil, by Berthier. II. A cold-blast slag, Tipton, Staffordshire, by D. Forbes. III. A hot-blast slag from coke-furnace, by Percy. IV. Average of slag from 13 blast furnaces at Dowlais, by Riley; the last three quoted from Percy's Metallurgy, vol. ii. pp. 497 and 499.

The oxygen in the bases of these slags is nearly equal in amount to that contained in the silica. Those quoted from

580

SMELTING OF CLAY IRONSTONE.

Percy approach the formula, 6 [2(CaMgMnFe)O‚§¡Ð,], 2Al‚Ð ̧

3 ᎦᎥᎾ,.

The composition of No. I. may be represented by the formula, 5 [3 (CaMgFe) 0,2 SiO„] · (2 Al ̧Ð ̧, SiO2).

2

There are several points which require nice adjustment in this process of reduction. The slag must not be of too fusible a description, otherwise the iron falls to the bottom before it has thoroughly combined with the carbon, and is not completely melted; a sufficiency of lime should always be present to neutralize the whole of the silica, for unless this be attended to, a ferrous silicate is formed, and iron runs off in waste. Indeed, a small excess of lime is advantageous, as it removes sulphur, if present, in the shape of sulphide of calcium. At the same time the calcareous matter must not be too abundant, otherwise the working of the furnace is obstructed; the slags which are formed being of a less fusible character are but imperfectly melted, the iron is entangled within them, it is again partially oxidated by the blast, and the product of the furnace is greatly diminished. Experience has shown that the slags (which are chiefly composed of the mixed silicates of aluminum and calcium) are most fusible when the oxygen of the silica amounts to double that in the bases with which it is combined, and when the proportion of lime employed as a flux is such as would be furnished by adding 2 parts of limestone for every 3 of clay contained in the ore; the ratio of lime to alumina being 6 Ca: Al,0,. A slag of this kind, however, can only advantageously be formed when the ore is smelted with charcoal, a fuel which contains but little sulphur, and which allows the reduction to be effected at a comparatively moderate temperature. When coal or coke is used as the fuel, an excess of lime is required to carry off the sulphur introduced by the pyrites of the coal, and the slag which is produced under these circumstances is found to work most advantageously when the proportion of oxygen in the bases is nearly equal to that of the silica. The temperature of a blast-furnace fed with coal or coke is much higher than that of one in which charcoal is used. Slags containing several bases are more fusible than when one or two only are present, the different silicates aiding the fusibility of each other. For a summary of an extensive experimental inquiry into the composition and properties of slags, the reader is referred to Percy's Metallurgy, vol. i. pp. 20-49, and vol. ii. loc. cit.

In the process of smelting it is also necessary to proportion the supply of air rightly; if too much be thrown in, the furnace becomes unduly cooled; if too little, the supply of oxygen is in

sufficient for the maintenance of a proper temperature by a due amount of combustion. These, however, are points the successful regulation of which can only be acquired by experience. The stream of air for the blast is not supplied in intermitting gusts, but is equalized as much as possible: where the cold blast is used, this object is attained by employing an air-chamber or reservoir; and where the hot blast is employed, the long pipes required for heating the air answer the same purpose.

(746) The Hot Blast.-The mass of air which passes through one of these furnaces is enormous, being not less than 160,000 cubic feet, or about 6 tons weight, per hour. It is evident, therefore, that this immense volume of air must exercise an extraordinary cooling effect upon the contents of the furnace. This evil has been much reduced of late years by the introduction of air which has been previously heated. In this contrivance, which is known as the hot blast, the air, before it reaches the furnace, is made to pass through a series of pipes which are maintained at a high temperature, either by means of a separate furnace, or by a portion of the waste heat of the blast furnace itself: in the latter case the hot gases are conveyed through flues which pass from the upper part of the furnace into the chamber which contains the pipes; the necessary draught being maintained by a chimney furnished with a damper. A jet of the blast as it enters the furnace should have a temperature sufficiently high to melt a strip of lead when held in it. The temperature of such a jet as it issues from the tuyère is somewhat higher than 600°. Mr. Siemens has improved upon this plan by transmitting the gases which escape from the furnace, through what he terms a regenerator. It is simply a chamber of brickwork filled with fire-bricks so arranged as to allow the heated gases to circulate freely around them. Two such chambers are prepared; as soon as the bricks in one of these chambers are redhot, the current of gas from the furnace is cut off, and directed into the other chamber, in order to heat it. In the meantime a current of cold air is forced through the heated chamber, and a hot blast of from 1200° to 1300° is thus obtained. Each chamber is worked alternately; the one becoming heated whilst the other is employed in heating the blast. In this way a large proportion of the heat of the waste gases may be economized.*

* This process, as its value becomes appreciated, will no doubt come into very extensive use in a great variety of operations in metallurgy. In many cases it effects an economy of one-half of the fuel employed, and it is possible to obtain by its means a steady and uniform temperature not exceeding that of a full red, up to the heat required for welding iron. It is now employed