REFINING OF CAST IRON.

587

a process of cementation the reverse of that which takes place during the manufacture of steel: the carbon is gradually removed from the outer layer of the metal, and is slowly transmitted from particle to particle through the solid bar, till it reaches the surface, where it undergoes oxidation at the expense of part of the oxygen of the hæmatite. The foregoing analyses contain the results furnished by a sample of iron both before and after it had been thus treated. The cast iron was obtained from the Lancashire brown hæmatite.

(748) Conversion of Cast Iron into Wrought Iron.-1. Refining. -The pig-iron as delivered from the furnace is, as already noticed, far from pure it contains variable quantities of carbon, silicon, sulphur, and phosphorus, besides traces of other metals, such as aluminum, calcium, and potassium. Before it can be converted into the wrought iron of commerce, it has to undergo a process for the removal of these extraneous matters. Many castings may be made at once with pig-iron, but it cannot be worked at the forge.

In order to effect the purification of the crude pig-iron, it is necessary to expose it to the regulated action of oxygen at a high temperature, so as gradually to burn off these oxidizable substances, and leave the iron. The pig iron is usually first remelted in quantities of from 25 to 30 cwt., upon the hearth of a sort of forge, termed the finery or refinery, the fire of which is animated by a cold blast from a double row of blast-pipes. The sides and back of the hearth are formed of hollow iron castings, through which water is kept continually flowing. During this operation, which lasts about two hours, and is one of the most wasteful both of fuel and of iron, the metal loses from 10 to 12 per cent. of its weight. The silicon is more readily oxidized than the carbon, so that it is the impurity which is first attacked in the refining process, but at the same time a small portion of the carbon contained in the iron is burned off as carbonic oxide; part of the iron also becomes converted into the protoxide, which unites with the silica furnished by the oxidation of the silicon, and with the sand which adhered to the surface of the cast metal: a fusible slag consisting of ferrous ortho-silicate (Fe,"Sie) is thus produced. The oxide of iron in this slag again reacts upon the melted metal, and by imparting a portion of its oxygen to the silicon and carbon disseminated through the mass, burns off an additional quantity of these substances; portions of sulphur and phosphorus are also separated by oxidation in this process, and accumulate in the slag. The melted iron is then run off, and formed into a flat

cake 2 or 3 inches thick, and as soon as it begins to solidify it is suddenly cooled by pouring water upon it; a hard white, brittle mass is thus obtained, which is broken up into fragments. In this operation coke is the combustible generally made use of, but where iron of superior quality is required, as in making tin-plate, charcoal is employed. Ordinary coke contains sulphur and earthy impurities which injure the quality of the iron.

The effect of the operation is well exhibited by the following analysis quoted by Regnault, giving the composition of a portion of cast iron before refining, and a portion of the same metal after it had passed through the refinery furnace.

(749) Puddling.-The refined metal still retains a considerable proportion of carbon and some silicon. In order to remove them it is next introduced, in charges of from 4 to 5 cwt., into the puddling furnace. This consists of a reverberatory furnace, connected with a chimney 40 or 50 feet in height, and capable of producing a powerful draught, which is under complete command by means of a damper. Fig. 346 represents a section of the puddling furnace; A, is the bed, or hearth, upon which the iron for puddling is placed; B, is the fire-place with the aperture for stoking, which is closed with coal, and not by a door as is usual in most furnaces: G is the bridge which separates the fuel from the metal: the hearth, A, is lined with cast-iron plates, E, E, which are prevented from melting by the free circulation of air beneath them; c is the flue leading to the chimney, D, at the top of which the damper is shown; H is the plate upon which the iron rests during the puddling process; it is protected from the heat by a coating of powdered hæmatite, of sand, or of slag ;* F is the working door of the furnace through the upper aperture in which the puddler works; the lower aperture is closed by sand during the operation, and is opened at intervals to allow the slag or tap

The employment of lime as a lining to the furnace has been recommended; it is said to improve the quality of the iron by removing sulphur more completely, and at the same time to diminish the rate of oxidation.

PUDDLING OR BOILING.

589

cinder to be drawn off, and 1 is the floss-hole, or aperture through which the overflow of slag is removed.

For the coarser kinds of iron the furnace is sometimes charged partially, or even wholly, with pig-iron that has not been refined.

Iron which has undergone the refining process never becomes so completely liquid in puddling as when crude pig-iron is employed, but the product is a metal of finer quality.

Supposing the crude pig-iron to be used, the pigs slowly become melted, and the metal when first heated forms a thick pasty mass, which gradually becomes fluid, and at length perfectly liquid. At this stage the metal becomes violently agitated, and assumes an appearance of boiling, owing to the escape of the carbonic oxide in jets, which take fire and burn with a blue flame, whilst the melted mass swells up to several times its original bulk. It is now briskly stirred by the puddler to promote oxidation.

When refined iron is used it is often mixed with a certain proportion of scales of oxide from the forge, and is then gradually brought into complete fusion, carefully avoiding the contact of fuel. The mass is well stirred, so as to incorporate the oxide of iron with the melted metal; oxygen is transferred from the oxide thus introduced, to the carbon of the melted iron, and carbonic

oxide is formed abundantly; but the appearance of boiling is less marked than when crude pig-iron is used. In either case, the metal by degrees becomes less fusible as the carbon diminishes in quantity, and at length it is converted into a granular, sandy mass. The heat is now raised till it becomes very intense, and air is carefully excluded by closing the damper and doors. The metal again begins to soften and agglomerate. The puddler gradually collects it into balls or blooms upon the end of an iron rod; he then removes it from the furnace in masses weighing about three-quarters of a hundredweight, and subjects it, whilst still intensely hot, to the action either of the steam hammer, or a powerful press, called the shingling press. The melted slag is thus forcibly squeezed out, the particles of metal are brought nearer together, and the density is increased. The iron is then fashioned into a bar by passing it between grooved rollers, and the bar thus obtained is cut into lengths, then piled up in a reverberatory furnace and re-heated; it is again rolled, doubled upon itself, and re-heated and rolled. Upon the best qualities of iron this process is repeated several times, in order to render its fibres parallel to each other, by which the toughness of the metal is much increased. The iron is now nearly pure; it contains from to of its weight of carbon, and about 300 of silicon. The presence even of this small proportion of carbon adds materially to the toughness and hardness of the metal. The process of puddling occupies about two hours; and provided it has been properly refined previously, the metal loses from 7 to 10 per cent. of its weight.*

50

400

*Calvert and Johnson (Phil. Mag. Sept. 1857) have made a series of analyses of the iron in different stages of the process of boiling. They employed in their experiments good cold-blast Staffordshire grey iron, No. 3, such as is used for making iron wire.

A charge of 2 cwt. of iron was introduced into the bed of the furnace,

MANUFACTURE OF WROUGHT IRON.

591

The slag produced during the operations of puddling and refining consists chiefly of ferrous ortho-silicate (FeSiO), and contains upwards of 60 per cent. of the metal. This slag or finery cinder is reduced in the blast furnace in the same manner as the original ore, but it is always found to produce a defective iron, technically known as cold short. Such iron may be forged well at a red heat, but when cold it is brittle and rotten. This. defect is attributed to the presence of phosphorus, which is separated from the crude metal in the form of phosphate of iron during the puddling. When the slag is reduced in the blast furnace, both the phosphorus and the iron are deprived of their oxygen, and by their union, as phosphide of iron, form the faulty metal in question.

Mr. Bessemer has attempted to substitute for the processes of puddling and refining a method of purification which consists in forcing cold air at a pressure of 10 or 12 lb. upon the square inch through melted cast iron, which, as it runs from the furnace, is received into a cylindrical vessel covered with an arched head and lined with fire clay, the air being driven in at the bottom, through several tuyères. An intense combustion occurs, attended with remarkable elevation of temperature, owing partly to the oxidation of the iron, and partly to that of the carbon; the latter, being converted into carbonic oxide, escapes at all points of the mass, throwing the whole into violent agitation, which subsides as

without any addition of oxide of iron in 40 minutes it became fused, and on cooling the sample suddenly, it yielded a brittle mass like white iron. It will be seen that whilst the carbon increases during the first stage of the process, the silicon undergoes a very rapid diminution. The 3rd sample was taken just before the beginning of the boil, when the iron was in its most fluid condition. No. 4 was taken during the full boil, and consisted of small detached brittle granules surrounded by slag. No. 5, the boil was completed. It was still in granules, but they were slightly malleable. No. 6, the iron was collecting into masses. No. 7 was taken during balling, and in No. 8 the balls were just ready for the shingling press. The puddled bar' was taken from the iron after it had been hammered; and the wire iron' was the same after it had been broken up into billets, reheated, and rolled as a preliminary to drawing.

The slag which was separated during the operation was found to have the following composition: