Roasting Iron-stone.—The ores of iron require different treatment in the smelting process, according to the nature and extent of the heterogeneous matter with which the metal is combined. In all ores the iron is in the state of an oxide, and would require a strong heat in contact with combustible matter for their reduction. In most species, the oxide of iron is combined with a considerable proportion of earthy or stony matter, and they are thence denominated iron-stones. But besides the earthy matter and oxygen, many of these contain sulphur, arsenic, and manganese ; and it is necessary that these should be extricated previously to melting, which is effected by calcination, called roasting. This is usually done by stratifying the ore, broken into small pieces, with small or refuse pit coal, and burning it in great heaps, either in the open air, or in a kiln ; which dissipates the sulphureous, arsenical, and other volatile matters, leaving behind the earth and oxide of iron, which are then easily broken into convenient fragments for melting.
Teague's Patent Roasting Furnaces.— Mr. Teague, of the Park End Iron Works, near Calford, in Gloucestershire, took out a patent in 1832, for improvements in smelting, in which he proposes to economise the process of roasting the ore, both as respects the labour and the fuel. Instead of making the calcina tion a distinct process, conducted in another part of the works, he combines the operations of roasting and smelting, which go on simul taneously in the same furnace. For this pur pose he constructs around the chimney shaft, near to the top or tunnel-head of an ordinary smelting furnace, a series of four or more small reverberatory furnaces or ovens, each provided with a chimney, a damper at top, and a lateral door, which opens externally. Through these doors the ores to be roasted are in troduced, and deposited upon iron plates, which form the bottoms, and incline down wards towards the shaft of the smelting fur nace. The ascending body of flame from the latter, when at work, is prevented from passing out vertically, as usual, by means of a valve or trap door, which closes the orifice, and the small chimneys to the surrounding reverbera tory roasting furnaces being opened, causes the flame and heated matters to pass through these, and in their progress to impinge upon the ore, and deprive it of its volatile com binations, When the ores have been thus sufficiently operated upon, they are thrust forward by a proper tool into the body of the smelting furnace ; and whilst the roasting furnaces are being recharged, the valve or trap to the smelting shaft is opened, that the flames may take that course instead. Thus it would appear that a considerable saving of labour is effected, and that all the fuel employed in roasting by the ordinary process is likewise saved, as the flame which usually passes out of the tunnel head to waste in the air, is made to calcine the ore, and this being dis charged into the blast furnace at the high temperature it had acquired in the oven, the subsequent fusion of it is greatly accelerated. In Fig. 1, on the preceding page, is given a vertical section of one of these roasting ovens, in connexion with the tunnel-head. a a represents a portion of the tunnel•heed, or upper end of the blast furnace, provided with a door or valve b, resting on a ledge at c ; d is one of the four ovens; e the iron plate that contains the ore or "mine," supplied through a doorway at f ; g the oven chimney, with its damper h. Fig. 2 exhibits, on a smaller scale, a plan of the whole building, taken just above the oven plates, as designated by the four letters e; the fifth compartment i of the pentagon being for supplying the fuel and fluxes into the tunnel a, into which all of the compartments directly lead; each of these are closed externally by massive iron doors, suspended to stout iron lever beams to the opposite ends of which are hung counterbalancing weights, that enable the workmen to move them with facility. The dotted rectangle upon the plan is designed to explain the precise nature of the sectional view given in Fig. 1.
Jefferies' Coke and Iron Ore Furnace.—A patent for economizing the process of roasting ores was, however, taken out prior to the last-mentioned, by Mr. W. Jefferies, of Ratcliff, which was specified in August 1827. By this plan the roasting of the ore and the forming of the coke are performed together in one furnace ; the process is thus described in the Journal of Patent Invenfions, Vol. II. p. 66 : " The ore is first broken by atampera, or crushed by rollers, until it is reduced to such fragments as will pass through a sieve of eight or ten holes to the square inch. After which, instead of introducing the pulverized ore into a roasting oven or furnace, it is incorporated with a sufficient quantity of small coal, and the mixture put into a coke oven, previously heated in the usual way. Herein the ore is calcined by the heat of the coal, the latter being thereby converted into coke ; for this purpose the door of the oven is left open until all the flame has passed off, in the ordinary manner, when the door is to be closed, and all access of air prevented. After this the charge is to be with drawn, as if it were coke merely, and, when cooled, broken down into manses of a proper size for the smelting furnace, into which it is thrown ; the metal is here smelted out' from the coke with which it was combined ; the coke serving as the fuel to fuse and extract it."
Fluxes.—The coke and iron-stone having been duly prepared, the next consider ation is the nature and extent of the flux required to separate the metal by fusion from those of its combinations which the previous process was inadequate to perform. These differ in every degree and form, according to the nature of the ore, and as their consideration at length would occupy a greater space than the assigned limits of this work will afford, we purpose availing ourselves of a few extracts from the valuable papers by Mr. David Mushet, inserted in Marl's Philosophical Magazine; and must refer those of our readers who may desire more information to the original source for it. " To deprive an ore of its iron" (says Mr. Mushet) " so that no portion of it shall escape in the emir unrevived, two things are indispensable. First, the metal contained in the ore must be presented to a portion of fuel sufficient to take up the oxygen of the metal. Second, as this revivification goes on in the manner of a metallic per spiration upon the softened surface of the ore, another agent ought to bepresent to facilitate the separation by uniting with the earthy parts of the ore, farming a thinly divided lava, no longer capable of retaining the lobules of metal, or of preventing the congenial affinity of the carbonic principle from taking full effect for the improvement of the quality of the iron."—" Experience has shown that an excess of any particular earth may be corrected by applying one opposite in its effects ; and that the addition of lime, in various proportions, is found in most cases to answer the desired end."—" If the various classes of iron ore are fused in contact with charcoal, without the addition of a flux or solvent, the result is, from calcareous iron-stone, a larger portion of iron propot tioned to the intrinsic richness of the ore, than from argillaceous iron-stone; and from the latter a larger produce of iron, than from an ore whose chief mixture was silex ; the scorise produced from the respective operations always demonstrate, from the colour and opacity, the probable quantity of iron which still remains to be taken up." There are some calcareous iron-stones which contain lime almost sufficient to form the necessary scoria', the colour of which, when freed from the metal, possesses a considerable degree of transparency. When a number 'of these stones are used in the blast furnace, a much less quantity of calcareous earths are necessary. It sometimes happens at iron works, whose chief supply is derived from a calcareous field of iron-stone, that by using a great proportion of an individual ore, surcharged with lime, the operations of the furnace are obstructed, and consequences entailed fatal to the interest of the manufacturer. From an excess of pure calcareous earth being present in the furnace, the acorn, thick and curdled, becomes attached to the sides and bottom of the furnace; the quantity hourly increases, till it has accumulated to such a degree as to intercept the ascent of the blast, and the descent of the materials. The remedy suggested by Mr. Mushet for these inconveniences, is, reducing the quantity of lime direct, or by an admixture of clay or sand, whether combined with iron or not. An excess of clay in the argillaceous ores has the same prejudicial effects which have led to their rejection, though it has been owing to improper application. The fusibility of lime and clay individually is much facilitated by the addition of sand. In all cases where these earths exist in excess in the ores, they ought either to be combined in the blast furnace with siliceous iron-stone, or treated with a lime-stone con taining a considerable portion of sand. When a scarcity of lime exists in the blast furnace, and a superior quantity of clay and silex is combined with the iron-stones, the lava will flow from the furnace comparatively cold, tenaceous, and of a brown or pale dirty green colour, containing iron; when the mixture is just, the colour of the scoria' is pure white, enamelled with a variety of blue shades, waving, circular, or formed in straight delicate lines, arising from a peculiar existing modification of a minute portion of the metal. Where nature has bestowed mixtures productive of every quality of crude iron, the proper management of ores would become simple and easy : just combinations super sede the necessity of changing the quality of the lime-stone added for a flux, or of having recourse to various qualities of it, in order to assist or correct the deficiency of the native mixture. Wherever the ores are of a structure thus deficient, it then becomes the province of the manufacturer to ascertain the mixture of the individual ores which compose his supply, and to restore that equilibrium of parts by the proper application of superadded earths which experiment and observation have proved to determine a certain quality of iron.