TIN MANUFACTURE AND TRADE. Referring to other articles for notices of the mineralogical, chemical, and medical characteristics of tin, we shall here treat the metal in its manufacturing and com mercial aspects.
Under 31rarso it is explained in what way the ores of tin are ex tracted from the mines of Cornwall and other places, and by what processes they are mechanically separated from some of the impurities, and broken into fragments. We shall now take up the details from that point.
Smelting and Refining.—After being reduced to a coarse powder, the ore is roasted or calcined in a reverberatory furnace, until it ceases to exhale arsenical vapours. By this, together with some subsequent processes, it is further cleansed from the admixture of foreign matter and prepared for smelting. The ores of tin raised In Cornwall and Devonshire are always smelted within those counties. The smelting works do not generally belong to the proprietors of the mines, but to other parties who purchase the ore from them. The value Is deter mined by a kind of assay. When several bags of ore, of nearly uniform quality, are conveyed to the smelting works, a small sample is taken from each. These samples, after being blended together, are mixed with about four per cent. of ground coal, placed in an open earthen crucible, and heated in an air-furnace until the ore is reduced. The melted metal is poured into a mould, and the drossy substances -which remain in the crucible are pounded in a mortar, in order to separate, and to add to the ingot, any minute granules of tin remain ing among them. The ratio of good tin to crude ore determines the value.
The smelting of tin-ores is effected by two different methods. In the first and moat common, the ore, previously mixed with culm, is exposed to heat upon the hearth of a reverberatory furnace, in which pit-coal is used as fuel; while in the second, which is applied merely to wean!, tin (the tin procured from stream-works), and which is followed in order to obtain tin of the finest quality, the ore is fused in a blast-furnace, in which wood-charcoal is used for fuel.
In the former process the prepared ore, called addich, Is mixed with from one-fifth to one-elghth of its weight of powdered anthracite, or culm, to which a little slaked lime is sometimes added as a flux. These substances are carefully blended together, and a little water is added to the mixture to facilitate the operation of charging the furnace, and to prevent the charge from being blown away by the draft at the com mencement of the smelting process. From twelve to twenty-four cwt. of the mixture forms a charge. The charge is spread upon the concave hearth of the furnace ; then the apertures by which it is inserted are closed and luted, and the furnace is gradually heated, and kept hot for six or eight hours, by which time the reduction of the ore is complete. When the fusion of the ore is finished, one of the apertures of the furnace is opened, and the melted mass is stirred up to complete the separation of the tin from the scoria. These scoria) consist principally of masses of refuse matter from which no more tin can be profitably extracted, and which are therefore immediately thrown away ; but among them are pieces which yet retain a considerable quantity of tin, and which are separated and reserved for further pro cesses. So soon as these refuse matters are removed, a channel is
opened, by which the molted tin flows from the hearth into a large vessel, where it is allowed to rent for some time, in order that the impurities may separate. When it has settled, the tin is ladled into moulds, so as to form it into large blocks or iogota. These ingots frequently contain portions of iron, copper, arsenic, tungsten, solphurets and arseniurets,unrcduced oxide of tin, and earthy matters which have not passed off with the seethe. To remove these the tin is exposed to the process of refining, which commences by placing the blocks or ingots on the hearth of a second reverberatory furnace, and applying a moderate heat. This causes the tin to melt and to flow into a basin provided for it, leaving upon the hearth a residuum consisting of a very ferruginous alloy. Fresh blocks are then placed in the furnace, without removing the unmelted remains of the former, until about five tons of tin have flowed into the basin. This part of the process is termed a liguation, and is followed by the actual refining, which is usually effected by plunging billets of green wood into the melted tin in the refining basin, by means of an apparatus erected near it. The heat occasions the disengagement of considerable volumes of gas from the wood, and thus a-kind of ebullition is produced in the tin, which causes the lighter impurities to rise to the surface in a frothy form, and the heavier to fall to the bottom. The scum is taken off and returned into the furnace ; and when the tin is sufficiently boiled, the green wood is lifted out, and the whole is allowed to settle, in doing which the purest tin rises to the top, that with a trifling admixture of foreign metals remains in the middle, while the foulest portion sinks to the bottom. When the mass becomes so cool that no further separation can take place, the tin is again ladled into moulds. Tho blocks formed from the lowest part of the tin are usually so impure as to need a repetition of the refining process. A similar effect is some times produced by an operation called tossing; in which, instead of the ebullition produced by the green wood, the mass of melted tin is agitated by a workman repeatedly lifting a quantity of tin in a ladle, and letting it fall into the basin from a considerable height. After continuing this agitation for some time, the surface is skimmed care fully ; and if the upper part of the tin be yet too impure for the market, the refining is rendered more perfect by keeping the metal in a fused state, without agitation, until tho impurities separate spon taneously. After refining, the tin is cast into blocks of about three cwt. each. The moulds used for this purpose are frequently made of granite ; and the tin thus prepared is sold as block tin.