Only 2 ores of tin have any commercial importance; these are the peroxide, known as cassiterite or tinstone, and the sulphide, called tin pyrites. The former is by far the more important, and is the chief source from which the metal is derived. It occurs in irregular veins, pockets, and bunches in undisturbed primary rocks, and in minute grains (stream tin) in alluvial deposits resulting from the denudation of rocks containing such veins. The mining of the ore is conducted in the same manner as that generally pursued with the ores of other metals already described, e. g. copper.

Cleaning And Sorting The Ore

As soon as the ore reaches the surface, it undergoes a rough cleansing process to free it from adhering earthy impurities. This is effected by passing it over a grating under a stream of water. Next it is hand broken to a size convenient for feeding into the stamps, and sorted into heaps according to its character, the main point being to separate the fragments of copper-, iron-, and arsenical pyrites found associated with tinstone, as well as the wolfram, the 3 kinds of pyrites are sold to sulphuric acid manufacturers for the sake of their sulphur, the "cinders' from the copper-pyrites being subsequently treated for the recovery of the copper. The ore mixed with wolfram is specially dealt with for the extraction of the tungsten (see p. 455).

Stamping The Ore

The cleaned tinstone is next crushed to powder in a stamp battery, generally of the most antiquated form, driven by a neighbouring stream. The battery consists essentially of a series of heavy pestles, which are raised in succession by a cogged shaft, and fall upon the ore as it is fed in between the pestles and the mortar or sole-plate beneath. As fast as the ore is reduced to powder, it is carried away in suspension by a stream of water flowing through gratings perforated with about 160 holes to the sq. in., and leading to a series of settling pits, where the superior weight of the tinstone causes it for the most part to separate from the other matters, which latter are carried farther away by the water. The separation thus effected is rendered more complete by washing the deposits ("tin witts ") on smooth tables termed "racks," where they are brushed about in a gentle stream of water. Various other contrivances are in use, and will be found fully described in Andrews ' Mining Machinery.'

Calcining The Ore

The object of this operation is the diminution of the sulphur imported by those portions of copper- and arsenical pyrites which it is impossible to completely separate from the tinstone before or after stamping. The calcining may be performed in a reverberatory furnace or in furnaces of special construction. The dimensions and form of the reverberatory furnace are subject to variation, but the general principle is the same in all. The furnace is built of atone, with an inside lining of firebrick, and is furnished with a working bottom of the same material. At the tear end is a narrow fireplace, communicating for its entire length with the chamber of the furnace, which may be 9 ft. long, 5 ft. wide in the middle, 4 ft. at the rear, 14 in. high, and £0 in. wide at the working door. The flame from the grate is deflected on to the floor, and escapes through a flue over the working door. The ore is placed in a hopper-like cavity in the crown of the furnace, whence it Is discharged as required into the furnace below. Under the floor is an arched pit for the reception of the calcined ore; while the working door is provided with a serrated plate to assist in the employment of a long bur for repeatedly stirring up the charge, which may amount to 10 or 12 cwt.

Such a charge requires about a day of 12 hours for its calcination, and consumes 2 cwt. of coal. The roasting converts the arsenic present into oxide (white arsenic), which is largely deposited in long condensing flues built for the purpose. Any copper sulphide in the ore is more or less completely oxidized to sulphate, and this change is farther perfected by ing arsenic condensing chambers o, grate; b, calcining hearth; c, fire bridge; d, chimney fur fire; e, flues for vapours issuing from the heath; f, flui leading to the condensing chambers g..

Fig. 174.

Another form of furance often used is that known as Brunton's calciner, illustrated in Fig. 175. It consists of a circular cast-iron table, supported on a central stout iron shaft, suitably stepped in bearings, to allow of its being slowly revolved by the gearing of a toothed wheel fastened on the under side of the revolving bed a, which is placed under the dome of a everberatory furnace, the fireplace b being on one side, and leaving the roasted mass for some days in a damp state exposed to the air. Fig. 174 shows the arrangement of a re-verberatory furnace and its accompanythe flue c, leading to the chimney, on the other. The ore is fed through a hopper d, situated in the crown of the arch, and is distributed, and regularly turned oyer and thrust from the centre to the, circumference. The revolution of the bed of the furnace is effected by the attachment of the machinery to a small water-wheel. The ore haying been constantly turned over, and every particle exposed to the oxidizing influence of the heated air passing through the furnace, when it has reached the discharging point e, it is there thrust out of the furnace into the "wrinkle " /, in a steady small stream, proportionate to a similar stream supplied through the hopper.

The rate of supply and discharge is regulated by the rapidity of motion imparted to the revolving bed, which is determined by the amount of time required by the various ores subjected to calcination. It is evident that much less labour is necessary for the working of this, than of the ordinary reverberatory furnace; and a less consumption of fuel is also sufficient for the production of the same effect.

Fig. 175..

The flue from the calciners is commonly conducted a considerable distance up the side of a hill, to a short chimney-stack, or, better still, a large square tower filled with brushwood or furze. For the first 100 or 150 ft. from the furnace, the flue is built much larger than farther off, with divisional walls to increase the length of circulation of the volatile products from the furnace; doorways are built in the sides of these chambers, and these are closed up with temporary brickwork, or with large stones. At convenient intervals of 1 to 3 months, the flues are opened, and the crude arsenic is removed, packed in casks, and sold to the arsenic manufacturer for refining. The sulphurous acid evolved with the arsenic passes on, and is discharged from the chimney into the surrounding atmosphere, not unfre-quently, during damp weather, causing much damage to adjacent vegetation. This might be entirely avoided by the filling of the tower with coke, kept moist with a small stream of water, which would absorb and condense the sulphurous acid.

The process of calcination is, with but few exceptions, employed exclusively on dressed ores; but great advantage in many instances would accrue from selecting those ores which contain sufficient iron-, copper-, or arsenical pyrites, to burn of themselves without fuel. The calcination in this case would be conducted in a furnace resembling a small lime-kiln, the ore being supplied at top, and, when burnt out, drawn at the bottom into the ash-pit below the fire-bars, in which water is kept, so that, by the hot burnt ore dropping into it, and being suddenly chilled, it falls to pieces. Ore thus burned is much more easily reduced to powder. From the side of the top of the furnace, the volatile product may be led off to the main flue, with which the reverberatory communicates. The partially calcined ore having been dressed in the usual manner for "witts," the final calcination may be effected in the reverberatory furnace, as already described.

Washing Roasted Ores

From the "wrinkle" of the calcining furnace, the burnt ore is removed to the burning-house floors, where it is subjected to a series of washing processes, much more carefully conducted than those in the preparation of "witts." The witts having been burned in separate parcels, according to their size, as "jigged," "fluran," smalls or "smales," " slime," and rough or " rows," they are operated on accordingly; the jigged being simply jigged over again in a copper sieve; the fluran passed through the buddle and tie; and the smaller sizes through a very much more complicated process,con-sisting first in washing carefully in the buddle, whence it is sized out according to the part of the buddle in which it settles; then of tossing or " tozing," and packing in a kieve, again washing on a hair-sieve in another kieve, or "dil-leughing," so as to throw off the light waste into the water, leaving the " crop " tin on the sieve, whence it is thrown into hand-barrows, and conveyed to " hutches," where it is stored.

If the witts have been "corrupted" with a "bad brood or mixture," such as iron-pyrites, copper-pyrites, arsenic, molybdenum, wolfram, 4c, the various operations lead to the production of a great number of temporarily refuse matters, which are treated over again according to the best process indicated by examination of samples of the respective heaps. The effect of the washing is first to remove the copper sulphate in solution, whence the metallic copper is recovered by prolonged contact with metallic iron; and secondly to separate the iron oxide in suspension, leaving a mass of "black " tin, containing about 60 per cent, of the metal. This is next smelted.