In some cases, the particles of proto- and peroxide of iron unremoved by the washing operation are extracted by means of a horse-shoe magnet, thus considerably increasing the value of the ore.

e. Treatment of the Purified Tin Schlich with Acids. -In order to avoid too great loss, the washing of the roasted ores must not be carried too far; hence some foreign oxides, chiefly of iron, copper, and bismuth, and also native bismuth, always remain in the tin ore.

*

At the Saxony tin works, according to Plattner's suggestion, the roasted ore containing bismuth is put into wooden vessels, together with 6 per cent of crude dilute muriatic acid, and stirred for some hours. The dissolved salts are extracted by means of water, and the basic chloride of bismuth thus precipitated is washed out, collected in lumps, and melted in plumbago crucibles, with burned lime and pulverised charcoal, for the reduction of bismuth. This treatment reduces the contaminations of tin schlich from nine-tenths to one-tenth. The Bohemian ores contain but very little bismuth, and are not therefore treated with acids, but are submitted to a sorting by hand to separate the wolfram.

Michell recommends that common muriatic acid should be diluted with an equal weight of water, and I cwt. of this dilute acid used for the purification of I ton of schlich. Pearce extracts native copper by dilute sulphuric acid with admission of air, with or without addition of muriatic acid; and he treats roasted ores containing sulphur, arsenical and copper pyrites, as well as grey copper ore, with hot muriatic acid in closed vessels.

Duclos introduced into Cornwall the process of treating tin ore with muriatic acid; good results were obtained when treating badly dressed ores, but the process offered no advantages in treating well-dressed ores. The dissolved copper is precipitated either by iron (Levant and Botallack mine) or by soda (Redruth).

* B. u. h. Ztg., 1852, p. 342, 702.

+ Ibid., 1844, p. 656.

Ibid., 1862, p. 263.

Ibid., 1862, p. 147.

Purification of Tin Ores from Wolfram.-Owing to its great specific gravity, and its refractory nature, wolfram cannot be separated from tin ore by mechanical dressing, by roasting, or by treatment with acids. If present in large quantity it alloys the tin and renders it difficult to fuse. In Cornwall, wolfram is separated by Oxland's* smelting method, which may be carried out with two modifications; namely, by employing carbonate of soda, or sulphate of soda, both for the purpose of forming soluble tungstate of soda.

At East Pool mine this process is carried on in reverberatory furnaces, furnished with an iron pan instead of a hearth, and so constructed that the flame from the grate passes first over the fire-bridge along the surface of the pan, then across a bridge which lies opposite the fire-bridge, and through flues under the bottom of the pan, into a channel leading into a chimney.

The use of the cast-iron bed effects considerable economy of fuel, and it is admirably adapted for the calcination of raw ores and the evolution of sulphur and arsenic contained in them, but it is especially useful, instead of fire-bricks or tile, by preventing the loss which would result from the reaction of the soda ash on the silica of the brick, giving rise to the formation of double silicate of soda and tin.

According to the fineness of the schlich, charges of from 6 to 9 cwts. of ore are made (the finer the ore the lighter the charges), and from 9 to 12 lbs. of carbonate of soda (containing 48 per cent of alkali) per cwt. of ore, are spread over the heated ore; one or two shovelfuls of coal are thrown upon that part of the hearth opposite the fire-bridge, in order to obtain as uniform a temperature as possible. The mass is kept at a white heat for about six hours; it is stirred every quarter of an hour, and every half hour some coal is thrown upon the back of the hearth. Half the charge is now removed from the furnace, and after one more stirring, the other half. About 1 tons of ore, with 330 lbs. of carbonate of soda are treated in four charges, yielding in 24 hours about 10 cwts. of pure tin ore. The mass when heated

*B. u. h. Ztg., 1852, p. 702; 1862, pp. 194, 294.

Preuss. Ztschr., ix., 254.

being in a pasty state, and after cooling frothy, is broken into pieces the size of an egg, mixed with 25 per cent of quartz, and pounded under a stamping mill, the quartz serving to separate the hard alkaline crust from the tin ore. The pounded mass is repeatedly washed, the coarser part pounded two or three times with quartz, and sometimes again treated with soda in the reverberatory furnace. This preparation is comparatively expensive, and it causes a considerable loss of tin ore both mechanically and chemically; the chemical loss arises from the formation of soluble stannate of soda.

When sulphate of soda is used instead of carbonate of soda, the loss of tin is lessened and the process cheapened, but it is rendered more difficult, as the reaction must be alternately oxidising and reducing. The dressed ore is mixed with sulphate of soda, according to the amount of tungsten in it, so that, approximately, tungstate of soda may be formed; some pulverised coal is added, and the mixture heated in a reverberatory furnace by a smoky reducing flame, whilst the mass is continually raked; the sulphuric acid of the sulphate of soda decomposes, and the liberated soda combines with tungstic acid when heated in an oxidising flame. The red-hot mass is removed from the furnace, and after about six hours, when the colour and other qualities of the roasted mass indicate its decomposition, thrown into. a cistern full of water. After 24 hours the solution of the tungstate is made to run off through a sieve attached to one of the sides of the cistern; the remaining tin stone is then washed, to remove the peroxide of iron mixed with it which originated from the wolfram. Four charges, equal to 36 cwts. of schlich, are treated in 24 hours, consuming 4 or 5 tons of coal.

The whole of the solution of tungstate of soda is sometimes boiled down, and the crystallised salt used for dyeing purposes, for rendering fabrics non-inflammable, for bleaching linen, for the production of bronze colours, &c.

At Zinnwald, in Bohemia, wolfram is separated mechanically, and sold at about 10s. 6d. per cwt. In 1859, 25 tons were produced; in 1860, only 5 tons.

At Schlaggenwald (vide Preuss. Zeitschr., 1862, Bd. x., Lief.

3, p. 165), 4 or 5 cwts. of ore, with an admixture of common salt, are roasted in a reverberatory furnace for eight hours, at a consumption of 25 cubic feet of coal. The chloride of copper and tungstate of soda thus formed are removed by lixiviation with water, the copper in the solution is precipitated by iron, and tungstate of lime by means of chloride of calcium, and the lixiviated ore is washed again.

II. SMELTING TIN ores.

This smelting is effected either in cupola or in reverberatory furnaces, according to the nature of the fuel. Reverberatory furnaces require good and cheap coal, and are exclusively employed in this country; they allow a larger production, a better control of the operations, and give a greater yield than cupola furnaces, as less tin becomes oxidised by the influence of the draught, and as the smelting mass remains longer at a higher temperature, which facilitates the separation of the metal. On the other hand, cupola furnaces yield a better product from impure ores, chiefly those containing arsenic, as the foreign substances are more readily induced to scorify and to volatilise; cupola furnaces also yield more, but do not always give poorer slags than reverberatory furnaces (vide Winkler in Freib. Jahrb., 1839, p. 13). Zirkel states in Preuss. Zeitschr., ix., 256, that the production of I ton of tin in reverberatory furnaces requires 13 tons of coal, and causes a loss of 5 per cent of ore, whilst cupola furnaces consume 3 tons of coal for the same production, and cause a loss of 15 per cent of ore.

The following products result from the smelting of tin

ores:

1. Crude Tin.-This contains more or less iron, copper, arsenic, antimony, bismuth, tungsten, molybdenum, &c., all of which substances render the tin hard, difficult to fuse, and more or less pasty when melted. To extract these impurities, the raw tin is submitted to a liquation process, which is effected in German smelting works in open hearths, and in English tin works in reverberatory furnaces. Both methods will be fully described further on.

Refined tin sometimes retains foreign substances, some of them prejudicial to the quality of the tin, even if present in small quantity, whilst others render the tin better fitted for different purposes. Tin is usually impure in proportion to the irregularities and the crystalline character of its surface. Pure tin solidifies, with a smooth, pure, and bright surface, and the characteristic tin-white colour.

Tin is very seldom free from iron, and as little as o'5 per cent influences its colour and lustre; I per cent injures its softness and smoothness, rendering it friable, dark, dull, and showing marks of iron rust, when long exposed to the air. Some sorts of tin, for instance Banca tin, when dissolved in muriatic acid, leave behind a definite alloy, FeSn2, in the form of fine acicular crystals; the same compound is likewise formed on the sole of the furnace in which this quality of tin is smelted, separating out during the slow cooling of the metal.

Small quantities of copper are almost without influence, but tin containing more than 1 per cent becomes harder and less ductile, without, however, losing its lustre. According to Stölzl (vide Dingl., Bd. 155, p. 124), an addition of from one-ninth to one-fourth of copper will increase the hardness of tin three- or five-fold. If the addition of copper exceeds a certain limit, it will injure the colour of the tin. The tin foil used in the manufacture of looking-glasses is intentionally alloyed with copper, as the foil then becomes harder, and better able to resist mechanical treatment, and is not too quickly attacked by mercury. According to analyses, tin foil for looking-glasses contains from 0.38 to 2.16 per cent of copper, from o'04 to 0.84 per cent of lead, and from o'10 to o 12 per cent of iron.

Bismuth and antimony in proportions not exceeding o'5 per cent, without considerably lessening its lustre, impair the ductility of tin in a greater degree than does the presence of iron; they also render the tin crystalline in texture; bismuth increases its fusibility.

Small quantities of lead render tin harder; I per cent injures its colour and lustre; the colour, which becomes greenish, may be restored by an addition of nickel.