49. GALVANIC ASSAY.1

Cadmium sulphide is precipitated from an acid solution with sulphuretted hydrogen. This (and also cadmium oxide) is dissolved in nitric acid, the free acid is neutralized with caustic potassa, with an addition of potassium cyanide solution, until the precipitate is dissolved. It is then diluted with a sufficient quantity of water, so that about 0.2 gramme (3 grains) of cadmium are contained in 75 cubic centimeters (4.57 cubic inches) of the liquid. The cadmium is then precipitated upon the platinum cone (p. 117) with 3 Bunsen elements, the glass containing the liquid being placed in a dish filled with cold water. The rate of precipitation should be from 80 to 90 milligrammes (1.23 to 1.38 grains) of metal per hour. The light gray cadmium is rinsed off with water, then with alcohol, and dried by placing it in a heated platinum dish (p. 118).

X. TIN.

50. ORES.

Tinstone (cassiterite), SnO2, with 78.7 per cent. tin.

51. DETERMINATION OF TINSTONE BY WASHING.

This method is used for testing borings from mines, in order to ascertain whether poor tin ores are worth working (Saxony); or in concentrating works, to determine the quantity of material worth smelting which may be obtained from an ore-heap (Cornwall). The specific gravity of tinstone 6.8 to 7.0.

=

1 Berichte der deutsch. chem. Ges. 1879, No. vii. p. 759.

A. Saxon assay of tin.-A sample of dust, taken by volume, is washed in the vanning trough (Fig. 4, p. 29).

B. Determination of tin by washing in Cornwall.'— 50 kilogrammes (110 pounds) of samples are taken from different parts of the heap. The mass is comminuted and thoroughly mixed. From this, another sample is taken, sifted, and dried, and 55 to 56 grammes (848.77 to 864.21 grains) of it are weighed off. This is placed upon an iron shovel, and washed, by imparting to it, first a rotary motion, and then a decided upward and downward movement. By these operations, the products free from tin will be washed away, while those yielding tin will, according to their specific gravity, be collected on different parts of the shovel. They are then removed, and, if necessary, roasted, and again washed. This manipulation requires considerable skill.

52. FIRE ASSAYS.

The object of these is to reduce the tin oxide (stannic acid) and slag off the admixtures of earths by solvent fluxes. The accuracy of the result is impaired, or the assay is made difficult, on account of the tin oxide being easily slagged off by acids and bases; by the difficulty of uniting the reduced particles of tin to a single button; and by the presence of many foreign metallic combinations and earths which promote the slagging off or the contamination of the tin.

The losses by the German method are less than by the English or Cornish method. The assay with potassium

cyanide gives the highest yield.

The tin buttons obtained

by the fire assay must be tested in the wet way for the

1 B. u. h. Ztg. 1859, p. 358. Muspratt's Chemie, vii. 1375.

presence of copper, iron, etc., by treating them with nitric acid of 1.3 specific gravity, adding water, digesting, filtering, drying, igniting, and weighing the tin oxide.

A. German assay.-5 grammes (77.16 grains) of clean ore are intimately rubbed together with 0.75 to 1 gramme (11.57 to 15.43 grains) of powdered charcoal. The mixture is poured into a suitable crucible (Fig. 42, p. 67), and covered with 12.5 to 15 grammes (192.90 to 231.48 grains) of carbonaceous black flux or potash with 50 per cent. of flour, 1 to 1.25 grammes (15.43 to 19.29 grains) of borax glass, and finally with a cover of common salt and a small piece of coal. The charge is exposed for three-quarters to one hour to a very strong red heat in the reverberatory (p. 59) or muffle furnace (p. 49), or for one-half to three-quarters of an hour in the blast furnace (p. 60). The crucible is then taken out and allowed to become entirely cold, as tin has a low fusing point. It is then freed from slag, and the result must be a single, ductile button of a tin-white color, which does not follow the magnet under water. In case the tin is distributed in the slag, this must be washed off and the metal collected.

Other charges: 25 grammes (385.80 grains) of ore, 5 grammes (77.16 grains) of argol, 20 grammes (308.64 grains) of soda, and 3 grammes (46.30 grains) of lime, are intimately mixed together, and covered with a layer of soda and 10 grammes (154.32 grains) of borax. The charge is smelted at a strong red heat and kept in fusion for twenty minutes. Or for siliceous ores: 10 grammes (154.32 grains) of ore, and from 10 to 20 grammes (154.32 to 308.64 grains) of fluor-spar or cryolite are placed in a charcoal-lined crucible and covered with charcoal. A lid is luted on, and the charge is then very strongly heated for one hour. This assay gives a good yield.

Modifications become necessary—

1. When the ore contains many earthy admixtures. The ore, before it is reduced, must be washed in a vanning trough (Fig. 4, p. 29), in spitz-glasses (Fig. 3, p. 28), or in a beaker-glass (as silicic acid especially promotes slagging off of tin). This manipulation is effectual on account of the high specific gravity of tin, but metallic admixtures cannot (or can only partly) be removed by it. Specific gravities: tinstone, 6.8 to 7; native bismuth, 9.6 to 9.8; tungsten, 7.2 to 7.5; arsenical pyrites, 6 to 6.4; copper glance, 5.5 to 5.8; iron pyrites, 4.9 to 5.1; copper pyrites, 4.1 to 4.3; molybdenite, 4.5 to 4.6; magnetic iron ore, 4.8 to 5.2; specular iron ore, 6 to 6.5; red hematite, 4.5 to 4.6; zinc blende, 3.9 to 4.2; quartz, 2.65 to 2.80; chlorite, 2.65 to 2.85; slate, 2.5.

2. When the ore contains foreign metallic sulphides, arsenides, and antimonides.

a. The unroasted ore is either digested with aqua regia for half an hour, and then washed by decantation, the tungstic acid, from tungsten ores, if any be present, removed by digesting with caustic ammonia for half an hour (the flask being frequently shaken), then washed by decantation and dried (Levol1), and then reduced:

b. Or, the dead-roasted ore is treated with hydrochloric acid as long as the acid, after the ore has been repeatedly decanted and washed, appears yellow, when a fresh addition is made at a boiling temperature; the ore is then washed by decantation, dried, and subjected to reducing and solvent fusion as above described (p. 215).

3. On account of the ease with which tin oxide is slagged off-For this reason

a. The ore is intimately rubbed together with powdered charcoal (p. 216), or carbonaceous black-flux, but too large a percentage of carbon will render the charge more refractory.

Polyt. Ctrbl. 1857, p. 406.

b. The tin oxide is reduced, before the reducing and solvent fusion, by mixing the ore with part of powdered wood charcoal and igniting it in the crucible. It is then charged as above.

4. When separate grains of tin are found.-These must be collected with copper (bronze being formed), by mixing 5 grammes (77.16 grains) of ore with 5 grammes (77.16 grains) of pure copper oxide (with 79.14 Cu). The mixture is placed in a suitable crucible (Fig. 42, p. 67) and 15 grammes (231.48 grains) of black flux and 1.25 grammes (19.29 grains) of borax-glass are added, a cover of common salt and a fragment of coal. The charge is gradually heated to a high temperature, and, after the "flaming" has ceased, is exposed for threefourths to one hour to a white heat in the muffle or wind furnace, or for one-half to three-fourths of an hour in the blast furnace. It is then taken out, and, when it has become cold, the brittle bronze button is freed from slag and weighed. The weight of the copper contained in the copper oxide, which was added, is deducted. If the copper oxide is not entirely pure, 5 grammes (77.16 grains) of it are fused with the same additions as given above and the weight of the resulting copper button is deducted from that of the bronze button.

5. When tin oxide is combined with silicate (as, for instance, in tin-ore slags), 5 to 25 grammes (77.16 to 385.80 grains) of slag are rubbed as fine as possible, the metallic tin is sifted out and the fine substance is gradu ally introduced in 12 to 15 times the quantity of potas sium bisulphate which has been previously fused in an iron or porcelain crucible under the muffle. The mixture is then fused until no more gas bubbles are formed. The fused mass is then extracted with boiling water, next washed with hot water, and the residue reduced as above.