A Text-book of Assaying: For the Use of Those Connected with Mines Part 45

The following a.n.a.lysis will ill.u.s.trate the composition of such a slag:--

Tungstic acid 1.3% Silica 39.4 Stannous oxide 8.1 Ferrous oxide 26.2 Alumina 14.8 Oxide of manganese traces Lime 7.9 Magnesia 0.5 Alkalies calculated as soda 1.7 ----- 99.9

t.i.tANIUM.

t.i.tanium only occurs as a mineral in its oxidised state, or as t.i.tanic oxide (TiO_{2}). It is a substance which has little commercial value, and is generally recognised as one of the rare bodies; although, in small quant.i.ties, it is widely disseminated. It occurs in granite, basalt, and other igneous rocks in quant.i.ties up to as much as 1 per cent. It is also met with in clays and iron ores, and in river sands, in which it is often a.s.sociated with stream tin. The proper minerals of t.i.tanium are rutile (TiO_{2}), t.i.taniferous iron (t.i.tanate of iron), and sphene (t.i.tanate and silicate of lime).

The oxide of t.i.tanium (like ca.s.siterite and quartz) is undecomposed by hydrochloric or nitric acid; so that it is generally found in the residue insoluble in acids. The t.i.tanates, however, are attacked, and a portion of the t.i.tanium dissolves; so that it must be looked for in both the filtrate and residue. Oxide of t.i.tanium in its native form, or after ignition, may be made soluble by fusing the finely-divided substance with fusion mixture in a platinum dish. The resulting t.i.tanate is dissolved out of the "melt" by cold hydrochloric acid.

The method most commonly used is fusion with bisulphate of potash. This renders the oxide of t.i.tanium soluble in cold water. The process is as follows:--The substance is extracted with hydrochloric and nitric acids, and the solution reserved for further treatment; the residue is dried, moistened with sulphuric acid, and evaporated once or twice to dryness with hydrofluoric acid. It is then fused with bisulphate of potash, and the "melt" extracted with cold water until all soluble matter is removed. The solution is filtered. The residue may consist of unremoved silica, and oxides of tantalum, niobium, and, perhaps, chromium. On the prolonged boiling of the filtrate, the oxide of t.i.tanium (and oxide of zirconium, if any) is precipitated.

Any t.i.tanium dissolved by the first extraction with acids is recovered in the following way:--Sulphuretted hydrogen is pa.s.sed into the acid solution, and any precipitate that may be formed is filtered off. The filtrate is oxidised, and the iron, aluminium, and t.i.tanium are separated as basic acetates (see under _Iron_). The precipitate is dried and fused with bisulphate of potash. The "melt" is extracted with cold water, filtered if necessary, and the solution rendered first faintly alkaline with ammonia, then very slightly acid with sulphuric acid. 30 or 40 c.c. of a saturated solution of sulphurous acid is added, and the oxide of t.i.tanium precipitated by prolonged boiling. It is filtered off, added to the precipitate previously got, ignited with ammonic carbonate towards the end, and then weighed.

~Detection.~--t.i.tanium is detected in an insoluble residue by fusing the residue for some time in a bead of microcosmic salt. In the reducing flame it gives a violet colour, which becomes reddish-brown if much iron is present. In the oxidising flame it gives a colourless or whitish bead. It is best detected in acid solutions by the deep brown or iodine colour developed on adding hydroxyl. A solution of this can be prepared by pouring peroxide of barium (BaO_{2}) diffused in water into dilute hydrochloric acid (a little at a time), and keeping the acid in excess.

~Separation.~--In the usual course of an a.n.a.lytical separation the hydrate of t.i.tanium will be thrown down with ferric hydrate, &c., on the addition of ammonic chloride and ammonia. It is best separated from this precipitate by fusion with bisulphate of potash, as already described, but it must be remembered that the presence of much mineral acid prevents complete precipitation when the solution is boiled. Further, if phosphates are present, the precipitate will contain phosphoric oxide; it may be freed from this by fusion with sodium carbonate. A very good method of separating t.i.tanium from iron is to add tartaric acid and ammonia to the solution, and then precipitate the iron (as sulphide) with sulphuretted hydrogen. The filtrate contains the t.i.tanium, which is recovered by evaporating and igniting. It may be separated from zirconia by the action of sodium carbonate, which precipitates both; but when concentrated, redissolves the zirconia. The separation from large quant.i.ties of silica is best effected by evaporating with hydrofluoric acid, which volatilises the silicon; but sulphuric acid must be present, otherwise some t.i.tanium also will be lost, as may be seen from the following experiments,[77] in which oxide of t.i.tanium (pure, ignited) was evaporated to dryness with a quant.i.ty of hydrofluoric acid known by experiment to be sufficient to volatilise 1 gram of silica.

_Without sulphuric acid_, 0.0466 gram of t.i.tanic oxide left 0.0340 gram, showing a loss of about 25 per cent.

_With sulphuric acid_ the following results were obtained:--

Oxide taken. Left after Evaporation and Ignition.

0.0340 gram 0.0340 gram 0.0414 " 0.0413 "

0.0520 " 0.0520 "

0.0352 " 0.0352 "

GRAVIMETRIC DETERMINATION.

The t.i.tanic hydrate thrown down by ammonia (or on boiling the solution from the bisulphate) is collected, washed, dried, ignited strongly with the addition of a little ammonic carbonate, and weighed. The substance is t.i.tanic oxide (TiO_{2}), and is generally reported as such. It contains 60.98 per cent. of t.i.tanium. It should be white, if pure (Holland), white, yellow, or brown (Fresenius), or black (Tidy).

VOLUMETRIC METHOD.

A method has been proposed based on the reduction of t.i.tanic oxide by zinc in hydrochloric acid solutions to the sesquioxide. The reduction is marked by the development of a violet or green colour, the former with chlorides and the latter when fluorides are present. The quant.i.ty of t.i.tanium reduced is measured by t.i.trating with permanganate of pota.s.sium solution. The water used must be free from dissolved oxygen.

TUNGSTEN AND TUNGSTATES.

Tungsten occurs in nature only in the oxidised state, or as tungstic acid (WO_{3}), either free, as in wolframine, or combined with oxides of manganese and iron, as in wolfram, or with lime, as in scheelite.

Wolfram occurs a.s.sociated with tin ores, the value of which is consequently lowered. Both wolfram and scheelite are of considerable importance as a source of tungstic acid for the manufacture of sodium tungstate, which is used as a mordant and for some other purposes, and as a source of metallic tungsten, which is used in steel-making.

The tungsten minerals have a high specific gravity (6 to 7.5). On treatment with hydrochloric acid or aqua regia they are decomposed; the yellow tungstic acid separates and remains insoluble.

Tungsten itself is insoluble in nitric acid or aqua regia; but is converted into tungstic acid (WO_{3}) by prolonged and strong ignition in air. Alloys containing tungsten leave tungstic acid after treatment with nitric acid or aqua regia. Tungstic acid may be got into solution after fusion with alkalies or alkaline carbonates. This solution gives with hydrochloric acid a white precipitate of tungstic acid, which becomes yellow on boiling, but the separation is not complete. Fusion with bisulphate of potash gives a residue, which does not dissolve in water, but is soluble in ammonic carbonate. For the a.s.say of minerals containing tungsten these reactions are only occasionally taken advantage of for testing or purifying the separated tungstic acid.

~Detection.~--The minerals are easily recognised by their physical characters, and the yellow tungstic acid separated by boiling with acids is the best test for its presence; this, after decanting and was.h.i.+ng, immediately dissolves in a few drops of dilute ammonia. A solution of tungstate acidulated with hydrochloric acid becomes intensely blue on the addition of stannous chloride and warming. Fused in a bead of microcosmic salt it gives a clear blue colour (reddish-brown if iron is also present) in the reducing flame, but is colourless in the oxidising flame.

~Solution and Separation.~--The decomposition and solution of natural tungstates is difficult to effect owing to the separation of tungstic acid; the method of treatment is as follows:--Boil the finely-powdered substance with hydrochloric acid or aqua regia till it apparently ceases to be attacked; dilute, filter, and wash with dilute hydrochloric acid.

Cover with dilute ammonia, and filter the solution, which contains ammonic tungstate, into an evaporating dish. Treat the residue again with acid, and again dissolve out the separated tungstic acid with ammonia, and repeat this operation until decomposition is complete. By this means there will be obtained--(1) a solution containing tungstate of ammonia; (2) an insoluble residue with silicates, and oxides of tin, niobium, tantalum, &c.; and (3) an acid solution containing the soluble bases. The tungstate of ammonia requires simple evaporation on the water-bath and gentle ignition in order to cause the tungstic acid to be left in an almost pure state; possibly, it may carry a little silica.

GRAVIMETRIC DETERMINATION.

The tungstic acid is dissolved, and separated as ammonic tungstate, and, after evaporation, is gently ignited, the heat being increased towards the end. The residual tungstic acid is fixed, so that when the ammonia has been driven off it may be strongly heated without loss. It is a dark yellow or brown powder whilst hot, which becomes a light yellow on cooling. If any reduction has taken place it will be more or less greenish. It is weighed when cold, and is the trioxide or "tungstic acid" (WO_{3}), which contains 79.31 per cent. of tungsten. After its weight has been taken its purity is checked by fusing with hydric pota.s.sic sulphate, extracting with water, and treating the residue with ammonic carbonate. Any silica present will be left undissolved; it should be separated and weighed, and its weight deducted from that of the tungstic acid found.

~Determination of Tungstic Acid in Wolfram.~--Take 2 grams of the finely-powdered sample and boil with 50 c.c. of hydrochloric acid for half an hour, adding 5 c.c. of nitric acid towards the end. Allow to stand overnight and boil again for 15 or 20 minutes; dilute with an equal volume of water, and filter. Wash with dilute hydrochloric acid, dissolve in a few c.c. of warm dilute ammonia, and dilute to 200 c.c.

with distilled water; allow to settle, and filter. Evaporate in a weighed dish, ignite, and weigh.

The following a.n.a.lysis will ill.u.s.trate the composition of a sample of Cornish wolfram as brought into the market:--

Tungstic acid 50.1% Ca.s.siterite 10.9 Ferrous oxide 24.6 Manganous oxide 5.4 Niobic oxide, alumina, &c. 3.5 Silica 1.2 Copper oxide 2.7 Zinc oxide 0.22 a.r.s.enic 0.51 Sulphur 0.20 ----- 99.33

NIOBIC AND TANTALIC OXIDES.

These oxides are commonly met with in samples of wolfram and tinstone, especially niobic. They are probably present in the form of columbite, a niobate of iron and manganese; and tantalite, a tantalate of the same metals.

On boiling with hydrochloric acid they are both liberated, and remain for the greater part (all the niobic) in the insoluble residue with the tungstic acid. On removing the latter with dilute ammonia they remain as a white insoluble precipitate, very p.r.o.ne to run through the filter on was.h.i.+ng. They may be dissolved in hydrofluoric acid either at once or after fusion with bisulphate of potash, and extraction with cold water.

To the solution in hydrofluoric acid gradually add a boiling solution of acid pota.s.sium fluoride (HF, KF.). Pota.s.sic fluotantalate (soluble in 200 parts of water) separates out first, and afterwards pota.s.sic fluoniobate (soluble in 12 parts of water). The separated salts (after heating with sulphuric acid and was.h.i.+ng out the pota.s.sium sulphate formed) are ignited with ammonic carbonate, and weighed as tantalic oxide (Ta_{2}O_{5}) and niobic oxide (Nb_{2}O_{5}) respectively.

They are both white powders. The oxide of niobium dissolved in a bead of microcosmic salt gives a bluish colour in the reducing flame. The oxide of tantalum dissolves in the bead, but gives no colour.

FOOTNOTES:

[76] This will give almost the whole of the tin; a further portion will be got in subsequent work, and must be added to this result.

[77] Published by P. Holland, in the _Chemical News_, vol. lix. p. 27.

CHAPTER XIII.

MANGANESE, CHROMIUM, &c.

MANGANESE.

Manganese occurs mainly as black oxide (MnO_{2}) in the mineral pyrolusite; and, in a less pure form, in psilomelane and wad. The value of the ore depends rather on the percentage of available oxygen than on the proportion of metal present. The results of a.s.says are generally reported as so much per cent. of the dioxide (MnO_{2}). In smaller quant.i.ties it is very widely distributed. Manganese itself has a value for steel-making; or, rather, for the making of spiegeleisen and ferro-manganese, which are used in the Bessemer and Siemens processes.

For this purpose the percentage of the metal (Mn) is required.

Consequently the minerals of manganese may be considered in two aspects--(1) as a source of oxygen; and (2) as a source of manganese.

These will require separate consideration.

The black oxide is mainly used in the preparation of chlorine, liberation of which it brings about when treated with hot hydrochloric acid, or with a mixture of common salt and sulphuric acid. The quant.i.ty of chlorine which is obtained depends upon the proportion of dioxide present;[78] and in a.s.saying may either be measured by its equivalent of iodine liberated, or by the oxidising effect on an acid solution of ferrous sulphate. When the ore also carries substances which have a reducing effect (such as ferrous compounds), such a.s.says will give, not the total dioxide (MnO_{2}), but less, by the amount required to oxidise these impurities; and this is exactly what is required in valuing such an ore for commercial purposes. Manganese compounds are characterised by the readiness with which they may be converted into highly-oxidised bodies. Solution of manganese in hydrochloric acid, rendered alkaline with ammonia, yields a clear solution,[79] which rapidly takes up oxygen from the air, forming a brown precipitate of the oxide (Mn_{2}O_{3}).

The addition of bromine or chlorine to such a solution determines the precipitation of a still higher oxide (approximately MnO_{2}). On treating a compound containing manganese with nitric acid and dioxide of lead (PbO_{2}), the oxidation is carried still further, a purple-coloured solution of permanganic acid (HMnO_{4} or H_{2}O.Mn_{2}O_{7}) being formed. On fusing minerals containing (even traces of) manganese with sodium carbonate in an open crucible, a green "melt" is obtained which owes its colour to sodium manganate (Na_{2}MnO_{4} or Na_{2}O.MnO_{3}). This salt is soluble in water, forming a green solution; which, when rendered acid, rapidly changes into the permanganate with the characteristic purple colour.

Permanganate of potash is a salt much used in a.s.saying, with some properties of which the student will have already become familiar.