A Text-book of Assaying: For the Use of Those Connected with Mines Part 33

1.0 c.c. at 15 C. showed the colour of 0.8 c.c. at 70 C.

2.5 " " " " 2.0 "

5.0 " " " " 5.0 "

~Effect of Free Acid.~--

2.5 c.c. with 5 c.c. of nitric acid equalled 2.5 c.c.

5.0 " " sulphuric acid " 5.0 "

Hydrochloric acid almost completely removes the colour, which, however, is restored by the addition of a few crystals of pota.s.sium iodide.

~Effect of Alkalies.~--Ammonia, soda, or potash destroys the colour, but it is restored on acidifying with nitric or sulphuric acid.

~Effect of Ammonic Salts.~--The following table shows the results after addition of ammonic salts:--

-------------+----------------+-----------------+----------------- C.c. present.|With 10 grams |With 10 grams |With 10 grams |Ammonic Nitrate.|Ammonic Sulphate.|Ammonic Chloride.

-------------+----------------+-----------------+----------------- 1.0 c.c. | 0.9 c.c. | 1.1 c.c. | -- 2.5 " | 2.5 " | 2.7 " | -- 5.0 " | 5.0 " | 5.5 " | -- -------------+----------------+-----------------+-----------------

Ammonic chloride, like hydrochloric acid, removes the colour, which may be restored on the addition of more pota.s.sium iodide. Nitrates and sulphates do not thus interfere.

~Effect of Foreign Salts.~--Sodic hyposulphite almost completely removes the colour. Copper salts liberate iodine; but when this has been removed by boiling and the cuprous iodide has been filtered off there is no further interference. Dilute solutions of lead salts give no colour.

PRACTICAL EXERCISES.

1. A fusible alloy is made up of 8 parts of bis.m.u.th, 5 of lead, and 3 of tin. What weight of oxide of bis.m.u.th, Bi_{2}O_{3}, would you get on the a.n.a.lysis of 1 gram of it?

2. What weight of bis.m.u.th can be got from 2 grams of the subnitrate BiONO_{3}.H_{2}O?

3. How would you detect and separate a.r.s.enic, lead, and copper in a sample of bis.m.u.th?

ANTIMONY.

Antimony occurs in the native state, but is rare; its common ore is antimonite, the sulphide (Sb_{2}S_{3}). Jamesonite and other sulphides of lead and antimony are frequently met with. Sulphide of antimony is also a const.i.tuent of fahlerz and of many silver ores.

Antimonite occurs generally in fibrous ma.s.ses, has a lead-like metallic l.u.s.tre, is easily cut with a knife, and melts in the flame of a candle.

Antimony itself has a very crystalline fracture, is brittle, and has a bluish-white colour. It is used in the preparation of alloys with lead and tin for the manufacture of type-metal. It is readily fusible, and imparts hardness and the property of taking a sharp cast to its alloys.

It is practically insoluble in hydrochloric acid. On boiling with strong nitric acid it is converted into antimonic oxide (Sb_{2}O_{5}), which is a powder almost insoluble in this acid or in water, but which may be got into solution with difficulty by the prolonged action of hydrochloric and tartaric acids. Antimonic oxide is converted on ignition into the tetroxide (Sb_{2}O_{4}) with loss of oxygen. Antimony forms two series of salts, antimonious and antimonic; and advantage is taken of this in its determination volumetrically. Either sulphide of antimony yields antimonious chloride on boiling with hydrochloric acid, sulphuretted hydrogen being given off; and, in the case of antimonic sulphide, sulphur is deposited. Antimonious is converted into antimonic chloride by treatment with permanganate of potash in an acid solution. Antimonic chloride and pota.s.sium iodide react, forming antimonious chloride and free iodine. This latter may be got rid of by boiling. Sulphide of antimony is separated from the ore by liquation; this regulus is met with in commerce as "crude antimony."

DRY a.s.sAY.

An approximate determination of the amount of sulphide of antimony in an ore may be made by fusing and liquating in a luted double crucible in the manner described under bis.m.u.th. This is unsatisfactory. The determination of metallic antimony in an ore is made either by fusion with pota.s.sium cyanide or by fusion with iron, as in the galena a.s.say.

Both methods yield poor results; and, where iron is used, it must be added in quant.i.ty only sufficient for desulphurising; this amounts to about 40 per cent. in pure ores. If the iron is in excess it alloys with the reduced antimony. If, on the other hand, it is insufficient, the metal will contain sulphur; or sulphide of antimony will be lost in the slag.

The following note, for which we are indebted to Mr. Bedford McNeill, A.R.S.M., gives a description of the method adopted in the commercial valuation of a parcel of antimony ore:--

The antimony smelter, when he wishes to determine the value of any parcel of ore--usually the sulphide--that may be offered for sale, practically has recourse to the smelting operation. That is, a quant.i.ty of 2 or 3 cwts. taken by his sampler having been obtained, he treats it under the immediate supervision of the foreman smelter as if it formed part of the ore in process of daily reduction at his works. He thus determines by actual trial the output which it may fairly be antic.i.p.ated will be yielded by the bulk, and upon the result of this trial or a.s.say, and the knowledge gained of the actual behaviour of the ore under treatment, he bases his tender, knowing that, should he secure the parcel, he may confidently expect a similar return.

Briefly, the process consists of the three ordinary operations of--

(a) Singling or removing most of the antimony from the ore; (b) Doubling; (c) Refining or "starring."

But in the a.s.say sufficient information is generally given by the first two of these.

A new pot having been taken and made hot in the furnace, 40 or 45 lbs.

of the ore is weighed in (the mineral from the necessities of sampling not exceeding walnut size); 1 to 3 lbs. of salt cake is now added to render the separation of the resulting sulphide of iron more easy, as also to a.s.sist in the fusion of the gangue; 20 to 25 lbs. of tin-plate sc.r.a.p, beaten more or less into ball shape, is weighed, placed on the top of the ore and salt cake, and the whole brought to a state of fusion. The foreman from time to time takes notice of the behaviour of the ore under the working conditions. Ores that manifest a tendency to "boil" or "froth " require the admixture of other more sluggish mineral in order to render their reduction economically practicable.

After 1-1/4 to 1-1/2 hours (the time depending mainly on the temperature), the contents of the crucible are usually in a state of tranquil fusion. The pot is now lifted from the fire, and its contents transferred to a conical iron mould, the empty pot being immediately put back into the fire, and the latter "mended" with sufficient c.o.ke for another run. The conical mould (when dealing with a "strange" ore, and the possibility of insufficient iron being present to satisfy the sulphur contents) is wiped inside with clay previous to pouring in the molten charge. Otherwise the mould itself will be attacked, and the contents after solidifying will require to be chiselled out piecemeal.

A further 40 lbs. of the ore is now charged into the crucible with iron as above; but before this second charge is ready to be drawn an inspection of the first may suggest the addition of either 3 or 5 lbs.

more iron, or 5 or 10 lbs. more ore.

It is a good fault rather to aim at an excess of iron as tending to clean the ore from antimony, any of the latter that (from an insufficiency of iron) may be left in the slag from the first process being irretrievably lost; whereas, if the iron be in excess, that which is combined with the crude antimony resulting from the first process is easily got rid of by adding 3 to 5 lbs. or so of ore in the second process.

This latter, as practised for the determination of the value of a parcel of ore, consists in selecting two of the best quality singles, resulting from perhaps four or five trials as above, and running them down with a few pounds of salt cake, or a mixture of salt cake with American potash, and (as is generally necessary) a small addition of ore.

Upon the final result (confirmed perhaps on another pair of singles, and, judging from the total weight or output of the metal as calculated from the ore used in "singling," plus any added in the "doubling," the crystalline fracture and face of the metal, its colour, etc.) the price to be offered for the parcel of ore is fixed.

WET METHODS.

~Detection.~--The antimony, if any, being got into solution by treating the ore with hydrochloric acid or aqua regia may be detected by evaporating with hydrochloric acid, diluting, and filtering into the cover of a platinum crucible or (better) a platinum dish. A small lump of zinc is then added, and, if antimony is present, _the dish_ will in a minute or so be stained black with a deposit of metallic antimony. This stain is removed by nitric, but not by hydrochloric, acid. The reaction is delicate and characteristic; a.r.s.enic under like conditions is evolved as a.r.s.eniuretted hydrogen, and tin is deposited as metal _on the zinc_.

~Solution.~--Ores, &c., containing antimony are best opened up by boiling with hydrochloric acid or aqua regia; treatment with nitric acid should be avoided wherever possible, since it forms antimonic acid, which is subsequently dissolved only with difficulty. Salts of antimony in solution have a tendency to form insoluble basic salts; so that care must be exercised in diluting. Compounds such as antimonite which are soluble in hydrochloric should be dissolved at once in that acid.

~Separation.~--To the solution add potash in excess and a little free sulphur, and pa.s.s a current of sulphuretted hydrogen for some minutes; allow to digest for an hour or so on a hot plate; filter; and wash the residue. Acidulate the filtrate with hydrochloric acid: the precipitate will contain the antimony (as Sb_{2}S_{5}), and possibly a.r.s.enic or tin.

The precipitate is transferred to a beaker and boiled with hydrochloric acid; the solution is filtered off and diluted. Add a few crystals of tartaric acid, and pa.s.s a current of sulphuretted hydrogen for some time. The first flocculent precipitate will become denser, and render the filtering more easy. Transfer the precipitate (after was.h.i.+ng free from chlorides) to a Berlin dish, and treat cautiously with fuming nitric acid. The action of this acid on the sulphide is very violent.

Evaporate and ignite, transfer to a silver dish, and fuse with four or five times its weight of caustic soda, cool and extract with a little water, then add an equal volume of alcohol, and allow to stand overnight. Filter, wash with dilute alcohol. (The filtrate will contain the tin.) The residue contains the antimony as antimonate of soda, and is dissolved off the filter with hot dilute hydrochloric, with the help of a little tartaric, acid. The filtrate is now ready for the gravimetric determination.

GRAVIMETRIC a.s.sAY.

Pa.s.s a current of sulphuretted hydrogen through the solution containing the antimony to which a little tartaric acid has been previously added.

Pa.s.s the gas till the precipitate becomes dense, and the antimony is all down. The solution must not be too strongly acid. Filter off the precipitate, wash with hot water, dry in the water oven, transfer to a weighed porcelain dish, and cautiously treat with fuming nitric acid.

Continue the action on the water bath till the sulphur and antimony are completely oxidised. Evaporate; ignite, gently at first, then strongly over the blast; cool, and weigh. The residue is a white infusible powder, and consists of antimony tetroxide, Sb_{2}O_{4}, containing 78.94 per cent. of the metal.

~Determination of Antimony as Bigallate.~--What appears to be a very good method has been worked out by M.A. Guyard, and is described in Crookes' _Select Methods_, p. 398.

The antimony must be in solution as antimonious chloride, and must not be accompanied by an excess of hydrochloric acid. To ensure these conditions, the solution is treated with pota.s.sium iodide until no more iodine is evolved, and is then evaporated to remove the excess of hydrochloric acid. To the concentrated, and nearly neutral, solution a freshly-prepared solution of gallic acid is added in slight excess. A bulky white precipitate is formed that settles rapidly. The solution is diluted with hot water and washed by decantation. Then the precipitate is collected on a weighed double filter, washed once or twice with hot water, and dried at 100 C. The dried substance is antimony bigallate, and contains 40.85 per cent. of antimony. It should be completely soluble in ammonium sulphide. The solution in which the antimony is precipitated need not be quite free from other metals.

VOLUMETRIC METHOD.

This is based on the reduction of antimonic chloride (SbCl_{5}) to antimonious (SbCl_{3}) by the action of pota.s.sium iodide in strong hydrochloric acid solution.[63] Iodine is at the same time liberated, and the amount of antimony reduced is got at by t.i.trating with sodium hyposulphite, which measures the iodine set free.

The standard solution of sodium hyposulphite is made by dissolving 41.32 grams of the salt (Na_{2}S_{2}O_{3}.5H_{2}O) in water, and diluting to 1 litre. One hundred c.c. will be equivalent to about 1 gram of antimony.