A Text-book of Assaying: For the Use of Those Connected with Mines Part 42

[69] About 5 inches in diameter.

[70] 61: 60:: 59: 58.13.

The iron in the ore is, then, the same in amount as that in 58.13 c.c.

of the ferric chloride solution; and since 100 c.c. of the latter contain 1 gram of iron, 58.13 c.c. of the same contains 0.5813 gram of iron; and, further, if 1 gram of ore carries this amount of iron, 100 grams of ore will obviously give 58.13 grams of iron.

[71] These compounds are Ni_{2}As and Co_{2}As.

[72] With large quant.i.ties of iron the ferric precipitate should be re-dissolved and re-precipitated. The filtrate must be added to the original filtrate.

[73] 4KCy + NiSO_{4} = K_{2}NiCy_{4} + K_{2}SO_{4} 2KCy + AgNO_{3} = KAgCy_{2} + KNO_{3} .'. 2AgNO_{3} = Ni

[74] Zn + H_{2}SO_{4} = H_{2} + ZnSO_{4}.

[75] These 104 c.c. are equivalent to 100 c.c. of dry air at 0 C. and 760 mm.

CHAPTER XII.

TIN--TUNGSTEN--t.i.tANIUM.

TIN.

Tin occurs in nature as ca.s.siterite (containing from 90 to 95 per cent.

of oxide of tin), which mineral is the source from which the whole of the tin of commerce is derived. Tin also occurs as sulphide combined with sulphides of copper and iron in the mineral stannine or bell-metal ore. It is a const.i.tuent of certain rare minerals, such as tantalite.

The methods of a.s.saying tin in actual use are remarkable when compared with those of other metals. The more strictly chemical methods are rendered troublesome by the oxide being insoluble in acids, resembling in this respect the gangue with which it is a.s.sociated. Moreover, it is not readily decomposed by fusion with alkalies. The oxide has first to be reduced to metal before the tin can be dissolved. The reduction may be performed by fusing with pota.s.sic cyanide, by heating to moderate redness in a current of hydrogen or coal gas, or by heating to a higher temperature with carbon. The reduced metal is only slowly dissolved by hydrochloric acid, and although it is readily soluble in aqua regia, the solution cannot be evaporated or freed from the excess of acids, by boiling, without loss of tin, because of the volatility of stannic chloride. There has long been a difficulty in getting a quick wet method.

The process of a.s.saying tin ores adopted in the mines of Cornwall is a mechanical one known as "vanning," the object of which is to find the percentage of "black tin," which, it is well to remember, is not pure ca.s.siterite, much less pure oxide of tin. Tin ore, as taken from the lode, contains from 2 to 5 per cent. of ca.s.siterite, and is mainly made up of quartz, felspar, chlorite, schorl, and other stony minerals, together with more or less mispickel, iron and copper pyrites, oxide of iron, and wolfram. The ca.s.siterite has a specific gravity (6.4 to 7.1) considerably higher than that of the vein-stuff (2.5 to 3.0), and is concentrated by a series of was.h.i.+ngs till it is free from the lighter material. Those minerals which have a specific gravity approaching that of the ca.s.siterite are not completely removed. The mispickel and copper and iron pyrites are converted into oxides by roasting, and are in great part removed by a subsequent was.h.i.+ng. The concentrated product is known as "black tin," and in this condition is sold to the smelter. The chief foreign matters in the black tin are silica, oxides of iron and copper, and wolfram, with traces of manganese and niobic acid; and in certain stream ores there may be as much as 6 or 7 per cent. of t.i.taniferous iron. The black tin from the mines contains from 5 to 12 per cent. of water, and is sold and a.s.sayed wet. A series of typical samples of black tin ranged as follows:--

--------------------------+---------------------+----------------- Source of Material. | Percentage of Metal |Specific Gravity.

| in Dry Ore. | --------------------------+---------------------+----------------- Good mine ore | 72.0 | 6.39 Inferior do. | 71.5 | 6.64 t.i.taniferous stream ore | 67.0 | 6.39 Mine ore with wolfram | 64.5 | 6.67 Ore from stream works | 58.5 | 5.99 --------------------------+---------------------+-----------------

It will be seen from these figures that black tin is a very variable substance; and that the specific gravity is largely influenced by the impurities; hence, it is only an indication of the percentage of metal when the same kind of ore is dealt with.

As already pointed out, the object of vanning is to determine the proportion of black tin in the lode stuff. The relation between the actual content in oxide of tin and the produce got by vanning has been tested on several occasions with results which show a fair degree of approximation.

The following are some published results of a.s.says of the same batch of ore. The vanning results were obtained by a Cornish vanner of recognised ability, and the wet a.s.says by two London firms of the highest standing:--

Vanning results: (Average) 91 lbs. of "black tin."

Wet a.s.say results: A 83.7 lbs. of stannic oxide.

B 79.7 lbs. "

The vanner reported his black tin as containing 70 per cent. of tin.

This will bring his result, if calculated as stannic oxide, to 80.9 lbs.

to the ton; which agrees with the others.

According to our experience the "van" a.s.say agrees fairly well with the "wet" one, if the black tin is a.s.sumed to contain 92.5 per cent. of stannic oxide (SnO_{2}).

Vanners are, as a rule, skilful men, and show remarkable dexterity in separating the black tin, with the help of their apparatus, which consists simply of a shovel and a kieve of water. An account of the process is given below. But different vanners, all good men, will get different results working on material new to them. The black tin weighed by the vanner is supposed to correspond in quality with the black tin returned from the floors of the mine for which he is a.s.saying, but this differs materially in different mines with the nature of the gangue. The process leaves too much to the judgment of the vanner. It is more than probable that in practice the returns from the dressing-floors check the a.s.sayer, instead of, as should properly be the case, the a.s.sayer checking the returns. It is only when this last is done that any control is had over the system of dressing. A correct a.s.say of this ore is a matter of some importance, because of the high price of the metal.

The method of a.s.saying the black tin is a dry one, and consists of mixing it with "culm," and submitting it in a black-lead crucible to the highest temperature of a wind furnace. The sample is taken wet as it arrives at the smelting house, and is a.s.sayed direct. The product of the a.s.say is examined, and a deduction of a considerable percentage is very properly made for impurities, since the a.s.say really determines the percentage, not merely of tin, but of the bodies present which are reducible at a white heat. The judgment as to how much is to be deducted is a.s.sisted partly by an examination of the metal got from the a.s.say, and partly by the experience acquired in smelting similar ores. The produce, which is that of the impure tin, is stated in parts in twenty; thus a produce of 14 is equivalent to 70 per cent., or to 14 cwt. per ton.

[Ill.u.s.tration: FIG. 57.]

MECHANICAL SEPARATION.--VANNING.

This process, which has already been referred to, is carried out as follows:--After sampling the ore in the ordinary way, a quant.i.ty (varying with its richness) is weighed out. Special weights are generally used. The standard weight, marked 200, weighs about an ounce; with poor ores this quant.i.ty is taken for an a.s.say, but with richer ores 100 or even 50 is sufficient. The unit of weight has no special name, but the parts in 200 are spoken of as the produce; thus, if 200 of ore were taken and 9.5 of black tin were separated, the produce would be 9-1/2: obviously half the "produce" will give the percentage. The weighed portion of the ore is placed on the vanning shovel. The vanner stands in front of a tub of water (kieve) and allows 30 or 40 c.c. of water to flow on to the ore. He then raises the shovel a little above the surface of the water, and, holding it nearly horizontal, briskly rotates the water by imparting to the shovel a slight circular motion, pa.s.sing into an elliptical one (front to back). This causes the finer mud to be suspended in the liquid, which is then run off, leaving the body of the ore in the centre of the shovel. This is repeated until the water after standing a moment is fairly clear. About half as much water as before is brought on; then, with a motion which is similar to the previous one, but with a jerk added in one direction, the heavier minerals are thrown up, and the stony matter brought back. The jerk is produced just as the wave of water is returning. The descending wave of water draws with it the bulkier and lighter particles of the ore, whilst the heavier matter lying on the bottom is scarcely affected by it. The jerky motion, however, carries it to the front of the shovel. The lighter stuff is washed off, and the residue dried by holding the shovel over the furnace. It now corresponds, more or less, to the stuff which on the mine is sent to the calciner. It is swept from the shovel into a scoop, and transferred to a hot crucible; in which it is calcined until free from sulphur. Some vanners calcine their samples before commencing to van. The calcined ore is shaken out of the crucible on to the shovel; rubbed up with a hammer; and washed (as at first) to get rid of the finer and lighter "waste." The separating motions are again gone through; and the "head" of the best of the black tin is thrown well up on one side of the shovel in the form of a crescent, so as to leave room on the shovel to work with the "tailings." The quant.i.ty of water used is kept low, to prevent this "crop" tin from being washed back again. The tailings are then crushed to free the tin from adherent oxide of iron; and again washed to throw up the remaining tin ore. As this tin is finely divided, it is more difficult to bring it up, so that a vigorous and rapid motion is required. The tailings are now washed off, and the whole of the black tin is brought into the centre of the shovel. It requires two or three was.h.i.+ngs more to free it from the waste it contains. Very small quant.i.ties of water are used. The purity of the black tin can be seen by its appearance on the shovel. The cleaned ore is dried as before, freed from particles of iron with the aid of a magnet, and weighed. The weighings are carried to 1/8th of the unit used. The following example ill.u.s.trates the method of calculation adopted on the mine. A parcel of 1 ton 2 cwt. 3 qrs. of tin ore with a produce of 45 (equal to 22-1/2 per cent.) contains 5 cwt. 0 qrs. 12 lbs.

of black tin. This result is obtained as follows:--

ton cwt. qrs.

1 2 3 9 } ----------------- } 10 4 3 } equivalent to multiplying by 45.

5 } ---------------- } 5.1 3 3 strike off the first figure to the right.

4 multiply by 4 to reduce to quarters.

--------- 4 12 3 --------- 4 15 28 multiply by 28 to reduce to pounds.

----- 112 15 ----- 12.7 strike off the first figure to the right.

Similarly, a parcel of 20 tons 10 cwt. with a produce of 9-1/2 contains 19 cwt. 1 qr. 25 lbs. of black tin. For the following information, as well as for much of that already given about vanning, we are indebted to Captain Reynolds, of Cook's Kitchen Mine. "To have a complete set of tools for all vanning purposes, it will be necessary to get the following:--A vanning shovel 14 inches long and 13 inches wide, weighing not over 2-3/4 pounds. It is made of hammered sheet iron of the shape shown in fig. 57. It must have a light wooden handle (preferably of deal) 3 feet long. A bruising hammer, weighing 2-1/2 pounds, with a handle 1 foot long. A pair of tongs (furnace) 2-1/2 feet long, made of 1/2-inch round iron. And a set of ordinary clay crucibles for calcining.

There ought to be two sets of scales and weights: the first should be confined to weighing the powdered tin stuff, and the second ought to be a much higher cla.s.s one, for weighing the black tin obtained. The furnace for roasting the sample should be 10 inches square and 12 inches deep, with the fire-bars at the bottom three-quarters of an inch apart.

The water-box for vanning in should be at least 4 feet long, 2 feet 6 inches wide, and 8 inches deep."

DRY METHODS.

For the following description of the process adopted in Cornwall we are indebted to Mr. A.K. Barnett, F.G.S., of Chyandour.

~Cornish Method.~--_Tin Ore a.s.say._--The ore to be smelted or a.s.sayed should be concentrated to say not less than 50 per cent. of metallic tin; though to obtain satisfactory results it should be brought nearer 70 per cent., as with ore containing less than 40 to 50 per cent. of metal there will be a considerable loss both in the a.s.saying and in the smelting. If the ore to be operated on does not contain this quant.i.ty of metal, then the sample (if coa.r.s.e) must be reduced to a fine state, the gangue being removed by vanning, and the ore saved for the fire a.s.say.

The method adopted for the determination of tin in the ore is as follows:--About 2-1/2 ounces troy (1200 grains, or about 80 grams) of the ore to be a.s.sayed is weighed out and mixed on a flat copper pan (shaped with a long lip) with one-fifth of its weight (240 grains, or 15.5 grams) of powdered culm (anthracite). The mixture of ore and culm is either transferred to a black-lead crucible before the latter is put into the furnace, or, as some prefer, it is carefully swept into a crucible which has been imbedded in the fire. Some a.s.sayers cover their pots with a flat cover placed loosely on, while others leave the mixture in the open pot. The furnace, which has been previously fired to a strong heat, is then covered, and the sample is subjected to a sharp fire for a period of from twelve to twenty minutes. No definite time can be stated, as, besides the strength of the fire, the quality and condition of the ore, and the impurities a.s.sociated with it, greatly affects the time required for the complete reduction of the ore. As soon as the mixture in the crucible has settled down to a uniform white heat, and any very slight ebullition which may have taken place has subsided, the crucible is gently shaken, removed from the fire (the culm-ash or slag which covers the metal being carefully drawn aside with an iron sc.r.a.per), and the metal is poured quickly into an iron ingot-mould, which is usually placed on a copper pan to save the culm-slag and the adherent metal which comes out with it. The crucible is then carefully sc.r.a.ped, and the sc.r.a.pings, together with the contents of the mould and pan, are transferred to a mortar. There the ingot of tin is freed from slag and then taken to the scales. The rest, after being finely powdered, is pa.s.sed through a sieve. The flattened particles of tin which remain on the sieve are weighed with the ingot (the _lump_, as it is called); whilst the siftings are vanned on a shovel, and (the slag being washed off) the fine tin is collected, dried, and weighed with the rest: the whole gives the produce or percentage of metal in the ore.

The results of the a.s.says are expressed in cwts. of metal in the ton of ore. The percentage is rarely given and never used in Cornwall.

Thus--"13-1/2 Produce" would mean that the a.s.say yielded results at the rate of 13-1/2 cwts. of metal for one ton of the ore. Some a.s.sayers use a little powdered fluor-spar to a.s.sist the fusion of refractory slags. A small quant.i.ty of borax will also occasionally be of service for ores containing silica in excess of any iron that may be present. The borax renders the slag more fusible, and a.s.sists the formation of a larger lump (with less fine tin in the slag) than would be obtained by the use of culm alone.

The quality and the percentage of _pure tin_ in the metal will vary considerably, according to the impurities that are a.s.sociated with the ore to be a.s.sayed.

The crude lump is then remelted in a small iron ladle at as low a temperature as possible, and the fused metal is poured into a shallow trench about 4 inches long by 3/4 of an inch wide cut in a block of white marble. The metal will be silvery-white if the temperature employed be correct; if too hot, the surface will show a yellow, red, or blue colour (according to the heat employed); in such case the metal should be remelted at a lower temperature. If the metal on cooling remains perfectly clear and bright, then it may be a.s.sumed that the tin is of good quality and commercially pure. A crystallised or frosted appearance of the metal indicates the presence of some alloy, say of iron, copper, zinc, lead, antimony, &c. The a.s.sayer who has had much practice can readily distinguish the metal or metals that are a.s.sociated with the ore by noting the appearance of the tin on cooling; and can fairly judge the quant.i.ty of impurity present by the amount of the crystallisation or stain.

Whilst the foregoing method of a.s.saying cannot lay claim to scientific accuracy, it is by no means so imperfect as some writers would have us believe, who state that a loss of 5 to 10 per cent. arises in the operation. It is certainly the most ready and expeditious mode of determining the commercial value of a parcel of tin ore, which, after all, is the main object of all a.s.saying operations.

The difficulty which beginners find in obtaining satisfactory results, and any loss of metal which those not accustomed to the process may incur, will invariably occur in the vanning of the powdered slag for the fine tin, the rest of the operations being easy of execution, and requiring only the ordinary care necessary for all metallurgical work.

There is no doubt that if low percentage ores containing silica are a.s.sayed in this manner, low results are obtained, as it is impossible to reduce the whole of the tin in the presence of free silica; with this cla.s.s of ores, care should be taken to remove some of the silica by preliminary vanning, or some flux should be added which will combine with the silica, and so prevent its entering into combination with the tin. Low quality tin ores containing iron, copper, lead, zinc, antimony, etc., combined with a.r.s.enic, sulphur, or oxygen, will give very much higher results than the actual percentage of tin in the sample. The other metals (being readily reduced in the presence of tin) alloy with it, and give a hard lump difficult to fuse in the iron ladle; where the quant.i.ty of foreign metals is large, the metal can only be melted to a stiff pasty ma.s.s; so that (in determining the value of a ton of tin ore, or even reporting on the percentage of tin it contains) not only must the weight of the a.s.say be the basis for calculation, but the quality and character of the metal obtained must also be considered. Thus two ores of tin might be a.s.sayed both yielding a similar _produce_, say 13-1/2 (67-1/2 per cent.), and yet one might contain 5 per cent. less tin than the other.

If it be required to obtain the pure metal from tin ores containing the ores of other metals a.s.sociated with them, the latter must be removed by digesting in strong hydrochloric acid, and was.h.i.+ng. The a.s.say may then be conducted in the usual way, and a fairly pure lump will be obtained.

If wolfram be present in any appreciable quant.i.ty in the ore, it considerably reduces the proportion of lump, and at the same time it increases the fine tin (or _prillion_, as it is termed) in the a.s.say.

This may be got rid of by boiling in aqua regia, and dissolving out the tungstic acid which has been liberated by means of ammonia.