A Text-book of Assaying: For the Use of Those Connected with Mines Part 43

It will be seen that this method of a.s.saying tin has its advantages and its drawbacks. It is quickly performed; with ores of good quality it gives results not to be excelled by any other process; and it gives the smelter the actual alloy and quality of metal he may expect to get in the smelting of the ore, which no other mode of a.s.saying will do: against which may be set the skill required to obtain accurate results with the vanning shovel; the loss of metal in poor ores containing an excess of silica; and the high results from ores containing a large quant.i.ty of metallic impurities.

~Cyanide Method.~--Weigh up 20 grams of the ore and dry it on a scoop over the Bunsen flame. When dry, weigh, and calculate the percentage of water from the loss in weight. Transfer the dried ore to an evaporating dish, and cover with 30 c.c. of hydrochloric acid; boil for 10 or 12 minutes, and then add 5 c.c. of nitric acid and boil again. Dilute with water, and filter. Transfer the filter and its contents to an E Battersea crucible, and calcine it for a few minutes. Cool, and weigh the residue. The loss equals the oxides soluble in acid. Transfer the residue to the crucible and mix it with its own weight of cyanide of pota.s.sium; add a similar amount of "cyanide" as a cover. Place in the furnace, and when the charge has attained the temperature of the furnace (in from 3 to 6 minutes), remove it at once; tap the pot _vigorously_ several times, and then pour its contents quietly into a mould. Dissolve the slag in water, clean, dry, and weigh the b.u.t.ton of tin.

WET METHODS.

~Detection.~--Tin ore is detected by its insolubility in acids, high specific gravity, and characteristic appearance in water. The powder is separated from the lighter gangue by was.h.i.+ng. It is fused in a Berlin crucible with five times its weight of pota.s.sic cyanide at a moderately high temperature in a m.u.f.fle, or over the blowpipe. The slag is washed off with water, and the metallic b.u.t.tons or residue treated with hydrochloric acid (not aqua regia), for some time. One portion of the solution strikes a purple colour with chloride of gold, another portion gives a white or grey precipitate or cloudiness with mercuric chloride.

These reactions are characteristic of tin as stannous chloride.

Metallic tin treated with nitric acid becomes converted into a white insoluble powder (metastannic acid). Aqua regia dissolves tin readily, forming stannic chloride, and in this solution the metal is detected by precipitation with sulphuretted hydrogen, which gives a yellow precipitate. Tin in solution as stannic or stannous chloride is precipitated as metal by means of zinc.

The fact that tin forms two well-defined series of compounds is taken advantage of in a.s.saying (just as in the case of iron), by determining how much of an oxidising agent is required to convert it from the stannous into the stannic state. For example, on the addition of a solution of permanganate of potash to a solution of stannous chloride the oxidation goes on rapidly, and the finis.h.i.+ng point is sharp and distinct; but acid solutions of stannous chloride quickly take up oxygen from that dissolved in the water used and from the air. Unfortunately, there is no obvious sign that such oxidation has taken place, except that (fatal to the a.s.say) a smaller volume of the permanganate is required. Great care is required with such solutions, both before and during t.i.tration. The addition of an excess of ferric chloride to the stannous solution, as soon as the whole of the tin has been dissolved, will lessen this liability to oxidation.

~Separation.~--If the tin is present in an alloy, the substance is boiled in an evaporating dish with dilute nitric acid until the whole of the material is attacked. Evaporate nearly to dryness, dilute, boil for a few minutes, and filter off the white insoluble residue. Under certain circ.u.mstances this residue will be nearly free from other metals, in which case it is ignited and weighed. If not known to be pure it must be ignited, reduced in a current of hydrogen, and treated as subsequently described.

When the tin is present as insoluble oxide in an ore, the substance is finely powdered, and from 1 to 5 grams of it (according to its richness) boiled with 30 c.c. of hydrochloric acid in an evaporating dish till the oxide of iron is seen to be dissolved. Then add 1 c.c. of nitric acid (or more if much pyrites, &c., is present) and continue the boiling till these are decomposed; dilute and filter off, was.h.i.+ng first with dilute acid and afterwards with a little dilute ammonia, dry, ignite, and place in a combustion tube (together with the filter-ash) and heat to redness for about thirty minutes in a current of dried hydrogen.

[Ill.u.s.tration: FIG. 58.]

The oxide of tin is placed in a porcelain boat (fig. 58), which is then introduced into a piece of combustion tube. The latter, wrapped in a piece of wire gauze, is supported on a couple of iron rings, and heated by one or two Bunsen burners in a furnace fitted up with loose fire-brick tiles, as shown in fig. 59.

[Ill.u.s.tration: FIG. 59.]

When the reduction is complete the tube is allowed to cool; the boat is removed and the tin dissolved. Add a rod of zinc to the freely-acid hot solution, and in a few minutes decant through a filter and wash with water, after having removed the zinc. Wash the precipitated metal back into the beaker, and dissolve in 10 c.c. of dilute nitric acid, evaporate off the excess of acid; dilute, boil, and filter. Wash, dry, ignite strongly in a porcelain crucible, and weigh.

In the absence of antimony the above separation works very well, but if this metal is present in quant.i.ty the metals precipitated on the zinc must be covered with hydrochloric acid and treated with a few drops of nitric. It is then warmed with iron wire until no more of the latter dissolves. The antimony is precipitated as metal, and the tin remains in solution as stannous chloride. The antimony is filtered off, and may be washed with alcohol, and weighed, whilst the tin in the filtrate is precipitated with zinc, and treated as already described.

GRAVIMETRIC METHOD.

If the tin is not already in the metallic state it is reduced to this condition by the method given (precipitation by zinc). Treat the finely-divided metal (washed free from chlorides) in a four-inch evaporating dish with 10 c.c. of dilute nitric acid, cover with a clock-gla.s.s, and apply a gentle heat until the precipitate appears of a white colour and the metal is completely attacked. Evaporate nearly to dryness on a water-bath; then add 50 c.c. of water, heat to boiling, and filter. Wash with hot water, dry, transfer to a weighed porcelain crucible, add the filter-ash, ignite strongly, and weigh. The precipitate after ignition is stannic oxide (SnO_{2}). It is a yellowish-white powder (darker whilst hot), insoluble in acids, and contains 78.67 per cent. of tin. Cold dilute nitric acid dissolves tin to a clear solution, which becomes a white enamel-like jelly on heating; this (filtered off, washed, and dried) forms an opal-like substance, which is converted on ignition into stannic oxide with evolution of nitrous fumes. Stannic oxide when ignited with chlorides is more or less completely converted into stannic chloride, which volatilises. The presence of chlorides during the evaporation with nitric acid causes a similar loss.

~Determination of Tin in an Alloy.~--(_Bronze._)--Take 2 grams, and attack with 20 c.c. of dilute nitric acid in a covered beaker with the aid of heat. Boil till the bulk is reduced by one-half, dilute with 50 c.c. of water, allow to settle for a few minutes, and filter; wash well first with water acidulated with a little nitric acid, and afterwards with water; dry, ignite, and weigh as stannic oxide.

~Determination of Tin in Tin Ore.~--Treat 5 grams of the dried and finely-powdered ore with 30 c.c. of hydrochloric acid in a four-inch evaporating dish. After the soluble oxides have been dissolved add 1 or 2 c.c. of nitric acid, boil off nitrous fumes, dilute, and filter. Dry the filter, transfer the cleaned ore to a piece of combustion tube ten or twelve inches long and narrowed at one end. Pa.s.s a current of hydrogen through the tube and heat to redness for 30 minutes; cool whilst the gas is still pa.s.sing. Dissolve in 20 c.c. of dilute hydrochloric acid and keep the solution tinted with permanganate of pota.s.sium. When the colour of the permanganate becomes permanent dilute to a bulk of 50 c.c. with water, filter, and wash. Heat; add a rod of zinc weighing about 3 grams; allow to stand for a few minutes; decant through a filter; and wash, removing the remaining zinc and returning the tin to the beaker. Treat with 5 c.c. of dilute nitric acid, boil for some time, take up with water, filter, wash, dry, ignite, and weigh as stannic oxide.

VOLUMETRIC METHOD.

~t.i.tration with Solution of Permanganate of Pota.s.sium.~--This t.i.tration may be made either directly on the solution of stannous chloride (prepared by dissolving the precipitated metal in hydrochloric acid), or indirectly, on a solution of ferrous chloride (produced by the reducing action of the precipitated metal on ferric chloride). The standard solution of permanganate of pota.s.sium is made by dissolving 5.356 grams of the salt in water and diluting to one litre. 100 c.c. are equivalent to 1.00 gram of tin.

The precipitated tin is transferred to a flask; and dissolved in 10 c.c.

hydrochloric acid, with the aid of heat and in an atmosphere of carbonic acid. The acid and metal are placed in the flask; which is then filled with the gas, and stopped with a cork provided with a rubber valve. When solution is complete the flask is again filled with carbonic acid. Fifty c.c. of water freed from air and saturated with carbonic acid are then added. This water is made by adding a gram of bicarbonate of soda and 2 c.c. of hydrochloric acid to 100 c.c. of water: the effervescence sweeps out the dissolved oxygen. The permanganate of pota.s.sium solution is then run in from a stop-c.o.c.k burette in the usual way until a faint pink tinge is obtained.

The following experiments show the effect of variations in the conditions of the a.s.say. A solution of stannous chloride equivalent in strength to the "permanganate" was made by dissolving 19.06 grams of the crystallised salt (SnCl_{2}.2H_{2}O.) in 50 c.c. of water and 10 c.c. of hydrochloric acid and diluting to 1 litre with water freed from dissolved oxygen. 100 c.c. contain 1 gram of tin. In the first experiments tap water was used and no precautions were taken for excluding air. Except when otherwise stated, 20 c.c. of the stannous chloride were used in each experiment with 10 c.c. of hydrochloric acid, and were diluted to 100 c.c. with water before t.i.tration.

~Effect of Varying Hydrochloric Acid.~

Acid added 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c.

"Permanganate" required 18.8 " 18.9 " 18.8 " 18.8 "

The only effect of the increase in quant.i.ty of acid was to give the brown of perchloride of manganese instead of the pink of permanganic acid to mark the finis.h.i.+ng point.

~Effect of Varying Temperature.~

Temperature 15 C. 50 C. 70 C. 100 C.

"Permanganate" required 18.8 c.c. 18.7 c.c. 18.6 c.c. 18.4 c.c.

~Rate of Atmospheric Oxidation.~--Solutions ready for t.i.tration were exposed to air at the ordinary temperature for varying lengths of time and then t.i.trated.

Time exposed 0 min. 5 min. 10 min. 20 min. 60 min.

"Permanganate"

required 18.8 c.c. 18.8 c.c. 18.8 c.c. 18.8 c.c. 18.6 c.c.

It is best to t.i.trate at once, although the loss by oxidation is only small after one hour's exposure.

~Effect of Varying Tin.~

Stannous chloride added 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

"Permanganate"

required 0.7 " 8.8 " 18.0 " 47.4 " 95.4 "

~Effect of Varying Bulk.~

Bulk 50.0 c.c. 100.0 c.c. 200.0 c.c. 500.0 c.c.

"Permanganate" required. 9.0 " 18.3 " 17.4 " 15.1 "

The two last series show an interference, which is due to the oxygen dissolved in the water, as may be seen from the following similar experiments, which were, however, performed with water freed from oxygen and in which the t.i.trations were effected in an atmosphere of carbonic acid.

~Effect of Varying Tin.~--A new solution of stannous chloride was used.

Stannous chloride added 1.0 c.c. 10.0 c.c. 20.0 c.c. 50.0 c.c. 100.0 c.c.

"Permanganate"

required 1.0 " 10.0 " 19.8 " 49.6 " 99.3 "

~Effect of Varying Bulk.~

Bulk 30.0 c.c. 50.0 c.c. 100.0 c.c. 200.0 c.c. 500.0 c.c.

"Permanganate"

required 19.8 " 19.8 " 19.8 " 19.8 " 19.8 "

It will be seen that in working under these conditions the results are proportional and the method satisfactory.

~Examination of Tin Phosphide.~--(_Phosphor Tin_.)--This substance is used in the manufacture of "phosphor bronze" and similar alloys. It is a crystalline, imperfectly-malleable, metallic substance. It is soluble in hydrochloric acid with effervescence; phosph.o.r.etted hydrogen, which inflames on the addition of a drop or two of nitric acid, being evolved.

It is attacked by nitric acid, yielding a white powder of stannic phosphate; this is not easily decomposed by ammonium sulphide or readily soluble in hydrochloric acid.

"Phosphor-tin" is made up only of tin and phosphorus. For the estimation weigh up 1 gram. Place in a weighed Berlin dish; and cover with 10 c.c.

of nitric acid and 3 or 4 c.c. of water. Let the reaction proceed (under a clock-gla.s.s) on the water-bath till complete. Remove the gla.s.s; evaporate to dryness, and ignite, at first gently over a Bunsen burner, and afterwards in the m.u.f.fle at a red heat. Cool in the desiccator, and weigh as quickly as possible when cold. The substance contains the tin as stannic oxide, SnO_{2}, and the phosphorus as phosphoric oxide, P_{2}O_{5}. The increase in weight on the gram of substance taken gives the weight of the oxygen taken up by the phosphorus and tin, and since 1 gram of tin takes up only 0.271 gram of oxygen, and 1 gram of phosphorus takes up 1.29 gram, the proportion of tin to phosphorus can be calculated from the increase in weight. For example, 1 gram of a sample gave 1.3410 gram of mixed oxides, which is 0.070 gram in excess of that which would be got with pure tin. If the substance was all phosphorus the excess would be 1.0190 gram; consequently the proportion of phosphorus in the substance is 0.070 / 1.019, or 6.87 per cent. The tin is calculated by difference, 93.13 per cent.