A Text-book of Assaying: For the Use of Those Connected with Mines Part 44

Another method of separating and determining the phosphorus is as follows:--Take 1 gram of the substance and add to it 15 c.c. of hot aqua regia. Boil till dissolved, dilute, and precipitate the tin with sulphuretted hydrogen. To the filtrate add ammonia and "magnesia mixture." Filter; wash the precipitate with dilute ammonia; dry, ignite, and weigh as magnesic pyrophosphate. Calculate the phosphorus, and take the tin by difference.

A sample of phosphor tin gave--

Tin 93.1 per cent. (by difference) Phosphorus 6.9 "

----- 100.0

~Tin a.r.s.enide.~--This is met with in tin-smelting; it closely resembles the phosphide, but the crystals have a duller grey appearance. It contains simply tin and a.r.s.enic. The determination is made by treating 1 gram of the substance with nitric acid and weighing the mixed oxides of tin and a.r.s.enic in the same manner as in the case of the phosphide. One gram of a.r.s.enic will give 1.533 gram of a.r.s.enic oxide, As_{2}O_{5}; consequently the excess of weight of the mixed oxides over 1.271 gram must be divided by 0.262; the result multiplied by 100 gives the percentage of a.r.s.enic. In consequence of the higher atomic weight of a.r.s.enic the results by this method are not so close as with the phosphide. Each milligram of excess weight (over 1.271) represents 0.38 per cent. of a.r.s.enic, As. Both in this and in the corresponding phosphide determination care must be taken to avoid absorption of moisture, by allowing the oxides to cool in a desiccator and weighing quickly.

The percentage of a.r.s.enic is better determined as follows:--Weigh up 1 gram of the substance, dissolve in aqua regia, dilute, and pa.s.s sulphuretted hydrogen. Render alkaline with ammonia, and add ammonium sulphide till the precipitate is dissolved. Add "magnesia mixture."

Filter off the precipitate, wash with dilute ammonia, ignite with a few drops of nitric acid, and weigh as magnesic pyra.r.s.enate. Calculate the a.r.s.enic and take the tin by difference. A sample treated in this way gave--

Tin 96.8 per cent. by difference a.r.s.enic 3.2 "

----- 100.0

~Examination of Black Tin.~--Dry the ore, and reduce it to a fine powder. Weigh up 2 grams, and boil with 20 c.c. of hydrochloric acid and 2 c.c. of nitric for ten or fifteen minutes. Filter, and reserve the filtrate.

~Tungstic Acid.~--Digest the residue with about 50 c.c. of water and a few c.c. of dilute ammonia for a few minutes, and filter; collect the filtrate in a weighed porcelain dish, evaporate to dryness, ignite, and weigh as tungstic acid, WO_{3}.

~Stannic Oxide.~--Dry, ignite, and weigh the insoluble residue. Transfer to a porcelain boat, and reduce in a current of hydrogen at a red heat for half an hour. Allow to cool whilst the hydrogen is still pa.s.sing.

Transfer the boat to a beaker, and dissolve up the tin in 10 c.c. of hydrochloric acid and a c.c. or so of nitric. Wash out the combustion tube with some acid and add the was.h.i.+ng to the contents of the beaker.

Warm gently, dilute with water, and filter. Collect, dry, ignite, and weigh the insoluble residue. Through the filtrate pa.s.s a rapid current of sulphuretted hydrogen, allow to settle, and filter. Wash the precipitate with hot water, dry, calcine gently; ignite with ammonium carbonate, and weigh as stannic oxide, SnO_{2}. The insoluble residue will in most cases retain some tin. Fuse it with fusion mixture, take up with hydrochloric acid, filter, pa.s.s sulphuretted hydrogen through the filtrate, collect and wash the sulphide of tin. Ignite and weigh as stannic oxide, and add it to that previously obtained.

~Copper.~--Pa.s.s sulphuretted hydrogen through the acid filtrate obtained in the first cleaning of the ore, collect the precipitate, and wash first with soda solution and then with hot water. Dry, ignite, and weigh as cupric oxide, CuO. Mix the filtrate with that from the main portion of the sulphide of tin.

~Ferric Oxide.~--Boil off the sulphuretted hydrogen from the mixed filtrates and peroxidise with nitric acid. Add ammonia in slight excess, boil, filter, dry, ignite, and weigh the precipitate as ferric oxide.

This will be practically pure, but the iron in it must be determined by dissolving and t.i.trating. The filtrate from the iron may contain zinc, lime, and magnesia, but rarely in quant.i.ties sufficient to be determined.

~Silica, &c.~---The silica may be calculated from the weight of the residue insoluble in acid, after the reduction of the tin in hydrogen, by deducting from it the weight of the oxide of tin subsequently found.

Or it may be determined as follows:--The insoluble portion is fused with fusion mixture, and taken up with hydrochloric acid, as already described. On filtering, the filter will retain a portion of the silica.

The rest is recovered, after the removal of the stannous sulphide, by evaporating to dryness, taking up with hydrochloric acid, and filtering through the same filter. It is washed, dried, ignited, and weighed as silica. The filtrate from the silica is boiled with a little nitric acid and precipitated with ammonia. The precipitate is collected, washed, ignited, and weighed as ferric oxide and alumina (but it frequently contains oxide of t.i.tanium). When the last is present it is determined by fusing with bisulphate of potash and extracting with cold water. The solution is nearly neutralised with ammonia, charged with sulphurous acid, and boiled. The precipitate is collected, washed, dried, ignited, and weighed as oxide of t.i.tanium, TiO_{2}. The difference between this weight and that of the combined oxides gives the ferric oxide and alumina. The filtrate from the mixed oxides is examined for lime and magnesia.

~Sulphur.~--Rub up 5 grams of the ore with 5 grams of nitre, transfer to a porcelain dish, and fuse over a Bunsen burner for fifteen minutes.

When cold, extract with water, and determine the sulphur volumetrically with standard barium chloride. The sulphur may be present as sulphide or sulphate.

~a.r.s.enic.~--Take 5 grams, and evaporate with nitric acid; dilute, add ammonia, pa.s.s sulphuretted hydrogen, and filter.

To the filtrate add "magnesia mixture." Collect the precipitate, ignite with nitric acid, and weigh as magnesic pyra.r.s.enate.

The following may be taken as an example of the composition of an impure black tin:--

Tungstic acid 1.8 % Stannic oxide 79.0 Silica 2.6 t.i.tanic oxide 0.8 Copper oxide 0.9 Ferric oxide 13.4 Sulphur 0.4 a.r.s.enic 0.3 ---- 99.2

~Examination of Hardhead.~--In the smelting of tin ores a quant.i.ty of speise, known as "hardhead," is produced. It is essentially an a.r.s.enide of iron, carrying a considerable quant.i.ty of tin. Much of this last is present in the form of small b.u.t.tons of metal distributed through the ma.s.s. The b.u.t.tons can be seen on careful inspection, and become evident on powdering.

In a.s.saying the substance, a variation in the usual method of sampling is required, because of the quant.i.ty of metal present which cannot be powdered. After powdering as finely as possible, the coa.r.s.e particles are sifted off and weighed. The weight of the powder is also taken. The method of working is best ill.u.s.trated by an example. A sample of hardhead weighed 155.1 grams, and gave 21.0 grams of coa.r.s.e particles, equivalent to 13.5 per cent. of the whole. The fine portion weighed 134 grams, which is equivalent to 86.5 per cent.

Thirteen and a half grams of the coa.r.s.e material were dissolved in aqua regia, and diluted with water to 1 litre. Ten c.c. of this contain 0.135 gram of the metallic portion, which is the amount contained in 1 gram of the original hardhead. If, in a determination, 1 gram of the substance is wanted, weigh up 0.865 gram of the powdered portion, and add to it 10 c.c. of the solution. It will be seen that these together make up 1 gram of the original sample. The solution of the metallic portion must be saved until the a.n.a.lysis is finished.

~Tin and Copper.~--Weigh up the portion of the powdered stuff equivalent to 1 gram of the sample. Transfer to a flask, and cover with 10 c.c. of the solution of the metallic portion and 10 c.c. of aqua regia. Boil gently till oxidation is complete and the nitric acid for the greater part driven off. Dilute to 100 c.c. with water, and pa.s.s sulphuretted hydrogen for some time. Filter, wash with hot water, and rinse through the funnel back into the flask. Digest with yellow sodium sulphide until only a light, flocculent, black precipitate is left. Filter this off, wash with hot water, dry, calcine, treat with a little nitric acid, ignite, and weigh as copper oxide, CuO. The weight multiplied by 0.7983 gives the weight of copper.

The filtrate containing the tin is rendered acid with hydrochloric acid, and filtered. The precipitate is rinsed into a half-pint beaker, covered with 20 c.c. of hydrochloric acid, and boiled down to about 20 c.c. The solution is filtered off from the sulphur and sulphide of a.r.s.enic, which, after was.h.i.+ng with hot water, is transferred to a flask labelled "a.r.s.enic." A strip of sheet zinc (2 in. by 1 in.) is placed in the solution. The evolution of hydrogen should be brisk. In five or ten minutes decant off a few c.c. of the liquid, and test with sulphuretted hydrogen for tin. If no yellowish precipitate is formed, decant off the rest of the liquid, and wash the precipitated metal with hot water two or three times by decantation. The metal should be in a lump; if there are any floating particles they must be made to sink by compression with a gla.s.s rod. Transfer the washed metal to an evaporating dish 3 or 4 in.

across, and cover with a few c.c. of hot water. Add nitric acid drop by drop till the tin is completely attacked. Evaporate nearly to dryness, and add a drop or two more of nitric acid and 20 c.c. of water. Boil and filter. Wash with hot water, dry, ignite, and weigh as stannic oxide, SnO_{2}. Calculate to metallic tin by multiplying by 0.7867.[76]

The filtrate from the first treatment with sulphuretted hydrogen will probably no longer smell of the gas. Warm and pa.s.s the gas for a few minutes longer. Filter off any precipitate of sulphide of a.r.s.enic, and transfer it to the flask for "a.r.s.enic." Boil the filtrate (ignoring any signs of a further precipitation of a.r.s.enic) with a few c.c. of nitric acid, and separate the iron as basic acetate. Wash; reserve the filtrate for cobalt.

~Iron.~--Rinse back the "basic acetate," precipitate into the flask, add ammonia, dilute with water to about 100 c.c., and pa.s.s sulphuretted hydrogen for a few minutes. Filter, and wash with hot water. Collect the filtrate in the flask labelled "a.r.s.enic." Boil the precipitate with dilute sulphuric acid, filter, and t.i.trate the filtrate with the permanganate of pota.s.sium solution after boiling off the sulphuretted hydrogen. Report the result as iron. The sulphuric acid will not effect complete solution, a light black residue will remain, chiefly sulphur; this must be rinsed into the filtrate from the acetate separation. It contains cobalt.

~Cobalt.~--The filtrate from the acetate separation will have a pink colour. Render it ammoniacal and pa.s.s sulphuretted hydrogen. Collect the precipitate on a filter, dry, and ignite. Dissolve in hydrochloric acid, and evaporate nearly to dryness with an excess of nitric acid.

Dilute with 10 or 20 c.c. of water and add potash solution in slight excess. Add acetic acid until the solution is acid and the precipitate is quite dissolved. Add 20 or 30 c.c. of a strong solution of pota.s.sium nitrite, and determine the cobalt, as described on pp. 254, 256. Boil the filtrate from the cobalt, precipitate with hydrochloric acid, render ammoniacal, and test for zinc, nickel, and manganese.

_The remainder of the tin_ will be contained in the flask labelled "a.r.s.enic." Acidify with hydrochloric acid and filter. Rinse into a beaker, and evaporate to a small bulk with 10 c.c. of nitric acid.

Dilute and filter. Dry the precipitate, consisting of stannic a.r.s.enate (2SnO_{2}.As_{2}O_{5}), ignite, and weigh. Calculate the tin it contains by multiplying by 0.4453, and add to that already found.

~a.r.s.enic.~--This is determined in a separate portion. Weigh up a portion of the powder equivalent to 1 gram of the hardhead, place in a pint flask, and boil with 10 c.c. of nitric acid. When action has ceased add 10 c.c. of the solution of the metallic portion and then hydrochloric acid (a few drops at a time) till solution is complete. Warm gently in dissolving, but do not boil. Dilute to about 100 c.c., render alkaline with ammonia, and add 20 c.c. of yellow ammonium sulphide. Digest at a gentle heat for about thirty minutes, filter, and wash. Add 50 c.c. of magnesia mixture, shake well, allow to stand for an hour, filter, and wash with dilute ammonia. The precipitate is dissolved and then t.i.trated with uranium acetate, or it is evaporated with nitric acid, ignited, and weighed as pyra.r.s.enate of magnesia. Calculate the result to a.r.s.enic, As.

~Sulphur.~--Weigh up a portion of the powder equivalent to 2 or 3 grams of the hardhead. Rub up in a mortar with 5 grams of nitre and fuse in a porcelain dish for ten minutes. Extract with water, add 20 or 30 c.c.

(as the case may be) of the solution of the "metallics." Add 10 grams of sodic acetate, and ferric chloride until the precipitate turns brown; dilute with water to half a litre, boil, and t.i.trate with standard baric chloride, as described under _Sulphur_. Report as sulphur.

A sample of hardhead examined in this way gave--

Sulphur 3.00% a.r.s.enic 27.10 Tin 22.2 Copper 1.64 Iron 43.2 Cobalt 2.6 ------ 99.74

~Examination of Tin Slags.~--In tin smelting works the term "slag" is applied to the unfused portion of the charge. It is made up of unburnt anthracite and small lumps of slag proper together with some b.u.t.tons of metallic tin. This is rarely, if ever, a.s.sayed. The slag proper (or, as it is generally called, "gla.s.s") is a silicate of iron, alumina, and lime, containing from 3 to 7 per cent. of tin. It is thus examined:--The sample after bruising on an iron plate, is reduced to a very fine powder by grinding in an agate mortar. In this state it is in most cases readily decomposed by hydrochloric acid.

~Determination of Tin.~--Where the percentage of tin only is required, take 2 grams of the powdered slag and well mix with it 20 c.c. of hydrochloric acid, and heat to boiling. Add 1 c.c. of nitric acid, allow to stand for fifteen minutes, dilute with water, and filter. Pa.s.s a rapid current of sulphuretted hydrogen for some time. Allow to settle, and filter. The precipitate, after was.h.i.+ng with hot water, is dried, and gently calcined until the greater part of the sulphur is burnt off. It is then strongly ignited in the m.u.f.fle (or over the blowpipe) with the addition of a small lump of ammonic carbonate. The residue is weighed as stannic oxide (SnO_{2}); and is calculated to metallic tin by multiplying by 0.787. The percentage on the slag is calculated in the usual way.

The tin is always best determined in the examination of slags by a separate a.s.say carried out in this way. The determination of the other const.i.tuents is thus made:--

~Silica.~--Take 2 grams of the powdered slag and cover them, in a small evaporating dish, with 20 c.c. of hydrochloric acid; mix well by stirring with a gla.s.s rod; and evaporate to dryness. If (as is generally the case) tungsten is present the solution will be blue. Take up with 20 c.c. of hydrochloric acid. Add 1 c.c. of nitric acid; and reduce by boiling to about half the bulk. Add about 20 c.c. of water, boil, and filter. Wash the residue with hot dilute hydrochloric acid. It consists of silica with the tungstic acid. Wash it back into the dish; and digest with 5 or 10 c.c. of a cold solution of ammonic carbonate. Filter; and collect the filtrate and was.h.i.+ngs in a weighed porcelain dish. Dry the residue, ignite strongly, and weigh as silica, SiO_{2}. In certain exceptional cases this may contain some unaltered ca.s.siterite, which is easily recognised by its appearance.

~Tungsten.~--The ammonic carbonate filtrate from the silica is evaporated to dryness, ignited strongly over the blowpipe, and weighed.

The residue is tungstic acid, WO_{3}. The tungsten may be conveniently reported in this form, although it is probably present as a lower oxide.

~Tin.~--The acid filtrate from the silica and tungstic acid is treated with sulphuretted hydrogen. The sulphide of tin is filtered off. Since the percentage of tin has been already determined, this precipitate may be neglected; or may be treated in the same way as the previous one, so as to check the result. Since some stannic chloride will have been lost in the evaporation, a low result may be expected. The tin should be reported as stannous oxide; and is calculated by multiplying the percentage of tin by 1.136.

The filtrate from the tin is boiled rapidly down to remove sulphuretted hydrogen; and then peroxidised with 1 or 2 c.c. of nitric acid. It is cooled, transferred to a graduated flask, and diluted with water to 200 c.c.

~Ferrous Oxide and Alumina.~--Half the filtrate from the tin (that is, 100 c.c.) is taken, nearly neutralised with soda, and treated with sodium acetate. The basic acetate precipitate obtained on boiling is filtered off and washed. Reserve the filtrate. The precipitate is dissolved off the filter with hot dilute hydrochloric acid; and the solution thus formed is treated with a slight excess of ammonia, and boiled. The precipitate is filtered off, washed with hot water, dried, ignited, and weighed as mixed ferric oxide and alumina. The ignited precipitate is then dissolved with sulphuric and hydrochloric acids; and the iron determined in the solution by t.i.tration with the solution of stannous chloride. The iron found is calculated to and reported as ferrous oxide, FeO (factor = 1.286). To find the alumina, which is best estimated by difference, multiply the iron by 1.428 to get the weight of ferric oxide, and deduct this from the weight of alumina and ferric oxide found. This, of course, gives the alumina. A direct determination may be made by removing the tin from the t.i.trated solution with sulphuretted hydrogen, filtering, nearly neutralising with ammonia, and boiling with a few grams of hyposulphite of soda. The precipitate, filtered, washed, and ignited, is the alumina, which is weighed. The direct determination gives a slightly low result.

~Oxides of Zinc and Manganese.~--These are determined in the filtrate from the basic acetate precipitate by rendering alkaline with ammonia, and pa.s.sing a current of sulphuretted hydrogen. Generally a small, but decided, precipitate of alumina comes down, together with sulphides of any zinc or manganese which is present. The precipitate is allowed to settle, dried, ignited, and weighed. The metals are separately determined in it; and the residue is counted as alumina, and added to that already found. The mixed precipitate amounts to from 1 to 2 per cent. of the sample.

~Lime.~--The filtrate from the last is treated with ammonic oxalate, boiled for a few minutes, allowed to settle, and filtered. The precipitate is washed with hot water; dried; ignited; and weighed as carbonate, after gentle ignition; or as lime, after strong ignition in the m.u.f.fle.

~Magnesia.~--The filtrate from the lime is treated with sodic phosphate and ammonia. It is well mixed by stirring, and allowed to stand overnight. The precipitate is washed with dilute ammonia, dried, ignited, and weighed as pyrophosphate.

~Soda and Potash.~--These are determined in the remaining half of the filtrate from the tin. The solution is rendered ammoniacal with ammonia; and treated, first with sulphuretted hydrogen, and then with ammonium oxalate. The precipitate is filtered off and rejected. The filtrate is evaporated in a small porcelain dish over a Bunsen burner, or on the sand bath; and towards the close (or earlier if the evaporation is not proceeding well) nitric acid is added. The evaporation is carried to dryness; and the residue heated nearly to redness. The residue, which consists of magnesia with carbonates and chlorides of the alkalies, is extracted with water; and filtered. The filtrate is evaporated with hydrochloric acid in a weighed platinum dish, ignited gently, and weighed. This gives the weight of the mixed chlorides of sodium and pota.s.sium; which are then separated and determined as described under _Potash_.

It must be remembered when calculating the percentage that (with the exception of the silica, tungstic acid, and tin) the determinations have been made on 1 gram of the sample.