A Text-book of Assaying: For the Use of Those Connected with Mines Part 65

~Alumina, &c.~--The filtrate from silica is treated by the basic acetate method. That is, it is first treated by a cautious addition of a solution of soda, almost to the point of producing a precipitate, in order to neutralise the excess of acid; 2 or 3 grams of sodium acetate are added, and the whole boiled for a minute or so. The precipitate is filtered off and washed only slightly. Save the filtrate. The precipitate is dissolved in hydrochloric, or, perhaps better, in nitric acid; and is reprecipitated by adding an excess of ammonia and boiling.

The precipitate is filtered and washed with water containing 2 per cent.

of ammonium nitrate. Both filtrates are evaporated separately to a small bulk, a drop or two of ammonia being added to the second towards the finish. They are next filtered into a 6 or 8-ounce flask through a small filter, the second filtrate coming after, and serving in a manner as wash water for the first[113]. The two washed alumina precipitates are dried and placed in the platinum crucible containing the residue from silica after treatment with hydrofluoric acid. They are then ignited in an oxidising atmosphere at a high temperature for about 10 minutes. The weight, including that of the residue from the silica, is noted as that of "alumina, &c."

The weighed oxides are next fused with bisulphate of potash for some hours. The bisulphate should have been first fused, apart, until the effervescence from the escape of steam has stopped. The melt is dissolved out with cold water and dilute sulphuric acid, and any insoluble residue is filtered off, washed, ignited and weighed. The filtrate is reserved for determinations of iron and t.i.tanium. The residue, after weighing, may be treated with hydrofluoric and sulphuric acids for any silica,[114] which would be determined by loss. It may be tested for barium sulphate by treatment with hot strong sulphuric acid; in which this salt dissolves, but is again insoluble (and so comes out as a white precipitate) on diluting with cold water; the acid also must be cold before adding the water. The filtrate containing the iron is reduced with sulphuretted hydrogen, boiled till free from that gas, filtered and t.i.trated with a standard solution of permanganate of pota.s.sium. The iron found is calculated to ferric oxide by dividing by .7. The iron solution after t.i.tration serves for the determination of t.i.tanium oxide (TiO_{2}). This is done colorimetrically, by adding peroxide of hydrogen free from hydrofluoric acid, and comparing the brown colour produced with that produced by the addition of a standard solution of t.i.tanium to an equal volume of water containing sulphuric acid.[115] The alumina is determined by difference. From the weight of the combined precipitate which has been recorded as "Alumina, &c.,"

deduct (1) the residue, insoluble, after fusion with bisulphate; (2) the ferric oxide; (3) the t.i.tanium oxide; and (4) the phosphoric oxide (P_{2}O_{5}), the amount of which is subsequently determined in a separate portion. This gives the alumina.

~Manganous oxide, &c.~--The filtrate from the "alumina, &c." contained in a 6 or 8-ounce flask, which it nearly fills, is made slightly alkaline with ammonia and treated with a small excess of ammonium sulphide; the flask is then corked and placed on one side for some time (a day or so) so that the manganese sulphide may separate. The precipitate is filtered off and washed with water containing ammonium chloride and a few drops of ammonium sulphide. The filtrate is reserved for lime, &c. The precipitate is digested with sulphuretted hydrogen water, to which one-fifth of its volume of strong hydrochloric acid has been added; this dissolves the sulphides of zinc and manganese; any black residue should be tested for copper and perhaps nickel. The solution is evaporated to dryness, taken up with a little water and treated with a small excess of solution of carbonate of soda. It is boiled and again evaporated, washed out with hot water and filtered on to a small filter, dried, ignited, and weighed as Mn_{3}O_{4}. It is calculated to MnO. It may contain, and should be tested for oxide of zinc, which, if present, must be deducted. If the dish becomes stained during evaporation, take up with a few drops of hydrochloric and sulphurous acids, evaporate, and then treat with carbonate of soda.

~Lime, &c.~--The filtrate from the manganese sulphide is boiled, and without cooling, treated with ammonium oxalate in solution, which also should be heated to boiling. The liquid is filtered off and reserved for magnesia. The precipitate is dissolved in very little hydrochloric acid and reprecipitated by adding ammonium oxalate and ammonia to the boiling solution. The filtrate and was.h.i.+ngs from this are reserved for magnesia.

The precipitate is either dissolved in dilute sulphuric and t.i.trated with permanganate of potash as described under Lime (p. 322); or it is ignited and weighed as oxide. In this last case it may be examined for barium and strontium, the former of which will rarely be present.

~Magnesia.~--The filtrate from the first lime precipitate is treated with sodium phosphate and ammonia, and allowed to stand overnight. It is then filtered. The precipitate is dissolved in hydrochloric acid; the solution is filtered into the beaker containing the solution from the second lime precipitate. Ammonia and sodium phosphate are again added, and the precipitate, after standing, is filtered off, washed with water containing ammonia; it is then dried, ignited and weighed as magnesium pyrophosphate. This is calculated into magnesia.

~Potash and Soda.~--Weigh out .5 gram of the dried ore, and mix with an equal quant.i.ty of ammonic chloride; and to the mixture add gradually 4 grams of calcium carbonate ("precipitated"). Introduce into a platinum-crucible and cover loosely. Heat, at first, gently; and then at a red heat for from forty to sixty minutes. Transfer to a porcelain dish, and digest with 60 or 80 c.c. of water; warm and filter: to the filtrate add ammonic carbonate and ammonia, and filter; evaporate the filtrate to dryness, adding a few drops more of ammonic carbonate towards the end; when dry, heat gently, and then raise the temperature to a little below redness. Dissolve in a small quant.i.ty of water, add a drop of ammonic carbonate, and filter through a small filter into a weighed platinum dish. Evaporate, ignite gently, and weigh. The residue contains the soda and potash of the mineral as chlorides.

To determine the proportion of pota.s.sium, dissolve this residue in a little water, add platinum chloride in excess, evaporate to a paste, extract with alcohol, decant through a small weighed filter, wash with alcohol, and dry at 100 C. Weigh. The substance is pota.s.sium platinic chloride (2KCl.PtCl_{4}). Its weight, multiplied by 0.1941, will give the weight of potash (K_{2}O).

To find the proportion of soda, multiply the weight of the pota.s.sium platinic chloride by 0.306; this gives the weight of pota.s.sium chloride.

Deduct this from the weight of the mixed chlorides first got; the difference will be the sodium chloride, which weight, multiplied by 0.53, will give the weight of soda (Na_{2}O).

~Ferrous Oxide.~--When a qualitative test shows both ferric and ferrous oxide to be present, the proportion of the ferrous oxide must be separately determined. The finely ground mineral mixed with dilute sulphuric acid is treated on a water bath with hydrofluoric acid.

Solution is best effected in an atmosphere of carbonic acid. In about an hour the decomposition is complete, and the solution is diluted with cold water, and t.i.trated with the solution of b.i.+.c.hromate or of permanganate of pota.s.sium. The iron found is multiplied by 1.286, and reported as ferrous oxide. To find the proportion of ferric oxide, the ferrous iron found is multiplied by 1.428, and this is deducted from the weight of ferric oxide obtained by precipitation with ammonia. The ammonia precipitate contains the whole of the iron as ferric oxide; hence the necessity for calculating the ferrous oxide as ferric, and deducting it.

~Phosphoric Oxide (P_{2}O_{5}).~--Weigh up 5 grams of the finely-divided and dry sample, and digest with 10 or 20 c.c. of nitric acid; evaporate to dryness on the water-bath; take up with a little dilute nitric acid; dilute with water; and filter. Add a few grams of ammonic nitrate and 10 c.c. of ammonium molybdate solution, heat nearly to boiling, and allow to settle; filter off, and wash the yellow precipitate. Dissolve with dilute ammonia, add "magnesia mixture," and allow to stand overnight.

Filter, wash with dilute ammonia, dry, ignite, and weigh as pyrophosphate of magnesia. The weight, multiplied by 0.6396, gives the weight of phosphoric oxide.

~Soluble Silica.~--Some silicates are acted on by hydrochloric acid, and leave on evaporation a residue; which, when the soluble salts have been washed out, consists generally of the separated silica with perhaps quartz and unattacked silicates. It should be ignited, weighed and boiled with a solution containing less than 10 per cent. of caustic soda: this dissolves the separated silica. The liquor is diluted, rendered faintly acid, and filtered. The residue is washed, ignited and weighed. The loss gives the soluble silica.

~Estimation of Silica in Slags~ (Ferrous silicates).--Take 1 gram of the powdered slag, treat with aqua regia, evaporate to dryness, extract with hydrochloric acid, filter, dry, ignite, and fuse the ignited residue with "fusion mixture," then separate and weigh the silica in the usual way. Slags are for the most part decomposed by boiling with aqua regia, but it will be found more convenient and accurate to first extract with acids and then to treat the residue as an insoluble silicate.

~Estimation of "Silica and Insoluble Silicates" in an Ore.~--Take 2 grams of the powdered mineral, evaporate with nitric acid (if sulphides are present), treat the dried residue (or the original substance if sulphides are absent) with 10 or 20 c.c. of hydrochloric acid; again evaporate to dryness, take up with dilute hydrochloric acid, filter, wash, ignite, and weigh.

~Estimation of Silicon in Iron.~--Place 2 grams of the metal (borings or filings) in a four-inch evaporating dish, and dissolve (with aid of heat) in 25 c.c. of dilute nitric acid. Evaporate to complete dryness, take up with 20 c.c. of hydrochloric acid, and allow to digest for one hour. Boil down to a small bulk, dilute with a 5 per cent. solution of hydrochloric acid, boil, and filter. Wash with acid and water, dry, ignite in a platinum crucible, and weigh the SiO_{2}. This, multiplied by 0.4673, gives the weight of the silicon. The percentage is calculated in the usual way.

PRACTICAL EXERCISES.

1. A certain rock is a mixture of 70 per cent. of quartz, 25 per cent.

of potash-felspar, and 5 per cent. of potash-mica. What per cent. of silica will it contain?

2. Two grams of a mixture of silica and ca.s.siterite left, after reduction in hydrogen, 1.78 grams. a.s.suming all the oxide of tin to have been reduced, what will be the percentage of silica?

3. The formula of a compound is 2FeO.SiO_{2}. What percentage of silica will it contain?

4. Two grams of a sample of cast-iron gave 0.025 gram of silica. Find the percentage of silicon in the metal.

5. What weights of quartz and marble (CaCO_{3}) would you take to make 30 grams of a slag having the formula CaO.SiO_{2}?

CARBON AND CARBONATES.

Carbon compounds enter so largely into the structure of organised bodies that their chemistry is generally considered apart from that of the other elements under the head of _Organic Chemistry_. Carbon occurs, however, among minerals not only in the oxidised state (as carbonates), but also in the elementary form (as in diamond and graphite), and combined with hydrogen, oxygen, &c. (as in petroleums, bitumens, lignites, shales, and coals). In small quant.i.ties "organic matter" is widely diffused in minerals and rocks. In shales and clays it may amount to as much as 10 or 20 per cent. (mainly as bituminous and coaly matters).

The a.s.sayer has only to take account of the organic matter when it is of commercial importance, so that in a.s.says it is generally included under "loss on ignition."

In coals, shales, lignites, &c., the carbon compounds are, on heating, split up into oils and similar compounds. The products of distillation may be cla.s.sified as water, gas, tars, c.o.ke, and ash. The a.s.say of these bodies generally resolves itself into a distillation, and, in the case of the shales, an examination of the distillates for the useful oils, paraffin, creosote, &c., contained in them.

Elementary carbon is found in nature in three different forms, but these all re-act chemically in the same way. They combine with oxygen to form the dioxide.[116] The weight of oxygen required to burn a given weight of any form of carbon is the same, and the resulting product from all three has the same characteristic properties. Carbon dioxide is the common oxide of carbon. A lower oxide exists, but on burning it is converted into the dioxide. Wherever the oxidation of carbon takes place, if there is sufficient oxygen, carbon dioxide (carbonic acid) is formed; this re-action is the one used for the determination of carbon in bodies generally. The dioxide has acid properties, and combines with lime and other bases forming a series of salts called carbonates.

The carbon-compounds (other than carbonates, which will be subsequently considered) occurring in minerals are generally characterised by their sparing solubility in acids. The diamond is distinguished from other crystals by its hardness, l.u.s.tre, and specific gravity. It may be subjected to a red heat without being apparently affected, but at a higher temperature it slowly burns away. Graphite, also, burns slowly, but at a lower temperature. The other bodies (coals, shales, &c.) differ considerably among themselves in the temperature at which they commence to burn. Some, such as anthracite, burn with little or no flame, but most give off gases, which burn with a luminous flame. They deflagrate when sprinkled on fused nitre, forming carbonate of potash. In making this test the student must remember that sulphur and, in fact, all oxidisable bodies similarly deflagrate, but it is only in the case of carbon compounds that carbonate of potash is formed. Carbon unites with iron and some of the metals to form carbides; combined carbon of this kind is detected by the odour of the carburetted hydrogen evolved when the metal is treated with hydrochloric acid; for example, on dissolving steel in acid.

The natural carbon compounds, although, speaking generally, insoluble in hydrochloric or nitric acids, are more or less attacked by aqua regia.

The a.s.sayer seldom requires these compounds to be in solution. The presence of "organic matter"[117] interferes with most of the reactions which are used for the determination of the metals. Consequently, in such cases, it should be removed by calcination unless it is known that its presence will not interfere. When calcination is not admissible it may be destroyed by heating with strong sulphuric acid and b.i.+.c.hromate or permanganate of potash or by fusion with nitre.

Carbon may be separated from other substances by conversion into carbon dioxide by burning. In most cases substances soluble in acids are first removed, and the insoluble residue dried, weighed, and then calcined or burned in a current of air. The quant.i.ty of "organic matter" may be determined indirectly by the loss the substance undergoes, but it is better to determine the "organic carbon" by confining the calcination in a tube, and collecting and weighing the carbon dioxide formed. Each gram of carbon dioxide is equivalent to 0.2727 gram of carbon.

[Ill.u.s.tration: FIG. 70.]

[Ill.u.s.tration: FIG. 71.]

Instead of a current of oxygen or air, oxide of copper may be more conveniently used. The operation is as follows:--Take a clean and dry piece of combustion tube drawn out and closed at one end, as shown in the figure (fig. 70), and about eighteen inches long. Fit it with a perforated cork connected with a ~U~-tube (containing freshly-fused calcium chloride in coa.r.s.e grains) and a set of potash bulbs (fig. 71) (containing a strong solution of potash), the exit of which last is provided with a small tube containing calcium chloride or a stick of potash. Both the ~U~-tube and bulbs should have a loop of fine wire, by which they may be suspended on the hook of the balance for convenience in weighing. They must both be weighed before the combustion is commenced; to prevent absorption of moisture during weighing, &c., the ends are plugged with pieces of tube and gla.s.s rod.

Fill the combustion tube to a depth of about eight inches with some copper oxide, which has been recently ignited and cooled in a close vessel. Put in the weighed portion for a.s.say and a little fresh copper oxide, and mix in the tube by means of an iron wire shaped at the end after the manner of a corkscrew. Put in some more oxide of copper, and clean the stirrer in it. Close loosely with a plug of recently ignited asbestos, place in the furnace, and connect the ~U~-tube and bulbs in the way shown in the sketch (fig. 72).

[Ill.u.s.tration: FIG. 72.]

See that the joints are tight, and then commence the combustion by lighting the burners nearest the ~U~-tube; make the first three or four inches red hot, and gradually extend the heat backwards the length of the tube, but avoid too rapid a disengagement of gas. When gas ceases to come off, open the pointed end of the tube and draw a current of dried air through the apparatus.

The carbon dioxide is absorbed in the potash bulbs, and their increase in weight multiplied 0.2727 gives the amount of carbon in the substance taken.

The increase in weight in the calcium chloride tube will be due to the water formed by the oxidation of the combined hydrogen. If this last is required the increase in weight multiplied by 0.111 gives its amount.

COALS.

The determination of the actual carbon in coals and shales is seldom called for; if required, it would be performed in the way just described.[118] The ordinary a.s.say of a sample of coal involves the following determinations--moisture, volatile matter, fixed carbon, ash, and sulphur. These are thus carried out:--

~Determination of Moisture.~--Take 3 grams of the powdered sample and dry in a water-bath for an hour or so. The loss is reported as moisture.

Coals carry from 1 to 2 per cent. If the drying is carried too far, coals gain a little in weight owing to oxidation, so that it is not advisable to extend it over more than one or two hours.

~Determination of Volatile Matter.~--This determination is an approximate one, and it is only when working under the same conditions with regard to time, amount of coal taken, and degree of heat used, that concordant results can be arrived at. It is a matter of importance whether the coal has been previously dried before heating or not, since a difference of 2 per cent. may be got by working on the dried or undried sample. Take 2 grams of the powdered, but undried, sample of coal, place in a weighed platinum crucible, and support this over a good Bunsen burner by means of a thin platinum-wire triangle. The heat is continued until no further quant.i.ty of gas comes off and burns at the mouth. This takes only a few minutes. The cover is tightly fitted on, and when cold the crucible is weighed. The loss in weight, after deducting the moisture, gives the "volatile matter," and the residue consists of "fixed carbon" and "ash."

~Determination of Ash.~--The c.o.ke produced in the last operation is turned out into a porcelain dish and ignited over a Bunsen burner till the residue is free from particles of carbon. Calcination is hastened by stirring with a platinum wire. The operation may be done in a m.u.f.fle, but this gives results a few tenths of a per cent. too low. The dish is cooled in a dessicator, and weighed. The increase in weight gives the amount of "ash," and the difference between this and the weight of the c.o.ke gives the "fixed carbon."

The a.s.say is reported as follows:--

Moisture at 100 C. ---- per cent.

Volatile matter ---- "

Fixed carbon ---- "