CHAPTER V.

PRELIMINARY EXAMINATION OF ORES.

By this time the student should be competent in general qualitative analysis, and he will now be able to apply his knowledge where necessary and the student who has advanced so far in mineralogy as to be able to distinguish some fifty of the commoner minerals will have little difficulty in obtaining a fair idea as to the contents of any ore.

Generally, it is not necessary that a preliminary examination be exhaustive, the presence or absence of some few elements or compounds being all that is sought for. Such preliminary examination may be carried out

by

(1) Inspection alone.

(2) Dry tests alone, or aided by inspection.

(3) Wet Tests alone, or aided by inspection and dry tests.

(1) Inspection alone. The experienced assayer or the mineralogical student will in many cases require no other aid than mere inspection, accompanied, if needful, by the usual physical tests of hardness, specific gravity, etc. etc. Quartz, galena, iron and copper pyrites, and many other minerals may be determined in this way. Further, by experience a shrewd estimation can generally be made of the percentage present of substances such as iron pyrites, and the experienced assayer need not conduct preliminary trials as to the oxidising or reducing nature of ores. He will judge quantitatively by inspection the approximate percentage of the various minerals present and flux his ores accordingly. The student, however, is recommended at the outset to rather distrust his mineralogical knowledge and make absolutely sure of such points.

(2) Dry Tests.-As the result of inspection the student may still be in doubt as to the composition of certain minerals present, and it frequently happens that the application of a few dry (blowpipe) tests will quickly and finally decide the doubtful point. The details of these tests have been given and need not be repeated, though here the student will require considerable skill in manipulation and in drawing conclusions from his tests, owing to the probable presence in many cases of conflicting substances. such difficulties arise the student is referred to Moses and Parsons, Eudlich, Cornwall, or other authorities on blowpipe work.

If

By the aid of such tests combined with inspection the student should in many cases be able to determine definitely the presence or otherwise of elements such as sulphur, arsenic, antimony, copper, lead, iron, etc. etc. (3) Wet Tests. These in this case need only be applied when the previous tests fail or only give partial satisfaction. In the case of certain alloys and complex ores the student's skill in dry reactions may fail him;

in such cases he must depend on wet methods, his knowledge of which should now be sufficient to solve any problem of ordinary difficulty coming within the scope of the assayer's work.

The student, then, on receiving an ore for assay, having labelled it, should if possible obtain at least some of it in lump form. If this be impossible he should carefully examine the powdered ore with a strong lens. If there be sufficient ore it will be instructive to 'pan off' a few pounds weight and examine the concentrates (see Mechanical Assays). The magnet may be applied to the powdered ore to detect any magnetic particles. Having noted any conclusions drawn from inspection, he will next proceed to dry tests, and then to wet if necessary.

In certain cases a modification of the usual methods of qualitative separations may be adopted, with the result that sufficient accuracy is attained with a considerable saving of time, a point of much importance to the assayer. The following table will be found convenient if the student wish to ascertain, for example, the impurities in a lead button or a sample of base bullion.

TABLE FOR ALLOYS AND BASE BULLION.

Suspected to contain (besides lead) antimony, tin, arsenic, copper, zinc, etc.

Method.-Place in a 100 c.c. conical beaker or flask about 5 gm. fine powder (if brittle) or fine shavings (if malleable) of the base bullion. Carefully add 10 c.c. of strong HNO, and 5 c.c. of strong H,SO. Gradually heat to boiling. Dilute, filter, and wash.

The scheme given is simply a modification of the methods previously employed in the qualitative analysis of alloys, and may be varied by the intelligent student as circumstances demand. The student will frequently find in practice that he cannot rigidly adhere to the schemes of text-books in every particular case; and in modifying such schemes to suit technical requirements, where accuracy and speed are necessary, it is absolutely essential that he should have a thorough grounding in theoretical and practical chemistry. Without this he

can never be more than a mere 'rule of thumb' assayer or analyst. It is essential that the student be acquainted with the accuracy of a reaction, and the effect of the presence of interfering elements on the reaction. Knowing these, he can proceed with some confidence to devise methods for particular cases. In quantitative analysis he should continually endeavour to ascertain the degree of accuracy of the methods employed. Certain methods of great accuracy he will find require a great expenditure of time; and in technical analysis, where results are required quickly, accuracy may to some extent be sacrificed in favour of speed. Where this is done, the degree of accuracy or value of the results must be carefully ascertained, and in such researches a thorough knowledge of qualitative analysis is the best possible basis from which an attack may be made on these and other problems in assaying and metallurgical analysis.

CHAPTER VI.

INTRODUCTORY EXPERIMENTAL WORK.

THE EFFECT OF VARYING FURNACE TEMPERATURES.

(a) The Wind or Melting Furnace.

Principle. According to the equation 2PbO+C= Pb,+ CO2, 446 grains of litharge fused with excess of charcoal should yield 414 grains of metallic lead. If the temperature be too high, PbO volatilises, and this loss, which varies with the temperature and air present, can be checked by weighing the button from a given quantity of litharge.

Apparatus.-Wind furnace, tongs, moulds, crucibles (C) and lids, pulp scales, weights, etc.

Reagents.-Litharge and finely ground charcoal.

Details of Operation.-Prepare a low red fire in the furnace. Adjust the pulp scales. Take 6 pieces of glazed paper, each about 25 c.m. x 30 c.m., and number them 1 to 6. Weigh out 6 charges, each consisting of 10 gm. (or 100 grains) powdered litharge. Place one charge on each of the 6 pieces of paper.

Weigh out 6 charges, each consisting of 5 gm. (8 grains) of finely powdered charcoal, placing one on each of the 6 pieces of paper. Carefully and thoroughly mix each of the 6 combined charges. This may be done either by transfer to a wedgwood mortar or by means of a spatula on the paper. The furnace will by this time require attention, and should be restoked to obtain a dull red heat.

Transfer charges 1 and 2 each to a crucible (Battersea round). On top of each charge, after transference to the crucible, add a cover of 2 gms. (20 grains) charcoal. Place the two crucibles in the furnace, sinking them till the coke is level with their top edges. This may be done either by building the coke up round the crucibles if the fire be low, or by making a cavity by aid of the tongs and inserting the crucible in the cavity; or, better still, two old crucibles (dummies) are built into the coke at the outset, and on removing them the crucibles containing the assays can be introduced into the cavities thus obtained. Place a lid on each crucible. Close the furnace door. Examine every few minutes till the crucibles become dull red, then allow them to remain in the furnace at this temperature for twenty minutes.

Meanwhile, clean two moulds, heat them till uncomfortable to the touch, oil or rub with graphite their interior surfaces. Remove each crucible in turn, and carefully pour into its respective mould. When cool (after five minutes or so), carefully remove the lead from each mould and weigh, taking care to collect any small prills of metal. Enter the results, with notes of the quantities of litharge, charcoal, time and heat of fusion, in your laboratory note-book.

Calculation of Results.-As 446 grams pure litharge should yield 414 grams 10 × 446 lead, 10 gms. litharge should yield gms. lead= 9.28 gms. lead. The 414 student will probably obtain buttons weighing about 8 gms., the remainder being lost in fusion. The results, though inaccurate, will be compared with those from the remaining tests. The student must remember that the litharge is rarely pure. Now transfer charges 3 and 4 to crucibles, and insert the charged crucibles in the furnace in the cavities left by 1 and 2. Urge the fire, and when the temperature becomes bright red, allow the assays to remain at that temperature for twenty minutes. Remove, pour, and collect the lead as before. Enter the results in your note-book.

Now proceed with charges 5 and 6, urging the fire to its utmost to produce a white heat. Allow the charged crucibles to remain at this temperature for 20 minutes. Remove, pour, and note the results as before.

The student will find some difficulty at first in regulating the temperature. If he has carefully followed the instructions given, he should obtain three pairs of buttons. The buttons of each pair should not differ more than a few grains from each other. The weights of each successive pair of buttons should diminish as the heat increases after a red heat has been obtained. (Probably the middle pair will give the highest results.)

In reducing lead from litharge by means of charcoal a certain temperature is required before the reaction will take place. Any considerable increase above this temperature can only lead to loss by volatilisation of the litharge. Also, at this certain temperature,' a certain time is required to reduce all the lead from a given quantity of litharge. The temperature being constant, the time depends largely on the intimacy of mixture of the charcoal and litharge; this, again, depends on the degree of fineness to which the two substances are ground. As a general rule, the finer the grinding and mixture the better the results. When once this necessary period of heating has elapsed, any further heating can only lead to loss, the lead being liable to reoxidation, though with the excess of charcoal present this loss must be slight.

So far the student has kept the time constant, and has varied the temperature. Time will now be considered.

The Effect of Variations in Time.

Weigh out 6 charges as before.

Place 1 and 2 in the furnace and keep at a bright red heat for five minutes. Remove, pour, collect and weigh buttons. Note results.

Place 3 and 4 in the furnace at the same temperature, but for twenty minutes. Treat as before, noting results.

Place 5 and 6 in the furnace at the same temperature, but for sixty minutes. Again note results.

These results will give the student some idea of the influence of time on an assay. Besides the losses mentioned, others, such as the formation of silicate of lead, occur. This loss may to some extent be avoided by coating the interior of the crucible with graphite. When considering the fire-assay of lead ores the student will see that certain precautions are taken to prevent the formation of silicates of lead. He will see that to obtain the least possible results other charges than that just given are adopted. The experiments just described are given not for the purpose of estimating the lead in litharge, but to show the student the effects of varying temperature and time on an assay.