A Text-book of Assaying: For the Use of Those Connected with Mines Part 19

The sulphocyanate solution is standardised by placing 50 c.c. of the silver nitrate solution in a flask with 20 c.c. of dilute nitric acid, diluting to 100 c.c. with water, and running in the sulphocyanate until the greater part of the silver is precipitated; then adding 2 c.c. of the ferric indicator, and continuing the t.i.tration until a reddish-brown colour is developed, and remains permanent after shaking continuously.

The a.s.say is similarly performed, the silver being used in the state of a nitric acid solution.

The effect of variations in the conditions of the a.s.say may be seen from the following experiments, in which 20 c.c. of standard silver nitrate were used:--

~Effect of Varying Temperature~:--

Temperature 10 C. 30 C. 70 C. 100 C.

Sulphocyanate reqd. 19.6 c.c. 19.3 c.c. 19.0 c.c. 18.6 c.c.

~Effect of Varying Nitric Acid~:--Varying nitric acid has no effect, except that with a fairly acid solution the finishing point is somewhat sharper.

Nitric acid added 5 c.c. 10 c.c. 20 c.c. 50 c.c.

Sulphocyanate reqd. 19.6 c.c. 19.5 c.c. 19.6 c.c. 19.6 c.c.

~Effect of Varying Bulk~:--

Bulk 50 c.c. 100 c.c. 200 c.c. 300 c.c.

Sulphocyanate reqd. 19.5 c.c. 19.6 c.c. 19.6 c.c. 19.7 c.c.

~Effect of Varying Ammonic Nitrate~:--

Ammonic nitrate 0 gram 1 gram 5 grams 10 grams Sulphocyanate reqd. 19.6 c.c. 19.6 c.c. 19.7 c.c. 19.9 c.c.

~Effect of Varying Silver~:--

Silver added 1 c.c. 10 c.c. 20 c.c. 50 c.c. 100 c.c.

Sulphocyanate reqd. 1.0 c.c. 9.70 c.c. 19.6 c.c. 49.4 c.c. 99.0 c.c.

This method is valuable for determining silver in salts, alloys, and solutions, where no more than an ordinary degree of accuracy is demanded. It is easy, and applicable under most of the usual conditions.

Its greatest disadvantage is the brown coloration produced by the sulphocyanate when the a.s.say is nearly, but not quite, finished; and the slowness with which this is removed on shaking up with the precipitate.

This is worse with large quant.i.ties of precipitate, and if about 1 gram of silver is present, it gives an indefiniteness to the finish which lowers the precision of the process to about 1 in 500; this is useless for the a.s.says of bullion. One writer states that this inconvenience is due to portions of liquid being entangled in the precipitate, but it appears much more likely to be due to the action of the precipitate itself. In attempting to apply the process to the a.s.say of bullion by working it on the principle of a Gay-Lussac a.s.say, it was found that a very considerable excess of silver was required to complete the reaction. In these experiments 100 c.c. of "sulphocyanate" (very accurately measured) was run into the solution containing the weighed portion of bullion (fine silver) and, after shaking the solution, was filtered. In the filtrate the remaining silver, if there should be any, was determined by the ordinary t.i.tration, but with "sulphocyanate" of one-tenth the strength. This final t.i.tration was quite satisfactory. The amount of silver precipitated by the first 100 c.c., however, varied with the quant.i.ty of silver present as in the following series.[17]

Silver present. Silver precipitated.

1.1342 gram. 1.1322 gram.

1.1375 " 1.1335 "

1.1405 " 1.1351 "

1.1484 " 1.1379 "

These, of course, preclude a method of the kind aimed at, and at the same time emphasise the importance of uniformity of work in the ordinary process. In the determination of chlorides in sea-water, Dittmar used a combined method: precipitating the bulk of the silver as chloride, and after filtering, determining the small excess of silver by sulphocyanate. This modification answers admirably when applied to the a.s.say of bullion. In the ordinary Gay-Lussac method, the precipitation of the bulk of the silver by the 100 c.c. of salt solution leaves nothing to be desired, either as to ease in working or accuracy of result; the silver precipitate settles quickly, and leaves a clear liquor admirably fitted for the determination of the few milligrams of silver remaining in solution. But the method of determining this residual silver by adding successive small quant.i.ties of salt so long as they continue to give a precipitate is unsatisfactory, and, judged on its own merits apart from the rest of the process, could hardly escape condemnation. It is clumsy in practice, for the continued adding of small portions of salt solution is laborious and becomes impossible with more than a few milligrams of silver in solution. The proposed modification is simple; having precipitated the silver with the 100 c.c.

of salt solution, as described under Gay-Lussac's method (page 120), shake till the liquor clears, and filter into a flask, washing with a little distilled water. Add 2 c.c. of "ferric indicator" to the filtrate and t.i.trate with a standard "sulphocyanate solution" made by diluting the ordinary standard solution to such an extent that 100 c.c. after diluting shall be equivalent to 0.1 gram of silver.[18] Calculate the weight of silver found by "sulphocyanate" and add it to the weight which 100 c.c. of the salt solution will precipitate.

An advantage of this modification is that an excess of 15 milligrams may be determined as easily and exactly as 5. In standardising the salt solution, then, weigh up, say 1.0150 gram of pure silver, dissolve and t.i.trate. Suppose 13.5 c.c. of "sulphocyanate" required; then these are equivalent to .0135 gram of silver, (100 c.c. = .1); the silver precipitated by the salt is 1.0150-.0135--_i.e._, 1.0015 gram, which is the standard.

~Application of the Method to a.s.says for a.r.s.enic.~--If silver nitrate be added to a neutral solution of an a.r.s.enate of one of the alkali metals, silver a.r.s.enate (Ag_{3}AsO_{4}), is thrown down as a dark-red precipitate. If, after adding excess of silver nitrate to insure a complete precipitation, the a.r.s.enate of silver be filtered off, the weight of the a.r.s.enic could be estimated from the weight of silver a.r.s.enate formed. But this may be done much more conveniently by dissolving the precipitate in nitric acid, and t.i.trating with sulphocyanate; the silver found will be to the a.r.s.enic present as 324 (1083) is to 75.

The mineral is best treated by the method given in the third paragraph on page 382; but the solution, after being acidified with nitric acid, should be made exactly neutral with ammonia. A small excess of silver nitrate should then be added, and since acid is liberated in the reaction, the liquor must again be neutralised.[19] The precipitate must then be filtered off, and washed with distilled water. Then dissolve it in the paper by slowly running over it 20 c.c. of dilute nitric acid.

Wash the filter with distilled water, collecting with the filtrate in a small flask. Add 2 c.c. of "ferric indicator" and t.i.trate.

If the sulphocyanate solution be made up with 11 or 12 grams of the pota.s.sium salt to the litre, and be then standardised and diluted, so that for 100 c.c. it shall equal 1.08 gram of silver, (see p. 38), then it will also equal .25 gram of a.r.s.enic (As). Except for ores rich in a.r.s.enic, it will be better to work with a solution one half this strength. The standard as calculated from an experiment with pure silver should be checked by another using pure resublimed white a.r.s.enic, As_{2}O_{3}, which contains 75.75 % of the metal. The quant.i.ty of white a.r.s.enic taken, .1 or .2 gram, should contain about as much a.r.s.enic as will be present in the a.s.says. It is converted into sodium a.r.s.enate by evaporating to a small bulk with nitric acid and neutralising with soda.

The precipitation and t.i.tration of the silver a.r.s.enate should be exactly as in the a.s.says.

The difficulty of the method is in the neutralising; which has to be very carefully done since silver a.r.s.enate is soluble in even faintly acid solutions; one drop of nitric acid in 100 c.c. of water is enough to produce an absolutely worthless result; and an excess of acid much less than this is still very prejudicial. The addition of a little sodium acetate to the solution after the final neutralising has a good effect.

~a.r.s.enic in Mispickel.~--Weigh up .250 gram of the finely-powdered ore, and place in a Berlin crucible about 1-1/4 or 1-1/2 inch in diameter.

Treat with 10 or 12 drops, one drop at a time, of strong nitric acid, warm very gently, but avoid much heating. Put on a thin layer of nitre, and rather more than half fill the crucible with a mixture of equal parts of soda and nitre. Heat quickly in the blow-pipe flame, and when the ma.s.s is fused and effervescing, withdraw and allow to cool. Boil out with water, filter and wash. Insert a piece of litmus paper and cautiously neutralise with nitric acid, using ammonia to neutralise any accidental excess of the acid. Add a gram or so of ammonium nitrate and silver nitrate in excess, neutralise again with ammonia and add two or three grams of sodium acetate. Filter off the precipitate, wash and t.i.trate. In the fusion care should be taken to avoid much effervescence (an excess of the soda mitigates this) and the operation should be stopped as soon as the whole has entered into fusion.

COLORIMETRIC DETERMINATION.

There is, properly speaking, no colorimetric method, but the following, which is sometimes used, is based on similar principles. It is useful for the determination of small quant.i.ties of silver in substances which yield clear solutions with nitric acid.

Dissolve a weighed quant.i.ty of the substance in nitric acid, and dilute to a definite bulk. Divide into two equal parts. To one, add a drop or two of dilute hydrochloric acid, stir and filter. To the other, add a similar amount of dilute acid, and then to the filtered portion run in from a burette standard silver nitrate (1 c.c. = 0.5 milligram silver) until the solutions are equally turbid. Calculate in the usual way.

GOLD.

Gold occurs in nature chiefly as metal. It always contains more or less silver, and, in alluvial sands, &c., may be a.s.sociated with platinum and iridium.

Gold is insoluble in hydrochloric or nitric acid, but is dissolved by aqua regia or by solutions of iodine, bromine, or chlorine. It is taken up by mercury, forming an amalgam, from which the mercury may be driven off by heat.

When gold occurs in particles of any size, it is readily detected by its appearance, but when finely disseminated through a large quant.i.ty of rock, it is separated and detected by the amalgamation a.s.say--described below--or by a process of washing somewhat similar to vanning, or by the following test:--Powder and, if necessary, roast 50 to 100 grams of the ore, put on it three or four crystals of iodine and enough alcohol to cover it; allow to stand for half an hour; a piece of filter paper moistened with the liquid and burnt leaves an ash with a distinctly purple tint if any gold is present. It is better, however, to filter off the solution, evaporate, and ignite. Then, either take up with mercury, and ignite the amalgam so as to get a speck of the metallic gold; or treat with a few drops of aqua regia, and test the solution with stannous chloride: a purple coloration indicates gold.

~AMALGAMATION a.s.sAY.~--This does not attempt to give the total produce of gold, but rather the quant.i.ty which can be extracted on a large scale; therefore it should imitate as closely as possible the process adopted in the mine or district for extracting the metal.

Take 2 lbs of the ore in powder and roast; make into a stiff paste with hot water and rub up for an hour or so with a little mercury. Wash off the sand carefully, and collect the amalgam. Drive off the mercury by heat, and weigh the residual gold. It is best to cupel it with lead before weighing.

In an experiment on a lot of ore which contained 0.189 gram of gold, 0.179 gram was obtained by the above process, equal to about 94-1/2 per cent. recovered. With ores generally, the yield may be from 80 to 90 per cent. of the actual gold present.

DRY a.s.sAY.

The dry a.s.say of gold ores resembles in its main particulars the dry a.s.say for silver by the crucible method; and for much that is of importance in its discussion the student is referred to what is written under Silver on pp. 90-113.

~Size of a.s.say Charges.~--Gold ores rarely contain more than a few ounces, often only a few pennyweights of gold to the ton; consequently, the b.u.t.ton of gold obtainable from such quant.i.ties of ore as may be conveniently worked by a.s.saying methods is often so small as to require more than ordinary care in its manipulation. One milligram of gold forms a b.u.t.ton of about the size of one of the full-stops on this page, and compared with a million similar particles of quartz (about four ounces), represents a produce of a quarter of an ounce to the ton: a proportion such as the a.s.sayer is frequently called on to determine. It is evident, therefore, that a charge of half an ounce or less of the ore, such as is usual with silver ores, would demand of the worker both skill and care in the handling of the minute quant.i.ty of gold to be obtained from it.

Fortunately the work is simple and precise, so that in practised hands and with only a 5-gram charge the a.s.say of a 5-dwt. ore is practicable; with so small a charge, however, the result is barely perceptible on a sensitive balance: the b.u.t.ton of gold should be measured under a microscope. It follows, therefore, that larger charges of say 50, 100, or even 200 grams, have an advantage in that they lessen the strain on the worker's attention, and, except in the case of the poorest mineral, bring the b.u.t.ton of gold within the scope of the balance. On the other hand, the inconvenience of the larger charges lies in the amount of fluxes and consequent size of the crucibles required to flux them.

~Sampling.~--A further consideration in favour of the larger charges is the matter of sampling. In preparing his ore, the student should ask himself what reasonable expectation he has that the portion he puts in the furnace will be of average richness. The larger charges are likely to be nearer than the smaller ones to the average of the parcel of ore from which they are taken. In explanation of this, let us suppose a large heap of 5-dwt. ore, in sand of the coa.r.s.eness of full-stops, and containing all its gold in particles of 1 milligram, as uniformly distributed as care and labour in the mixing can accomplish. Such a heap could not possibly occur in practice, but it will serve for purposes of ill.u.s.tration. Now, one ton of the sand, however taken, would contain appreciably the same quant.i.ty of gold as any other ton. For a ton would contain about 8000 particles of gold; and even if two separate tons differed by as much as 100 particles (which they are just likely to do), this would mean only a difference of 1 or 2 grains to the ton. On the other hand, two portions of 14 lbs., which should contain on the average 50 particles of gold, are likely enough to differ by 10 particles, and this, calculated on a ton, means a difference of 1 dwt. It is easy to see that something like this should be true; for on calculating the 14-lb. lot up to a ton, the deviation from the average, whatever it may be, is multiplied by 160; whereas, if the ton were made up by adding 14-lb. lot to 14-lb. lot, up to the full tale, then a large proportion of the errors (some being in excess and some in defect) would neutralise each other. An average which is practically true when dealing with thousands, and perhaps sufficiently exact with hundreds, would be merely misleading when applied to tens and units. Reasonable safety in sampling, then, is dependent largely on the number of particles of gold in the charge taken, and the risk of an abnormal result is less, the larger the charge taken.

By doubling the charge, however, we merely double the number of particles. Powdering finely is much more effective; for, since the weight of a particle varies as the cube of the diameter, halving the diameter of the particles increases their number eight-fold. If, now, we modify our ill.u.s.tration by a.s.suming the particles to have only one-sixth the diameter of a full-stop (which would represent a powder of a fineness not unusual in ores prepared for a.s.saying), we should multiply the number of particles by 200 (6 6 6 = 216). We should then reasonably expect a 14-lb. parcel of the powder to give as safe a sample as a ton of the sand would give; and portions of a size fit for crucible work, say 50 or 100 grams, would be as safe as 10 or 20-lb. samples of the coa.r.s.er stuff. For example, 60 grams of such powder would contain, for a 5-dwt. ore, about 100 particles; and in the majority of cases the error due to sampling would be less than 10 or 12 grains to the ton, and would only occasionally exceed a pennyweight. With richer ores the actual deviation stated as so much to the ton of ore might be greater, but it would represent a smaller proportion, stated in percentage of the gold actually present, and would ultimately fall within the limits of unavoidable error.

It will be seen that the size of the quartz particles has no direct bearing on the argument; and, in fact, the coa.r.s.eness of the quartz only interferes by preventing the uniform mixing of the sand and by binding together several particles of gold; in this last case, particles so united must, of course, count as one larger particle. Now, there are some natural ores in which the gold particles are all very small; with these fine powdering and mixing yields a product from which a sample may be safely taken. Then, again, in "tailings," before or after treatment with cyanide, we have a similar material, inasmuch as the coa.r.s.er gold has been removed by previous amalgamation. With these, it is not unusual to take the portion for a.s.say without any further powdering, since they are poor in gold, and have already been stamped and pa.s.sed through a sieve of say thirty holes to the inch (linear).

But there are other ores, in lump showing no visible gold, which contain the gold in all possible degrees of fineness, from say prills of a milligram or so down to a most impalpable powder. The treatment of these cannot be so simple and straightforward. Suppose a parcel of 1000 grams (say 2 lbs.) of such ore in fine powder, containing on an average 1 particle of 1 milligram (the presence or absence of which makes a difference of .6 dwt. on the ton), 10 others of about .5 milligram (each representing .3 dwt.), and 100 others, which are too coa.r.s.e to pa.s.s through an 80 sieve, and having an average weight of .1 milligram (each .06 dwt.), and that the rest of the gold, equivalent altogether to 2 ounces to the ton, is so finely divided that a charge of 50 grams may be taken without any considerable risk of its interfering with the sampling. Then in a 50-gram charge there would be one chance in twenty of getting the milligram particle, in which case the result would be 12.35 dwts. too high; on the other hand, if it were not present the result would on this account be .65 dwt. too low. Of the ten .5-milligram particles, it is as likely as not that one will be present, and its presence or absence would cause an error of 3.3 dwts., more or less. Of the 100 particles of .1 milligram, there would probably be from 3 to 7, instead of 5, the proper number; this would mean a variation of 2.6 dwts. from the true proportion. So that the probable result would range about 5 dwts. more or less than the 2-1/2 ozs., which is the true produce, and there are possibilities of astounding results. It is true that the majority of the results would be well within these limits, and now and again the heart of the student would be gladdened by a beautiful concordance in duplicate a.s.says; nevertheless, there can be no reasonable expectation of a good a.s.say, and to work in this way, on a 50-gram charge, would be to court failure. The coa.r.s.e gold must ruin the a.s.say.

The difficulty may be met by concentrating the whole of the coa.r.s.e gold in a small fraction of the ore, by sifting and making a separate a.s.say of this fraction. A portion of the ore, of about 1000 grams, is ground to a very fine powder and pa.s.sed through an 80 sieve, re-grinding when necessary, until only 20 or 30 grams is left of the coa.r.s.er powder. This is mixed with fluxes and carried through as a separate a.s.say. The sifted portion is _thoroughly mixed_, and a portion of it, say 30 or 50 grams, taken for a.s.say. The weights of the two portions must be known, and care must be taken that nothing is lost in the powdering. The method of calculating the mean result from the two a.s.says is shown on page 109. In this way of working there is no advantage in continuing the grinding until the coa.r.s.er fraction is reduced to a gram or so--rather the contrary; and rubbing on until all the gold is sent through the sieve is to be distinctly avoided. The student must bear in mind that what he is aiming at is the exclusion of all coa.r.s.e gold from the portion of ore of which he is going to take only a fraction.

The question of the smaller sampling of gold ores has been dwelt on at considerable length, as befits its importance, in order that the student may be impressed with a sense of its true meaning. Sampling is not a mystery, nor does the art lie in any subtle manner of division. It is, of course, absolutely necessary that the stuff to be sampled shall be well mixed, and the fractions taken, so that each part of the little heap shall contribute its share to the sample. Moreover, it must be remembered that tossing about is a poor sort of mixing, and that everything tending to separate the large from the small, the light from the heavy, or the soft from the hard (as happens in sifting), must be avoided, or, if unavoidable, must be remedied by subsequent mixing.

With a well-taken sample, we may rely on a great majority of our results falling within normal limits of error; but nothing can be more certain than that, in a moderately large experience we shall get, now and again, deviations much more considerable. These erratic a.s.says can only be met by the method of working duplicates, which call attention to the fault by discordant results. Such faulty a.s.says should be repeated in duplicate, so that we may rest the decision on three out of four determinations.