A Text-book of Assaying: For the Use of Those Connected with Mines Part 17

_Sifted Portion._--Weight 399 grams.

20 grams of it yielded 0.105 gram of silver.

.'. Produce = 0.525 per cent.

Produce multiplied by weight (0.525 399) is ~209.475~.

Add together; and divide by 400, the weight of the whole sample--

84.2 209.475 ------- 400) 293.675 (0.7342

0.7342 is the total produce of the ore in per cents.

Referring to the 100-gram column in the table we find 239.84 ounces to the ton as the produce.

0.7 = 228.67 0.03 = 9.80 0.004 = 1.31 0.0002 = 0.06 ------ 239.84

Comparing this with the result calculated by the first method--viz., 240.26, we see that that was 0.38 oz., or between 7 and 8 dwts. too high.

With ores containing "metallics" it is of great importance to powder the whole of the selected sample without loss during the process; and of even greater importance to well mix the sifted portion, of which the last portions to come through the sieve are apt to be more than ordinarily rich through the grinding down of some portions of the metallic prills.

~Remarks on Cupellation.~--Cupellation is at once the neatest and the most important of the dry methods of a.s.saying. Its purpose is to remove easily oxidisable metals, such as lead and copper, from silver and gold, which are oxidisable with difficulty. Metals of the first cla.s.s are often spoken of as _base_, and gold and silver as _n.o.ble_ metals.

When lead is exposed to the action of air at a temperature a little above redness, it combines with the oxygen of the air to form litharge, an oxide of lead, which at the temperature of its formation is a _liquid_. Consequently, if the lead rests on a porous support, which allows the fused litharge to drain away as fast as it is formed, a fresh surface of the lead will be continually exposed to the action of the air, and the operation goes on until the whole of the lead has been removed. Silver or gold exposed to similar treatment does not oxidise, but retains its metallic condition; so that an alloy of lead and silver similarly treated would yield its lead as oxide, which would sink into the support, while the silver would remain as a b.u.t.ton of metal.

The porous support, which is called _a cupel_(fig. 5), should absorb the slag (oxide of lead, etc.) just as a sponge absorbs water, but must be sufficiently fine-grained to be impervious to the molten metal. At first sight it appears difficult to filter, as it were, a fluid slag from a fluid metal; but an ordinary filter-paper damped with oil will allow oils to run through and yet retain the water; but damped with water it will allow water to run through and retain oils. Similarly, fused slags damp and filter through a cupel, but the molten metal not damping it withdraws itself into a b.u.t.ton, which is retained. Although, of course, if the cupel is very coa.r.s.e-grained the metal may sink into the hollows.

Copper, antimony, tin, and most other metals, form powdery oxides, which are not of themselves easily fusible, and it is necessary when these are present to add some solvent or flux to render the oxide sufficiently fluid. Fortunately, oxide of lead is sufficient for the purpose; hence, mixed oxides of copper and lead, provided the lead is present in proper proportion, form a fluid slag. In separating copper from silver or gold, advantage is taken of this fact; for, although we cannot cupel an alloy of copper and silver, it is easy to cupel an alloy of copper, silver and lead. If, however, the lead is not present in sufficient quant.i.ty, the whole of the copper will not be removed, and the b.u.t.ton of silver, still retaining copper, will be found embedded in a coating of black oxide of copper. Copper oxidises less easily than lead does; and, consequently, the alloy which is being cupelled becomes relatively richer in copper as the operation proceeds. It is on this account that the ill-effects of the copper make themselves felt at the close of the operation, and that the oxide of copper is found acc.u.mulated around the b.u.t.ton of silver.

Tin and antimony, on the other hand, are more easily oxidised; and the tendency of their oxides to thicken the slag makes itself felt at the commencement: if the b.u.t.ton of alloy once frees itself from the ring or crust of unfused oxide first formed, the cupellation proceeds quietly, and leaves a clean b.u.t.ton of silver in the centre. But in either case the cupellation is imperfect, and should be repeated with a larger proportion of lead. An unfused and, consequently, unabsorbed slag tends to retain small b.u.t.tons of alloy or metal, and thus cause serious loss.

There is a principle underlying many of the phenomena of dry silver a.s.saying which the student should endeavour to understand; and which serves to emphasise and explain some facts which without an explanation may present difficulties. If a b.u.t.ton of melted lead be covered with a layer of slag rich in oxide of lead, and a second metal be added, this other metal distributes itself between the metal and slag in proportions which depend mainly upon the ease with which it is oxidised, and to a large extent upon the relative quant.i.ties of material present. Easily oxidisable metals such as zinc, iron, antimony and tin, will go mainly into the slag, and, if the proportion of the slag is large, very little will go into the metal. On the other hand, with metals oxidisable with difficulty, such as silver, gold, and platinum, the reverse holds true; nearly the whole of the metals will go into the lead, and very little into the slag. If, however, the slag be very rich, say in antimony, the lead will contain antimony; and, on the other hand, if the lead be very rich in silver, the slag will contain silver in appreciable quant.i.ty.

Copper, which is near lead in the facility with which it is oxidised, will serve for the purpose of a detailed example. The results of actual a.n.a.lyses of metal and slag formed in contact with each other are shown in the following table:--

---------------------------+-------------------------- Percentage Composition | Percentage Composition of the Metal. | of the Slag.

----------+----------------+-----------+-------------- Lead. | Copper. | Lead. | Copper.

6.8 | 93.2 | 71.4 | 21.4 20.0 | 80.0 | 78.0 | 17.0 28.0 | 72.0 | 80.0 | 12.5 32.0 | 68.0 | 86.0 | 6.7 85.0 | 15.0 | 90.0 | 3.6 ----------+----------------+-----------+--------------

It will be seen from this table that the slag is always much richer in lead and poorer in copper than the metal with which it is in contact.

The ratio of lead to copper in these five samples is:--

In the Metal. In the Slag.

1 : 14 1 : 0.3 1 : 4 1 : 0.2 1 : 2.5 1 : 0.16 1 : 2 1 : 0.08 1 : 0.16 1 : 0.04

a.s.suming these figures to be correct, the following statement is approximately true. On oxidising an alloy of 10 grams of copper and 10 grams of lead, and pouring off the slag when 3 grams of lead have gone into it, there will be a loss of (owing to the slag carrying it off) about 0.2 gram of copper. On repeating the operation, the next 3 grams of lead will carry with them about 0.5 gram of copper; and on again repeating, 3 grams of lead will remove 0.8 gram of copper. Finally, the last gram of lead will carry with it 0.3 gram of copper, and there will be left a b.u.t.ton of copper weighing 8.3 grams. The slag will have carried off altogether 1.7 gram of copper, which is 17 per cent. of the metal originally present.

With the more perfect exposure to the air, and quicker removal of the slag, which results from heating on a cupel, the loss would be heavier.

Karsten got by actual experiment on cupelling copper and lead in equal proportions, a loss of 21.25 per cent.

Going back to the example: if the slag were collected and fused with a suitable reducing agent so as to convert, say, half of it into metal, that half would contain nearly the whole of the copper (such a reduction is called "cleaning the slag"). On reoxidising this metal, another b.u.t.ton of copper is formed which, added to the first, would reduce the loss from 17 per cent. to, say, 7 or 8 per cent. And it is conceivable that by a series of similar operations, almost the whole of the 10 grams of copper originally taken might be recovered. In practice the problem is (as far as the copper is concerned) not how to save, but how most easily to remove it; and since the removal of this metal is quicker from an alloy containing not too much lead, it is evident that two or three operations with small quant.i.ties of lead will be more effectual than a single treatment with a larger quant.i.ty. With those metals (tin, antimony, &c.) which pa.s.s quickly into the slag, the contrary is true; hence with these it is necessary to have enough lead present, so that the slag formed at the outset shall contain enough oxide of lead to make it fluid. As silver is so much less easily oxidised than copper, we should reasonably expect that the proportion of silver carried off in the oxide of lead would be considerably less than that of the copper indicated in the above example. Indeed, there are one or two facts which tend to encourage the hope that the operation may be conducted without any loss. If a piece of pure silver foil is exposed on a cupel to air at the usual temperature of cupellation, it undergoes very little change; it does not even fuse; it loses nothing in weight, and does not oxidise. In fact, even if oxide of silver were formed under these conditions, it could not continue to exist, for it is decomposed into silver and oxygen at a temperature considerably below redness. On the other hand, oxide of silver is not reduced to metal by heat alone, when mixed with an excess of oxide of lead; while metallic silver is converted into oxide when heated with the higher oxides of lead, copper, and some other metals. That silver, and even gold (which is more difficult to oxidise than silver), may be carried off in the slag in this way, is in agreement with general experience. If 10 grams of silver are cupelled with 10 grams of lead, there will be a loss of about 50 milligrams of silver, which is in round numbers 1-30th of the corresponding copper loss; with 10 grams of gold and 10 grams of lead, the loss will be 4 or 5 milligrams, which is about 1-12th of the corresponding silver loss.

~Determination of Silver in a.s.say Lead.~--Scorify 50 grams of the lead with 0.5 gram of powdered quartz or gla.s.s at not too high a temperature.

When the eye has "closed in," pour; reject the slag, and cupel the b.u.t.ton of lead. Remove the cupel from the m.u.f.fle immediately the operation is finished. Weigh, and make a prominent note of the result in the a.s.say book, as so many milligrams of silver contained in 100 grams of lead.

~Determination of Silver in Red Lead or Litharge.~--Fuse 100 grams of the oxide with from 10 to 20 grams of borax; and in the case of litharge with 2 grams or with red lead 4 grams of flour. Cupel the lead, and weigh the b.u.t.ton of silver. Note the result as in the last case.

~Determination of Silver in Argentiferous Lead.~--Be careful in taking the sample, since with rich silver lead alloys the error from bad sampling may amount to several parts per cent. Cupel two lots of 20 grams each, and weigh the b.u.t.tons of silver. Add to these the estimated cupel loss, and calculate the result. Or wrap each b.u.t.ton of silver in 20 grams of a.s.say lead, and re-cupel side by side with two fresh lots of 20 grams each of the alloy. Calculate the loss incurred, and add on to the weight of the two fresh b.u.t.tons got.

~Determination of Silver in Bullion.~--The remarks made under the last heading as to the importance of correct sampling apply with equal force here. Make a preliminary a.s.say by cupelling 0.1 gram of the alloy with 1 gram of a.s.say lead; calculate the percentage composition. Refer to the table on page 105 to find what weight of lead is required for cupelling 1 gram of alloy.

Weigh out four lots of 1 gram each, and wrap them in the required quant.i.ty of lead. Make two check pieces by weighing up two lots of fine silver equal to that which you believe to be present in the a.s.say pieces; add copper to make up the weight to 1 gram, and wrap in the same quant.i.ty of lead as was used for the a.s.says.

[Ill.u.s.tration: FIG. 43.]

Prepare six cupels and charge them in the annexed order (fig. 43), and cupel. Guard against spirting. Clean and weigh the b.u.t.tons of silver.

Add the mean loss on the two check pieces to the mean weight of the four a.s.say pieces; this multiplied by 1000 will give the degree of fineness.

~Determination of Silver in Copper.~--The silver is best separated in the wet way before cupelling, but if the proportion is not too small, it can be found by cupellation. Weigh up 3 grams of the metal, wrap in 30 grams of sheet lead, and cupel; when the cupellation has proceeded for fifteen minutes, add 20 grams more lead, and continue till finished.

Weigh the b.u.t.ton of silver.

The cupellation loss will be five or six per cent. of the silver present. Determine it by powdering the saturated portion of the cupel and fusing in a large Cornish crucible with 30 grams each of soda and borax, 10 grams of fluor spar, and 1-1/2 gram of charcoal. Cupel the resulting b.u.t.ton of lead, and add 10 grams more of lead towards the close of the operation. Deduct the weight of silver contained in the lead used from the weight of the two b.u.t.tons, and calculate to ounces to the ton.

In an experiment in which 0.1975 gram of silver was present, the weight of the b.u.t.ton from the first cupellation was 0.1867, and that of the b.u.t.ton from the second, after correcting for the lead added, was 0.0110 gram.

~Determination of Silver in Galena.~ _By Pot a.s.say._--Mix 20 grams of the powdered ore with 30 grams of red lead, 20 grams of soda, and 5 grams of borax, as also with from 7 to 10 grams of nitre. Fuse and pour.

Clean the slag if the ore is rich. Cupel the b.u.t.tons of lead. Make the usual corrections and calculate in ounces to the ton.

_By Scorification._--Take 10 grams of the ore, 30 grams of lead, and 0.5 gram of borax. Scorify, clean the slag by adding anthracite after the "eye" has closed in: cupel the b.u.t.ton of lead. Weigh the b.u.t.ton of silver, make the necessary corrections, and calculate to ounces to the ton.

The determination may also be made by cupelling the b.u.t.ton of lead got in the dry lead a.s.say.

A sample of galena determined by the three methods gave the following results:--

By pot a.s.say 7.18 ozs. per ton.

" scorification 7.02 "

" lead a.s.say 6.72 "

~Determination of Silver in an Ore.~ _By Pot a.s.say._--Take 20 grams of the powdered ore and mix with 30 grams of soda, 40 grams of red lead, and 5 grams of borax, as also with from 2 to 3 grams of flour. Fuse: pour. Clean the slag by fusing with 20 grams of red lead and two grams of flour. Cupel the b.u.t.tons of lead; weigh; make the necessary corrections, and calculate to ounces to the ton.

_By Scorification._--Take 5 grams of the powdered ore, 50 grams of lead, and 0.5 gram of "soda" or borax. Scorify. Clean the slag by fusing in a crucible as in the pot a.s.say. Cupel, &c.

_Examples._--_By Pot a.s.say._--Ore taken 20 grams.

Silver got 0.2893 gram Silver from slag 0.0060 "

Silver lost in cupellation 0.0100 "

------ 0.3053 "

Deduct silver in red lead 0.0017 "

------ Silver in ore 0.3036 " = 495.9 ozs. per ton.

_By Scorification._--Ore taken, 3 grams.

Silver got. 0.0425 gram Silver from slag 0.0022 "

Silver lost in cupellation 0.0020 "

------ 0.0467 "