A Text-book of Assaying: For the Use of Those Connected with Mines Part 15

The following are some results obtained which will ill.u.s.trate the rule:--

Red Lead used. Flour used. Lead got.

40 grams 3 grams 25.0 grams 100 " 3 " 13.5 "

80 " 4 " 30.0 "

80 " 5 " 40.0 "

_On the Reducing Effect of Metallic Sulphides, and the Counteracting Effect of Nitre._--The sulphides found in ores will reduce a b.u.t.ton of lead from oxide of lead just as flour does; and, as charcoal, flour and tartar differ in their reducing power, so equal weights of the different mineral sulphides throw down different weights of lead.

One gram of iron pyrites yields about 11 grams of lead. One gram of copper pyrites, blende, fahlerz, or mispickel, yields 7 or 8 grams of lead, whilst 1 gram of antimonite will give 6, and 1 gram of galena only a little over 3 grams. It is evident that if an ore carries much of these sulphides, the quant.i.ty of lead reduced will be very much larger than that required for an a.s.say. To counteract this effect nitre is added; _1 gram is added for each 4 grams of lead in excess of that required_. For example: with a 20-gram charge of an ore containing 50 per cent. of pyrites, if no nitre were added, 110 grams of lead would be got; or, if there was not sufficient oxide of lead to yield this quant.i.ty of metal, the b.u.t.ton would be sulphury. To reduce the weight of the b.u.t.ton by 80 grammes, we should add 20 grams of nitre, if litharge were used; or if red lead were used, we should add 16 grams of nitre, _since the oxidizing effect of 20 grams of red lead is equivalent to that of 1 of nitre_, and since 80 grams of red lead are generally used in a charge. Two a.s.says of an ore of this kind with these quant.i.ties of nitre gave 26.0 grams of lead with litharge, and 22.5 grams with red lead.

It is best to use in these a.s.says 80 grams of red lead, 20 of soda, and 5 of borax, with 20 grams of the ore. If the lead got by the preliminary fusion in a small crucible with litharge (described under "_ores containing much sulphides_") is known, the following table will indicate the quant.i.ty of nitre, or flour, to be added with this charge:--

-------------------------------+-------------------+------------------ Lead got in Preliminary Fusion | Flour to be added | Nitre to be added with 3 grams of Ore. | to the a.s.say. | to the a.s.say.

-------------------------------+-------------------+------------------ 0.0 gram | 3.3 grams | none 3.0 grams | 1.3 gram | -- 6.0 " | none | 4.0 grams 9.0 " | -- | 9.0 "

12.0 " | -- | 14.0 "

15.0 " | -- | 19.0 "

18.0 " | -- | 24.0 "

21.0 " | -- | 29.0 "

If litharge is used in the a.s.say instead of red lead 4 grams more nitre, or 1.5 gram less flour must be used. When more than a few grams of nitre are added to a charge the proportion of "soda" and borax should be increased, because one of the products of the reaction of nitre upon sulphides in the presence of soda is sulphate of soda, and because the "soda" thus used up no longer serves as a flux; more borax should be added, as it is the best flux for the metallic oxides which are formed in the process. If in an a.s.say too large a b.u.t.ton of lead is got, even after this calculation has been made, and the a.s.say is repeated, add 1 gram more nitre for each 4 grams of lead in excess. Sometimes the a.s.say appears tranquil before the nitre has produced its full effect; in such cases it is well to seize the crucible with the tongs and mix its fused contents by rotating them; if this causes an effervescence, the crucible should be replaced in the fire and the fusion continued. The following experiments will ill.u.s.trate the extent to which the above rules may be relied on. In all of them the standard flux was used, viz.:--80 grams of red lead, 20 of soda, and 5 of borax.

_Pyrites_ 5 5 5 5 2.5 5 10 15 20 Quartz -- 20 -- 20 17.5 15 10 5 Nitre -- -- 5 5 -- 4 16 28.5 41 Lead got 42.5 36.0 16.0 19.0 11.5 22.5 22.5 26.5 27.5

_Copper Pyrites_ 8 8 8 8 Quartz -- 12 -- 12 Nitre -- -- 4 4 Lead got 47.5 34.0 33.0 26.0

_Antimonite_ 8 8 8 8 Quartz -- 12 -- 12 Nitre -- -- 4 4 Lead got 29.0 26.0 13.0 13.0

_Galena_ 10 10 10 10 15 20 Quartz. -- 15 -- 15 5 -- Nitre -- -- 3 3 3.5 7 Lead got 17.0 19.0 8.0 8.0 18.5 18.5

A similar set of experiments, with 80 grams of litharge instead of 80 grams of red lead, gave:--

_Pyrites_ 4 4 4 4 7 10 Quartz -- 15 -- 15 13 10 Nitre -- -- 5 5 12.5 20 Lead got 46.5 40.5 25.5 24.5 27.0 26.5

_Copper Pyrites_ 5 5 5 5 Quartz -- 15 -- 15 Nitre -- -- 5 5 Lead got 44.5 32.5 23.0 25.0

_Blende_ 5 5 5 5 10 Quartz -- 15 -- 15 10 Nitre -- -- 5 5 15 Lead got 41.5 38.5 21.5 22.5 21.6

_Antimonite_ 5 5 5 5 10 Quartz -- 15 -- 15 10 Nitre -- -- 5 5 10 Lead got 31.0 32.5 11.5 12.5 18.7

_Galena_ 10 10 10 10 15 20 Quartz -- 15 -- 15 5 -- Nitre -- -- 5 5 7.5 11 Lead got 33.5 33.5 13.0 14.0 19.5 22.7

The variation in some of these experiments, in which we might have expected similar results, is due to the fact that the sulphur, and in some cases the metals, are capable of two degrees of oxidation. For example: theoretically 1 gram of iron pyrites (FeS_{2}) would yield 8.6 grams of lead if the sulphur were oxidised to sulphurous oxide (SO_{2}), and the iron to ferrous oxide (FeO); whilst if the sulphur were oxidised to sulphate (SO_{3}), and the iron to ferric oxide, 12.9 grams of lead will be thrown down. Similarly the yield with copper pyrites would be 7.5 or 11.6; with blende, 6.4 or 8.5; with antimonite, 5.5 or 8; and with galena, 2.6 or 3.4. As regards the metals, the lower oxide will always be formed if the a.s.say is carried out properly (fused under a cover, and with a sufficiency of reducing agent). But the proportion of sulphur oxidised completely will vary with the conditions of the a.s.say.

With a slag containing much soda the tendency will be to form sulphate, and, in consequence, a big reduction of lead; whilst with an acid slag containing much quartz the tendency will be for the sulphur to go off as sulphurous oxide (SO_{2}). In a fusion with litharge alone all the sulphur will be liberated as the lower oxide, whilst with much soda it will be wholly converted into sulphate. For example: 3 grams of an ore containing a good deal of pyrites and a little galena, gave, when fused with litharge, 16.5 grams of lead. A similar charge, containing in addition 20.0 grams of soda, gave 22.5 grams of lead.

It will be noted from the experiments that 1 gram of nitre kept up on the average 4 grams of lead; the range being from 3.2 with acid slags to 5.3 with very basic ones. These facts serve to explain some apparently irregular results got in practice.

CUPELLATION.

The process is as follows:--The cupels, which should have been made some time before and stored in a dry place, are first cleaned by gentle rubbing with the finger and blowing off the loose dust; and then placed in a hot m.u.f.fle and heated to redness for from 5 to 10 minutes before the alloy to be cupelled is placed on them. The reasons for this are sufficiently obvious: the sudden evolution of much steam will blow a cupel to pieces; and, if the whole of the water has not been removed before the cupel is filled with molten lead, the escaping steam will bubble through, and scatter about particles of the metal. If some particles of unburnt carbon remain in the bone ash, a similar result will be produced by the escape of bubbles of carbonic acid as soon as the fused litharge comes in contact with them. The cupels having been prepared are arranged in a definite order in the m.u.f.fle, and the a.s.say b.u.t.tons are arranged in a corresponding order on some suitable tray (cupel tray, fig. 41); the heat of the m.u.f.fle being at bright redness.

Then with the help of the tongs (fig. 42) the a.s.say b.u.t.tons should be placed each in its proper cupel; a note having been previously made of the position it is to occupy, and the door of the m.u.f.fle closed.

[Ill.u.s.tration: FIG. 41.]

This part of the work should be done promptly, so as not to unduly cool the m.u.f.fle: the start requires a fairly high temperature, and is a critical part of the process. A black crust forms at once on the surface of the lead; but this ought soon to fuse and flow in greasy drops from off the face of the metal, so as to leave the latter fluid with a well-defined outline, and much brighter than the cupel. If this clearing does not take place, the b.u.t.tons are said to be frozen; in which case the temperature must be raised, some pieces of charcoal put in the m.u.f.fle, and the door closed. If they still do not clear, the heat must have been much too low, and it is best to reject them and repeat the a.s.says.

[Ill.u.s.tration: FIG. 42.]

When the b.u.t.tons have cleared it is well to check the draught of the furnace, and to partly open the door of the m.u.f.fle, so as to work at as low a temperature as is compatible with the continuation of the process.[11] Too low a temperature is indicated by the freezing of the b.u.t.tons and the consequent spoiling of the a.s.says. Experience soon enables one to judge when the heat is getting too low. A commoner error is to have the heat too high: it should be remembered that that which was high enough to clear the b.u.t.tons at starting is more than sufficient to keep the process going. At the finish a higher temperature is again required: therefore the door of the m.u.f.fle should be closed and the furnace urged. The finish is easily recognised. The drops of litharge which in the earlier stages flow steadily from the surface of the alloy, thin off later to a luminous film. At the end this film appears in commotion, then presents a brilliant play of colours, and, with a sudden extinction, the operation is finished. The metal again glows for an instant whilst becoming solid.

If the b.u.t.ton is a small one the cupel is withdrawn at once and placed on that square of the cupel tray which corresponds to the position it occupied in the m.u.f.fle. If, however, it is fairly large precautions must be taken to prevent spirting.

Molten silver dissolves oxygen from the air and gives it off on solidifying; the escape of the gas on sudden cooling is violent and, by throwing off particles of the metal, may cause loss. This is called "vegetation" or "spirting." The silver is apparently solid when spirting takes place; the crust breaks suddenly and some of the metal is forced out. The evil is best guarded against by slow cooling and avoiding draughts. With large b.u.t.tons of silver precautions should never be omitted. One plan is to allow the cupels to cool in the m.u.f.fle itself, the mouth being closed with hot charcoal. Another is to cover the cupel with another cupel previously heated to redness; in this case the silver cools between two hot cupels, and, of course, cools slowly. A third plan is to withdraw the cupel to the door of the m.u.f.fle, holding it until it begins to get solid and then immediately to put it back into the hotter part of the m.u.f.fle.

Silver remains after cupellation in flattened elliptical b.u.t.tons, adhering but only slightly to the cupel. Its upper surface should show faint markings as if it were crystalline. The presence of platinum renders it still more crystalline, but removes the characteristic l.u.s.tre and renders the metal dull and grey. Copper, if not completely removed, has a very marked effect on the appearance of the b.u.t.ton: the metal is spread out, damping, as it were, and firmly adhering to the cupel, which latter in the neighbourhood of the metal is almost black with oxide of copper. Sometimes the silver b.u.t.ton is globular, or even more sharply rounded on its under than on its upper surface; it is said that this is due to the presence of lead. Gold may be present even to the extent of 50 per cent. without showing any yellow colour.

The appearance of the cupel affords some useful information. The presence of cracks evidently due to shrinkage indicates a badly made cupel. If, however, they are accompanied by a peculiar unfolding of the cupel, the margin losing its distinctness, it is because of the presence of antimony. When lead is the only easily oxidisable metal present, the stained portion of cupel is yellow when cold. A greenish tint may be due to small quant.i.ties of copper or, perhaps, nickel, cobalt, or platinum.

Larger quant.i.ties of copper give a greenish grey or almost black colour.

A dark green and corroded cupel may be due to iron. Rings of pale-coloured scoria may be due to tin, zinc, antimony, or a.r.s.enic. When the cupel shows signs of the presence of these metals in objectionable quant.i.ty, it is well to repeat the a.s.say and scorify so as to remove them before cupellation.

The b.u.t.ton should be detached from the cold cupel by seizing with a pair of pliers: the under surface should be distorted by squeezing or hammering the b.u.t.ton so as to loosen the adhering bone ash. The cleaning is easily completed by rubbing with a clean hard brush. After cleaning the b.u.t.tons are best put on a tray of marked watch-gla.s.ses, and then taken to the balance and weighed. The weight of silver got needs a small correction; (1) by deducting for the amount of silver introduced by the lead or oxide of lead used in the a.s.say;[12] and (2) by adding for the cupellation loss.

~Loss in Cupellation.~--During the whole process of cupelling a silver lead alloy a more or less abundant fume may be observed rising from the cupel. This furnishes an evident loss of lead and a possible loss of silver; for although silver at the temperature of cupellation gives off no appreciable vapour, it is known that such fume formed on a large scale contains silver. It is, however, difficult to believe that the small amount of lead vapourised carries with it a weighable amount of silver. That it does not do so in the ordinary way of working is shown by the fact that a b.u.t.ton of silver equal in weight to the silver lost in cupelling may be got by smelting the cupel and cupelling the resulting b.u.t.ton of lead. The loss of silver by volatilisation is altogether inconsiderable, unless the temperature at which the operation is performed is much too high.

Another possible source of loss is the infiltration of small particles of alloy into the cupel. The cupel is necessarily porous, and particles of metal may perhaps drain into it, more especially if the bone ash is not in fine powder; but if this is the main source of loss it is hard to see why, in cupelling equal weights of silver and gold, the loss is not equal in each case. It is not easy to believe that the mere filtration of the fused alloy will effect such a change in the proportion of the metals as that which actually occurs. For example: a cupel on which an alloy consisting of 0.80 gram of silver, 0.47 gram of gold, and 25 grams of lead had been cupelled, was found to contain 7-1/2 milligrams of silver, and rather less than half a milligram of gold. a.s.suming, for the sake of argument, that the gold present had filtered into the cupel in the form of small drops of alloy, it would have been accompanied by less than a milligram of silver, and the presence of the extra 6 or 7 milligrams of silver must have been due to a different cause. There can, thus, be little doubt that the cause of the greater part of the "cupellation loss" is a chemical one and cannot be counteracted by a mechanical contrivance.[13] In cupellation, then, there is a loss, apart from imperfect working, inherent in the process itself; and as the amount of this loss varies under different conditions, it is necessary to study it somewhat in detail.

The following experiments are taken without selection from the work of one student. Three experiments were made for each determination, and the mean result is given. By "range" is meant the difference between the highest and lowest result and the percentage loss is calculated on the silver present. The silver added in the lead used has been deducted.

~Effect of Varying Lead.~--In each experiment 0.4 gram of silver was taken and cupelled with the lead. The silver loss and "range" are expressed in milligrams.

------------+--------------+--------+------------------ Lead Used. | Silver Lost. | Range. | Percentage Loss.

------------+--------------+--------+------------------ Grams. | | | 10 | 6.5 | 1.0 | 1.62 20 | 7.0 | 1.0 | 1.75 40 | 12.0 | 1.5 | 3.00 60 | 12.7 | 0.5 | 3.17 ------------+--------------+--------+------------------

The loss increases with the lead used.

~Effect of Varying Temperature.~--0.4 gram of silver was cupelled with 20 grams of lead.

Temperature. Silver Lost. Range. Percentage Loss.

Bright red 7.0 1.0 1.75 Clear yellow 17.3 1.7 4.32

The difference in temperature in these experiments was much greater than would occur even with careless work.

~Effect of Varying Silver.~--20 grams of lead were used in each cupellation.

---------------+--------------+--------+------------------ Silver Taken. | Silver Lost. | Range. | Percentage Loss.

---------------------------------------------------------- Milligrams. | | | 12.5 | 0.7 | 0.2 | 5.6 25.0 | 1.4 | 0.1 | 5.6 50.0 | 1.6 | 0.4 | 3.2 100.0 | 2.9 | 0.3 | 2.9 200.0 | 5.6 | 0.5 | 2.8 400.0 | 7.0 | 1.0 | 1.7 800.0 | 9.7 | 1.0 | 1.2 ---------------+--------------+--------+------------------