A Text-book of Assaying: For the Use of Those Connected with Mines Part 14

[8] The difference of 20 or 30 milligrams is disregarded here because it detracts equally from the actual weight of the water and liquid to be determined. If the liquid is a heavy one the difference shows itself in the third or second place of decimals. The correction may be made by deducting from the weight of the flask 0.0012 grams for each gram of water it holds.

PART II.--THE METALS.

CHAPTER IX.

SILVER, GOLD, CYANIDES, PLATINUM, MERCURY.

SILVER.

Silver is widely diffused, and has been found in most mining districts.

It occurs native in sufficient quant.i.ty to const.i.tute one of the chief ores of the metal. It also occurs combined with sulphur (as in argent.i.te), with sulphur and antimony (as in stephanite or brittle silver ore, and in pyrargyrite or ruby silver), and with copper, sulphur, antimony, and a.r.s.enic, as in polybasite. Chloride of silver occurs native as horn silver or kerargyrite. Silver is found in the ores of other metals, such as fahlerz, which sometimes contains from two to ten per cent. of the metal, and galena, which is an important source of it; in fact, galena is never found entirely free from silver. It is present also in greater or less quant.i.ty in the ores of copper and zinc.

Silver dissolves readily in nitric acid, forming silver nitrate. It only forms one family of salts, and of these the chloride and nitrate are of chief importance to the a.s.sayer. The formation of the chloride of silver on the addition of hydrochloric acid or a soluble chloride to the nitric acid solution, serves for the recognition and separation of silver. The precipitated chloride is white (becoming violet on exposure to light), insoluble in nitric acid, soluble in ammonia, hyposulphite of soda, or concentrated solutions of chlorides. The best confirmatory test is made by wrapping the precipitate in a little sheet lead, and cupelling, when the silver will be left in the metallic state, and is easily recognized.

~Dry a.s.say.~--This a.s.say is made up of two parts: (1) the concentration of the silver in a b.u.t.ton of lead; and (2) the cupellation of the resulting alloy. The concentration of the b.u.t.ton of lead may be effected either by scorification or by fusion in a crucible.

The scorification a.s.say is performed in a scorifier, which is a shallow open-mouthed dish about 2-1/2 inches across, with a very thick bottom to enable it to withstand the corrosive action of the slag. A charge of more than 3 or 5 grams of the ore cannot be worked in one, and with such small charges the unavoidable variations have a serious effect on the figures reported. A difference of one milligram on the weight of the b.u.t.ton of silver got represents a difference of 6 or 10 ounces per ton.

With rich ores such variation is unavoidable under any conditions, and the only safe plan is to take the mean of several a.s.says. But with poorer ores the accuracy of the a.s.say, as well as convenience in working, is much increased by working in a crucible with larger charges.

In scorification the proportion of lead required for scorifying 1 gram of ore is in average cases from 10 to 15 grams, sinking in the case of galena to 2 grams, and rising with earthy and refractory substances to from 30 to 40 grams. But by fusing in a crucible with well-selected fluxes, a proportion of 4 of flux to 1 of ore is generally sufficient; and not only is the proportion of added matter less, but it is also easier to manipulate large quant.i.ties in crucibles, so that, although in some cases the crucible a.s.say is more troublesome and less satisfactory, yet with poor and earthy ores it is the best method of dealing with them; while when properly worked it yields results as accurate as scorification does. As a general rule, if more than 5 grams of ore must be taken, the crucible a.s.say should be adopted.

[Ill.u.s.tration: FIG. 38.]

~Scorification a.s.say.~--The charge of ore is usually 3 grams, sometimes 5; the lead varies from 30 to 70 grams, and the quant.i.ty of soda, borax, or powdered gla.s.s added varies from 0.3 to 3 or 4 grams. It is generally recommended to have the lead granulated,[9] and to mix the ore with about half of it in the scorifier; then to put on the rest of the lead; and finally to sprinkle the borax or gla.s.s on the top. It answers just as well, however, to use the lead in the shape of foil, and wrap the ore up in it; and if the ore contains much sulphur, the borax may with advantage be added (wrapped in a little tissue paper) some five or ten minutes after the operation has started.

[Ill.u.s.tration: FIG. 39.]

The process of scorification is as follows:--A scorifier (fig. 38) of convenient size having been selected (one 2-1/2 inches across is most generally useful), it is dried at a gentle heat for about ten minutes.

The charge is then put into it, and it is introduced, with the help of a scorifier tongs (fig. 39), into a m.u.f.fle heated considerably above redness. The m.u.f.fle is then closed, and when the metal has melted down, it is opened, but the temperature is kept up. A ring of slag will, after a time, form around the metal, and when this appearance (known as the eye) presents itself, the temperature may be lowered. When the eye has disappeared--that is, when the layer of slag has quite closed in--a pinch of powdered culm wrapped in tissue paper is added. As soon as the slag has again become tranquil, the scorifier is taken out, and its contents are poured into a mould (fig. 40), the slag is detached, and saved. If the b.u.t.ton of metal weighs more than 30 grams, its size is reduced by another scorification in the same scorifier, which should have been replaced in the m.u.f.fle immediately after the contents had been poured out. If the ore is not a very rich one, the b.u.t.ton of lead will carry practically all the silver; but with rich ores it is more satisfactory to save the slag, and subsequently to melt it down with the cupel on which the lead has been treated, so as to recover the silver lost in the slag, together with that absorbed in the cupel, at one operation. Or, if the cupellation loss is neglected or calculated in some other manner, the slag or slags from the scorifier may be powdered and mixed with 20 grams of oxide of lead, 5 grams of borax, and 1 gram of charcoal. This should be melted down in a small crucible, and the resulting b.u.t.ton of lead cupelled.

[Ill.u.s.tration: FIG. 40.]

If the scorification has been unsatisfactory, the quant.i.ty of silver obtained from the slag will be by no means inconsiderable. The usual explanation is that with sulphury ores compounds of metallic oxides and sulphides (oxysulphides) are formed, which remain in the slag, retaining considerable quant.i.ties of the precious metal. It is said that under certain conditions such a slag may contain as much as 10 per cent. of silver. An excess of lead and a high temperature prevents the formation of these oxysulphides. But if much silver is present in the ore, the slag cannot be safely thrown away, even if sulphur is absent, and the process has been satisfactorily performed.

If the crust which appears on the surface of the lead does not clear, add a small lump of borax and 20 grams more lead; then close the m.u.f.fle, and keep the temperature as high as possible. If the slag forms properly, but shows unfused or only half-fused lumps, even when the scorification has proceeded for some time, add more borax, and stir with an iron rod. The slag adhering to the rod must be detached by hammering, and replaced in the scorifier.

If the ore consists largely of quartz, soda should be added instead of borax; or, if it contains much copper, powdered quartz may be used. If the scorifier at the end of an operation is more than usually corroded, the borax should be replaced in subsequent a.s.says on similar ores by powdered gla.s.s or quartz.

If a fairly fluid slag is formed which does not clear from the metal and show the eye, more lead and a higher temperature is wanted.

As a general rule, it may be stated that when a scorification is unsatisfactory, what is wanted is more heat, more lead, or more borax.

It is a safe plan when work has to be done on a strange ore, to make three or four a.s.says with varying quant.i.ties of lead. The proportion of lead is right when a further addition does not yield a higher result.

The proper proportion having been found, a note of it should be made for future use.

POT a.s.sAYS.

The object of the fusion in a crucible, like that of scorification, is to concentrate the silver in a b.u.t.ton of lead which is to be subsequently cupelled; and to retain the earthy and waste matters in the slag. It is necessary to consider the quality of the slag and the weight and quality of the lead. The slag when fused should be liquid and h.o.m.ogeneous, and not too corrosive on the crucible. The b.u.t.ton of lead should be soft, malleable, and free from a coating of regulus.[10] In weight it should not differ much from the ore taken. With 20 grams of ore, for example, a b.u.t.ton of lead weighing from 18 to 25 grams will be satisfactory: less than this would leave an undue proportion of silver in the slag; and more would be unnecessarily large for cupelling, and would increase the loss in that operation.

With average ores, take 20 grams of the powdered ore and mix with 30 grams of "soda," 40 grams of red-lead or litharge, 5 grams of borax, and from 2 to 2.5 grams of flour, and place in an E crucible (Battersea round). Put these in the furnace at a red heat, cover the crucible, and gradually raise the temperature until the whole charge has melted down and is in a state of tranquil fusion. Pour into a mould, and replace the crucible in the furnace. As soon as the lead is solid, detach the slag and put it back into the crucible; and when it is again fluid, charge on to it with a copper scoop a mixture of 20 grams of oxide of lead, and 1 gram of charcoal: when fusion has again become tranquil, pour and detach the b.u.t.ton of lead. The lead b.u.t.tons should be hammered into discs with rounded edges, and be freed from slag; if too big for a cupel they may be scorified together in a small scorifier, but it is better to cupel them separately.

~Ores containing Metallic Oxides.~--Peroxides of iron, manganese, and copper interfere by counteracting the effect of the charcoal or flour, and thus reducing the size of the lead b.u.t.ton. Peroxide of iron will reduce the weight of lead by a little more than its own weight; and peroxide of manganese has about twice this effect. When these oxides are present an additional quant.i.ty of flour must be used, and precautions must be taken to prevent reoxidation of the slag by the furnace gases.

This may best be prevented by using a layer of common salt as a cover to the charge. When the ores contain a good deal of quartz or stony matter, the fluxes just given (for average ores) will do; but the proportion of soda should be diminished, and that of the borax, oxide of lead, and flour increased as the quant.i.ty of metallic oxides become greater. If the ore contains practically no quartz, the soda may be altogether omitted, and some gla.s.s or powdered quartz added. The following charge may be taken as an example: weigh up 20 grams of the powdered ore, 15 grams each of "soda" and borax, 60 grams of oxide of lead, and 5 grams of flour. Mix and place them in an E crucible, and cover with a layer of from a quarter to half an inch of common salt. Place in the furnace as before. The salt will give off a considerable amount of fume, which will, to a certain extent, conceal the state of the charge: when the crucible has been in the furnace for about 25 minutes remove it and pour out the contents immediately. With ores that produce a thick slag the addition of 5 grams of fluor spar will be an advantage. It may happen that with an unknown ore the first a.s.say will be more or less unsatisfactory: but from it the necessity for adding more or less flour will be learnt, and a second a.s.say, with the necessary modification of the charge, should give a good result.

~Ores containing much Sulphides.~--Ores of this cla.s.s may be easily recognized, either by the appearance of the minerals they contain or by the odour of sulphurous oxide (SO_{2}) which they evolve when roasted on a spatula. The sulphides most commonly present, in addition to the sulphurized minerals of silver, are pyrites, galena, blende, and mispickel. When they are present in only a moderate amount, their effect is simply to increase the weight of the b.u.t.ton of lead; and this is easily counteracted by reducing the amount of flour, or by omitting it.

When in larger amounts, they not only yield large b.u.t.tons, but also render the metal sulphury, sometimes even giving a b.u.t.ton of regulus instead of lead. This last evil may be remedied (1) by putting in a rod of iron as soon as the charge has fused, or (2) it may be counteracted by a proper addition of nitre, or (3) when the sulphides present are only those of iron or copper the sulphur may be removed by calcining, and the ore converted into one of the cla.s.s containing metallic oxides.

The calcination is effected as follows:--Weigh up 20 grams of the powdered ore and place it in a wide-mouthed crucible sufficiently large to perform the subsequent melting down in. The roasting must be done at a gentle heat at first, so as to avoid clotting: the mouth of the crucible should project considerably above the c.o.ke, and should slope forward towards the worker. The charge must be occasionally stirred with the stirrer (fig. 10) so as to expose fresh surfaces to the action of the air, and to prevent adhesion to the sides of the crucible. The stirrer should not be removed till the calcination is finished. The temperature should be raised at the end to a good red heat; and (to ensure the decomposition of any sulphate that may be formed) the roasted ore should be rubbed up in a mortar with a pinch of anthracite, and again calcined. It is then mixed with fluxes as described, and fused in the same crucible.

The calcination of an ore is a work occupying a good deal of time, and, in most cases, it is better to take advantage of the desulphurizing power of red lead or nitre. Red lead by itself will do, but a large quant.i.ty of it will be required; 1 part of a metallic sulphide needs from 20 to 50 parts of red lead to yield a b.u.t.ton free from sulphur; whereas at most from 2 to 2-1/2 parts of nitre are sufficient. There is sometimes an advantage in having a considerable excess of oxide of lead in the slag, but where there is no such reason, 2 parts of red lead to 1 of ore is enough. A charge which will do for most sulphides is the following: 20 grams of ore, 40 to 100 grams of red lead, 20 grams of "soda," 5 of borax, and sufficient nitre (or perhaps flour) to give a b.u.t.ton of about 25 grams of lead. How much this must be (if not already known) may be approximately determined by fusing 3 grams of the ore and 3 grams of "soda" in a small crucible (C) with 50 grams of litharge (not red lead) under a cover of salt, and weighing the resulting b.u.t.ton of lead. Subtract 3 from the weight of lead obtained, and the difference multiplied by 1.3 will give the quant.i.ty in grams of nitre required. If the b.u.t.ton of lead weighs less than 3 grams flour must be added. If this is not satisfactory repeat the a.s.say, adding an extra gram of nitre for each 4 grams of lead in excess of that required, or 1 gram of flour for a 12-gram deficiency.

In the method in which iron is used as a de-sulphurising agent, only as much oxide of lead should be added as will give a b.u.t.ton of lead of the required size. Rather a large b.u.t.ton of lead should be got, and the slag should be strongly alkaline; if the ore does not already carry a large amount of sulphur some should be added. The fusion should be performed at a low temperature (similar to that for a galena a.s.say), and should be continued for some time after it has become tranquil. Take 20 grams of the ore, 40 grams of "soda," 40 grams of oxide of lead, and 5 or 10 grams of borax; place this mixture in a crucible (with a rod of iron, as in the galena a.s.say), cover, and fuse for about half an hour. Take out the rod, was.h.i.+ng it in the slag, and, in a minute or two, pour. Clean and cupel the b.u.t.ton of lead.

~General Remarks on the Fusion.~--Other things being equal, the smaller the quant.i.ty of the slag the better, provided there is sufficient to cover the metal. The presence of peroxides of the heavy metals is prejudicial, since they tend to increase the quant.i.ty of silver retained in the slag. It may be given as a general rule that when iron, copper, manganese, &c., are present, there is a more than ordinary need for cleaning the slags, and care must be taken to keep these metals in the state of lower oxide.

In selecting the fluxes, it should be remembered that soda is the best for quartz, and borax for lime and metallic oxides. And that with ores almost free from gangue some quartz or gla.s.s should be added to protect the crucible. Two parts of soda are enough to flux 1 part of quartz; whilst of borax, or oxide of lead, 4 parts are barely sufficient. Oxide of lead has the advantage of being heavy and so does not occupy much s.p.a.ce in the crucible; on the other hand, if the melting down be performed too quickly, or if oxide of lead only is used, this high specific gravity is a disadvantage, for the lighter earthy matter floats as a pasty ma.s.s on the more fluid oxide of lead, and thus escapes its action.

When metallic sulphides are present in the ore, an excess of oxide of lead helps to keep the sulphur out of the b.u.t.ton of metal. In addition to the oxide of lead required as a flux, some will be required to provide the lead in which the silver is to be collected. Oxide of lead, mixed with charcoal or flour, yields, when heated, a mult.i.tude of minute b.u.t.tons of metal uniformly distributed through the ma.s.s of the charge; as the charge melts down these run together and fall to the bottom; this shower of lead collects the silver more easily than a single b.u.t.ton at the bottom of the crucible could do. Only that portion of the oxide of lead which remains in the slag can be considered as a flux; very often the first indication of an excessive reduction of lead is the pastiness of the slag rendered thick by the withdrawal of the oxide of lead which would have kept it fluid. If, in an a.s.say, it is found that 5 parts of flux are not sufficient for 1 part of ore, the remedy lies in using a different flux rather than in taking a larger quant.i.ty.

_On the Reducing Effect of Charcoal, Flour, and Tartar._--The weight to be got from a given charge will depend (provided sufficient oxide of lead is present) upon the proportion of the reducing agents in it. We have thought it well to ill.u.s.trate this part of the subject by a series of experiments which the learner will do well to practise for himself before proceeding to the a.s.say of actual ores. Take 80 grams of litharge and 20 grams of a mixture of borax and soda. Fuse three lots (1) with 1.5 gram of charcoal, (2) with 3 grams of flour, and (3) with 7.5 grams of tartar. Weigh the b.u.t.tons of lead obtained, and divide each by the weight of reducing agent used. The results will differ somewhat with the dryness and quality of the flour, etc., used; in one series of experiments they were as follows:--

Gram. Grams. Gram. Grams.

1.5 charcoal gave 34.0 lead .'. 1 charcoal = 22.6 lead.

3.0 flour " 33.5 " .'. 1 flour = 11.2 "

7.5 tartar " 38.0 " .'. 1 tartar = 5.0 "

The use of flour as a reducing agent has many advantages, and it is well to remember that _1 gram of flour reduces about 11 grams of lead_; and that charcoal has twice, and tartar one-half, this reducing effect.

_On the Reducing Effect of Charcoal, &c., on Red Lead._--It is often easier to obtain red lead of good quality than it is litharge, and by a large number of a.s.sayers red lead is the form of oxide of lead always used. Red lead, however, contains an excess of oxygen which will use up some of the reducing agent before lead separates out. On making a series of experiments (similar to the last, but using 80 grams of red lead instead of the litharge) the results were, with the same quant.i.ties of the reducing agents:--

With charcoal, 18 grams of lead.

" flour, 18 " "

" tartar, 22 " "

Comparing these with the results with litharge, in the previous table it will be seen that the same quant.i.ty of reducing agent has in each case brought down 16 grams less of lead, so that a larger amount of the reducing agent must be added to get a b.u.t.ton of the same weight as that obtained with litharge. To get a b.u.t.ton of a desired weight, say 22 grams, we must add reducing agent sufficient to throw down 22 + 16 or 38 grams of lead, which would require 3.4 grams of flour. If this amount of flour is fused with 80 grams of red lead, a b.u.t.ton of lead weighing 22 grams will be formed, the other 16 grams being kept up by the oxygen of the red lead.

If the quant.i.ty of red lead differs from 80 grams, this rule must be modified. With 40 grams of red lead, for example, we should add an excess of reducing agent sufficient to throw down 8 grams of lead instead of 16. Similarly, with 160 grams of red lead, we should add enough to throw down 32 grams.

The following rule will enable one to calculate the weight of flour required to produce a b.u.t.ton of lead of any desired weight from any given quant.i.ty of red lead. Each 5 grams of red lead present diminishes the weight of the lead by 1 gram. If then we _divide the weight of red lead in a charge by 5, and add this to the weight of lead required, the sum divided by 11 will give the weight of flour which must be added_.

Using 80 grams of red lead and wanting a b.u.t.ton of 20 grams, we should add 3.3 grams of flour.

80/5 = 16; 16+20 = 36; 36/11 = 3.3 nearly.