A Text-book of Assaying: For the Use of Those Connected with Mines Part 3

The difference between solution and fusion is easily ill.u.s.trated: a lump of sugar heated over a candle-flame melts or fuses; suspended in water it dissolves. Many substances which are insoluble or infusible of themselves, become soluble or fusible when mixed with certain others; thus, in this way, solution is got with the aid of reagents, and fusion with the help of fluxes. For example, lead is insoluble in water, but if nitric acid be added, the metal rapidly disappears. It is convenient, but somewhat inaccurate, to say that the acid dissolves the lead. If the lead be acted on by nitric acid alone, without water, it is converted into a white powder, which does not dissolve until water is added; in this case it is obvious that the water is the solvent. The function of the acid is to convert the lead into a soluble compound.

~Fluxes~ may act as true solvents. Fused carbonate of soda dissolves baric carbonate, and perhaps in many slags true solution occurs; but in the great majority of cases a flux is a solid reagent added for the purpose of forming a fusible _compound_ with the earthy or stony minerals of the ore. Few of the minerals which occur in the gangue of an ore are fusible; and still fewer are sufficiently fusible for the purposes of the a.s.sayer, consequently the subject is one of importance, and it ought to be treated on chemical principles. An idea of the composition of some of the more frequently occurring rocks may be gathered from the following table, which represents rough averages:--

----------------------------------------------------------------- | | |Oxide|Lime and | |Silica.|Alumina.| of |Magnesia.|Alkalies.

| | |iron | | ----------------------------------------------------------------- | % | % | % | % | % Sandstone, grit, | | | | | quartzite, &c. |80-100 | -- | -- | -- | -- Granite, gneiss, | | | | | quartz-porphyry, | | | | | fire-clay, &c. | 70-75 | 13-20 | 2 | 2 | 5-8 | | | | |Less in | | | | |fire-clay.

Mica-schist | 65 | 18 | 5 | 3 | 3 Trachyte, syenite | 60 | 17 | 7 | 4-7 | 6-9 Clay-slate | 60 | 18 | 10 | 8 | 3 Diorite | 54 | 17 | 12 | 9 | 3-4 Horneblende-rock | 50 | 18 | 15 | 12 | 3-4 Brick-clay | 50 | 34 | 8 | 6 | -- China-clay | 47 | 39 | -- | -- | -- Basalt, dolerite, &c.| 50 | 15 | 15 | 16 | 3 Serpentine | 44 | -- | -- | 44 | -- Chalk, limestone, | | | | | dolomite, &c. | -- | -- | -- | 45-55 | -- -----------------------------------------------------------------

Silica itself, and the silicates of alumina, of lime, and of magnesia, are practically infusible; the silicates of soda, of potash, and of iron are easily fusible if the base (soda, potash, or oxide of iron) be present in sufficient quant.i.ty, and if, in the case of the iron, it is present mainly as lower oxide (ferrous silicate). The addition of lime, oxide of iron, or alkali to silicate of alumina results in the formation of a double silicate of alumina and lime, or of alumina and iron, &c., all of which are easily fusible. Similarly, if to a silicate of lime we add oxide of iron, or soda, or even alumina, a fusible double silicate will be formed. Thus lime, soda, oxide of iron, and clay, are _fluxes_ when properly used; but since lime, clay (and oxide of iron if there be any tendency to form peroxide), are of themselves infusible, any excess of these fluxes would tend to stiffen and render pasty the resulting slag. So, too, soda, which is a very strong base, may act prejudicially if it be in sufficient excess to set free notable quant.i.ties of lime and magnesia, which but for that excess would exist in combination as complex fusible silicates. There are many minerals which with but little soda form a gla.s.s, but with more yield a lumpy scoriacious ma.s.s. There are many minerals, too, which are already basic (for example, calcite), and which, when present, demand either a less basic or an acid flux according to the proportions in which they exist. For purposes of this kind borax, or gla.s.s, or clay with more or less soda may be used, and of these borax is by far the most generally useful. An objection to too basic a slag (and a very important one) is the speed with which it corrodes ordinary crucibles. These crucibles, consisting of quartz and clay, are rapidly attacked by lime, soda and bases generally.

[Ill.u.s.tration: FIG. 4.]

In considering what is and what is not a good slag, certain chemical properties are of importance. If a mixture of many substances be fused and allowed to solidify in a crucible, there will be found some or all of the following. At the bottom of the crucible (fig. 4) a b.u.t.ton of metal, resting on this a speise; then a regulus, next a slag made up of silicates and borates and metallic oxides, and lastly, on the top another layer of slag, mainly made up of fusible chlorides and sulphates. In a.s.saying operations the object is generally to concentrate the metal sought for in a b.u.t.ton of metal, speise or regulus, and to leave the earthy and other impurities as far as possible in the slag; whether there be one or two layers of slag is a matter of indifference;[3] but the chemical action of the lower layer upon the speise, or regulus, or metal, is of great importance.

A _regulus_ is a compound of one or more of the metals with sulphur; it is usually brittle, often crystalline, and of a dull somewhat greasy l.u.s.tre. It is essential that the slag, when solid, shall be so much more brittle than the regulus, that it shall be easy to crumble, and remove it without breaking the latter; and it must not be basic. The effect of fusing a regulus with a basic slag is well seen when _sulphide of lead_ is fused with _carbonate of soda_; the result is a b.u.t.ton of metal (more or less pure), and a slag containing sulphides of lead and sodium; and again, if sulphide of lead be fused with an excess of oxide of lead, a b.u.t.ton of lead will be got, and a slag which is simply oxide of lead (with whatever it may have taken up from the crucible), or if a sufficient excess has not been used, oxide of lead mixed with some sulphide. When (as is most frequently the case) the desire is to prevent the formation of regulus, these reactions may be taken advantage of, but otherwise the use of a flux having any such tendency must be avoided. A good slag (from which a regulus may be easily separated) may be obtained by fusing, say, 20 grams of ore with borax 15 grams, powdered gla.s.s 15 grams, fluor spar, 20 grams, and lime 20 grams; by quenching the slag in water as soon as it has solidified, it is rendered very brittle.

Sulphide of iron formed during an a.s.say will remain diffused through the slag, instead of fusing into a b.u.t.ton of regulus, if the slag contain sulphide of sodium. The same is true of other sulphides if not present in too great a quant.i.ty, and if the temperature is not too high.

_Speises_ are compounds of a metal or metals with a.r.s.enic. They are chiefly of interest in the metallurgy of nickel, cobalt, and tin. They are formed by heating the metal or ore in covered crucibles with a.r.s.enic and, if necessary, a reducing agent. The product is fused with more a.r.s.enic under a slag, consisting mainly of borax. They are very fusible, brittle compounds. On exposure to the air at a red heat the a.r.s.enic and the metal simultaneously oxidize. When iron, cobalt, nickel, and copper are present in the same speise, they are eliminated in the order mentioned.

_Slags_ from which metals are to be separated should not be too acid; at least, in those cases in which the metal is to be reduced from a compound, as well as separated from earthy impurities. Where the object is simply to get a b.u.t.ton of metal from a substance in which it is already in the metallic state, but mixed with dross (made up of metallic oxides, such as those of zinc or iron), from which it is desired to separate it, an acid flux like borax is best; or, if the metal is easily fusible, and there would be danger of loss of metal by oxidation or volatilising, it may be melted under a layer of resin or fat. Common salt is sometimes used with a similar object, and is often useful. Under certain conditions, however, it has a tendency to cause the formation of volatile chlorides with a consequent loss of metal.

In the great majority of cases, the fusion of the metal is accompanied by reduction from the state of oxide; in these the slag should be basic.

It is not easy to reduce the whole of a reducible oxide (say oxide of copper or of iron) from a slag in which it exists as a borate or silicate; there should be at least enough soda present to liberate it.

When the object is to separate one metal, say copper, without reducing an unnecessary amount of another (iron) at the same time, a slag with a good deal of borax is a distinct advantage. The slag then will probably not be free from copper, so that it will be necessary to powder and mix the slag with some soda and a reducing agent, and to again fuse the slag in order to separate this residual metal. In all those cases in which the slag retains an oxide of a heavy metal, this cleaning of the slag is advisable, and in the case of rich ores necessary. Slags containing sulphides are especially apt to retain the more easily reducible metals.

The following are the ordinary and most useful fluxes:--

~Soda.~--The powdered bicarbonate, sold by druggists as "carbonate of soda," is generally used. It gives off its water and excess of carbonic acid readily and without fusion. Where the melting down is performed rapidly, the escaping gas is apt to cause trouble by frothing, and so causing waste of the material. Ordinary carbonate of soda, when hydrated (soda crystals), melts easily, and gives off its water with ebullition.

It is unfit for use in a.s.saying, but when dried it can be used instead of the bicarbonate. One part of the dried carbonate is equivalent to rather more than one and a half parts of the bicarbonate. From two to four parts of the flux are amply sufficient to yield a fluid slag with one part of earthy matter. This statement is also true of the fluxes which follow.

~Borax~ is a hydrated biborate of soda, containing nearly half its weight of water. When heated it swells up, loses its water, and fuses into a gla.s.s. The swelling up may become a source of loss in the a.s.say by pus.h.i.+ng some of the contents out of the crucible. To avoid this, _fused_ or _dried borax_ may be used, in which case a little more than half the amount of borax indicated will suffice. Borax will flux almost anything, but it is especially valuable in fluxing lime, &c., and metallic oxides; as also in those cases in which it is desired to keep certain of the latter in the slag and out of the b.u.t.ton of metal.

~Oxide of Lead~, in the form of red lead or litharge, is a valuable flux; it easily dissolves those metallic oxides which are either infusible or difficultly fusible of themselves, such as oxides of iron or copper. The resulting slag is strongly basic and very corrosive; no crucible will long withstand the attack of a fused mixture of oxides of lead and copper. With silicates, also, it forms very fusible double silicates; but in the absence of silicates and borates it has no action upon lime or magnesia. Whether the lead be added as litharge or as red lead, it will exist in the slag as monoxide (litharge); the excess of oxygen of the red lead is thus available for oxidising purposes. If this oxidising power is prejudicial, it may be neutralised by mixing the red lead with 1 per cent. of charcoal.

~Gla.s.s~: broken beakers and flasks, cleaned, dried, and powdered will do. It should be free from lead.

~Fluor~: fluor-spar as free as possible from other minerals, powdered.

It helps to flux phosphate of lime, &c., and infusible silicates.

~Lime~: should be fresh and powdered. It must not be slaked. Powdered white marble (carbonate of lime) will do; but nearly double the quant.i.ty must be taken. One part of lime produces the same effect as 1.8 parts of the carbonate of lime.

~Tartar~ and "black flux," are reducing agents as well as fluxes. The "black flux," which may be obtained by heating tartar, is a mixture of carbonate of potash and charcoal.

REDUCING AGENTS.--The distinction between reducing agents and fluxes (too often ignored) is an important one. Fluxes yield slags; reducing agents give b.u.t.tons of regulus or of metal. The action of a reducing agent is the separation of the oxygen or sulphur from the metal with which it is combined. For example, the mineral anglesite (lead sulphate) is a compound of lead, sulphur, and oxygen; by carefully heating it with charcoal the oxygen is taken away by the charcoal, and a regulus of lead sulphide remains. If the regulus be then fused with metallic iron the sulphur is removed by the iron, and metallic lead is left. The charcoal and the iron are reducing agents. But in defining a reducing agent as one which removes oxygen, or sulphur, from a metallic compound so as to set the metal free, it must be remembered that sulphur itself will reduce metallic lead from fused litharge, and that oxygen will similarly set free the metal in fused lead sulphide. There is no impropriety in describing sulphur as a reducing agent; but it is absurd to call oxygen one. Some confusion will be avoided if these substances and those which are opposite to them in property be cla.s.sed as oxidising and de-oxidising, sulphurising, and de-sulphurising agents. Most oxidising agents also act as de-sulphurisers.

_The de-oxidising agents_ most in use are the following:--

~Charcoal.~--Powdered wood charcoal; it contains more or less hygroscopic moisture and about 3 or 4 per cent. of ash. The rest may be considered carbon. Carbon heated with metallic oxides takes the oxygen; at low temperatures it forms carbon dioxide, and at higher ones, carbon monoxide. Other conditions besides that of temperature have an influence in producing these results; and as the quant.i.ty of charcoal required to complete a definite reaction varies with these, it should be calculated from the results of immediate experience rather than from theoretical considerations.

~Flour.~--Ordinary wheat flour is convenient in use. On being heated it gives off inflammable gases which have a certain reducing effect, and a residue of finely divided carbon is left. It is likely to vary in the quant.i.ty of moisture it contains. Two parts of flour should be used where one part of charcoal would be otherwise required.

~Tartar.~--This is crude hydric pota.s.sic tartrate; the purified salt, cream of tartar, may be used. On being heated it gives off inflammable gases, and leaves a residue formed of pota.s.sic carbonate mixed with finely divided carbon. Five parts of tartar should be used in the place of one of charcoal.

~Anthracite~ or ~Culm~ is a kind of coal containing 90 per cent. or more of carbon. It gives off no inflammable gas. It is denser, and takes longer in burning, than charcoal. Its reducing effect is little inferior to that of charcoal. Almost any organic substance can be used as a reducing agent, but it is well not to select one which melts, swells up, or gives off much water and gas when heated in the furnace.

~Pota.s.sic Cyanide~ is an easily fusible and somewhat volatile salt, which, when fused, readily removes oxygen and sulphur from metallic compounds, and forms pota.s.sic cyanate or sulphocyanate as the case may be. Commercial samples vary much in purity; some contain less than 50 per cent. of the salt. For a.s.saying, only the better qualities should be used.

~Iron~ is a de-sulphurising rather than a de-oxidising agent. Iron is used in the form of rods, 1/2-inch in diameter, or of nails, or of hoop iron. In the last case it should be thin enough to be bent without difficulty. Wrought iron crucibles are very useful in the processes required for making galena a.s.says.

_The chief oxidising agents (which are also de-sulphurisers)_ are the following:--

~Nitre~, or Pota.s.sic Nitrate.--This salt fuses very easily to a watery liquid. It oxidises most combustible substances with deflagration, and thereby converts sulphides into sulphates, a.r.s.enides into a.r.s.enates, and most metals into oxides. In the presence of strong bases, such as soda, the whole of the sulphur is fully oxidised; but in many cases some a.r.s.enic is apt to escape, and to give rise to a peculiar garlic-like odour. The sulphates of soda and potash are thus formed, and float as a watery liquid on the surface of the slag.

~Red lead~ is an oxide of lead. About one-quarter of its oxygen is very loosely held, and, hence, is available for oxidising purposes, without any separation of metallic lead. The rest of the oxygen is also available; but for each part of oxygen given off, about 13 parts of metallic lead are deposited. In silver a.s.says this power of readily giving up oxygen is made use of. The residual oxide (litharge) acts as a flux.

~Hot air~ is the oxidising agent in roasting operations. The sulphur and a.r.s.enic of such minerals as mispickel and pyrites are oxidised by the hot air and pa.s.s off as sulphur dioxide and "white a.r.s.enic." The metals generally remain in the form of oxide, mixed with more or less sulphate and a.r.s.enate. The residue may remain as a powdery substance (a calx), in which case the process of roasting is termed calcination; or it may be a pasty ma.s.s or liquid. In the calcination of somewhat fusible minerals, the roasting should be done at a low temperature to avoid clotting; a.r.s.enic and sulphur being with difficulty burnt off from the clotted mineral. A low temperature, however, favours the formation of sulphates; and these (if not removed) would reappear in a subsequent reduction as sulphides. These sulphates may be decomposed by a higher temperature towards the end of the operation; their removal is rendered more certain by rubbing up the calx with some culm and re-roasting, or by strongly heating the calx after the addition of solid ammonic carbonate. In roasting operations, as large a surface of the substance as possible should be exposed to the air. If done in a crucible, the crucible should be of the Cornish type, short and open, not long and narrow. For calcinations, _roasting dishes_ are useful: these are broad and shallow, not unlike saucers, but unglazed. In those cases in which the products of the roasting are liquid at the temperature used, a _scorifier_ (fig.

38) is suitable if it is desired to keep the liquid; but if the liquid is best drained off as quickly as it is formed, a _cupel_ (fig. 5) should be used.

[Ill.u.s.tration: FIG. 5.]

A scorifier is essentially a roasting dish sufficiently thick to resist, for a time, the corrosive action of the fused metallic oxides it is to contain. The essential property of a cupel is, that it is sufficiently porous to allow the fused oxide to drain into it as fast as it is formed. It should be large enough to absorb the whole of the liquid; and of course must be made of a material upon which the liquid has no corrosive action. Cupels do not bear transport well; hence the a.s.sayer generally has to make them, or to supervise their making. A quant.i.ty of bone ash is carefully mixed with water so that no lumps are formed, and the mixture is then worked up by rubbing between the hands. The bone ash is sufficiently wet when its cohesion is such that it can be pressed into a lump, and yet be easily crumbled into powder. Cupel moulds should be purchased. They are generally made of turned iron or bra.s.s. They consist of three parts (1) a hollow cylinder; (2) a disc of metal; and (3) a piston for compressing the bone ash and shaping the top of the cupel. The disc forms a false bottom for the cylinder. This is put in its place, and the cylinder filled (or nearly so) with the moistened bone ash. The bone ash is then pressed into shape with the piston, and the cupel finished with the help of three or four smart blows from a mallet. Before removing the piston, turn it half-way round upon its axis so as to loosen and smooth the face of the cupel. The cupel is got out by pressing up the disc of metal forming the false bottom; the removal is more easily effected if the mould is somewhat conical, instead of cylindrical, in form. The cupels are put in a warm place to dry for two or three days. A conveniently sized cupel is 1-1/4 inches in diameter and about 3/4 inch high. The cavity of the cupel is about 1/4 inch deep, and something of the shape shown in fig. 5.

[Ill.u.s.tration: FIG. 6.]

[Ill.u.s.tration: FIG. 7.]

[Ill.u.s.tration: FIG. 8.]

There are two kinds of furnaces required, the "wind" and "m.u.f.fle"

furnaces. These are built of brick, fire-brick, of course, being used for the lining. They are connected with a chimney that will provide a good draught. Figure 6 shows a section of the wind furnace, fig. 7 a section of the m.u.f.fle furnace, and fig. 8 a general view of a group comprising a m.u.f.fle and two wind furnaces suitable for general work.

When in operation, the furnaces are covered with iron-bound tiles. The opening under the door of the m.u.f.fle is closed with a loosely fitting brick. The floor of the m.u.f.fle is protected with a layer of bone-ash, which absorbs any oxide of lead that may be accidentally spilt. The fire bars should be easily removable.

Few tools are wanted; the most important are some cast-iron moulds, tongs (fig. 9), stirrers for calcining (fig. 10), and light tongs of a special form for handling scorifiers and cupels (_see_ SILVER).

[Ill.u.s.tration: FIG. 9.]

[Ill.u.s.tration: FIG. 10.]

The c.o.ke used should be of good quality; the formation of a fused ash (clinker), in any quant.i.ty, causes ceaseless trouble, and requires frequent removal. The c.o.ke should be broken into lumps of a uniform size (about 2 in. across) before being brought into the office. The furnace should be well packed by stirring, raising the c.o.ke and not ramming it, and it should be uniformly heated, not hot below and cold above. In lighting a furnace, a start is made with wood and charcoal, this readily ignites and sets fire to the c.o.ke, which of itself does not kindle easily.

In commencing work, add (if necessary) fresh c.o.ke, and mix well; make hollows, and into these put old crucibles; pack around with c.o.ke, so that the surface shall be concave, sloping upwards from the mouths of the crucibles to the sides of the furnace; close the furnace, and, when uniformly heated, subst.i.tute for the empty crucibles those which contain the a.s.says. It is rarely advisable to have a very hot fire at first, because with a gradual heat the gases and steam quietly escape through the unfused ma.s.s, while with too strong a heat these might make some of the matter in the crucible overflow. Moreover, if the heat should be too strong at first, the flux might melt and run to the bottom of the crucible, leaving the quartz, &c., as a pasty ma.s.s above; with a gentler heat combination is completed, and the subsequent fiercer heat simply melts the fusible compound into h.o.m.ogeneous slag.

The fused material may be left in the crucible and separated from it by breaking when cold. It is generally more convenient to pour it into cast-iron moulds. These moulds should be dry and smooth. They act best when warmed and oiled or black-leaded.

Air entering through the fire-bars of a furnace and coming in contact with hot c.o.ke combines with it, forming a very hot mixture of carbonic acid and nitrogen; this ascending, comes in contact with more c.o.ke, and the carbonic acid is reduced to carbonic oxide; at the top of the furnace, or in the flue, the carbonic oxide meeting fresh air, combines with the oxygen therein and re-forms carbonic acid. In the first and third of these reactions, much heat is evolved; in the second, the furnace is cooled a little. It must always be remembered, that the carbonic oxide of the furnace gases is a reducing agent. When these gases are likely to exert a prejudicial effect, and a strongly oxidising atmosphere is required, the work is best done in a _m.u.f.fle_.

FOOTNOTES:

[3] There is an exception to this, as when the slag is liable to be acted on when exposed to the air and to the gases of the furnace. In this case a layer of fused common salt floating on the slag, so as to protect it from the air and furnace gases, is a distinct advantage.