METHODS OF ASSAYING.--DRY GRAVIMETRIC METHODS.
The methods of assaying are best classed under two heads, Gravimetric
and Volumetric, in the former of which the final results are weighed,
whilst in the latter they are measured. A commoner and older division is
expressed in the terms much used in practice--wet assays and dry assays.
Wet assays include all those in which solvents, &c. (liquid at the
ordinary temperature), are mainly used; and dry assays, those in which
solid re-agents are almost exclusively employed. Dry assays form a
branch of gravimetric work, and we shall include under this head all
those assays requiring the help of a wind furnace. Wet assays, as
generally understood, would include not only those which we class as wet
gravimetric assays, but also all the volumetric processes.
~Gravimetric Methods~ aim at the separation of the substance from the
other matters present in the ore, so that it may be weighed; and,
therefore, they must yield the _whole_ of the substance in a pure state.
It is not necessary that a metal should be weighed as metal; it may be
weighed in the form of a compound of definite and well known
composition. For example, one part by weight of silver chloride contains
(and, if pure, always contains) 0.7527 part of silver; and a quantity of
this metal can be as exactly determined by weighing it as chloride as by
weighing it in the metallic state. But in either case the metal or its
chloride must be pure.
Exact purity and complete separation are not easily obtained; and
methods are used which are defective in one or both of these respects.
It is well to note that an impure product increases the result, whilst a
loss of the substance decreases it; so that if both defects exist in a
process they tend to neutralise each other. Of dry methods generally, it
may be said that they neither give the whole of the substance nor give
it pure; so that they are only calculated to show the amount of metal
that can be extracted on a manufacturing scale, and not the actual
quantity of it present. Their determinations are generally rough and
always low. The gold and silver determinations, however, will compare
very favourably with any of the other processes for the estimation of
these metals in their ores.
The calculation of the results of a gravimetric assay has already been
referred to. If the result is to be stated as percentage, it may always
be done by the following rule:--_Multiply the weight of the substance
got by the percentage of metal it contains, and divide by the weight of
ore taken._
Gravimetric methods are divided into three groups: (1) mechanical
separations; (2) dry methods; and (3) wet methods.
~Mechanical Separations.~--Under this head are classed the method of
assaying tin ores, known as vanning, and the amalgamation assay for
gold. A set of sieves to determine the relative proportion of powders of
different degrees of fineness is sometimes useful. A set with 10, 20, 40
and 80 meshes to the inch is convenient.
~Dry Assays.~--An important distinction between wet and dry methods of
assaying is, that in the former the substance is got into the liquid
state by solution, whilst in the latter fusion is taken advantage of.
The difference between solution and fusion is easily illustrated: 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 assayer, 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:--
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| | |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 | --
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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 quantity, 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 quantities 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 glass, but with more yield a lumpy scoriacious mass. 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 glass, 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.
[Illustration: 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 button 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 assaying operations the object is generally to concentrate
the metal sought for in a button 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
lustre. 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 button 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
button 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 glass 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 assay will remain diffused through
the slag, instead of fusing into a button of regulus, if the slag
contain sulphide of sodium. The same is true of other sulphides if not
present in too great a quantity, and if the temperature is not too high.
_Speises_ are compounds of a metal or metals with arsenic. 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 arsenic
and, if necessary, a reducing agent. The product is fused with more
arsenic under a slag, consisting mainly of borax. They are very fusible,
brittle compounds. On exposure to the air at a red heat the arsenic 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 button 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 assaying, 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 glass. The swelling up may become a source of loss in the assay
by pushing 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 button 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.
~Glass~: 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 quantity
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 buttons 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 classed 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 quantity 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
quantity 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 potassic tartrate; the purified salt,
cream of tartar, may be used. On being heated it gives off inflammable
gases, and leaves a residue formed of potassic 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.
~Potassic Cyanide~ is an easily fusible and somewhat volatile salt,
which, when fused, readily removes oxygen and sulphur from metallic
compounds, and forms potassic 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 assaying, 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 assays.
_The chief oxidising agents (which are also de-sulphurisers)_ are the
following:--
~Nitre~, or Potassic Nitrate.--This salt fuses very easily to a watery
liquid. It oxidises most combustible substances with deflagration, and
thereby converts sulphides into sulphates, arsenides into arsenates, 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
arsenic 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 assays 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
arsenic of such minerals as mispickel and pyrites are oxidised by the
hot air and pass off as sulphur dioxide and "white arsenic." The metals
generally remain in the form of oxide, mixed with more or less sulphate
and arsenate. 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 mass or liquid. In the calcination of somewhat fusible minerals,
the roasting should be done at a low temperature to avoid clotting;
arsenic 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.
[Illustration: 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 assayer
generally has to make them, or to supervise their making. A quantity 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 brass. 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.
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
There are two kinds of furnaces required, the "wind" and "muffle"
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 muffle furnace, and fig. 8 a general view of a group
comprising a muffle 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 muffle is closed with a loosely fitting
brick. The floor of the muffle 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).
[Illustration: FIG. 9.]
[Illustration: FIG. 10.]
The coke used should be of good quality; the formation of a fused ash
(clinker), in any quantity, causes ceaseless trouble, and requires
frequent removal. The coke 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 coke 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 coke, which of itself does not kindle
easily.
In commencing work, add (if necessary) fresh coke, and mix well; make
hollows, and into these put old crucibles; pack around with coke, 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, substitute for the empty crucibles those which contain
the assays. 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 mass, 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 mass above; with a gentler
heat combination is completed, and the subsequent fiercer heat simply
melts the fusible compound into homogeneous 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 coke combines with it, forming a very hot mixture of carbonic
acid and nitrogen; this ascending, comes in contact with more coke, 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 _muffle_.
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.