The Assay of Silver Ores.
A large proportion of the silver of commerce is extracted from ores
(which are too poor to allow of their being smelted or fused) by a
process called amalgamation. Founded on the ready solubility of silver,
&c., in metallic mercury, the ore is first crushed to powder, then
mixed with common salt, and afterwards roasted. By the adoption of this
plan the silver is reduced to a state of chloride. The roasting is
done in a reverberatory furnace, in which the heat is very gradually
raised, the ore being constantly stirred; the heat is then increased
sufficiently to raise the ore to a good red heat. It is then put into
wooden barrels, revolving on iron axles attached to the ends, and
scraps of iron are then added to it; both are then agitated together by
rotary motion, the effect of which is to reduce the chloride of silver
to a metallic state. When this is effected, it is again agitated with
mercury, and a fluid amalgam is formed with the metal, together with
any other metallic ingredient that may happen to be present in the
roasted ore. Subsequently, to recover the silver, the mercury is driven
off by heat, and the silver is thus left behind in an impure state.
There are three ways of assaying silver ores; they are in the _test_
assay as follows:--
1. Melting in a crucible.
2. Scorification.
3. Cupellation.
In the crucible assay the ore is commonly run down with a suitable
flux, those most frequently employed being litharge, carbonate of soda,
borax, and charcoal. These four substances are all that are required by
the practical assayer in the treatment of the regular ores of silver.
[Illustration: Fig. 2. Fire-clay Crucible.]
The assaying of the genuine ores is performed in the following manner;
that is, if they contain but little earthy matter. They may then be
conveniently treated by fusing with carbonate of soda, on account
of its cheapness, and borax, in a fire-clay crucible (Fig. 2). The
dimensions of the crucible should be as follows: 4-1/2 inches in
height, and 2-1/2 inches in its greatest diameter, which should be
at the top. A quantity of litharge (a semi-vitrious substance, oxide
of lead), more than is actually necessary to take up the whole of
the silver in the ore, should be added, so as to promote fusion, and
collect the ingredients into one mass at the bottom of the crucible.
In preparing the ore for the crucible, it must be well pounded, and
intimately mixed with the undermentioned chemicals:--
Pounded silver ore 240 grains.
Litharge 800 "
Carbonate of soda 700 "
Borax 300 "
Charcoal 50 "
Place two crucibles to warm during the time occupied in the preparation
of the mixture, then put it into the warm crucible; take 100 grains
more of litharge, and powder it over the contents in the vessel.
Prepare in this manner a second mixture for the other crucible, place
them both in the furnace, and put plenty of coke round them. The
mixtures may be melted in an ordinary wind or melting furnace, such
as is used by jewellers in the preparation of their material for
art working. The fusion should take place very gradually at first,
as silver in combination with lead is sensibly volatile at a high
temperature: it may then be continued at a low heat for twenty-five
minutes, and finally the operation may be completed with a full red
heat for five minutes longer.
During the process of fusing the contents of the crucible may be
watched by removing one of the bricks from the top of the furnace,
and when the whole mass has become quite liquid the crucible must
be seized with a pair of suitable tongs, tapped once or twice very
lightly against the side of the furnace to procure the settlement of
the contents, and immediately poured into an iron mould, previously
warmed and greased to prevent adhesion and spitting. Allow the mould
to remain for some time, in order to partially cool, and then plunge
it into a vessel of cold water. On cooling, the metallic elements
will be found incorporated into a button, the slag can then easily be
removed by tapping with a hammer on the edge, and the plunging into
cold water greatly facilitates this separation. The whole mass has then
to be cupelled, in order to separate the silver from the lead and other
metals.
[Illustration: Fig. 3. Fire-clay Fusing Cup.]
Silver ores, containing a large proportion of the sulphides (chemical
combinations of sulphur with metallic substances) of other metals,
may be easily assayed by the scorification process, which is, without
exception, applicable to the assay of all kinds of argentiferous ores;
and is one of the best, most simple, and most exact methods that can
possibly be employed in the extraction of silver from its ores. This
process, like that of fusion with litharge, already described, has the
effect of producing an alloy, and subsequently requires cupellation.
The ore is first well pounded, and then put into a small shallow vessel
made of close-grained refractory fire-clay (Fig. 3), with an excess
of finely granulated lead and some borax. The fusing cup or scorifier
employed in this process should be about 1-1/2 in. high and 2-1/2 ins.
in its greatest diameter; some assayers, however, use them deeper in
proportion to their width, and representing in form the end of an egg.
The object of this shape is to preserve the bath of molten metal at
the bottom, and that it may always be well covered and protected by
the slag on the top during the process of fusing. In the scorification
method the principles are exactly the reverse of those of the crucible
assay; for in the latter the object is to reduce the oxide of lead to
a metallic state, whereas in the former the metallic lead added to the
pounded ore in the scorifier is oxidized by being fused in contact with
the air. The charge for this assay may be as follows:--
Well pounded ore 60 grains.
Finely granulated lead 600 "
Borax anhydrous 100 "
Powdered anthracite 5 "
The cups or scorifiers should be charged in the following manner: well
mix the silver ore with 300 grains of granulated lead; place this
mixture in a scorifier, and add 300 grs. more of granulated lead, and
over the top of the whole put the burnt borax. The vessel may then be
placed in an ordinary assay furnace or muffle, as many being introduced
at one time as there is room for in the furnace, and submitted to the
strongest heat for about thirty minutes; during the greater portion
of this time the door should be kept closed, especially for the first
fifteen minutes. On opening the muffle-door a current of air passes
through the furnace, converting a portion of the lead into litharge;
this enters into combination with the earthy portions of the ore, the
other metallic sulphides, and also the borax, producing a fusible slag
on the surface of the metallic bath, extending over the whole surface
of the scorifier. The excess of lead is thus protected by this film or
flux from the oxidizing effects of the currents of air admitted into
the furnace, and remains united with whatever silver there may be in
the ore, in a metallic state.
The fusing should be continued longer than the thirty minutes--in
fact until the slag or flux is reduced into a perfectly liquid
state; stirring it well with a slender iron rod will facilitate the
operation, as it will tend to mix with the mass any hard portions
remaining undissolved and attached to the sides or other parts of
the vessels. This condition of the flux is absolutely indispensable;
when the slags are quite liquid, which with a strong fire will take
place in from thirty to forty minutes, wrap up in a piece of paper the
powdered anthracite, and drop it into the scorifier while still in the
furnace or muffle. The object of adding the anthracite at the last
moment is to reduce any minute portions of the metal that may exist
in the slags, and remain separated from the bulk. When the anthracite
has burnt off, which process usually takes about five minutes, this
point is considered to have been attained, and the operation is then
complete. The scorifier may be immediately withdrawn from the fire,
and the contents poured into a suitable casting-mould, of the form
represented in Fig. 4, a button of silver lead being the result. When
cold, the metallic mass is readily separated from the slag or flux by
slightly tapping with a hammer; the former may then be passed on to
the next operation, viz. to be purified of its lead by the process of
cupellation, which will be presently described.
When there is not enough borax present the assayer will observe an
infusible skin floating upon the surface; should this be the case more
borax must at once be employed, in order to dissolve such impurity.
When a chloride of silver ore is to be assayed, carbonate of soda must
be added to the mixture to prevent sublimation.
The following method of assaying is adopted in several large
Continental establishments, where the ores have, beside the usual
earthy matter and the sulphides of lead, an admixture of zinc, iron,
and copper. The process is precisely similar to the crucible assay,
in the case of genuine silver ores, as already described--with this
exception, that no more lead is added than the ores then contain--that
is, if we are treating _galena_ or _silver lead_; other ores require
different treatment according to their known composition. In this
process wrought-iron crucibles are employed having the form and shape
as shown in Fig. 5. They are made of thick iron plate, and are rendered
secure by welding the edges firmly together. Their dimensions are as
follows: a depth of 4-1/2 ins., with a thickness of iron at the bottom
of 1-1/2 in., and a 1/4 of an inch in the sides; the diameter at the
top of the crucible should be about 2-1/2 ins., and at the bottom
between 2 and 2-1/4 ins. A mechanical mixture or flux is prepared
to use with the ores to which we have referred, consisting of the
following chemicals, all of which should be finely powdered and well
mixed with the ore to be assayed:--
Carbonate of soda 6 parts.
Tartar 3 "
Saltpetre 2 "
Borax 1 part.
The furnace used for this assay is the ordinary one, having rather a
high chimney, to insure a perfect draught. In effecting the reduction
of the silver, the crucible is first placed as before on the fire, and
allowed to become hot; when this is accomplished, take
Well powdered ore 480 grains.
Prepared flux 500 "
These ingredients should be thoroughly mixed together, and put into the
red hot crucible. Fuse at a low heat for about twenty minutes, when
the whole will be in a perfect state of fusion; then give about five
minutes strong heat, and at the end of that time the crucible may be
withdrawn, and its contents poured into an iron mould, as represented
in Fig. 4, having one or two conical holes for the reception of the
fused mass. The silver and lead collect at the bottom of the mould by
reason of its high specific gravity. It may be removed by reversing
the position of the latter, when a gentle tap or two will deprive it
of that slag or flux which is usually attached to it. A large quantity
of silver can be readily collected from its ores by an alternate use
of crucibles, in which case it is possible to make a regular number of
fusions per hour. Wrought-iron crucibles, when strongly prepared and
carefully made, will stand about thirty of these fusions, giving way in
the end on account of the action of the sulphur contained in the ores.
[Illustration: Fig. 4. Iron Casting-moulds.]
Another kind of crucible, in addition to those already mentioned, is
used by the trade, and is recommended by many assayers as superior to
all others. Fig. 6 represents the form of it. It is about 4-1/2 ins.
high, and 2 ins. in its greatest interior diameter, being in the form
of a skittle. The charge consists of the following in this assay:
Finely powdered ore 60 grains.
Small pieces of iron 12 "
Black flux 180 "
Common salt 50 "
Put the powdered ore into the crucible, and place upon it the iron,
which should not be in the form of filings or dust, but in small
pieces; upon the ore and iron should be put the black flux, and
lastly the common salt must be placed above all these substances as
a protection against the air. The crucibles, as many as convenient,
may now be introduced into the furnace, and slowly raised to a strong
red heat, at which temperature they should be kept for about half an
hour; at the end of that period they should be removed from the fire,
slightly tapped to settle the contents, and then placed aside to cool.
When this has taken place, a few blows with a hammer near the base of
the crucibles, each in turn, will soon expose the button of silver
attached to the undecomposed iron; the latter substance may, however,
be easily detached by a few well-directed blows with the hammer.
[Illustration: Fig. 5. Iron Crucible for Assay.]
[Illustration: Fig. 6. Fire-clay Crucible for Assay.]
In order to ascertain the exact amount of the precious metal--that is,
the silver--contained in the buttons of lead obtained as the results of
the foregoing operations, they are subjected to a purifying process by
the metallurgist, called cupellation. By this means the lead and other
impurities are driven off by heat in contact with a current of air, and
the silver is left behind in a pure state. To perform this operation it
is necessary to expose the buttons on some absorbing medium or porous
support, and this support is commonly known as a _cupel_. No doubt
many porous substances could be made available for the formation of
cupels, but bone-ash is the best for all practical purposes, such as
are required by the assayer. The bone-ash, in the condition of a very
fine powder, is mixed with a little water in which has been dissolved
a small quantity of potash, and moulded into the desired shape. The
cupels are tightly consolidated by pressure in an iron mould of the
form shown in Fig. 7, which is the best in use, being well adapted for
the manufacture of cupels. It consists of a slightly conical steel
ring, 2 ins. in depth, and about 1-1/2 in. in diameter at the top
internally; a steel die with a wooden handle (Fig. 8) is made to fit
the mould. To make a cupel the space in the ring is nearly filled with
the moistened bone-ash, and pressed down by the hand, and afterwards
by the die, the latter being driven into the ring by the application
of a wooden mallet (Fig. 9) to the handle of the die. It will be seen
from the illustration that the die forms a cavity in the cupel capable
of receiving the charge of metal for assay. When the bone-ash has been
sufficiently compressed, the die is withdrawn, and the cupel removed
from the ring. This is a delicate operation, as sometimes the edges of
the cupel are liable to be injured; to prevent which and facilitate
the removal a loose plate of iron, exactly fitting the bottom of the
mould, should be introduced previous to putting in the bone-ash. The
iron plate of course being removed with the cupel, it must be replaced
before another can be made. By introducing a cylindrical piece of wood
to the lower aperture of the steel ring, the cupel can be removed
without difficulty.
[Illustration: Fig. 7. Cupel Mould.]
[Illustration: Fig. 8. Die for Cupel.]
[Illustration: Fig. 9. Wooden Mallet.]
The size of the cupel should always be regulated according to the
quantity of foreign matter to be absorbed, it being generally
understood that the material of which it is formed takes up double its
weight of lead. The process of cupelling is conducted in the furnace of
the assayer, an apparatus of peculiar construction, the most important
part of which, however, is the muffle (Fig. 10), consisting of a
small arched oven of fire-clay closed at one end, and furnished with
perpendicular slits in the sides, in order to allow of a free access of
air to the cupels inside.
[Illustration: Fig. 10. Assayer's Muffle for Cupels.]
[Illustration: Fig. 11. Cupel Tongs.]
[Illustration: Fig. 12. Cupels, section and perspective views.]
The position of the muffle in the furnace is so arranged that it can
be readily heated on every side; and when it has become red hot, six
or eight cupels, previously well dried, are taken and placed on the
floor of it, which should be covered with a thin layer of bone-ash.
The form of tongs required for this purpose is shown in Fig. 11. When
the cupels have been raised to the temperature of the muffle itself,
the assays are put in by a very slender pair of tongs, the door of
the furnace is then closed for a few minutes, when the metal will
have become fused, and the litharge will begin to be taken up by the
bone-ash of which the cupel is composed. The temperature of the furnace
is now lowered as much as possible, although not to such an extent that
it will retard the progress of oxidation and absorption. When nearly
the whole of the lead has been thus absorbed, the bead remaining will
have become very rich in silver, and, as the oxidation proceeds, will
appear much agitated, assuming a rapid circular movement, and revolving
with great rapidity. The silver gradually concentrates itself in the
centre of the cupel, taking the form of a globule, and at this stage
the fire should be made sharper, the operation being carefully watched.
When the last particle of lead leaves the silver, the agitation will
suddenly cease, and a beautiful phenomenon be witnessed, called by
assayers the _brightening_. The button of silver then becomes brilliant
and immovable, and the operation, when this takes place, is complete.
The cupel must be cooled with very great care, in order to prevent the
silver from _sprouting_; which if allowed to take place would result
in considerable loss, besides destroying the accuracy of the assay.
To prevent this sprouting it is a good plan immediately to cover the
cupel by another, which has been heated for that purpose; the two are
withdrawn together, and allowed to remain at the mouth of the muffle
until the silver has become solid; the metal is then in a state of
almost chemical purity, and may be detached and weighed. Previous to
the latter, however, it should be carefully cleansed from all foreign
matter, and flattened on a smooth-faced anvil, this process greatly
assisting in the removal of any oxide of lead, which not unfrequently
attaches itself to the globule of silver. The weighing is conducted
with a pair of scales having an extremely delicate balance; and where
any commercial transaction depends upon the accuracy of the assay, it
is always imperative to make several tests of the same sample, to avoid
the consequences of any accident or mistake.
The chief element in combination with silver on the large scale is
lead. Formerly the plan adopted in the separation of this metal
was cupellation alone. This process on the large scale is somewhat
different from that just described; and as it may appear to the reader
interesting and instructive, a brief explanation of it may not be
considered out of place.