Refining and Coining of the Precious Metals
By SVEN GUMBINNER
(excerpted from the Ninth Annual Report of
the State Mineralogist for the year ending December 1, 1889, by the California State
Mining Bureau.)
All California crude gold, gold bars, etc., received at the mints,
that portion which is not gold and the alloy, silver, constitutes the so called base
metals, of which there are generally from ten to twenty parts in one thousand, of antimony
and lead, copper, iron, often bismuth, traces of platinum and palladium, and frequently a
small percentage of iridium.
All gold bars received in San Francisco from
the Pacific Coast are stamped with the gold fineness and value thereof by the assayers
casting the same, the silver contained therein allowed for at the market rate.
California gold bullion generally contains,
on an average, 12 per cent of silver in weight, while that of Australia has averaged but 5
per cent, though in the past few years silver in the Australian gold has materially
increased.
Generally silver bullion, as recovered from
their ores, are more or less auriferous. The silver produced from pyritic ore, such as
contain antimony, arsenic, and copper, is often very rich in gold, the poorest being
silver from lead ores. But, unlike gold, large quantities of silver have been extracted
from the ores in which not a trace of gold could be detected, notably at White Pine,
Nevada. The gold contained in the Comstock bullion has averaged fully one third its value,
frequently as much as one half.
"Doré bars" is a term applied to
such silver bars as contain gold, as well as being alloyed with a small percentage of base
metals. These bars are generally in the form of rectangular, oblong bars of various sizes,
from one thousand to one thousand five hundred ounces each, differing in the contents of
silver from six hundred to nine hundred and ninety per thousand, the base present chiefly
consisting of copper, lead, often antimony, and sometimes sulphur.
Silver bars are those which are free from gold, and sufficiently free from alloys to
render them fit for coinage and for the use in the arts.
"Base bars" contain a large
percentage of base alloys, usually comprising lead and antimony, or copper, differing in
their contents in silver from one hundred to six hundred in one thousand, and often
containing gold.
Melting and Preliminary Refining of
Deposited Bullion
At the mints deposits of every description
are invariably remelted, as a necessary precaution to a uniform mixture that a correct
sample for the assay may be obtained.
"Tough gold bars" and "fine
silver bars" are received at the mints without being remelted. The same are chipped
and the chips assayed to ascertain the correctness of the stamped fineness.
When the gold is received in dust, gold bars,
or the spongy gold from amalgamation, etc., it has to be subjected to a series of
metallurgical operations to obtain a state of purity sufficient for coinage or use in the
arts.
The separation of gold and silver is mostly
resorted to by means of sulphuric acid, which method is a very simple one, but it is
frequently rendered difficult by too large an admixture of base metals, owing to the
formation of salts from. base alloys; hence, the necessity of preliminary refining in
crucibles.
By this means there is obtained an alloy of
gold and silver, sufficiently free from base metals and suitable for the separation of the
gold from the silver.
Gold melts at 1,102 degrees C., and silver at
1,023 degrees C.
When gold is in a state of fusion, even in
the strongest heat of a blast furnace it is not appreciably volatile; it proves, however,
to be so in the heat of the oxyhydrogen jet, as a wire of gold placed therein soon
disperses in vapor.
Silver volatilizes more readily when in a
molten state, and rapidly so when in free contact with air.
The fusion of the deposits is performed in
black lead crucibles, placed in the ordinary wind furnace, by means of a stone-coal fire.
The deposits are placed in the crucible with a small amount of borax, which latter,
besides taking up the earthy impurities, assists in the fusion of the metals.
When the deposit is melted, a small quantity
of niter is added, which exerts a powerful oxidizing action on the base metals. If the
deposit is very base the addition of niter is repeated several times, caution being taken
to use but little at a time, as by too rapid an action the contents would overflow.
The molten mass is well stirred with a black
lead spatula heated to bright red, which insures a perfect uniformity of the bar after
casting, and immediately after the last stirring the crucible is lifted out of the furnace
by means of a pair of long tongs, and the metal poured into a mold previously warmed and
oiled.
It requires practice and dexterity to take
the melting-pot from the furnace and cast the bar, so that there should be no spilling or
chilling of the mass. The gold immediately sets in the mold, and the slag, consisting of
the grosser impurities, accumulates on the surface.
The bar, while red, is turned out of the
mold, the slag is removed from the metal, the bar immersed in the pickling vat, in which
there is water acidulated with sulphuric acid, then dipped into clean water to remove the
acid stain, the bar retaining warmth enough after removal to expel all moisture; after
which it is cleansed of any adherent particles of slag, and hammered smooth. The bar is
then chipped; that is, cuttings are taken from two diagonal corners, the chips are
flattened, the rough edges of which are removed by shears, and these chips, or
representatives of the melt, are assayed to ascertain the fineness of the bar.
The slags are ground finely in a mortar, or,
as in the San Francisco Mint, under chili rollers, and vanned; the very small proportion
of recovered grains are dried, weighed, and allowed for with the chips in computing the
value of the assayed bar. This admirable system secures the depositor the full value of
his gold. Naturally, in the metallurgical treatment of the precious metals, some loss is
always sustained, but that incurred in the process above described is very rarely more
than a mechanical loss of ten cents on a deposit of one hundred ounces.
The residual from the vannings is allowed to
accumulate, and when sufficiently large, having been dried, is melted with a small
addition of borax in an old black lead crucible at a high heat. The crucible and contents
are allowed to remain in the furnace over night to cool. The crucible is then broken and
the small particles of metal found adhering to the bottom are often principally silver
from the action of the niter. It will be found that invariably the amount of gold
recovered from these slags does not return sufficient to pay for fuel and labor involved.
Frequently the spongy gold obtained by
amalgamation contains such a large quantity of iron that it is necessary to separate the
latter from the former by sulphur. This sulphurization is effected in a plumbago crucible.
The metal is kept at a melting heat, for when
the temperature is too high the molten sulphur is wastefully volatilized. Into the molten
metal sulphur is sprinkled near the sides of the crucible, because if the sulphur is
thrown upon the center, particles of gold are projected to the sides and adhere thereto,
and are very difficult to remove. The mass is carefully stirred with a black lead stirrer
(previously heated to bright redness); the stirring is resorted to after each small
addition of sulphur, the latter being added till the gold is entirely freed from its iron
impurities. Sulphur, when intermixed with molten gold, has no action upon the latter, but
upon the iron present it acts energetically, forming sulphide of iron; the sulphur also
having a strong affinity for silver, takes up a small proportion thereof in the form of
sulphide of silver, but not rapidly until nearly all of the iron has first been converted
into sulphide.
The gold being now almost freed of iron, will
subside to the bottom of the crucible. The crucible is now removed from the fire, gently
tapped, and allowed to cool. When cold the mass is removed by inverting the crucible. It
is then struck with a hammer; the gangue separates from the gold; the latter is then
remelted and cast into a bar. This plan is superior to pouring the metal into a mold,
because the lump of gold is better formed — is level on the top, and separates
readily from the gangue.
When a large quantity of lead is present with
the metal to be operated upon it frequently happens, owing, most likely, to the presence
of basic sulphide of lead, that the niter employed acts but slowly in oxidizing the
latter. By a free use of repeated additions of sub-ammoniac (chloride of ammonia), and
alternate additions of niter, the lead will become more readily oxidized. Should any
perceptible osm-iridium be present, the bar is remelted in a crucible clean of adherent
flux. After melting it is kept at a high state of fusion for several minutes, the
osm-iridium will settle, intermixed with the gold, in the bottom of the crucible. The
crucible is then gently lifted out of the furnace and the greater portion carefully, but
rapidly, poured into a mold. The remaining small portion of gold, now containing nearly
all the osm-iridium, is allowed to cool in the crucible till it is set. It is then
removed, chipped, and assayed separately, to determine the amount of osm-iridium. When
sufficient quantities of these cones of gold with osm-iridium have accumulated to warrant
the extraction of the osm-iridium, the following are the plans adopted:
To the gold with the osm-iridium, three parts
of silver are added, melted, and again proceeded with, as in the first operation. The
remaining silver and gold alloy, rich in osm-iridium, is several times melted with silver,
whereby the osm-iridium becomes more and more concentrated with the silver and replacing
to a great extent the original gold. After the last addition of silver the metal is
granulated, the silver extracted by boiling concentrated sulphuric acid, and the separated
gold washed out, leaving the osm-iridium. Or the gold containing the osm-iridium is
dissolved by aqua regia, which does not attack the osm-iridium. The gold is afterward
precipitated from its solution by sulphate of protoxide of iron, the residue well washed
with boiling water, dried and fused with niter in a French clay crucible. Such osm-iridium
is worth from $2 to $5 per ounce; pure iridium being worth about $20 per ounce; whereas
selected grains of osm-iridium, suitable for pen points, are valued at from $50 to $70 per
ounce, according to their size and quality. The osm-iridium grains are used for the points
of gold pens called " diamond points.” The latest discoveries have rendered the
metal iridium available in the arts, many useful applications being made, such as
"iridium bearings,” made for fine assay scales and balances; iridium draw plates
in place of ruby plates, hyperdermic needles for physicians and surgeons, which are made
of gold and tipped with iridium in place of steel; and, also, for points of
surveyors’ and engineers’ instruments.
Gold bars that are received at the mints and
are of sufficient purity for the subsequent "parting” process are simply
remelted without addition of any fluxes, stirred and cast into bars, chipped and assayed
to determine their value.
In making a bar of cleaned skimmed metal it
is necessary to throw a little powdered charcoal into the crucible before pouring into a
mold, which insures a free flow of the metal from the crucible; it furthermore prevents
spattering in pouring.
Gold bars received from the chlorination works are more or less brittle, therefore
rendering this otherwise fine gold unfit for coinage, and this brittleness is generally
ascribed to the presence of very small quantities of such metals as lead, antimony,
arsenic, and bismuth, which metals have to be removed from the fine gold in order to
render it "tough.”
These kind of bars are remelted in black lead
crucibles, in which no fluxing had been done. When in a state of fusion repeated additions
of sal-ammoniac is sprinkled on the metal, which exerts its influence on the lead
principally, after which repeated additions are made of small quantities of powdered corrosive sublimate (bichloride of mercury). Care must
be taken that upon the application of the latter the furnace door be at once closed, as
dense and highly injurious fumes are immediately evolved. The corrosive sublimate is found
to effect the complete toughening of the gold, by which the base metals are converted into
volatile chlorides, and although the corrosive sublimate is only thrown on the surface of
the molten gold (be there even two hundred to three hundred pounds of gold in a crucible),
yet the whole mass is toughened by its action. When the gold is supposed to be toughened,
a small portion of it is poured into a long, flat, iron mold. When set and cold it is
folded back upon itself by hammering, and if not the slightest fracture is sustained the
gold is considered tough; a little powdered charcoal is then sprinkled into the crucible,
the mass well stirred, and poured into an iron mold. Silver bars are often very brittle,
this condition being caused by the presence of antimony, arsenic, etc., rendering the
silver unfit for coinage. In this case the bars are remelted with gold for subsequent
parting.
A large amount of sulphur in silver bars
renders the condition of the silver such that a correct assay cannot be had. The sulphur
is reduced by introducing into the molten silver, with frequent stirring, wrought iron
bars; the sulphur acting upon the metallic iron converts it into a sulphide, by which a
complete desulphurization takes place; the metal is then skimmed, well stirred, and cast
into bars. In the mints, all silver bars, excepting those which are the so termed fine
silver bars, are remelted in black lead crucibles.
The metal, when fused, is kept constantly
covered with charcoal, to prevent the too easily volatilization of the silver; it is
diligently stirred with a black lead spatula; a sample is then taken from the metal by a
plumbago dipper and poured into water; half of the contents of the crucible is then dipped
out and poured into a mold, another sample is taken by the dipper, and the balance of the
metal poured from the crucible into another mold.
The two representative samples are dried,
assayed preliminarily by cupellation, and the exact value determined by the " humid
" method. The bars are melted with gold bars for separation.
During the period of the free coinage of
silver, or when a demand is created in the market for refined silver bars, which latter
are principally sent to India and China, the heretofore mentioned auriferous silver bars,
in place of being cast into bars again, were at some establishments cast into slabs for
the direct separation by nitric or sulphuric acid, in place of being inquarted with gold
and then granulated. These slabs were generally sixteen inches long, nine inches wide, and
from three fourths to one inch in thickness.
The former method of using basket tongs, or
the crane; for removing melts from the furnace, has been superseded by the following:
Two men grasp the crucible on opposite sides
with tongs whose jaws are made to suit the thickness of the crucible. The crucible is
quickly lifted high enough out of the furnace to allow the metal to be poured into the
mold in front of them; taking, therefore, less time than even to adjust the basket tongs.
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