1. In division A, B; _a_, _a_, is the magazine of salt; _b_, _b_, is the
hall of preparation of the ores; on the floor of which they are sorted,
interstratified, and mixed up with salt; _c_, _c_, are the roasting
furnaces; in each of which we see, 1, the fireplace; 2, 3, the
reverberatory hearth, divided into two portions, one a little higher
than the other, and more distant from the fireplace, called the _drier_.
The materials to be calcined fall into it, through a chimney 6. The
other part 2, of the hearth is the calcining area. Above the furnace are
chambers of sublimation 4, 5, for condensing some volatile matters which
escape by the opening 7. _e_ is the main chimney.
2. In the division B, C, we have _d_, the floor for the coarse sifting;
beneath, that for the fine sieves; from which the matters fall into the
hopper, whence they pass down to _g_, the mill-house, in which they are
ground to flour, exactly as in a corn-mill, and are afterwards boulted
through sieves, _p_, _f_, is the wheel machinery of the mill.
3. The compartment C, D, is the amalgamation work, properly speaking,
where the casks are seen in their places. The washing of the residuums
is effected in the shop _l_, below. _k_, _k_, is the compartment of
revolving casks.
4. In the division D, E, the distillation process is carried on. There
are four similar furnaces, represented in different states, for the sake
of illustration. The wooden drawer is seen below, supporting the
cast-iron basin, in which the tripod with its candelabra for bearing the
amalgam saucers is placed. _q_ is a store chamber.
At B, are placed the pulleys and windlass for raising the roasted ore,
to be sifted and ground; as also for raising the milled flour, to be
transported to the amalgamation casks. At D, the crane stands for
raising the iron bells that cover the amalgamation candelabra.
_Details of the Amalgamation Process, as practised at Halsbrücke._--All
ores which contain more than 7 lbs. of lead, or 1 lb. of copper, per
cent., are excluded from this reviving operation (_anquickverfahren_);
because the lead would render the amalgam very impure, and the copper
would be wasted. They are sorted for the amalgamation, in such a way
that the mixture of the poorer and richer ores may contain 7-1/2, or, at
most, 8 loths (of 1/2 oz. each) of silver per 100 lbs. The most usual
constituents of the ores are, sulphur, silver, antimonial silver
(speissglanzsilber), bismuth, sulphurets of arsenic, of copper, iron,
lead (nickel, cobalt), zinc, with several earthy minerals. It is
essential that the ores to be amalgamated shall contain a certain
proportion of sulphur, in order that they may decompose enough of sea
salt in the roasting to disengage as much chlorine as to convert all the
silver present into a chloride. With this view, ores poor in sulphur are
mixed with those that are richer, to make up a determinate average. The
ore-post is laid upon the _bed-floor_, in a rectangular heap, about 17
ells long, and 4-1/2 ells broad (13 yards and 3-1/2); and upon that
layer the requisite quantity of salt is let down from the floor above,
through a wooden tunnel; 40 cwts. of salt being allotted to 400 cwts. of
ore. The heap being made up with alternate strata to the desired
magnitude, must be then well mixed, and formed into small bings, called
_roast-posts_, weighing each from 3-1/2 to 4-1/2 cwts. The annual
consumption of salt at Halsbrücke is 6000 cwts.; it is supplied by the
Prussian salt-works.
_Roasting of the Amalgamation Ores._--The furnaces appropriated to the
roasting of the ore-posts are of the reverberatory class, provided with
soot chambers. They are built up alongside of the _bed-floor_, and
connected with it by a brick tunnel. The prepared ground ore (_erzmehl_)
is spread out upon the hearth, and dried with incessant turning over;
then the fire is raised so as to kindle the sulphur, and keep the ore
redhot for one or two hours; during which time, dense white-gray vapours
of arsenic, antimony, and water, are exhaled. The desulphuration next
begins, with the appearance of a blue flame. This continues for three
hours, during which the ignition is kept up; and the mass is diligently
turned over, in order to present new surfaces, and to prevent any
caking. Whenever sulphurous acid ceases to be formed, the finishing
calcination is to be commenced with increased firing; the object being
now to decompose the sea salt by means of the metallic sulphates that
have been generated, to convert them into chlorides, with the
simultaneous production of sulphate of soda. The stirring is to be
continued till the proofs taken from the hearth no longer betray the
smell of sulphurous, but only of muriatic acid gas. This roasting stage
lasts commonly three quarters of an hour, 13 or 14 furnaces are worked
at the same time at Halsbrücke; and each turns out in a week 5 tons
upon an average. Out of the _nicht_ chambers or soot vaults of the
furnaces, from 96 to 100 cwts. of ore-dust are obtained, containing 32
marcs (16 lbs.) of silver. This dust is to be treated like unroasted
ore. The fuel of the first fire is pitcoal; of the finishing one,
fir-wood. Of the former 115-1/2 cubic feet, and of the latter, 294-1/4,
are, upon an average, consumed for every 100 cwts. of ore.
During the last roasting, the ore increases in bulk by one fourth,
becomes in consequence a lighter powder, and of a brown colour. When
this process is completed, the ore is raked out upon the stone pavement,
allowed to cool, then screened in close sieve-boxes, in order to
separate the finer powder from the lumps. These are to be bruised, mixed
with sea salt, and subjected to another calcination. The finer powder
alone is taken to the millstones, of which there are 14 pairs in the
establishment. The stones are of granite, and make from 100 to 120
revolutions per minute. The roasted ore, after it has passed through the
boulter of the mill, must be as impalpable as the finest flour.
_The Amalgamation._--This (the _verquicken_) is performed in 20
horizontal casks, arranged in 4 rows, each turning upon a shaft which
passes through its axis; and all driven by the water-wheel shown in the
middle of _fig._ 1006. The casks are 2 feet 10 inches long, 2 feet 8
inches wide, inside measure, and are provided with iron ends. The staves
are 3-1/2 inches thick, and are bound together with iron hoops. They
have a double bung-hole, one formed within the other, secured by an iron
plug fastened with screws. They are filled by means of a wooden spout
terminated by a canvas hose; through which 10 cwts. of the boulted
ore-flour (_erzmehl_) are introduced after 3 cwts. of water have been
poured in. To this mixture, from 3/4 to 7/8 of a cwt. of pieces of iron,
1-1/2 inch square, and 3/8 thick, are added. When these pieces get
dissolved, they are replaced by others from time to time. The casks
being two thirds full, are set to revolve for 1-1/2 or 2 hours, till the
ore-powder and water become a uniform pap; when 5 cwts. of quicksilver
are poured into each of them. The casks being again made tight, are put
in geer with the driving machinery, and kept constantly revolving for 14
or 16 hours, at the rate of 20 or 22 turns in the minute. During this
time they are twice stopped and opened, in order to see whether the pap
be of the proper consistence; for if too thick, the globules of
quicksilver do not readily combine with the particles of ore; and if too
thin, they fall and rest at the bottom. In the first case, some water
must be added; in the second, some ore. During the rotation, the
temperature rises, so that even in winter it sometimes stands so high as
104° F.
The chemical changes which occur in the casks are the following:--The
metallic chlorides present in the roasted ore are decomposed by the
iron, whence results muriate of iron, whilst the deutochloride of copper
is reduced partly to protochloride, and partly to metallic copper, which
throw down metallic silver. The mercury dissolves the silver, copper,
lead, antimony, into a complex amalgam. If the iron is not present in
sufficient quantity, or if it has not been worked with the ore long
enough to convert the copper deutochloride into a protochloride,
previously to the addition of the mercury, more or less of the last
metal will be wasted by its conversion into protochloride (calomel). The
water holds in solution sulphate of soda, undecomposed sea salt, with
chlorides of iron, manganese, &c.
As soon as the revivification is complete, the casks must be filled with
water, set to revolve slowly (about 6 or 8 times in the minute), whereby
in the course of an hour, or an hour and a half at most, a great part of
the amalgam will have collected at the bottom; and in consequence of the
dilution, the portion of horn silver held in solution by the sea salt
will fall down and be decomposed. Into the small plug in the centre of
the bung, a small tube with a stopcock is now to be inserted, to
discharge the amalgam into its appropriate chamber. The cock must be
stopped whenever the brown muddy residuum begins to flow. The main bung
being then opened, the remaining contents of the casks are emptied into
the _wash-tun_, while the pieces of iron are kept back. The residuary
ore is found to be stripped of its silver within 5/32 or 7/40 of an
ounce per cwt. The emptying of all the casks, and charging them again,
takes 2 hours; and the whole process is finished within 18 or 20 hours;
namely, 1 hour for charging, 14 to 16 hours for amalgamating; 1-1/2 hour
for diluting; 1 hour for emptying. In 14 days, 3200 cwts. of ore are
amalgamated. For working 100 cwts. of ore, 14-1/2 lbs. of iron, and 2
lbs. 12-1/2 ounces of mercury, are required; whence, for every pound of
silver obtained, 0·95 of an ounce of mercury are consumed.
Trials have been made to conduct the amalgamation process in iron casks,
heated to 150° or 160° Fahrenheit, over a fire; but though the
de-silvering was more complete, the loss by mercury was so much greater
as to more than counterbalance that advantage.
_Treatment of the Amalgam._--It is first received in a moist canvas bag,
through which the thin uncombined quicksilver spontaneously passes. The
bag is then tied up and subjected to pressure. Out of 20 casks, from 3
to 3-1/2 cwts. of solid amalgam are thus procured, which usually consist
of 1 part of an alloy, containing silver of 12 or 13 _loths_ (in 16),
and 6 parts of quicksilver. The foreign metals in that alloy are,
copper, lead, gold, antimony, cobalt, nickel, bismuth, zinc, arsenic,
and iron. The filtered quicksilver contains moreover 2 to 3 loths of
silver in the cwt.
[Illustration: 1007]
_Fig._ 1007. represents the apparatus for distilling the amalgam in the
Halsbrücke works, marked _m_ in _fig._ 1006. _a_ is the wooden drawer,
sliding in grooves upon the basis _q_; B is an open basin or box of cast
iron, laid in the wooden drawer; _y_ is a kind of iron candelabra,
supported upon four feet, and set in the basin B; under _d_ are five
dishes or plates, of wrought iron, with a hole in the centre of each,
whereby they are fitted upon the stem of the candelabra, 3 inches apart,
each plate being successively smaller than the one below it. 3 indicates
a cast-iron bell, furnished with a wrought-iron frame and hook, for
raising it by means of a pulley and cord. _s_ is a sheet-iron door for
closing the stove, whenever the bell has been set in its place.
The box _a_, and the basin B, above it, are filled with water, which
must be continually renewed, through a pipe in the side of the wooden
box, so that the iron basin may be kept always submersed and cool. The
drawer _a_, being properly placed, and the plates under _d_ being
charged with balls of amalgam (weighing altogether 3 cwts.), the bell 3
is to be let down into the water, as at _y_, and rested upon the lower
part of the candelabra. Upon the ledge 1, which defines the bottom of
the fireplace, a circular plate of iron is laid, having a hole in its
middle for the bell to pass through. Upon this plate chips of fir-wood
are kindled, then the door _s_, which is lined with clay, is closed and
luted tight. The fuel is now placed in the vacant space _k_, round the
upper part of the bell. The fire must be fed in most gradually, first
with turf, then with charcoal; whenever the bell gets red, the mercury
volatilizes, and condenses in globules into the bottom of the basin B.
At the end of 8 hours, should no more drops of mercury be heard to fall
into the water, the fire is stopped. When the bell has become cool, it
is lifted off; the plates are removed from the candelabra _d_; and this
being taken out, the drawer _a_ is slid away from the furnace. The
mercury is drained, dried, and sent again into the amalgamation works.
The silver is fused and refined by cupellation.
The solid amalgam which is distilled in the above apparatus, would be
distilled more profitably out of iron trays set in the mercurial retorts
described and figured in pages 809, 810.
From 3 cwts. of amalgam, distilled under the bell, from 95 to 100 marcs
(1/2 lbs.) of _teller_ silver (dish silver) are procured, containing
from 10 to 13-1/2 parts of fine silver out of 16; one-fifth part of the
metal being copper. The _teller_ silver is refined in quantities of 160
or 170 marcs, in black-lead crucibles filled within two inches of their
brims, and submitted to brisk ignition. The molten mass exhales some
vapours, and throws up a liquid slag, which being skimmed off, the
surface is to be strewed over with charcoal powder, and covered with a
lid. The heat having been briskly urged for a short time, the charcoal
is then removed along with any fresh slag that may have risen, in order
to observe whether the vapours have ceased. If not, fresh charcoal must
be again applied, the crucible must be covered, and the heat increased,
till fumes are no longer produced, and the surface of the silver becomes
tranquil. Finally, the alloy, which contains a little gold, and much
copper, being now from 11 to 13 _löthig_ (that is, holding from 11 to 13
parts of fine silver in 16 parts), is cast into iron moulds, in ingots
of 60 marcs. The loss of weight by evaporation and skimming of the slag
amounts to 2 per cent.; the loss in silver is quite inconsiderable.
The dust from the furnace (_tiegelöfen_) is collected in a large
condensation chamber of the chimney, and affords from 40 to 50 marcs of
silver per cwt. The slags and old crucibles are ground and sent to the
small amalgamation mill.
The earthy residuum of the amalgamation casks being submitted to a
second amalgamation, affords out of 100 cwts. about 2 lbs. of coarse
silver. This is first fused along with three or four per cent. of a
mixture of potashes and calcined quicksalz, (impure sulphate of soda),
and then refined. The supernatant liquor that is drawn out of the tanks
in which the contents of the casks are allowed to settle, consists
chiefly of sulphate of soda, along with some common salt, sulphates of
iron and manganese, and a little phosphate, arseniate, and fluate of
soda. The earthy deposit contains from 1/4 to 9/32 of a _loth_ of silver
per cwt., but no economical method of extracting this small quantity has
yet been contrived.
[Illustration: 1008 1009 1010 1011]
The argentiferous or _rich lead_ is treated in Germany by the
cupellation furnace represented in _figs._ 1008, 1009, 1010, and 1011.
These figures exhibit the cupellation furnace of the principal smelting
work in the Hartz, where the following parts must be distinguished;
(_fig._ 1010.) 1, masonry of the foundation; 2, flues for the escape of
moisture; 3, stone covers of the flues; 4, bed of hard rammed scoriæ; 5,
bricks set on edge, to form the permanent area of the furnace; 6, the
sole, formed of wood ashes, washed, dried, and beaten down; _k_, dome of
iron plate, movable by a crane, and susceptible of being lined two
inches thick with loam; _n_, _n_, tuyères for two bellows _s_; having
valves suspended before their orifices to break and spread the blast;
_q_, door for introducing into the furnace the charge of lead, equal to
84 quintals at a time; _s_, _fig._ 1011., two bellows, like those of a
smith’s forge; _y_, door of the fireplace, through which billets of wood
are thrown on the grate; _x_, small aperture or door, for giving issue
to the frothy scum of the cupellation, and the litharge; _z_, basin of
safety, usually covered with a stone slab, over which the litharge
falls: in case of accident the basin is laid open to admit the _rich
lead_.
The following is the mode of conducting the cupellation. Before putting
the lead into the furnace, a floor is made in it of ashes beat carefully
down (see 6, _fig._ 1010.); and there is left in the centre of this
floor a circular space, somewhat lower than the rest of the hearth,
where the silver ought to gather at the end of the operation. The cupel
is fully six feet in diameter.
In forming the floor of a cupel, 35 cubic feet of washed wood ashes,
usually got from the soap works, are employed. The preparation of the
floor requires 2-1/2 hours’ work; and when it is completed, and the
movable dome of iron plate has been lined with loam, 84 quintals (cwt.)
of lead are laid on the floor, 42 quintals being placed in the part of
the furnace farthest from the bellows, and 42 near to the fire-bridge;
to these, scoriæ containing lead and silver are added, in order to lose
nothing. The movable lid is now luted on the furnace, and heat is slowly
applied in the fireplace, by burning fagots of fir-wood, which is
gradually raised. Section 1010. is in the line C, D, of 1009.
At the end of three hours, the whole lead being melted, the instant is
watched for when no more ebullition can be perceived on the surface of
the bath or melted metal; then, but not sooner, the bellows are set
a-playing on the surface at the rate of 4 or 5 strokes per minute, to
favour the oxidizement.
In five hours, reckoned from the commencement of the process, the fire
is smartly raised; when a grayish froth (_abstrich_) is made to issue
from the small aperture _x_ of the furnace. This is found to be a
brittle mixture of oxidized metals and impurities. The workman now
glides the rake over the surface of the bath, so as to draw the froth
out of the furnace; and, as it issues, powdered charcoal is strewed upon
it, at the aperture _x_, to cause its coagulation. The froth skimming
lasts for about an hour and a half.
After this time, the litharge begins to form, and it is also let off by
the small opening _x_; its issue being aided by a hook. In proportion as
the floor of the furnace gets impregnated with litharge, the workman
digs in it a gutter for the escape of the liquid litharge: it falls in
front of the small aperture, and concretes in stalactitic forms.
By means of the two movable valves suspended before the tuyères _n_,
_n_, (_fig._ 1010.) the workman can direct the blast as he will over the
surface of the metal. The wind should be made to cause a slight curl on
the liquid, so as to produce circular undulations, and gradually propel
a portion of the litharge generated, towards the edges of the cupel, and
allow this to retain its shape till the end of the operation. The stream
of air should drive the greater part of the litharge towards the small
opening _x_, where the workman deepens the outlet for it, in proportion
as the level of the metal bath descends, and the bottom of the floor
rises by the apposition of the litharge formed. Litharge is thus
obtained during about 12 hours; after which period the cake of silver
begins to take shape in the centre of the cupel.
Towards the end of the operation, when no more than four additional
quintals of litharge can be looked for, and when it forms solely in the
neighbourhood of the silver cake in the middle of the floor, great care
must be taken to set apart the latter portions, because they contain
silver. About this period, the fire is increased, and the workman places
before the little opening _x_ a brick, to serve as a mound to the efflux
of litharge. The use of this brick is,--1, to hinder the escape of the
silver in case of any accident; for example, should an explosion take
place in the furnace; 2, to reserve a magazine of litharge, should that
still circulating round the silver cake be suddenly absorbed by the
cupel, for in this dilemma the litharge must be raked back on the
silver; 3, to prevent the escape of the water that must be thrown on the
silver at the end of the process.
When the argentiferous litharge, collected in the above small magazine,
is to be removed, it is let out in the form of a jet, by the dexterous
use of the iron hook.
Lastly, after 20 hours, the silver cake is seen to be well formed, and
nearly circular. The moment for stopping the fire and the bellows is
indicated by the sudden disappearance of the coloured particles of oxide
of lead, which, in the latter moments of oxidation, undulate with
extreme rapidity over the slightly convex surface of the silver bath,
moving from the centre to the circumference. The phenomenon of their
total disappearance is called the _lightning_, or fulguration. Whenever
this occurs, the plate of silver being perfectly clean, there is
introduced into the furnace, by the door _q_, a wooden spout, along
which water, previously heated, is carefully poured on the silver.
The cupellation of 84 quintals of argentiferous lead takes in general 18
or 20 hours’ working. The promptitude of the operation depends on the
degree of purity of the leads employed, and on the address of the
operator, with whom also lies the economy of fuel. A good workman
completes the cupellation of 84 quintals with 300 billets, each
equivalent to a cubic foot and eight-tenths of wood (Hartz measure);
others consume 400 billets, or more. In general, the cupellation of 100
quintals of lead, executed at the rate of 84 quintal charges, occasions
a consumption of 790 cubic feet of resinous wood billets.