§ 1. The mechanical preparation of ores, including _picking_,
_stamping_, and different modes of washing.
§ 2. The chemical preparation, consisting especially in the roasting or
calcination of the ores.
§ 3. The assay of ores, comprehending the mechanical part: that is, by
washing; the chemical part, or assays by the _dry way_; and the assays
by the _moist way_.
_Section_ 1. _Of the mechanical preparation or dressing of ores._--I.
The first picking or sorting takes place in the interior, or
underground, workings, and consists in separating the fragments of
rocks, that apparently contain no metallic matter, from those that
contain more or less of it. The external aspect guides this separation;
as also the feeling of density in the hand.
The substances when turned out to the day, undergo another _sorting_,
with greater or less care, according to the value of the included metal.
This operation consists in breaking the lumps of ore with the hammer,
into fragments of greater or less size, usually as large as the fist,
whereby all the pieces may be picked out and thrown away that contain no
metal, and even such as contain too little to be smelted with advantage.
There is for the most part, a building erected near the output of the
mine, in which the breaking and picking of the ores are performed. In a
covered gallery, or under a shed, banks of earth are thrown up, and
divided into separate beds, on each of which a thick plate of cast iron
is laid. On this plate, elderly workmen, women, and children, break the
ores with hand hammers, then pick and sort them piece by piece. The
matters so treated, are usually separated into three parts; 1. the rock
or sterile gangue, which is thrown away; 2. the ore for the stamping
mill, which presents too intimate a mixture of rock and metallic
substance to admit of separation by breaking and picking; and 3. the
pure ore, or at least the very rich portion, called the _sorted mine_ or
the _fat ore_. On the sorting floors there remains much small rubbish,
which might form a fourth subdivision of ore, since it is treated in a
peculiar manner, by sifting, as will be presently mentioned.
The distribution of fragments more or less rich, in one class or
another, is relative to the value of the included metal, taking into
account the expenses necessary for its extraction. Thus in certain lead
mines, pieces of the gangues are thrown away, which judged by the eye
may contain 3 per cent. of galena, because it is known that the greater
portion of this would be lost in the washings required for separating
the 97 parts of the gangue, and that the remainder would not pay the
expenses.
II. The very simple operations of _picking_ are common to almost all
ores; but there are other operations requiring more skill, care, and
expense, which are employed in their final state of perfection only upon
ores of metals possessing a certain value, as those of lead, silver, &c.
We allude to the _washing_ of ores.
The most simple and economical washings are those that certain iron
ores, particularly the alluvial, are subjected to, as they are found
near the surface of the ground agglutinated in great or little pieces.
It is often useful to clean these pieces, in order to pick out the
earthy lumps, which would be altogether injurious in the furnaces.
This crude washing is performed sometimes by men stirring in the midst
of a stream of water, with iron rakes or shovels, the lumps of ore
placed in large chests, or basins of wood or iron.
In other situations, this washing is executed more economically by a
machine called a _buddle_ or dolly-tub by our miners. A trough of wood
or iron, with a concave bottom, is filled with the ore to be washed.
Within the tub or trough, arms or iron handles are moved round about,
being attached to the arbor of a hydraulic wheel. The trough is kept
always full of water, which as it is renewed carries off the earthy
matters, diffused through it by the motion of the machine, and the
friction among the pieces of the ore. When the washing is finished, a
door in one of the sides of the trough is opened, and the current
removes the ore into a more spacious basin, where it is subjected to a
kind of picking. It is frequently indeed passed through sieves in
different modes. See LEAD and TIN, for figures of _buddles_ and
_dollies_.
[Illustration: 670]
III. _Stamping._ Before describing the refined methods of washing the
more valuable ores of copper, silver, lead, &c., it is proper to point
out the means of reducing them into a powder of greater or less
fineness, by _stamping_, so called from the name _stamps_ of the pestles
employed for that purpose. Its usefulness is not restricted to preparing
the ores; for it is employed in almost every smelting house for pounding
clays, charcoal, scoriæ, &c. A stamping mill or pounding machine, _fig._
670., consists of several movable pillars of wood _l l l_, placed
vertically, and supported in this position between frames of carpentry K
K K. These pieces are each armed at their under end with a mass of iron
_m_. An arbor or axle _a a_, moved by water, and turning horizontally,
tosses up these wooden pestles, by means of wipers or cams, which lay
hold of the shoulders of the pestles at _l l l_. These are raised in
succession, and fall into an oblong trough below _m m_, scooped out in
the ground, having its bottom covered either with plates of iron or hard
stones. In this trough, beneath these pestles, the ore to be stamped is
allowed to fall from a hopper above, which is kept constantly full.
The trough is closed in at the sides by two partitions, and includes
three or four pestles; which the French miners call a battery. They are
so disposed that their ascent and descent take place at equal intervals
of time.
Usually a stamping machine is composed of several batteries (two, three,
or four), and the arrangement of the wipers on the arbor of the
hydraulic wheel is such that there is constantly a like number of
pestles lifted at a time; a circumstance important for maintaining the
uniform going of the machine.
The matters that are not to be exposed to subsequent washing are stamped
dry, that is without leading water into the trough; and the same thing
is sometimes done with the rich ores, whose lighter parts might
otherwise be lost.
Most usually, especially for ores of lead, silver, copper, &c., the
trough of the stamper is placed in the middle of a current of water, of
greater or less force; which, sweeping off the pounded substances,
deposits them at a greater or less distance onwards, in the order of the
size and richness of the grain; constituting a first washing, as they
escape from beneath the pestles.
In the dry stamping, the fineness of the powder depends on the weight of
the pestles, the height of their fall, and the period of their action
upon the ore; but in the stampers exposed to a stream of water, the
retention of the matters in the trough is longer or shorter, according
to the facility given for their escape. Sometimes these matters flow out
of the chest over its edges, and the height of the line they must
surmount has an influence on the size of the grain; at other times, the
water and the pounded matter which it carries off, are made to pass
through a grating, causing a kind of sifting at the same time. There
are, however, some differences in the results of these two methods.
Lastly, the quantity of water that traverses the trough, as well as its
velocity, has an influence on the discharge of the pounded matters, and
consequently on the products of the stampers.
The size of the particles of the pounded ore being different, according
to the variable hardness of the matters which compose them, suggests the
means of classing them, and distributing them nearly in the order of
their size and specific gravity, by making the water, as it escapes from
the stamping trough, circulate in a system of canals called a
_labyrinth_, where it deposits successively, in proportion as it loses
its velocity, the earthy and metallic matters it had floated along.
These metalliferous portions, especially when they have a great specific
gravity like galena, would be deposited in the first passages, were it
not that from their hardness being inferior to that of the _gangue_,
they are reduced to a much finer powder, or into thin plates, which seem
to adhere to both the watery and earthy particles; whence they have to
be sought for among the finest portions of the pulverised gangue, called
slime, _schlich_, or _schlamme_.
There are several methods of conducting the stamps; in reference to the
size of the grains wished to be obtained, and which is previously
determined agreeably to the nature of the ore, and of the gangue; its
richness, &c. The height of the slit that lets the pounded matters
escape, or the diameters of the holes in the grating, their distance,
the quantity of water flowing in, its velocity, &c., modify the result
of the stamping operation.
When it is requisite to obtain powder of an extreme fineness, as for
ores that are to be subjected to the process of amalgamation, they are
passed under millstones, as in common corn mills; and after grinding,
they are bolted so as to form a species of flour; or they are crushed
between rolls. See LEAD and TIN.
_Washing of ores._
IV. The ores pounded under the stamps are next exposed to very delicate
operations, both tedious and costly, which are called the _washings_.
Their purpose is to separate mechanically the earthy matters from the
metallic portion, which must therefore have a much higher specific
gravity; for otherwise, the washing would be impracticable.
The medium employed to diminish the difference of specific gravity, and
to move along the lightest matters, is water; which is made to flow with
greater or less velocity and abundance over the schlich or pasty mud
spread on a table of various inclination.
But as this operation always occasions, not only considerable expense,
but a certain loss of metal, it is right to calculate what is the degree
of richness below which washing is unprofitable; and on the other hand,
what is the degree of purification of the _schlich_ at which it is
proper to stop, because too much metal would be lost comparatively with
the expense of fusing a small additional quantity of gangue. There
cannot, indeed, be any fixed rule in this respect, since the elements of
these calculations vary for every work.
Before describing the different modes of washing, we must treat of the
sifting or riddling, whose purpose, like that of the labyrinth
succeeding the stamps, is to distribute and to separate the ores (which
have not passed through the water stamps) in the order of the coarseness
of grain. This operation is practised particularly upon the debris of
the mine, and the rubbish produced in breaking the ores. These
substances are put into a riddle, or species of round or square sieve,
whose bottom is formed of a grating instead of a plate of metal pierced
with holes. This riddle is plunged suddenly and repeatedly into a tub or
cistern filled with water. This liquid enters through the bottom, raises
up the mineral particles, separates them and keeps them suspended for an
instant, after which they fall down in nearly the order of their
specific gravities, and are thus classed with a certain degree of
regularity. The sieve is sometimes dipped by the immediate effort of the
washer; sometimes it is suspended to a swing which the workman moves; in
order that the riddling may be rightly done, the sieve should receive
but a single movement from below upwards; in this case the ore is
separated from the gangue, and if there be different specific gravities,
there is formed in the sieve as many distinct strata, which the workman
can easily take out with a _spatula_, throwing the upper part away when
it is too poor to be re-sifted. This operation by the hand-sieve, is
called _riddling in the tub_, or riddling by deposit.
We may observe, that during the sifting, the particles which can pass
across the holes of the bottom, fall into the tub and settle down there;
whence they are afterwards gathered out, and exposed to washing when
they are worth the trouble.
Sometimes, as at Poullaouen, the sieves are conical, and held by means
of two handles by a workman; and instead of receiving a single movement,
as in the preceding method, the sifter himself gives them a variety of
dexterous movements in succession. His object is to separate the poor
portions of the ore from the richer; in order to subject the former to
the stamp mill.
Among the siftings and washings which ores are made to undergo, we must
notice among the most useful and ingenious, those practised by _iron
gratings_, called on the Continent _grilles anglaises_, and the
_step-washings_ of Hungary, _laveries à gradins_. These methods of
freeing the ores from the pulverulent earthy matters, consist in placing
them, at their out-put from the mine, upon gratings, and bringing over
them a stream of water, which merely takes down through the bars the
small fragments, but carries off the pulverulent portions. The latter
are received in cisterns, where they are allowed to rest long enough to
settle to the bottom. The washing by steps is an extension of the
preceding plan. To form an idea, let us imagine a series of grates
placed successively at different levels, so that the water, arriving on
the highest, where the ore for washing lies, carries off a portion of
it, through this first grate upon a second closer in its bars, thence to
a third, &c., and finally into labyrinths or cisterns of deposition.
[Illustration: 671]
The _grilles anglaises_ are similar to the _sleeping tables_ used at
Idria. The system of these _en gradins_ is represented in _fig._ 671.
There are 5 such systems in the works at Idria, for the sorting of the
small morsels of quicksilver ore, intended for the stamping mill. These
fragments are but moderately rich in metal, and are picked up at random,
of various sizes, from that of the fist to a grain of dust.
These ores are placed in the chest _a_, below the level of which 7
grates are distributed, so that the fragments which pass through the
first _b_, proceed by an inclined conduit on to the second grate _c_,
and so in succession. (See the conduits _l_, _o_, _p_). In front, and on
a level with each of the grates _b_, _c_, _d_, &c., a child is stationed
on one of the floors, 1, 2, 3, to 7.
A current of water, which falls into the chest _a_, carries the
fragments of ore upon the grates. The pieces which remain upon the two
grates _b_ and _c_, are thrown on the adjoining table _v_, where they
undergo a sorting by hand; there the pieces are classified, 1. into
gangue to be thrown away; 2. into ore for the stamp mill; 3. into ore to
be sent directly to the furnace. The pieces which remain on each of the
succeeding grates, _d_, _e_, _f_, _g_, _h_, are deposited on those of
the floors 3 to 7, in front of each. Before every one of these shelves a
deposit-sieve is established, (see _t_, _u_,) and the workmen in charge
of it stand in one of the corresponding boxes, marked 8 to 12. The sieve
is represented only in front of the chest _h_, for the sake of
clearness.
Each of the workmen placed in 8, 9, 10, 11, 12, operates on the heap
before him; the upper layer of the deposit formed in his sieve, is sent
to the stamping house, and the inferior layer directly to the furnace.
As to the grains which, after traversing the five grates, have arrived
at the chest _x_, they are washed in the two chests _y_, which are
analogous to the German chests to be presently described. The upper
layer of what is deposited in _y_ is sent to the furnace; the rest is
treated anew on three tables of percussion, similar to the English
brake-sieves, also to be presently described.
After several successive manipulations on these tables, an upper stratum
of _schlich_ is obtained fit for the furnace; an intermediate stratum,
which is washed anew by the same process; and an inferior stratum, that
is sent to the system of stamps, _fig._ 672.
[Illustration: 672]
This figure represents the general ground plan of a stamping and washing
mill. The stamps F are composed of two batteries similar to _fig._ 670.
The ore passes in succession under three pestles of cast iron, each of
which is heavier the nearer it is to the sieve through which the _sand_
or pounded matter escapes.
In the upper part of the figure we see issuing from the stamps, two
conduits destined to receive the water and the metalliferous sand with
which it is loaded. The first, marked F, S, _w_, is used only when a
certain quality of ore is _stamped_, richer in metal than is usually
treated by means of the second conduit, the first being closed. The
second conduit, or that employed for ordinary manipulation when the
other is shut, is indicated by F, 0·7, B; then by 0·58 and 0·29. These
numbers express the depth of the corresponding portions of this conduit.
From F to B, the conduit or water-course is divided into three portions
much shallower, called the _rich conduit_, the _middle conduit_, and the
_inferior_. Beyond the basin B, the conduit takes the name of labyrinth.
There the muddy sediments of ore are deposited; being the finer the
further they are from the stamps F. Darts indicate the direction of the
stream in the labyrinth. On the _German chests_, placed at 3, the sand
derived from the rich and middle conduits is treated, in order to obtain
three distinct qualities of _schlich_, as already mentioned. P is a
cloth-covered table, for treating the deposit of the German chests at 3.
M N are two sweep tables (_à balai_), for treating the ore collected in
the lower conduit, which precedes the midmost of the three German
chests. Upon the three similar tables R T V, are treated in like manner
the muddy deposits of the labyrinth, which forms suite to three parallel
German chests situated at 3, not shown for want of room in the figure,
but connected in three rectangular zigzags with each other, as well as
by a transverse branch to the points 0·7 and P. At the upper part of
these five sweep tables, the materials which are to undergo washing are
agitated in two boxes O O, by small paddle-wheels.
We shall now describe the _percussion-tables_ used in the Hartz, for
treating the sand of ore obtained from the conduits represented above.
[Illustration: 673 674 675]
_Figs._ 673, 674. and 675. exhibit a plan, a vertical section, and
elevation, of one of these tables, taken in the direction of its length.
The _arbor_ or great shaft in prolongation from the stamps mill, is
shown in section perpendicularly to its axis, at A. The _cams_ or wipers
are shown round its circumference, one of them having just acted on _n_.
These cams, by the revolution of the arbor, cause the alternating
movements of a horizontal bar of wood _o_, _u_, which strikes at the
point _u_ against a table _d_, _b_, _c_, _u_. This table is suspended by
two chains _t_, at its superior end, and by two rods at its lower end.
After having been pushed by the piece _o_, _u_, it rebounds to strike
against a block or bracket B. A lever _p_, _q_, serves to adjust the
inclination of the movable table, the pivots _q_ being points of
suspension.
The ore-sand to be washed, is placed in the chest _a_, into which a
current of water runs. The ore floated onwards by the water, is carried
through a sieve on a sloping small table _x_, under which is concealed
the higher end of the movable table _d_, _b_, _c_, _u_; and it thence
falls on this table, diffusing itself uniformly over its surface. The
particles deposited on this table form an oblong _talus_ (slope) upon
it; the successive percussions that it receives, determine the weightier
matters, and consequently those richest in metal, to accumulate towards
its upper end at _u_. Now the workman by means of the lever _p_, raises
the lower end _d_ a little in order to preserve the same degree of
inclination to the surface on which the deposit is strewed. According as
the substances are swept along by the water, he is careful to remove
them from the middle of the table towards the top, by means of a wooden
roller. With this intent, he walks on the table _d b c u_, where the
sandy sediment has sufficient consistence to bear him. When the table is
abundantly charged with the washed ore, the deposit is divided into
three bands or segments _d b_, _b c_, _c u_. Each of these bands is
removed separately and thrown into the particular heap assigned to it.
Every one of the heaps thus formed becomes afterwards the object of a
separate manipulation on a percussion table, but always according to the
same procedure. It is sufficient in general to pass twice over this
table the matters contained in the heap, proceeding from the superior
band _c u_, in order to obtain a pure _schlich_; but the heap preceding
from the intermediate belt _b c_, requires always a greater number of
manipulations, and the lower band _d b_ still more. These successive
manipulations are so associated that eventually each heap furnishes pure
_schlich_, which is obtained from the superior band _c u_. As to the
lightest particles which the water sweeps away beyond the lower end of
the percussion table, they fall into conduits; whence they are lifted to
undergo a new manipulation.
[Illustration: 676]
_Fig._ 676. is a profile of a plan which has been advantageously
substituted, in the Hartz, for that part of the preceding apparatus
which causes the jolt of the piece _o u_ against the table _d b c u_. By
means of this plan, it is easy to vary, according to the circumstances
of a manipulation always delicate, the force of percussion which a bar
_x y_, ought to communicate by its extremity _y_. With this view, a
slender piece of wood _u_ is made to slide in an upright piece, _v x_,
adjusted upon an axis at _v_. To the piece _u_ a rod of iron is
connected, by means of a hinge _z_; this rod is capable of entering more
or less into a case or sheath in the middle of the piece _v x_, and of
being stopped at the proper point, by a thumb-screw which presses
against this piece. If it be wished to increase the force of percussion,
we must lower the point _z_; if to diminish it, we must raise it. In the
first case, the extremity of the piece _u_, advances so much further
under the cam of the driving shaft _t_; in the second, it goes so much
less forwards; whereby the adjustment is produced.
[Illustration: 677 678]
_Figs._ 677. and 678. represent a complete system of _sleeping tables_,
_tables dormantes_; such as are mounted in Idria. _Fig._ 678. is the
plan, and _fig._ 677. a vertical section. The mercurial ores, reduced to
a sand by stamps like those of _fig._ 672., pass into a series of
conduits _a a_, _b b_, _c c_, which form three successive floors below
the level of the floor of the works. The sand taken out of these
conduits is thrown into the cells _q_; whence they are transferred into
the trough _e_, and water is run upon them by turning two stopcocks for
each trough. The sand thus diffused upon each table, runs off with the
water by a groove _f_, comes upon a sieve _h_, spreads itself upon the
board _g_, and thence falls into the slanting chest, or sleeping table
_i k_. The under surface _k_ of this chest is pierced with holes, which
may be stopped at pleasure with wooden plugs. There is a conduit _m_, at
the lower end of each table, to catch the light particles carried off by
the water out of the chest _i k_, through the holes properly opened,
while the denser parts are deposited upon the bottom of this chest. A
general conduit _n_ passes across at the foot of all the chests _i k_;
it receives the refuse of the washing operations.
[Illustration: 679]
_Fig._ 679. is a set of stamping and washing works for the ores of
argentiferous galena, as mounted at _Bockwiese_, in the district of
Zellerfeldt, in the Hartz.
A is the stamp mill and its subsidiary parts; among which are _a_, the
driving or main shaft; _b_, the overshot water-wheel; _c c_, six strong
rings or hoops of cast iron, for receiving each a cam or tappet; _g_,
the brake of the machine; _k_, _k_, _k_, the three standards of the
stamps; _l l_, &c. six pestles of pine wood, shod with lumps of cast
iron. There are two chests, out of which the ore to be ground falls
spontaneously into the two troughs of the stamps. Of late years,
however, the ore is mostly supplied by hand; the watercourse terminates
a short distance above the middle of the wheel _b_. There is a stream of
water for the service of the stamps, and conduits proceeding from it, to
lead the water into the two stamp troughs; the conduit of discharge is
common to the two batteries or sets of stamps through which the water
carries off the sand or stamped ore. There is a movable table of
separation, mounted with two sieves. The sands pass immediately into the
conduit placed upon a level with the floor, and separated into two
compartments, the first of which empties its water into the second.
There are two boards of separation, or tables, laid upon the ground,
with a very slight slope of only 15 inches from their top to their
bottom. Each of these boards is divided into four cases with edges; the
whole being arranged so that it is possible, by means of a flood-gate or
sluice, to cause the superfluous water of the case to pass into the
following ones. Thus the work can go on without interruption, and
alternately upon the two boards. There are winding canals in the
labyrinth, N, N, N, in which are deposited the particles carried along
by the water which has passed upon the boards. The depth of these canals
gradually increases from 12 to 20 inches, to give a suitable descent for
maintaining the water-flow. At D, two percussion tables are placed. F G
are two German chests. H J are two percussion tables, which are driven
by the cams _z z_, fixed upon the main shaft _x y_. K K´ are two sloping
sweep tables (_à balai_).
The _German chests_ are rectangular, being about 3 yards long, half a
yard broad, with edges half a yard high; and their inclination is such
that the lower end is about 15 inches beneath the level of the upper. At
their upper end, usually called the bolster, a kind of trough or box,
without any edge at the side next the chest, is placed, containing the
ore to be washed. The water is allowed to fall upon the bolster in a
thin sheet.
The _sleeping tables_ have upright edges; they are from 4 to 5 yards
long, nearly 2 yards wide, and have fully a yard of inclination.
The preceding tables are sometimes covered with cloth, particularly in
treating ores that contain gold, on a supposition that the woollen or
linen fibres would retain more surely the metallic particles; but this
method appears on trial to merit no confidence, for it produces a very
impure _schlich_.
[Illustration: 680]
_Fig._ 680. is a swing-sieve employed in the Hartz, for sifting the
small fragments of the ore of argentiferous lead. Such an apparatus is
usually set up in the outside of a stamp, and washing mill; its place
being denoted by the letter A, _fig._ 672. The two movable chests or
boxes A B, of the sieve, are connected together, at their lower ends,
with an upright rod, which terminates at one of the arms of a small
balance beam, mounted between the driving shaft of the stamps and the
sieve, perpendicularly to the length of both. The opposite arm of this
beam carries another upright rod, which ears (cams or _mentonnets_),
placed on purpose upon the driving shaft, may push down. During this
movement the two lower ends A, B, are raised; and when the peg-cam of
the shaft quits the rod which it had depressed, the swing chests fall by
their own weight. Thus they are made to vibrate alternately upon their
axes. The small ore is put into the upper part of the chest A, over
which a stream of water falls from an adjoining conduit. The fragments
which cannot pass through a cast-iron grid in the bottom of that chest,
are sorted by hand upon a table in front of A, and they are classed by
the workman, either among the ores to be stamped, whether dry or wet, or
among the rubbish to be thrown away, or among the copper ores to be
smelted by themselves. As to the small particles which fall through the
grid upon the chest B, supplied also with a stream of water, they
descend successively upon two other brass wire sieves, and also through
the iron wire _r_, in the bottom of B.
In certain mines of the Hartz, tables called _à balais_, or _sweeping
tables_, are employed. The whole of the process consists in letting
flow, over the sloping table, in successive currents, water charged with
the ore, which is deposited at a less or greater distance, as also pure
water for the purpose of washing the deposited ore, afterwards carried
off by means of this sweeping operation.
At the upper end of these _sweep-tables_, the matters for washing are
agitated in a chest, by a small wheel with vanes, or flap-boards. The
conduit of the muddy waters opens above a little table or shelf; the
conduit of pure water, which adjoins the preceding, opens below it. At
the lower part of each of these tables, there is a transverse slit,
covered by a small door with hinges, opening outwardly, by falling back
towards the foot of the table. The water spreading over the table, may
at pleasure be let into this slit, by raising a bit of leather which is
nailed to the table, so as to cover the small door when it is in the
shut position; but when this is opened, the piece of leather then hangs
down into it. Otherwise the water may be allowed to pass freely above
the leather, when the door is shut. The same thing may be done with a
similar opening placed above the conduit. By means of these two slits,
two distinct qualities of _schlich_ may be obtained, which are deposited
into two distinct conduits or canals. The refuse of the operation is
turned into another conduit, and afterwards into ulterior reservoirs,
whence it is lifted out to undergo a new washing.
In the percussion tables, the water for washing the ores is sometimes
spread in slender streamlets, sometimes in a full body, so as to let two
cubic feet escape per minute. The number of shocks communicated per
minute, varies from 15 to 36; and the table may be pushed out of its
settled position at one time, three quarters of an inch, at another
nearly 8 inches. The coarse ore-sand requires in general less water, and
less slope of table, than the fine and pasty sand.
The _mechanical_ operations which ores undergo, take place commonly at
their out-put from the mine, and without any intermediate operation.
Sometimes, however, the hardness of certain _gangues_ (vein-stones), and
of certain iron-ores, is diminished by subjecting them to calcination
previously to the breaking and stamping processes.
When it is intended to wash certain ores, an operation founded on the
difference of their specific gravities, it may happen that by slightly
changing the chemical state of the substances that compose the ore, the
earthy parts may become more easily separable, as also the other foreign
matters. With this view, the ores of tin are subjected to a roasting,
which by separating the arsenic, and oxidizing the copper which are
intermixed, furnishes the means of obtaining, by the subsequent washing,
an oxide of tin much purer than could be otherwise procured. In general,
however, these are rare cases; so that the washing almost always
immediately succeeds the picking and stamping; and the roasting comes
next, when it needs to be employed.
The operation of roasting is in general executed by various processes,
relatively to the nature of the ores, the quality of the fuel, and to
the object in view. The greatest economy ought to be studied in the
fuel, as well as the labour; two most important circumstances, on
account of the great masses operated upon.
Three principal methods may be distinguished; 1. the roasting in a heap
in the open air, the most simple of the whole; 2. the roasting executed
between little walls, and which may be called case-roasting
(_rost-stadeln_, in German); and 3. roasting in furnaces.
We may remark, as to the description about to be given of these
different processes, that in the first two, the fuel is always in
immediate contact with the ore to be roasted, whilst in furnaces, this
contact may or may not take place.
1. The roasting in the open air, and in heaps more or less considerable,
is practised upon iron ores, and such as are pyritous or bituminous. The
operation consists in general in spreading over a plane area, often
bottomed with beaten clay, billets of wood arranged like the bars of a
gridiron, and sometimes laid crosswise over one another, so as to form a
uniform flat bed. Sometimes wood charcoal is scattered in, so as to fill
up the interstices, and to prevent the ore from falling between the
other pieces of the fuel. Coal is also employed in moderately small
lumps; and even occasionally, turf. The ore either simply broken into
pieces, or even sometimes under the form of _schlich_, is piled up over
the fuel; most usually alternate beds of fuel and ore are formed.
The fire, kindled in general at the lower part, but sometimes, however,
at the middle chimney, spreads from spot to spot, putting the operation
in train. The combustion must be so conducted as to be slow and
suffocated, to prolong the ustulation, and let the whole mass be equably
penetrated with heat. The means employed to direct the fire, are to
cover outwardly with earth the portions where too much activity is
displayed, and to pierce with holes or to give air to those where it is
imperfectly developed. Rains, winds, variable seasons, and especially
good primary arrangements of a calcination, have much influence on this
process, which requires, besides, an almost incessant inspection at the
beginning.
Nothing in general can be said as to the consumption of fuel, because it
varies with its quality, as well as with the ores and the purpose in
view. But it may be laid down as a good rule, to employ no more fuel
than is strictly necessary for the kind of calcination in hand, and for
supporting the combustion; for an excess of fuel would produce, besides
an expense uselessly incurred, the inconvenience, at times very serious,
of such a heat as may melt or vitrify the ores; a result entirely the
reverse of a well-conducted ustulation.
[Illustration: 681 682 683]
_Figs._ 681, 682, 683. represent the roasting in mounds, as practised
near Goslar in the Hartz, and at Chessy in the department of the Rhone.
_Fig._ 681. is a vertical section in the line _h c_ of _figs._ 682. and
683. In _fig._ 682. there is shown in plan, only a little more than one
half of the quadrangular truncated pyramid, which constitutes the heap.
_Fig._ 683. shows a little more than one fourth of a bed of wood,
arranged at the bottom of the pyramid, as shown by _a a_, _fig._ 681.,
and _c g h_, _fig._ 683. C is a wooden chimney, formed within the heap
of ore, at whose bottom _c_ there is a little parcel of charcoal, _d d_
are large lumps of ore distributed upon the wooden pile _a a_; _e e_ are
smaller fragments, to cover the larger; _f f_ is rubbish and clay laid
smoothly in a slope over the whole. _g_, _fig._ 683., a passage for air
left under the bed of billets; of which there is a similar one in each
of the four sides of the base _a a_, so that two principal currents of
air cross under the upright axis C _c_, of the truncated pyramid
indicated in _fig._ 681.
The kindling is thrown in by the chimney C. The charcoal _c_, and the
wood _a a_, take fire; the sulphureous ores _d e f_ are heated to such a
high temperature as to vaporize the sulphur. In the Lower Hartz, a heap
of this kind continues roasting during four months.
2. The second method. The difficulty of managing the fire in the
roasting of substances containing little sulphur, with the greater
difficulty of arranging and supporting in their place the _schlichs_ to
be roasted, and last of all, the necessity of giving successive fires to
the same ores, or to inconsiderable quantities at a time, have led to
the contrivance of surrounding the area on which the roasting takes
place with three little walls, or with four, leaving a door in the one
in front. This is what is called a _walled area_, and sometimes,
improperly enough, a roasting furnace. Inside of these little walls,
about 3 feet high, there are often vertical conduits or chimneys made to
correspond with an opening on the ground level, in order to excite a
draught of air in the adjacent parts. When the roasting is once set
agoing, these chimneys can be opened or shut at their upper ends,
according to the necessities of the process.
Several such furnaces are usually erected in connexion with each other
by their lateral walls, and all terminated by a common wall, which forms
their posterior part; sometimes they are covered with a shed supported
partly by the back wall, built sufficiently high for this purpose. These
dispositions are suitable for the roasting of _schlichs_, and in general
of all matters which are to have several fires; a circumstance often
indispensable to a due separation of the sulphur, arsenic, &c.
3. The furnaces employed for roasting the ores and the _mattes_ differ
much, according to the nature of the ores, and the size of the lumps.
We shall content ourselves with referring to the principal forms.
When iron ores are to be roasted, which require but a simple calcination
to disengage the combined water and carbonic acid, egg-shaped furnaces,
similar to those in which limestone is burned in contact with fuel, may
be conveniently employed; and they present the advantage of an operation
which is continuous with a never-cooling apparatus. The analogy in the
effects to be produced is so perfect, that the same furnace may be used
for either object. Greater dimensions may, however, be given to those
destined for the calcination of iron ores. But it must be remembered
that this process is applicable only to ores broken into lumps, and not
to ores in grains or powder.
It has been attempted to employ the same method a little modified, for
the roasting of ores of sulphuret of copper and pyrites, with the view
of extracting a part of the sulphur. More or less success has ensued,
but without ever surmounting all the obstacles arising from the great
fusibility of the sulphuret of iron. For sometimes it runs into one
mass, or at least into lumps agglutinated together in certain parts of
the furnace, and the operation is either stopped altogether, or becomes
more or less languid; the air not being able to penetrate into all the
parts, the roasting becomes consequently imperfect. This inconvenience
is even more serious than might at first sight appear; for, as the
ill-roasted ores now contain too little sulphur to support their
combustion, and as they sometimes fall into small fragments in the
cooling, they cannot be passed again through the same furnace, and it
becomes necessary to finish the roasting in a reverberatory hearth,
which is much more expensive.
In the Pyrenees, the roasting of iron ores is executed in a circular
furnace, so disposed that the fuel is contained and burned in a kind of
interior oven, above which lie the pieces of ore to be calcined.
Sometimes the vault of this oven which sustains the ore, is formed of
bricks, leaving between them openings for the passage of the flame and
the smoke, and the apparatus then resembles certain pottery kilns; at
other times the vault is formed of large lumps of ore, carefully
arranged as to the intervals requisite to be left for draught over the
arch. The broken ore is then distributed above this arch, care being
taken to place the larger pieces undermost. This process is simple in
the construction of the furnace, and economical, as branches of trees,
without value in the forests, may be employed in the roasting. See
_Lime-kiln_ figures.
In some other countries, the ores are roasted in furnaces very like
those in which porcelain is baked; that is to say, the fuel is placed
exteriorly to the body of the furnace in a kind of brick shafts, and the
flame traverses the broken ore with which the furnace is filled. In such
an apparatus the calcination is continuous.
When it is proposed to extract the sulphur from the iron pyrites, or
from pyritous minerals, different furnaces may be employed, among which
that used in Hungary deserves notice. It is a rectangular parallelopiped
of four walls, each of them being perforated with holes and vertical
conduits which lead into chambers of condensation, where the sulphur is
collected. The ore placed between the four walls on billets of wood
arranged as in _figs._ 681, 682, 683., for the great roastings in the
open air, is calcined with the disengagement of much sulphur, which
finds more facility in escaping by the lateral conduits in the walls,
than up through the whole mass, or across the upper surface covered over
with earth; whence it passes into the chambers of condensation. In this
way upwards of a thousand tons of pyrites may be roasted at once, and a
large quantity of sulphur obtained. See COPPER.
[Illustration: 684 685]
_Roasting of Pyrites._--_Figs._ 684, 685. represent a furnace which has
been long employed at Fahlun in Sweden, and several other parts of that
kingdom, for roasting iron pyrites in order to obtain sulphur. This
apparatus was constructed by the celebrated Gahn. _Fig._ 684. is a
vertical section, in the line _k d n o_ of _fig._ 685., which is a plan
of the furnace; the top being supposed to be taken off. In both figures
the conduit may be imagined to to be broken off at _e_; its entire
length in a straight line is 43 feet beyond the dotted line _e n_,
before the bend, which is an extension of this conduit. Upon the slope
_a b_ of a hillock _a b c_, lumps _r_ of iron pyrites are piled upon the
pieces of wood _i i_ for roasting. A conduit _d f e_ forms the
continuation of the space denoted by _r_, which is covered by stone
slabs so far as _f_; and from this point to the chamber _h_ it is
constructed in boards. At the beginning of this conduit, there is a
recipient _g_. The chamber _h_ is divided into five chambers by
horizontal partitions, which permit the circulation of the vapours from
one compartment to another. The ores _r_ being distributed upon the
billets of wood _i i_, whenever these are fairly kindled, they are
covered with small ore, and then with rammed earth _l l_. Towards the
point _m_, for a space of a foot square, the ores are covered with
movable stone slabs, by means of which the fire may be regulated, by the
displacement of one or more, as may be deemed necessary. The liquid
sulphur runs into the recipient _g_, whence it is laded out from time to
time. The sublimed sulphur passes into the conduit _f e_ and the chamber
_h_, from which it is taken out, and washed with water, to free it from
sulphuric acid with which it is somewhat impregnated; it is afterwards
distilled in cast-iron retorts. The residuum of the pyrites is turned to
account in Sweden, for the preparation of a common red colour much used
as a pigment for wooden buildings.
The reverberatory furnace affords one of the best means of ustulation,
where it is requisite to employ the simultaneous action of heat and
atmospherical air to destroy certain combinations, and to decompose the
sulphurets, arseniurets, &c. It is likewise evident that the facility
thus offered of stirring the matters spread out on the sole, in order to
renew the surfaces, of observing their appearances, of augmenting or
diminishing the degree of heat, &c., promise a success much surer, a
roasting far better executed, than by any other process. It is known,
besides, that flame mingled with much undecomposed air issuing from the
furnace, is highly oxidizing, and is very fit for burning away the
sulphur, and oxidizing the metals. Finally, this is almost the only
method of rightly roasting ores which are in a very fine powder. If it
be not employed constantly and for every kind of ore, it is just because
more economy is found in practising calcination in heaps, or on areas
enclosed by walls; besides, in certain mines, a very great number of
these furnaces, and many workmen, would be required to roast the
considerable body of ores that must be daily smelted. Hence there would
result from the construction of such apparatus and its maintenance a
very notable outlay, which is saved in the other processes.
But in every case where it is desired to have a very perfect roasting,
as for blende from which zinc is to be extracted, for sulphuret of
antimony, &c., or even for ores reduced to a very fine powder, and
destined for amalgamation, it is proper to perform the operation in a
reverberatory furnace. When very fusible sulphurous ores are treated,
the workman charged with the calcination must employ much care and
experience, chiefly in the management of the fire. It will sometimes,
indeed, happen, that the ore partially fuses; when it becomes necessary
to withdraw the materials from the furnace, to let them cool and grind
them anew, in order to recommence the operation. The construction of
these furnaces demands no other attention than to give to the sole or
laboratory the suitable size, and so to proportion to this the grate and
the chimney that the heating may be effected with the greatest economy.
The reverberatory furnace is always employed to roast the ores of
precious metals, and especially those for amalgamation; as the latter
often contain arsenic, antimony, and other volatile substances, they
must be disposed of in a peculiar manner.
The sole, usually very spacious, is divided into two parts, of which the
one farthest off from the furnace is a little higher than the other.
Above the vault there is a space or chamber in which the ore is
deposited, and which communicates with the laboratory by a vertical
passage; which serves to allow the ore to be pushed down, when it is
dried and a little heated. The flame and the smoke which escape from the
sole or laboratory pass into condensing chambers, before entering into
the chimney of draught, so as to deposit in them the oxide of arsenic
and other substances. When the ore on the part of the sole farthest from
the grate has suffered so much heat as to begin to be roasted, has
became less fusible, and when the roasting of that in the nearer part of
the sole is completed, the former is raked towards the fire-bridge, and
its ustulation is finished by stirring it over frequently with a paddle,
skilfully worked, through one of the doors left in the side for this
purpose. The operation is considered to be finished when the vapours and
the smell have almost wholly ceased; its duration depending obviously on
the nature of the ores.
When this furnace is employed to roast very arsenical ores, as the tin
ores of Schlackenwald in Bohemia, and at Ehrenfriedersdorf in Saxony,
the arsenical pyrites of Geyer (in Saxony), &c., the chambers of
condensation for the arsenious acid are much more extensive than in the
furnaces commonly used for roasting galena, copper, or even silver ores.
[Illustration: 686 687 688]
_Figs._ 686, 687, 688. represent a reverberatory furnace employed in the
smelting works of Lautenthal, in the Hartz, for roasting the schlichs of
lead ores, which contain much blende or sulphuret of zinc. In _fig._
686. we see that the two parts A B, B C, are absolutely like, the two
furnaces being built in one body of brickwork. _Fig._ 687. is the plan
of the furnace B C, taken at the level E F of _fig._ 686. _Fig._ 688. is
a vertical section of the similar furnace A B, taken in the prolongation
of the line G H in _fig._ 687.
_a_ is the fire-place of the furnace, its grate and ash-pit. _b_ is the
conduit of vaporization, which communicates with the chambers _c_; _c_,
chambers into which the vaporized substances are deposited; _d_, chimney
for the escape of the smoke of the fire-place _a_, after it has gone
through the space _b c c_; _e´_, is the charging door, with a hook
hanging in front to rest the long iron rake upon, with which the
materials are turned over; _f_, chamber containing a quantity of schlich
destined for roasting; this chamber communicates with the vaulted
corridor (gallery) D, seen in _fig._ 686.; _g_, orifice through which
the schlich is thrown into the furnace; _h_, area or hearth of the
reverberatory furnace, of which the roof is certainly much too high;
_i_, channels for the escape of the watery vapours; _k l_, front arcade,
between which and the furnace, properly speaking, are the two orifices
of the conduits, which terminate at the channels _m_, _m´_. _m_ is the
channel for carrying towards the chimney _d_, the vapours which escape
by the door _e´_. _n_ is a walled-up door, which is opened from time to
time, to take out of the chambers _c_, _c_, the substances that may be
deposited in them.
At the smelting works of Lautenthal, in such a roasting furnace, from 6
to 9 quintals (cwts.) of schlich are treated at a time, and it is
stirred frequently with an iron rake upon the altar _h_. The period of
this operation is from 6 to 12 hours, according as the schlich may be
more or less dry, more or less rich in lead, or more or less charged
with blende. When the latter substance is abundant, the process requires
12 hours, with about 60 cubic feet of cleft billets for fuel.
In such furnaces are roasted the cobalt ores of Schneeberg in Saxony,
the tin ores of Schlackenwald in Bohemia, of Ehrenfriedersdorf in
Saxony, and elsewhere; as also the arsenical pyrites at Geyer in Saxony.
But there are poison towers and extensive condensing chambers attached
in the latter case. See ARSENIC.
_Figs._ 689, 690, 691. represent the reverberatory furnace generally
employed in the Hartz, in the district of Mansfeldt, Saxony, Hungary,
&c., for the treatment of black copper, and for refining rose copper
upon the great scale. An analogous furnace is used at Andreasberg for
the liquefaction or purification of the mattes, and for workable lead
when it is much loaded with arsenic.
[Illustration: 689 690 691]
_Fig._ 689. presents the elevation of the furnace parallel to the line I
K, of the plan _fig._ 690.; which plan is taken at the level of the
tuyère _n_, of _fig._ 691.; _fig._ 691. is a vertical section in the
line L M, _fig._ 690. _k_ represents one of two basins of reception,
brasqued with clay and charcoal; _n_, _n_, two tuyères, through which
enters the blast of two pairs of bellows, like those shown at
Cupellation of SILVER; _q_, door by which the matter to be melted is
laid upon the sole of the furnace; _v_, _v_, two points where the sole
is perforated, when necessary to run off the melted matter into either
of the basins _h_; _x_, door through which the slags or cinders floating
upon the surface of the melted metal are raked out; _y_, door of the
fire-place. The fuel is laid upon a grate above an ash-pit, and below
the arch of a reverberatory which is contiguous to the dome or cap of
the furnace properly so called. In the section, _fig._ 691., the
following parts may be noted: 1, 2, 3, mason-work of the foundation; 4,
vapour channels or conduits, for the escape of the humidity; 5, bed of
clay; 6, brasque composed of clay and charcoal, which forms the
concavity of the hearth.
[Illustration: 692 693 694]
_Figs._ 692, 693, 694., show the furnace employed for liquation in one
of the principal smelting works of the Hartz. _Fig._ 694. exhibits the
working area charged with the liquation cakes and charcoal, supported by
sheets of wrought iron; being an image of the process in action. _Fig._
693. is the plan, in the line F, G, of _fig._ 692.