4. The system of washing used in Cumberland differs essentially from
that of Brittany. The slime pits are constructed with much less care
than in France and Germany. They never present, as in these countries,
those long windings backwards and forwards, whence they have been called
labyrinths; probably because the last deposits, which are washed with
profit in France and Germany, could not be so in Cumberland. There is
reason to believe, however, that the introduction of _brake tables_,
(_tables à secousses_, see METALLURGY) would enable deposits to be
saved, which at present run to waste in England.
5. From what we have now said about the system of washing, and the
basins of deposit or settling cisterns, it may be inferred that the
operation followed in Cumberland is more expeditious than that used in
Brittany, but it furnishes less pure ores, and occasions more
considerable waste; a fact sufficiently obvious, since the refuse stuff
at Poullaouen is often resumed, and profitably subjected to a new
preparation. We cannot however venture to blame this method, because in
England, fuel being cheap, and labour dear, there may possibly be more
advantage in smelting an ore somewhat impure, and in losing a little
galena, than in multiplying the number of washing processes.
6. Lastly, the _dolly tub_ ought to be adopted in all the establishments
where the galena is mixed with much blende (sulphuret of zinc); for
_schlich_ (metallic slime) which appears very clean to the eye, gives
off a considerable quantity of blende by means of the _dolly tub_. While
the vane is rapidly whirled, the sludge is gradually let down into the
revolving water, till the quantity is sufficiently great. Whenever the
ore is thoroughly disseminated in the liquid, the dolly is withdrawn.
The workmen then strike on the sides of the tub for a considerable time,
with mallets or wooden billets, to make the slime fall fast to the
bottom. The lighter portions, consisting almost entirely of refuse
matter, fall only after the knocking has ceased; the water is now run
away; then the very poor slime upon the top of the deposit is skimmed
off; while the pure ore found at the bottom of the tub is lifted out,
and laid on the _bingstead_. In this way the blende, which always
accompanies galena in a greater or smaller quantity, is well separated.
_Smelting of lead ores._--The lead ores of Derbyshire and the north of
England were antiently smelted in very rude furnaces, or _boles_, urged
by the natural force of the wind, and were therefore placed on the
summits or western slopes of the highest hills. More recently these
furnaces were replaced by blast hearths, resembling smith’s forges, but
larger; and were blown by strong bellows, moved by men or water-wheels.
The principal operation of smelting is at present always executed in
Derbyshire in _reverberatory furnaces_, and at _Alston Moor_ in furnaces
similar to those known in France by the name of Scotch furnaces. Before
entering into the detail of the founding processes, we shall give a
description of the furnaces essential for both the smelting and
accessory Operations.
1. The reverberatory furnace called cupola, now exclusively used in
Derbyshire for the smelting of lead ores, was imported thither from
Wales, about the year 1747, by a company of Quakers. The first
establishment in this county was built at Kalstedge, in the district of
Ashover.
In the works where the construction of these furnaces is most improved,
they are interiorly 8 feet long by 6 wide in the middle, and two feet
high at the centre. The fire, placed at one of the extremities, is
separated from the body of the furnace by a body of masonry, called the
_fire-bridge_, which is two feet thick, leaving only from 14 to 18
inches between its upper surface and the vault. From this, the highest
point, the vault gradually sinks towards the further end, where it
stands only 6 inches above the sole. At this extremity of the furnace,
there are two openings separated by a triangular prism of _fire-stone_,
which lead to a flue, a foot and a half wide, and 10 feet long, which is
recurved towards the top, and runs into an upright chimney 55 feet high.
The above flue is covered with stone slabs, carefully jointed with
fire-clay, which may be removed when the deposit formed under them
(which is apt to melt), requires to be cleaned out. One of the sides of
the furnace is called the labourers’ side. It has a door for throwing
coal upon the fire-grate, besides three small apertures each about 6
inches square. These are closed with movable plates of cast iron, which
are taken off when the working requires a freer circulation of air, or
for the stirring up of the materials upon the hearth. On the opposite
side, called the working side, there are five apertures; namely, three
equal and opposite to those just described, shutting in like manner with
cast iron plates, and beneath them two other openings, one of which is
for running out the lead, and another for the scoriæ. The ash pit is
also on this side, covered with a little water, and so disposed as that
the grate-bars may be easily cleared from the cinder slag.
The hearth of the furnace is composed of the reverberatory furnace
slags, to which a proper shape has been given by beating them with a
strong iron rake, before their entire solidification. On the labourers’
side, this hearth rises nearly to the surface of the three openings, and
falls towards the working side, so as to be 18 inches below the middle
aperture. In this point, the lowest of the furnace, there is a tap-hole,
through which the lead is run off into a large iron boiler (lea-pan),
placed in a recess left outside in the masonry. From that lowest point,
the sole gradually rises in all directions, forming thus an inside
basin, into which the lead runs down as it is smelted. At the usual
level of the metal bath, there is on the working side, at the end
furthest from the fire, an aperture for letting off the slag.
In the middle of the arched roof there is a small aperture, called the
_crown-hole_, which is covered up during the working with a thick cast
iron plate. Above this aperture a large wooden or iron hopper stands,
leading beneath into an iron cylinder, through which the contents of the
hopper may fall into the furnace when a trap or valve is opened.
2. _The roasting furnace._--This was introduced about 30 years ago, in
the neighbourhood of Alston Moor, for roasting the ore intended to pass
through the Scotch furnace, a process which greatly facilitates that
operation. Since its first establishment it has successively received
considerable improvements.
[Illustration: 631 632 633]
_Figs._ 631, 632, 633., represent the cupola furnace at the Marquess of
Westminster’s lead smelting works, two miles from Holywell. The hearth
is hollowed out below the middle door of the furnace; it slopes from
the back and ends towards this basin. The distance from the lowest point
of this concavity up to the sill of the door, is usually 24 inches, but
it is sometimes a little less, according to the quality of the ores to
be smelted. This furnace has no hole for running off the slag, above the
level of the top hole for the lead _i_, like the smelting furnace of
Lea, near Matlock. A single chimney stalk serves for all the
establishments; and receives all the flues of the various roasting and
reducing furnaces. _Fig._ 633. gives an idea of the distribution of
these flues. _a a a_, &c. are the furnaces; _b_, the flues, 18 inches
square; these lead from each furnace to the principal conduit _c_, which
is 5 feet deep by 2-1/2 wide; _d_ is 6 feet deep by 3 wide; _e_ is a
round chamber 15 feet in diameter; _f_ is a conduit 7 feet high by 5
wide; _g_ another, 6 feet high by 3 wide. The chimney at _h_ has a
diameter at bottom of 30 feet, at top of 12 feet, including the
thickness of its sides, forming a truncated cone 100 feet high; whose
base stands upon a hill a little way from the furnaces, and 62 feet
above their level.
_a_, _figs._ 631, 632., is the grate; _b_, the door of the fire-place;
_c_, the fire-bridge; _d_, the arched roof; _e_, the hearth; _f f f_,
&c., the working doors; _g g_, flues running into one conduit, which
leads to the subterranean condensing chamber, _e_, and thence to the
general chimney; _h_, a hopper-shaped opening in the top of the furnace,
for supplying it with materials.
This magnificent structure is not destined solely for the reduction of
the ores, but for dissipating all the vapours which might prove noxious
to the health of the work-people and to vegetation.
The ores smelted at Holywell are very refractory galenas, mixed with
blende, calamine, pyrites, carbonate of lime, &c., but without any
fluate of lime. They serve mutually as fluxes to one another. The coal
is of inferior quality. The sole of each furnace is formed of slags
obtained in the smelting, and they are all of one kind. In constructing
it, 7 or 8 tons of these slags are first of all thrown upon the brick
area of the hearth; are made to melt by a brisk fire, and in their
stiffening state, as they cool, they permit the bottom to be sloped and
hollowed into the desired shape. Four workmen, two at each side of the
furnace, perform this task.
The ordinary charge of ore for one smelting operation is 20 cwt., and it
is introduced through the hopper; see COPPER, _fig._ 304. An assistant
placed at the back doors spreads it equally over the whole hearth with a
rake; the furnace being meanwhile heated only with the declining fire of
a preceding operation. No regular fire is made during the first two
hours, but a gentle heat merely is kept up by throwing one or two
shovelfuls of small coal upon the grate from time to time. All the doors
are closed, and the register-plate of the chimney is lowered.
The outer basin in front of the furnace is at this time filled with the
lead derived from a former process, the metal being covered with slags.
A rectangular slit above the tap hole is left open, and remains so
during the whole time of the operation, unless the lead should rise in
the interior basin above the level of that orifice; in which case a
little mound must be raised before it.
The two doors in front furthest from the fire being soon opened, the
head-smelter throws in through them, upon the sole of the furnace, the
slags swimming upon the bath of lead, and a little while afterwards he
opens the tap-hole, and runs off the metallic lead reduced from these
slags. At the same time his assistant turns over the ore with his
paddle, through the back doors. These being again closed, while the
above two front doors are open, the smelter throws a shovelful of small
coal or coak cinder upon the lead bath, and works the whole together,
turning over the ore with the paddle or iron oar. About three quarters
of an hour after the commencement of the operation, he throws back upon
the sole of the hearth the fresh slags which then float upon the bath of
the outer basin, and which are mixed with coaly matter. He next turns
over these slags, as well as the ore with the paddle, and shuts all the
doors. At this time the smelter runs off the lead into the pig-moulds.
The assistant now turns over the ore once more through the back doors. A
little more than an hour after the operation began, a quantity of lead
proceeding from the slag last remelted, is run off by the tap; being
usually in such quantity as to fill one half of the outer basin. Both
the workmen then turn over the ore with the paddles, at the several
doors of the furnace. Its interior is at this time of a dull red heat;
the roasting being carried on rather by the combustion of the sulphurous
ingredients, than by the action of the small quantity of coal in the
grate. The smelter, after shutting the front doors, with the exception
of that next the fire-bridge, lifts off the fresh slags lying upon the
surface of the outside bath, drains them, and throws them back into the
furnace.
An hour and a half after the commencement, the lead begins to ooze out
in small quantities from the ore; but little should be suffered to flow
before two hours have expired. About this time the two workmen open all
the doors, and turn over the ore, each at his own side of the furnace.
An hour and three quarters after the beginning, there are few vapours
in the furnace, its temperature being very moderate. No more lead is
then seen to flow upon the sloping hearth. A little coal being thrown
into the grate to raise the heat slightly, the workmen turn over the
ore, and then close all the doors.
At the end of two hours, the _first fire_ or roasting being completed,
and the doors shut, the register is to be lifted a little, and coal
thrown upon the grate to give the _second fire_, which lasts during 25
minutes. When the doors are now opened, the inside of the furnace is of
a pretty vivid red, and the lead flows down from every side towards the
inner basin. The smelter with his rake or paddle pushes the slags upon
that basin back towards the upper part of the sole, and his assistant
spreads them uniformly over the surface through the back doors. The
smelter next throws in by his middle door, a few shovelfuls of quicklime
upon the lead bath. The assistant meanwhile, for a quarter of an hour,
works the ore and the slags together through the three back doors, and
then spreads them out, while the smelter pushes the slags from the
surface of the inner basin back to the upper parts of the sole. The
doors being now left open for a little, while the interior remains in
repose, the metallic lead, which had been pushed back with the slags,
flows down into the basin. This occasional _cooling_ of the furnace is
thought to be necessary for the better separation of the products,
especially of the slags from the lead bath.
In a short time the workmen resume their rakes, and turn over the slags
along with the ore. Three hours after the commencement, a little more
fuel is put into the grate, merely to keep up a moderate heat of the
furnace during the paddling. After three hours and ten minutes, the
grate being charged with fuel for the _third fire_, the register is
completely opened, the doors are all shut, and the furnace is left in
this state for three quarters of an hour. In nearly four hours from the
commencement, all the doors being opened, the assistant levels the
surfaces with his rake, in order to favour the descent of any drops of
lead; and then spreads the slags, which are pushed back towards him by
the smelter. The latter now throws in a fresh quantity of lime, with the
view not merely of covering the lead bath and preventing its
oxidizement, but of rendering the slags less fluid.
Ten minutes after the third fire is completed, the smelter puts a new
charge of fuel in the grate, and shuts the doors of the furnace to give
it the _fourth fire_. In four hours and forty minutes from the
commencement, this fire being finished, the doors are opened, the
smelter pierces the tap-hole to discharge the lead into the outer basin,
and throws some quicklime upon the slags in the inner basin. He then
pushes the slags thus _dried up_ towards the upper part of the hearth,
and his assistant rakes them out by the back doors.
The whole operation of a _smelting shift_ takes about four hours and a
half, or at most five hours, in which four periods may be distinguished.
1. The _first fire_ for roasting the ores, requires very moderate
firing, and lasts two hours.
2. The _second fire_, or the smelting, requires a higher heat, with shut
doors; at the end the slags are _dried up_ with lime, and the furnace is
also allowed to cool a little.
3, 4. The last two periods, or the _third and fourth fires_, are
likewise two smeltings or foundings, and differ from the first only in
requiring a higher temperature. The heat is greatest in the last. The
form and dimensions of the furnace are calculated to cause a uniform
distribution of heat over the whole surface of the hearth. Sometimes
billets of green wood are plunged into the metallic lead of the outer
basin, causing an ebullition which favours the separation of the slags,
and consequently the production of a purer lead; but no more metallic
metal is obtained.
Ten cwts. of coal are consumed at Holywell in smelting one ton of the
lead-ore _schlich_ or sludge; but at Grassington, near Skipton in
Yorkshire, with a similar furnace worked with a slower heat, the
operation taking from seven hours to seven hours and a half, instead of
five, only 7-1/2 cwts. of coal are consumed. But here the ores are less
refractory, have the benefit of fluor spar as a flux, and are more
exhausted of their metal, being smelted upon a less sloping hearth.
_Theory of the above operations._--At Holywell, Grassington, and in
Cornwall, the result of the first graduated roasting heat, is a mixture
of undecomposed sulphuret of lead, with sulphate and oxide of lead, in
proportions which vary with the degree of care bestowed upon the
process. After the roasting, the heat is raised to convert the sludge
into a pasty mass; in which the oxide and sulphate re-act upon the
sulphuret, so as to produce a sub-sulphuret, which parts with the metal
by liquation. The _cooling of the furnace_ facilitates the liquation
every time that the sub-sulphuret is formed, and the ore has passed by
increase of temperature from the pasty into the liquid state. _Cooling_
brings back the sludge to the pasty condition, and is therefore
necessary for the due separation of the different bodies. The drying up
of the thin slags by lime is intended to liberate the oxide of lead, and
allow it to re-act upon any sulphuret which may have resisted roasting
or decomposition. It is also useful as a _thickener_, in a mechanical
point of view. The iron of the tools, which wear away very fast, is also
serviceable in reducing the sulphuret of lead. The small coal added
along with the lime at Grassington, and also sometimes at Holywell, aids
in reducing the oxide of lead, and in transforming the sulphate into
sulphuret.
[Illustration: 634]
3. _The smelting furnace or ore hearth._--This furnace, called by the
French _écossais_, is from 22 to 24 inches in height and 1 foot by 1-1/2
in area inside; but its horizontal section, always rectangular, varies
much in its dimensions at different levels, as shown in _fig._ 634.
The hearth and the sides are of cast iron; the sole-plate A B is also of
cast iron, 2-1/2 inches thick, having on its back and two sides an
upright ledge, A C, 2-1/2 inches thick, and 4-1/4 high. In front of the
hearth there is another cast iron plate M N, called the _work-stone_,
surrounded on every side excepting towards the sole of the furnace, by a
ledge one inch in thickness and height. The plate slopes from behind
forwards, and its posterior ledge, which is about 4-1/2 inches above the
surface of the hearth, is separated from it by a void space _q_, which
is filled with a mixture of bone ash and galena, both in fine powder,
moistened and pressed down together. The melted lead cannot penetrate
into this body, but after filling the basin at the bottom of the
furnace, flows naturally out by the gutter (nearly an inch deep) through
a groove in the _work-stone_; and then passes into a cauldron of
reception P, styled the _melting-pot_, placed below the front edge of
the _work-stone_.
The posterior ledge of the sole is surmounted by a piece of cast iron C
D, called the _back-stone_, 28 inches long, and 6-1/2 high; on which the
_tuyère_ or blast-pipe is placed. It supports another piece of cast iron
E, called _pipe-stone_, scooped out at its under part, in the middle of
its length, for the passage of the _tuyère_. This piece advances 2
inches into the interior of the furnace, the back wall of which is
finally crowned by another piece of cast iron E H, called also
_back-stone_.
On the ledges of the two sides of the sole, are placed two pieces of
cast iron, called _bearers_, each of which is 5 inches in breadth and
height, and 26 inches long. They advance an inch or two above the
posterior and highest edge of the _work-stone_, and contribute
effectually to fix it solidly in its place. These bearers support,
through the intervention of several ranges of fire-bricks, a piece of
cast iron called a _fore-stone_, which has the same dimensions as the
piece called the _back-stone_, on which the base of the blowing-machine
rests. This piece is in contact, at each of its extremities, with
another mass of cast iron, 6 inches cube, called the _key-stone_,
supported on the masonry. Lastly, the void spaces left between the two
_key-stones_ and the back part of the furnace are filled up with two
masses of cast iron exactly like the key-stones.
The front of the furnace is open for about 12 inches from the lower part
of the front cross-piece called _fore-stone_, up to the superior part of
the _work-stone_. It is through this opening that the smelter operates.
The gaseous products of the combustion, on escaping from this
ore-hearth, are frequently made to pass through a long flue, sloped very
slightly upwards, in which they deposit all the particles of ore that
they may have swept along; these flues, whose length is sometimes more
than 100 yards, are usually 5 feet high and 3 feet wide in the inside,
and always terminate in a chimney stalk. The matters deposited near the
commencement of the flue require to be washed; but not the other dusty
deposits. The whole may then be carried back to the roasting furnace, to
be calcined and re-agglutinated, or introduced without any preparation
into the _slag-hearth_.
[Illustration: 635 636]
4. _Figs._ 635, 636. represent a slag-hearth, the _fourneau à manche_
(elbow furnace) of the French, and the _krummofen_ (crooked furnace) of
the Germans; such as is used at Alston Moor, in Cumberland, for the
reduction of the lead-slag. It resembles the Scotch furnace. The shaft
is a parallelopiped, whose base is 26 inches by 22 in area inside, and
whose height is 3 feet; the sole-plate _a_, of cast iron, slopes
slightly down to the basin of reception, or the fore-hearth _b_. Upon
both of the long sides of the sole-plate there are cast iron beams,
called _bearers_ C C, of great strength, which support the side walls
built of a coarse grained sand-stone, as well as the cast-iron plate _d_
(_fore-stone_), which forms the front of the shaft. This stands 7
inches off from the sole-plate, leaving an empty space between them. The
back side is made of cast iron, from the sole-plate to the horizontal
tuyère in its middle; but above this point it is made of sand-stone. The
tuyère is from 1-1/5 to 2 inches in diameter. In front of the
fore-hearth _b_, a cistern _e_, is placed, through which water
continually flows, so that the slags which spontaneously overflow the
fore-hearth may become inflated and shattered, whereby the lead
disseminated through them may be readily separated by washing. The lead
itself flows from the fore-hearth _b_, through an orifice, into an iron
pot _f_, which is kept hot over a fire. The metal obtained from this
slag-hearth is much less pure than that extracted directly from the ore.
The whole bottom of the furnace is filled to a height of 17 inches, that
is, to within 2 or 3 inches of the tuyère, with the rubbish of coke
reduced to coarse powder and beat strongly down. At each _smelting
shift_, this bed must be made anew, and the interior of the furnace
above the tuyère repaired, with the exception of the front, consisting
of cast iron. In advance of the furnace there is a basin of reception,
which is also filled with coke rubbish. Farther off is a pit, full of
water, replenished by a cold stream, which incessantly runs in through a
pipe. The scoriæ, in flowing out of the furnace, pass over the coke bed
in the basin of reception, and then fall into the water, whose coolness
makes them fly into small pieces, after which they are easily washed, so
as to separate the lead that may be entangled among them.
[Illustration: 637]
These furnaces are urged, in general, by wooden bellows; _fig._ 637. But
at the smelting works of Lea, near Matlock, the blowing-machine consists
of two casks, which move upon horizontal axes. Each of these casks is
divided into two equal parts by a fixed plane that passes through its
axis, and is filled with water to a certain height. The water of one
side communicates with that of the other by an opening in the lower part
of the division. Each cask possesses a movement of oscillation, produced
by a rod attached to a crank of a bucket-wheel. At each
demi-oscillation, one of the compartments, being in communication with
the external air, is filled; whilst the other, on the contrary,
communicates with the nozzle, and supplies wind to the furnace.