6. _Smelting by the reverberatory furnace_, is adopted exclusively in
Derbyshire, and in some works at Alston-moor. The charge in the hopper
consists commonly of 16 cwt., each weighing 120 lbs. avoirdupois,
composed of an intimate mixture of 5, 6, 7 or even 8 kinds of ore,
derived from different mines, and prepared in different ways. The
proportions of the mixture are determined by experience, and are of
great consequence to the success of the work.
The ore is rather in the form of grains than of a fine _schlich_; it is
sometimes very pure, and affords 75 _per cent._; but usually it is mixed
up with a large proportion of carbonate and fluate of lime; and its
product varies from 65 to 23 _per cent._
After scraping the slaggy matters out of the furnace, a fresh smelting
shift is introduced at an interval of a few minutes; and thus, by means
of two alternate workmen, who relieve each other every seven or eight
hours, the weekly operations continue without interruption. The average
product in lead of the reverberatory furnaces in Derbyshire, during
several years, has been 66 per cent. of the ore. Very fine ore has,
however, afforded 76.
7. _Smelting of the drawn slag, on the slag-mill hearth._--The black
slag of the reverberatory furnace is broken by hammers into small
pieces, and mixed in proper proportions with the coal cinders that fall
through the grate of the reverberatory fire. The leaden _matts_ that
float on the surface of the bath, and the dust deposited in the chimney,
are added, along with some poor ore containing a gangue of fluor spar
and limestone, which had been put aside during the mechanical
preparation. With such a mixture, the slag-hearth, already described,
_figs._ 635, 636., is charged. By the action of heat and coal, the lead
is revived, the earthy matters flow into very liquid scoriæ, and the
whole is made to pass across the body of fire into a basin of reception
placed beneath. The scoriæ are thickened by throwing quicklime upon
them, and they are then raked away. At the end of the operation the lead
is cast into pigs or ingots of a peculiar form. This is called
slag-lead. It is harder, more sonorous than the lead obtained from the
reverberatory furnace, and is preferred for the manufacture of minium,
lead shot, and some other purposes.
8. _Treatment of lead ores by the Scotch furnace, or ore-hearth._--This
furnace is generally employed in the counties of Northumberland,
Cumberland, and Durham, for the smelting of lead ores, which were
formerly carried to them without any preparation, but now they are
exposed to a preliminary calcination. The roasted ore yields in the
Scotch furnace a more considerable product than the crude ore, because
it forms in the furnace a more porous mass, and at the same time _it
works drier_, to use the founders’ expression; that is, it allows the
stream of air impelled by the bellows to diffuse itself more completely
across the matters contained in the furnace.
The charge of the _roasting_ furnace, _figs._ 631, 632, 633., is from 9
to 11 cwt. of ore, put into the furnace without any addition. Three such
shifts are usually passed through in eight hours. The fire should be
urged in such a manner as to produce constantly a dense smoke, without
letting any part of the ore melt and form a slag; an accident which
would obstruct the principal end of the process, which is to burn off
the sulphur and antimony, and to expel the carbonic acid of the
carbonate of lead. The ore must be frequently turned over, by moving it
from the bridge to the other end and back again. To prevent the ore from
running into masses as it cools, it is made to fall out of the furnace
into a pit full of water, placed below one of the lateral doors.
_Smelting of the lead ores in the Scotch furnace._--When a _smelting
shift_ has been finished in the Scotch furnace, a portion of the ore,
called _browse_, remains in a semi-reduced state, mixed with coke and
cinders. It is found of more advantage to preserve the browse for
beginning the following operation, than to take raw or even roasted ore.
To set the furnace in action, the interior of it is filled with peats,
cut into the form of bricks. The peats towards the posterior part are
heaped up without order, but those near the front are piled up with care
in the form of a wall. A kindled peat is now placed before the nozzle of
the bellows, which are made to blow, and the blast spreads the
combustion rapidly through the whole mass. To increase the heat, and to
render the fire more steady and durable, a few shovelfuls of coals are
thrown over the turf. A certain quantity of the browse is to be next
introduced; and then (or sometimes before all the browse is put in) the
greater part of the matters contained in the furnace is drawn over on
the _work-stone_, by means of a large rake called a _gowelock_; the
refuse of the ore called _gray slag_, which a skilful smelter knows by
its shining more than the browse, is taken off with a shovel, and thrown
to the right hand into a corner outside of the furnace. The browse left
on the work-stone is to be now thrown back into the furnace, with the
addition of a little coal, if necessary. If the browse be not well
cleaned from the slag, which is perceived by the whole mass being in a
soft state, and shewing a tendency to fuse, quicklime must be added,
which by its affinity for the argillaceous, siliceous, and ferruginous
substances, dries up the materials, as the smelters say, and gives to
the earthy parts the property of concreting into lumps or balls; but if,
on the other hand, the siliceous, argillaceous, or ferruginous parts
contained in the ore be too refractory, lime is also to be added, but in
smaller quantity, which, by rendering them more fusible, communicates
the property of concreting into balls. These lumps, called gray slag,
contain from one-tenth to one-fifteenth of the lead which was present in
the ore. They must be smelted afterwards at a higher temperature in the
slag hearth, to extract their lead. After the browse has been thrown
back into the furnace, as has been said, a few shovelfuls of ore are to
be strewed over it; but before doing this, and after removing the
scoriæ, there must be always placed before the tuyère half a peat, a
substance which, being extremely porous and combustible, not only
hinders any thing from entering the nozzle of the bellows, but spreads
the blast through all the vacant parts of the furnace. After an interval
of from 10 to 15 minutes, according to circumstances, the materials in
the furnace are drawn afresh upon the work-stone, and the gray slag is
removed by the rake. Another peat being placed before the tuyère, and
coal and quicklime being introduced in suitable proportions, the browse
is thrown back into the furnace, a fresh portion of ore is charged above
it, and left in the furnace for the above mentioned time.
This mode of working, continued for 14 or 15 hours, forms what is called
a _smelting shift_; in which time from 20 to 40 cwt. of lead, and even
more, are produced.
By this process the purest part of the lead, as well as the silver, are
sweated out, as it were, from the materials, with which they are mixed,
without any thing entering into fusion except these two metals in the
state of alloy. It is probable that the moderate temperature employed in
the Scotch furnace is the main cause of the purity of the lead which it
yields.
9. _Smelting of the scoriæ of the Scotch furnace on the slag
hearth._--Before putting fire to the slag hearth already described,
_figs._ 635, 636., its empty space is to be filled with peats, and a
lighted one being placed before the tuyère, the bellows are made to
play. A layer of coke is to be now thrown upon the burning peats, and as
soon as the heat is sufficiently high, a layer of the _gray slag_ is to
be introduced, or of any other scoriæ that are to be reduced. From time
to time, as the fit moment arrives, alternate strata of coke and slag
are to be added. In this operation, though the slag and the lead are
brought to a state of perfect fluidity; the metal gets separated by
filtering down through the bed of peat cinders, which the slag cannot do
on account of its viscidity. Whenever that coke bed becomes covered with
fluid slag, the workman makes a hole in it, of about an inch diameter,
by means of a kneed poker; and runs it off by this orifice, as it cannot
sink down into the hard rammed cinders, which fill the basin of
reception. The slag flows over it in a glowing stream into the pit
filled with water, where it gets granulated and ready for washing.
When lead is obtained from galena without the addition of combustible
matter, we have an example on the great scale, of the mutual
decomposition of the oxides and sulphates formed during the roasting
heat, by the still undecomposed galena, especially when this action is
facilitated by working up and skilfully mingling the various matters, as
happens in the reverberatory and Scotch furnaces. It is therefore the
sulphuret of lead itself which serves as the agent of reduction in
regard to the oxide and sulphate, when little or no charcoal has been
added. Sometimes, however, towards the end of the operation in the
reverberatory hearth, it becomes necessary to throw in some wood or
charcoal, because the oxidizement having become too complete, there does
not remain a sufficient body of sulphuret of lead to effect the
decompositions and reductions just mentioned, and therefore it is
requisite to regenerate some galena by means of carbonaceous matter,
which immediately converts the sulphate of lead into the sulphuret. The
sulphur and oxygen are eventually all separated in the form of
sulphurous acid. Roasted galena contains sometimes no less than 77 per
cent. of sulphate of lead.
At Viconago in the Valais, the process of smelting lead ore in the
reverberatory furnace with the addition of iron, as practised at Vienne
on the Isère, was introduced; but the difficulty of procuring a
sufficient supply of old iron has led to an interesting modification.
On the hearth of the reverberatory furnace, 10 quintals of moderately
rich ore are spread; these are heated temperately for some time, and
stirred about to promote the sublimation of the sulphur. After three or
four hours, when the ore seems to be sufficiently de-sulphuretted, the
heat is raised so as to melt the whole materials, and whenever they flux
into a metallic glass, a few shovelfuls of bruised charcoal or cinders
are thrown in, which soon thicken the liquid, and cause metallic lead to
appear. By this means three-fourths of the lead contained in the ore are
usually extracted; but at length the substance becoming less and less
fluid, yields no more metal. Stamped and washed carbonate of iron
(sparry iron ore) is now added, in the proportion of about 10 per cent.
of the lead ore primarily introduced.
On stirring and working together this mixture, it assumes the
consistence of a stiff paste, which is raked out of the furnace. When
this has become cold, it is broken into pieces, and thereafter smelted
in a slag-hearth, without the addition of flux. By this operation,
almost the whole lead present is obtained. 100 quintals of schlich yield
45 of argentiferous lead; and in the production of 100 quintals (cwts.)
of marketable lead, 140 cubic feet of beech-wood, and 357-1/2 quintals
of charcoal are consumed.
This process is remarkable for the use of iron-ore in smelting galena.
10. _Reduction in the reverberatory furnace, of the litharge obtained in
the refining of lead._--The litharge of Alston Moor is seldom sold as
such, but is usually converted into lead, in a reverberatory furnace.
In commencing this reduction, a bed of coal about 2 inches thick is
first of all laid on the hearth; which is soon kindled by the flame of
the fire-place, and in a little while is reduced to red hot cinders.
Upon these a certain quantity of a mixture of litharge and small coal is
uniformly spread; the heat of the fire-place being meanwhile so managed
as to maintain in the furnace a suitable temperature for enabling the
combustible to deprive the litharge of its oxygen, and to convert it
into lead. The metal is run out by the tap-hole into an iron pot; and
being cast into pigs of half a hundred weight, is sold under the name of
refined lead at a superior price.
The quantity of small coal mixed with the litharge, should be somewhat
less than what may be necessary to effect the reduction, because if in
the course of the process, a deficiency of it is perceived in any part
of the furnace, more can always be added; whereas a redundancy of coal
necessarily increases the quantity of slag, which, at the end of the
shift, must be removed from the furnace before a new operation is begun,
whereby lead is lost. In the reverberatory furnace, six fodders of lead
may be revived in nine or ten hours; during the first six of which the
mixture of litharge and coal is added at short intervals. A fodder is
from 21 to 24 cwts.
It deserves to be remarked that the work does not go on so well nor so
quick when the coal and litharge are in a pulverulent form; because the
reduction in this case takes place only at the surface, the air not
being able to penetrate into the body, and to keep up its combustion,
and the mutual action of the litharge and carbon in the interior. But on
the other hand, when the litharge is in porous pieces as large as a
hen’s egg, the action pervades the whole body, and the sooty fumes of
the coal effect the reduction even in the centre of the fragments of the
litharge, penetrating into every fissure and carrying off the oxygen.
The heat ought never to be urged so far as to melt the litharge.
The grounds of the cupel, and the slag of the reduction furnace, being a
mixture of small coke, coal ash, and oxide of iron, more or less
impregnated with lead, are smelted upon the _slag hearth_, along with
coke, and by way of flux, with a certain quantity of the black scoriæ
obtained from the same furnace, prepared for this purpose, by running it
out in thin plates, and breaking it into small pieces. The lead thus
obtained is usually very white, very hard, and not susceptible of
refinement.
MM. Dufrénoy and Beaumont consider the smelting of lead ore by the
reverberatory furnace as practised in Derbyshire, as probably preferable
to that with the slag hearth as carried on in Brittany; a process which
seldom gives uniform products, while it occasions a more considerable
waste of lead, and consumption of fuel.
The mixed process employed in Cumberland of roasting the ore, and
afterwards smelting it in a small furnace resembling that called the
Scotch, apparently yields a little less lead than if both operations
were executed in the reverberatory furnace; but according to Mr.
Forster, (see his _Treatise on a Section of the Strata from Newcastle
upon Tyne_, &c.) this slight loss is more than compensated by the
smaller consumption of fuel, the increased rapidity of the operation,
and especially by the much greater purity of the lead obtained from the
Scotch furnace. When it comes to be refined, the loss is only about
one-twelfth or one-thirteenth, whereas the lead revived in the
reverberatory furnace, loses frequently a ninth. Moreover, the lead
furnished by the first method admits of being refined with profit, when
it yields only 5 ounces of silver _per_ fodder of 20 quintals, _poids de
marc_, while that produced by the reverberatory furnace cannot be
cupelled unless it gives 10 ounces per fodder; and as in the English
cupellation, lead is constantly added anew without skimming, the
litharge obtained in the second case can never be brought into the
market, whereas the litharge of the leads from the Scotch furnace is of
good quality. See the new method of enriching lead for cupellation,
under SILVER.
As the _smelting_ of galena, the principal ore of lead, is not a little
complex, the following tabular view of the different processes may prove
acceptable to the metallurgist:--