The greater part of the zinc works are situated in the neighbourhood of
Birmingham and Bristol. The manufacture of brass, which has been long
one of the staple articles of these towns, was probably the cause of the
introduction of this branch of industry, at the period when brass began
to be made by the direct union of copper with metallic zinc, instead of
calamine. A few zinc furnaces exist also in the neighbourhood of
Sheffield, amid the coal-pits surrounding that town. Bristol and
Birmingham derive their chief supply of ores from the Mendip Hills and
Flintshire; and Sheffield, from Alston-moor.
The calamine, freed from the galena by sorting with the hand, is
calcined before its introduction into the smelting-furnaces, by being
exposed, coarsely bruised, in reverberatory ovens, 10 feet long, and 8
broad, in a layer 6 inches thick. In some establishments the calcination
is omitted, and the calamine, broken into pieces about the size of a
pigeon’s egg, is mixed with its bulk of small coal.
Zinc is smelted in England, likewise from blende (sulphuret of zinc).
This ore, after being washed, and broken into pieces of the size of a
filbert, was sold a few years ago at the mine of Holywell for 3_l._ a
ton, or half the price of calamine. It is roasted, without any other
preparation, in reverberatory furnaces; which are about 8 feet wide, and
10 long; the distance between the roof and the sole being 30 inches, and
the height of the fire-bridge 18. The layer of blende, which is placed
on the hearth, is about 4 or 5 inches thick; and it is continually
stirred up with rakes. One ton of it requires, for roasting, four tons
of coals; and it suffers a loss of 20 per cent. The operation takes from
10 to 12 hours. The mixture of reducing consists of one-fourth part of
the desulphuretted oxide, one-fourth of calcined calamine, and one-half
part of charcoal; which affords commonly 30 per cent. of zinc.
[Illustration: 1231 1232]
The English furnaces for smelting zinc ores are sometimes quadrangular,
sometimes round; the latter form being preferable. They are mounted with
from 6 to 8 crucibles or pots (see _fig._ 1231.), arched over with a
cupola _a_, placed under a conical chimney _b_, which serves to give a
strong draught, and to carry off the smoke. In this cone there are as
many doors _c_, _c_, _c_, as there are pots in the furnace; and an equal
number of vents _d_, _d_, _d_, in the cupola, through which the smoke
may escape, and the pots may be set. In the surrounding walls there are
holes for taking out the pots, when they become unserviceable; after the
pots are set, these holes are bricked up. The pots are heated to
ignition in a reverberatory furnace before being set, and are put in by
means of iron tong machinery supported upon two wheels, as is the case
with glass-house pots, _e_, is the grate; _f_, the door for the fuel;
_g_, the ash-pit. The pots _h_, _h_, _h_, have a hole in the centre of
their bottom, which is closed with a wooden plug, when they are set
charged with calamine, mixed with one-seventh of coal; which coal
prevents the mixture from falling through the orifice, when the heat
rises and consumes the plug. The sole of the hearth _i_, _i_, upon which
the crucibles stand, is perforated under each of them, so that they can
be reached from below; to the bottom orifice of the pot, when the
distillation begins, a long sheet-iron pipe _k_, is joined, which dips
at its end into a water-vessel _l_, for receiving in drops the condensed
vapours of the zinc. The pot is charged from above, through an orifice
in the lid of the pot, which is left open after the firing, till the
bluish colour of the flame shows the volatilization of the metal;
immediately whereupon the hole is covered with a fire-tile _m_. The iron
tubes are apt to get obstructed during the distillation, and must
therefore be occasionally cleared out with a redhot rod. When the
distillation is finished, the iron pipes must be removed; the coaly and
other contents of the pot cleared away. A pot lasts about four months
upon an average. Five distillations may be made in the course of 14
days, in which from 6 to 10 tons of calamine may be worked up, and from
22 to 24 tons of coals consumed, with a product of two tons of zinc. The
metal amounts to from 25 to 40 per cent. of the ore.
1, 2, is the level of the upper floor; 3, 4, level of the lower ceiling
of the lower floor.
_Fig._ 1232., ground plan on the level of 1, 2: only one-half is here
shown.
The zinc collected in this operation, is in the form of drops, and a
very fine powder, mingled with some oxide. It must be melted in an iron
pot or boiler, set in a proper furnace; and the oxide is skimmed off the
surface, to be returned into the crucibles. The metal is, lastly, cast
into square bars or ingots.
The crucibles are discharged at the end of each operation, by
withdrawing the condenser, breaking with a rake the piece of charcoal
which shuts their bottom, and then emptying them completely, by shaking
their upper part. In replacing the condenser-pipe _k_ (see second pot
from the right hand, _fig._ 1231), the flange at its top is covered with
a ring of loam-lute, pressed against the conical bottom of the crucible,
and secured in its place by means of two parallel rods _o_, _o_, which
can be clamped by screws projecting horizontally from the vertical
tunnel. See their places, indicated by two open dots near _o_, _o_.
A smelter and two labourers are employed in conducting a furnace; who
make, with a mixture of equal parts of fire-clay, and cement of old
pounds finely ground, the pots or crucibles, which last about four
months. Five charges are made in 15 days; these work up from 6 to 10
tons of calamine, consume from 22 to 24 tons of coals, and produce 2
tons of zinc, upon an average. The following estimate of prices was made
a few years ago:--
3 tons of calamine, at _£_6. _£_18 0 0
24 ditto coal, at 5_s._ 6 0 0
A smelter, at 2 guineas a week 2 2 0
Two labourers, each at 4_s._ per day 2 16 0
Incidental expenses 1 0 0
--------
_£_29 18 0
The calamine of Alston-moor, used at Sheffield, is not so rich; it
produces at most only 25 per cent. of zinc. The coals are laid down at a
cost of 5_s._ 8_d._ per ton; and the calamine laid down there 5_l._;
whence the zinc will amount to 32_l._ 14_s._ per ton. The considerable
importations of zinc from Belgium and Germany, for some years back, have
caused a considerable fall in its price.
At Lüttich, where the calamine of Altenberg, near Aix-la-Chapelle, is
smelted, a reduction furnace, containing long horizontal earthen tubes,
is employed. The roasted calamine is finely ground, and mixed with from
one-third to two-thirds its volume of coke or charcoal, broken to pieces
the size of nuts.
[Illustration: 1233 1234]
_Fig._ 1233. represents this zinc furnace in elevation; and _fig._ 1234.
in a vertical section through the middle. From the hearth to the bottom
of the chimney it is 9 feet high, and the chimney itself is 18 or 20
feet high. _a_, is the ash-pit; _b_, the grate; _c_, the fireplace; _d_,
the hearth; _e_, _e_, the laboratory; _f_, the upper arch, which closes
in the laboratory; _f_, the second arch, which forms the hood-cap of the
furnace; _g_, the chimney; _h_, the fire-wall, which rests against a
supporting wall of the smelting-house. Through the vaulted hearth the
flame of the fire draws through ten flues _i_, _i_, two placed in one
line; betwixt these 5 pairs of draught openings, upon the sole of the
hearth, the undermost earthen tubes _k_, immediately rest. The second
and third rows of tubes _k_, _k_, lie in a parallel direction over each
other, at about one inch apart; in the sixth row there are only two
tubes; so that there are 22 tubes altogether in one furnace. At their
two ends these tubes rest upon fire-tiles, which form, with the
side-walls, a kind of checquer-work _l_, _l_. The tubes are 4 feet long,
4 to 5 inches in diameter within, 5/4 of an inch thick. The fire, which
arrives at the laboratory through the flues _i_, _i_, plays round the
tubes, and passes off through the apertures _m_, _m_, in both arches of
the furnace, into the chimney. _n_, is an opening in the front wall
between the two arches, which serves to modify the draught, by admitting
more or less of the external air.
The two slender side walls _o_, _o_, of the furnace, are a foot distant
from the chequer-work, so that on the horizontal iron bars _q_, _q_,
supported by the hooks _p_, _p_, the iron receivers _r_, _r_, may have
room to rest at their fore part. These receivers are conical pipes of
cast iron, 1-1/2 foot long, posteriorly 1-1/2 inch, and anteriorly 1
inch wide at the utmost. After the earthen tubes have been filled with
the ore to be smelted, these conical pipes are luted to them in a
slightly slanting position. These cones last no more than three weeks;
and are generally lengthened with narrow-mouthed wrought-iron tubes, to
prevent the combustion of the zinc, by contact of air. When the furnace
is in activity, a blue flame is to be seen at the mouths of all these
pipes. Every two hours the liquefied metal is raked out into a shovel
placed beneath; and in 12 hours the charge is distilled; after which the
tubes are cleared out, and re-charged. 100 pounds of metallic zinc are
the product of one operation. It is remelted at a loss of 10 per cent.,
and cast into moulds for sale.
[Illustration: 1235 1236]
_Fig._ 1235. is a longitudinal section of the furnace for calcining
calamine in Upper Silesia; _fig._ 1236. is a ground-plan of the furnace.
_a_, is the orifice in the vault or dome, for the introduction of the
ore; _b_, _b_, apertures in the side-walls, shut with doors, through
which the matter may be turned over; _c_, the chimney; _d_, the
fire-bridge; _e_, the grate; _f_, the feed opening of the fire, the fuel
being pitcoal. The calamine is stirred about every hour; and after being
well calcined during 5 or 6 hours, it is withdrawn; and a new charge is
put in. These Silesian furnaces admit of 30 cwt. at a time; and for
roasting every 100 cwt. 15 Prussian bushels of fuel, equal to 23 English
bushels, are employed. These calcining furnaces are sometimes built
alongside of the zinc smelting-furnaces, and are heated by the waste
flame of the latter. The roasting is performed in the Netherlands in
shafts, like small blast iron-furnaces, called schachtofen.
[Illustration: 1237 1238]
The hearth _a_, in _figs._ 1237, 1238., is constructed for working with
5 muffles, one of which is long, and four short. The muffles are made
upon moulds, of fire-clay mixed with ground potsherds. The receivers are
stoneware bottles. The grate is 10 inches beneath the level of the
hearth. _b_, the fire-bridge, is proportionally high, to diminish the
force of the flame upon the hearth, that it may not strike the muffles.
_c_, is the opening through which the muffles are put in and taken out;
during the firing it is partly filled with bricks, so that the smoke and
flame may escape between them; _d_, _d_, are openings for adjusting the
positions of the muffles; _e_, cross hoops of iron, to strengthen the
brick arch; _f_, is a bed of sand under the sole of the hearth. During
the first two days, the fire is applied under the grating; the heat must
be very slowly raised to redness, at which pitch it must be maintained
during two days. From 8 to 10 days are required for the firing of the
muffles.
[Illustration: 1239 1240 1241]
The furnace shown in _figs._ 1239, 1240, 1241. is for the melting of the
metallic zinc. _Fig._ 1240. is a front view; _fig._ 1239. a transverse
section; _fig._ 1241. a view from above: _a_, is the fire-door; _b_, the
grate; _c_, the fire-bridge; _d_, the flue; _e_, the chimney; _f_, _f_,
_f_, cast-iron melting-pots, which contain each about 10 cwt. of the
metal. The heat is moderated by the successive addition of pieces of
cold zinc. The inside of the pots should be coated with loam, to prevent
the iron being attacked by the zinc. When the zinc is intended to be
laminated, it should be melted with the lowest possible heat, and poured
into hot moulds.
When the zinc ores contain cadmium, this metal distils over in the form
of brown oxide, with the first portions, being more volatile than zinc.
Under BRASS and COPPER, the most useful alloys of zinc are described.
The sulphate, vulgarly called white vitriol, is made from the sulphuret,
by roasting it gently, and then exposing it upon sloping terraces to the
action of air and moisture, as has been fully detailed under SULPHATE OF
IRON. The purest sulphate of zinc is made by dissolving the metal in
dilute sulphuric acid, digesting the solution over some of the metal,
filtering, evaporating, and crystallizing.
Sulphate of zinc is added as a drier to japan varnishes.
The ordinary zinc found in the market is never pure; but contains lead,
cadmium, arsenic, copper, iron, and carbon; from some of which, it may
be freed in a great degree by distillation; but even after this process
it retains a little lead, with all the arsenic and cadmium. The
separation of the latter is described under CADMIUM. Zinc, free from
other metals, may be obtained by distilling a mixture of charcoal and
its subcarbonate, precipitated from the crystallized sulphate by
carbonate of soda. By holding a porcelain saucer over the flame of
hydrogen produced from the action of dilute sulphuric acid upon any
sample of the zinc of commerce, the presence of arsenic in it may be
made manifest by the deposit of a gray film of the latter metal.
Antimony, however, produces a somewhat similar effect to arsenic.
Zinc is extensively employed for making water-cisterns, baths, spouts,
pipes, plates for the zincographer, for voltaic batteries, filings for
fire-works, covering roofs, and a great many architectural purposes,
especially in Berlin; because this metal, after it gets covered with a
thin film of oxide or carbonate, suffers no further change by long
exposure to the weather. One capital objection to zinc as a roofing
material, is its combustibility.
Chloride of zinc has been recently used with great advantage as an
escharotic for removing cancerous tumours, and healing various
ill-constitutioned ulcers. It, as also the nitrate, forms an ingredient
in the resist pastes for the pale blues of the indigo vat.
Spelter (zinc) imported for home consumption--in 1835, 52,604 cwts.; in
1836, 47,406 cwts. Duty,--in cakes, 2_s._; not in cakes, 10_s._ per cwt.