3. Smelting the bottoms of the cupels, to extract the lead which had
soaked into them, in a glassy state.
Cupellation and its two complementary operations were, in many respects,
objectionable processes; from the injurious effects of the lead vapours
upon the health of the workmen; from the very considerable loss of
metallic lead, amounting to 7 per cent. at least; and, lastly, from the
immense consumption of fuel, as well as from the vast amount of manual
labour incurred in such complicated operations. Hence, unless the lead
were tolerably rich in silver, it would not bear the expense of
cupellation.
The patent process lately introduced by Mr. Pattinson, of Newcastle, is
not at all prejudicial to the health of workmen; it does not occasion
more than 2 per cent. of loss of lead, and in other respects it is so
economical, that it is now profitably applied in Northumberland to
alloys too poor in silver to be treated by cupellation. This process is
founded upon the following phenomena.
After melting completely an alloy of lead and silver, if we allow it to
cool very slowly, continually stirring it meanwhile with a rake, we
shall observe at a certain period a continually increasing number of
imperfect little crystals, which may be taken out with a drainer,
exactly as we may remove the crystals of sea salt deposited during the
concentration of brine, or those of sulphate of soda, as its agitated
solution cools. On submitting to analysis the metallic crystals thus
separated, and also the liquid metal deprived of them, we find the
former to be lead almost alone, but the latter to be rich in silver,
when compared with the original alloy. The more of the crystalline
particles are drained from the metallic bath, the richer does the
_mother_ liquid become in silver. In practice, the poor lead is raised
by this means to the standard of the ordinary lead of the litharge
works; and the better lead is made ten times richer. This very valuable
alloy is then submitted to cupellation; but as it contains only a tenth
part of the quantity of lead subjected to crystallization, the loss in
the cupel will be obviously reduced to one-tenth of what it was by the
former process; that is, 7/10 of a per cent., instead of 7.
These nine-tenths of the lead separated by the drainer, are immediately
sent into the market, without other loss than the trifling one, of about
1/2 per cent., involved in reviving a little dross skimmed off the
surface of the melted metal at the beginning of the operation. Hence the
total waste of lead in this method does not exceed 2 per cent. And as
only a small quantity of lead requires to be cupelled, this may be done
with the utmost slowness and circumspection; whereby loss of the
precious metal, and injury to the health of the workpeople, are equally
avoided.
The crystallization refinery of Mr. Pattinson is an extremely simple
smelting-house. It contains 3 hemispherical cast-iron pans, 41 inches in
diameter, and 1/4 of an inch thick. The three pans are built in one
straight line, the broad flange at their edge being supported upon
brickwork. Each pan has a discharge pipe, proceeding laterally from one
side of its bottom, by which the melted metal may be run out when a plug
is withdrawn, and each is heated by a small separate fire.
Three tons of the argentiferous lead constitute one charge of each pan;
and as soon as it is melted, the fire is withdrawn; the flue,
grate-door, and ash-pit, are immediately closed, and made air-tight with
bricks and clay-lute. The agitation is now commenced, with a round bar
of iron terminated with a chisel point, the workman being instructed
merely to keep moving that simple rake constantly in the pan, but more
especially towards the edges, where the solidification is apt to begin.
He must be careful to take out the crystals, progressively as they
appear, with an iron drainer, heated a little higher than the
temperature of the metal bath. The liquid metal lifted in the drainer,
flows readily back through its perforations, and may be at any rate
effectually detached by giving the ladle two or three jogs. The solid
portion remains in the form of a spongy, semi-crystalline, semi-pasty
mass.
The proportion of crystals separated at each melting, depends upon the
original quality of the alloy. If it be poor, it is usually divided in
the proportion of two-thirds of poor crystals, and one-third of rich
liquid metal; but this proportion is reversed if the alloy contain a
good deal of silver.
Let us exemplify, by the common case of a lead containing 10 ounces of
silver per ton. Operating upon three tons of this alloy, or 60 cwt.,
containing 30 oz. of silver, there will be obtained in the first
operation--
(_a_) 40 cwt. at 4-1/4 ounces of silver per ton; in whole 9 oz.}30 oz.
(_b_) 20 cwt. at 21 -- -- 21 }
Each of these alloys (_a_) and (_b_) will be joined to alloys of like
quality obtained in the treatment of one or several other portions of
three tons of the primitive alloy. Again, three tons of each of these
rich alloys are subjected to the crystallization process, and thus in
succession. Thus poorer and poorer lead is got on the one hand, and
richer and richer alloys on the other. Sometimes the _mother_ metal is
parted from a great body of poor crystals, by opening the
discharge-pipe, and running off the liquid, while the workman keeps
stirring, to facilitate the separation of the two.
25 fodders, 15 cwts., 49 lbs. = 540 cwts., 49 lbs. of alloy, holding 5
oz. of silver per fodder, in the whole 130 oz., afforded, after three
successive crystallizations--
oz.
440 cwts. of poor lead, holding 1/2 oz. of silver per fodder;
in all 10-1/2
15 cwt. 49 -- holding the original quantity, nearly 3-1/2
84 cwts. of lead for the cupel, holding 29 oz. 116
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Total 130