[Footnote: Lately read before the Institute of Mechanical Engineers.]
By NORMAN C. COOKSON, of Newcastle.
The author began by stating that probably in few trades have a smaller
number of changes been made during recent years, in the processes
employed, than in that of lead smelting and manufacturing. He then
briefly noted what these changes are, and went on to describe the "steam
desilverizing process," as used in the works of the writer's firm, and
in other works licensed by them, which process is the invention of
Messrs. Luce Fils et Rozan, of Marseilles. It is one which should
commend itself especially to engineers, as in it mechanical means
are employed, instead of the large amount of hand-labor used in the
Pattinson process. It consists in using two pots only, of which the
lower is placed at such a height that the bottom of it is about 12
in. to 15 in. above the floor level, while the upper is placed at a
sufficiently high level to enable the lead to be run out of it into
the lower pot. The capacity of the lower pot, in those most recently
erected, is thirty-six tons--double that of the upper one. Round each
pot is placed a platform, on which the workmen--of which there are two
only to each apparatus--stand when skimming, slicing, and charging the
pots. The upper pot is open at the top, but the lower one has a cover,
with hinged doors; and from the top of the cover a funnel is carried
to a set of condensers. At a convenient distance from the two pots is
placed a steam or hydraulic crane, so arranged that it can plumb each
pot, and also the large moulds which are placed at either side of the
lower pot. The mode of working is as follows:
The silver lead is charged into the upper pot by means of the crane.
When melted, the dross is removed, and the lead run into the lower, or
working pot, among the crystals remaining from a previous operation.
When the whole charge is thoroughly melted, it is again drossed; and in
order to keep the lead in a thoroughly uniform condition, and prevent it
setting solid on the top and the outside, a jet of steam is introduced.
To enable this steam to rise regularly in the working pot, a disk-plate
is placed above the nozzle, which acts as a baffle-plate; and uniform
distribution of the steam is the result. To quicken the formation of
crystals, and thus hasten the operation, small jets of water are allowed
to play on the surface of the lead.
This, it might be thought, would make the lead set hard on the surface;
but the violent action of the steam acts in the most effectual manner
in causing the regular formation of crystals. Owing to the ebullition
caused by this action of the steam, small quantities of lead are forced
up, and set on the upper edges and cover of the pot. From time to time
the valve controlling the thin stream of water playing on the top of the
charge is closed, and the workman, opening the doors of the cover in
rotation, breaks off this solidified lead, which falls among the rest of
the charge, and instantly becomes uniformly mixed with it.
Very little practice enables an ordinary workman to judge when
two-thirds of the contents of the big pot are in crystals, and one-third
liquid; and when he sees this to be the case, instead of ladling out the
crystals ladleful by ladleful, as in the old Pattinson process, he taps
out the liquid lead by means of two pipes, controlled by valves, the
crystals being retained in the pot by means of perforated plates.
The liquid lead is run into large cone-shaped moulds on either side of
the pot; and a wrought iron ring being cast into the blocks thus formed,
they are readily lifted, when set, by the crane. To give some idea of
the rapidity of the process, it may be mentioned that from the time the
lead is melted and fit to work in the big pot, to the time that it is
crystallized and ready for tapping, is, in the case of a 36 ton pot,
from thirty-five to forty-five minutes; and the time required for
tapping the liquid lead into the large moulds is about eight minutes.
Before the lead begins to crystallize, the upper pot is charged with
lead of half the richness of that in the lower pot. Thus, when the
liquid lead has been tapped out of the lower pot, it is replaced by a
similar amount of lead of the same richness as the remaining crystals,
by simply tapping the upper or melting pot, and allowing the contents to
run among the crystals.
The same operation is repeated from time to time, until the crystals are
so poor in silver that they are fit to be melted, and run into pigs for
market.
The large blocks of partially worked lead are placed by the crane in
a semicircle round it, and pass successively through the subsequent
operations. The advantages of the steam process, as compared to the old
six-ton Pattinson pots formerly used by the writer's firm, are: (1) a
saving of two-thirds amount of fuel used; (2) the saving of cost of
calcination of the lead to the extent of at least four-fifths of
all that is used; (3) above all, a saving in labor to the extent of
two-thirds. The process has its disadvantages, and these are a larger
original outlay for plant, and a constant expense in renewals and
repairs. This is principally caused by the breakage of pots; but with
increased experience this item has been very much reduced during the
last two or three years.
The "zinc process" of desilverizing, which is largely used by Messrs.
Locke, Blackett & Co., and was patented in the form adopted by them
about fourteen years since. The action of this process is dependent on
the affinity of zinc for silver. The following is a brief description of
it:
A charge of silver lead, usually about fifteen tons, is heated to a
point considerably above that which is used in either the Pattinson or
the steam process. The quantity of zinc added is regulated by the amount
of silver contained in the lead; but for lead containing 50 oz. to the
ton, the quantity of zinc used is in most cases about 1½ per cent, of
the charge of lead. The lead being melted as described, a portion of
this zinc, usually about half of the total quantity required for the
charge, is added to the melted lead, and thoroughly mixed with it by
continued stirring. The lead is now allowed to cool, when the zinc is
seen gradually to rise to the top, having incorporated with it a large
proportion of the silver. The setting point of zinc being above that
of lead, a zinc crust is gradually formed, and this is broken up and
carefully lifted off into a small pot conveniently placed, care being
taken to let as much lead drain off as possible. The fire is again
applied strongly to the pot, and when the lead is sufficiently heated, a
further quantity of zinc, about one-third of the whole quantity used, is
added, when the same process of cooling and removing the zinc crust is
repeated. This operation is gone through a third time with the remaining
portion--¼ per cent.--of zinc; and if each of these operations has
been carefully carried out, the lead will be found to be completely
desilverized, and will only show a very small trace of zinc. In some
works this trace of zinc is allowed to remain in the market lead, but
at Messrs. Locke, Blackett & Co.'s works it is invariably removed by
subjecting the lead to a high heat in a calcining furnace. The zinc
crusts, rich in silver, are freed as far as possible from the lead by
allowing this to sweat out in the small pot, after which the crusts are
placed in a covered crucible, where the zinc is distilled off, and a
portion of it recovered. The lead remaining, which is extremely rich in
silver, is then taken to the refinery, and treated in the usual manner.
The writer is given to understand that the quantity of zinc recovered is
as high as from 50 to 60 per cent. of the total quantity used.
Although it was said that the rolling or milling of lead remains
unchanged in its main features since the first mill was established, yet
the writer's firm have introduced many important improvements. When lead
is required for sheet making, instead of running out the market lead
into the usual pigs of about one hundredweight each, it is run into
large blocks of 3½ tons. These 3½ ton blocks are taken on a bogie to the
mill-house, where the mill melting pot is charged with them by means
of a double-powered hydraulic crane, lifting, however, with the single
power only.
Three such blocks fill the pot, and when melted are tapped on to a large
casting plate, 8 ft. 4 in. by 7 ft. 6 in., and about 7 in. thick. This
block, weighing 10½ tons, is lifted on to the mill table by the same
crane as fills the pot, but using the double power; and is moved along
to the rolls in the usual manner by means of a rope working on a surging
head. The mill itself, as regards the roll, is much the same as those
of other firms; but instead of an engine with a heavy fly-wheel, always
working in one direction, and connected to the rolls by double clutch
and gearing, the work is done by a pair of horizontal reversing engines,
in connection with which there is a very simple, and at the same time
extremely effectual, system of hydraulic reversing. On the usual method
there is no necessity for full or delicate control of lead mill engines;
but with this system it is essential, and the hydraulic reversing gear
contributes largely to such control. This may be explained as follows:
In all other mills with which the writer is acquainted, when the lead
sheet, or the original block, has passed through the rolls, and before
it can be sent back in the opposite direction, a man on either side of
the mill must work it into the grip of the rolls with crowbars.
In the writer's system this labor is avoided, and the sheet or block is
fed in automatically by means of subsidiary rolls, which are driven by
power. When it is required to cut the block or sheet by the guillotine,
or cross-cutting knife, instead of the block being moved to the desired
point by hand-labor, the subsidiary driven rolls work it up to the
knife; and such perfect control does the engine with its hydraulic
reversing gear possess, that should the sheet overshoot the knife
1/8 in., or even less, the engine would bring it back to this extent
exactly.
Another point, which the writer looks upon as one of the greatest
improvements in this mill, is its being furnished with circular knives,
which can be set to any desired width, and put in or out of gear at
will; and which are used for dressing up the finished sheet in the
longitudinal direction. This is a simple mechanical arrangement, but
one which is found to be of immense benefit, and which, in the writer's
opinion, is far superior to the usual practice of marking off the sheet
with a chalk line, and then dressing off with hand knives. The last
length of the mill table forms a weighbridge, and a hydraulic crane
lifts the sheet from it either on to the warehouse floor or the tramway
communicating with the shipping quay.
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