Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
The present invention concerns the preparation of a
metal by electrolysis, and especially the preparation of lead
from lead chloride. It concerns in particular the electrolysis
of very pure solutions of lead chloride.
The processes currently used for metallurgical
treatment of lead ores permit the preparation of solutions of
lead chloride which are very pure, for example, after purification
by a solvent or crystallization. The invention concerns the
preparation of the lead by such solutions. Published French
patent no. 73-30.657 describes a process for deposit of
metallic lead by aqueous solutions of lead chloride. More
specifically this patent describes the electrolysis of such
a solution in a cell with a diaphragm, in the presence of ferrous
chloride which oxidizes into ferric chloride during the operation;
in example 1 of this patent, the concentration of the lead
in the electrolyte is reduced to a value between 25 and 11
grams per liter, in a 3M solution in ferrous chloride, with
a current density of 323 A/m2 and a faradic return of 70~.
The patent does not indicate the properties of the deposit
of metal7ic lead such as its DENSITY, its adherence to the
cathodic support (leaf of lead), its compact nature, or dusty
nature, or its purity, nor the method of extraction of the
lead.
The wor~ "Electr~metallurgy of Chloride Solutions"
of V. V. Stender, Consultants Bureau, New York (1965) indicates
that a deposlt of lead, non compact, with gross crystallization,
having a metallic appearance, can be obtained from a concentrated
brine of sodium chloride containing lead. The concentration
of the lead decreases from 40 to 10 grams per liter in the
course of an electrolysis carried out with a current density
between 500 and 1,000 A/m2. The work does not give precise
* now French Patent 2,240,956 - Hazen Research Inc.
information on the properties of the lead such as its purity,
its density, its adherence to the cathode which is composed
of a leaf of lead, nor on the method of extraction of the
deposit. The faradic yield return obtained would be between
85 and 90~. The same work indicates that a powder of lead can
be obtained from so~utions containing 300 grams per liter of
NaCl and 10 grams per liter of lead in the form of chloride,
with a faradic return of approximately 80%.
The report "Aqueous electrolysis of lead chloride"
by F. P. Haver, D. L. Bixby and M. M. Wong, U.S. ~.M. Report of
Investigations, 8276 (1978) described the electrolysis of lead
chloride, crystallized on a horizontal cathode placed at the
bottom of the cell, so that the concentration of lead in
solution remains constant. This document indicates that, as
soon as ~he last crystal of lead chloride disappears, the
deposit becomes spongy, with a non-metallic appearance and
sticky appearance. In the presence of crystals, the faradic
return obtained is 96% for a current density of 150 A/m2, in a
solution of 20% HCl at 25C. Published French patent no.
79-12.867 describes a process of extracting lead from sulphurated
ores. This process assures the regeneration of the reagent,
the ferric chloride, to the anode of an electrolyser having
neither diaphragm nor membraneO The lead is deposited on a
cathode formed from an assemblage of shafts mounted in special
supports so that shocks can be applied to the shafts from
the rotation of these or their mounting setup. The lead
formed detaches itself under the action of the shocks and
falls to the bottom of the vat. It is then removed. This
patent does ~ot descri~ the effects of the electrolysis
current in the neighborhood of the electrode and describes
neither the recovery of the lead fragments nor the treatment
of the lead before fusion.
French patent no. 2,386,349 describes a process and
* now French Patent No. 2,427,401 - Kammel et al
1;~;3~
an apparatus for recovery of metallic particles by electrolysis.
This patent concerns essentially copper and secondarily
transition metals, those which are indicated as preferable
being two of the groups VIII, lb and 2b, of the Periodic
Classification of the Elements. This patent thus does not
concern the treatment of lead. According to this patent, the
metal, essentially copper, forms, on the cathodes, particles
which are removed by use of vigorous rubbing done by mechanical
agitators moved in front of the cathodes. According to an
essential characteristic of the process described in this
patent, the powder must undergo washing before being removed
from the electrolysis cell.
Thus, the processes described have some drawbacks
and the documents mentioned above possess certain gaps. In
particular, it is not known what is the quality of the powder
obtained, particularly its purity, its density, its properties
of oxidation by the air, all of which are essential properties
in the industrial exploitation of such a powder.
These documents do not indicate any process of
extraction of the lead formed capable of being used in an
industrial electrolyser.
The faradic returns obtained are most often lower
than 90~. The anodic reaction is not described in general and
it is not indicated if the chlorine disengages itself from the
anode or if on the contrary this disengagement is avoided, and
in what manner.
OBJECTS OF THE INVENTION
The invention concerns the preparation by electrolysis
of a very pure metal, preferably lead.
The invention concerns the preparation by electrolysis
of a metal present in the electrolyte under a non-cationic
~3~
and especially anionic form.
It concerns such a process which applies a continuous
removal of the metal which is formed on the cathodes.
It concerns such a process which permits the
preparation of a particular metal, especially lead specifically
which is not pyrophoric and which can be easily turned into
an economically profitable by-product.
It concerns also such a process which permits a very
high faradic return.
SUMMARY OF THE INVENTION
_
According to the invention, the electrolyte
circulates parallel to the cathodes which are placed vertically
with a speed such that its flowing is of a laminar type or
weakly turbulent, so that this current, in cooperation with the
apparent weight of the particles, ensures the removal of these
from the cathodes and, simultaneously, the renewal of the
electrolyte next the surfaces of the electrodes.
More specifically, the invention concerns a process
of preparation of a metal, preferably of lead, by electrolysis
in a cell with a diaphragm, of the type which includes the
formation of an electrolyte containing a chloride of the metal
to be prepared and an alkali metal or an earth-alkaline
metal chloride, and the circulation of the electrolyte between
the electrodes and parallelto the surface of the cathodes.
According to the invention, the cathodic surface is arranged
in a mainly vertical direction and has a density sufficiently
weak in sites of nucleation so that the metallic particles
which are formed from these sites keep their individuality
vis-a-vis the adjacent particles, until they reach a dimension
of at least 100 micrometers; the flowing of the electrolyte
the length of the cathodic surface is of a laminar type or
weakly turbulent type, so that, under the action of their
1'~3~
weight and of the forces attracting them exercised by the
electrolyte current, the metal particles detach themselves
and fall into the electrolyte; the process includes in addition
the removal of the metallic particles gathered together at
the bottom of the cell.
When the metal of the electrolysis is lead, it is
present, in the form of chloride, in a quantity betwPen
approximately 5 and 50 grams per liter, preferably between 15
and 25 grams per liter in the electrolyte.
The chloride of alkali or earth-alkaline metal is
preferably sodium chloride. Its concentration in the electrolyte
is preferably between 230 and 300 grams per liter.
In the course of the electrolysis, the density of the
current of electrolysis is between 500 and l,500 A/m2, preferably
between 700 and l,000 A/m2. It is preferable that this density
of current increase progressively from the beginning of the
electrolysis.
During the electrolysis, the temperature of the
electrolyte is beneficially between 70 and 95C.
The cathodic surface having a low density of sites
of nucleation is preferab]y formed of titanium, of stainless
steel, or of graphite.
It is advantageous that the elec1:rolyte contain also
some iron under the form of chloride. The concentration of
the iron is then advantageously higher than lO grams per liter
and preferably between 20 and 60 grams per liter.
The flow of laminar type or weakly turbulent type
of the electrolyte the length of the cathodic surface is
obtained when the current of the electrolyte circulates around
the cathodes at a speed between O.Ol and 0.15 meters per second.
The removal of the particles gathered at the bottom
of the cell is advantageously done by moving the particles out
of the cell, then by densification of the particles by compression~
,' ~7
In addition, the thickened particles can be subjected to a
lamination (e.g. rolling mill) step, dèsigned to expel the
inclusions of electrolyte. The thickened particles can also
undergo a fusion in the presence of solder.
The invention also concerns an apparatus for preparing
a metal by electrolysis, specifically lead, in which the
cathodes and the anodes are arranged vertically. According
to the invention, the cathodes are formed of a material chosen
from the group which includes titanium, stainless steel and
graphite, and the apparatus includes at least one pump designed
to circulate a current of electrolyte of laminary or weakly
turbulent type the length of the cathodes, and a transport
mechanism designed to withdraw the solid, divided material
which can fall to the bottom of the cell.
In one manner of advantageous application of the
invention, the electrodes are bipolar.
The device includes advantageously a recovery hood
when gaseous chlorine separates from the anodes.
The anodes are advantageously formed of a metal which
cannot be attacked by the electrolyte and in a spread-out form.
The transport device can be advantageously an endless
screw, an elevator of cups, or a transporting roller and
preferably the gooseneck system described below.
The device can also include an extruder designed to
receive the particles extracted and to thicken them.
The invention concerns also a by-product of lead,
prepared by the previously mentioned process and containing
less than 0.2~ of electrolyte in the form of inclusions.
The process and the apparatus according to the
invention have all the advantages of apparatus in which the
metal removes itself automatically from the cathodes. The
main advantage is the almost total elimination of manipulation
~;~3~0~0
of the electrodes. This reduction in manipulation increases
the time of useful service of the electrolysers so much that
the number of electrolysis cells can be reduced, with corresponding
reduction in investment.
In addition, thanks to the very high faradic return
ensured by the invention, higher than 90%, and often 95%, the
energy losses are reduced.
PREFERRED EMB DIMENTS
The different parameters which influence the application
of the process will now be considered in more detail, first
of all in the case where the beginning solution contains iron
in the form of chloride, in addition to lead.
The solution which composes the electrolyte contains
lead chlorides, alkali or earth-alkaline metal chlorides, iron
chlorides and possibly chlorides of other metals, for example
zinc.
The solution of lead chloride is advantageously
formed from a concentrate of sulphurated lead ore which, in
addition to lead, contains small amounts of zinc, copper, iron,
calcium, and magnesium. After purification, for example
according to the techniques described in French patentsno.
2,323,766, 2,359,211 and 2,387,293 and in the European patent
no. 0,024,987, granted 17 November, 19~5, the solution contains
practically nothing but lead and iron, with the other metals
being in negligible quantities.
The amount of lead in the form of chloride, present
in the electrolyte, is preferably higher than 5 grams per
liter and it preferably does not go higher than 50 grams per
liter. These two values are determined according to the
densities of current used in the course of electrolysis and
~23~0~0
the speeds of circulation of the electrolyte in the neighborhood
of the electrodes, so that the faradic return and the production
capacity are optimal.
The electrolyte contains also, in strong concentration,
an alkaline or earthy~alkaline chloride. The most advantageous
is sodium chloride, for reasons of cost and availability.
The quantity of this chloride in solution is advantageously
chosen so that the concentration in chloride ion is higher than
3 gram-equivalents (i.e. in the ease of sodium chloride
approximately 200 grams per liter), preferably between 4 and 5
gram equivalent (i.e. for sodium chloride between 230 and 300
grams per liter). The role of this chloride is to increase the
eoncentration of chloride ions in the eleetrolyte, which permits
the dissolution of the metals whose chlorinated complexes are
soluble, and to reduce the losses because of the Joule effect.
In the method of applieation considered here, the
electrolyte contains also some iron under the form of chloride.
n the absence of iron, the chloride ions oxidize at the anode
into gaseous chlorine with an electrode potential of 1.2 V in
relation to the electrode of saturated mercurous chloride
(calomel) (ECS). The device must inelude therefore a convenient
collecting system. When it is not desirable that chlorine
detach itself, the electrolyte contains advantageously some
iron so that, at the anode, the ferrous ion is oxidized into
ferric ion potential around 0.6 V/ECS. It is thus necessary
that the electrolyte contain some iron in the form of ferrous
iron. Not only does the chlorine no longer disengage at the
anode, but the energy return is clearly increased. In addition,
the ferric chloride formed at the anode is recovered and can
be used again for the treatment of sulphurated lead ores and
the transformation of the galena into elementary sulphur and
into lead chloride.
-- 8 --
~34070
The concentration of iron in the electrolyte, under
the form of chloride, is preferably between 20 and 60 grams per
liter, and advantageously it is on the order of 40 grams per
liter. It is important that this concentration be at least
equal to 20 grams per liter in the anolyte, i.e. at proximity
to the anodes.
Table I which follows shows the main characteristics
of the composition of the electrolyte.
TABLE I
Optimal
Useful region Conditions
. ~
Acidity, pH 1 - 2
Concentrations, g/l
-- sodium chloride230 - 300 250
-- iron (Fe++/Fe+++)
in chloride form 20 - 60 40
-- lead in chloride form 5 - 50 15 - 25
-- 2inc in chloride form 0 - 20
-- Ferrous iron in chloride
form in the anode 20 20 - 30
electrolyte
The nature of the electrodes used and especially of
the cathodes is important for the application of the invention.
One notes in fact that many materials are too "active", i.e.
form sites of nucleation in too great a number. Consequently,
particles of lead begin to form at an excessively large number
of places on the surface of the cathodes and cannot grow
individually. For this reason, it is essential according to
the invention that the density of sites of nucleation, in the
~23~C~O
conditions of electrolysis used, be sufficiently low for the
particles to be able to reach a dimension of at least 100
micrometers without sticking to adjacent particles. Preferably,
the particles keep their individuality until they reach a
dimension of at least 600 micrometers and preferably one
millimeter. In these conditions, the particles individually
have a surface sufficiently large so that, when the electrolyte
moves the length of the surface of the cathode, it exercises
a drawing force which, in combination with the force of weight,
suffices to detach the particles when they have a size of
several hundred micrometers.
This density of sites is important according to the
invention because, if the number of sites is too large, the
particles formed are small and numerous and, when they are
later put into the air, they oxidize easily because they form a
pyrophoric powder. On the other hand, if the density of sites
of nucleations is too low, the production capacity is reduced.
One observes that one obtains a convenient density
of nucleation sites by the use of cathodes whose surface is
formed of smooth titanium. One can also use surfaces of stainless
steel or graphite. Of course, one can also use other materials,
when these have the suitable density of sites of nucleation.
This density can be obtained by a treatment of activation or,
most often, of deactivation according to the known technical
techniques.
The anodes can be formed of graphite. However, as
it is desirable that the transport of the anode electrolyte
be facilitated, it is preferable that the anodes be formed
of an expanded sheet metal, for example, ruthenized titanium.
However, the nature of the anode has much less importance for
the application of the process of the invention than that of
the cathode.
Obtaining high faradic returns requires control of
-- 10 --
~ Z3~ 0
the transport of ferric iron formed at the anode toward the
cathode electrolyte. The choice of a suitable diaphragm,
having a weak permeability, the use of a high density current
and the maintenance of a difference in hydrostatic pressure
between the cathode electrolyte and the anode electrolyte (this
difference in pressure being at least 20 millimeters of liquid
column) permit the avoidance of the passage of Fe III toward
the cathode electrolyte. Thus, in practice all of the delivery
of the anode electrolyte of the cell passes across the diaphragm.
The diaphragm is advantageously formed of chemically inert
textile fibers in the electrolyte. Materials which are suitable
are polyester fibers covered with silicone, TEFLON (trade mark)
covered glass fibers, and preferably synthetic fibers of a base
of polymers containing fluorine.
The phenomena of transport of materials in the course
of electrolysis have an overriding importance on the morphology
of the particles formed and on the faradic return thereby
obtained. It has already been noted that it was advantageous
that the anodes be formed of expanded metal, permitting a
good transport of electrolyte, possibly by promoting turbulence.
However, this phenomenon of accentuation of turbulences is
advantageously used only at the level of the anodes. It is
preferable, for obtaining particles of suitable morphology,
that the electrolyte current the length of the cathodes be
of laminar type or at least weakly turbulent only, while ensuring
a sufficient renewal of the electrolyte at the level of the
cathodes. It is in fact important that the concentration of
lead vary only weakly in the entire electrolyte. Obtaining
a laminary or weakly turbulent flow at the level of the cathode
depends not only on the nature of the surface of the cathodes
but also on the speed of the liquid the length of the cathodes.
It is therefore desirable, according to the invention, that
~23~L~3~)
the linear speed of the cathode electrolyte, parallel to the
cathodes, be at least 0.01 meter per second and preferably
between 0.01 and 0.15 meter per second. At the level of
the anodes, the speed of circulation of the solution which
can be nil, is preferably at least 0.01 meter per second; the
maximum value can be moderated, for example 0.05 meter per
second, given the form of the anodes which favour the creation
of turbulences.
The temperature of the electrolyte is advantageously
put between 70 and 95C, preferably between 70 and 90C. No
heating is necessary because the normal losses by the Joule
effect suffice to maintain the temperature in the mentioned
range.
The application of the process of the invention
permits the use of current densities which are very high. They
can be between 500 and 1,500 A/m . Preferably, they are included
between 700 and 1,000 A/m .
Table II summarizes the various conditions cited
above:
TABLE II
Optimal
Useful range Conditions
. _ _ _ _
Temperature, centigrade 70 - 9570 - 90
Linear speed of circulation
m/s
-- at the anode 0.010.01 - 0.05
-- at the cathode 0.010.01 - 0.15
Density of current A/m 500 - 1500700 - 1000
It is desirable, at the time of the application of
12 -
~3f~0~0
the process according to the invention, that the electrolysis
begin at a weak density of current, lower than the values indicated
above, and the density grows progressively up to the chosen
value, included in the mentioned range. In fact, when the
solution contains ferrous chloride, the iron can deposit itself
at the same time as the lead, on the cathodes, when the density
of the current is initially very high. The metal adheres then
on the entire surface of the cathodes, so that they no longer
possess a density suita~le for sites of nucleation.
When the electrolyte contains practically no iron
chloride, the initial use of a high density of current can
produce the disengagement of hydrogen whose bubbles have a
tendency to stick to metallic particles so that the latter,
instead of falling to the bottom of the electrolysis cell, have
a tendency to float.
The period during which the density of current
increases progressively or by steps, up to the final desired
value, is advantageously several hours.
As indicated above, it has been noted that it was
desirable that the product obtained be under the form of
individual particles having a dimension of several hundreds of
micrometers, for example 300 to 600 micrometers. Their form
can be spreading and relatively flat, but their surface is
relatively weak for their volume. It is this characteristic
which gives t~ the particles formed their non pyrophoric
character.
~ he particles are formed of very pure lead. For
example, met~ ~uch as zinc, copper, cadmium, magnesium, etc-
are present in a quantity lower than 1 ppm in weight. The
amount of iron is lower than several ppm in weight. Therefore,
lead which requires no later refining at all is produced for
most applications. The examples which follow give the purity
of lead obtained in different conditions.
- 13 -
~23~0~)
One important parameter for the application of an
electrolysis is the faradic return obtained since this shows
the importance of electric losses. According to the invention,
this faradic return is at leas~ equal to 90% and it reaches and
exceeds in general 95%. Of course, these returns are obtained
only when the different parameters have the desired values,
corresponding for example to the tables cited previously i.e.
Tables I and II.
The particles of lead which are deposited on the
bottom of the cell are then extracted with the aid of a suitable
device, with reference to an apparatus designed for the
application of the process according to the invention. The
particles of lead, when they are withdrawn, are associated
with the occluded electrolyte, present in an amount between
20 and 30~ in weight, approximately. It is thus desirable that
the material undergo a compacting process or lamination. It
is in particular desirable that the particles be thickened
by extrusion, in a piston or roller press, exercising pressures
higher than approximately 70 MPa. The filtration of the product
is scarcely desirable given that the smallest particles run
the risk of partially oxidizing in the air.
~ hen the particles are removed from the bottom of
the vat, they have an apparent density on the order of 1.5 to
2Ø After extrusion, this density surp~sses 10.5. The lead
by-product thereby obtained, for example in the form of foil,
is stable vis-a-vis oxidation by the air. It can be used "as
is" in certain applications.
In a variant, the lead can undergo fusion in the
presence of solder, according to a well-known technique.
The description which precedes concerns the electrolysis
of a solution containing ferrous chloride. This characteristic
is not indispensible. When the process is applied without
- 14 -
1~340~0
ferrous chloride in the electrolyte, it removes some chlorine
at the level of the anodes. The device used must thus include
a collecting system for chlorine. Such systems are well known
in the electrochemical industry and they are therefore not
described in detail here.
The density of the current can then have an increased
value, between 800 and 2,000 A/m2, preferably between 800 and
1,200 A/m .
The pH of the electrolyte is at an equilibrium value
between 1.2 and 1.7, at a temperature from 70 to 80C. This pH
depends on the concentration of sulphate ions and on the density
of the current. It may be advantageous, however, to work at
a pH ranging from 2 to 3, so as to prevent the reactions of
electrolysis of the proton, in order to produce hydrogen. In
this case, it is therefor necessary to provide a system for the
adjustment of the pH by means of adding a base that is, by
preference, chosen, in such a way as not to add foreign ions
to the electrolytes. One would make use by preference of the
basic compounds of sodium (soda, sodium carbonate, or even basic
compounds of lead, hydroxide of lead, lead monoxide, basic
lead carbonate, etc.).
Besides the different parameters considered above
must have mainly the same values. They are therefore not
described in detail.
Although the process has been described in reference
to the deposit of lead, it is not limited to this single metal.
In fact, the process also permits the formation of particles of
copper, in similar conditions and especially with copper chloride.
Another object of the present invention consists in
providing a device containing numerous pairs of anodes and
cathodes that have been arranged, in such a way that they are
not isopotential. As a matter of fact, the method which has
been explained above, implies the use of numerous pumps for
. ,
123~ [3t70
recycling. Numerous pumps cause investment costs that may be
considerable. That is thereason why we have striven to
perfPct a device for electrolysis that will make it possible to
reduce the number of pumps for the recycling of the electrolytes,
and that, in a general way, will reduce the investment cost
of electrolysis devices. Non-isopotential devices, that are
derived from those that have already been put into operation
for the electro refining of copper in a sulfate medium, may
be used. We may make reference to the "Mining Annual Review
1982", page 282, as well as to the article "Technologically
Advanced Smelter Incorporates Latest Design Concepts", Journal
of Metals, July 1978, pages 16 to 26. The electrolysis tank
mentioned above and commonly called "tank-bath" is equipped
with a series of practically parallel rows, of electrolytic
cells. Each cell consists of a couple formed by an anodic
surface and a cathodic surface. Each anode and each cathode
- except those at the end of each row - belong to two cells.
In one and the same row, all the cells have been mounted parallel
in the form of a comb (or of a rake), i.e. that all the anodes
of one and the same row are of the same voltage, while all the
cathodes of one and the same row are, likewise, of the same
voltage. The rows of said electrolytic tank-bath have been
mounted in electric series. For that reason, there is a voltage
gradient in the tank-bath. But a technoloay of that type
can be used for the electroplating and refining of copper
only because of the very small voltage differential between
anode and cathode, viz. of the order of 0.25 V; and even so,
it is necessary that the distance separating two rows amount
to approximately 0.5 m; if that is not the case, the energy losses
due to the parasitic current and/or leakage from one cell to
another will be considerable.
That is the reason why the devices described in the
articles mentioned above had to be thoroughly modified, in
~23~
order to be adapted to the methods in accordance with the
invention, so as to minimize the leakage currents and the
various energy losses caused by those leakage currents.
According to the invention, it was observed that
when cathodes of two different rows, situated within the same
plane, are connected by insulating partitions of non-conductive
material, and when one forms "anodic channels" by interconnecting
the anodic boxes by means of a non-conductive material, in such
a way that the anode electrolyte is separated from the cathode
electrolyte, it is possible to realize these non-equipotential
means, in such a way that the energy losses will not be too
high as long as the distance separating two rows of electrodes
is comprised between 0.8 and 2 m and, by preference, between
1.0 and 1.5 m. The insulating partitions have, by and large,
the same height as the electrodes.
Conditions such as the ones described in Examples 6
and 7 make it possible to achieve excellent results.
Accordingly, the present invention, which one may call
a "tank-channel" consists of a series of rows of anodes and
cathodes mounted in parallel, wherein the anodes of each row
are staggered, so as to be parallel to one another, by a distance
of 5 to 2 cm in the direction of the decreasing voltages of
the electrolysis tank, while the distance between two rows
ranges from 0.8 to 2.0 m. The cathodes of the various rows
located within the same plane are separated by means of partitions
that consist of insulating material, in order to limit parasitic
currents or leakages. Even though this is less important, the
anodes of the various rows, situated within one and the same
anodlc channel, may be separated by means of partitions consisting
of insulating material, in such a way as to limit parasitic
currents and/or leakages.
The whole forms a juxtaposition of channels that are
parallel to one another and perpendicular to the rows of electrodes.
~3~070
Pump systems similar to the ones that have been
described in the present application, enforce a circulation of
cathode electrolyte and anode electrolyte, whereas each cathode
and each anode are separated by the diaphragm that has been
described above in the description of he method. A device of
this type can be used not only for starting the opration of
the present invention, but also in any device for electrolysis
in which a forced ciruclation of the electrolytes is desired.
The characteristics of a device designed for the
application of the process of the invention will now be considered.
The characteristics of the cathodes and the anodes, which have
already been specified, will not be considered again. The
device can be of monopolar or bipolar type. The bipolar
arrangement has some advantages because it reduces the losses
of energy in the electrodes; it reduces the cost of the electrodes
since they have a double role, and it simplifies the arrangement
of all of the electrodes, while permitting a better energy
return. This advantageous arrangement however poses certain
problems of configuration at the ends of the electrodes,
especially for avoiding stray currents, as it well known to
specialists in this field.
In the drawings which form a part of this application,
Fig. l is a schematic drawing of an apparatus designed
for the application of the process, according to this invention;
Fig. 2 shows schematically an experimental set-up
for an electrolytic laboratory cell for deposition of copper;
Fig. 3 shows the electric connections between the
anodes 7 and the cathodes 8 in schematic form;
Fig. 4 is a graph showing the profile of the mean
density, as obtained by cathodes 8 for test 2;
Figs. 5 and 6 represent profiles of the true density
distribution of the current on the cathodes, with or without
staggering.
- 18 -
~.Z3~07(~
~ igure 1 in the attaehed drawing is a design of
an example of an apparatus designed for the application of the
process according to the invention, and includes an electrolysis
vat 1 containing an anodic box 2. The diaphragm is represented
by 3.
The circuit of cireulation of the eathode eleetrolyte
ineludes a reservoir 4, and a pump 5, for eireulation. The
eathode electrolyte eirculates parallel to the plane of the
cathodes whieh are mounted in the vat 1.
The circuit of the anode eleetrolyte ineludes a
reservoir 6, and a pump 7 r whieh makes the anode electrolyte
circulate.
Pump 8 is designed to extract a part of the anode
eleetrolyte which is concentrated as ferric ehloride and is suited
for the treatment of sulphurated lead ores. The solution of
supply 9, returns to the eathode eleetrolyte the suitable
eomposition in the reservoir 4. The particles which detach
themselves from the cathodes fall to the bottom of the cell and
are taken up by an endless screw 10, mounted on a trunk 11,
drivenin rotation by a motor 12. The particles arriving at the
end of the screw enter a receptacle 13, and are then treated
as described above.
In the apparatus represented schematieally the figure,
the nature of the electrodes and their arrangement are such ~s
described above. The diaphragm and the anodes also have
properties indicated above. When the solution contains no
ferrous chloride, a collecting hood must be mounted above the
anodes so that it gathers the chlorine which is produced.
The adjustment of the overflow makes it possible to
maintain a difference of the level between cathode electrolyte
and anode electrolyte as indicated above. The flows of the
pumps 5 and 7 are regulated, in such a way that the speeds of
the anode electrolyte and the cathode electrolyte, the length
-- 19 --
~340~0
of the anodes and the cathodes, have the values shown befoxe,
i.e., at least equal to 0.01 meter per second. The flow which
crosses the diaphragm is practically equal to the flow of the
supply solution. In this manner, the ferric iron cannot
practically pass into the cathode electrolyte.
The cell has, preferably, a trapezoidal bottom, or
rounded bottom so that the particles which fall may be guided
toward the endless screw.
Although the figure represents an endless screw driven
by a motor, other mechanisms are also suitable. For example,
elevators with cups or movable belts may also be used advantageously.
The product may also pass into an extruder where it is prelim-
inarily thickened, up to a density from 3 to 6. The extruder
may also be equipped with a drawplate long enough to ensure
watertightness.
According to the preferred application of the
invention, the metallic particles formed are recovered with the
help of a gooseneck which is operated intermittently. In
this case, one gives the bottom of the cell a pyramidal type
shape in order to direct the lead particles toward a gooseneck
which mounts vertically the length of the cell. The liquid
level in the gooseneck is in hydrostatic equilibrium with that
of the cell of electrolysis, i.e. the throwing-out point of the
gooseneck is located from 2 to 20 centimeters above the ~evel
of the surface of the cathode electrolyte: the aggregates of
lead accumulate }n the lower part of the gooseneck, thus
constitu~ng a veritable cork. One or severalejector liq~ids, which
may be ~arried out by discharge pipes, are fed intermittently
with cathode electrolyte, without solid matter, at a flow
sufficiently great to create in effect a suction at the bottom
of the cell and to reach a linear speed of flow of the liquid
in the gooseneck of at least 0.5 meter per second. The lead is
led away and recovered after separation of the liquid in an
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~3~0~0
appropriate system which is disconnected hydraulically from
the cell of electrolysis.
One may also carry away the agglomerates of lead by
air-lift. The ejector(s) are arranged under the gooseneck at
appropriate places known to those s~illed in this field, to
obtain a good effect of suction or air-lift.
The following examples of carrying out the present
invention, though not exhaustive, have the purpose of letting
specialists determine easily the operating conditions which are
suitable to use in each specific case.
Example 1
One treats with a solution of ferric chloride and of
sodium chloride, a sulphurated raw material composed of a
concentrate of galena, containing 75.5% lead, 0.70% zinc,
0.85~ copper, 1.40~ iron, 1.0% calcium, and 0.6% magnesium.
After purification, the feeding solution of the
electrolyser and the electrolyte have the following compositions:
-
Elements Na Pb Fe Cu Ag Ni
g/l g/l g/l mg/l mg/l mg/l
Feeding
Solution 100 47 35 0.2 0.1 0.5
Electrolyte 100 20 35 0.2 0.1 0.5
Electrolysis is carried out in an installation of thetype shown in Figure l; the speed of circulation of the cathode
electrolyte is 0.06 meter per second and that of the anode
electrolyte 0.01 meter per second. The cathodes are formed of
smooth titanium. The density of current, as a rule is 550A/m2.
The distance separating the electrodes is 70 millimeters.
- 21 -
1~3~0~7(~
One observes that the lead obtained is in the form
of particles having a length on the order of 300 to 600
micrometers and does not adhere to the cathodes. The faradic
yield observed is 95%, and the energy yield is 0.57 kWh per
kilo of lead.
The purity of the lead o~tained in
a simple lamination forming step and melted and formed into an
ingot is as follows:
Elements (ppm) Na Fe Cl- Cu Ag Ni Cd As Sn Bi
.
~aminated 120 159 2000 2 2 7.0
Ingot 10 5 30 2 5 7.0 5 15 20 5
Example 2
One uses the same installation and an electrolyte of
the same composition as in example 1. The speed of circulation
of the cathode electrolyte is 0.10 meter per second and that of
the anode electrolyte 0 02 meter per second. The density of
current used is 850 A/m and the distance between the electrodes
is the same as in example 1.
The lead produced is similar to that described in
example 1. The energy return of the electrolysis is 0.74 kWh
per kilo.
Example 3
One uses an installation similar to that of example 1.
The cathodes are formed of smooth titanium and the anodes of
spread out titanium covered with ruthenium oxide. The distance
which separates them is equal to 70 millimeters. The anodes
are arranged in an anodic box in which the anode electrolyte
does not circulate. The difference in pressure between the
anode electrolyte and the cathode electrolyte is 20 millimeters
~3~ao~ [)
of a column of water. The installation is designed to permit
the recovery of chlorine.
In this e~ample, the density in lead of the electrolyte
is maintained by the continuous introduction of crystallized
lead chloride. The crystals contain the following impurities,
expressed in grams per ton:
Fe: 15 Ni: 1 Mg: 4.4
Na: 10-50 Zn: 4.0 Ca: 5.0
Cu: 2.5 Cd: 5
Ag: 1 S: 40
The conditions of electrolysis are the following:
density of current: 1,000 A/m
temperature: 75C
linear speed of the cathode electrolyte: 0.04 meter per second.
The energy return of the electrolysis is 1 kWh per
kilo of lead. The lead particles form a power of apparent
density between 1.5 and 2.5 and contain 20 to 30% in weight of
occluded electrolyte. After thickening in the laminator, this
electrolyte is extracted from the powder.
The table which follows shows not only the composition
of the electrolyte but also the purity of the products obtained,
on the one hand after laminating, and on the other hand after
the formati~n of an ingot.
Electrolyte Pb laminated Ingot of Pb
mg/l g/t g/t
_ _ _ . . _
Na 98,000 100 30
so4 10 - _
Mg 14 .1
Ca 40
Cd 0.5 2 2
Cu 0.7
zn 8 2 2
Fe 16 3 3 - 10
Ag 0.5
Ni 0.7 1 1 - 3
- 23 -
~23~
Example 4
The operating conditions are identical to those of
example 3, but the electrolyte contains 10 grams per liter of
sulphate. At this concentration, the electrolysis is not
disturbed by the sulphate ions and the energy return remains
mainly equal to 1 kWh per kilo of deposited lead.
The lead particles obtained have the same purity and
the same rate of occluded electrolyte as in example 3.
Example 5
One uses operating conditions identical to those of
example 4, but one carries the density of current to 1,500 A/m2.
The energy return reaches 1.24 kWh per kilo of lead
deposited. The purity of lead particles obtained and the
characteristics before thickening remain the same as in the
preceding example.
Example 6
Electrodes of the same type of electric series are
mounted in the same tank to prepare an electrolytic laboratory
cell that is 2 m long, 0.15 m high, and 0.03 m wide. The amount
of the leakage currents between two cells has been evaluated.
Each cell consists of one anode and one cathode. Electrolysis
of copper sulfate has been chosen, so as to facilitate the
measures that essentially pertain to the evolution of leakage
cu:crents, and to the distribution of the density of the current
on the surface of the cathodes.
As a matter of fact, in a sulfate medium, the deposits
of copper are compact and the faradic yield of the deposits
is very close to the unit within a range of density of the
current ranging from 200 to 300 amp per m2.
~ nder those conditions, it is possible, by cutting
the deposit again into bands of equal length, to determine, on
the basis of the weight of each band, the average current
- 24 -
~3~'76~
density of electrolysis on each surface element, and to find,
in that way, the profile of distribution of the average current
density over the surface of the cathode.
Figure 2 shows the experimental set-up used. The
solution of copper sulfateis kept in circulation between the
supply tank 4, the means of heating 5, and the electrolysis tank
of the canal type 1 by means of the centrifugal pump 6.
Each cell 2 consists of a lead anode and of a stainless
steel cathode, which are mounted at a distance of 1.6 cm. In
the tank 1, one, two, or three cells 2 may be mounted in electric
series, and the distance L from one cell to the other may vary.
The schematic drawing of Fig. 3 shows mainly the
electric connections between the anodes 7 and the cathodes 8.
Each cell 2 is connected externally by means of a conductor 9.
There exists between each cell 2 a leakage current 1 F that
reduces the overall energy yield of the electrolyser, and that
disturbs the distribution of the current density on the edges
of the electrodes, mainly between the anode of one cell and the
cathode of the adjoining cell.
The following Table shows the most important results
achieved by an electrolyte that contains 40 g of copper per
liter and 165 g of sulfuric acid per liter. All the tests have
been conducted at a temperature of 40C over periods of 15 to
20 hours.
Test Distance Between Intensity Amount of Leakage
Cell L (M) (A) Electricity (Cb) Current lF
1 0.25 1 54,374 0.66
2 0.50 1 23,190 0.38
3 0.75 1 58,951 0.32
4 0.55 1 60,543 0.19
0.55 1.5 29,980 0.22
On a small scale, the leakage current represent a
- 25 -
~3~
relatively important value in relation to the intensity of the
current supplied by the rectifier 3. Relative importance of the
leakage current will be reduced considerably on a larger scale.
The following graph (Figure 4) presents, by way of an example,
the profile of the mean density as obtained on the cathodes 8
for test 2.
Example 7
_ _ _
The excessive density of the current on the edges of
the cathodes is not acceptable because of the increase in the
local excess voltage of the electrode that carries the risk of
causing the appearance of parasitic reactions.
This major disadvantage has been alleviated by means
of staggering the vertical axes of the anodes and of the cathodes
of each cell, and by the use of a current strength taking into
account the density of the current chosen and of the electrode
surfaces opposite. The aim is the capability of ensuring a
true current density on the surface of the cathodes that is
lower than the density of the current chosen. This type of
assembly has been tested with the experimental set-up as
described above.
In the following Table, we have shown the comparative
results of two electric assemblies consisting of three cells
placed at distances of 0.63 m from one another; one assembly
comprises the staggered electrodes, and the other assembly
shows the electrodes in each cell non-staggered.
The following graphs 5, 6 represent the profiles of
the true density distribution of the current on the cathodes with
or without staggering.
No. of Space between Electrode Staggering Int. Leak
Testcell 2 cells surface amp
L (M) DM2(MM) (A) lFl lF2
6 3 0.63 0.72 01.44 0.49 0.48
7 3 0.63 0.54 251.08 0.49 0.46
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