Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
L;~Y~'7~
~ he invention relates to a coated metal anode
for the electro]y-tic recovery of metals, the working
surface of which is represented by rods which are
arranged in a plane in spaced, parallel relationship to
each other, and which are electrically connected to a
current supply rail.
In the field of the electrolytic recovery of
metals, especially non-ferric metals, from acidic solu-
tions containing the metal to be recovered, anodes of
iron or iron alloy should be replaced by other metal
anodes. The core of each of these metal anodes consists of a
valve metal, such as e~g. titanium, whereas the coating
is formed of e.g. platinum or platinum oxide. The
essential advantage of the metal anodes in question is
to be seen in the saving of electric energy as compared
to the conventional iron or graphite anodes. The saving
of energy results from the larger surface obtainable
with metal anodes, the high activity of the coating and
the stability of form, which permit a considerable re-
duction in anode voltage. A further operational economyis achieved with metal anodes in that the cleaning and
neutralization of the electrolyte is simplified, as the
coating of the metal anodes Cl, NO3 or free H2SO4 is
not destroyed. An additional economy in costs results
from the fact that with the use of metal anodes, it is
not necessary to add expensive additives, e.g. cobalt,
to the electrolyte, as is necessary with the use of iron
anodes. Furthermore, the contamination of the electro-
lyte and the recovered metal by iron, which is unavoid-
able in the case of iron anodes, is no longer applicable.
Finally, the metal anodes permit an increase in current
density and thus in productivity.
In a known metal anode of the given type (German
application DE-OS 24 04 167 filed January 29, 1974, pub-
35 lished August 1, 1974 in the name of Diamond Shamrock
~A~
~ ... :i
2-
Technologies, Zurich), the working surEace is repre-
sented by vertically arranged rods which are spaced from
each other in the ver-tical plane and are in parallel
relationship to each other. The most essent:ial feature
of this known anode is that the anode surface opposing
the cathode is 1.5 to 20 times smaller than the opposing
cathode surface and the anode is operated with a current
density which is 1.5 to 20 times greater than the cathode
current density. Thus, supposedly, in an econo~ical
manner, a relatively pure metal deposit of the desired
crystalline structure and purity should be obtained on
the cathode. The economy with which the known anode
should operate should evidently be seen in the fact that
on account of the reduced area of the anode as compared
to the cathode, the requirement of material for the pro-
duction of the anode is reduced and thus expensive
material saved. The reduction in costs is paid for
dearly, however, by considerable disadvantages during
production, as follows:
The considerably reduced surface of the known
anode as compared to the cathode, and the resultant neces-
sity to work with high current densities, cause the course
and uniformity of the current paths in the cell to be
very difficult to control. However, a non-uniform dis
tribution and a course of current paths which is not
precisely foreseeable result in a non-uniform deposit of
the metal on the cathode.
Since the known anode works with a high current
density, the anodic portion of the cell voltage is high.
This results in the substantial disadvantage of a high
energy requirement for the cells equipped with such an
anode.
The large current density and the reduced con-
ductor cross-section of the known anode on account of the
reduced surface and thus of the small volume cause a
--3--
large inner IR drop, which results in a Eurther increase
of -the necessary electric ener~y. In order to elimin-
ate this disadvantage, the known anodes require a plur-
ality of current supply rai]s of complicated construction,
which considerably increase the construction costs.
In the known anodes, the rods are formed with a
xound profile, i.e. they have a circular cross-section.
~ Thus, a considerable portion of the surface of the rods
-~ ` which bear the active coating liesin the current shadow
region of the cathode. This portion of the surface of the
rods contributes vexy little to the working surface of
the anode. Thus, the known anode has only a low effici-
ency due to the reduced utili~ation of the coating, i.e.
the working surface of the anode.
Furthermore, the known anode has the disadvantage
that the sensitive and expensive coating on the round
rods is relatively freely accessible with the consequence
that the coating can easily be destroyed mechanically,
e.g. when the anode or cathode is being built in or out.
During the assembly and disassembly of the known
anodes, also the necessary current supply rails, which
extend partly parallel to and partly perpendicular to the
rods, have a disadvantageous effect, as they increase the
width of the construction so that the danger of damage
of both the anode and also the cathode when withdrawing
e.g. the anode from the cell is increased.
Furthermore, in the case of the known anodes, no
sufficient measures have been taken to form a rigid rod
construction so that there is the possibility that the
rods will arch out beyond the plane of arrangement re-
sulting in a contact with the cathode and thus a short-
circuit.
As compared thereto, it is the object of the
invention to provide a coated metal anode of the above-
given type, which effects a compromise meeting all the
demands between a material-saving cons-truction on the
one hand and an operation with acceptable current den-
si-ty on the other hand, and which permits, with a simple,
constructive assembly, an economic deposit of metal with
high purity on the oppositely disposed cathode.
This object is solved in the case of a coated
metal anode of the above-described type in that the total
surface area of all the rods FA and the surface area F
assumed by the entire arrangement of all the rods fulfills
the relationship 6 ~ FA / Fp > 2.
The solution according to the invention is pro-
vided by an anode which, on the one hand, offers a large
working surface and nevertheless, on the other hand, can
be produced with the smallest possible requirement of
material. The large working surface permits an operation
of the anode according to the invention with relatively
small current densities, even upon large application of
voltage. This guarantees an economic deposit of
the desired metal with great purity on the cathode. The
saving in energy is achieved primarily by the reduction
of the anodic portion of the cell voltage obtained as
compared to the known solutions.
The large surface of the anode according to the
invention leads also to a large conductor total cross-
section of the rods and thus causes only a relativelysmall inner voltage drop of the electrical current when
flowing through the rods from the current supply rail to
the ends of the rods remote therefrom. For this reason,
besides the main current supply rail, no further current
supply rails are necessary, so that the anode construction
according to the invention is not only relatively small,
but also material, and thus production costs are saved.
In the anode according to the invention, the rods
are arranged vertically in the usual manner. The surface
assumed by the rods corresponds approximately with the
'~
.~#~
surface of the cathode facing the anode. Precise]y on
account oE the last-mentioned measure, a uniform, easlly
controllable distribution of the current paths between
anode and cathode results.
An especially advantageous configuration of the
anQde ~ac~ording to the invention is to be seen in the
~c~-~t the'rods have an essentially rectangular cross-
section and are arranged in such a manner that the larger
~ e~ ~ ~ the cross-section of the rods extends perpen-
dicular to the arrangement plane assumed by the rods.
Due to this measure, a large portion of the work-
ing surface oE the anode, i.e. the rods or their coating,
respectively, lies perpendicular to the arrangement plane
of the rods of the anode, or the surface of the cathode
facing the anode, respectively. This results in a series
of advantages. One advantage is that the portion of the
working surface which lies in the current shadow reyion
of the cathode is relatively small. Thus, already geo-
metrically a large effective surface results. This
causes an optimal utilization of the coatiny, and thus a
very large physical surface of the anode of the invention.
Moreover, a large portion of the coating, namely that on
the surfaces of the rods perpendicular to the arrangement
plane of the anode, is protected from a mechanical de-
struction, so that not only can the anode according tothe invention be built in and out without any problems,
but also the cathode can be withdrawn from and re~inserted
into the cell without difficulty. The anode structure
according to the invention also reduces the danger of
short-circuits and mechanical destructions due to a for-
mation of dendrites on the cathode surface.
It has proven to be advantageous to provide a
ratio of the short side to the long side of the rectangular
cross-section of the rods which amounts to 1:2 to 1:10.
In this respect, it is especially advantageous to provide a width B
~ ~ ~ 7~
--6--
of t.he rods measu.red parallel to the arrangement p]ane
which amounts to about 0.5 mm to about 2.5 mm. This
measure contributes no-t only to the desired state of
the surface ratio, but also permits the use of convention-
al profiles for the rods and thus a construction favour~able to costs and further, a practical production of
the anode.
It is also advantageous with respect to the named
aspects that the depth T of the rods measured perpendi-
cular to the anode plane ~ amounts to about 5 mm to25 mm.
It has also proven to be expedient tha-t the ratio
of the width of one of each rods to the distance of
two adjacent rods is 1:4 to 1:6. In this respect, it is
especially recommendable if the clear distance A between
two adjacent rods amounts to A 7 2 mm. This construction
of the anode of the invention permits a good circulation
of the electroly-te between the rods.
An especially advantageous constructive configur-
ation of the anode of -the invention is to be seen in the
fact that the rods lie in a plane with the current supply
rail, connect with their one face end to the current sup-
ply rail, and both the electrical and mechanical con-
nection of the rods with the current supply rail takes
place via at least one connecting strip extending parallel
to the latter, the one marginal region of which is con-
nected with the current supply rail and the other marginal
region of which is connected with the rods. This solution
ensures with a large geometrical surface not only a large
mechanical strength of the anode of the invention, but
simultaneously a configuration of the electrical connection
of the component parts of the anode of the invention in
such a manner that at the contact ~ones the current den-
sity or current load always assumes acceptable values and
thus the drop of voltage in the contact zones is slight,
~7--
even in -the event of long operating t.imes. The attained
mechanical strength of the anode of the invention
simplifies not only the installa-tion and removal of same,
but also reduces the danger of short circuits due to an
arching out of the anode struc-ture with -the consequence
of a contact with the ca-thode.
An especially preferred embodiment of this solution
is that on both sides of the current supply rail or the
rods, respectively, one connecting strip respectively is
arranged. By means of this measure, the current density
in the contact zones between the individual component
parts of the anode according to the invention can be kept
especially low.
It is expedient if the connecting strips are
secured to the current supply rails by screw connecti.ons.
This provides an especially simple exchangeability of the
anode arrangement of the invention. It is further pos-
sible with this measure to introduce the anode according
to the invention instead of e.g. an iron anode using the
same current supply rail in a cell already provided.
Accordingly, it is especially economical and simple to
exchange the conventional iron anodes with a coated metal
anode according to the invention.
According to the invention, the screw connection
is constructed such that the contact area between the
connecting strip or strips and the current supply rail is
selected to be so large that the reduction of the contact
area caused by-the bores of the screw connection have no
substantial effect on the current density or current l.oad
in the contact area.
Whereas expediently the connecting strips are
screwed together with the current supply rail, it is
advantageous that the rods are connected to the connecting
strips by means of spot-welding. This permits an especially
economical connection of the rods to the connecting strips.
7~
~8
In order to increase the mechanical strength of
the anode structure of the inven-tion, it is moreover
advantageous that the rods are connected with each other
by a plurallty of crossbars. This especially applies if
the successive crossbars are arranged alternatively on
the one and on the other side of the rods. In -this
respect, it is expedient that the crossbar be secured to
the rods by means of spot-welding. The crossbars should
be integrated extensively into the rod structure so that
no projecting edges are formed which would cause an
especially rapid formation of dendrite by the ca-thode.
Functionally, this aim can be achieved especially simply
if the crossbars are ~lattened on their outwardly lying
surfaces so that these surfaces either do not arch beyond
the outline of the anode rods, or only to an insignificant
extent.
Advantageously, the core of the rods is formed of
valve metal, especially titanium, whereas the coating is
formed of platinum metal and/or platinum metal oxide and/or
an electrically conductive, non-stoichiometric oxide and/
or a base metal and/or its oxide and/or mixtures of the
above substances.
An embodiment of the coated metal anode according
to the invention is explained in more detail on the basis
of the enclosed drawings. These show:
Fig. 1 a plan view of the arrangement plane of
the anode according to the invention,
Fig. 2 an end view of the arrangement according
to the invention,
Fig. 3 an enlarged representation of the detail
III of Fig. 2, and
Fig. 4 an enlarged representation of the detail
IV of Fig. lo
As shown by Fig. 1, a cell tank represented only
schematically is indicated with 1. On bearing blocks 2
_9_
at the opening edge of the cel] tank 1, a current supply
rail 3 is positioned, which is connected v.ia a contact
rail 5 to the source of current. The current rail 3
bears a series of rods 4, which represents the working
surface of -the anode. The rods with the length LS have
a rectangular cross-section with the width B and the depth
T. In this respect, the rods are orientated such that
their depth T extends perpendicular to -the arrangement
plane of Fig. 1. The surface area sssumed by the arrange-
ment of the rods is defined by the length of the rods LSand by the distance LG of the outer sides of the two outer
rods of the anode structure. The rods 4 are arranged with
a clear distance A to each other.
The electrical and mechanical connection of the
current supply rail, comprised e.g. of copper, with the
rods 4/ comprised e.g. of coated titanium, is best shown
by Fig. 3. According to same, the current supply rails
3 and the rods 4 are arranged in a plane such that the
upper end faces of the rods 4 border against the lower
surface of the current supply rail 3. The connection of
the current supply rail 3 with the rods 4 takes place via
two connecting strips 6 arranged on both sides of the
current supply rail and parallel thereto, whereby said
strips 6 can also be of coated titanium. The connecting
strips 6 are secured by means of screws 7a and nuts 7b
to the current supply rail 3. The connection of the rods
4 with the connecting strips 6 takes place by welding
spots 8. For the further stiffening of the anode structure,
a plurality of crossbars 9, which are also of coated
titanium, are connected to the rods 4 by spot welding.
In this respect, the successive crossbars 9 are arranged
alternatively on the one or the other side oE the arrange-
ment plane of the rods 4.
With the described construction, the rods 4 have a
35 length LS of 1170 mm, whereas their width B is 2 mm and
.
-10~
their depth T l2 mm. The clear dis-tance A between two
adjacent rods 4 is 8 mm. The entire length LG of the
anode structure is 852 mm. 82 rods are provided.
The described anode is designed for a current of
600 A corresponding with an anode-side current density
of 355 A/m (Fp). With a current of 600 A, merely an IR
drop of about 100 rnV occurs in the anode.
The anode construction is stiff and robust. This
results not only from the described connection of the
rods 4 with the current supply rail 3 by means of the
connecting strips 6 and from the spot welding of the
rods with these connecting strips 6, but also from the
additional arrangement of the crossbars 9, which have
a diameter of 4 mrn in the embodiment. In this manner,
each lamella-like rod 4 is held by seven welding spots.
The anode is simple in construction, relatively
inexpensive to produce on account of the smallest pos-
sible amount of material, and has a very large geometrical
surfaee. Without the current supply rail 3, it weighs
about 12 kg. The total surface of the rods FA, to which
the coating is applied, is about 3 m , inclusive of the
contaets. The working surface of the anode, i.e. that
which immerses in the electrolyte, is about 2.4 m2,
which provides at 600 A a DA value (anodic current density)
of about 250 A/m (FA~. The actual physical anode current
density which results from the extremely large BET surfaee
of the eoating amounts to only a few 5%o of the DA value.
Therefrom, and from the catalytic effectivity of the
active components of the eoating, a constant, low oxygen
voltage results at the anode according to the invention
for a long period of operation.
The coating of the anode surface which projects
from the bath serves for the protection against corrosion
of the component parts of the anode consisting of titanium.
3S The relatively small current load of the current
`.~
3~3
supply rail 3 consistincJ of copper of abou-t 0.8 A/mm
with a current of 600 A at the anode permits the pro-
vision of nine bores 3a in the current supply rail 3
over a length LG of 852 mm. Each bore 6a in the con-
necting strip 6 has an individual current of about 33 A.On account of this small individual current in the con-
tact zones and the good contact coating, the voltage
drop in these regions remains constant for long periods
of operation.
-; ; s:, .
