Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ELECTRODE FOR MEMBRANE ELECTROLYSIS CELLS
BACKGROUND
The invention relates to an electrode for electrochemical applications, in
particular to
an electrode for membrane electrolysis cells made on a metal support.
Electrolytic processes carried out in cells separated by ion-exchange
membranes are
among the most relevant industrial electrochemical applications. Some examples
of
such applications are the electrolysis of alkali chloride brines (chlor-alkali
electrolysis), with particular reference to the electrolysis of sodium
chloride brine for
the production of chlorine and caustic soda, and the electrolysis of
hydrochloric acid
solutions.
In the following description, reference will be made to sodium chloride
electrolysis as
the most representative example in terms of overall production, but the
present
invention shall not be understood as limited to such application.
In membrane chlor-alkali electrolysis, the anodic compartment of the
electrolysis cell
is separated from the cathodic compartment by means of an ion-exchange
membrane. The anodic compartment of the cell is fed with a sodium chloride
brine,
for instance at a concentration of about 300 g/l; chlorine evolution takes
place on the
anode surface, at a current density usually not above 4 kA/mZ, while brine is
consequently depleted down to an outlet concentration usually comprised
between
200 and 220 g/l. Sodium ions are transported by the electric field across the
membrane to the cathodic compartment, where the caustic product is generated
at a
concentration usually not higher than 33% by weight. The caustic product is
then
extracted and concentrated by evaporation outside the cell. Hydrogen evolution
also
takes place on the cathode surface. The need of decreasing the capital
investment
has led to the design of plants operating at higher current density: in fact,
while older
plants usually work at 3 kA/m2, those of newer construction operate at about 5
kA/m2.
The current trend in plant design is to further increase such values up to 6
kA/mz or
more. The evolution of gas in form of bubbles, whose flow-rate increases at
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increasing current densities, may cause pressure fluctuations potentially
dangerous
for the mechanical integrity of the membrane: for this reason, the pressure
differential
across the two compartments is usually controlled in an accurate fashion and
maintained below 3000 Pa, which complicates the cell operation. Moreover, the
product gas has the tendency to build-up between the membrane and the
electrode
surfaces facing the same, increasing the ohmic drop in the contact zone and
locally
depleting the chloride-ion concentration due to poor electrolyte renewal.
Brine dilution
favours the local evolution of oxygen with consequent acidification. The
combination
of these different aspects (chlorine build-up, oxygen build-up, depletion of
trapped
brine, acidification) accounts for the early deterioration of the membranes,
particularly
in form of blister generation especially in correspondence of interstitial
zones
between anode and membrane, leading to voltage increase and electrolysis
efficiency decrease. A similar deterioration may also take place in the
interstitial
zones between membrane and cathode: in this case, liquid stagnation leads to
an
increase in the caustic product concentration, which may reach a value up to
40-
45%. Such a high alkalinity can damage the membrane chemical structure, with
consequent voltage increase going along with the onset of localised
blistering, as
described for the anode side.
A few measures have been proposed to improve brine circulation near the
electrode
surface in order to mitigate the problems associated with gas bubble
stagnation: US
4,608,144 disclosed an anode surface equipped with vertical parallel channels
alternatively directed to brine feed and withdrawal, and further equipped with
horizontal channels of lower section reciprocally connecting the feed and
withdrawal
channels. In this way a forced brine circulation is achieved, somehow
preventing the
adhesion of chlorine bubbles. US 5,114,547 discloses an anode aimed at
promoting
brine circulation at the membrane-anode interface in order to obviate the
increase in
the electrical resistance associated with the depletion of stagnating brine at
the
interface by means of a structure consisting of vertical channels connected
with
slanted secondary channels disposed in a herringbone pattern. US 2006/0042935
addresses the same problem by providing an irregular anode surface obtained by
sandblasting or acid etching in order to improve the brine supply to the
anode.
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While all of the proposed measures might contribute to some extent to prevent
deterioration of ion-exchange membranes in the usual process conditions, they
fail to
guarantee an optimal functioning in the exasperated process conditions needed
to
meet the current market requirements aimed at a higher cell productivity.
It would therefore be desirable to have an electrode for membrane electrolytic
cells
overcoming the limitations of the prior art, particularly as regards the
possibility to
operate a membrane electrolysis cell with higher performances in terms of
parameters such as membrane lifetime, higher applicable current density,
operative
voltage, concentration of the caustic product obtained in the cell, degree of
brine
utilisation or maximum applicable pressure differential.
SUMMARY
Various aspects of the invention are set out in the accompanying claims.
One embodiment provides an electrode obtained on a metal substrate having a
multiplicity of locally parallel grooves with a depth of 0.005 to 0.02 mm and
a pitch -
defined as the distance between adjacent grooves - of 0.01 to 0.5 mm.
By locally parallel grooves it is hereby intended a multiplicity of grooves,
of open or
closed shape, running in parallel at least in part of their length; the path
of the locally
parallel grooves may assume a generally parallel trend across the whole
electrode
structure, in straight lines or with curvatures of any type. In one
embodiment, the
electrode surface presents locally parallel grooves having a closed shape and
intersecting one another reciprocally.
The electrode as hereinbefore defined can be advantageous in any electrolytic
application, especially for working in direct contact with an ion-exchange
membrane;
in the case of chlor-alkali electrolysis, the above electrode can be assembled
with its
grooved surface in direct contact with the membrane, with surprisingly
advantageous
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results both used as the anode and/or as the cathode. The metal substrate may
be
made of different materials, including but not limited to titanium and
titanium alloys for
anode application and nickel, nickel alloys and stainless steels for cathode
application. The substrate geometry can be of any type: as a non limiting
example,
the grooved surface can be provided on punched or expanded sheets, meshes and
structures comprised of parallel strips optionally rotated along the
horizontal axis,
also called louvered electrodes.
The electrode substrate can be provided with a known catalytic coating on its
grooved surface: for instance, when use as anode for chlorine evolution in
chlor-alkali
cells is intended, the electrode substrate may be provided with a coating
based on
noble metals or oxides thereof. Electrodes obtained on the substrate as
hereinbefore
defined can be particularly useful in chlor-alkali electrolysis cells, both as
anodes for
chlorine evolution and as cathodes for hydrogen evolution, especially when
assembled with the grooved surface in direct contact with the membrane. In
case of
straight grooves running parallel across the whole structure, orienting the
grooves in
the vertical direction can provide an improved circulation of electrolyte and
gas-
bubble release from the surface. In the case of cells assembled according to
the
configuration known in the art as zero-gap, wherein both electrodes are in
direct
contact with the membrane, the inventors observed that manufacturing both the
anode and the cathode on grooved substrates as defined made possible to
operate
at current densities largely exceeding 6 kA/m2, up to 10 kA/m2, with totally
acceptable
cell voltages. Life-tests were also carried out with excellent results at
anolyte
concentrations below 200 g/I (in particular down to 150 g/1), with caustic
product
concentrations above 33% (in particular up to 37%) and maintaining pressure
differentials across the two compartments higher than 3000 Pa (in particular
up to
10000 Pa), conditions which normally led to a quick deterioration of the
membranes
when prior art electrodes were employed.
Without wishing to be limited by any particular theory, it might be supposed
that the
electrode obtained on a grooved substrate as defined allows a particularly
efficient
release of the gas bubbles, also in comparison with grooved electrodes of the
prior
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art, possibly because the densely packed and shallow grooves favour capillary
transport phenomena as opposed to an electrolyte circulation.
The electrode as defined can be obtained by simple and cheap methods such as a
5 superficial erosion carried out by means of abrasive paper or fabric -
optionally in a
continuous rolling process - lamellar grinding wheels or grindstones; other
techniques include the use of draw-benches or rolling mills, besides more
sophisticated technologies such as laser etching or lithographic techniques,
according to the selected geometry. The erosion by grindstone for instance can
be
suitable for obtaining locally parallel grooves of closed shape and
intersecting one
another, while a lamellar grinding wheel, a draw-bench or a rolling mill can
be more
suitable for obtaining generally parallel grooves along the whole surface.
An electrode obtained with the above mentioned techniques can allow a sensible
cost reduction compared to other grooved electrodes known in the art and
characterised by a much higher groove depth, which cannot be obtained by
simple
abrasion.
EXAMPLE 1
Six 1 mm thick and 600 mm x 800 mm wide sheets of titanium grade 1 were
degreased and subjected to an erosion treatment with a lamellar grinding
wheel,
obtaining grooves of 0.2 mm pitch on all samples at various depths; the sheets
were
expanded according to a known technique, obtaining a rhomboidal-mesh geometry
of
10 mm x 5 mm diagonals and 1.6 mm displacement step. Upon completion of the
expansion procedure, the grooves measured with a profilometer displayed
average
depths as reported in table 1:
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TABLE I
Sample ID Groove depth (mm)
Al 0.003
A2 0.006
A3 0.01
A4 0.02
A5 0.05
A6 0.2
Similarly, three 1 mm thick and 600 mm x 800 mm wide sheets of nickel were
degreased and subjected to the same erosion treatment and subsequent
expansion,
so as to obtain an identical geometry. Upon completion of the expansion
procedure,
the grooves measured with a profilometer displayed average depths as reported
in
table 2:
TABLE 2
Sample ID Groove depth (mm)
C1 0.002
C2 0.01
C3 0.05
One sheet of titanium and one of nickel, having the same size as the previous
samples, identified as A0 and CO respectively, were subjected to the same
expansion treatment as the above samples, after sandblasting with corundum and
subsequent etching in HCI as known in the art; no additional abrasive
treatment was
effected on these samples.
All titanium samples were subsequently coated with a ruthenium and titanium
oxide-
based catalyst for anodic evolution of chlorine, with an overall catalyst
loading of 12
g/m2. A new check of the groove depth did not show any significant variation
introduced by the coating step.
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EXAMPLE 2
All samples prepared in the previous example were cut into 150 mm x 200 mm
wide
pieces and characterised, coupled in various combinations, in a multiple bench
for
chlor-alkali electrolysis accelerated lifetime tests. Each station of the
multiple bench
was equipped with one membrane electrolysis cell suitable for accommodating
one
anode and one cathode of 1 mm thickness in direct contact with a reference
sulphonic/carboxylic double layer membrane (Nafion 982 produced by DuPont,
U.S.A.). The electrode samples of tables 1 and 2 were assembled with
vertically
oriented grooves. The lifetime test was carried out simultaneously starting-up
all cells
with the various combinations of anodes and cathodes at process conditions
much
more severe than the common industrial practice, determining the time of ion-
exchange membrane decay, defined as the time required for the cell voltage to
increase by 0.5 V with respect to the initial value at the process current
density.
Process conditions were set as follows:
- brine concentration at the anodic compartment outlet: 150 g/l
- concentration by weight of product caustic soda: 37%
- pressure differential across the two compartments: 5000 Pa
- current density: 12 kA/m2
The results obtained are reported in table 3:
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TABLE 3
Test number Anode Cathode Duration (h)
1 AO CO 514
2 AO CO 562
3 AO C2 580
4 AO C3 565
Al CO 729
6 A2 CO 904
7 A3 CO 1213
8 A4 CO 1417
9 A5 CO 866
A6 CO 578
11 A2 C1 940
12 A3 C1 1283
13 A4 C1 1646
14 A5 C1 1108
Al C2 887
16 A2 C2 959
17 A3 C2 1682
18 A4 C2 1704
19 A5 C2 1011
A6 C2 622
21 A3 C3 1088
22 A4 C3 1544
23 A3 C1 1305
24 A4 C1 1593
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EXAMPLE 3
An electrolysis cell as in example 2, equipped with an anode sample A4 and a
cathode sample C2, and a second analogous electrolysis cell equipped with a
non-
grooved anode sample A0 and a non-grooved cathode sample CO were subjected to
a lifetime test at process conditions sensibly more severe than the common
industrial
practice.
Process conditions were set as follows:
- brine concentration at the anodic compartment outlet: 180 g/l
- concentration by weight of product caustic soda: 35%
- pressure differential across the two compartments: 4000 Pa
- current density: 10 kA/m2
After about 900 hours of testing, the cell equipped with electrode samples AO
and CO
had to be shut down because the progressive deterioration of the membrane had
caused a strong increase in the cell voltage, which attained high values
strongly
fluctuating in time. The cell disassembly evidenced a general formation of
blisters on
the surface, with a higher population in correspondence of the brine exhaust
outlet
nozzle, where an incipient local delamination of the two layers of the
membrane
could also be observed.
The cell equipped with anode A4 and cathode C2 was dismantled after 2400 hours
of
continuous testing at practically constant voltage. Upon disassembling the
cell, no
particular phenomenon of membrane deterioration was observed.
The previous description shall not be intended as limiting the invention,
which may be
practised according to different embodiments without departing from the scopes
thereof, and whose extent is solely defined by the appended claims.
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Throughout the description and claims of the present application, the term
"comprise"
and variations thereof such as "comprising" and "comprises" are not intended
to
exclude the presence of other elements or additives.
5 The discussion of documents, acts, materials, devices, articles and the like
is
included in this specification solely for the purpose of providing a context
for the
present invention. It is not suggested or represented that any or all of these
matters
formed part of the prior art base or were common general knowledge in the
field
relevant to the present invention before the priority date of each claim of
this
10 application.
