Note: Descriptions are shown in the official language in which they were submitted.
CA 02437671 2003-08-05
WO 02/070786 PCT/IB02/00667
- 1 -
METAL-BASED ANODES FOR ALUMINIUM PRODUCTION CELLS
Field of the Invention
This invention relates to metal-based anodes for
aluminium production cells, aluminium production cells
operating with such anodes as well as operation of such
cells to produce aluminium.
Background Art
The technology for the production of aluminium by the
electrolysis of alumina, dissolved in molten cryolite, at
temperatures around 950°C is more than one hundred years
old. This process, conceived almost simultaneously by Hall
and Heroult, has not evolved as many other electrochemical
processes.
The anodes are still made of carbonaceous material
and must be replaced every few weeks. During electrolysis
the oxygen which should evolve on the anode surface
combines with the carbon to form polluting C02 and small
amounts of CO and fluorine-containing dangerous gases. The
actual consumption of the anode is as much as 450 Kg/Ton
of aluminium produced which is more than 1/3 higher than
the theoretical amount of 333 Kg/Ton.
Using metal anodes in aluminium electrowinning cells
would drastically improve the aluminium process by
reducing pollution and the cost of aluminium production.
US Patent 6,077,415 (Duruz/de Nora) discloses a
metal-based anode comprising a metal-based core covered
with an oxygen barrier layer and an electrochemically
active outer layer, the barrier layer and the outer layer
being separated by an intermediate layer to prevent
dissolution of the oxygen barrier layer.
US Patents 4,614,569 (Duruz/Derivaz/Debely/Adorian),
4,966,674 (Bannochie/Sheriff), 4,683,037 and 4,680,094
(both in the name of Duruz) describe metal anodes for
aluminium electrowinning coated with a protective coating
CA 02437671 2003-08-05
WO 02/070786 PCT/IB02/00667
of cerium oxyfluoride, formed in-situ in the cell or pre-
applied, this coating being maintained by the addition of
small amounts of cerium to the molten cryolite.
Along the same lines, EP Patent application 0 306 100
and US Patents 5,069,771, 4,960,494 and 4,956,068 (all in
the name of NyguenlLazounilDoan) disclose aluminium
production anodes having an alloy substrate protected with
an oxygen barrier layer that is covered with a copper
nickel layer for anchoring a cerium oxyfluoride operative
surface coating.
Although the above mentioned prior art metal-based
anodes showed a significantly improved lifetime over known
oxide and cermet anodes, they have not as yet found
commercial acceptance.
Ob-iects of the Invention
A major object of the invention is to provide an
anode for aluminium electrowinning which has no carbon so
as to eliminate carbon-generated pollution and increase
the anode life.
An important object of the invention is to reduce the
solubility of the surface of an aluminium electrowinning
anode, thereby maintaining the anode dimensionally stable
without excessively contaminating the product aluminium.
Another object of the invention is to provide a cell
for the electrowinning of aluminium utilising metal-based
anodes, and a method to produce aluminium in such a cell
and preferably maintain the metal-based anodes
dimensionally stable.
A main object of the invention is to provide a metal
based anode for the production of aluminium which is
resistant to fluoride attack.
A subsidiary object of the invention is to prevent
diffusion of chromium in a metal-based anode that
comprises chromium as an oxygen barrier layer.
CA 02437671 2003-08-05
WO 02/070786 PCT/IB02/00667
- 3 -
Summary of the Invention
It has been observed that prior art aluminium
production metal-based anodes are attacked during use by
fluorides. Also when aluminium production cells are
operated with an electrolyte at reduced temperature, i.e.
below 960°C, fluoride attack increases, as the fluoride
content is higher.
Without being bound to any theory, it is believed
that metal oxides present at the surface of metal-based
anodes, like oxides of iron, nickel, copper, chromium
etc..., combine during use with fluorides of the electrolyte
to produce soluble oxyfluorides.
The invention is based on the observation that silver
can be used as a barrier layer to fluoride attack. At high
temperature, i.e. above 450°C, silver does not form an
oxide and remains as a metal. It follows from the above
theory that during use fluorides cannot form oxyfluorides
by exposure to the silver layer which is devoid of oxide,
and the fluorides cannot corrode the silver layer.
Therefore, the invention relates to a metal-based
anode substrate of a cell for the electrowinning of
aluminium from alumina dissolved in a fluoride-containing
molten electrolyte. The substrate comprises a nickel-alloy
based core, a layer of silver on the core and a layer
comprising nickel and iron covering the silver layer and
ser~ring as an anchorage layer for anchoring an
electrochemically active surface coating on top of the
anode substrate. The silver layer inhibits diffusion of
fluoride species into the core and prevents interdiffusion
of constituents of the core and constituents of the
anchorage layer.
The silver layer may have an average thickness in the
range of 5 to 100 micron. .
As silver is not an efficient barrier to oxygen, when
the core of the anode substrate is not by itself
sufficiently resistant to oxygen attack, it is preferable
to protect it with an oxygen barrier layer, such as a
chromium barrier layer.
CA 02437671 2003-08-05
WO 02/070786 PCT/IB02/00667
- 4 -
Therefore, in one embodiment, the anode substrate
comprises a further layer of silver and a layer of
chromium, the chromium layer being located between the
core and the anchorage layer and separated therefrom by
the silver layers. The chromium layer may have an average
thickness in the range of 10 to 100 micron.
Thus, the oxygen barrier layer-is separated from the
core by a first layer of silver and from the anchorage
layer by a second layer of silver. The silver layers
prevent interdiffusion of chromium from the barrier layer
with constituents of the core and with constituents of the
anchorage layer.
As silver is insoluble in chromium and vice-versa,
the silver layers confine the chromium within the barrier
layer and do not mix with the chromium, thereby securing a
long-lasting integrity of the chromium barrier layer.
By having an oxygen barrier layer of chromium
separated by silver layers from miscible anode
constituents, this embodiment of the anode according to
the present invention is efficiently protected against
oxidation for a longer period of time than prior art
anodes.
Conversely, in prior art anodes, e.g. as disclosed in
US Patent s 5,069,771, 4,960,494 and 4,956,068 mentioned
above, the chromium barrier layer contacts miscible metals
such as nickel and/or copper. During use, a slow
interdiffusion of chromium with nickel and/or copper takes
place, and the contaminated chromium barrier layer becomes
pervious to oxygen permitting oxidation of the anode
material underneath.
The anchorage layer and/or the core may comprise one
or more additives selected from yttrium, tantalum and
niobium in a total amount of 0.1 to 5 weight%. Preferably,
the anchorage layer and/or the core comprise yttrium, for
instance in an amount of less than 1 weighto.
It has been observed that when a nickel-iron alloy is
used as anode material, the iron of the alloy slowly
diffuses to the surface, becomes oxidised by anodically
CA 02437671 2003-08-05
WO 02/070786 PCT/IB02/00667
- 5 -
evolved oxygen and dissolves in the electrolyte during
use. The addition of yttrium to the nickel-iron alloy
greatly inhibits diffusion of iron from inside the alloy
to its surface. Indeed, in such an alloy, yttrium is
mainly located at the joints between the grains forming
the nickel-iron alloy and constitutes a mechanical
obstacle to diffusion of the grains within the alloy.
The anchorage layer may have an average thickness in
the range of 30 to 300 micron.
The anchorage layer can be made of a bottom layer of
nickel and iron. and a top layer of copper. In this case,
the nickel-iron bottom layer may have an average thickness
in the range of 30 to 300 micron and the copper top layer
an average thickness in the range of 5 to 50 micron. Upon
heat treatment, the copper top layer is usually partly
interdiffused with the nickel-iron layer adjacent to it.
Usually, upon exposure to an oxidising atmosphere
and/or during use, one or more of the layers on the anode
core are at least partly oxidised.
The invention also relates to a metal-based anode
that comprises an anode substrate as described above which
is coated with an electrochemically active surface coating
made of one or more cerium compounds, in particular cerium
oxyfluoride.
The electrochemically active surface coating may
comprise at least one additive selected from yttrium,
tantalum and niobium.
The electrochemically active surface coating can be
an electrolytically deposited coating or applied before
use, for instance from a cerium-based slurry.
Another aspect of the invention is a cell, for the
electrowinning of aluminium from alumina dissolved in a
fluoride-based molten electrolyte. The cell according to
the invention comprises at least one of the above
described metal-based anodes.
CA 02437671 2003-08-05
WO 02/070786 PCT/IB02/00667
- 6 -
The electrolyte of the cell preferably comprises
cerium species to maintain the electrochemically active
surface coating dimensionally stable.
The cell of the invention may be operated with or
without a crust and/or a sideledge of frozen electrolyte.
Advantageously, the cell has an electrolyte at
reduced temperature, i.e. below 960°C, for instance in the
range from 860° to 930°C.
A further aspect of the invention is a method of
producing aluminium in the above described cell. The
method of the invention comprises dissolving alumina in
the electrolyte and passing an electrolysis current
between the or each anode and a facing cathode whereby
oxygen is anodically evolved on the electrochemically
active surface coating and aluminium is cathodically
reduced.
The invention will be further described in the
following Examples:
Example 1
Anode substrate:
An anode substrate made of a nickel-iron core covered
with a silver barrier layer and a nickel-iron anchorage
layer according to the invention was prepared. as follows:
A hemi-spherical nickel-containing anode core having
a diameter of 20 mm and a length of 30 mm was machined
from a nickel-iron alloy rod made of 80 weight% nickel and
20 weighto iron. The surface of the anode core was sand-
blasted, degreased and rinsed carefully with deionised
water.
The anode core was then immersed in an AgCN/KCN bath
at room temperature and polarised in order to
electrolytically deposit silver thereon from a silver
counter electrode. A cathodic current with a current
density of about 50 mA/cm~ was passed at the surface of
anode core. During the electrolytic deposition, the
AgCN/KCN bath was moderately agitated.
CA 02437671 2003-08-05
WO 02/070786 PCT/IB02/00667
_ 7 -
After 10 minutes electrodeposition was interrupted.
The anode core was removed from the AgCN/KCN bath and
carefully rinsed with deionised water. An electrodeposited
silver layer having an average thickness of about 25 to 30
micron had been formed on the anode core.
The silver plated anode core was then immersed and
polarised in a FeS04-NiS04-NiCl2/Boric - Salicylic acid
bath at a temperature of 55°C. A nickel-iron alloy was
deposited onto the silver plated anode core from an alloy
counter electrode made of a 50 weighto nickel and 50
weighto iron. An electrolysis current was passed between
the plated anode core and the counter-electrode at a
current density of about 60 mA/cm2 at the surface of the
plated anode core. As before, the bath was moderately
agitated during the electrolytic deposition.
After 30 minutes electrodeposition was interrupted.
The plated anode core was removed from the bath and rinsed
carefully with deionised water. A nickel iron anchorage
layer made of 54 weighto nickel and 46 weight% iron with
an average thickness of about 30 to 35 micron had been
deposited onto the silver plated anode core which thus
constituted an anode substrate according to the invention.
The anode substrate was oxidised in air at a
temperature of about 1100°C for 1 hour. An iron oxide
based black adherent layer consisting of 95-97 weight%
iron oxide and 3-5% nickel oxide was formed on the anode
substrate.
Electrolysis_Testin~_
The oxidised anode substrate was then immersed and
anodically polarised in ' a laboratory aluminium
electrowinning cell operating with a cryolite-based
electrolyte consisting of about 21 weighto A1F3, 4 weighto
A1203, 3 weighto CeF3 and 72 weighto Na3A1F6 at a
temperature of about 920°C. The cell used an aluminium
pool as a cathode.
At the beginning of electrolysis, to permit formation
of an electrochemically active cerium oxyfluoride coating
on the anode substrate, a reduced electrolysis current was
CA 02437671 2003-08-05
WO 02/070786 PCT/IB02/00667
_ g _
passed between the anode substrate and the aluminium
cathodic pool 'at an anodic current density of about 0.5
A/cm2. After 5 hours the current density was increased to
about 0.8 A/cm2.
To compensate depletion of CeF3 and A1a03 during
electrolysis, the cell was periodically supplied with a
powder feed of A1203 containing 2 .tweight% CeF3. The feeding
rate corresponded to 300 of the cathodic current
efficiency. After 24 hours the anode was removed from the
molten bath and cooled down to room temperature.
Examination After_Testin~_
Visual examination of the anode showed that a blue
and uniform cerium oxyfluoride coating had been deposited
on the part of the anode substrate that had been immersed
in the cryolite-based electrolyte.
The anode was cut perpendicular to a cerium
oxyfluoride coated surface and the section was examined
under a SEM microscope.
It was observed. that the cerium-based coating had a
thickness of about 500 to 700 micron. Underneath the
cerium-based coating, the nickel-iron anchorage layer had
been completely oxidised and transformed. into a black and
adherent matrix of iron-nickel mixed oxyfluorides. The
electroplated silver layer had remained un-oxidised.
Underneath the silver layer, the anode core showed no sign
of corrosion or exposure to fluorides. However, a surface
layer containing a uniform distribution of iron oxide
inclusions and having a thickness of about 200 micron had
been formed on the core.
Example 2
Anode substrate:
Another anode substrate made of a nickel-iron core
covered with a silver barmier layer, a chromium barrier
layer, a further silver layer and a nickel-iron anchorage
layer according to the invention was prepared as follows:
CA 02437671 2003-08-05
WO 02/070786 PCT/IB02/00667
- 9 -
An anode core was plated with a layer of silver as in
Example 1.
An oxygen barrier layer of chromium was then formed
on the plated anode core by immersing and polarising it in
a Cr03/H2S04 bath at a temperature of 35°C. A dimensionally
stable counter electrode was used. An electrolysis current
was passed between the plated anode core and the counter-
electrode at a current density of about 300 mA/cm2 on the
plated anode core in order to deposit chromium from the
bath onto the anode core. The bath was moderately agitated
during the electrolytic deposition.
After 30 minutes electrodeposition was interrupted.
The plated anode core was removed from the bath and rinsed
carefully with deionised water. A mat chromium
electrodeposited layer of about 15 micron had been
deposited onto the silver layer.
Then, the chromium layer was activated in a NiCl2/HCl
bath by anodic polarisation at a current density of about
30 mA/cm2 for 3 minutes followed by a cathodic
polarisation at the same current density for 6 minutes. A
layer of nickel having a thickness of about 1 micron was
deposited onto the chromium coating.
After activation, the plated anode core was removed
from the activation bath, rinsed carefully with deionised
water and immediately plated with a further layer of
silver following the above-described silver plating
procedure and then with a nickel-iron anchorage layer air
oxidised as described in Example 1.
Electrolysis _Tes~ir~.~_
The anode substrate was coated with a cerium
oxyfluoride electrochemically active layer to form an
anode according to the invention and then used for 24
hours in a cell as described in Example 1.
Examinati~n _Af ter_ Testing
Visual examination of the anode showed that a blue
and uniform cerium oxyfluoride coating had been deposited
CA 02437671 2003-08-05
WO 02/070786 PCT/IB02/00667
- 10 -
on the part of the anode substrate that had been immersed
in the cryolite-based electrolyte.
The anode was cut- perpendicular to a cerium
oxyfluoride coated surface and the section was examined
under a SEM microscope.
It was observed that the cerium-based coating had a
thickness of about 500 to 700 micron. Underneath the
cerium-based coating, the nickel-iron anchorage layer had
been completely oxidised and transformed into a black and
adherent matrix of iron-nickel mixed oxyfluorides. The
electroplated silver layers had remained un-oxidised. The
chromium oxygen barrier layer was oxidised to a depth of
about 2 to 5 micron.
No nickel or iron from the core or the anchorage
layer was found in the chromium layer which demonstrated
that silver acts as an efficient barrier preventing
interdiffusion of constituents of the core and
constituents of the anchorage layer.
Underneath the silver layers and the chromium layer,
the anode core showed no sign of corrosion or exposure to
fluorides. No oxide was found in the anode core
demonstrating the efficiency of the chromium oxygen
barrier layer.
