Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD OF CONDITIONING IRON ALLOY-BASED ANODES
FOR ALUMINIUM ELECTROWINNING CELLS
Field of the Invention
This invention relates to the conditioning of iron
alloy based aluminium electrowinning anodes to form an
integral electrochemically active iron-based oxide layer
thereon, anodes so conditioned, aluminium electrowinning
cells with such anodes and the production of aluminium in
such cells.
Background Art
The technology for the production of aluminium by
the electrolysis of alumina, dissolved in molten cryolite
containing salts, at temperatures around 950°C is more
than one hundred years old. This process and the cell
design have not undergone any great change or improvement
and carbonaceous materials are still used as electrodes
and cell linings.
Using metal anodes in aluminium electrowinning
cells would drastically improve the aluminium process by
reducing pollution and the cost of aluminium production.
Many patents have been filed on non-carbon anodes but none
has found commercial acceptance, also because of
economical reasons.
US Patents 4,614,569 (Duruz/Derivaz/Debely/
Adorian), 4,680,094 (Duruz), 4,683,037 (Duruz) and
4,966,674 (Bannochie/Sherriff) describe non-carbon anodes
for aluminium electrowinning coated with a protective
coating of cerium oxyfluoride, formed in-situ in the cell
or pre-applied, this coating being maintained by the
addition of a .cerium compound to the molten cryolite
electrolyte. EP Patent application 0 306 100 (Nyguen/
Lazouni/Doan), US Patents 5,069,771, 4,960,494 and
4,956,068 (all Nyguen/Lazouni/Doan) describe metallic
anode substrates which may be further covered with such an
in-situ formed protective cerium oxyfluoride layer.
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US Patent 5,510,008 (Sekhar/Ziu/Duruz) discloses
an anode made from an inhomogeneous porous metallic body
obtained by micropyretically reacting a powder mixture of
nickel, iron, aluminium and possibly copper and other
elements. An electrochemically active oxide-based outer
portion is formed by in-situ polarisation.
W000/06803 (Duruz/de Nora/Crottaz), WO00/06804
(Crottaz/Duruz) and W001/42534 (de Nora/Duruz) disclose
anodes produced from nickel-iron alloys which are surface
oxidised to form a coherent and adherent outer iron oxide-
based layer whose surface is electrochemically active
surface .
W001/42534 (Duruz/Nguyen/de Nora) discloses a
nickel-iron alloy aluminium electrowinning anode with an
openly porous electrochemically active surface produced by
removal of iron from the surface, in particular by
electrolytic dissolution of iron.
WO00/06805 (de Nora/Duruz) discloses an aluminium
electrowinning anode having a metallic anode body which
can be made of various alloys, for example a nickel-iron-
copper alloy. The surface of the anode body is oxidised by
anodically evolved oxygen to form an integral
electrochemically active oxide-based surface layer. The
oxidation rate of the anode body is equal to the rate of
dissolution of the surface layer into the electrolyte.
This oxidation rate is controlled by the thickness and
permeability of the surface layer which limits the
diffusion of anodically evolved oxygen to the anode body.
W001/42536 (Duruz/Nguyen/de Nora) discloses an
aluminium electrowinning anode made form a nickel-iron
alloy having an openly porous outer portion which consists
predominantly of nickel metal and which is obtainable by
removal of iron from the alloy.
W001/42534 (de Nora/Duruz), W002/070785 (Nguyen/de
Nora), W002/083990 (de Nora/Nguyen), W002/083991 (Nguyen
/de Nora), W003/014420 (Nguyen/Duruz/de Nora), W003/078695
(Nguyen/de Nora) and W003/087435 (Nguyen/de Nora) disclose
further metal-based aluminium electrowinning anodes.
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Metal or metal-based anodes are highly desirable
in aluminium electrowinning cells instead of carbon-based
anodes. Usually, the anodes are pre-oxidised in an
oxidising atmosphere before use to produce an
electrochemically active oxide layer thereon. In practice,
prior to insertion into the cell, the metal-based anodes
are usually pre-heated above the molten electrolyte to
inhibit thermal shocks in the electrolyte and its
solidification when the anode is immersed.
Objects of the Invention
A major object of the invention is to provide a
method of conditioning an iron-based alloy anode for
aluminium electrowinning to increase its lifetime and
improve its electrical conductivity during use.
Another object of the invention is to provide a
method of conditioning an iron-based alloy anode to form
an active anode surface that has a high electrochemical
activity for the oxidation of oxygen ions for the
formation of bimolecular gaseous oxygen and a low
solubility in the electrolyte.
A further object of the invention is to provide a
method of conditioning an iron-based alloy anode to form
an integral electrochemically active oxide layer with an
improved density and coherence and with limited ionic
conductivity for oxygen ions and a low electrical
resistance.
Yet another object of the invention is to provide
a method of conditioning an aluminium electrowinning anode
which is made of readily available alloys(s).
Yet a further object of the invention is to
provide an aluminium electrowinning anode which is made of
_ readily available alloys(s) and an aluminium
electrowinning cell having such an anode.
Summarv of the Invention
The invention is based on the observation that an
integral oxide layer formed by oxidation of an iron-based
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alloy, in particular an iron-based alloy comprising nickel
and/or cobalt, is significantly denser and more coherent
when the oxidation of the iron-based alloy is carried out
by polarisation thereof in a molten electrolyte under
specific conditions compared to oxide layers produced by
conventional oxidation in air or by polarisation in a
molten electrolyte as disclosed in the prior art.
To produce such a denser and more coherent oxide
layer, the anode's active surface is up to immersion into
the electrolyte essentially metallic and substantially
unreacted with reactive species that form ceramic
compounds with. metals from the iron-based alloy. Such
reactive species include oxygen at or above ambient
temperature or fluorine-containing gases that could
contact the anode while pre-heating the anode above the
cell.
It has been found that the formation of such
ceramic compounds on the iron-based alloy before
polarisation in the electrolyte impairs the coherence and
density of the oxide layer subsequently formed in-situ,
which reduces its capacity to limit diffusion of oxygen
and leads to thicker oxide layers with greater electrical
resistance. It has been observed that this effect is most
detrimental when the iron-based alloy is pre-oxidised
before electrolysis and also when the iron-based alloy is
pre-heated above the electrolyte in the fluorine-
containing fumes prior to immersion into the electrolyte.
The invention relates to a method of conditioning
a metallic anode structure for producing aluminium in a
molten electrolyte containing dissolved alumina. The
metallic anode structure has initially an iron-based alloy
outer part with an active anode surface which is
essentially metallic and free of any ceramic compounds, in
particular oxides and fluoride s, of metals from the
metallic anode structure.
According to the invention, the method comprises
the ordered steps of: (a) substantially preventing the
essentially metallic active surface free of said ceramic
compound from reacting with any reactable species, in
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particular oxygen and fluorine species, until immersion
into a molten electrolyte containing oxygen ions; (b)
immersing into the molten electrolyte the metallic anode
structure with its essentially metallic active surface
free of said ceramic compounds; and (c) polarising the
immersed metallic anode structure at a potential above the
potential of oxidation of oxygen thereby evolving oxygen
on the active anode surface and oxidising the active anode
surface to form on the metallic anode structure a dense
and coherent integral iron-based oxide layer which is
electrochemically active for the oxidation of oxygen and
which inhibits diffusion of oxygen towards the metallic
anode structure.
The prevention of the metallic active surface from
reacting with any reactable species, in particular oxygen
and fluorine species, until immersion into a molten
electrolyte can be achieved either by protecting the
active surface against reaction, for instance by coating
the surface with a temporary protective layer, or by
avoiding exposure of the active surface to a reactive
environment for an extended period of time during which a
significant amount of anode constituents at the active
surface can react with the environment.
For instance, exposure of a cast iron-based alloy
to air at room temperature for a few days would not lead
to significant formation of ceramic (oxide) compounds at
its surface. However, when the iron-based alloy is cast
and then left unprotected in the atmosphere for a long
period of time before use, for example stored several
months on a shelf after casting, the surface of the iron-
based alloy can become noticeably altered which has been
found to reduce the anode's performances, in particular
the stability, lifetime and energy efficiency. Moreover,
it has been discovered that even a short exposure, e.g. a
few minutes, to fluorine-containing gases while pre-
heating the anode above a fluoride-based molten
electrolyte has a significant deleterious effect.
As opposed to the prior art anode conditioning
methods in which the iron-based alloy anodes after casting
were kept unprotected until immersion into the molten
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electrolyte and also preferably were pre-oxidised in a
high temperature oxidising atmosphere and/or pre-heated
unprotected in fluorine-containing fumes above the molten
electrolyte before immersion into the molten electrolyte,
the method of conditioning the iron-based alloy anodes of
the present invention prevents substantially any reactive
interactions between the anode and the environment before
immersion into the molten electrolyte.
In case, before use, the anode's alloy contains at
its surface a noticeable amount of ceramic compounds
produced by reaction of metals from the alloy's surface
with a reactive environment, such ceramic compounds should
be removed from the alloy's surface before carrying out
the method of the invention.
Preferably, the method of the invention includes
the step of pre-heating the metallic anode structure,
prior to its immersion, to a temperature at which it can
be immersed into the molten electrolyte without
substantial solidification thereof. During this pre-
heating step, the structure's essentially metallic active
surface is maintained free of said ceramic compounds by
substantially preventing the active anode surface from
reacting with the environment during pre-heating before
immersion. This is not the case with prior art methods,
when the anodes are pre-heated without the inventive
conditioning.
Protection of the active anode surface prior to
immersion into the molten electrolyte can be achieved by
covering the active surface with a temporary protective
layer which is substantially impermeable to any species
reactable with the active surface and which is removed
prior to immersion into the molten electrolyte or
dissolves therein as a fugitive coating.
When the anode is pre-heated in a reactive
atmosphere prior to immersion, the temporary layer should
be heat stable, for instance applied from a ceramic paint,
e.g. an alumina-based paint, which is stable also at high
temperature. Of course, a heat-stable protective coating
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can be used even if the anode is not subj ected to a pre-
heat treatment.
When the anode structure is not pre-heated in a
reactive atmosphere, the temporary layer for longlasting
protection at ambient temperature before use, e.g. for
storage, can be an organic paint or a substantially
impervious wrapping, e.g. made of plastic.
The anode structure can also be pre-heated prior
to immersion in an inert atmosphere that is substantially
free from any species reactable with the active surface.
In this case, no heat stable temporary protective layer is
needed during the pre-heating.
The iron-based alloy outer part may comprise one
or more elements selected from nickel, cobalt, copper,
molybdenum, manganese, titanium, tantalum, tungsten,
hafnium, vanadium, zirconium, niobium, chromium, cobalt,
aluminium, silicon, carbon and the rare earth metals, in
particular yttrium.
In one embodiment, the iron-based alloy outer
part, consists essentially of: 45-55 weighto iron; 15-55
weighto in total of nickel and/or cobalt; 0-30 weighto
copper; and 0-10 weighto in total of one or more further
elements, usually selected from molybdenum, manganese,
titanium, tantalum, tungsten, hafnium, vanadium,
zirconium, niobium, chromium, cobalt, aluminium, silicon,
carbon and the rare earth metals, in particular yttrium.
Further suitable iron-based alloy compositions for
the anode structure are disclosed in W000/40783 (de Nora/
Duruz), W000/06803 (Duruz/de Nora/Crottaz), W000/06804
(Crottaz/Duruz), W001/42534 (de Nora/Duruz), W001/42536
(Duruz/Nguyen/de Nora), W002/014420 (Nguyen/Duruz/de Nora)
and W003/078695 (Nguyen/de Nora).
The metallic anode structure can be a cast alloy.
Casting can be advantageously used to produce anodes of
complex shapes, e.g. specially adapted for the circulation
of electrolyte. Examples of such anode shapes are
disclosed in W099/02764 (de Nora/Duruz), W000/40781,
W000/40782 and W003/006716 (all de Nora).
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The metallic anode structure can be quenched
and/or annealed prior to immersion into the molten
electrolyte, taking care however that the anode's active
surface is not exposed to an environment that can
substantially react with anode constituents at the active
surface.
The invention also relates to a method of
conditioning or reconditioning a metallic anode structure
for producing aluminium in a molten electrolyte containing
dissolved alumina, for example when the metallic anode
structure has been impaired by exposure to a reactive
environment before use or when the anode structure has
been worn during use.
Such a used or impaired metallic anode structure
comprises an iron-based alloy outer part having a surface
which is covered with ceramic compounds, in particular
oxides and fluorides, of metals from the outer part. The
conditioning or reconditioning method comprises the steps
of: removing substantially all ceramic compounds from the
surface of the outer part to form an essentially metallic
active anode surface; and then conditioning as described
above the metallic anode structure with its essentially
metallic active anode surface free of any ceramic
compounds.
A further aspect of the invention relates to
method of electrowinning aluminium. This method comprises
the steps of: conditioning a metallic anode structure as
described above including polarising it in a molten
electrolyte to form on the anode structure a dense and
coherent integral iron-based oxide layer; and
electrolysing dissolved alumina in the same or a different
molten electrolyte using the conditioned anode structure
to evolve oxygen thereon and produce aluminium on a facing
cathode.
The dense and coherent integral iron-based oxide
layer of the anode structure can be further formed during
electrolysis by slow oxidation of the metallic anode
structure at the metallic structure/oxide layer interface.
Constituents of the dense and coherent integral iron-based
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oxide layer at the oxide layer/electrolyte interface may
slowly dissolve into the electrolyte during electrolysis,
preferably at a rate corresponding to the oxidation rate
of the metallic anode structure at the metallic
structure/oxide layer interface, as disclosed in
W000/06805 (de Nora/Duruz).
As disclosed in W000/06802 (Duruz/de Nora/Crottaz)
the aluminium production molten electrolyte can comprise
an amount of dissolved iron species and dissolved alumina
sufficient to inhibit significantly dissolution of
constituents of the dense and coherent integral iron-based
oxide layer at the oxide layer/electrolyte interface.
The anode can be protected against the electrolyte
with a cerium oxyfluoride-based outermost coating deposited
and/or maintained during use, for example as disclosed in
the abovementioned US Patents 4,614,569, 4,680,094,
4, 683, 037 and 4, 966, 674.
Preferably, the aluminium production molten
electrolyte is maintained at a temperature below 960°C,
preferably between 840°C and 940°C.
The aluminium production molten electrolyte can
contain NaF and AlF3 in a molar ratio in the range from 1.2
to 2.4. The alumina content in the aluminium production
molten electrolyte is usually below 10 weighto, typically
between 5 weight% and 8 weighto.
It is preferred that alumina-depleted electrolyte
is circulated away from the electrochemically active iron-
based oxide layer, enriched with alumina, and alumina-
enriched electrolyte is circulated towards the
electrochemically active iron-based oxide layer. Such an
electrolyte circulation can be achieved by following the
teachings of W099/41429 (de Nora/Duruz), W099/41430
(Duruz/Belld), W000/40781, W000/40782, W001/31088 and
W003/006716 (all de Nora).
Another aspect of the invention relates to an
aluminium electrowinning anode structure. This structure
comprises an iron-based alloy metallic outer part covered
with a dense and coherent integral iron-based oxide layer
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obtainable by conditioning by the above described method a
metallic anode structure having an iron-based alloy outer
part with an active anode surface which is essentially
metallic and free of any ceramic compounds of metals from
the metallic anode structure.
Compared to pre-oxidised anodes or anodes that are
pre-heated unprotected in fumes above the molten
electrolyte, the anode of the invention exhibits upon use
in an aluminium electrowinning cell a more protective
denser and more coherent oxide layer which is thinner and
more conductive, as demonstrated in the example.
The invention also relates to an aluminium
electrowinning anode structure having an iron-based alloy
metallic outer part with an active anode surface. Before
use, the active surface is essentially metallic and free
of any ceramic compounds of metals from the metallic anode
structure and is covered with a temporary protection
medium. This protective medium substantially prevents
ceramic-forming reactions at the essentially metallic
anode surface and is separable from the active surface
prior to immersion into the molten electrolyte or by
contact with the molten electrolyte. Usually, the
temporary protection medium is removable prior to
immersion into the electrolyte or soluble in the
electrolyte.
The temporary protection medium may comprises one
or more substantially impervious solid layers. Suitable
layers comprise at least one of ceramics, such as alumina,
including ceramics applied from colloids, for instance
colloidal alumina precursor, such as NyacolTM and/or
CondeaTM, and other colloid precursors of ceramics; metals,
in particular reactable metals, such as aluminium, iron,
copper, chromium or yttrium, for reacting with possibly
diffusing reactive gases polymers, e.g. plastic, in
particular wrapping the anode structure under vacuum or
inert gas or carrying one or more of the above layer
constituents. Furthermore, the temporary protection medium
can comprise an inert liquid or viscous material, such as
oil or grease, or an inert gas, such as nitrogen or carbon
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dioxide, for example within a polymer enclosure or
wrapping around the anode structure.
The protective layer can comprise alumina, in
particular applied from a paint or slurry, and/or a
polymer. The protective layer may also contain metallic
particles that trap a possible oxygen diffusion before it
reaches the metallic anode surface.
Yet another aspect of the invention relates to an
aluminium electrowinning cell comprising at least one
oxygen-evolving anode structure as described above.
Preferably, the cell comprises an aluminium-
wettable cathode preferably having an aluminium-wettable
coating, in particular a drained cathode. Suitable,
aluminium-wettable coatings are disclosed in US Patent
5,651,874 (de Nora/Sekhar), and PCT publications
W098/17842 (Sekhar/Duruz/Ziu), W001/42168 (de Nora/Duruz)
and W001/42531 (Nguyen/Duruz/de Nora). Suitable drained
cathode designs are disclosed in US Patents 5,683,559,
5,888,360, 6,093,304 (all de Nora), 6,258,246 (Duruz/de
Nora), as well as PCT publications W099/02764 (de Nora/
Duruz), W099/41429 (de Nora/Duruz), W000/63463 (de Nora),
W001/31086 (de Nora/Duruz) and W001/31088 (de Nora).
Detailed Description
The invention will be further described in the
following Examples:
Comparative Example
Conditioning_
A prior art-type conditioning of an anode was
carried out as follows.
An anode was made by pre-oxidising in air at
1100°C for 3 hour a substrate of a cast nickel-iron alloy
consisting of 50 weighto nickel, 0.3 weighto manganese,
0.5 weighto aluminium, 0.05 weighto C and 49.15 weight%
iron, to form a very thin oxide surface layer on the
alloy.
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The surface-oxidised anode was cut perpendicularly
to the anode operative surface and the resulting section
of the anode was subjected to microscopic examination.
The anode before use had an outer portion that
comprised an electrolyte-pervious, electrochemically
active iron-rich nickel-iron oxide surface layer having a
thickness of up to 10-20 micron and, underneath, an iron-
depleted nickel-iron alloy having a thickness of about 10-
micron containing generally round cavities filled with
10 iron-rich nickel-iron oxide inclusions and having a
diameter of about 2 to 5 micron. The nickel-iron alloy of
the outer portion contained about 75 weighto nickel.
Underneath the outer portion, the nickel-iron
alloy had remained substantially unchanged.
15 Testing:
An anode prepared as above was pre-heated
unprotected in electrolyte fumes above an aluminium
electrowinning cell for 20 minutes and then tested
therein.
The cell contained a molten electrolyte at 880-
890°C consisting essentially of NaF and AlF3 in a weight
ratio NaF/A1F3 of about 0.7 to 0.8, i.e. an excess of AlF3
in addition to cryolite of about 24 weighto of the
electrolyte, and approximately 5 weighto alumina. The
alumina concentration was maintained at a substantially
constant level throughout the test by adding alumina at a
rate adjusted to compensate the cathodic aluminium
reduction. The test was run at a current density of about
0.8 A/cm~, and the electrical potential of the anode
remained in the range of 4.2 to 4.5 volts throughout the
test.
During electrolysis aluminium was cathodically
produced while oxygen was anodically evolved which was
derived from the dissolved alumina present near the
anodes.
After 24 hours, electrolysis was interrupted and
the anode was extracted from the cell. The external
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dimensions of the anode had remained substantially
unchanged during the test and the anode showed no signs of
damage.
The anode was cut perpendicularly to the anode
operative surface and the resulting section of the used
anode was subjected to microscopic examination.
It was observed that the anode was covered with an
iron oxide surface layer having a thickness of 100 to 120
micron and consisting essentially of Fe203 with less than 5
weighto nickel oxide.
Example 1
An anode was made of the same metals as in the
comparative Example except that it had an active surface
essentially metallic and free of any ceramic compounds, in
particular neither oxides nor fluorides, of metals from
the anode's surface.
The cold anode was immersed in the molten
electrolyte of a cell without prior reaction of the
anode's active surface with a reactive environment, i.e.
with the active surface essentially metallic and free of
said ceramic compounds.
The molten electrolyte was at a temperature of
880-890°C and consisted essentially of NaF and AlF3 in a
weight ratio NaF/AlF3 of about 0.7 to 0.8, i.e. an excess
of A1F3 in addition to cryolite of about 24 weighto of the
electrolyte, and approximately 5 weighto alumina.
Upon immersion of the anode, the electrolyte froze
around the anode which prevented electrolysis. After 10 to
15 minutes in the electrolyte, the temperature of the
anode had reached the melting point of the electrolyte and
the electrolyte contacting the anode had molten thereby
permitting electrolysis.
During the test, the alumina concentration was
maintained at a substantially constant level throughout
the test by adding alumina at a rate adjusted to
compensate the cathodic aluminium reduction. The test was
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run at a current density of about 0.8 A/cm2, and the
electrical potential of the anode remained in the range of
4.0-4.3 volts throughout the test, i.e. about 0.2 V lower
in average than in the above comparative test with a pre-
y oxidised and fume pre-heated anode.
During electrolysis aluminium was cathodically
produced while oxygen was anodically evolved which was
derived from the dissolved alumina present near the
anodes.
After 24 hours, electrolysis was interrupted and
the anode was extracted from the cell. The external
dimensions of the anode had remained substantially
unchanged during the test and the anode showed no signs of
damage.
The anode was cut perpendicularly to the anode
operative surface and the resulting section of the used
anode was subjected to microscopic examination.
It was observed that the anode was covered with an
iron oxide surface layer having a thickness of about 100
micron and consisting essentially of Fe203 with less than 5
weighto nickel oxide and additionally an outermost layer
of oxides of iron and aluminium having a thickness of
about 25 micron.
The iron oxide surface layer, with pores of 3 to 5
micron, was denser than the oxide layer of the above
comparative anode that had pores of 5 to 10 micron. This
greater density of the oxide layer of the anode
conditioned according to the invention provides a better
protection for the alloy located underneath oxidation and
electrolyte attack.
Example 2
Example 1 was repeated with a pre-heating step of
the anode above the molten electrolyte prior to immersion
into the electrolyte.
To prevent reaction of the essentially metallic
anode surface with electrolyte fumes, the anode was
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covered with a protective layer of alumina applied from an
alumina-based colloidal slurry.
The colloidal slurry comprised an alumina colloid
carrier consisting essentially of water and colloidal
alumina particles in an amount of 20 weight% of the
colloid, for example NyacolTM, and suspended alumina
particles in an amount of 60 weighto of the colloidal
slurry.
The anode was dipped into the slurry and allowed
to dry to produce an alumina coating having a thickness of
0.6 to 1.0 mm covering and protecting the essentially
metallic anode surface against reaction with the
environment.
After pre-heating the protected anode for about 1
hour in the electrolyte fumes above the molten
electrolyte, the anode was immersed with its alumina
coating into the molten electrolyte. The protective
alumina coating was dissolved almost instantaneously and
normal electrolysis could start.
During electrolysis, the anode behaved like the
anode in Example 2. After 24 hours, the anode was removed
and examined. The anode upon use was not significantly
different to the anode of Example 2.
In a variation, the protective effect of the
alumina slurry can be improved by substituting half of the
suspended alumina particles with suspended metallic
particles, such as aluminium, iron and/or copper
particles, which trap a possible oxygen diffusion before
it reaches the metallic anode surface. Thus, in case the
temporary protective coating is not perfectly impervious
to the environment, reactive constituents of the
environment will react with the metallic particles of the
coating and substantially not with .the metallic active
anode surface.
