Note: Descriptions are shown in the official language in which they were submitted.
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HIGH STABILITY FLOW-THROUGH NON-CARBON ANODES
FOR ALUMINIUM ELECTROWINNING
Field of the Invention
This invention relates to the use in a molten
electrolyte for the electrowinning of aluminium of a non-
carbon anode having a flow-through active structure with
an enhanced stability.
Background Art
Using non-carbon anodes - i.e. anodes which are not
made of carbon as such, e.g. graphite, coke, etc..., but
possibly contain carbon in a compound - for the
production of aluminium in electrolytic cells should
drastically improve the aluminium production process by
reducing pollution and the cost of aluminium production.
The developments of non-carbon anode materials, in
particular metals, led to the design of new anode shapes
that are better adapted to the cell's fluid mechanisms
and electromagnetic effects than the conventional anodic
solid carbon blocks.
Several designs for oxygen-evolving anodes for
aluminium electrowinning cells were proposed in the
following documents. US Patent 4,681,671 (Duruz)
discloses vertical anode plates or blades operated in low
temperature aluminium electrowinning cells. US Patent
5,310,476 (Sekhar/de Nora) discloses oxygen-evolving
anodes consisting of roof-like assembled pairs of anode
plates. US Patent 5,362,366 (de Nora/Sekhar) describes
non-consumable anode shapes including roof-like assembled
pairs of anode plates. US Patent 5,368,702 (de Nora)
discloses vertical tubular or frustoconical oxygen-
evolving anodes for multimonopolar aluminium cells. US
Patent 5,683,559 (de Nora) describes an aluminium
electrowinning cell with oxygen-evolving bent anode
plates which are aligned in a roof-like configuration
facing correspondingly shaped cathodes. US Patent
5,725,744 (de Nora/Duruz) discloses vertical oxygen-
evolving anode plates, preferably porous or reticulated,
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in a multimonopolar cell arrangement for aluminium
electrowinning cells operating at reduced temperature.
W000/40781, W000/40782 and W003/006716 (all de Nora) both
disclose aluminium production anodes with a series of
parallel spaced-apart elongated anode members which are
electrochemically active for the oxidation of oxygen.
For the dissolution of the raw material alumina, a
highly aggressive fluoride-based electrolyte, such as
cryolite, is required. Various modified electrolytes have
been proposed to improve cell operation and reduce wear
of non-carbon metal-based anode, particularly caused by
corrosion by the electrolyte.
W000/06804 (Crottaz/Duruz) teaches that a nickel-
iron anode can be used in an electrolyte at a temperature
of 8200 to 870 C containing 23 to 26.5 weight% A1F3, 3 to
5 weight% A1203, 1 to 2 weight% LiF and 1 to 2 weighto
MgF2. US Patents 5,006,209 and 5,284,562 (both
Beck/Brooks), 6,258,247 and 6,379,512 (both Brown/
Brooks/Frizzle/Juric), 6,419,813 (Brown/Brooks/Frizzle)
and 6,436,272 (Brown/Frizzle) all disclose the use of
nickel-copper-iron anodes in an aluminium production
electrolyte at 660 -800 C containing 6-26 weight% NaF, 7-
33 weight% KF, 1-6 weight% LiF and 60-65 weight% AlF3.
The electrolyte may contain A1203 in an amount of up to
30 weight%, in particular 5 to 10 or 15 weight%, most of
which is in the form of suspended particles and some of
which is dissolved in the electrolyte, i.e. typically 1
to 4 weighto dissolved A1203. In US Patents 6,258,247,
6,379,512, 6,419,813 and 6,436,272 such an electrolyte is
said to be useable at temperatures up to 900 C. In US
Patents 6,258,247 and 6,379,512 the electrolyte further
contains 0.004 to 0.2 weight% transition metal additives
to facilitate alumina dissolution and improve cathodic
operation. US Patent 5,725,744 (de Nora/Duruz) discloses
an aluminium production cell having anodes made of
nickel, iron and/or copper in a electrolyte at a
temperature from 680 to 880 C containing 42-63 weight%
AlF3, up to 48 weight% NaF, up to 48 weight% LiF and 1 to
5 weight o A1203. MgF2, KF and CaF2 are also mentioned as
possible bath constituents. W02004/035871 (de
Nora/Nguyen/Duruz) discloses a metal-based anode
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containing at least one of nickel, cobalt and iron. The
anode is used for electrowinning aluminium in a fluoride-
containing molten electrolyte consisting of: 5 to 14 wt%
dissolved alumina; 35 to 45 wt% aluminium fluoride; 30 to
45 wt% sodium fluoride; 5 to 20 wt% potassium fluoride; 0
to 5 wt% calcium fluoride; and 0 to 5 wt% of further
constituents.
The materials having the greatest resistance to
oxidation are metal oxides which are all to some extent
soluble in cryolite. Oxides are also poorly electrically
conductive, therefore, to avoid substantial ohmic losses
and high cell voltages, the use of non-conductive or
poorly conductive oxides should be minimal in the
manufacture of anodes. Whenever possible, a good
conductive material should be utilised for the anode
core, whereas the surface of the anode is preferably made
of an oxide having a high electrocatalytic activity.
Several attempts have been made in order to develop non-
carbon anodes for aluminium electrowinning cells,
resistant to chemical attacks of the bath and by the cell
environment, and with an electrochemical active surface
for the oxidation of oxygen ions to atomic and molecular
gaseous oxygen and having a low dissolution rate. Many
patents have been filed on non-carbon anodes but none has
found commercial acceptance yet, also because of
economical reasons.
US Patent 4,614,569 (Duruz/Derivaz/Debely/Adorian)
describes metal 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 during electrolysis by the addition of
small amounts of a cerium compound to the molten cryolite
electrolyte so as to protect the surface of the anode
from the electrolyte attack. Several patents disclose the
use of an electrically conductive metal anode core with
an oxide-based active outer part, in particular US
patents 4,956,069, 4,960,494, 5,069,771 (all
Nguyen/Lazouni/Doan), 6,077,415 (Duruz/de Nora),
6,103,090 (de Nora), 6,113,758 (de Nora/Duruz) and
6,248,227 (de Nora/Duruz), 6,361,681 (de Nora/Duruz),
6,365,018 (de Nora), 6,372,099 (Duruz/de Nora), 6,379,526
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(Duruz/de Nora), 6,413,406 (de Nora), 6,425,992 (de
Nora), 6,436,274 (de Nora/Duruz), 6,521,116 (Duruz/de
Nora/Crottaz), 6,521,115 (Duruz/de Nora/Crottaz),
6,533,909 (Duruz/de Nora), 6,562,224 (Crottaz/Duruz) as
well as PCT publications W000/40783 (de Nora/Duruz),
WO01/42534 (de Nora/Duruz), W001/42535 (Duruz/de Nora),
WO01/42536 (Nguyen/Duruz/ de Nora), W002/070786
(Nguyen/de Nora), W002/083990 (de Nora/Nguyen),
W002/083991 (Nguyen/de Nora), W003/014420
(Nguyen/Duruz/de Nora), W003/078695(Nguyen/de Nora),
W003/087435 (Nguyen/de Nora).
US 4,374,050 (Ray) discloses numerous multiple oxide
compositions for electrodes. Such compositions inter-alia
include oxides of iron and cobalt. The oxide compositions
can be used as a cladding on a metal layer of nickel,
nickel-chromium, steel, copper, cobalt or molybdenum. US
4,142,005 (Cadwell/Hazelrigg) discloses an anode having
a substrate made of titanium, tantalum, tungsten,
zirconium, molybdenum, niobium, hafnium or vanadium. The
substrate is coated with cobalt oxide Co304.
US 6,103,090 (de Nora), 6,361,681 (de Nora/Duruz),
6,365,018 (de Nora), 6,379,526 (de Nora/Duruz), 6,413,406
(de Nora) and 6,425,992 (de Nora), and W004/018731
(Nguyen/de Nora) disclose anode substrates that contain
at least one of chromium, cobalt, hafnium, iron,
molybdenum, nickel, copper, niobium, platinum, silicon,
tantalum, titanium, tungsten, vanadium, yttrium and
zirconium and that are coated with at least one of
ferrites of cobalt, copper, chromium, manganese, nickel
and zinc. WO01/42535 (Duruz/de Nora) and W002/097167
(Nguyen/de Nora), disclose aluminium electrowinning
anodes made of surface oxidised iron alloys that contain
at least one of nickel and cobalt. US 6,638,412 (de
Nora/Duruz) discloses the use of anodes made of a
transition metal-containing alloy having an integral
oxide layer, the alloy comprising at least one of iron,
nickel and cobalt.
Non-carbon anodes have not as yet been commercially
and industrially applied and there is still a need for a
metal-based anodic material and an appropriate anode
shape that can be used for electrowinning aluminium.
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Summary of the Invention
The present invention generally relates to aluminium
electrowinning with metal-based anodes having a shape for
promoting an electrolyte circulation and having an
5 electrochemically active outer part that has an enhanced
stability against corrosion by the highly aggressive
circulating electrolyte and/or against oxidation by
anodically evolved oxygen, the enhanced stability being
provided by a layer that contains predominantly cobalt
oxide CoO.
In particular, the invention relates to a cell for
the electrowinning of aluminium from alumina dissolved in
a fluoride-containing molten electrolyte. The cell
comprises at least one non-carbon metal-based anode
having an electrically conductive metallic structure that
comprises an outer part with an electrochemically active
anode surface on which, during electrolysis, oxygen is
anodically evolved, and which is suspended in the
electrolyte substantially parallel to a facing cathode.
This metallic structure has one or more flow-through
openings extending from the active anode surface through
the metallic structure, the flow-through opening(s) being
arranged for guiding a circulation of electrolyte driven
by the fast escape of anodically evolved oxygen. The
outer part of the anode comprises the abovementioned
layer that contains predominantly cobalt oxide CoO to
enhance the stability of the anode.
In other words, the invention concerns a cell having
an anode that has a shape that promotes electrolyte
circulation and that has an electrochemically active
outer part that is resistant to the circulating
electrolyte and/or to anodically evolved oxygen by the
presence of a layer made predominantly of a special form
of cobalt oxide, i.e. CoO.
There are several forms of stoichiometric and non-
stoichiometric cobalt oxides which are based on:
- CoO that contains Co(II) and that is formed
predominantly at a temperature above 920 C in air;
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- Co203 that contains Co(III) and that is formed at
temperatures up to 895 C and at higher temperatures
begins to decompose into CoO;
- Co3O4 that contains Co(II) and Co(III) and that is
formed at temperatures between 300 and 900 C.
It has been observed that - unlike Co203 that is
unstable and Co304 that does not significantly inhibit
oxygen diffusion - CoO forms a well conductive
electrochemically active material for the oxidation of
oxygen ions and for inhibiting diffusion of oxygen. Thus
this material forms a limited barrier against oxidation
of the metallic cobalt body underneath.
The anode's CoO-containing layer can be a layer made
of sintered particles, especially sintered CoO particles.
15' Alternatively, the Co0-containing layer may be an
integral oxide layer on a Co-containing metallic layer or
anode core. Tests have shown that integral oxide layers
have a higher density than sintered layers and are thus
preferred to inhibit oxygen diffusion.
When CoO is to be formed by oxidising metallic
cobalt, care should be taken to carry out a treatment
that will indeed result in the formation of CoO. It was
found that using Co2O3 or Co3O4 in a known aluminium
electrowinning electrolyte does not lead to an
appropriate conversion of these forms of cobalt oxide
into CoO. Therefore, it is important to provide an anode
with the CoO layer before the anode is used in an
aluminium electrowinning electrolyte.
The formation of CoO on the metallic cobalt is
preferably controlled so as to produce a coherent and
substantially crack-free oxide layer. However, not any
treatment of metallic cobalt at a temperature above 895 C
or 900 C in an oxygen-containing atmosphere will result
in optimal coherent and substantially crack-free CoO
layer that offers better electrochemical properties than
a C02O3/CO3O4.
For instance, if the temperature for treating the
metallic cobalt to form Co0 by air oxidation of metallic
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cobalt is increased at an insufficient rate, e.g. less
than 200 C/hour, a thick oxide layer rich in Co304 and in
glassy Coz03 is formed at the surface of the metallic
cobalt. Such a layer does not permit optimal formation of
the CoO layer by conversion at a temperature above 895 C
of Co203 and Co304 into CoO. In fact, a layer of CoO
resulting from such conversion is not preferred but still
useful despite an increased porosity and may be cracked.
Therefore, the required temperature for air oxidation,
i.e. above 900 C, usually at least 920 C or preferably
above 940 C should be attained sufficiently quickly, e.g.
at a rate of increase of the temperature of at least
300 C or 600 C per hour to obtain an optimal CoO layer.
The metallic cobalt may also be placed into an oven that
is pre-heated at the desired temperature above 900 C.
Likewise, if the anode is not immediately used for
the electrowinning of aluminium after formation of the
CoO layer but allowed to cool down, the cooling down
should be carried out sufficiently fast, for example by
placing the anode in air at room temperature, to avoid
significant formation of Co304 that could occur during
the cooling, for instance in an oven that is switched
off.
An anode with a CoO layer obtained by slow heating
of the metallic cobalt in an oxidising environment will
not have optimal properties but still provides better
results during cell operation than an anode having a
COZO3-CO3O4 layer and therefore also constitutes an
improved aluminium electrowinning anode according to the
invention.
The anode structure can be foraminate. For instance,
the anode structure can have any of the shapes disclosed
in the abovementioned W000/40781, W000/40782 and
WO03/006716. For example, the anode structure comprises a
series of parallel anode members, in particular
horizontal anode members having electrochemically active
surfaces in a generally coplanar arrangement to form said
active anode surface, the anode members being spaced
apart to form longitudinal flow-through openings for the
circulation of electrolyte driven by the fast escape of
anodically evolved oxygen. Typically these anode members
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are blades, bars, rods or wires. The active anode surface
can be substantially horizontal, substantially vertical
or inclined to the horizontal, for example as disclosed
in W000/40782 or W003/023092 (both de Nora).
In one embodiment, the molten electrolyte is at a
temperature below 950 C, in particular in the range from
9100 to 940 C, and consists of:
- 6.5 to 11 weight% dissolved alumina, in particular 7
to 10 weighto;
- 35 to 44 weight s aluminium fluoride, in particular 36
to 42 weight% aluminium fluoride, such as 36 to 38
weight;
- 38 to 46 weight% sodium fluoride, in particular 39 to
43 weight%;
- 2 to 15 weight% potassium fluoride, in particular 3 to
10 weight% potassium fluoride, such as 5 to 7 weight%;
- 0 to 5 weight% calcium fluoride, in particular 2 to 4
weight% calcium fluoride; and
- 0 to 5 weight% in total of one or more further
constituents, in particular up to 3 weight%.
The presence in the electrolyte of potassium
fluoride in the above amount has two effects. On the one
hand, it leads to a reduction of the operating
temperature by up to several tens of degrees without
increase of the electrolyte's aluminium fluoride content
or even a reduction thereof compared to standard
electrolytes operating at about 950 C with an aluminium
fluoride content of about 45 weight%. On the other hand,
it maintains a high solubility of alumina, i.e. up to
above about 8 or 9 weighto, in the electrolyte even
though the temperature of the electrolyte is reduced
compared to conventional temperature.
Hence, in contrast to prior art low temperature
electrolytes which carry large amounts of undissolved
alumina in particulate form, a large amount of alumina is
in a dissolved form in the above electrolyte.
Without being bound to any theory, it is believed
that combining a high concentration of dissolved alumina
in the electrolyte and a limited concentration of
aluminium fluoride leads predominantly to the formation
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of (basic) fluorine-poor aluminium oxyfluoride ions
([A1202F4] 2-) instead of (acid) fluorine-rich aluminium
oxyfluoride ions ([A120F6]2-) near the anode. As opposed
to acid fluorine-rich aluminium oxyfluoride ions, basic
fluorine-poor aluminium oxyfluoride ions do not
significantly dissolve the anode's CoO and do not
noticeably passivate or corrode metallic cobalt. The
weight ratio of dissolved alumina/aluminium fluoride in
the electrolyte should be above 1/7, and often above 1/6
or even above 1/5, to obtain a favourable ratio of the
fluorine-poor aluminium oxyfluoride ions and the
fluorine-rich aluminium oxyfluoride ions.
It follows that the use of the above described
electrolyte with metal-based anodes that contains CoO
inhibits its dissolution, passivation and corrosion.
Moreover, a high concentration of alumina dissolved in
the electrolyte further reduces dissolution of oxides of
the anode, in particular CoO.
The electrolyte may consist of: 7 to 10 weight%
dissolved alumina; 36 to 42 weight% aluminium fluoride,
in particular 36 to 38 weight%; 39 to 43 weight% sodium
fluoride; 3 to 10 weight% potassium fluoride, such as 5
to 7 weight%; 2 to 4 weight% calcium fluoride; and 0 to 3
weight% in total of one or more further constituents.
This corresponds to a cryolite-based (Na3AlF6) molten
electrolyte containing an excess of aluminium fluoride
(AlF3) that is in the range of about 8 to 15 weight% of
the electrolyte, in particular about 8 to 10 weight%, and
additives that can include potassium fluoride and calcium
fluoride in the abovementioned amounts.
The electrolyte can contain as further
constituent(s) at least one fluoride selected from
magnesium fluoride, lithium fluoride, cesium fluoride,
rubidium fluoride, strontium fluoride, barium fluoride
and cerium fluoride.
Advantageously, The electrolyte contains alumina at
a concentration near saturation on the active anode
surface.
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In order to maintain the alumina concentration above
a given threshold in the abovementioned range during
normal electrolysis, the cell is preferably fitted with
means to monitor and adjust the electrolyte's alumina
content.
The CoO-containing anode layer can be integral with
a core made of cobalt or a cobalt alloy. Such an anode
core can be made of the same materials as the Co-
containing alloys described below. The cobalt-containing
anode core can advantageously be cast.
Alternatively, the anode comprises an electrically
conductive substrate that is covered with an applied
electrochemically active coating that comprises the CoO-
containing layer.
The Co0-containing layer can be a layer of sintered
particles. In particular, the Co0-containing layer can be
formed by applying a layer of particulate CoO to the
anode and sintering. For instance, the Co0-containing
layer is applied as a slurry, in particular a colloidal
and/or polymeric slurry, and then heat treated. Good
results have been obtained by slurring particulate
metallic cobalt or CoO, optionally with additives such as
Ta, in an acqueous solution containing at least one of
ethylene glycol, hexanol, polyvinyl alcohol, polyvinyl
acetate, polyacrylic acid, hydroxy propyl methyl
cellulose and ammonium polymethacrylate and mixtures
thereof, followed by application to the anode, e.g.
painting or dipping, and heat treating.
The Co0-containing layer can be an integral oxide
layer on an applied Co-containing metallic layer of the
coating.
The Co0-containing layer can be formed by applying a
Co-containing metallic layer to the anode and subjecting
the metallic layer to an oxidation treatment to form the
CoO-containing layer on the metallic layer, the CoO-
containing layer being integral with the metallic layer.
Conveniently, the oxidation treatment can be carried
out in an oxygen containing atmosphere, such as air. The
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treatment can also be carried out in an atmosphere that
is oxygen rich or consists essentially of pure oxygen.
It is also contemplated to carry out this oxidation
treatment by other means, for instance electrolytically.
However, it was found that full formation of the CoO
integral layer cannot be achieved in-situ during
aluminium electrowinning under normal cell operating
conditions. In other words, when the anode is intended
for use in a non-carbon anode aluminium electrowinning
cell operating under the usual conditions, the anode
should always be placed into the cell with a preformed
integral oxide layer containing predominantly CoO.
As the conversion of Co(III) into Co(II) occurs at a
temperature of about 895 C, the oxidation treatment
should be carried out above this temperature. Usually,
the oxidation treatment is carried out at a treatment
temperature above 895 C or 920 C, preferably above 940 C,
in particular within the range of 950 C to 1050 C. The
Co-containing metallic layer can be heated from 'room
temperature to this treatment temperature at a rate of at
least 300 C/hour, in particular at least 450 C/hour, or
is placed in an environment, in particular in an oven,
that is preheated to said temperature. The oxidation
treatment at this treatment temperature can be carried
out for more than 8 or 12 hours, in particular from 16 to
48 hours. Especially when the oxygen-content of the
oxidising atmosphere is increased, the duration of the
treatment can be reduced below 8 hours, for example down
to 4 hours.
The Co-containing metallic layer can be further
oxidised during use. However, the main formation of CoO
is preferably achieved before use and in a controlled
manner for the reasons explained above.
The method for forming the CoO-containing layer on
the Co-containing metallic layer can be used to form the
Co0-containing layer on the previously mentioned Co-
containing anode core.
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The Co-containing metallic layer can contain
alloying metals for further reducing oxygen diffusion
and/or corrosion through the metallic layer.
In one embodiment, the anode comprises an oxygen
barrier layer between the CoO-containing layer and the
electrically conductive substrate. The oxygen barrier
layer can contain at least one metal selected from
nickel, copper, tungsten, molybdenum, tantalum, niobium
and chromium, or an oxide thereof, for example alloyed
with cobalt, such as a cobalt alloy containing tungsten,
molybdenum, tantalum and/or niobium, in particular an
alloy containing: at least one of nickel, tungsten,
molybdenum, tantalum and niobium in a total amount of 5
to 30 wto, such as 10 to 20 wt%; and one or more further
elements and compounds in a total amount of up to 5 wt%
such as 0.01 to 4 weighto, the balance being cobalt.
These further elements may contain at least one of
aluminium, silicon and manganese.
Typically, the oxygen barrier layer and the CoO-
containing layer are formed by oxidising the surface of
an applied layer of the abovementioned cobalt alloy that
contains nickel, tungsten, molybdenum, tantalum and/or
niobium. The resulting CoO-containing layer is
predominantly made of CoO and is integral with the
unoxidised part of the metallic cobalt alloy that forms
the oxygen barrier layer.
When the CoO layer is integral with the cobalt
alloy, the nickel, when present, should be contained in
the alloy in an amount of up to 20 weight%, in particular
5 to 15 weight%. Such an amount of nickel in the alloy
leads to the formation of a small amount of nickel oxide
NiO in the integral oxide layer, in about the same
proportions to cobalt as in the metallic part, i.e. 5 to
15 or 20 weight%. It has been observed that the presence
of a small amount of nickel oxide stabilises the cobalt
oxide CoO and durably inhibits the formation of Co2O3 or
Co3O4. However, when the weight ratio nickel/cobalt
exceeds 0.15 or 0.2, the advantageous chemical and
electrochemical properties of cobalt oxide CoO tend to
disappear. Therefore, the nickel content should not
exceed this limit.
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Alternatively, an oxygen barrier layer, for example
made of the above cobalt alloy that contains nickel,
tungsten, molybdenum, tantalum and/or niobium, can be
covered with an applied layer of CoO or a precursor
thereof, as discussed above. In this case the oxygen
barrier layer can be an applied layer or it can be
integral with the electrically conductive substrate.
In another embodiment, the Co-containing metallic
layer consists essentially of cobalt, typically
containing cobalt in an amount of at least 95 wt%, in
particular more than 97 wt% or 99 wt%.
Optionally the Co-containing metallic layer contains
at least one additive selected from silicon, nickel,
manganese, niobium, tantalum and aluminium in a total
amount of 0.1 to 2 wt%.
Such a Co-containing layer can be applied to an
oxygen barrier layer which is integral with the
electrically conductive substrate of the flow-through
anode structure or applied thereto.
The electrically conductive substrate can comprise.
at least one metal selected from chromium, cobalt,
hafnium, iron, molybdenum, nickel, copper, platinum,
silicon, titanium, tungsten, molybdenum, tantalum,
niobium, vanadium, yttrium and zirconium, or a compound
thereof, in particular an oxide, or a combination
thereof. For instance, the electrically conductive
substrate may have an outer part made of cobalt or an
alloy containing predominantly cobalt to which the
coating is applied. For instance, this cobalt alloy
contains nickel, tungsten, molybdenum, tantalum and/or
niobium, in particular it contains: nickel tungsten,
molybdenum, tantalum and/or niobium in a total amount of
5 to 30 wt%, e.g. 10 to 20 wt%; and one or more further
elements and/or compounds in a total amount of up to 5
wt%, the balance being cobalt. These further elements may
contain at least one of aluminium, silicon and manganese.
The electrically conductive substrate, or an outer part
thereof, may contain or consist essentially of at least
one oxidation-resistant metal, in particular one or more
metals selected from nickel, tungsten, molybdenum,
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cobalt, chromium and niobium, and for example contains
less than 1, 5 or 10 wt% in total of other metals and
metal compounds, in particular oxides. Alternatively, the
electrically conductive substrate can be made of an alloy
of nickel, iron and copper, in particular an alloy
containing: 65 to 85 weight% nickel; 5 to 25 weight%
iron; 1 to 20 weight% copper; and 0 to 10 weight% further
constituents. For example, the alloy contains about: 75
weight% nickel; 15 weight% iron; and 10 weight% copper.
Advantageously, the anode's Co0-containing layer, in
particular when the CoO layer is integral with the
applied Co-containing metallic layer or the anode body,
has an open porosity of below 12%, such as below 7%.
The anode's Co0-containing layer can have a porosity
with an average pore size below 7 micron, in particular
below 4 micron. It is preferred to provide a
substantially crack-free Co0-containing layer so as to
protect efficiently the anode's metallic outer part which
is covered by this CoO-containing layer.
Usually, the Co0-containing layer contains cobalt
oxide CoO in an amount of at least=80 wt%, in particular
more than 90 wt% or 95 wt% or 98 wt%.
Advantageously, the Co0-containing layer is
substantially free of cobalt oxide Co203 and
substantially free of Co3O4, and contains preferably
below 3 or 1.5% of these forms of cobalt oxide.
The Co0-containing layer may be electrochemically
active for the oxidation of oxygen ions during use, in
which case this layer is uncovered or is covered with an
electrolyte-pervious layer.
Alternatively, the CoO-containing layer can be
covered with an applied protective layer, in particular
an applied oxide layer such as a layer containing cobalt
and/or iron oxide, e.g. cobalt ferrite. The applied
protective layer'may contain a pre-formed and/or in-situ
deposited cerium compound, in particular cerium
oxyfluoride, as for example disclosed in the
abovementioned US patents 4,956,069, 4,960,494 and
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5,069,771. Such an applied protective layer is usually
electrochemically active for the oxidation of oxygen ions
and is uncovered, or covered in turn with an electrolyte
pervious-layer.
The anode's electrochemically active surface can
contain at least one dopant, in particular at least one
dopant selected from iridium, palladium, platinum,
rhodium, ruthenium, silicon, tungsten, molybdenum,
tantalum, niobium, tin or zinc metals, Mischmetal and
metals of the Lanthanide series, as metals and compounds,
in particular oxides, and mixtures thereof. The dopant(s)
can be present at the anode's surface in a total amount
of 0.1 to 5 wt%, in particular 1 to 4 wt%.
Such a dopant can be an electrocatalyst for
fostering the oxidation of oxygen ions on the anode's
electrochemically active surface and/or can contribute to
inhibit diffusion of oxygen ions into the anode.
The dopant may be added to the precursor material
that is applied to form the active surface or it can be
applied to the active surface as a thin film, for example
by plasma spraying or slurry application, and
incorporated into the surface by heat treatment.
The cell can have a cathode that has an aluminium-
wettable surface, in particular a horizontal or inclined
drained surface. This surface can be formed by an
aluminium-wettable material that comprises a refractory
boride and/or an aluminium-wetting oxide. Examples of
such materials are disclosed in WO01/42168, WO01/42531,
W002/070783, W002/096830 and W002/096831 (all in the name
of MOLTECH).
The anode can be suspended in the electrolyte by a
stem, in particular a stem having an outer part
comprising a layer that contains predominantly cobalt
oxide CoO.
Another aspect of the invention relates to a method
of electrowinning aluminium in a cell as described above
The method comprises electrolysing the dissolved alumina
to produce oxygen on the anode and aluminium
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cathodically, and supplying alumina to the electrolyte to
maintain therein a concentration of dissolved alumina of
6.5 to 11 weight%, in particular 7 to 10 weight%.
Oxygen ions may be oxidised on the anode's CoO-
containing layer that contains predominantly cobalt oxide
CoO and/or, when present, on an active layer applied to
the anode's CoO layer, the Co0 layer inhibiting oxidation
and/or corrosion of the anode's metallic outer part.
The invention also relates to a non-carbon metal-
based anode for the electrowinning of aluminium from
alumina dissolved in a fluoride-containing molten
electrolyte. This anode comprises an electrically
conductive metallic structure that comprises an outer
part with an=electrochemically active anode surface on
which oxygen is anodically evolved and which is suspended
in the electrolyte substantially parallel to a facing
cathode during use. This metallic structure has one or
more flow-through openings extending from the active
anode surface through the metallic structure. These flow-
through opening(s) are arranged for guiding during use a
circulation of electrolyte driven by the fast escape of
anodically evolved oxygen. The outer part of the anode
comprises the abovementioned layer that contains
predominantly cobalt oxide CoO to enhance the stability
of the anode. This anode can include any of the above
described anode features or a combination thereof.
Brief Description of the Drawings
The invention will now be described with reference
to the schematic drawings, wherein:
- Figures la and lb show respectively a side elevation
and a plan view of an anode according to the invention;
- Figures 2a and 2b show respectively a side elevation
and a plan view of another anode according to the
invention;
- Figures 3, 4, 5 and 6 show side elevations of
variations of the anode shown in Figures la and lb;
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- Figures 7 and 8 show cross-sections of multi-part anode
members according to the invention;
- Figure 9 shows an aluminium electrowinning cell
operating with anodes according to the invention fitted
with electrolyte guide members;
- Figures 10, 11 and 12 are enlarged views of parts of
variations of the electrolyte guide members shown in
Figure 9, Figure 10 illustrating cell operation;
- Figure 13 is a cross section of another anode according
to the invention with electrolyte guide members only one
of which is shown;
- Figure 14 shows a plan view of half of an assembly of
several electrolyte guide members like the one shown in
Figure 13;
- Figure 15 is a plan view of the anode shown Figure 13
with half of an assembly of electrolyte guide members as
shown in Figure 14; and
- Figure 16 is a plan view of a variation of the anode of
Figure 15.
Detailed Description
Figures la and lb schematically show an anode 10 of
a cell for the electrowinning of aluminium according to
the invention.
The anode 10 comprises a vertical current feeder 11
for connecting the anode to a positive bus bar, a cross
member 12 and a pair of transverse connecting members 13
for connecting a series of anode members 15.
The anode members 15 have an electrochemically
active lower surface 16 where oxygen is anodically
evolved during cell operation. The anode members 15 are
in the form of parallel rods in a coplanar arrangement,
laterally spaced apart from one another by inter-member
gaps 17. The inter-member gaps 17 constitute flow-through
openings for the circulation of electrolyte and the
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escape of anodically-evolved gas released at the
electrochemically active surfaces 16.
The anode members 15 are transversally connected by
the pair of transverse connecting members 13 which are in
turn connected together by the cross member 12 on which
the vertical current feeder 11 is mounted. The current
feeder 11, the cross member 12, the transverse connecting
members 13 and the anode members 15 are mechanically
secured together by welding, rivets or other means.
In accordance with the invention, the
electrochemically active surface 16 of the anode members
is formed by an outer part that comprises a layer
containing predominantly CoO. This CoO layer can form the
electrochemically active surface 16 and be directly
15 exposed to the electrolyte during use or the CoO layer
can be covered with a further layer, for instance a layer
containing predominantly a cerium compound such as cerium
oxyfluoride.
The cross-member 12 and the transverse connecting
members 13 are so designed and positioned over the anode
members 15 to provide a substantially even current
distribution through the anode members 15 to their
electrochemically active surfaces 16. The current feeder
11, the cross-member 12 and the transverse connecting
members 13 do not need to be electrochemically active and
their surface may passivate when exposed to electrolyte.
However they should be electrically well conductive to
avoid unnecessary voltage drops and should not
substantially dissolve in electrolyte. The
electrochemically-inactive current-carrying elements
(11,12,13) can have an outer part with a protective layer
containing predominantly CoO.
Figures 2a and 2b schematically show a variation of
the anode 10 shown in Figures la and lb.
Instead of having transverse connecting members 13,
a cross-member 12 and a current feeder 11 for
mechanically and electrically connecting the anode
members 15 to a positive bus bar as illustrated in
Figures la and lb, the anode 10 shown in Figures 2a and
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2b comprises a pair of cast or profiled support members
14 fulfilling the same function. Each cast support member
14 comprises a lower horizontally extending foot 14a for
electrically and mechanically connecting the anode
members 15, a stem 14b for connecting the anode 10 to a
positive bus bar and a pair of lateral reinforcement
flanges 14c between the horizontally extending foot 14a
and stem 14b.
The anode members 15 may be secured by force-fitting
or welding in the horizontal foot 14a. As an alternative,
the shape of the anode members 15 and corresponding
receiving slots in the foot 14a may be such as to allow
only longitudinal movements of the anode members. For
instance the anode members 15 and the foot 14a may be
connected by dovetail joints.
Figures 3 to 6 show a series of anodes 10 according
to the invention which are similar to the anode 10 shown
in Figures la and lb. However the cross-sections of the
anode members 15 of the anodes 10 shown in Figures 3 to 6
differ to the circular cross-section of the anode members
10 shown in Figures la and lb.
The anode members 15 of the anode shown in Figure 3
have in cross-section a generally semi-circular upper
part and a flat bottom which constitutes the
electrochemically active surface 16 of each anode member
15.
Figure 4 illustrates anode members 15 in the form of
rods which have a generally bell-shaped or pear-shaped
cross-section. The electrochemically active surface 16 of
the anode members 10 is located along the bottom of the
bell-shape or pear-shape.
The anode members 15 shown in Figure 5 are rods
having a generally rectangular cross-section. The
electrochemically active surface 16 is located along the
bottom narrow side of the rod.
Figures 6 and 7 show an anode 10 having assembled
multi-part anode members 15 comprising a first member 15b
supporting an electrochemically active second member 15a.
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The electrochemically active member 15a has an
electrochemically active surface 16 and is connected
along it whole length to the electrically well-conductive
support member 15b by an intermediate connecting member
15c such as a flange.
Figure 7 shows an enlarged view of the assembled
anode member 15 of Figure 6, comprising a generally
cylindrical electrochemically active member 15a with an
electrochemically active surface 16, a generally
cylindrical electrically conductive support member 15b
and an intermediate connecting member or flange 15c
electrically and mechanically connecting the support
member 15b to the electrochemically active member 15a.
Alternatively, the connecting member 15c may be an
extension of either the electrochemically active member
15a or the support member 15b as shown in Figure 8.
The intermediate connecting member 15c shown in
Figure 7 may be connected to the electrochemically active
member 15a and to the support member 15b by force-fitting
or welding. However, these parts may be mechanically
connected by providing a suitable geometry of the
connecting members 15c and the corresponding receiving
slots of the electrochemically active member 15a and the
support member 15b, for instance with dovetail joints.
In accordance with the invention, the
electrochemically active member 15a shown in Figures 7
and 8 has an outer part that comprises a layer containing
predominantly CoO. As mentioned above, this CoO layer can
form the electrochemically active surface 16 and be
directly exposed to the electrolyte during use or the CoO
layer can be covered with a further layer, for instance a
layer containing predominantly a cerium compound such as
cerium oxyfluoride.
The support member 15b shown in Figures 7 and 8 and
the connecting member 15c shown in Figure 7 are
preferably highly conductive and may comprise a metallic
core, for instance nickel covered with an oxidised cobalt
layer (having a CoO-based surface).
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Figure 9 shows an aluminium electrowinning cell
according to the invention having a series of generally
horizontal anodes 10 which are similar to those shown in
Figures la and lb, immersed in an electrolyte 30. The
anodes 10 face a horizontal cathode cell bottom 20
connected to a negative busbar by current conductor bars
21. The cathode cell bottom 20 is made of conductive
material such as graphite or other carbonaceous material
coated with an aluminium-wettable refractory cathodic
coating 22 on which aluminium 35 is produced and from
which it drains or on which it forms a shallow pool, a
deep pool or a stabilised pool. The molten aluminium 35
produced is spaced apart from the facing anodes 10 by an
inter-electrode gap.
Pairs of anodes 10 are connected to a positive bus
bar through a primary vertical current feeder 11' and a
horizontal current distributor 11" connected at both of
its ends to a foraminate anode 10 through a secondary
vertical current distributor il"'.
The secondary vertical current distributor 11"' is
mounted on the anode structure 12,13,15, on a cross
member 12 which is in turn connected to a pair of
transverse connecting members 13 for connecting a series
of anode members 15. The current feeders 11',11",11"',
the cross member 12, the transverse connecting members 13
and the anode members 15 are mechanically secured
together by welding, rivets or other means.
The anode members 15 have an electrochemically
active lower surface 16 on which during cell operation
oxygen is anodically evolved. The anode members 15 are in
the form of parallel rods in a foraminate coplanar
arrangement, laterally spaced apart from one another by
inter-member gaps 17. The inter-member gaps 17 constitute
flow-through openings for the circulation of electrolyte
and the escape of anodically-evolved gas from the
electrochemically active surfaces 16.
The cross-member 12 and the transverse connecting
members 13 provide a substantially even current
distribution through the anode members 15 to their
electrochemically active surfaces 16. The current feeder
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11, the cross-member 12 and the transverse connecting
members 13 do not need to be electrochemically active and
their surface may passivate when exposed to electrolyte.
However they should be electrically well conductive to
avoid unnecessary voltage drops and should not
substantially dissolve in the molten electrolyte.
In accordance with the invention, the active surface
16 of the anode members 15 can be CoO-based.
The CoO-based surface may extend over all immersed
parts 11"',12,13,15 of the anode 10, in particular over
the immersed part of the secondary vertical current
distributor 11"' which is preferably covered with a CoO-
based layer at least up to 10 cm above the surface of the
electrolyte 30.
The anodes 10 are further fitted with means for
enhancing dissolution of fed alumina in the form of
electrolyte guide members 5 formed of parallel spaced-
apart inclined baffles 5 located above and adjacent to
the foraminate anode structure 12,13,15. The baffles 5
provide upper downwardly converging surfaces 6 and lower
upwardly converging surfaces 7 that deflect gaseous
oxygen which is anodically produced below, the
electrochemically active surface 16 of the anode members
15 and which escapes between the inter-member gaps 17
through the foraminate anode structure 12,13,15. The
oxygen released above the baffles 5 promotes dissolution
of alumina fed into the electrolyte 30 above the
downwardly converging surfaces 6. Further details of such
baffles are disclosed in the abovementioned W000/40781
and W000/40782.
The aluminium-wettable cathodic coating 22 of the
cell shown in Figure 9 can advantageously be a slurry-
applied refractory hard metal coating as disclosed in US
Patent WO01/42534 (de Nora/Duruz), WO01/42531 (Nguyen/
Duruz/de Nora) and W002/096831 (Nguyen/de Nora).
During cell operation, alumina is fed to the
electrolyte 30 all over the baffles 5 and the metallic
anode structure 12,13,15. The fed alumina is dissolved
and distributed from the bottom end of the converging
-e
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surfaces 6 into the inter-electrode gap through the
inter-member gaps 17 and around edges of the metallic
anode structure 12,13,15, i.e. between neighbouring pairs
of anodes 10 or between peripheral anodes 10 and
sidewalls 25. By passing an electric current between
anodes 10 and facing cathode cell bottom 20 oxygen is
evolved on the electrochemically active anode surfaces 16
and aluminium is produced which is incorporated into the
cathodic molten aluminium 35. The oxygen evolved from the
active surfaces 16 escapes through the inter-member gaps
17 and is deflected by the upwardly converging surfaces 7
of baffles 5. The oxygen escapes from the uppermost ends
of the upwardly converging surfaces 7 enhancing
dissolution of the alumina fed over the downwardly
converging surfaces 6.
The aluminium electrowinning cells partly shown in
Figures 10, 11 and 12 are similar to the aluminium
electrowinning cell shown in Figure 9.
In Figure 10 the guide members are inclined baffles
5 as shown in Figure 9. In this example the uppermost end
of each baffle 5 is located just above mid-height between
the surface of the electrolyte 30 and the transverse
connecting members 13.
Also shown in Fig. 10, an electrolyte circulation 31
is generated by the escape of gas released from the
active surfaces 16 of the anode members 15 between the
inter-member gaps 17 and which is deflected by the upward
converging surfaces 7 of the baffles 5 confining the gas
and the electrolyte flow between their uppermost edges.
From the uppermost edges of the baffles 5, the anodically
evolved gas escapes towards the surface of the
electrolyte 30, whereas the electrolyte circulation 31
flows down through the downward converging surfaces 6,
through the inter-member gaps and around edges of the
metallic anode structure 12,13,15 to compensate the
depression created by the anodically released gas below
the active surfaces 17 of the anode members 15. The
electrolyte circulation 31 draws down into the inter-
electrode gap dissolving alumina particles 32 which are
fed above the downward converging surfaces 6.
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Figure 11 shows part of an aluminium electrowinning
cell with baffles 5 operating as electrolyte guide
members like those shown in cell of Figure 9 but whose
surfaces are only partly converging. The lower sections,4
of the baffles 5 are vertical and parallel to one
another, whereas their upper sections have upward and
downward converging surfaces 6,7. The uppermost end of
the baffles 5 are located below but close to the surface
of the electrolyte 30 to increase the turbulence at the
electrolyte surface caused by the release of anodically
evolved gas.
Figure 12 shows a variation of the baffles shown in
Figure 11, wherein parallel vertical sections 4 are
located above the converging surfaces 6,7.
By guiding and confining anodically-evolved oxygen
towards the surface of the electrolyte 30 with baffles or
other confinement means as shown in Figures 11 and 12,
oxygen is released so close to the surface as to create
turbulences above the downwardly converging surfaces 6,
promoting dissolution of alumina fed thereabove.
It is understood that the electrolyte confinement
members 5 shown in Figures 9, 10, 11 and 12 can either be
elongated baffles, or instead consist of a series of
vertical chimneys of funnels of circular or polygonal
cross-section, for instance as described below.
Figures 13 and 15 illustrate an anode 10' having a
circular bottom, the anode 10' being shown in cross-
section in Figure 5 and from above in Figure 15. On the
right hand side of Figures 13 and 15 the anode 10' is
shown with electrolyte guide members 5'. The electrolyte
guide members 5' represented in Figure 15 are shown
separately in Figure 14.
The anode 10' shown in Figures 13 and 15 has several
concentric circular anode members 15. The anode members
15 are laterally spaced apart from one another by inter-
member gaps 17 and connected together by radial
connecting members in the form of flanges 13 which join
an outer ring 13'. The outer ring 13' extends vertically
from the outermost anode members 15, as shown in Figure
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13, to form with the radial flanges 13 a wheel-like
structure 13,13', shown in Figure 15, which secures the
anode members 15 to a central anode current feeder 11.
As shown in Figure 13, the innermost circular anode
member 15 partly merges with the current feeder 11, with
ducts 18 extending between the innermost circular anode
member 15 and the current feeder 11 to permit the escape
of oxygen produced underneath the central current feeder
11.
Each electrolyte guide member 5' is in the general
shape of a funnel having a wide bottom opening 9 for
receiving anodically produced oxygen and a narrow top
opening 8 where the oxygen is released to promote
dissolution of alumina fed above the electrolyte guide
member 5'. The inner surface 7 of the electrolyte guide
member 5' is arranged to canalise and promote an upward
electrolyte flow driven by anodically produced oxygen.
The outer surface 6 of the electrolyte guide member 5' is
arranged to promote dissolution of alumina fed thereabove
and guide alumina-rich electrolyte down to the inter-
electrode gap, the electrolyte flowing mainly around the
foraminate structure.
As shown in Figures 14 and 15, the electrolyte guide
members 5' are in a circular arrangement, only half of
the arrangement being shown. The electrolyte guide
members 5' are laterally secured to one another by
attachments 3 and so arranged to be held above the anode
members 15, the attachments 3 being for example placed on
the flanges 13 as shown in Figure 15 or secured as
required. Each electrolyte guide member 5' is positioned
in a circular sector defined by two neighbouring radial
flanges 13 and an arc of the outer ring 13' as shown in
Figure 15.
The arrangement of the electrolyte guide members 5'
and the anode 10' can be moulded as units. This offers
the advantage of avoiding mechanical joints and the risk
of altering the properties of the materials of the
electrolyte guide members 5' or the anode 10' by welding.
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The anodes 10' and electrolyte guide members 5' can
be made of the same materials, in particular they can be
made of a metallic body having an outer part with a layer
containing predominantly CoO.
Figure 16 illustrates a square anode 10' as a
variation of the round anode 10' of Figures 13 and 15.
The anode 10' of Figure 16 has generally rectangular
concentric parallel anode members 15 with rounded
corners. The anode 10' shown in Figure 16 can be fitted
with electrolyte guide members similar to those of
Figures 13 to 15 but in a corresponding rectangular
arrangement.
The anodes 10' of Figures 13 and 16 can be made by
casting a metal or an alloy. Typically, an anode
substrate, for example consisting manly of nickel, can be
cast to form the anode's internal structure which is then
coated as described above to form the anode's outer part
having a layer that consists predominantly of CoO.
Alternatively, cobalt or a cobalt alloy can be cast to
form the anode's internal structure which is then
oxidised as described above to form an outer part with a
layer that consist predominantly of CoO. It is also
contemplated to cast cobalt or a cobalt alloy around an
anode core or skeleton having a different composition
than the cast cobalt or cobalt alloy, for example iron or
steel. The electrolyte guide members 5' can be
manufactured by the same method and with the same or
different materials.
Useful variations of this anode structure are
disclosed in the abovementioned W000/40782. Further
suitable anode designs are disclosed W099/027064 (de
Nora/Duruz), W001/31088, W003/006716 and W003/023092 and
US 5,368,702 (all de Nora)
The manufacturing and behaviour in an aluminium
electrowinning cell of the cobalt-oxide containing
material used for the anode of the present invention will
be further described in the following examples:
Comparative Example 1
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A cylindrical metallic cobalt sample was oxidised to
form an integral cobalt oxide layer that did not
predominantly contain CoO. The cobalt samples contained
no more than a total of 1 wt% additives and impurities
and had a diameter of 1.94 cm and a height of 3 cm.
Oxidation was carried out by placing the cobalt
sample into an oven in air and increasing the temperature
from room temperature to 850 C at a rate of 120 C/hour.
After 24 hours at 850 C, the oxidised cobalt sample
was allowed to cool down to room temperature and
examined.
The cobalt sample was covered with a greyish oxide
scale having a thickness of about 300 micron. This oxide
scale was made of: a 80 micron thick inner layer that had
a porosity of 5% with pores that had a size of 2-5
micron; and a 220 micron thick outer layer having an open
porosity of 20% with pores that had a size of 10-20
micron. The outer oxide layer was made of a mixture of
essentially Co203 and Co304. The denser inner oxide layer
was made of CoO.
As shown in Comparative Examples 2 and 3, such
oxidised cobalt provides poor results when used as an
anode material in an aluminium electrowinning cell.
Example la
A cobalt sample which can be used to manufacture for
an anode according to the invention was prepared as in
Comparative Example 1 except that the sample was oxidised
in an oven heated from room temperature to a temperature
of 950 C (instead of 850 C) at the same rate
(120 C/hour).
After 24 hours at 950 C, the oxidised cobalt sample
was allowed to cool down to room temperature and
examined.
The cobalt sample was covered with a black glassy
oxide scale having a thickness of about 350 micron
(instead of 300 micron). This oxide scale had a
continuous structure (instead of a layered structure)
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with an open porosity of 10% (instead of 20%) and pores
that had a size of 5 micron. The outer oxide layer was
made of CoO produced above 895 C from the conversion into
CoO of Co304 and glassy Co2O3 formed below this
temperature and by oxidising the metallic outer part of
the sample (underneath the cobalt oxide) directly into
CoO. The porosity was due to the change of phase during
the conversion of Co2O3 and Co3O4 to CoO.
Such a material can be used for making an aluminium
electrowinning anode according to the invention. However,
the density of the CoO layer and the performances of this
material can be further improved as shown in Examples ic
and id.
In general, to allow appropriate conversion of the
cobalt oxide and growth of CoO from the metallic outer
part of the substrate, it is important to leave the
sample sufficiently long at a temperature above 895 C.
The length of the heat treatment will depend on the
oxygen content of the oxidising atmosphere, the
temperature of the heat treatment, the desired amount of
CoO and the amount of Co2O3 and Co3O4 to convert into CoO.
Example lb
Example la was repeated with a similar cylindrical
metallic cobalt sample. The oven in which the sample was
oxidised was heated to a temperature of 1050 C (instead
of 950 C) at the same rate (120 C/hour).
After 24 hours at 1050 C, the oxidised cobalt sample
was allowed to cool down to room temperature and
examined.
The cobalt sample was covered with a black
crystallised oxide scale having a thickness of about 400
micron (instead of 350 micron) . This oxide scale had a
continuous structure with an open porosity of 20%
(instead of 10%) and pores that had a size of 5 micron.
The outer oxide layer was made of CoO produced above
895 C like in Example la.
Such a oxidised cobalt is comparable to the oxidised
cobalt of Example la and can likewise be used as an anode
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material to produce aluminium according to the present
invention.
In general, to allow appropriate conversion of the
cobalt oxide and growth of CoO from the metallic outer
part of the substrate, it is important to leave the
sample sufficiently long at a temperature above 895 C.
The length of the heat treatment above 895 C will depend
on the oxygen content of the oxidising atmosphere, the
temperature of the heat treatment, the desired amount of
CoO and the amount of Co203 and Co304 (produced below
895 C) which needs to be converted into CoO.
Example lc (improved material)
Example la was repeated with a similar cylindrical
metallic cobalt sample. The oven in which the sample was
.oxidised was heated to the same temperature (950 C) at a
rate of 360 C/hour (instead of 120 C/hour).
After 24 hours at 950 C, the oxidised cobalt sample
was allowed to cool down to room temperature and
examined.
The cobalt sample was covered with a dark grey
substantially non-glassy oxide scale having a thickness
of about 350 micron. This oxide scale had a continuous
structure with an open porosity of less than 5% (instead
of 10%) and pores that had a size of 5 micron.
The outer oxide layer was made of CoO that was
formed directly from metallic cobalt above 895 C which
was reached after about 2.5 hours and to a limited extent
from the conversion of previously formed Co2O3 and Co3O4.
It followed that there was less porosity caused by the
conversion of Co2O3 and Co304 to CoO than in Example la.
Such an oxidised cobalt sample has a significantly
higher density than the samples of Examples la and lb,
and is substantially crack-free. This oxidised cobalt
constitutes a preferred material for making an improved
aluminium electrowinning anode for use in a cell
according to the invention.
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Example id (improved material)
Example lc was repeated with a similar cylindrical
metallic cobalt sample. The oven in which the sample was
oxidised was heated to the same temperature (1050 C) at a
rate of 600 C/hour (instead of 120 C/hour in Example la
and lb and 360 C/hour in Example ic).
After 18 hours at 1050 C, the oxidised cobalt sample
was allowed to cool down to room temperature and
examined.
The cobalt sample was covered with a dark grey
substantially non-glassy oxide scale having a thickness
of about 300 micron (instead of 400 micron in Example lb
and 350 micron in Example lc). This oxide scale had a
continuous structure with a crack-free open porosity of
less than 5% (instead of 20% in Example lb) and pores
that had a size of less than 2 micron (instead of 5
micron in Example lb and in Example lc).
The outer oxide layer was made of CoO that was
formed directly from metallic cobalt above 895 C which
was reached after about 1.5 hours and to a marginal
extent from the conversion of previously formed Co203 and
Co304. It followed that there was significantly less
porosity caused by the conversion of Co203 and Co304 to
CoO than in Example lb and in Example lc.
Such an oxidised cobalt sample has a significantly
higher density than the samples of Examples la and lb,
and is substantially crack-free. This oxidised cobalt
constitutes a preferred material for making an improved
aluminium electrowinning anode according to the
invention.
Comparative Example 2 (overpotential testing)
An anode made of metallic cobalt oxidised under the
conditions of Comparative Example 1 was tested in an
aluminium electrowinning cell.
The cell's electrolyte was at a temperature of 925 C
and made of 11 wt% A1F3, 4 wt% CaF2, 7 wt% KF and 9.6 wt%
A1203, the balance being cryolite Na3AlF6.
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The anode was placed in the cell's electrolyte at a
distance of 4 cm from a facing cathode. An electrolysis
current of 7.3 A was passed from the anode to the cathode
at an anodic current density of 0.8 A/cm2.
The electrolysis current was varied between 4 and 10
A and the corresponding cell voltage measured to estimate
the oxygen overpotential at the anode.
By extrapolating the cell's potential at a zero
electrolysis current, it was found that the oxygen
overpotential at the anode was of 0.88 V.
Example 2 (overpotential testing)
A test was carried out under the conditions of
Comparative Example 2 with two anodes made of metallic
cobalt oxidised under the conditions of Example lc and
id, respectively, in cells according to the invention
using the same electrolyte as in Comparative Example 2.
The estimated oxygen overpotential for these anodes were
at 0.22 V and 0.21 V, respectively, i.e. about 75% lower
than in Comparative Example 2.
It follows that the use of metallic cobalt covered
with an integral layer of CoO instead of Co203 and Co304
as an aluminium electrowinning anode material in a cell
according to the invention leads to a significant saving
of energy.
Comparative Example 3 (aluminium electrowinning)
Another anode made of metallic cobalt oxidised under
the conditions of Comparative Example 1, i.e. resulting
in a Co203 and Co304 integral surface layer, was tested in
an aluminium electrowinning cell. The cell's electrolyte
was at 925 C and had the same composition as in
Comparative Example 2. A nominal electrolysis current of
7.3 A was passed from the anode to the cathode at an
anodic current density of 0.8 A/cm2.
The cell voltage at start-up was above 20 V and
dropped to 5.6 V after about 30 seconds. During the
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initial 5 hours, the cell voltage fluctuated about 5.6 V
between 4.8 and 6.4 V with short peaks above 8 V. After
this initial period, the cell voltage stabilised at 4.0-
4.2 V.
Throughout electrolysis, fresh alumina was fed to
the electrolyte to compensate for the electrolysed
alumina.
After 100 hours electrolysis, the anode was removed
from the cell, allowed to cool down to room temperature
and examined.
The anode's diameter had increased from 1.94 to 1.97
cm. The anode's metallic part had been heavily oxidised.
The thickness of the integral oxide scale had increased
from 350 micron to about 1.1-1.5 mm. The oxide scale was
made of: a 300-400 micron thick outer layer containing
pores having a size of 30-50 micron and having cracks; a
1-1.1 mm thick inner layer that had been formed-during
electrolysis. The inner layer was porous and contained
electrolyte under the cracks of the outer layer.
Example 3 (aluminium electrowinning)
An anode made of metallic cobalt oxidised under the
conditions of Example lc, i.e. resulting in a CoO
integral surface layer was tested in an aluminium
electrowinning cell under the conditions of Comparative
Example 3. A nominal electrolysis current of 7.3 A was
passed from the anode to the cathode at an anodic current
density of 0.8 A/cm2.
At start-up the cell voltage was 4.1 V and steadily
decreased to 3.7-3.8 V after 30 minutes (instead of 4-4.2
in Comparative Example 3). The cell voltage stabilised at
this level throughout the test without noticeable
fluctuations, unlike in Comparative Example 3.
After 100 hours electrolysis, the anode was removed
from the cell, allowed to cool down to room temperature
and examined.
The anode's external diameter did not change during
electrolysis and remained at 1.94 cm. The metallic cobalt
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.inner part underneath the oxide scale had slightly
decreased from 1.85 to 1.78 cm. The thickness of the
cobalt oxide scale had increased from 0.3 to 0.7-0.8 mm
(instead of 1-1.1 mm of Comparative Example 3) and was
made of: a non-porous 300-400 micron thick external
layer; and a porous 400 micron thick internal layer that
had been formed during electrolysis. This internal oxide
growth (400 micron thickness over 100 hours) was much
less than the growth observed in Comparative example 3
(1-1.1 mm thickness over 100 hours).
It follows that the anode's CoO integral surface
layer inhibits diffusion of oxygen and oxidation of the
underlying metallic cobalt, compared to the CoZ03 and
Co304 integral surface layer of the anode of Comparative
Example 3.
Example 4 (Variations)
The anode material of Examples la to ld, 2 and 3 can
be covered upon formation of the integral CoO layer with
a slurry applied layer, in particular containing CoFe2O4
particulate in a iron hydroxide colloid followed by
drying at 250 C to form a protective layer on the CoO
integral layer.
Example 5
A coated anode for use in a cell according to the
invention was made by covering a metallic cobalt
substrate with an applied electrochemically active
coating comprising an outer CoO layer and an inner layer
of tantalum and cobalt oxides.
The coating was formed by applying cobalt and
tantalum using electrodeposition. Specifically, tantalum
was dispersed in the form of physical inclusions in
cobalt electrodeposits.
The electrodeposition bath had a pH of 3.0 to 3.5
and contained:
- 400 g/l CoSO4.7H2O;
- 40 g/1 H3BO3;
- 40 g/1 KC1; and
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- 7-10 g/l Ta particles.
The tantalum particles had a size below 10 micron
and were dispersed in the electrodeposition bath.
Electrodeposition on the cobalt substrate was
carried out at a current density of 35 mA/cm2 which led
to a cobalt deposit containing Ta inclusions, the deposit
growing at a rate of 45 micron per hour on the substrate.
After the deposit had reached a total thickness of
250-300 micron, electrodeposition was interrupted. The
deposit contained 9-15 wt% Ta corresponding to a volume
fraction of 4-7 v%.
To form a coating, the substrate with its deposit
were exposed to an oxidation treatment at a temperature
of 950 C. The substrate with its deposit were brought
from room temperature to 950 C at a rate of 450-
500 C/hour in an oven to optimise the formation of Co0
instead of Co203 or Co304.
After 8 hours at 950 C, the substrate and the
coating that was formed by oxidation of the deposit were
taken out of the oven and allowed to cool down to room
temperature. The coating had an outer oxide layer CoO on
an inner oxide layer of Co-Ta oxides, in particular
CoTaO41 that had grown from the deposit. The innermost
part of the deposit had remained unoxidised, so that the
Co-Ta oxide layer was integral with the remaining
metallic Co-Ta deposit. The Co-Ta oxide layer and the CoO
layer had a total thickness of about 200 micron on the
remaining metallic Co-Ta.
As demonstrated in Example 6, this CoO outer layer
can act as an electrochemically active anode surface. The
inner Co-Ta oxide layer inhibits oxygen diffusion towards
the metallic cobalt substrate.
Example 6
A coated anode was made of a cobalt substrate
covered with a Co-Ta coating as in Example 5 and used in
a cell for the electrowinning aluminium according to the
invention.
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The anode was suspended in the cell's electrolyte at
a distance of 4 cm from a facing cathode. The electrolyte
contained 11 wt% AlF3, 4 wt% CaF2, 7 wt% KF and 9.6 wt o
A1203, the balance being Na3AlF6. The electrolyte was at a
temperature of 925 C.
An electrolysis current was passed from the anode to
the cathode at an anodic current density of 0.8 A/cm2.
The cell voltage remained remarkably stable at 3.6 V
throughout electrolysis.
After 150 hours electrolysis, the anode was removed
from the cell. No significant change of the anode's
dimensions was observed by visual examination.
Example 7
Example 5 was repeated by applying a Co-Ta coating
onto an anode substrate made of a metallic alloy
containing 75 wt% Ni, 15 wt% Fe and 10 wt% Cu.
The anode was tested as in Example 6 at an anodic
current density of 0.8 A/cm2. At start-up, the cell
voltage was at 4.2 V and decreased within the first 24
hours to 3.7 V and remained stable thereafter.
After 120 hours electrolysis, the anode was removed
from the cell. No sign of passivation of the nickel-rich
substrate was observed and no significant change of
dimensions of the anode was noticed by visual examination
of the anode.
Example 8
Examples 5 to 7 can be repeated by substituting
tantalum with niobium.
Example 9
Another anode for use in a cell according to the
invention was made by applying a coating of Co-W onto an
anode substrate made of a metallic alloy containing 75
wt% Ni, 15 wt% Fe and 10 wt% Cu.
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The coating was formed by applying cobalt and
tungsten using electrodeposition. The electrodeposition
bath contained:
- 100 g/l CoC12.6H20;
- 45 g/l Na2WO4.2H20;
- 400 g/l KNaC4H4O6.4H20; and
- 50 g/1 NH4C1.
Moreover, NH4OH had been added to this bath so that
the bath had reached a pH of 8.5-8.7.
Electrodeposition on the Ni-Fe-Cu substrate was
carried out at a temperature of 82-90 C and at a current
density of 50 mA/cm2 which led to a cobalt-tungsten alloy
deposit on the substrate, the deposit growing at a rate
of 35-40 micron per hour at a cathodic current efficiency
of about 90%.
After the deposit had reached a total thickness of
about 250 micron, electrodeposition was interrupted. The
deposited cobalt alloy contained 20-25 wt% tungsten.
To form a coating, the substrate with its deposit
were exposed to an oxidation treatment at a temperature
of 950 C. The substrate with its deposit were brought
from room temperature to 950 C at a rate of 450-
500 C/hour in an oven to optimise the formation of CoO
instead of Co203 or Co3O4.
After 8 hours at 950 C, the substrate and the
coating that was formed by oxidation of the deposit were
taken out of the oven and allowed to cool down to room
temperature. The coating contained at its surface cobalt
monoxide and tungsten oxide.
The structure of the coating after oxidation was
denser and more coherent than the coating obtained by
oxidising an electrodeposited layer of Ta-Co as disclosed
in Example 1.
As demonstrated in Example 10, this coating can act
as an electrochemically active anode surface. The
presence of tungsten inhibits oxygen diffusion towards
the metallic cobalt substrate.
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Example 10
An anode was made as in Example 9 and used in a cell
for the electrowinning of aluminium according to the
invention.
The anode was suspended in the cell's electrolyte at
a distance of 4 cm from a facing cathode. The electrolyte
contained 11 wt% AlF3, 4 wt% CaF2, 7 wt o KF and 9.6 wt%
A1203, the balance being Na3AlF6. The electrolyte was at a
temperature of 925 C.
An electrolysis current was passed from the anode to
the cathode at an anodic current density of 0.8 A/cm2.
The cell voltage remained stable at 3.5-3.7 V throughout
electrolysis. After 100 hours electrolysis, the anode was
removed from the cell. No change of the anode's
dimensions was observed by visual examination.
Example 11
Examples 9 and 10 can be repeated with an anode
substrate made of cobalt, nickel or an alloy of 92 wt%
nickel and 8 wt% copper.
Comparative tests show that the use in a
conventional cryolite-based electrolyte at 960 C of a
metal-based anode having an electrochemically active
outer part comprising a layer that contains predominantly
cobalt oxide CoO, leads to accelerated oxidation of the
anode and dissolution into the electrolyte of oxides of
the anode, in particular CoO. Moreover, use of such an
anode in an electrolyte at 910 -940 C without potassium
fluoride leads to corrosion or passivation the anode.
These Examples demonstrate that a material having an
outer part with a layer that contains predominantly
cobalt oxide CoO as described above, provides an enhanced
stability during use in an aluminium electrowinning cell
and is therefore suitable to protect anodes having a
flow-through structure which is exposed to the fluoride-
based molten electrolyte that is rendered more aggressive
to the anodes by its circulation through the anodes.
