U.S. patent application number 09/728581 was filed with the patent office on 2001-08-16 for slow consumable non-carbon metal-based anodes for aluminium production cells.
Invention is credited to De Nora, Vittorio, Duruz, Jean-Jacques.
Application Number | 20010013474 09/728581 |
Document ID | / |
Family ID | 26318736 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010013474 |
Kind Code |
A1 |
De Nora, Vittorio ; et
al. |
August 16, 2001 |
Slow consumable non-carbon metal-based anodes for aluminium
production cells
Abstract
A non-carbon, metal based slow-consumable anode of a cell for
the electrowinning of aluminum self-forms during normal
electrolysis an electrochemically-active oxide-based surface layer
(20). The rate of formation (35) of the layer (20) is substantially
equal to its rate of dissolution (30) at the surface
layer/electrolyte interface (25) thereby maintaining its thickness
substantially constant, forming a limited barrier controlling the
oxidation rate (35). The anode (10) usually comprises an alloy or
iron at least one of nickel, copper, cobalt or zinc which during
use forms an oxide surface layer (20) mainly containing
ferrite.
Inventors: |
De Nora, Vittorio; (Nassau,
BS) ; Duruz, Jean-Jacques; (Geneva, CH) |
Correspondence
Address: |
Jayadeep R. Deshmukh
6 Meetinghouse Court
Princeton
NJ
08540
US
|
Family ID: |
26318736 |
Appl. No.: |
09/728581 |
Filed: |
December 1, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09728581 |
Dec 1, 2000 |
|
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PCT/IB99/01358 |
Jul 30, 1999 |
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Current U.S.
Class: |
205/188 ;
204/243.1; 204/290.01; 204/290.03; 204/290.1; 204/290.12;
204/290.13; 205/333; 205/383; 205/384 |
Current CPC
Class: |
C25C 3/12 20130101 |
Class at
Publication: |
205/188 ;
204/290.01; 204/290.1; 204/290.12; 204/290.13; 204/290.03; 205/333;
204/243.1; 205/383; 205/384 |
International
Class: |
C25C 003/12; C25C
007/02; C25D 003/66; C25D 009/06; C25D 011/00 |
Claims
1. A non-carbon, metal-based slow-consumable anode of a cell for
the electrowinning of aluminium by the electrolysis of alumina
dissolved in a molten fluoride-based electrolyte, such anode
self-forming during normal electrolysis an electrochemically-active
oxide-based surface layer, the rate of formation of said layer
being substantially equal to its rate of dissolution at the surface
layer/electrolyte interface thereby maintaining its thickness
substantially constant forming a limited barrier controlling the
oxidation rate.
2. The anode of claim 1, which comprises an iron-containing alloy
which is oxidised to form the oxide-based surface layer.
3. The anode of claim 2, comprising a hematite-based surface
layer.
4. The anode of claim 3, wherein said iron-containing alloy is a
low-carbon high-strength low-alloy (HSLA).
5. The anode of claim 4, wherein the high-strength low-alloy steel
comprises 94 to 98 weight % iron and carbon, the remaining
constituents being one or more further metals selected from
chromium, copper, nickel, silicon, titanium, tantalum, tungsten,
vanadium, zirconium, aluminium, molybdenum, manganese and niobium,
and optuionally a small amount of at least one additive selected
from boron, sulfur, phosphorus and nitrogen.
6. The anode of claim 2, wherein the iron-containing alloy is
oxidised into a mixed ferrite-hematite layer forming the
oxide-based surface layer.
7. The anode of claim 2, wherein said alloy comprises cerium which
is oxidised to ceria in the formation of the oxide-based surface
layer to provide on the surface of the layer a nucleating agent for
the in-situ formation of an electrolyte-generated protective
layer.
8. The anode of claim 1, wherein the oxide-based surface layer
comprises ceramic oxides.
9. The anode of claim 1, comprising a metallic anode body or layer
which progressively forms the oxide-based surface layer on an
electronically conductive, inert, inner core.
10. The anode of claim 9, wherein the inner core is selected from
metals, alloys, intermetallic compounds, cermets and conductive
ceramics or combinations thereof.
11. The anode of claim 9, wherein the inner core is covered with an
oxygen barrier layer.
12. The anode of claim 14, wherein the oxygen barrier layer
comprises at least one oxide selected from chromium, niobium and
nickel oxide.
13. The anode of claim 12, wherein the inner core is covered with
an oxygen barrier layer which is covered in turn with at least one
protective layer consisting of copper or copper and at least one of
nickel and cobalt, and/or oxides thereof to protect the oxygen
barrier layer by inhibiting its dissolution into the
electrolyte.
14. A method of producing a non-carbon, metal-based,
slow-consumable anode according to claim 1, the method comprising
immersing an anode with an oxide-free or a pre-oxidised surface
into a molten fluoride-containing electrolyte and self-forming or
growing the electrochemically active oxide-based surface layer.
15. The method of claim 14, wherein the anode is pre-oxidised prior
to its immersion into an electrolyte where the electrolysis of
alumina takes place.
16. The method of claim 15, wherein the anode is pre-oxidised in an
oxidising atmosphere prior to its immersion into an electrolyte
where the electrolysis of alumina takes place.
17. The method of claim 15, wherein the anode is pre-oxidised in a
first molten electrolyte before being transferred in a second
molten electrolyte containing dissolved alumina for the production
of aluminium.
18. A method of restoring a non-carbon, metal-based anode according
to claim 9 when said anode is worn and/or damaged, the method
comprising clearing at least the parts of the anode which are worn
and/or damaged; reconstituting the anode; immersing it into an
electrolyte; and self-forming or growing an electrochemically
active oxide-based surface layer.
19. The method of claim 18, comprising pre-oxidising the anode
after reconstitution and immersing it into the electrolyte.
20. A cell for the electrowinning of aluminium by the electrolysis
of alumina dissolved in a molten fluoride-containing electrolyte
comprising a cathode facing at least one anode according to claim 1
which during normal electrolysis is oxidised, self-forming the
electrochemically active oxide-based surface layer.
21. The cell of claim 20, comprising an aluminium-wettable
cathode.
22. The cell of claim 21, which is in a drained configuration.
23. The cell of claim 20, which is in a bipolar configuration.
24. The cell of claim 20, wherein during operation the electrolyte
is at a temperature of 700.degree. C. to 970.degree. C.
25. A method of producing aluminium in a cell according to claim
20, comprising dissolving alumina in the electrolyte and
electrolysing the alumina-containing electrolyte to produce
aluminium on the cathode and oxygen on the facing anodes.
26. A method preparing an anode and using it for producing
aluminium in a cell for the electrowinning of aluminium by the
electrolysis of alumina dissolved in a molten fluoride-containing
electrolyte, the method comprising preparing an anode according to
the method of claim 14, and then utilising the anode to electrolyse
dissolved alumina in a molten electrolyte contained in an aluminium
electrowinning cell to produce aluminium by passing a current
between the anode and a facing cathode of the cell.
27. The method of claim 26, wherein the anode is pre-oxidised
in-situ, or in a different electrolytic cell and then transferred
to an aluminium production cell.
28. The method of claim 26, wherein the anode is pre-oxidised in an
oxygen containing atmosphere.
29. The method of claim 26, wherein after introduction of the anode
into the cell and before steady operation the rate of formation of
the anode's oxide-based surface layer is initially smaller than its
rate of dissolution, thereby decreasing the thickness of the
surface layer.
30. The method of claim 26, wherein after introduction of the anode
into the cell and before steady operation the rate of formation of
the anode's oxide-based surface layer is initially greater than its
rate of dissolution, thereby increasing the thickness of the
surface layer.
31. The method of claim 26, wherein the anode is replaced when worn
or necessary with a new anode or a restored anode.
Description
[0001] This application is a continuation of co-pending
international application designating the USA, PCT/IB99/01358,
filed on Jul. 30, 1999.
FIELD OF THE INVENTION
[0002] This invention relates to non-carbon, metal-based, slow
consumable anodes for use in cells for the electrowinning of
aluminium by the electrolysis of alumina dissolved in a molten
fluoride-containing electrolyte, and to methods for their
fabrication and reconditioning, as well as to electrowinning cells
containing such anodes and their use to produce aluminium.
BACKGROUND ART
[0003] The technology for the production of aluminium by the
electrolysis of alumina, dissolved in molten cryolite, at
temperatures around 950.degree. C. is more than one hundred years
old.
[0004] This process, conceived almost simultaneously by Hall and
Hroult, has not evolved as many other electrochemical
processes.
[0005] The anodes are still made of carbonaceous material and must
be replaced every few weeks. During electrolysis the oxygen which
should evolve on the anode surface combines with the carbon to form
polluting CO.sub.2 and small amounts of CO and fluorine-containing
dangerous gases. The actual consumption of the anode is as much as
450 Kg/Ton of aluminium produced which is more than 1/3 higher than
the theoretical amount of 333 Kg/Ton.
[0006] Using metal anodes in aluminium electrowinning cells would
drastically improve the aluminium process by reducing pollution and
the cost of aluminium production.
[0007] U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian)
describes 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 cerium to the molten cryolite electrolyte. This made it possible
to have a protection of the surface only from the electrolyte
attack and to a certain extent from the gaseous oxygen but not from
the nascent monoatomic oxygen.
[0008] EP Patent application 0 306 100 (Nyguen/Lazouni/Doan)
describes anodes composed of a chromium, nickel, cobalt and/or iron
based substrate covered with an oxygen barrier layer and a ceramic
coating of nickel, copper and/or manganese oxide which may be
further covered with an in-situ formed protective cerium
oxyfluoride layer.
[0009] Likewise, U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068
(all Nyguen/Lazouni/Doan) disclose aluminium production anodes with
an oxidised copper-nickel surface on an alloy substrate with a
protective oxygen barrier layer. However, full protection of the
alloy substrate was difficult to achieve.
[0010] Metal or metal-based anodes are highly desirable in
aluminium electrowinning cells instead of carbon-based anodes. As
mentioned hereabove, many attempts were made to use metallic anodes
for aluminium production, however they were never adopted by the
aluminium industry.
OBJECTS OF THE INVENTION
[0011] An object of the present invention is to provide a
non-carbon, metal-based anode for the electrowinning of aluminium
so as to eliminate carbon-generated pollution and reduce the
frequency of anode replacement, such an anode having an outside
layer well resistant to chemical electrolyte attack whose surface
is electrochemically active for the oxidation of oxygen ions
contained in the electrolyte and for the formation of gaseous
oxygen.
[0012] A further object of the invention is to provide a
metal-based anode capable of generating during normal electrolysis
at its surface an electrochemically active oxide layer which slowly
and progressively dissolves into the electrolyte.
[0013] A major object of the invention is to provide an anode for
the electrowinning of aluminium which has no carbon so as to
eliminate carbon-generated pollution and reduce the high cell
voltage.
SUMMARY OF THE INVENTION
[0014] The invention relates to a non-carbon, metal-based
slow-consumable anode of a cell for the electrowinning of aluminium
by the electrolysis of alumina dissolved in a molten fluoride-based
electrolyte. The anode self-forms during normal electrolysis an
electrochemically-active oxide-based surface layer, the rate of
formation of said layer being substantially equal to its rate of
dissolution at the surface layer/electrolyte interface thereby
maintaining its thickness substantially constant forming a limited
barrier controlling the oxidation rate.
[0015] In this context, metal-based anode means that the anode
contains at least one metal as such or as an alloy, intermetallic
and/or cermet.
[0016] During normal operation, the anode thus comprises a metallic
(un-oxidised) anode body (or layer) on which and from which the
oxide-based, surface layer is formed.
[0017] The electrochemically active oxide-based surface layer may
contain an oxide as such, or in a multi-compound mixed oxide and/or
in a solid solution of oxides. The oxide may be in the form of a
simple, double and/or multiple oxide, and/or in the form of a
stoichiometric or non-stoichiometric oxide.
[0018] The oxide-based surface layer has several functions. Besides
protecting in some measure the metallic anode body against chemical
attack in the cell environment and its electrochemical function for
the conversion of oxygen ions to molecular oxygen, the oxide-based
surface layer controls the diffusion of oxygen which oxidises the
anode body to further form the surface layer.
[0019] When the oxide-based surface layer is too thin, in
particular at the start-up of electrolysis, the diffusion of oxygen
towards the metallic body is such as to oxidise the metallic anode
body at the surface layer/anode body interface with formation of
the oxide-based surface layer at a faster rate than the dissolution
rate of the surface layer into the electrolyte, allowing the
thickness of the oxide-based surface layer to increase. The thicker
the oxide-based surface layer becomes, the more difficult it
becomes for oxygen to reach the metallic anode body for its
oxidation and therefore the rate of formation of the oxide-based
surface layer decreases with the increasing thickness of the
surface layer. Once the rate of formation of the oxide-based
surface layer has met its rate of dissolution into the electrolyte
an equilibrium is reached at which the thickness of the surface
layer remains substantially constant and during which the metallic
anode body is oxidised at a rate which substantially corresponds to
the rate of dissolution of the oxide-based surface layer into the
electrolyte.
[0020] In contrast to carbon anodes, in particular pre-baked carbon
anodes, the consumption of the non-carbon, metal-based anodes
according to the invention is at a very slow rate. Therefore, these
slow consumable anodes in drained cell configurations do not need
to be regularly repositioned in respect of their facing cathodes
since the anode-cathode gap does not substantially change.
[0021] To practically realise the invention, the anode body can
comprise an iron alloy which when oxidised will form an oxide-based
surface layer containing iron oxide, such as hematite or a mixed
ferrite-hematite, some of which adheres to the iron alloy,
providing a good electrical conductivity and electrochemical
activity, and a low dissolution rate in the electrolyte.
[0022] Optionally, the anode body may also comprise one or more
additives selected from beryllium, magnesium, yttrium, titanium,
zirconium, vanadium, niobium, tantalum, chromium, molybdenum,
tungsten, manganese, rhodium, silver, aluminium, silicon, tin,
hafnium, lithium, cerium and other Lanthanides.
[0023] Suitable kinds of anode materials which may be used for
forming the oxide-based surface layer comprise high-strength
low-alloy (HSLA) steels.
[0024] It has been observed that low-carbon HSLA steels such as
Cor-Ten.TM., even at high temperature, form under oxidising
conditions an iron oxide-based surface layer which is dense,
electrically conductive, electrochemically active for oxygen
evolution and, as opposed to oxide layers formed on standard steels
or other iron alloys, is highly adherent and less exposed to
delamination and limits diffusion of ionic, monoatomic and
molecular oxygen.
[0025] HSLA steels are known for their strength and resistance to
atmospheric corrosion especially at lower temperatures (below
0.degree. C.) in different areas of technology such as civil
engineering (bridges, dock walls, sea walls, piping), architecture
(buildings, frames) and mechanical engineering
(welded/bolted/riveted structures, car and railway industry, high
pressure vessels). However, these HSLA steels have never been
proposed for applications at high temperature, especially under
oxidising or corrosive conditions, in particular in cells for the
electrowinning of aluminium.
[0026] It has been found that the iron oxide-based surface layer
formed on the surface of a HSLA steel under oxidising conditions
limits also at elevated temperatures the diffusion of oxygen
oxidising the surface of the HSLA steel. Thus, diffusion of oxygen
through the surface layer decreases with an increasing thickness
thereof.
[0027] If the HSLA steel is exposed to an environment promoting
dissolution or delamination of the surface layer, in particular in
an aluminium electrowinning cell, the rate of formation of the iron
oxide-based surface layer (by oxidation of the surface of the HSLA
steel) reaches the rate of dissolution or delamination of the
surface layer after a transitional period during which the surface
layer grows or decreases to reach an equilibrium thickness in the
specific environment.
[0028] High-strength low-alloy (HSLA) steels are a group of
low-carbon steels (typically up to 0.5 weight % carbon of the
total) that contain small amounts of alloying elements. These
steels have better mechanical properties and sometimes better
corrosion resistance than carbon steels.
[0029] The surface of the high-strength low-alloy steel body may be
oxidised in an electrolytic cell or in an oxidising atmosphere, in
particular a relatively pure oxygen atmosphere. For instance the
surface of the high-strength low-alloy steel body may be oxidised
in a first electrolytic cell and then transferred to an aluminium
production cell. In an electrolytic cell, oxidation would typically
last 5 to 15 hours at 800 to 1000.degree. C. Alternatively, the
oxidation treatment may take place in air or in oxygen for 5 to 25
hours at 750 to 1150.degree. C.
[0030] In order to prevent thermal shocks causing mechanical
stresses, a high-strength low-alloy steel body may be tempered or
annealed after pre-oxidation. Alternatively, the high-strength
low-alloy steel body may be maintained at elevated temperature
after pre-oxidation until immersion into the molten electrolyte of
an aluminium production cell.
[0031] The high-strength low-alloy steel body may comprise 94 to 98
weight % iron and carbon, the remaining constituents being one or
more further metals selected from chromium, copper, nickel,
silicon, titanium, tantalum, tungsten, vanadium, zirconium,
aluminium, molybdenum, manganese and niobium, and possibly small
amounts of at least one additive selected from boron, sulfur,
phosphorus and nitrogen.
[0032] Advantageously, the anode comprises cerium which is oxidised
to ceria in the formation of the oxide-based surface layer to
provide on the surface of the oxide-based surface layer a
nucleating agent for in-situ formation of an electrolyte-generated
protective layer. Such electrolyte-generated protective layer
usually comprises cerium oxyfluoride when cerium ions are contained
in the electrolyte and may be obtained by following the teachings
of U.S. Pat. No. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) which
describes a protective anode coating of cerium oxyfluoride, formed
in-situ in the cell or pre-applied, and maintained by the addition
of small amounts of cerium to the molten electrolyte.
[0033] The oxide-based surface layer may alternatively comprise
ceramic oxides containing combinations of divalent nickel, cobalt,
magnesium, manganese, copper and zinc with divalent/trivalent
nickel, cobalt, manganese and/or iron. The ceramic oxides can be in
the form of perovskites or non-stoichiometric and/or partially
substituted or doped spinels, the doped spinels further comprising
dopants selected from the group consisting of Ti.sup.4+, Zr.sup.4+,
Sn.sup.4+, Fe.sup.4+, Hf.sup.4+, Mn.sup.4+, Fe.sup.3+, Ni.sup.3+,
Co.sup.3+, Mn.sup.3+, Al.sup.3+, Cr.sup.3+, Fe.sup.2+, Ni.sup.2+,
CO.sup.2+, Mg.sup.2+, Mn.sup.2+, Cu.sup.2+, Zn.sup.2+ and
Li.sup.+.
[0034] The anode can also comprise a metallic anode body or layer
which progressively forms the oxide-based surface layer on an
inert, inner core made of a different electronically conductive
material, such as metals, alloys, intermetallics, cermets and
conductive ceramics.
[0035] In particular, the inner core may comprise at least one
metal selected from copper, chromium, nickel, cobalt, iron,
aluminium, hafnium, molybdenum, niobium, silicon, tantalum,
tungsten, vanadium, yttrium and zirconium, and combinations and
compounds thereof. For instance, the core may consist of an alloy
comprising 10 to 30 weight % of chromium, 55 to 90 weight % of at
least one of nickel, cobalt and/or iron and up to 15 weight % of at
least one of aluminium, hafnium, molybdenum, niobium, silicon,
tantalum, tungsten, vanadium, yttrium and zirconium.
[0036] Resistance to oxygen may be at least partly achieved by
forming an oxygen barrier layer on the surface of the inner core by
surface oxidation or application of a precursor layer and heat
treatment. Known barriers to oxygen are chromium oxide, niobium
oxide and nickel oxide.
[0037] Advantageously, the inner core is covered with an oxygen
barrier layer which is in turn covered with at least one protective
layer consisting of copper, or copper and at least one of nickel
and cobalt, and/or oxide(s) thereof to protect the oxygen barrier
layer by inhibiting its dissolution into the electrolyte.
[0038] The invention also relates to a method of producing such
anodes. The method comprises immersing an anode with an oxide-free
or pre-oxidised surface into a molten fluoride-containing
electrolyte and self-forming or growing an electrochemically active
oxide-based surface layer as described hereabove.
[0039] An anode according to the invention can be restored when the
metallic anode body or layer is worn and/or damaged. The method for
restoring the anode comprises clearing and cleaning at least the
worn and/or damaged parts of the anode; reconstituting the anode
and optionally pre-oxidising the surface of the anode; immersing it
into a molten fluoride-containing electrolyte; and self-forming or
growing an electrochemically active oxide-based surface layer as
described above.
[0040] A further aspect of the invention is a cell and a method for
the electrowinning of aluminium comprising at least one anode which
during normal electrolysis is oxidised, self-forming the
electrochemically active oxide-based surface layer as described
above.
[0041] Preferably, the cell comprises an aluminium-wettable
cathode. Even more preferably, the cell is in a drained
configuration by having a drained cathode on which aluminium is
produced and from which aluminium continuously drains, as described
in U.S. Pat. Nos. 5,651,874 (de Nora/Sekhar) and 5,683,559 (de
Nora).
[0042] The cell may be of monopolar, multi-monopolar or bipolar
configuration. A bipolar cell may comprise the anodes as described
above as a terminal anode or as the anode part of a bipolar
electrode.
[0043] Preferably, the cell comprises means to improve the
circulation of the electrolyte between the anodes and facing
cathodes and/or means to facilitate dissolution of alumina in the
electrolyte. Such means can for instance be provided by the
geometry of the cell as described in co-pending application
PCT/IB99/00222 (de Nora/Duruz) or by periodically moving the anodes
as described in co-pending application PCT/IB99/00223
(Duruz/Bello).
[0044] The cell may be operated with the electrolyte at
conventional temperatures, such as 950 to 970.degree. C., or at
reduced temperatures as low as 700.degree. C.
[0045] The invention also relates to a method of producing
aluminium in a cell for the electrowinning of aluminium. The method
comprises immersing a metallic anode having an oxide-free or a
pre-oxidised surface into a molten fluoride-containing electrolyte,
self-forming an electrochemically active oxide-based surface layer
as described hereabove, and then electrolysing the dissolved
alumina to produce aluminium in the same or a different
fluoride-based electrolyte.
[0046] The surface of the anode may be in-situ or ex-situ
pre-oxidised, for instance in air or in another oxidising
atmosphere or media, or it may be oxidised in a first electrolytic
cell and then transferred into an aluminium production cell.
[0047] Another aspect of the invention is an anode comprising an
oxide-free or a pre-oxidised surface which when (further) oxidised
during cell operation as described above gives origin to the above
described self-formed, electrochemically active oxide-based surface
layer.
[0048] When the anode has a pre-oxidised surface layer which is
thicker than its thickness during steady operation, the rate of
formation of the oxide-based surface layer is initially less than
its rate of dissolution but increases to reach it. Conversely, when
the anode has an oxide-free surface or a pre-oxidised surface
forming an oxide-based layer which is thinner than its thickness
during steady operation, the rate of formation of the oxide-based
surface layer is initially greater than its rate of dissolution but
decreases to reach it.
[0049] The pre-oxidised surface layer may be of such a thickness
that after immersion into the electrolyte and during electrolysis
the thick oxide-based surface layer prevents the penetration of
nascent monoatomic oxygen beyond the oxide-based surface layer.
Therefore the mechanism for forming new oxide by further oxidation
of the anode is delayed until the existing pre-oxidised surface
layer has been sufficiently dissolved into the electrolyte at the
surface layer/electrolyte interface, no longer forming a barrier to
nascent oxygen.
[0050] Anodes made according to the invention when worn can be
replaced during normal use of a cell with new anodes or restored
anodes.
[0051] A further aspect of the invention is a method for preparing
an anode and using it for producing aluminium in a cell for the
electrowinning of aluminium by the electrolysis of alumina
dissolved in a molten fluoride-containing electrolyte, the method
comprising preparing an anode as described above, and then
utilising the anode to electrolyse dissolved alumina in a molten
electrolyte contained in an aluminium electrowinning cell to
produce aluminium by passing an ionic current between the anode and
a facing cathode of the cell.
[0052] The anode may be pre-oxidised in-situ, or in a different
electrolytic cell and then transferred to an aluminium production
cell. Alternatively, the anode may be pre-oxidised in an oxygen
containing atmosphere, such as air.
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] Reference is made to the drawings wherein:
[0054] FIGS. 1(a), 1(b) and 1(c) are schematic representations of
the evolution in time of an anode according to the invention with a
self-formed oxide-based surface layer;
[0055] FIGS. 2(a) and 2(b) are schematic representations of the
evolution in time of an anode similar to the anode shown in FIGS.
1(a), 1(b) and 1(c) which further comprises an inner metal
core.
DETAILED DESCRIPTION
[0056] FIGS. 1(a), 1(b) and 1(c) show an anode comprising a
metallic (un-oxidised) anode body 10 which is slowly consumed as a
self-formed electrochemically active oxide-based surface layer 20
progresses according to the invention when the anode is anodically
polarised in an electrolytic bath 40, such as a fluoride-based
electrolyte 40 at about 950.degree. C. containing 1 to 10%
dissolved alumina in a cell for the electrowinning of aluminium.
The anode for example comprises an alloy of iron with nickel,
copper and/or cobalt which forms an oxide-based surface layer 20
containing ferrites.
[0057] FIG. 1(a) shows part of a pre-oxidised anode according to
the invention shortly after its immersion into the electrolyte 40.
In FIG. 1(a) the anode is in a transitional period during which the
pre-oxidised surface layer 20' is grown from the metallic anode
body 10 at the surface layer/anode body interface 15 at a faster
rate than its dissolution 30 into the electrolyte 40 at the surface
layer/electrolyte interface 25, thereby progressively increasing
its thickness. The dashed line 25' shows the initial position of
the surface layer/electrolyte interface 25 at or shortly after
immersion of the anode into the electrolyte 40.
[0058] FIGS. 1(b) and 1(c) illustrate the situation where the anode
has reached its steady state of operation. The oxide-based surface
layer 20 has grown from its original thickness shown in FIG. 1(a)
to its equilibrium thickness as shown in FIGS. 1(b) and 1(c). The
rate of dissolution 30 of the surface layer 20 into the electrolyte
40 at the surface layer/electrolyte interface 25 is substantially
equal to its rate of formation 35 at the surface layer/anode body
interface 15, consuming the metallic anode body 10 at an equivalent
rate. Furthermore, the surface layer/electrolyte interface 25
slowly withdraws from its initial position 25' while the
oxide-based surface layer 20 is dissolved into the electrolyte
40.
[0059] FIGS. 2(a) and 2(b) show an anode comprising an
electronically conductive and oxidation resistant inner core 5, for
instance nickel-based, supporting a metallic anode layer 10' having
an electrochemically active oxide-based surface layer 20 as
described previously.
[0060] FIG. 2(a) illustrates the oxide-based surface layer 20 grown
from the metallic anode layer 10' at the surface layer/anode layer
interface 15. The formation rate 35 of the surface layer is equal
to its dissolution rate 30 into the electrolyte 40 as illustrated
in FIGS. 1(b) and 1(c).
[0061] In FIG. 2(b), the oxide-based surface layer 20 has
progressed until the metallic anode layer 10' covering the inner
core 5 has been nearly completely consumed. Since the inner core 5
is resistant to oxidation, further dissolution 30 of the
oxide-based surface layer is not replaced by oxidation of the inner
core once the metallic anode layer 10' has worn away. The remaining
surface layer 20 will slowly dissolve into the electrolyte 40 at
the surface layer/electrolyte interface 25 and its thickness slowly
decreases.
[0062] An anode having an oxidisable metallic anode layer 10'
covering an inner core 5 may still remain in the electrolyte 40
after its metallic anode layer 10' is completely consumed, provided
the inner core 5 is not fully passivated when exposed to oxygen,
until the oxide-based surface layer 20 is too thin to allow the
conversion of ionic oxygen to molecular oxygen. When this
conversion is no longer possible the anode needs to be extracted
and replaced or restored. However, the anode can be removed earlier
if desired.
[0063] The invention will be further described in the following
Examples.
EXAMPLE 1
[0064] Electrolysis was carried out in a laboratory scale cell
equipped with an anode according to the invention.
[0065] The anode was made with a Cor-Ten.TM. type low-carbon
high-strength (HSLA) steel doped with niobium, titanium, chromium
and copper in a total amount of less than 4 weight %, which is
commercially available from US-Steel. The anode was pre-oxidised in
air at about 1050.degree. C. for 15 hours to form a dense
hematite-based outer layer constituting an oxide-based surface
layer on an unoxidised anode body.
[0066] The anode was then tested in a fluoride-containing molten
electrolyte at 850.degree. C. containing cryolite and 15 weight %
excess of AlF.sub.3 and approximately 3 weight % alumina at a
current density of about 0.7 A/cm.sup.2.
[0067] To maintain the concentration of dissolved alumina in the
electrolyte, fresh alumina was periodically fed into the cell. The
alumina feed contained sufficient iron oxide to slow down the
dissolution of the hematite-based anode surface layer.
[0068] After 140 hours electrolysis was interrupted and the anode
extracted. Upon cooling the anode was examined externally and in
cross-section. No corrosion was observed at or near the surface of
the anode.
[0069] The produced aluminium was also analysed and showed an iron
contamination of about 700 ppm which is below the tolerated iron
contamination in commercial aluminium production.
[0070] As variations, other HSLA steel may be used as anodes, such
as a HSLA steel doped with manganese 0.4 weight %, niobium 0.02
weight %, molybdenum 0.02 weight %, copper 0.3 weight %, nickel
0.45 weight % and chromium 0.8 weight %, or a HSLA steel doped with
nickel, copper and silicon in a total amount of less than 1.5
weight %.
EXAMPLE 2
[0071] A non-carbon metal-based anode according to the invention
was obtained from a 15.times.15.times.80 mm sample of a nickel-iron
based alloy. The sample was made of cast alloy consisting of 79
weight % nickel, 10 weight % iron and 11 weight % copper.
[0072] The sample was pre-oxidised in air at about 1100.degree. C.
for 5 hours in a furnace to form the anode with a pre-oxidised
surface layer.
[0073] After pre-oxidation, the anode was immersed in molten
cryolite contained in a laboratory scale cell. The molten cryolite
contained approximately 6 weight % of dissolved alumina. Current
was passed through the anode sample at a current density of 0.5
A/cm.sup.2. After 100 hours, the anode was extracted from the cell
for analysis.
[0074] The anode was crack-free and its dimensions remained
substantially unchanged. On the surface of the anode a well
adherent oxide surface layer of a thickness of about 0.6 mm had
grown providing an adequate protection.
EXAMPLE 3
[0075] This Example illustrates the wear rate of the nickel-iron
containing anode of Example 2 and is based upon observations made
on dissolution of nickel-based samples in a fluoride-based
electrolyte.
[0076] An estimation of the wear rate is based on the following
parameters and assumptions:
[0077] With a current density of 0.7 A/cm.sup.2 and a current
efficiency of 94% an aluminium electrowinning cell produces daily
53.7 kg aluminium per square meter of active cathode surface.
[0078] Assuming a contamination of the produced aluminium by 200
ppm of nickel, which corresponds to the experimentally measured
quantities in typical tests, the wear rate of a nickel-iron sample
corresponds to approximately 1.2 micron/day. Therefore, it will
theoretically take about 80 to 85 days to wear 0.1 mm of the
anode.
* * * * *