U.S. patent application number 10/165611 was filed with the patent office on 2003-12-11 for aluminium electrowinning with metal-based anodes.
Invention is credited to Duruz, Jean-Jacques, Nora, Vittorio de.
Application Number | 20030226760 10/165611 |
Document ID | / |
Family ID | 29710478 |
Filed Date | 2003-12-11 |
United States Patent
Application |
20030226760 |
Kind Code |
A1 |
Duruz, Jean-Jacques ; et
al. |
December 11, 2003 |
Aluminium electrowinning with metal-based anodes
Abstract
A process for the electrowinning of aluminium from alumina
dissolved in a fluoride-based molten electrolyte in a cell
operating at reduced temperature, typically below 870.degree. C.,
utilising nickel-alloy based anodes, in particular nickel-iron
alloy anodes. The electrolyte contains AlF.sub.3 in such a high
concentration, usually above 20 weight %, in addition to cryolite,
that fluorine-containing ions rather than oxygen ions are oxidised
on the anodes. However, only oxygen is evolved, the evolved oxygen
being derived from the dissolved alumina present near the anodes.
The anodes may be porous at the surface so as to provide a high
active surface area for operation at low current density.
Inventors: |
Duruz, Jean-Jacques;
(Geneva, CH) ; Nora, Vittorio de; (Nassau,
BS) |
Correspondence
Address: |
JAYADEEP R. DESHMUKH
SUITE 2100
600 COLLEGE ROAD EAST
PRINCETON
NJ
08540
US
|
Family ID: |
29710478 |
Appl. No.: |
10/165611 |
Filed: |
June 8, 2002 |
Current U.S.
Class: |
205/373 ;
205/385 |
Current CPC
Class: |
C25C 3/12 20130101; C25C
3/06 20130101; C25C 3/18 20130101 |
Class at
Publication: |
205/373 ;
205/385 |
International
Class: |
C25C 003/12 |
Claims
1. A process for the electrowinning of aluminium from alumina
dissolved in a fluoride-based molten electrolyte in a cell
operating at reduced temperature and utilising metal-based anodes
comprising an alloy of nickel and an alloying metal having an outer
part consisting predominantly of nickel which forms an
electrochemically active surface for the oxidation of ions, in
which the electrolyte contains AlF.sub.3 in such a high
concentration that fluorine-containing ions predominantly rather
than oxygen ions are oxidised on the electrochemically active
surfaces, however, only oxygen is evolved, the evolved oxygen being
derived from the dissolved alumina present near the
electrochemically active anode surfaces.
2. The process of claim 1, wherein dissolved alumina predominantly
combines with oxidised fluorine ions to produce aluminium fluoride
and oxygen.
3. The process of claim 2, wherein dissolved alumina combines with
monoatomic nascent fluorine formed by oxidation of fluorine ions to
produce oxygen gas and partly dissociated aluminium fluoride.
4. The process of claim 1, wherein aluminium oxyfluoride ions
predominantly rather than oxygen ions are oxidised.
5. The process of claim 4, wherein aluminium oxyfluoride ions
resulting from the combination of aluminium fluoride and alumina
rather than oxygen ions are oxidised on the electrochemically
active surfaces into transient aluminium oxyfluoride which
decomposes into oxygen and aluminium fluoride.
6. The process of claim 1, wherein the operating temperature of the
electrolyte is below 900.degree. C., preferably below 880.degree.
C., and even more preferably below 870.degree. C.
7. The process of claim 1, wherein the electrolyte contains
cryolite and, in addition to cryolite, an excess of AlF.sub.3 in an
amount of at least 20 weight % of the electrolyte, preferably
between 25 and 35 weight % of the electrolyte.
8. The process of claim 1, wherein the electrolyte further contains
CaF.sub.2 and/or MgF.sub.2.
9. The process of claim 1, wherein said alloying metal of the
nickel alloy is iron.
10. The process of claim 1, wherein the outer part of the anode
comprises more than 75 weight % nickel, preferably between 85 and
95 weight % nickel.
11. The process of claim 1, wherein the outer part has an open
porosity defining a high surface area electrochemically active
surface, current being passed at a low current density on the high
surface area electrochemically active surface.
12. The process of claim 11, wherein part of said alloying metal of
the nickel alloy dissolves into the electrolyte to form said open
porosity.
13. The process of claim 1, comprising circulating electrolyte
containing dissolved aluminium to constantly maintain dissolved
alumina near the electrochemically active anode surfaces.
14. A cell for the electrowinning of aluminium from alumina
dissolved in a fluoride-based molten electrolyte operating at
reduced temperature and utilising metal-based anodes comprising an
alloy of nickel and an alloying metal having an outer part
consisting predominantly of nickel which forms an electrochemically
active surface for the oxidation of ions, in which the electrolyte
contains AlF.sub.3 in such a high concentration that
fluorine-containing ions predominantly rather than oxygen ions are
oxidised on the electrochemically active surfaces, however, only
oxygen is evolved, the evolved oxygen being derived from the
dissolved alumina present near the electrochemically active anode
surfaces.
15. The cell of claim 14, wherein the temperature of the
electrolyte is below 900.degree. C., preferably below 880.degree.
C., even more preferably below 870.degree. C.
16. The cell of claim 14, wherein the electrolyte contains cryolite
and, in addition to cryolite, an excess of AlF.sub.3 in an amount
of at least 20 weight % of the electrolyte, preferably between 25
and 35 weight % of the electrolyte.
17. The cell of claim 14, wherein the electrolyte further contains
CaF.sub.2 and/or MgF.sub.2.
18. The cell of claim 14, wherein said alloying metal of the nickel
alloy is iron.
19. The cell of claim 14, wherein the outer part of the anodes
comprises more than 75 weight % nickel, preferably between 85 and
95 weight % nickel.
20. The cell of claim 14, wherein the nickel alloy has a decreasing
concentration of said alloying metal towards the electrochemically
active surface layer.
21. The cell of claim 20, wherein the nickel alloy has a nickel
metal rich outer part with a porosity defining a high surface area
electrochemically active surface, said porosity containing cavities
which are partly or completely filled during use with fluorides of
at least one metal selected from nickel, said alloying metal and
aluminium.
22. The cell of claim 20, wherein the nickel metal rich outer part
comprises nickel metal and said alloying metal in a nickel/alloying
metal atomic ratio of more than 3 where it reaches the
electrochemically active surface.
23. The cell of claim 14, wherein the alloy of nickel with said
alloying metal has before use a nickel/alloying metal ratio below
1.
24. The cell of claim 14, wherein the alloy of nickel with said
alloying metal has before use a nickel/alloying metal ratio of at
least 1, in particular from 1 to 4.
25. The cell of claim 14, wherein the alloy of nickel with said
alloying metal contains one or more additives, the alloy before use
containing nickel with said alloying metal in a total amount of at
least 85 weight %, preferably at least 95 weight %, and the balance
said additive(s).
26. The cell of claim 24, wherein one or more additives are
selected from chromium, copper, cobalt, silicon, titanium,
tantalum, tungsten, vanadium, yttrium, molybdenum, manganese,
aluminium and niobium in a total amount of up to 10 weight % in
particular up to 5 weight %, of the alloy before use.
27. The cell of claim 25, wherein one or more additives are
catalytically active and selected from iridium, palladium,
platinum, rhenium, rhodium, ruthenium, tin or zinc metals,
Mischmetals and their oxides and metals of the Lanthanide series
and their oxides as well as mixtures and compounds thereof in a
total amount of up to 5 weight % of the alloy before use.
28. The cell of claim 14, wherein before anodic polarisation the
nickel alloy is covered with an integral oxide film obtainable by
oxidising the alloy in an oxidising atmosphere.
29. The cell of claim 14, wherein each anode is a nickel iron
alloy-based anode.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a process and cell for the
electrowinning of aluminium from alumina dissolved in a
fluoride-containing molten electrolyte using non-carbon,
metal-based anodes.
BACKGROUND ART
[0002] The production of aluminium since Hall and Heroult has been
carried out by dissolving the feed material consisting of pure
alumina obtained from bauxite in a cryolite-based electrolyte at
about 950.degree. C. This process has not evolved for more than one
hundred years as many other electrochemical processes.
[0003] Different types of carbon have been used as anode, cathode
and sidewall material. All attempts to utilise other materials have
failed with the exception of silicon carbide for sidewalls and more
recently TiB.sub.2 protective coatings on carbon cathodes instead
of or in addition to a thick pool of aluminium protecting the
cathodes against cryolite attack.
[0004] The carbonaceous anodes 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.
[0005] Using metal anodes in aluminium electrowinning cells would
drastically improve the aluminium process by reducing pollution and
the cost of aluminium production.
[0006] U.S. Pat. No. 4,374,050 (Ray) discloses inert anodes made of
specific multiple metal compounds which are produced by mixing
powders of the metals or their compounds in given ratios followed
by pressing and sintering, or alternatively by plasma spraying the
powders onto an anode substrate. The possibility of obtaining the
specific metal compounds from an alloy containing the metals is
mentioned.
[0007] U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian)
describes non-carbon anodes for aluminium electrowinning coated
with a protective coating of cerium oxyfluoride, formed in-situ in
the cell or pre-applied, this coating being maintained by the
addition of a cerium compound to the molten cryolite electrolyte.
This made it possible to have a protection of the surface 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. 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.
[0009] U.S. Pat. No. 4,681,671 (Duruz) discloses aluminium
production from alumina dissolved in an electrolyte between
680.degree. and 690.degree. C. in a cell utilising metal anodes
that have an electrochemically active surface whose area is
increased at least 5 times compared to conventional anodes. The
anodes are arranged for the discharge of oxide ions preferentially
to fluorine ions using a low current density at the anode. Use of
such a process with a multimonopolar arrangement of non-consumable
electrodes that are vertical or at a slope, is described in U.S.
Pat. No. 5,725,744 (Duruz/de Nora).
[0010] In Belyaev & Studentsov: Electrolysis of Alumina in
Fused Cryolite with Oxide Anodes, Legkie Metali 6 No. 3, 1937, pp.
17-24 and Belyaev: Electrolysis of Alumina with Ferrite Anodes,
Legkie Metali 7 No. 1, 1938, pp. 7-20, it has been established in
tests using anodes made of precious metals such as platinum, and
bulk ceramic oxides such as ferrites that the primary anodic
product resulting from the electrolysis of cryolite-alumina melts
is oxygen.
[0011] Metal or metal-based anodes are highly desirable in
aluminium electrowinning cells instead of carbon-based anodes. Many
attempts were made to use metallic anodes for aluminium production,
however they were never adopted by the aluminium industry because
they had a short life and contaminated the aluminium produced.
[0012] All efforts made to utilise non-carbon anodes and avoid
pollution by CO.sub.2 and organic fluorides have not succeeded
because all non-noble metal oxides, which are the only materials
commercially acceptable and resistant to oxygen, are more or less
soluble in cryolite which was chosen and is still used as the
electrolyte because it is a good solvent of oxides such as
alumina.
OBJECTS OF THE INVENTION
[0013] An object of the invention is to provide a process and cell
for aluminium electrowinning using long-lasting non-carbon anodes
so as to eliminate carbon-generated pollution.
[0014] Another object of the invention is to provide a process and
cell for aluminium electrowinning using metal-based anodes, in
which the conditions are such as to inhibit corrosion or oxidation
of the anodes.
[0015] A further object of the invention is to provide an aluminium
electrowinning process and cell with anodes having a high
electrochemical activity and a low or no solubility in the
electrolyte.
[0016] Another object of the invention is to provide an aluminium
electrowinning process and cell utilising improved metal-based
anodes made of readily available material(s).
[0017] A major object of the invention is to provide an aluminium
electrowinning process and cell using metal anodes and operating
under such conditions that the contamination of the product
aluminium is limited.
SUMMARY OF THE INVENTION
[0018] The present invention concerns an aluminium electrowinning
process in a cell containing alumina dissolved in a fluoride-based
molten electrolyte and utilising specific metal alloy-based anodes
which do not require to be made of oxides in order to be
electrochemically active and resistant to the attack of the molten
electrolyte and of oxygen gas.
[0019] Several models of anodic reactions can be considered to
explain the production of oxygen gas during the electrowinning
process of the invention, namely:
2O.sup.2--4e=C [1]
2AlO.sub.3.sup.3--6e=Al.sub.2O.sub.3+3/2O.sub.2 [2]
2AlO.sub.2.sup.--2e=Al.sub.2O.sub.3+1/2O.sub.2 [3]
2F.sup.--2e=F.sub.2;
and
2Al.sub.2O.sub.3+6F.sub.2=4AlF.sub.3+O.sub.2 [4]
F.sup.--e=F;
and
Al.sub.2O.sub.3+6F=2AlF.sub.3+O;
and
O+O=O.sub.2 [5]
2AlF.sub.6.sup.3-+Al.sub.2O.sub.3-6e=2Al.sub.2F.sub.6+3/2O.sub.2
[6]
2AlF.sub.3+Al.sub.2O.sub.3=Al.sub.2F.sub.6O.sub.2.sup.-+Al.sup.3+
or
2AlF.sub.3+AlO.sub.2.sup.-=Al.sub.2F.sub.6O.sup.2-+Al.sup.3+
or
2AlF.sub.3+O.sup.2-=Al.sub.2F.sub.6O.sup.2-;
and
Al.sub.2F.sub.6O.sup.2--2e=Al.sub.2F.sub.6O;
and
Al.sub.2F.sub.6O=Al.sub.2F.sub.6+1/2O.sub.2 [7]
[0020] Whereas mechanisms [1] to [7] have been defined in terms of
stoichiometric compounds, it is possible that corresponding
mechanisms involving non-stoichiometric compounds may occur during
electrolysis.
[0021] The present invention is based on the observation that under
specific cell operating conditions, i.e. reduced electrolysis
temperature and high fluoride content in the electrolyte, the
electrochemical oxidation reaction of oxygen ions or fluorine-free
ionic oxides to form oxygen gas, i.e. reactions [1] to [3], can be
minimised or even suppressed. Hence, the oxidation of fluorine ions
or ionic fluorine-containing compounds, i.e. reactions [4], [5],
[6] and [7], in particular the reaction involving the oxidation of
F.sup.- to nascent fluorine F and/or of aluminium oxyfluoride ions
[7], become the main or only electrochemical reactions occurring on
the electrochemically active anode surface. This inhibits direct
contact of reactive oxygen species, in particular nascent
monoatomic oxygen, with the electrochemically active surface, which
greatly reduces the risk of oxidation and corrosion of the anode by
these oxygen species.
[0022] Furthermore, it has been observed that nickel alloys, in
particular nickel-iron metal alloys, are electrochemically active
with a low overvoltage for the oxidation of fluorine ions or ionic
fluorine-containing compounds such as aluminium oxyfluoride ions
and, surprisingly, are stable and substantially do not react with
the product of the anodic electrolysis even after several hundred
hours of electrolysis under specific cell operating conditions.
[0023] The anodes used in this invention consist essentially of a
nickel alloy, in particular of a nickel-iron based alloy, and can
be used as such for efficient and successful operation in a melt
having a high concentration of aluminium fluoride and operated at
reduced temperature.
[0024] Cermet anodes which have been described in the past in
relation to aluminium production have an oxide content which forms
the major phase of the anode. Conversely, the anode according to
the invention is made predominantly of metal, possibly covered with
a thin oxide layer. For the first time, this invention permits
utilisation of a non-noble metal anode which is resistant to a
fluoride-based molten electrolyte, electrochemically active and has
a very long life the limit of which has not been determined
yet.
[0025] The invention relates to a process for the electrowinning of
aluminium from alumina dissolved in a fluoride-based molten
electrolyte in a cell operating at reduced temperature and
utilising metal-based anodes. The anodes comprise an alloy of
nickel and an alloying metal, in particular iron, having an outer
part consisting predominantly of nickel which forms an
electrochemically active surface for the oxidation of ions. In this
process the electrolyte contains AlF.sub.3 in such a high
concentration that fluorine-containing ions, such as aluminium
oxyfluoride ions, predominantly rather than oxygen ions are
oxidised on the electrochemically active surfaces. However, only
oxygen is evolved, the evolved oxygen being derived from the
dissolved alumina present near the electrochemically active anode
surfaces.
[0026] As in the fluorine oxidation reactions [4], [5], [6] and [7]
listed above, the oxidation of fluorine-containing ions covers
oxidation of ions of fluorine as such as well as ions contained in
a fluorine compound such as AlF.sub.6.sup.3- or
Al.sub.2F.sub.6O.sup.2-.
[0027] As explained below, the outer part of the nickel alloy
advantageously has an open porosity defining a high surface area
electrochemically active surface. The total amount of electrolysis
current passed between the anode and facing cathode which
corresponds to about to 0.5 to 1.5 A/cm.sup.2 at the cathode
surface of an industrial cell corresponds to a lower current
density on the high surface area electrochemically active surface.
The actual current density on the surface of the pores of the anode
is typically 5 to 50 times smaller than the corresponding density
on the cathode.
[0028] To prevent anode effects and corrosion of the anode by
fluorine-containing ions oxidised on the electrochemically active
anode surface, a sufficient concentration of dissolved alumina is
permanently present in the molten electrolyte near the
electrochemically active anode surfaces so that fluorine-containing
ions react before or after their oxidation with oxygen ions from
the dissolved alumina to evolve oxygen gas instead of fluorine.
[0029] The cell is preferably operated with a crustless and
ledgeless electrolyte, as described in co-pending application
PCT/IB99/01739 (de Nora/Duruz). To ensure sufficient dissolution of
alumina in the electrolyte at reduced temperature, the cell is
preferably fitted with an alumina spraying device to spray and
distribute alumina over substantially the entire surface of the
molten electrolyte, as disclosed in PCT/IB99/00697 (de
Nora/Berclaz). To promote circulation of molten electrolyte rich in
dissolved alumina to the electrochemically active anode surface,
the electrodes may be designed as disclosed in WO99/41429 (de
Nora/Duruz) and in PCT/IB99/01740 (de Nora). Preferably, the anodes
have a foraminate electrochemically active structure to permit
circulation of the molten electrolyte therethrough, as disclosed in
PCT/IB99/00018 (de Nora), which is advantageously fitted with a
funnel-like arrangement to guide the molten electrolyte from and to
the electrochemically active anode surfaces as disclosed in
PCT/IB99/00017 (de Nora).
[0030] Normally, the molten electrolyte contains cryolite and, in
addition to cryolite, an excess of AlF.sub.3 in an amount of at
least 20 weight % of the electrolyte typically 23 weight % or more,
preferably between 25 and 35 weight %, in particular between 27 to
30 weight %, for example about 28 weight % of the electrolyte. The
electrolyte may further contain CaF.sub.2 and/or MgF.sub.2.
[0031] The reduced temperature of the molten electrolyte should be
at 900.degree. C. or 910.degree. C. at the most, typically below
880.degree. C. and preferably below 870.degree. C., and above the
melting point of aluminium, but usually above 730.degree. C.
[0032] As stated above, the cell may advantageously be fitted with
means to circulate electrolyte containing dissolved alumina to
constantly maintain a sufficient concentration of dissolved alumina
near the electrochemically active anode surfaces.
[0033] The invention also relates to a cell for the electrowinning
aluminium from alumina dissolved in a fluoride-based molten
electrolyte operating at reduced temperature and utilising
metal-based anodes. The anodes comprise an alloy of nickel and an
alloying metal, in particular iron, having an outer part consisting
predominantly of nickel which forms an electrochemically active
surface for the oxidation of ions. The electrolyte contains
AlF.sub.3 in such a high concentration that fluorine-containing
ions, such as aluminium oxyfluoride ions, predominantly rather than
oxygen ions are oxidised on the electrochemically active surfaces,
but only oxygen is evolved, the evolved oxygen being derived from
the dissolved alumina present near the electrochemically active
anode surfaces.
[0034] Preferably, aluminium is produced on an aluminium-wettable
cathode, in particular on a drained cathode, for instance as
disclosed in U.S. Pat. No. 5,683,559 (de Nora) or in PCT
application WO99/02764 (de Nora/Duruz).
[0035] In one embodiment of the cell, each anode is a nickel-iron
alloy based anode. The anode before use has an electrochemically
active surface with an oxide film. When it is polarised in a molten
electrolyte of a cell, it becomes electrochemically active for the
oxidation of fluorine ions rather than oxygen ions. However, only
oxygen is evolved which is derived from the dissolved alumina
present near the electrochemically active anode surfaces.
[0036] Before use, the alloy of which the anode is made may have a
Ni/Fe, or more generally nickel/alloying metal, atomic ratio below
1. Alternatively, the Ni/Fe atomic ratio may be at least 1, in
particular from 1 to 4. As described below, when the outer part of
the anode is made porous by oxidation and removal of the alloying
metal, a higher content of alloying metal leads to a greater
porosity whereas a lower content of alloying metal leads to a
smaller removal and formation of a reduced porosity.
[0037] The alloy can further contain one or more additives. Before
use, the alloy may contain nickel and the alloying metal, in
particular iron, in a total amount of at least 85 weight %, in
particular at least 95 weight %, and the balance additive(s). For
example, one or more additives can be selected from chromium,
copper, cobalt, silicon, titanium, tantalum, tungsten, vanadium,
yttrium, of at least 85 weight %, in particular at least 95 weight
%, and the balance additive(s). For example, one or more additives
can be selected from chromium, copper, cobalt, silicon, titanium,
tantalum, tungsten, vanadium, yttrium, molybdenum, manganese,
aluminium and niobium in a total amount of up to 5 or even 10
weight % of the alloy before use. One or more additives may be
catalytically active for the desired reaction(s) and selected from
iridium, palladium, platinum, rhenium, rhodium, ruthenium, tin or
zinc metals, Mischmetals and their oxides and metals of the
Lanthanide series and their oxides as well as mixtures and
compounds thereof in a total amount of up to 5 weight % of the
alloy before use.
[0038] The outer part of the anodes may comprise more than 75
weight % nickel, preferably between 85 and 95 weight % nickel.
[0039] The nickel metal rich outer part typically has a porosity
defining a high surface area electrochemically active surface and
which can be obtained by oxidation in an oxidising atmosphere
before use. Usually, the porosity contains cavities which are
partly or completely filled before use with nickel and/or iron
oxides or more generally oxides of nickel and/or the alloying metal
and during use with one or more fluorine-containing compounds of at
least one metal selected from nickel, iron or other alloying metal,
and aluminium.
[0040] The porosity defining a high surface area electrochemically
active surface can alternatively be obtained or can be completed by
dissolving part of the iron or other alloying metal into the
electrolyte of the aluminium electrowinning cell, or of another
electrolytic cell and then transferred into the aluminium
electrowinning cell, this dissolution taking place usually soon
after electrolysis start-up. During use, the porosity usually
contains cavities which are partly or completely filled with
fluorides of at least one metal selected from nickel, iron or other
alloying metal and aluminium.
[0041] In one embodiment the nickel alloy underlying the
electrochemically active surface has a decreasing concentration of
iron or other alloying metal(s) towards the electrochemically
active surface layer.
[0042] The nickel metal rich outer part can comprise nickel metal
and iron or other alloying metal in a Ni/Fe or more generally
nickel/alloying metal atomic ratio of more than 3 where it reaches
the electrochemically active surface.
[0043] A suitable nickel-iron alloy based anode for such a cell can
be produced as follows. A nickel alloy substrate, in particular a
nickel-iron alloy substrate, is heat treated in an oxidising
atmosphere to form a nickel alloy based anode having an integral
thin oxide film and anodically polarised in a molten electrolyte
contained in a cell as described above, whereby fluorine-containing
ions predominantly rather than oxygen ions are oxidised on the
electrochemically active surface of the nickel-iron anode.
[0044] When the alloy is covered with a thin oxide film obtainable
by oxidation before use, during use the oxides of nickel and iron
or other alloying metal present on and possibly in the alloy
substrate originating from the oxidation treatment in the oxidising
atmosphere may be dissolved in the molten electrolyte without being
replaced, or may be substituted with one or more
fluorine-containing compounds of aluminium from the electrolyte and
of iron and nickel from the anode.
[0045] The nickel-iron or other nickel alloy substrate can be heat
treated in an oxidising atmosphere for 20 minutes to 5 hours or
even 6 hours, preferably 30 to 240 minutes, for example about 120
minutes during use, at a temperature of 900 to 1200.degree. C. It
can be heat treated in an oxidising atmosphere containing 10 to 100
molar % O.sub.2 and the balance one or more inert gases. The
nickel-iron or other nickel alloy substrate can also be heat
treated in air.
[0046] After formation of the integral oxide film, the nickel-iron
or other nickel alloy substrate may further be heat treated in an
inert atmosphere.
[0047] As nickel and cobalt behave very similarly under the above
described cell conditions, in a modification of the above aspects
of the invention, the nickel of the metal-based anodes, in
particular of their outer part, is wholly or predominantly
substituted by cobalt. For example, the anode is made from a
nickel-cobalt-iron alloy or a cobalt-iron alloy, in which case its
outer part is rich in nickel and cobalt metal, or rich in cobalt
metal only, respectively.
[0048] The invention also relates to the use of a nickel alloy, in
particular a nickel-iron alloy, which comprises a surface
electrochemically active for the oxidation of fluorine ions as an
anode of a cell for the electrowinning of aluminium from alumina
dissolved in a fluoride-based molten electrolyte. The
electrochemically active surface of the anode is a surface of the
nickel alloy as such or oxidised before or during electrolysis.
DETAILED DESCRIPTION
[0049] The invention will be further described in the following
Examples:
EXAMPLE 1
[0050] Anode Preparation:
[0051] An anode suitable for producing aluminium according to the
invention was made by pre-oxidising in air at 1100.degree. C. for
30 minutes a substrate of a nickel-iron alloy consisting of 50
weight % nickel and 50 weight % iron, to form a very thin oxide
surface film on the alloy.
[0052] The surface oxidised anode was cut perpendicularly to the
anode operative surface and the resulting section of the anode was
subjected to microscopic examination.
[0053] Before use, the anode had an external oxide surface layer
having a thickness of up to 20-25 micron. This layer in the given
example of a nickel-iron alloy consisted of an iron-rich
nickel-iron oxide and, underneath, an iron-depleted nickel-iron
alloy outer part containing generally round columnar pores filled
with iron-rich nickel-iron oxide. The pores had a diameter of about
2 to 5 micron. The nickel-iron alloy of the outer part contained
about 80-85 weight % nickel.
[0054] Underneath this outer part, the nickel-iron alloy had
remained substantially unchanged.
EXAMPLE 2
[0055] Electrolysis Testing:
[0056] An anode prepared as in Example 1 was tested in an aluminium
electrowinning cell containing a molten electrolyte at 850.degree.
C. consisting essentially of NaF and AlF.sub.3 in a weight ratio
NaF/AlF.sub.3 of about 0.7 to 0.8, i.e. an excess of AlF.sub.3 in
addition to cryolite of about 26 to 30 weight % of the electrolyte,
and approximately 3 weight % alumina. The alumina concentration was
maintained at a substantially constant level throughout the test by
adding alumina at a rate adjusted to compensate the cathodic
aluminium reduction. The test was carried out at an apparent
current density of about 0.6 A/cm.sup.2 which generally corresponds
to a current density of less than about 0.06 A/cm.sup.2 on the
surface of the pores. The electrical potential of the anode
remained substantially constant at 4.2 volts throughout the
test.
[0057] During electrolysis aluminium was cathodically produced
while fluorine and/or fluorine-containing ions, such as aluminium
oxyfluoride ions, rather than oxygen ions were oxidised on the
nickel-iron anodes. However, only oxygen was evolved which was
derived from the dissolved alumina present near the anodes.
[0058] After 72 hours, electrolysis was interrupted and the anode
was extracted from the cell. The external dimensions of the anode
had remained unchanged during the test and the anode showed no
signs of damage.
[0059] The anode was cut perpendicularly to the anode operative
surface and the resulting section of the anode was subjected to
microscopic examination, as in Example 1.
[0060] It was observed that the anode had an electrochemically
active surface covered with a discontinuous, macroporous, non
adherent iron oxide layer of the order of between 500 to 1000
micron thick, hereinafter called the "excess iron oxide layer". The
excess iron oxide layer was pervious to and contained molten
electrolyte, indicating that it had been formed during
electrolysis.
[0061] The excess iron oxide layer resulted from the excess of iron
contained in the part of the nickel-iron alloy underlying the
electrochemically active surface and which diffuses therethrough.
In other words, the excess oxide layer resulted from an iron
migration from inside to outside the anode during the
electrolysis.
[0062] Such an iron oxide layer has no or little electrochemical
activity. It slowly diffuses and dissolves into the electrolyte
until the part of the anode underlying the electrochemically active
surface reaches an iron content of about 15-20 weight %
corresponding to an equilibrium under the operating conditions at
which iron ceases to diffuse, and thereafter the layer continues to
dissolve into the electrolyte.
[0063] The anode's aforesaid outer part had been transformed during
electrolysis. Its thickness had grown from 20-25 micron to about
500 to 1000 micron and the cavities had also grown in size to
vermicular form but were only partly filled with nickel and iron
compounds. The cavities had a length of about 10 to 20 micron and a
diameter of about 2 to 5 micron. The nickel and iron oxides filling
the cavities had been fluorised to form fluoride-containing nickel
and iron ceramic compounds.
[0064] The presence of the fluoride-containing nickel and iron
ceramic compounds attests the anodic fluorine reaction, i.e.
mechanisms [4], [5], [6] and/or [7].
[0065] The cavities also contained aluminium fluoride but no
electrolyte was detected and no sign of corrosive damage appeared
throughout the anode.
[0066] Underneath the outer part, the nickel-iron alloy had
remained unchanged.
[0067] The shape and external dimensions of the anode remained
unchanged after electrolysis which demonstrated stability of this
anode structure under the operating conditions in the molten
electrolyte.
[0068] In another test a similar anode was operated under the same
conditions for several hundred hours at a substantially constant
current and cell voltage which demonstrated the long anode life
compared to known non-carbon anodes.
EXAMPLE 3
[0069] Anode Preparation:
[0070] Another anode suitable for producing aluminium according to
the invention was prepared by coating a nickel-rich nickel-iron
alloy substrate with a layer of nickel-iron alloy richer in iron,
and heat treating this coated substrate. The alloy substrate
consisted of 80 weight % nickel and 20 weight % iron. The alloy
layer consisted of about 50 weight % nickel and 50 weight %
iron.
[0071] The alloy layer was electrodeposited onto the alloy
substrate using an appropriate electroplating bath prepared by
dissolving the following constituents in deionised water at a
temperature of about 50.degree. C.:
1 a. Nickel sulfate hydrate (NiSO.sub.4.7H.sub.2O): 130 g/l b.
Nickel chloride hydrate (NiCl.sub.2.6H.sub.2O): 90 g/l c. Ferrous
sulfate hydrate (FeSO.sub.4.78H.sub.2O): 52 g/l d. Boric acid
H.sub.3BO.sub.3: 49 g/l e. 5-Sulfo-salicylic acid hydrate
(C.sub.7H.sub.6O.sub.6S.2H.sub.2O): 5 g/l f. o-Benzoic acid
sulfimide Sodium salt hydrate 3.5 g/l
(C.sub.7H.sub.4NaO.sub.3S.aq): g. 1-Undecanesulfonic acid Sodium
salt (C.sub.11H.sub.23NaO.sub.3S): 3.5 g/l
[0072] To assist dissolution, the constituents were stirred in the
deionised water.
[0073] The alloy layer was electrodeposited onto the cathodically
polarised alloy substrate from a nickel-iron alloy anode consisting
of 50 weight % nickel and 50 weight % iron, immersed in the
electroplating bath at a temperature of 50 to 55.degree. C. After 4
hours electrodeposition at a cathodic current density of 0.060
A/cm.sup.2, the deposited layer had an average thickness of about
250 to 280 micron with an average composition of 47.5 weight %
nickel and 52.5 weight % iron.
[0074] After deposition, the coated alloy substrate was surface
oxidised at 1100.degree. C. in air for 1 hour and cooled to room
temperature. The surface-oxidised anode was then cut
perpendicularly to the anode operative surface and the resulting
section of the anode was subjected to microscopic examination as in
Example 1.
[0075] It was observed that the external anode surface was covered
with iron-rich nickel-iron oxides over a thickness of about 20 to
25 micron.
[0076] The alloy layer had an iron-depleted nickel-iron alloy outer
part with a thickness of about 50 micron, this outer part
containing vermicular iron-rich nickel-iron oxide inclusions in a
nickel-iron alloy containing about 70 to 75 weight % nickel metal.
Underneath this outer part, the composition of the alloy layer had
remained substantially unchanged.
[0077] Some minor interdiffusion of iron was also observed at the
interface between the alloy layer and the alloy substrate enhancing
the adherence of the layer on the substrate.
EXAMPLE 4
[0078] Electrolysis Testing:
[0079] An anode prepared as in Example 3 was tested in an aluminium
electrowinning cell as in Example 2 except that the electrolyte
contained approximately 4 weight % alumina and that the anode was
tested during 75 hours.
[0080] During electrolysis aluminium was produced and oxygen
evolved. The anode when inspected showed no signs of having been
subjected to the usual type of oxidation/passivation mechanisms
observed with prior art process. This lead to the conclusion that
predominantly fluorine and/or fluorine-containing ions, such as
aluminium oxyfluoride ions, rather than oxygen ions were oxidised
on the nickel-iron anodes. However, only oxygen was evolved which
was derived from the dissolved alumina present near the anodes.
[0081] After electrolysis the anode was extracted from the cell and
examined.
[0082] The external surfaces of the anode were crust free and its
external dimensions were practically unchanged. No sign of damage
was visible.
[0083] The anode was cut perpendicularly to the operative surface
and the resulting section of the anode was subjected to the
microscopic examination as in Example 1.
[0084] It was observed that the anode surface was covered with an
iron rich oxide over a thickness of less than 25 to 50 micron. The
thinness of this oxide layer attested the fact that the anode had
not, or only marginally, been exposed to nascent monoatomic oxygen,
hence that the oxidation process of fluorine-containing ions was
predominant over the process of oxygen ions.
[0085] The anode's outer part (depleted in iron metal) had grown
from 50 to about 250 micron containing mainly empty pores. The
pores were vermicular with a length limited to the thickness of the
overall alloy layer and a diameter of about 10 micron. The outer
part was further depleted in iron metal and had a composition of
about 75 weight % nickel and 25 weight % iron.
[0086] The structure and composition of the alloy substrate had
remained substantially unchanged, with the exception of empty pores
of random shape having a size of about 5 to 10 micron that were
located at the substrate/layer interface and up to a depth of 100
to 150 micron. The empty pores resulted from the internal oxidation
and diffusion towards the anode's surface of iron during
electrolysis.
EXAMPLE 5
[0087] Anode Preparation:
[0088] A metallic anode consisting of an alloy of 70 weight %
nickel and 30 weight % iron was conditioned to be suitable for
electrolysis according to the invention by anodic polarisation in
an electrolytic cell. The electrolytic cell contained a molten
electrolyte at 850.degree. C. consisting essentially of NaF and
AlF.sub.3 in a weight ratio NaF/AlF.sub.3 of about 0.7 to 0.8, i.e.
an excess of AlF.sub.3 in addition to cryolite of about 26 to 30
weight % of the electrolyte. The electrolyte contained no alumina
other than that included in impurities of the added AlF.sub.3
making about 2 weight % of the electrolyte.
[0089] Before immersion into the electrolyte, the anode was
pre-heated for 0.5 hour over the cell to a temperature of about
750.degree. C.
[0090] After immersion into the conditioning electrolyte, the anode
was polarised at an initial current density of about 0.06-0.1
A/cm.sup.2 which decreased over time to less than about 0.01
A/cm.sup.2. The cell voltage was about 2.2 volt and the anode
potential was below 2 volt Thus, substantially no oxygen could be
evolved during polarisation. The current passed during polarisation
was essentially due to selective anodic dissolution of iron present
at and close to the surface of the anode.
[0091] After 24 hours, polarisation was interrupted and the anode
was extracted from the cell. The external dimensions of the anode
had remained unchanged and was covered with black oxide.
[0092] This conditioned anode was ready to be used for the
production of aluminium according to the invention. The anode's
composition was ascertained by cutting it perpendicular to the
operative surface and the resulting section of the anode was
subjected to the microscopic examination, as in Example 1.
[0093] It was observed that the anode surface was covered with a
very thin film of iron-rich oxide having a thickness of less than 1
micron. Underneath, the anode had an outer iron-depleted
nickel-iron alloy part which had an average thickness of 100 to 150
micron. This outer alloy part had vermicular pores with a diameter
of 10 to 30 micron that were empty except for small oxide
inclusions.
[0094] The average metal composition of the outer alloy part was
about 80 weight % nickel and 20 weight % iron. Below the outer
alloy part, the initial nickel-iron alloy composition had remained
substantially unchanged.
[0095] In a variation of this Example, the composition of the anode
can be changed. For instance, the starting alloy contains 30 weight
% nickel and 70 weight % iron or 80 weight % nickel and 20 weight %
iron.
[0096] A coated substrate as described in Example 3 can also be
conditioned to form an anode suitable for the production of
aluminium according to the invention by dissolving part of the iron
of the anode as described in Example 5.
[0097] All or part of the nickel content of the anodes of Examples
1, 3 and 5 can be replaced by cobalt.
EXAMPLE 6
[0098] Electrolysis Testing:
[0099] An anode as prepared in Example 5 was used in an aluminium
electrowinning cell containing a molten electrolyte as described in
Example 4.
[0100] As in Example 4, during electrolysis aluminium was produced
and oxygen evolved. The anode inspection also led to the conclusion
that fluorine-containing ions predominantly rather than oxygen ions
were oxidised on the anode surface.
[0101] After 75 hours, electrolysis was interrupted and the anode
was extracted from the cell. The external surfaces of the anode
were crust free and its external dimensions were practically
unchanged. No sign of damage was visible.
[0102] The anode was cut perpendicularly to the operative surface
and the resulting section of the anode was subjected to the
microscopic examination as in Example 1.
[0103] It was observed that the anode surface was covered with a
iron rich oxide over a thickness of less than 25 to 50 micron. The
anode surface was covered by a very thin film of iron-rich oxide
having a thickness of less than 100 micron, which indicated that
the iron depletion during electrolysis was less than for a
pre-oxidised anode as in Example 2.
[0104] The anode outer part had grown from 150 micron to about 500
to 750 micron and contained pores that were substantially empty in
their majority. Below this outer part, the alloy composition had
remained unchanged.
EXAMPLE 7
[0105] Anode Construction and Electrolysis Testing:
[0106] An anode having an active structure of 210 mm diameter was
made of three concentric rings spaced from one another by gaps of 6
mm. The rings had a generally triangular cross-section with a base
of about 19 mm and were connected to one another and to a central
vertical current supply rod by six members extending radially from
the vertical rod and equally spaced apart from one another around
the vertical rod. The gaps were covered with chimneys for guiding
the escape of anodically evolved gas to promote the circulation of
electrolyte and enhance the dissolution of alumina in the
electrolyte as disclosed in PCT publication WO00/40781 (de
Nora).
[0107] The anode and the chimneys were made from cast nickel-iron
alloy containing 50 weight % nickel and 50 weight % iron that was
heat treated as in Example 1. The anode was then tested in a
laboratory scale cell containing an electrolyte as described in
Example 2 except that it contained approximately 4 weight %
alumina.
[0108] During the test, a current of approximately 280 A was passed
through the anode at an apparent current density of about 0.8
A/cm.sup.2 on the apparent surface of the anode which generally
corresponds to a current density of less than about 0.08 A/cm.sup.2
on the surface of the columnar pores of the anode. The electrical
potential of the anode remained substantially constant at
approximately 4.2 volts throughout the test.
[0109] The electrolyte was periodically replenished with alumina to
maintain the alumina content in the electrolyte close to
saturation. Every 100 seconds an amount of about 5 g of fine
alumina powder was fed to the electrolyte. The alumina feed was
periodically adjusted to the alumina consumption based on the
cathode efficiency, which was about 67%.
[0110] As in Examples 4 and 6, during electrolysis aluminium was
produced and oxygen evolved. The anode inspection also led to the
conclusion that fluorine-containing ions predominantly rather than
oxygen ions were oxidised on the anode surface.
[0111] After more than 1000 hours, i.e. 42 days, electrolysis was
interrupted and the anode was extracted from the cell and allowed
to cool. The external dimensions of the anode had not been
substantially modified during the test but the anode was covered
with iron-rich oxide and bath. The anode showed no sign of
damage.
[0112] The anode was cut perpendicularly to the anode operative
surface and the resulting section of a ring of the active structure
was subjected to microscopic examination, as in Example 1.
[0113] It was observed that the porous outer alloy part had grown
inside the anode ring to a depth of about 7 mm leaving only an
inner part of about 5 mm diameter unchanged, i.e. consisting of a
non-porous alloy of 50 weight % nickel and 50 weight % iron. The
outer porous alloy part of the anode had a concentration of nickel
varying from 85 to 90 weight % at the anode surface to 70 to 75
weight % nickel close to the non-porous inner part, the balance
being iron. The iron depletion in the porous alloy outer part
corresponded about to the accumulation of iron present as oxide on
the surface of the anode, which indicated that the iron oxide had
not substantially dissolved into the electrolyte during the
test.
SUMMARY OF EXAMPLES
[0114] In summary, the analysis of the anodes tested in all the
above Examples showed that, at equal anode current, the oxidation
rate of nickel-alloy anodes was between about 20 and 100 times
smaller than the oxidation rate under conventional conditions in
which the oxidation of oxygen ions is the sole or the predominant
mechanism occurring at the surface of the anode, so in the above
described Examples the nickel-alloy anodes should last several
thousand hours, whereas in a normal cryolite electrolyte the anodes
last less than 50 hours.
[0115] It is believed that the greatly reduced oxidation of iron at
the anode surface under the present electrolysis conditions can
have two causes. The first possible cause of oxidation is exposure
to nascent oxygen produced by the oxidation of oxygen ions at the
anode surface which may marginally occur in parallel to the
oxidation of fluorine-containing ions and which might represent
less than 1% of the overall oxidation mechanism at the anode
surface. The second cause of oxidation is exposure to dissolved
molecular oxygen which is marginally present in the electrolyte at
a theoretical pressure of about 10.sup.-10 atm under the test
conditions.
[0116] If the surface of nickel-iron alloy anodes described above
were exposed to significant oxygen concentration in the
electrolyte, the nickel of the anode would be rapidly oxidised into
NiO which would passivate the anode and prevent electrolysis. The
absence of such oxidation/passivation confirms that no or
substantially no oxygen ions are oxidised at the surface of the
nickel-alloy anodes.
[0117] In addition, the presence of sodium-free fluorides, such as
nickel, iron and aluminium fluorides and oxyfluorides, was observed
in the pores of the tested anodes. This indicates that not
electrolyte but fluorine or fluorides from the active anode surface
penetrated into these pores, and confirms that the mechanism of
oxidation of fluorine-containing ions took place at the surface of
the anodes.
* * * * *