U.S. patent application number 10/485035 was filed with the patent office on 2005-01-06 for aluminium production cells with iron-based metal alloy anodes.
Invention is credited to De Nora, Vittorio, Duruz, Jean-Jacques, Nguyen, Thinh T..
Application Number | 20050000823 10/485035 |
Document ID | / |
Family ID | 26318630 |
Filed Date | 2005-01-06 |
United States Patent
Application |
20050000823 |
Kind Code |
A1 |
Nguyen, Thinh T. ; et
al. |
January 6, 2005 |
Aluminium production cells with iron-based metal alloy anodes
Abstract
An iron-based metal anode for the electrowinning of aluminium by
the electrolysis of alumina in a molten fluoride electrolyte has an
electrochemically active integral outside oxide layer on an
iron-based alloy that consists of 75 to 90 weight % iron; 0.5 to 5
weight % in total of at least one rare earth metal, in particular
yttrium; 1 to 10 weight % aluminium; 0 to 10 weight % copper; 0 to
10 weight % nickel; and 0.5 to 5 weight % of other elements. The
total amount of aluminium, copper and nickel is in the range from 5
to 20 weight %; and the total amount of rare earth metal(s),
aluminium and copper is also in the range from 5 to 20 weight %.
The electrochemically active surface layer is predominantly of iron
oxide that slowly dissolves into the electrolyte during operation
and is maintained by progressive slow oxidation of iron at the
interface of the bulk metal of the alloy with the oxide layer. This
progressive slow oxidation of iron corresponds to the dissolution
of iron into the electrolyte which remains at or below saturation
level at the operating temperature, the operating temperature being
maintained sufficiently low to limit the contamination of the
product aluminium to an acceptable level, and the electrolyte being
circulated to maintain a sufficient concentration of alumina in the
anode cathode gap.
Inventors: |
Nguyen, Thinh T.; (Onex,
CH) ; Duruz, Jean-Jacques; (Geneva, CH) ; De
Nora, Vittorio; (Nassau, BS) |
Correspondence
Address: |
Jayadee R Deshmukh
458 Cherry Hill Road
Princeton
NJ
08540
US
|
Family ID: |
26318630 |
Appl. No.: |
10/485035 |
Filed: |
July 28, 2004 |
PCT Filed: |
August 2, 2002 |
PCT NO: |
PCT/IB02/03088 |
Current U.S.
Class: |
205/380 ;
204/290.01; 205/387 |
Current CPC
Class: |
C25C 3/12 20130101; C25C
3/06 20130101 |
Class at
Publication: |
205/380 ;
205/387; 204/290.01 |
International
Class: |
C25C 003/06; C25C
003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 6, 2001 |
IB |
01/01453 |
Oct 3, 2001 |
IB |
01/01838 |
Claims
1. An iron-based metal anode for the electrowinning of aluminium by
the electrolysis of alumina in a molten fluoride electrolyte,
having an electrochemically active integral outside oxide layer on
an iron-based alloy that consists of: 75 to 90 weight % iron; 0.5
to 5 weight % in total of one or more rare earth metals, in
particular yttrium; 1 to 12 weight % aluminium; 0 to 10 weight %
copper; 0 to 10 weight % nickel; and 0 to 5 weight % of other
elements, wherein the total amount of aluminium, copper and nickel
is in the range from 5 to 20 weight %; and the total amount of rare
earth metal(s), aluminium and copper is in the range from 5 to 20
weight %.
2. The anode of claim 1, wherein the iron-based alloy contains
yttrium in an amount of 0.5 to 3 weight %.
3. The anode of claim 1 or 2, wherein the iron-based alloy contains
aluminium in an amount of 2 to 10 weight %, preferably 4 to 8
weight %.
4. The anode of claim 1, 2 or 3, wherein the iron-based alloy
contains copper in an amount of 0.5 to 8 weight %.
5. The anode of any preceding claim, wherein the iron-based, alloy
contains nickel in an amount of 0.5 to 8 30 weight %.
6. The anode of any preceding claim, wherein the iron-based alloy
contains, as said other element(s), at least one of molybdenum,
manganese, titanium, tantalum, tungsten, vanadium, zirconium,
niobium, chromium, cobalt, silicon and carbon.
7. The anode of any preceding claim, wherein the iron-based alloy
contains said other element(s) in an amount up to 2 weight %.
8. The anode of any preceding claim, wherein the iron-based alloy
contains copper in an amount of 2 to 6 weight % and/or nickel in an
amount of 2 to 8 weight %.
9. The anode of any preceding claim, wherein the iron-based alloy
contains aluminium in an amount of 4 to 6 weight %.
10. The anode of any preceding claim, wherein the total amount of
aluminium, copper and nickel is in the range from 8 to 18 weight
%.
11. The anode of any preceding claim, wherein the total amount of
rare earth metal, aluminium and copper is in the range from 8 to 18
weight %.
12. The anode of any preceding claim, wherein the iron-based alloy
consists of: 80 to 90 weight % iron; 0.5 to 3 weight % yttrium; 2
to 6 weight % aluminium; 1 to 8 weight % copper; 1 to 8 weight %
nickel; and 0 to 5 weight % of other elements.
13. The anode of any preceding claim wherein the iron-based alloy
contains copper and nickel in a weight ratio Cu:Ni in the range 1:3
to 3:1.
14. The anode of any preceding claim, wherein the iron-based alloy
is made by casting iron together with said metals as additives.
15. A cell for the electrowinning of aluminium by the electrolysis
of alumina in a molten fluoride electrolyte utilising an iron-based
metal anode having an ectrochemically active integral outside oxide
layer according to any one of the preceding claims.
16. The cell of claim 15, wherein during operation the
electrochemically active integral outside oxide layer of the anode
slowly dissolves into the electrolyte and is maintained by
progressive slow oxidation of iron at the interface of the metal
bulk of the alloy with the oxide layer.
17. The cell of claim 15 or 16, wherein the concentration of
alumina dissolved in the electrolyte is below 10 weight %,
preferably between 5 weight % and 8 weight %.
18. The cell of claim any one of claims 1 to 17, comprising an
aluminium-wettable cathode.
19. The cell of any one of claims 16 to 18, wherein the progressive
slow oxidation of iron at the interface of the bulk of the alloy
with the oxide layer corresponds to the dissolution of iron into
the electrolyte at a rate such that the maximum concentration of
iron species in the electrolyte is at or below the saturation level
of iron species in the electrolyte at the operating
temperature.
20. The cell of claim 19, wherein the operating temperature is
maintained sufficiently low to control the dissolution of iron into
the electrolyte and limit the contamination of the product
aluminium to an acceptable level.
21. The cell of claim 20, wherein the operating temperature is
below 930.degree. C., preferably between 840.degree. C. and
22. The cell of any one of claims 15 to 21, wherein the electrolyte
contains NaF and AlF.sub.3 in a molar ratio in the range from 1.2
to 2.4.
23. The cell of any one of claims 15 to 22, which is arranged to
circulate alumina-depleted electrolyte away from the
electrochemically active oxide layer of the anode(s), enrich the
electrolyte with alumina, and circulate alumina-enriched
electrolyte towards the electrochemically active oxide layer of the
anode(s).
24. A method for the electrowinning of aluminium by the
electrolysis of alumina in a molten fluoride electrolyte,
comprising dissolving alumina in the electrolyte and electrolysing
the alumina-containing electrolyte to produce aluminium on a
cathode and oxygen on an iron-based metal anode as claimed in any
one of claims 1 to 14.
25. The method of claim 24, wherein the electrochemically active
integral outside oxide layer of the anode slowly dissolves into the
electrolyte and is maintained by progressive slow oxidation of iron
at the interface of the metal bulk of the alloy with the oxide
layer providing a dissolution of iron into the electrolyte at a
rate such that the maximum concentration of iron species in the
electrolyte is at or below the saturation level of iron species in
the electrolyte at the operating temperature.
26. The method of claim 24 or 25, wherein the operating temperature
is maintained sufficiently low to control the dissolution of iron
into the electrolyte and limit the contamination of the product
aluminium to an acceptable level.
27. The method of claim 26, wherein the operating temperature is
below 930.degree. C., preferably between 840.degree. C. and
890.degree. C.
28. The method of any one of claims 24 to 27, wherein the
electrolyte contains NaF and AlF.sub.3 in a molar ratio in the
range from 1.2 to 2.4.
29. The method of any one of claims 24 to 28, wherein the
concentration of alumina dissolved in the electrolyte is below 10
weight %, preferably between 5 weight % and 8 weight %.
30. The method of any one of claims 24 to 29, wherein
alumina-depleted electrolyte is circulated away from the
electrochemically active. oxide layer of the anode(s), enriched
with alumina, and alumina-enriched electrolyte is circulated
towards the electrochemically active oxide layer of the
anode(s).
31. The method of any one of claims 24 to 30, wherein aluminium is
produced on an aluminium-wettable cathode.
32. The method of claim 31, wherein the product aluminium is
continuously drained from the aluminium-wettable cathode.
Description
FIELD OF THE INVENTION
[0001] This invention relates to iron-based metal anodes for the
electrowinning of aluminium by the electrolysis of alumina
dissolved in a molten fluoride-containing electrolyte and to a cell
and method for the electrowinning of aluminium using such
iron-based metal anodes.
BACKGROUND ART
[0002] Using non-carbon anodes in aluminium electrowinning cells
should drastically improve the aluminium production process by
reducing pollution and the cost of aluminium production. Many
attempts have been made to use oxide anodes, cermet anodes and
metal-based anodes for aluminium production, however they were
never adopted by the aluminium industry.
[0003] U.S. Pat. No. 6,248,227 (de Nora/Duruz) discloses a
non-carbon, metal-based slow-consumable anode of a cell for the
electrowinning of aluminium that self-forms during normal
electrolysis an electrochemically-active oxide-based surface layer.
The rate of formation of this layer is maintained substantially
equal to its rate of dissolution at the surface layer/electrolyte
interface thereby maintaining its thickness substantially
constant.
[0004] In some embodiments, the anode body comprised an iron alloy,
in particular an HSLA steel comprising 94 to 98 weight % iron with
small amounts of alloying elements and less than 0.5 weight %
carbon, which when oxidised formed an oxide-based surface layer
containing iron oxide, such as hematite or a mixed
ferrite-hematite. It was disclosed that the anode body may comprise
one or more additives selected from beryllium, magnesium, yttrium,
titanium, zirconium, vanadium, niobium, tantalum, chromium,
molybdenum, tungsten, manganese, rhodium, silver, aluminium,
copper, nickel, silicon, tin, hafnium, lithium, cerium and other
Lanthanides.
[0005] A different approach was taken in WO 00/06802. (Duruz/de
Nora/Crottaz) where anodes comprising a transition metal-based
oxide active surface of iron oxide, cobalt oxide, nickel oxide or
combinations thereof, were kept dimensionally stable during
electrolysis by continuously or intermittently feeding to the
electrolyte a sufficient amount of alumina and transition metal
species that are present as oxides at the anode surface. The cell
was operated at a sufficiently low temperature so as to limit the
solubilisation of the transition metal species.
[0006] WO 00/40783 (de Nora/Duruz) further describes the use of
HSLA steel with a coherent and adherent oxide surface as an anode
for aluminium electrowinning, preferably using an external supply
of iron to maintain the anode surface as described in WO
00/06802.
[0007] Nickel-iron alloy anodes with various additives are further
described in WO 00/06803 (Duruz/de Nora/Crottaz), WO 00/006804
(Crottaz/Duruz), WO 01/42534 (de Nora/Duruz), WO 01/42535,
(Duruz/de Nora) and WO 01/42536 (Duruz/Nguyen/de Nora).
[0008] Despite the progress achieved, there is still a need, in
particular with iron-rich steels or alloys (>75% iron), to
reduce contamination of the product aluminium. For this reason,
most effort was directed to alloys with lower iron content, such as
60-40 weight % iron-nickel and 65-35 weight % iron-nickel: see the
examples in the above-mentioned mentioned patent publications on
nickel-iron alloy anodes.
SUMMARY OF THE INVENTION
[0009] An object of the invention is to provide an iron-based metal
anode with an iron-rich alloy having an integral outside oxide
layer which can be progressively formed during use at a rate
corresponding to a controlled dissolution into the electrolyte at
the operating temperature, or which can even be stabilised by
maintaining an amount of iron species in the electrolyte, leading
to an acceptably low contamination of the product aluminum.
[0010] According to the invention, an iron-based metal anode for
the electrowinning of aluminium by the electrolysis of alumina in a
molten fluoride electrolyte has an electrochemically active
integral outside oxide layer on an iron-based alloy that consists
of:
[0011] 75 to 90 weight % iron, preferably 80 to 90 weight %;
[0012] 0.5 to 5 weight % in total of one or more rare earth metals,
in particular yttrium preferably in an amount of 0.5 to 3 weight
%;
[0013] 1 to 12 weight % aluminium, usually 2 to 10 weight % and
preferably 4 to 8 weight %;
[0014] 0 to 10 weight % copper, preferably 0.5 to 8; weight %;.
[0015] 0 to 10 weight % nickel, preferably 0.5 to 8 weight %;
and
[0016] 0 to 5 weight % of other elements, usually at least one of
molybdenum, manganese, titanium, tantalum, tungsten, vanadium,
zirconium, niobium, chromium, cobalt, silicon and carbon, and
preferably up to 2 weight %.
[0017] In this iron-based alloy according to the invention, the
total amount of aluminium, copper and nickel is in the range from 5
to 20 weight %, and the total amount of rare earth metal(s),
aluminium and copper is also in the range from 5 to 20 weight
%.
[0018] The electrochemically active oxide-based surface layer on
the iron-based metal anode is predominantly iron oxide, in the form
of hematite, or in a multi-compound mixed oxide and/or in solid
solution of oxides, depending on the additive metals included in
the bulk of the alloy. The oxide can be in the form of a simple,
double and/or multiple oxide, and/or in the form of a
stoichiometric or non-stoichiometric oxide.
[0019] Suitable rare earth metals include Actinides, such as
scandium or yttrium, and Lanthanides, such as cerium and ytterbium.
The preferred rare earth metal is yttrium and preferably the
iron-based metal anode contains yttrium in an amount of 0.5 to 3
weight %.
[0020] The rare earth metals--which are substantially insoluble in
iron--are present in the grain boundaries of the metal bulk of the
anode in an amount which provides during use controlled diffusion
of oxygen into the metal bulk, and hence the controlled oxidation
and dissolution rate of the anode. When the iron alloy is cast, the
presence of the rare earth metal refines the structure of the alloy
by reducing the grain size, for example from about 0.5-1 cm to
about 50-100 micron when yttrium is used as an additive.
[0021] Such a rare earth metal migrates predominantly to the grain
boundaries of the iron or iron alloy and acts as a barrier against
diffusion of oxygen. At the grain boundaries, the rare earth metals
can be present before oxidation as a substantially distinct metal
phase and after oxidation as oxides, in particular mixed oxides
with iron and the other alloying metals. To be effective, oxidation
of the rare earth metal should be avoided during casting before it
has reached the grain boundaries.
[0022] When cerium is included as a rare earth (preferably in
combination with yttrium), it 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. 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. 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.
[0023] The further metals in the iron-rich alloy include aluminium
and usually at least one of copper and/or nickel.
[0024] Aluminium, copper and nickel are soluble in iron and can
form alloys therewith, and in addition may form intermetallic
compounds or mixed oxides with the rare earth metals.
[0025] The presence of aluminium in an amount up to 10 or 12 weight
%, normally up to about 8 weight % of the iron-rich alloy and
preferably from 2 to 6 weight %, has the effect of controlling the
oxidation of the bulk iron by reinforcing the oxygen barrier at the
grain boundaries through forming stable intermetallics with the
rare earths.
[0026] The inclusion of copper in an amount up to 10 weight %,
normally from 1 to 8 weight % of the iron-rich alloy, has the
effect of improving the compactness of the oxide layer formed,
thereby reducing its imperviousness and improving its resistance to
further oxidation. The migration of copper to the surface inhibits
the formation of a non-conductive layer of fluoride compounds such
as NiF.sub.2 on the surface of the iron bulk under the desired
hematite layer which is dense and protective, and further reduces
the inward migration of oxygen.
[0027] The inclusion of nickel in amounts up to 10 weight %
stabilises the iron against oxidation by the formation of stable
intermetallics with aluminium and the rare earth metals in
particular Yttrium.
[0028] In embodiments with; copper and nickel, the weight ratio of
copper to nickel is preferably in the range 1:3 to 3:1. The
combination of copper with nickel, in particular copper from 2 to 6
weight % and nickel from 2 to 8 weight %, produces copper-nickel
alloys that inhibit the formation of unwanted nickel fluoride
(NiF.sub.2).
[0029] Usually the total amount of aluminium, copper and nickel is
in the range from 8 to 18 weight %, and the total amount of rare
earth metal, aluminium and copper is also in the range from 8 to 18
weight %. n one embodiment of the anode, the iron-based alloy
consists of:
[0030] 180 to 90 weight % iron;
[0031] 0.5 to weight % yttrium;
[0032] 2 to 6 weight % aluminium;
[0033] 1 to 8 weight % copper;
[0034] 1 to 8 weight % nickel; and
[0035] 0 to 5 weight % (usually 0.5 to 2 weight %) of other
elements, subject to the aforesaid minimum and maximum combined
amounts of the groups of additives.
[0036] Possible additives constituting these other alloying
elements in amounts up to 5 weight % and preferably below 2 weight
% in total of the iron-based alloy, include:
[0037] molybdenum (usually up to 1 weight %);
[0038] manganese, titanium, tantalum, tungsten, vanadium,
zirconium, and niobium (usually each up to 1 weight %);
[0039] chromium and cobalt (usually each up to 2 weight %);
[0040] silicon up to about 2 weight %;
[0041] as well as traces of carbon and of the usual impurities.
[0042] In a variation of the invention, an iron-based metal anode
for the electrowinning of aluminium by the electrolysis of alumina
in a molten fluoride electrolyte has an electrochemically active
integral outside oxide layer on an iron-based alloy that consists
of:
[0043] 75 to 90 weight % iron;
[0044] 1 to 12 weight % aluminium, usually 2 to 10 weight %;
[0045] 1 to 10 weight % copper;
[0046] 1 to 10 weight % nickel; and
[0047] 1 to 10 weight % of one or more additional elements selected
from:
[0048] 0 to 5 weight % in: total of one or more rare earth metals,
in particular yttrium; and
[0049] 0 to 5 weight % of other elements, as listed above.
[0050] In this modified composition and the previous compositions,
the total amount of copper and nickel is preferably at least 4
weight %; the weight ratio of copper to nickel is in the range 1:3
to 3:1; and the weight ratio of the total amount of (a) copper and
nickel to (b) the total amount of said additional elements is in
the range (a):(b) from 20:1 to 4:10; preferably from 10:1 to
1:6.
[0051] The anode is preferably made by casting iron containing the
specified metals as additives, i.e. where the final anode shape is
produced by casting the molten iron with additives in a mould,
usually a sand mould. As mentioned above, when the iron alloy is
cast, the presence of a rare earth metal refines the structure of
the alloy by reducing the grain size. Moreover, casting is
particularly advantageous for forming the anodes into structures of
the desired shape.
[0052] The anode may have an active part consisting of a body made
of the described iron-rich alloy, however its active part can
comprise a layer of the iron-rich alloy on an electronically
conductive, inert, inner core made of a different electronically
conductive material, such as metals, alloys, intermetallics,
cermets and conductive ceramics. Such inner core can be selected
from metals, alloys, intermetallic compounds, cermets and
conductive ceramics or combinations thereof and may be covered with
an oxygen barrier layer, as described in U.S. Pat. No. 6,248,227
(de Nora/Duruz).
[0053] 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 in particular non-stoichiometric nickel
oxide. As described in U.S. Pat. No. 6,248,227 (de Nora/Duruz), the
inner core may be 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.
[0054] The anode according to the invention can be pre-oxidised
prior to its immersion into an electrolyte where the electrolysis
of alumina takes place, by oxidation in an oxidising atmosphere or
by electrolysis in a conditioning molten electrolyte before being
transferred in a production molten electrolyte containing dissolved
alumina for the electrowinning of aluminium. However, in general
with the anode compositions according to the invention, it is
possible to self-form the electro-chemially active integral outside
oxide layer on the alloy during use.
[0055] Another aspect of the invention is a cell for the
electrowinning of aluminium by the electrolysis of alumina in a
molten fluoride electrolyte utilising an iron-based metal anode
with an electrochemically active integral outside layer
predominantly of iron oxide as discussed above.
[0056] This integral outside layer can slowly dissolve into the
electrolyte during operation and be maintained by progressive slow
oxidation of iron at the interface of the metal bulk of the alloy
with the oxide layer. Alternatively, such a layer can be inhibited
from dissolving by maintaining an amount or iron species in the
electrolyte as disclosed in the abovementioned WO00/06802.
[0057] The cell preferably comprises at least one
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. No. 5,651,874 (de Nora/Sekhar) and U.S.
Pat. No. 5,683,559 (de Nora).
[0058] 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.
[0059] During operation of the cell, the concentration of alumina
dissolved in the electrolyte is below 10 weight %, preferably
between 5 weight % and 8 weight %.
[0060] 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 applications WO
99/41429 (de Nora/Duruz) and WO 01/31088 (de Nora), or by
periodically moving the anodes as described in co-pending
application WO 99/41430 (Duruz/Bell). Preferably, the iron-based
metal anodes have a foraminate electrochemically active structure
provided with openings to permit circulation of the electrolyte
therethrough, as disclosed in WO 00/40782 (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 described in WO 00/40781 (de Nora).
[0061] According to one mode of operation of the invention, the
progressive slow oxidation of iron at the interface of the bulk of
the alloy with the oxide layer corresponds to the dissolution of
iron into the electrolyte at a rate such that the maximum
concentration of iron species in the electrolyte does not exceed
the saturation level of iron species in the electrolyte at the
operating temperature.
[0062] During operation, the progressive slow oxidation of iron at
the interface of the metal alloy with the oxide layer provides a
compensation for dissolution of iron into the electrolyte which
takes place at a rate depending on the electrolyte composition, the
temperature of the electrolyte and the composition of the oxide
layer. On the other hand, the rate of dissolution of iron can be so
low that contamination of the aluminium can be kept at an
acceptable level and so that the rate of oxidation can be
controlled. To achieve this, the operating temperature should be
maintained sufficiently low to control the dissolution of iron into
the electrolyte.
[0063] The anode with the specified additives provides a slow
oxidation which corresponds to the slow controlled dissolution of
iron into the electrolyte from the anodes that supply current for
the electrolysis of alumina.
[0064] Whether the anode is operated as discussed above in slow
dissolution mode or in a dimensionally stable mode, the operating
temperature is preferably maintained sufficiently low to control
the solubility of iron in the electrolyte and to limit the
contamination of the product aluminium to an acceptable level, for
example the operating temperature is below 930.degree. C.,
preferably between 840.degree. C. and 890.degree. C.
[0065] The operating temperature can also be in the range of
930.degree. to 960.degree. C., preferably around 940.degree. C.
[0066] A further aspect of the invention is a method for the
electrowinning of aluminium by the electrolysis of alumina in a
molten fluoride electrolyte. This method comprises dissolving
alumina in the electrolyte and electrolysing the alumina-containing
electrolyte to produce aluminium on the cathode and oxygen on the
facing anodes utilising iron-based metal anodes as discussed
above.
[0067] The method can be implemented by immersing the 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 previously and then,
as mentioned above, electrolysing the dissolved alumina to produce
aluminium in the same or a different fluoride-based
electrolyte.
[0068] The anode has an electrochemically active surface layer
predominantly of iron oxide that during operation slowly dissolves
into the electrolyte. The surface layer is maintained by
progressive slow oxidation of iron at the interface of the bulk of
the alloy. There is a corresponding controlled dissolution of iron
into the electrolyte at such a low rate that the contamination of
the product aluminium by iron is at an acceptable level.
[0069] For example, the operating temperature is below 930.degree.
C., preferably between 840.degree. C. and 890.degree. C. and
typically the electrolyte contains NaF and AlF.sub.3 in a molar
ratio comprised between 1.2 and 2.4. The electrolyte may also
contain other fluorides such as LiF, CaF.sub.2 or MgF.sub.2. The
concentration of alumina dissolved in the electrolyte is below 10
weight %, preferably between 5 weight % and 8 weight %.
[0070] During operation, alumina-depleted electrolyte is circulated
away from the electrochemically active oxide layer of the anode(s),
enriched with alumina, and alumina-enriched electrolyte is
circulated towards the electrochemically active oxide layer of the
anode(s) to provide a constant supply of alumina to be electrolysed
(i.e. maintain a sufficient concentration of alumina in the
anode-cathode gap) and to reduce dissolution of the anode.
[0071] The aluminium is preferably produced on an
aluminium-wettable cathode from which the product aluminium is
continuously drained. As the consumption of the non-carbon,
metal-based anodes according to the invention is at a very slow
rate, 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.
[0072] In summary the anode, cell and method according to the
invention all provide or make use of an iron-based metal anode with
an iron-rich alloy containing selected additives in the given
ranges whereby the anode's integral outside oxide layer can be
formed during use at a rate corresponding to a controlled
dissolution into the electrolyte at the operating temperature, or
can be stabilised during use by maintaining an amount of iron
species in the electrolyte, leading to an acceptably low
contamination of the product aluminium. In either case, such an
anode has a long lifetime. The alloy can be produced economically
in particular by casting. Its high iron content further contributes
to its economic attractiveness. Moreover, the contamination of the
product aluminium associated with prior iron-rich anodes has been
reduced.
DETAILED DESCRIPTION
[0073] Examples of anode compositions according to the invention
are given in Table I, which shows the weight percentages of the
indicated metals for each specimen A-J.
1 TABLE I Fe Y Al Cu Ni Mn A 82 2 10 6 -- -- B 88 2 10 -- -- -- C
87 4.5 4 4 -- 0.5 D 84 1 10 4.5 -- 0.5 E 88 2 6 4 -- -- F 84 2 6 6
2 -- G 80 2 6 6 6 -- H 81.5 0.5 2 8 8 -- I 84 1 3 5 7 -- J 86 1 4 4
5 --
[0074] The invention will be further described in the following
Examples.
EXAMPLE 1
[0075] An anode rod of diameter 20 mm and total length 200 mm was
prepared by casting the composition of Sample D of Table I, using a
sand mould.
[0076] Electrolysis was carried out in a laboratory scale cell
equipped with this anode immersed to a depth of 50 mm in a
fluoride-containing molten electrolyte at 880.degree.C. The
electrolyte contained cryolite with 24 weight % excess of AlF.sub.3
and further containing 4 weight % of CaF.sub.2.
[0077] The current density was about 0.7 A/cm.sup.2 and the
concentration of dissolved alumina in the electrolyte was 5 weight
%. This concentration of alumina was maintained during the entire
electrolysis by periodically feeding fresh alumina into the
cell.
[0078] After 22 hours electrolysis was interrupted and the anode
extracted. Upon cooling the anode was examined externally and in
cross-section. The anode was covered by an external scale of
Fe.sub.2O.sub.3 about 10-15 micron thick on top of a layer of Fe
aluminate (probably Fe(FeAl).sub.2O.sub.4 spinel phase) about
150-200 micron thick. No corrosion was observed at or near the
surface of the anode. The presence of the Fe(FeAl).sub.2O.sub.4
phase indicates the mechanism of internal oxidation at the
interface of the Fe--Al core during the anode operation.
[0079] At the operating temperature 880.degree. C., the saturation
limit of iron in the electrolyte is approximately 450 ppm.
[0080] The produced aluminium was analysed and showed an iron
contamination of approximately 800 ppm which is below the tolerated
iron contamination in commercial aluminium production.
EXAMPLE 2
[0081] This Example illustrates the wear rate of the iron-based
metal anode of Example 1.
[0082] An estimation of the wear rate is based on the following
parameters and assumptions:
[0083] With a current density of 0.7 A/cm.sup.2 and a current
efficiency of 90% an aluminium electrowinning cell produces daily
approximately 50 kg aluminium per square meter of active cathode
surface.
[0084] Assuming a contamination of the produced aluminium by 800
ppm of iron, which corresponds to the experimentally measured
quantities in typical tests, the wear rate of an iron sample
corresponds to approximately 5 micron/day.
EXAMPLE 3
[0085] An anode rod of diameter 20 mm and total length 20 mm was
prepared by casting the composition of Sample F of Table I, using a
sand mould.
[0086] The anode was subjected to testing as in Example 1 but with
electrolysis during 72 hours with the cell voltage maintained
stable at 3.8 to 4.0 volts. After 72 hours, the electrolysis was
stopped, the anode was extracted and, upon cooling, was examined
externally and in cross-section. The anode was covered by a
coherent and dense external scale of Fe.sub.2O.sub.3 20 to 30
micron thick, over a cermet zone about 50 micron thick composed of
a Ni--Al and Ni--Fe network with inclusions of
Ni.sub.XFe.sub.2-xO.sub.3 and Al.sub.2O.sub.3, or a mixture
thereof.
[0087] Comparing with the results described in Example 1, the
decrease of the Al content from 10 to 6 weight % and he presence of
nickel increased the oxidation resistance of the alloy core by
formation of protective scales of Ni.sub.XFe.sub.2-XO.sub.3 and
Al.sub.2O.sub.3.
* * * * *