Note: Descriptions are shown in the official language in which they were submitted.
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SURFACE OXIDISED NICKEL-IRON METAL ANODES
FOR ALUMINIUM PRODUCTION
Field of the Invention
This invention relates to surface oxidised
nickel-iron metal anodes for the electrowinning of
aluminium by the electrolysis of alumina dissolved in a
molten fluoride-containing electrolyte, an aluminium
electrowinning cell with such an anode and its use to
produce aluminium.
Background Art
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.
US Patents 6,248,227 and 6,436,274 (both de
Nora/Duruz) disclose 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.
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.
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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.
Nickel-iron alloy anodes with various additives
are further described in WO 00/06803 (Duruz/de Nora/
Crottaz), WO 00/006804 (Crottaz/Duruz), WO 01142534 (de
Nora/Duruz), WO 01/42535, (Duruz/de Nora), WO 01/42536
(Duruz/Nguyen/de Nora) and W002/083991 (Nguyen/de Nora).
Summary of the Invention
An object of the invention is to provide a
nickel-iron alloy-based anode for aluminium
electrowinning having a long life, which anode during use
does not contaminate the product aluminium beyond an
acceptable level.
The invention relates to an alloy-based anode for
the electrowinning of aluminium by the electrolysis of
alumina in a molten fluoride electrolyte. The anode has
an electrochemically active integral outside oxide layer
obtainable by surface oxidation of a metal alloy having a
composition adjusted to achieve the effect described
below. This metal alloy consists of:
- 20 to 60, preferably 35 to 60, weight% nickel;
- 5 to 15, preferably 6 to 12, weight% copper;
- 1.5 to 5, preferably 1.5 to 4, weight% aluminium;
- 0 to 2, preferably 0.2 to 0.5, weight% in total of
one or more rare earth metals, in particular
yttrium;
- 0 to 2, usually 0.5 to 1.5, weight% of further
elements, in particular manganese, silicon and
carbon; and
- the balance being iron,
the metal alloy having a copper/nickel weight ratio in
the range of 0.1 to 0.5, preferably 0.2 to 0.3.
When such a metal alloy is exposed to an
oxidising atmosphere at elevated temperature, e.g. above
600°C, typically 700° to 1000°C, for a duration of up to
36 hours depending on the temperature, and/or during use
in an aluminium production cell, the iron migrates from
an outer part to the surface where it is oxidised.
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When the anode's alloy is oxidised before use,
the integral oxide layer formed thereon usually consists
essentially of iron oxides and up 30 weight% nickel
oxide, in particular from 1 to 10, weight%.
Whether or not the alloy is oxidised before use,
the integral oxide layer typically comprises during use
in a cell an iron-rich outer portion which consists
essentially of non-stoichiometric well conductive iron
oxide (FeOX) and nickel oxide in a metal equivalent
weight ratio that is at least 9 iron for 1 nickel, and an
iron-rich inner portion which consists essentially of a
mixture of oxides of iron, nickel, copper and aluminium
which are present in metal equivalent weight percentages
of 65 or 70 to 80% iron, 15 to 25 or 30% nickel 2 to 3%
copper and up to 1% aluminium. Usually, the outer portion
of the integral oxide layer makes about 1/3 of the
thickness of the layer, whereas the inner portion makes
about 2/3 of the thickness of the integral oxide layer.
Underneath the electrochemically active oxide
surface, the (iron-depleted) alloy outer.part is rich in
copper and nickel metal in a ratio derived from the
nickel-copper ratio of the alloy's nominal composition
and contains a limited amount of iron metal. The copper-
nickel outer part controls the iron diffusion from inside
the anode to its electrochemically active surface so as
to compensate slow dissolution of iron oxides from the
anode's active surface into the electrolyte while it
prevents excessive iron diffusion to the anode' s surface
and dissolution into the electrolyte of an excess of iron
oxide from the anode's surface, which would lead to
premature iron depletion of the anode's alloy and
unnecessary and unwanted contamination of the product
aluminium.
Typically, the nickel-copper metal outer part has
a nickel/copper weight ratio in the range of 1.8 to 4
upon heat treatment and/or during use in a cell.
The small amount of aluminium contained in the
anode's alloy diffuses to the grain joints of the nickel-
iron alloy inside the anode where it is oxidised to form
a partial barrier against oxygen diffusion into the
alloy's grains and iron diffusion therefrom. Thus, the
combined effect of the alloy's aluminium on the one hand
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and of the anode's nickel-copper outer part on the other
hand leads to a control of the supply of iron to the
anode's active surface.
Small amounts of rare earth metals, such as
yttrium, are preferably used in the anode's alloy to
improve the anchorage of the integral oxide layer on the
nickel-copper outer part. For example, the metal alloy
contains 0.3 to 0.4 weight% yttrium.
The anode's metal alloy can contain 16 to 73.5
weighto iron, usually from 20 to 70 weight%. In
particular in this case, the nickel/iron weight ratio can
be in the range of 0.3 to 2.5.
In one embodiment the anode's metal alloy
contains 30 to 70 weight% iron, preferably 40 to 60
weight%. Especially in this case, the nickel/iron weight
ratio can be in the range of 0.3 or 0.4 to 1.5,
preferably 0.7 to 1.2.
In another embodiment, the anode's metal alloy
contains 20 to 40 weight% iron, preferably 25 to 35
weight%. Particularly in this case, the nickel/iron
weight ratio may be in the range of 1.5 to 3, preferably
2 to 2.5.
Especially when the anode is used with an
electrolyte in a reduced temperature range, e.g. from
850-880° to 940°C, the anode's alloy preferably contains
at least one of the metals nickel, copper, aluminium and
iron in the respective amounts: 35 to 50 weight% nickel;
6 to 10 weighto copper; 3 to 4 weight% aluminium; and 32
to 56 weight% iron, in particular 35 to 55 weight% iron.
For instance, the alloy contains: 35 to 50 weighto
nickel; 6 to 10 weight% copper; 3 to 4 weight% aluminium;
32 to 56 weight% iron, in particular 35 to 55 weight%
iron; and 0 to 4 weight% in total of further elements,
i.e. the rare earth metals plus the abovementioned
further elements.
Especially when the anode is used with an
electrolyte in a higher temperature range, e.g. from 910°
to 960°C such as 930° to 950°C, the anode's alloy
preferably contains at least one of the metals nickel,
copper, aluminium and iron in the respective amounts: 50
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to 60 weight% nickel, in particular 55 to 60 weight%; 7
to 12 weight% copper; 1.5 to 3 weight% aluminium; and 21
to 41.5 weight% iron, preferably 21 to 36.5 weight%. In
particular, the alloy contains: 50 to 60 weight% nickel,
in particular 55 to 60 weight%; 7 to 12 weighto copper;
1.5 to 3 weight% aluminium; and 21 to 41.5 weight% iron,
preferably 21 to 36.5 weighto; and 0 to 4 weight% in
total of further elements (the rare earth metals plus the
abovementioned further elements).
Advantageously, the metal alloy contains
manganese to trap and solubilise in the alloy sulphur
that can be present as an impurity in the electrolyte. In
the absence of manganese, sulphur combines with nickel to
form NiS instead of MnS and migrates to the grain joints
of the alloy and impairs its properties. The alloy
preferably contains manganese in an amount of less than 1
weight%, in particular from 0.2 to 0.5 weight%.
When the metal alloy is cast, especially to
produce complex shapes, silicon can be used to lower the
viscosity of the alloy and enhance its castability. It is
not unusual to find 0.2 to 0.7 weight% silicon in the
metal alloy when it is cast.
Furthermore, to avoid oxidation of the metal
alloy when it is cast, carbon can be used to trap any
oxygen to which the alloy may be exposed during casting.
Therefore, residual amounts of carbon, typically 0.01 to
0.2 weight%, is commonly found in such alloys.
For example, the metal alloy consists of 41 to 49
weight% nickel, 41 to 49 weight% iron, 6 to 8 weight%
copper, 2.5 to 3.5 weight% aluminium and 0 to 2 weight%
in total of further elements (the rare earth metals plus
the abovementioned further elements). The metal alloy can
also consist of 33 to 39 weight% nickel, 49 to 59 weighto
iron, 6 to 8 weight% copper, 2.5 to 3.5 weighto aluminium
and 0 to 2 weight% in total of further elements (the rare
earth metals plus the abovementioned further elements).
The anode's metal alloy can contain 0 to 1.5
weight% in total of further elements (the rare earth
metals plus the abovementioned further elements),
preferably no more than about 1 weighto.
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In another embodiment, the anode's alloy consists
of 56 to 58 weight% nickel, 28 to 32 weight% iron, 9 to
11 weight% copper, 1.5 to 2.5 weight% aluminium and 0 to
1 or 1.5 weight% in total of further elements (the rare
earth metals plus the abovementioned further elements).
The anode is preferably covered with a protective
coating on the integral oxide layer, in particular a
protective oxide coating. Suitable oxide coatings may
contain iron oxide such as hematite (Fe203), in
particular a coating made of hematite and at least one
oxide selected from oxides of titanium, yttrium,
ytterbium and tantalum as disclosed in PCT/IB02/02973
(Nguyen/de Nora) . Other suitable coatings can be used to
protect the anode's alloy, in particular oxide coatings
as disclosed in W099/36594 (de Nora/Duruz), US Patents
6,077,415 (Duruz/de Nora), 6,103,090 (de Nora) 6,361,681
(de Nora/Duruz), 6,365,018 (de Nora), or cerium-based
coatings, especially for use in an electrolyte in a
higher temperature range, e.g. in the range of 910° to
960°C, for example the cerium-based coatings disclosed in
US Patents 4,614,569 (Duruz/Derivaz/Debely/Adorian),
4,966,674 (Bannochie/Sheriff), 4,683,037 and 4,680,094
(both in the name of Duruz), 4,960,494, 4,956,068 and
5,069,771 (all in the name of Nyguen/Lazouni/Doan), and
W002/070786 (Nguyen/de Nora) and W002/083990 (de
Nora/Nguyen).
Unless specified otherwise, all the above
mentioned metal percentages of the alloy refer to the
nominal alloy composition, i.e. before any heat treatment
or use in a cell.
The invention relates also to an aluminium
electrowinning cell comprising at least one anode as
described above.
Advantageously, the cell comprises an aluminium-
wettable cathode, in particular a drained cathode.
Suitable aluminium-wettable cathode materials are
disclosed in W001/42168 (de Nora/Duruz), W001/42531
(Nguyen/Duruz/de Nora), W002/070783 (de Nora),
W002/096830 (Duruz/Nguyen/de Nora) and W002/096831
(Nguyen/de Nora). Suitable drained cathode designs are
disclosed in US Patents 5,683,559 (de Nora) and 6,258,246
(Duruz/de Nora), and in PCT applications W099/02764,
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W099/41429 (both de Nora/Duru~), W000/63463 (de Nora),
W001/31086 (de Nora/Duruz), W001/31088 (de Nora),
W002/070785 (de Nora), W002/097168 (de Nora) and
W002/097169 (de Nora).
Another aspect of the invention relates to a
method of electrowinning aluminium. The method comprises
passing an electrolysis current in a molten electrolyte
containing dissolved alumina between a cathode and an
anode as described above to produce aluminium
cathodically and oxygen anodically.
During cell operation, oxides of the anode's
oxide layer may slowly dissolve in the electrolyte, the
oxide layer being maintained by slow oxidation of the
anode's metal alloy at the oxide layer/metal alloy
interface. Advantageously, the dissolution rate of the
anode's oxides is substantially equal to the oxidation
rate of the metal alloy at the oxide layer/metal alloy
interface, as taught in US Patent 6,248,227 and
W000/06805 (both de Nora/Duruz).
Alternatively, dissolution of oxides of the
anode's oxide layer can be inhibited, in particular
prevented, by maintaining in the electrolyte an amount of
alumina and iron species, preferably at a level close to
or at saturation, as disclosed in W000/06802 (Duru~/de
Nora/Crottaz).
Preferably, the electrolyte has a temperature
which is maintained sufficiently low to limit the
solubility of iron species in the electrolyte and the
contamination of the product aluminium to an acceptable
level. The electrolyte temperature of the cell may be in
a reduced temperature range, typically from 850°C to
940°C, preferably between 880°C and 930°C. Alternatively,
the electrolyte temperature may be in a higher
temperature range, typically in the range of 910°C to
960°C, in particular from 930°C to 950°C.
The electrolyte can contain sodium fluoride (NaF)
and aluminium fluoride (A1F3) in a molar ratio in the
range from 1.2 to 2.4, in particular from 1.4 to 1.9 with
an electrolyte in a reduced temperature range and from
1.7 to 2.3 with an electrolyte in a higher temperature
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range. Suitable electrolyte compositions are disclosed in
W002/097168 (de Nora).
Advantageously, the electrolyte is continuously
circulated from an alumina feeding area where it is
enriched with alumina to the anode where the alumina is
electrolysed and from the anode back to the alumina
feeding area so as to maintain a high alumina
concentration near the anode. Means for providing such a
circulation are disclosed in W099/41429 (de Nora/Duruz),
W000/40781, W000/40781 and W003/006716 (all de Nora).
A further aspect of the invention relates to an
alloy, in particular for use to produce an anode for the
electrowinning of aluminium. The alloy consists of:
- 20 to 60, preferably 35 to 60, weighto nickel;
- 5 to 15, preferably 6 to 12, weighty copper;
- 1.5 to 5, preferably 1.5 to 4, weight% aluminium;
- 0 to 2 , preferably 0 . 2 to 0 . 5 , weight% in total of
one or more rare earth metals, in particular
yttrium;
- 0 to 2, usually 0.5 to 1.5, weights of further
elements, in particular manganese, silicon and
carbon; and
- the balance being iron,
the alloy having a copper/nickel weight ratio in the
range of 0.1 to 0.5, preferably 0.2 to 0.3.
The alloy can contain at least one of the metals
nickel, copper, aluminium and iron in the respective
amounts: 35 to 50 weight% nickel; 6 to 10 weight% copper;
3 to 4 weighty aluminium; and 32 to 56 weight% iron, in
particular 35 to 55 weight% iron. In particular, the
alloy contains: 35 to 50 weight% nickel; 6 to 10 weight%
copper; 3 to 4 weight% aluminium; 32 to 56 weight% iron,
in particular 35 to 55 weight% iron; and 0 to 4 weight o
in total of further elements (the rare earth metals plus
the abovementioned further elements).
The alloy may also contain at least one of the
metals nickel, copper, aluminium and iron in the
respective amounts: 50 to 60 weight% nickel, in
particular 55 to 60 weight%; 7 to 12 weight% copper; 1.5
to 3 weight% aluminium; and 21 to 41.5 weights iron,
preferably 21 to 36.5 weight%. In particular, the alloy
contains: 50 to 60 weighto nickel, in particular 55 to 60
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weighto; '7 to 12 weighto copper; and 1.5 to 3 weights
aluminium; 21 to 41.5 weight% iron, preferably 21 to 36.5
weight%; and 0 to 4 weighto in total of further elements
(the rare earth metals plus the abovementioned further
elements).
Another aspect of the invention relates to an
anode starter for the electrowinning of aluminium having
an outer part made of the alloy described above which is
oxidisable before and/or during use to form an integral
electrochemically active oxide outer layer.
A further aspect of the invention relates to a
component of an aluminium electrowinning cell, in
particular an anode support member or a current
distribution member. This cell component has an outer
part made of the alloy described above which is
oxidisable before and/or during use to form an integral
oxide outer layer.
Detailed Description
Examples of anode alloy compositions according to
the invention are given in Table I, which shows the
weight percentages of the indicated metals for each
specimen A-R.
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TABLE I
Ni Fe Cu Al Y Mn Si C
A 48 38 10 3 -- 0.5 0.45 0.05
B 49 40 7 3 -- 0.5 0.45 0.05
C 36 50 10 3 -- 0.5 0.45 0.05
D 36 50 10 3 0.35 0..3 0.3 0.05
E 36 53 7 3 -- 0.5 0.45 0.05
F 36 53 '7 3 0.35 0.3 0.3 0.05
G 48 38 10 3 0.35 0.3 0.3 0.05
H 48 38 10 3 0.2 0.3 0.45 0.05
I 22 68 5.5 4 -- 0.25 0.2 0.05
J 22 69 5.5 3 -- 0.25 0.2 0.05
K 42 42 12 2 1 0.5 0.45 0.05
L 42 40 12.5 4 0.4 0.45 0.6 0.05
M 45 44 7 3 -- 0.5 0.45 0.05
N 55 30 12 2 0.2 0.3 0.45 0.05
O 53 36 8 2.3 0.1 0.2 0.35 0.05
P 55 32 10 2 0.2 0.3 0.45 0.05
Q 57 30 10 2 0.2 0.3 0.45 0.05
R 59 27 10 3 0.2 0.3 0.45 0.05
The invention will be further described in the
following Examples.
Example 1
An anode rod of diameter 20 mm and total length
200 mm was prepared by casting the composition of Sample
A of Table I, using a sand. mould. The anode was oxidised
in air for 24 hours at 700°C.
Electrolysis was carried out in a laboratory
scale cell equipped with this oxidised anode immersed to
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a depth of 50 mm in a fluoride-containing molten
electrolyte at 920° to 930°C. The electrolyte consisted
of 16 weight% aluminium fluoride (AlF3) and 7 weighto
alumina A1203 and 4 weight% CaF2, the balance being
cryolite (3NaF-AlF3) .
The current density was about 0.8 A/cm2 at a cell
voltage of 3.5 to 3.8 V. The concentration of dissolved
alumina in the electrolyte was maintained during the
entire electrolysis by periodically feeding fresh alumina
into the cell.
After 150 hours electrolysis was interrupted and
the anode extracted. Upon cooling the anode was examined
externally and in cross-section.
The anode's outer dimensions had remained
substantially unchanged.
The anode was covered with an external oxide
scale having a thickness of about 50-100 micron. The
oxide scale had an outer portion that consisted
essentially of non-stoichiometric iron oxide (FeOx) with
small amounts of nickel oxide (metal equivalent of about
90 weight% Fe and 10 weight% Ni) at its surface which is
electrochemically active during use. Below the outer
portion, the external oxide scale had an inner portion
that consisted essentially of a mixture of hematite
(Fe203) and mixed oxides of nickel, iron and aluminium.
Underneath the oxide scale, the anode's alloy had
become vermicular over a depth of about 1500 micron and
contained 75 weighto nickel and 15 weight% copper, the
balance being essentially iron (below 10 weight%). The
vermicular outer part of the alloy had elongated pores
having a diameter of 3 to 5 micron and a length of 10 to
30 micron and containing oxides essentially of iron.
Below the anode's vermicular part the alloy was non
vermicular but had the same metal alloy composition as
the vermicular outer part over a depth of about 50 micron
followed by an unchanged inner part having the nominal
composition of the alloy before heat treatment.
The alloy grain joints were oxidised all over the
vermicular outer part and to a depth of about 100 micron
therebelow.
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Example 1a
An anode rod of diameter 20 mm and total length
20 mm was prepared by casting the composition of Sample B
of Table I, using a sand mould. The anode was oxidised in
air for 24 hours at 700°C and then tested in a laboratory
scale cell as in Example 1.
Similar results were obtained as in Example 1
except that the wear rate of the anode had increased to
about 1 mm per 100 hours of use.
Example 2
An anode rod of diameter 20 mm and total length
200 mm was prepared by casting the composition of Sample
N of Table I, using a sand mould. The anode was oxidised
in air for 24 hours at 750°C.
Electrolysis was carried out in a laboratory
scale cell equipped with this oxidised anode immersed to
a depth of 50 mm in a fluoride-containing molten
electrolyte at about 940°C. The electrolyte consisted of
15 weighto aluminium fluoride (A1F3) and 7 weight%
alumina A1203 and 4 weight% CaF2, the balance being
cryolite (3NaF-A1F3) .
The current density was about 0.8 A/cm2 at a cell
voltage of 3.5 to 3.8 V. The concentration of dissolved
alumina in the electrolyte was maintained during the
entire electrolysis by periodically feeding fresh alumina
into the cell.
After 200 hours electrolysis was interrupted and
the anode extracted. Upon cooling the anode was examined
externally and in cross-section.
The anode's outer dimensions had remained
substantially unchanged.
The anode was covered with an external oxide
scale having a thickness of about 50-100 micron. The
oxide scale had an outer portion that consisted
essentially of non-stoichiometric iron oxide (FeOX) with
small amounts of nickel oxide (metal equivalent of about
70 weighto Fe and 30 weight% Ni) at its surface which is
electrochemically active during use. Below the outer
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portion, the external oxide scale had an inner portion
that consisted essentially of a mixture of hematite
(Fe203) and mixed oxides of nickel, iron anal aluminium.
Underneath the oxide scale and over a.depth of
about 150 micron, the anode's alloy was nearly non-porous
and contained about .70-75 weight% nickel and 20 weighto
copper, the balance being essentially iron (below 10
weight%). Therebelow, the anode's alloy had remained
unchanged (nominal composition of sample N before heat
treatment).
The alloy grain joints were nearly not oxidised,
unlike those of Example 1a.
Example 3
An anode rod of diameter 20 mm and total length
200 mm was prepared by casting the composition of Sample
N of Table I, using a sand mould.
A slurry for the application of a protective
coating onto the anode rod was prepared by suspending a
particle mixture of Fe203 particles (-325 mesh, i.e.
smaller than 44 micron) and Ti02 particles (-325 mesh) in
colloidal alumina (NYACOL~ Al-20, a milky liquid with a
Colloidal particle size of about 40 to 60 nanometer and
Containing 20 weighto colloidal particle and 80 weighto
liquid solution) in a weight ratio Fe203:Ti02:colloid of
40:20:40. The pH of the slurry was adjusted at 4 by
adding a few drops of HN03 to avoid gelling of the
slurry.
The anode rod was covered with several layers of
this slurry using a brush. The applied layers were dried
for 10 hours at 140°C. The dried layers formed a coating
of about 350-450 micron thick on the anode rod.
The anode rod was pre-heated over a molten
electrolyte for an hour. During pre-heating at about
900°-950°C, the coating was further consolidated by
reactive sintering of the iron oxide and the titanium
oxide. During the pre-heating or at the latest at the
beginning of use in the electrolyte, the coating became
substantially continuous and thoroughly reacted forming a
protective multiple oxide matrix of Fe203 and Ti02.
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Underneath the protective coating, an integral oxide
scale mainly of iron oxide was grown from the alloy rod
during the heat treatment and reacted with Ti02 from the
coating to firmly anchor the coating to the anode rod.
The reacted integral oxide scale contained titanium oxide
in an amount of about 10 metal weight%. Minor amounts of
copper, aluminium and nickel were also found in the oxide
scale (less that 5 metal weighto in total).
Electrolysis was carried out as in Example 2. The
current density was about 0.8 A/cm2 at a reduced cell
voltage of 3.1 to 3.3 V.
After 200 hours electrolysis was interrupted and
the anode extracted. Upon cooling the anode was examined
and no significant change was observed.
Samples of the used electrolyte and the product
aluminium were analysed. It was found that the
electrolyte was nickel-free and. the produced aluminium
contained less than 300 ppm nickel. This demonstrated
that the Fe203-Ti02 coating constituted an efficient
barrier against nickel dissolution from the anode's
alloy.
Exam~l a 4
Anode rods can be prepared, as in Examples 1, 1a
and 2, respectively, by casting using sand moulds and
oxidising in air the composition of Table I's Samples C
to M and O to R, respectively, and as in Example 3 by
casting and coating the composition of Table I's Samples
A to M and 0 to R. Thereafter, the anode rods can be
tested in laboratory scale cells as in Examples 2 to 3.
Exa~~le 5
Examples 1, 1a and 2 and their variations
disclosed in Example 4 can be repeated without oxidation
of the anode rods before use.
