Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
Inert anodes for aluminum electrolysis and method of production thereof
FIELD OF THE INVENTION
[0001] The
present invention relates to inert anodes for aluminum electrolysis. More
specifically, the present invention is concerned with a composition of anodes
for aluminum electrolysis
and a method of production thereof.
BACKGROUND OF THE INVENTION
[0002] The
primary aluminum industry is a high producer of greenhouse gases with mean
emissions of 5.7 to 19.2 tons of CO2-equivalent per ton of produced Al,
depending on the electric power
source. A significant contribution, i.e. about 3.7 tons CO2-eq /ton Al,
originates from the use of
consumable carbon anodes in the Hall-Heroult electrolysis process. In this
context, the development of
alternative cells consisting in a combination of inert anodes, also referred
to as 02-evolving anodes, and
wetted cathodes is a first R&D priority of primary aluminum producers.
Successful research in this field
promises significant environmental benefits, energy savings and cost
reductions.
[0003] Among
possible inert anode materials, i. e. metals, ceramics and cermets, metal-
based anodes are currently promising candidates because they offer high
electrical conductivity,
excellent thermal shock resistance, mechanical robustness, ease of manufacture
and simplicity of
electrical connection to current leads. However, obtaining a metal-based inert
anode with a long-term
viability, i.e. typically of at least several months, in the highly corrosive
conditions of the Al electrolysis, is
very challenging. The use of a low-temperature, i.e. between about 700 and
about 800 C instead of a
temperature of about 950 C as standardly used for cryolite electrolyte for Al
electrolysis should
significantly increase the utility of metal-based anodes and would offer a
larger selection of alloys that
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could be used as inert anodes. However, the decrease of the alumina solubility
in low-temperature NaF-
A1F3 electrolytes causes operational difficulties. KF-AIF3-based electrolytes
have been proposed as a
way to operate at lower temperatures due to its relatively high alumina
solubility at low-temperatures, in
amounts of about 5 wt. % at 700 C.
[0004] Metals
are chemically unstable in cryolitic bath and, as a result, metallic anodes
must be permanently covered by a protective, self-repairing and relatively
thin oxide layer during Al
electrolysis. For that purpose, the metallic anode composition must be
optimized in order to achieve an
adequate balance between the oxidation rate of the metal substrate and the
dissolution rate of the oxide
layer in the electrolyte. Cu-Ni-Fe based alloys have shown promising
properties as inert anodes US
patent 5,284,562 to Beck et al., 1994) due to their ability to form an
adherent, electronically conducting
nickel ferrite plus copper scale during the operation of the electrolysis
cell. However, Cu-Ni-Fe alloys
present a two-phased microstructure, comprising a Cu-rich phase and a Fe-Ni-
rich phase, over a large
composition range. This chemical inhomogeneity decreases their corrosion
resistance because the iron-
rich phase is preferentially corroded upon Al electrolysis inducing the
formation of iron fluoride corrosion
tunnels in the anode scale as recently shown by Beck et al. (T.R. Beck, C.M.
MacRae and N.C. Wilson,
Metal!. Mat. Trans. B, 42, 807 (2011)).
[0005]
Homogenization of the alloys through an appropriate thermal treatment is said
to
improve their corrosion resistance for Al production (T.R. Beck, C.M. MacRae
and N.C. Wilson, Metall.
Mat. Trans. B, 42, 807 (2011), US Patent 7,7077,945 to Bergsma et al., 2006).
[0006] There
is still a need in the art for a composition for anodes for aluminum
electrolysis and a method of production thereof.
SUMMARY OF THE INVENTION
[0007] More
specifically, in accordance with the present invention, there is provided an
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inert anode for Al electrolysis, made of Cu-Ni-Fe-0 based materials,
comprising Fe in a range between
about 10 and 20 % by weight, Cu in a range between about 60 and about 80% by
weight, Ni in a range
between about 20 and about 30% by weight, and oxygen in a range between about
1 and about 3% by
weight.
[0008] There is further provided a method for producing metallic inert
anodes, comprising
mechanically alloying metallic elements; oxygen doping; and consolidation.
[0009] Other objects, advantages and features of the present invention
will become more
apparent upon reading of the following non-restrictive description of specific
embodiments thereof, given
by way of example only with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] In the appended drawings:
[0011] Figure la is a diagrammatic representation of a method for
producing
(Cu6sNi2oFei5)100-x0x powders according to an embodiment of an aspect of the
present invention;
[0012] Figure lb is a diagrammatic representation of an
electrochemical reactor
according to an embodiment of an aspect of the present invention;
[0013] Figure 2 shows XRD patterns of (Cu65Ni2oFe15)100-x0x materials
for different values
of x in a) as-milled state; and b) after powder consolidation treatment; ;
[0014] Figure 3 shows evolution of the lattice parameter of the rphase
as function of x in
as-milled and consolidated (Cu65Ni2oFei5)loo-x0x samples of Figure 2;
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[0015] Figures 4 show BSE surface images of the consolidated
(Cu65Ni2oFe15)100-x0x
materials with a) x = 0.3, b) x= 1.4, c) x= 3.3 and d) x= 7.2;
[0016] Figure 5 shows variation of the mass gain Arnim (%) with
respect to the oxidation
time of the consolidated (Cu65Ni2oFe15)100-x0x materials with x= 0.3, 1.4, 3.3
and 7.2;
[0017] Figures 6 show cell voltage versus electrolysis time for
(Cu65Ni2oFe15)100-x0x
anodes with a) x = 0.3, b) x= 1.4, c) x= 3.3 and d) x= 7.2;
[0018] Figure 7 shows the evolution of the ohmic drop with the
electrolysis time at
(Cu65Ni2oFe15)100-x0x anodes with x= 0.3, 1.4, 3.3 and 7.2;
[0019] Figures 8 show BSE cross-sectional images of the
(Cu65Ni2oFei5)10o-x0x anodes
with a) x = 0.3, b) x = 1.4, c) x = 3.3 and d) x= 7.2 after 20h of
electrolysis;
[0020] Figures 9 are a diagrammatic representations of the composition
of the scale
formed on the Cu65Ni2oFe15)100-x0x anodes with a) x = 0.3, b) x = 1.4, c) x =
3.3 and d) x = 7.2 after 20h
of electrolysis;
[0021] Figure 10 shows Cu, Ni and Fe concentrations (wt.%) in the
produced Al versus x
in (Cu65Ni2oFe15)100-x0x anodes after 20 hours of electrolysis;
[0022] Figure 11 shows the evolution of the Cu, Ni and Fe
concentrations (ppm) in the
electrolyte as a function of the electrolysis time with the
(Cus5Ni2oFei5)98.601.4 anode; and
[0023] Figure 12 shows BSE surface image of a consolidated composite
anode made
from a mixture milled for 4 hours of 95.3 wt.% Cu67.1Ni20.6Fe12.3 (pre-milled
for 10 h) + 4.7 wt.% Fe70030
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(50 nm in size).
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0024] In a nutshell, there is provided an anode composition and a
method for producing
metallic anodes having an improved resistance to corrosion, by preparing the
anodes starting from alloys
synthesized by mechanical alloying, optimizing the stoichiometry, and oxygen
doping.
[0025] In particular, it was found that mechanically preparing
Cu65Ni2oFe15 alloys under an
oxygen atmosphere yielded nanostructured alloys with a resistance to corrosion
in cryolitic environment
at 700 C increased compared to corrosion resistance of Cu65Ni2oFel5 alloys
synthesized under inert
atmosphere.
[0026] (Cu65Ni2oFei5)100-x0x materials were prepared by mechanical
alloying under
oxygen atmosphere. Their structural and chemical characteristics were studied
at different stages of their
preparation and after 20 hours of electrolysis in a low-temperature (700 C) KF-
AIF3 electrolyte. It was
shown that oxygen, when added in appropriate amount during the mechanical
alloying process, has a
significant beneficial effect on the electrode corrosion resistance.
[0027] (Cu65Ni2oFe15)100-x0x materials with different 0 contents were
synthesized by
grinding powders in two steps as shown in Figure la.
[0028] In a first step, shown in grey in Figure la, the metallic
Cu65Ni2oFel5 alloy was
synthesized by high energy ball milling (HEBM) of Cu, Ni and Fe powders (Cu
purity ..?.99.5 /0, Ni and Fe
purity a 99.9%, -325 mesh). The powder mixture (11.35 g) was weighted in a
glove box under Ar
atmosphere and placed in a vial (55 ml) with three hardened steel balls (two
balls with a diameter of 14
mm and one ball with a diameter of 11 mm for a total mass of 22.7 g). The ball-
to-powder mass ratio
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(BPR) was 2:1. Stearic acid (0.5 wt. %) was also added in order to prevent
excessive cold welding. The
vial was sealed under Ar atmosphere and placed in a vibratory miller (SPEXTM
8000M). The milling time
was set at 10 h, leading to the completion of the alloying process between the
Cu, Ni and Fe elements.
[0029] In a
second step, the Cu65Ni2oFei5 powder was oxidized by a subsequent HEBM
performed under 02 atmosphere. The oxygen amount in the samples was varied
with the number of
times (0, 4, 9 and 18 times) that the vial was filled with 02 (Figure la). For
each 02 filling, the vial was
opened and then sealed in a glove bag under oxygen atmosphere at a pressure of
1 atm. The milling
duration after each 02 filling was 30 minutes. The oxygen contents x (measured
with a LECOTM oxygen
analyzer) in the resulting (Cu65Ni2oFei5)100-x0x samples were 0.3, 1.4, 3.3
and 7.2 wt. %, respectively, as
indicated in Figure la. The 0 contamination in the sample only milled under Ar
atmosphere (x = 0.3
wt.%) may originate from the native oxide layer present on the starting
powders and/or to the oxidation of
the powder surface with ambient atmosphere once the sample is taken out of the
vial. The Cu, Ni and Fe
concentrations measured by energy dispersive X-ray (EDX) analysis in the four
samples were in
accordance (within 1-2 wt. %) with their nominal composition. The structure of
the (Cu65Ni2oFe15)100-x0x
powders was determined by X-ray diffraction (XRD) using a Bruckerrm D8
diffractometer with Cu Ka
radiation.
[0030] Powder
consolidation was then performed to obtain pellet samples for oxidation
and electrolysis tests. The as-milled powder was first sieved to select only a
powder fraction with a
particle size between 20 and 75 pm. Then, it was introduced into a quartz
cylinder pre-form and heated
from room temperature to 1000 C under Ar atmosphere for a thermal softening
treatment. The resulting
sample was cold pressed at 26 tons cm-2 for 10 minutes and then sintered at
1000 C under Ar
atmosphere for one hour. The pellet was removed from the heating zone of the
furnace and left to cool
down to room temperature. The obtained pellets had a diameter of about 11.3 mm
and a thickness of
about 5 mm for the electrolysis tests, and of about 1 mm for the oxidation
tests. Their porosity was
assessed according to the following equation:
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(d.7. ¨ dexp F
porosity ¨ x100
dt
where cit is the theoretical density determined from XRD measurements, and
dexp is the experimentally
measured density obtained by weighing and measuring the thickness of the
pellet. The porosity of the
samples was thus determined to be of 5 2%. The structure of the consolidated
samples was
determined by XRD. Backscattered electron (BSE) images of the sample surface
were carried out using
a JEOLTM JSM-6300F scanning electron microscope (SEM).
[0031] Thermogravimetric analyses (TGA) were performed using a
ThermaxTm 500
equipment. The samples were first heated up from room temperature to 7000C at
100C min-1 under Ar
atmosphere. Oxidation experiments were then conducted at 7000C under Ar-20%02
with a flow rate of
240 cc min-1. The mass variation of the samples was recorded for 20 hours. The
nature of the oxides
formed during these oxidation tests was determined by XRD analyses.
[0032] For electrolysis tests, a hole was drilled and tapped into the
edge of the pellet in
order to insert an electrical connection rod protected by an alumina-based
cement coating. Electrolyses
were performed at about 700 C under argon atmosphere using a two-electrode
configuration cell
controlled by a VMP3 Multichannel Potentiostat/Galvanostat (by BioLogic
Instruments). The
electrochemical reactor contained three electrochemical cells and thus, three
electrolysis tests could be
conducted in parallel.
[0033] Figure ibis a diagrammatic representation of such
electrochemical reactor, which
allows running three experiments at the same time with three electrochemical
cells controlled by a multi-
channel potentiostat/galvanostat. Only one cell set-up is represented here for
clarity.
[0034] The reactor comprises a stainless steel container 4 receiving
crucibles 11,
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provided with a stainless steel cover 3, which temperature was controlled by a
furnace controller and a
water cooling system.
[0035] In the cell geometry and electrode arrangement illustrated in
Figure lb, the
geometric surface area of the inert anode 7 immersed in the KF-AIF3-A1203
electrolyte 9 was about 4
cm2. The counter electrode 8 was a graphite rod of about 13 cm2 immersed in
the electrolyte 9. The
anode-cathode distance was 2.3 cm. The crucibles 11 containing the electrolyte
were made of sintered
alumina. The electrolyte composition was 50 wt. % AlF3-45 wt. % KF-5 wt. %
A1203. No alumina was
added during the electrolysis since its consumption is assumed to be
compensated by the dissolution of
the alumina crucibles. Electrolyses were performed at an anode current density
of 0.5 A cm-2 for 20
hours. Before measurement, the anode 7 was maintained above the electrolyte 9
for 30 minutes and
then immersed in the electrolyte 9 at open circuit conditions for 10 minutes.
[0036] Current interruption measurements were performed after 0.25, 5
and 20 hours of
electrolysis to determine the different voltage components, i.e., Nernst
potential, polarization potential
and ohmic drop. The current was interrupted for 30 seconds with a voltage
sampling rate of 1 ms. The
Nernst potential was defined as the voltage measured at 30 seconds after the
current interruption. The
ohmic drop was defined as the difference between the operating voltage
measured before the current
interruption and the voltage taken at 1 ms after the current interruption. The
polarization potential was
defined as the difference between the Nernst potential and the voltage taken
at 1 ms after the current
interruption.
[0037] The figure shows a graphite connecting rod 5 connected to the
sintered alumina
tubes 2 supported by stainless steel rods 1 connected to a thermocouple 12. As
the connection rod 6
was partially immersed in the electrolyte, an undesired part of the
contamination of the produced
aluminum 10 came from the corrosion of the connection rod 6. Thus, in order to
determine the Cu, Ni and
Fe contamination levels only coming from the anode corrosion, two series of
electrolysis tests were
performed: a first series using an Inconel 718 rod for Cu quantification and a
second series using an
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aluminum-bronze (C63000) rod for Ni and Fe quantifications. The Cu, Ni and Fe
contents in the
produced Al and electrolyte were measured by neutron activation. The
experiment was repeated at least
twice for each anode composition. The annual wear rate of the anode was
calculated according to the
equation:
(mbwb mAiwAdx 365 x 24
Wear rate (cm year-1) = _______________________________________ (2)
100 x pa X SaX t
where mb is the mass of electrolyte (g); wb is the mass fraction of
contaminants (Cu+Ni+Fe) in the
electrolyte (wt.%); mAi is the mass of produced Al (g); WA/ is the mass
fraction of contaminants
(Cu+Ni+Fe) in the produced Al (wt.%); pa is the anode density (g cm-3); Sa is
the geometric surface area
of the anode immersed in the electrolyte (cm2); and t is the electrolysis time
(h).
[0038] The
composition and structure of the oxide layers formed on the anode during Al
electrolysis were determined by EDX and XRD analyses recorded after polishing
the electrode for
different limes in order to reveal the successive oxide layers. The surface
and cross section of the
electrodes were observed by SEM.
[0039] Figure
2a shows the XRD patterns of as-milled (Cu65Ni2oFel5)100-x0x powders for x
= 0.3, 1.4, 3.3 and 7.2 wt. %. All XRD patterns exhibit one series of peaks
which corresponds to a face-
centered-cubic (fcc) phase (y-phase) attributed to a solid solution of
Cu(Ni,Fe,0). The lattice parameter
of the ?-phase (calculated from the (111) peak position) increases slightly
with x, as shown in Figure 3,
which may reflect the insertion of 0 atoms in the 'y-phase.
[0040] Figure
2b displays the XRD patterns of the (Cu65Ni2oFel5)100-x0x materials after the
powder consolidation treatment. As expected, this treatment generates grain
growth and strain release
as illustrated by a decrease of the full width at half maximum (FWHM) of the
diffraction peaks. On the
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basis of Williamson-Hall plots (not shown), the lattice strain is about 0.3%
and the crystallite size is about
30 nm for the consolidated samples compared to about 0.5% and about 15 nm
before consolidation.
Furthermore, a new series of peaks for x = 7.2 was observed, which correspond
to a Fe203 phase. This
phase is also observable for x= 3.3 but the intensity of the peaks is much
smaller. In addition, it is noted
that the diffraction peaks of the 7-phase slightly shift towards higher 20
angles with increasing the oxygen
content in the material, indicating a decrease of the 7-phase lattice
parameter as x increases in the
consolidated samples (see Figure 3). This can be attributed to the decrease of
the Fe content in the y-
phase due to the formation of iron oxides during the consolidation treatment.
[0041]
Moreover, it appears in Figure 3 that the 7-phase lattice parameter after
consolidation decreases linearly with x. Thus, it is assumed that all the
consolidated (Cu65Ni2oFe15)loo-x0x
materials milled under 02 contain some amount of iron oxides even if the XRD
pattern for x = 1.4 does
not show any discernable FeOx diffraction peaks (see Figure 2b).
[0042] This
is supported by BSE images of the surface of the consolidated
(Cu65Ni2oFei5)100-x0x materials (see Figures 4). Indeed, for x = 1.4, 3.3,
7.2, the micrographs supported
by EDX analyses reveal the presence of micrometric Fe203 precipitates, shown
as dark grey areas, well
distributed in the Cu(Ni,Fe) phase matrix, which appears as clear grey areas.
It can be seen that the
number and size of the iron oxide precipitates increase with x. Their sizes
are typically at most 0.5, 0.5-3
and 1-5 pm for x = 1.4, 3.3 and 7.2, respectively.
[0043] Figure
5 shows TGA curves expressed as the mass gain (Amin)) with respect to
the oxidation time performed at 700 C under 1 atm Ar:02 (80:20) for the
different consolidated
(Cu65Ni2oFei5)100-x0x materials. For x = 0.3, a very fast increase of the
sample mass is observed during
the first hour of oxidation, which is attributed to the formation of CuO as
confirmed by XRD analysis (not
shown). The oxidation rate drastically slows down for further oxidation time
due to the formation of NiO
and NiFe204 phases which act as barrier to the copper flux at the oxide-alloy
interface. After 20 hours of
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oxidation, the mass gain reaches 5.2%. The addition of a small amount of 0 in
the Cu-Ni-Fe alloy (x =
1.4) decreases drastically the oxidation kinetics by preventing the rapid mass
gain during the first stage
of oxidation related to the formation of CuO, which leads to a mass gain of
only 1.1% after 20 hours of
oxidation.
[0044] A
possible explanation is that the presence of finely dispersed Fe203 inclusions
in
the Cu-Ni-Fe matrix (Figures 4) favors the rapid formation of NiFe204 from NiO
+ Fe203 thanks to a
minimization of the diffusion distance between each compounds. The formation
of NiFe204 is assumed to
limit the outward diffusion of Cu in Cu oxides, resulting in a lower oxidation
rate as shown in Figure 5.
More added oxygen in the Cu-Ni-Fe alloy (x = 3.3 and 7.4) induces minor
additional improvement of the
alloy oxidation resistance, with a mass gain of 0.7% for both samples after 20
hours of oxidation. The
XRD patterns of the (Cu65Ni2oFei5)100-x0x samples after the oxidation test
(not shown) confirm the
formation of CuO, NiO and NiFe204 in all cases.
[0045]
Figures 6 show the evolution of the cell voltage for 20 hours of electrolysis
in a
low-temperature (700 C) KF-AIF3 electrolyte at 'anode= 0.5 A cm-2 with the
different (Cu65Ni2oFel5)loo-x0x
electrodes. For x = 0.3, the cell voltage gradually increases from 3.8 to 4.4
V during the 20 hours of
electrolysis. For x =1.4 and 3.3, the cell voltage is more stable with a
slight increase from ca. 3.9 to 4.1 V.
For x = 7.2, the cell potential is less stable and higher with a rapid
decrease from 4.5 to 4.1 V during the
first hour of electrolysis followed by a slow increase to reach 4.4 V at the
end of electrolysis.
[0046] The
current interruption method was performed after 0.25, 5 and 20 hours of
electrolysis in order to determine the different voltage components, i.e.,
Nernst potential, polarization
potential and ohmic drop. For all electrodes, the measured Nernst potential is
initially around 2.1 V and
reaches a stable value of ca. 2.4 V after a few hours of electrolysis which is
in accordance with the
theoretical voltage (E = 2.37 V) for the decomposition reaction of alumina
(A1203 = 2AI + 3/202) at 700 C
under 1 atm. 02. The polarization potential at the end of the electrolysis is
in the range 0.15-0.3V.
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[0047] The evolution of the ohmic drop with the electrolysis time for
the four electrodes is
showed in Figure 7. Assuming that the external circuit, electrolyte and
electrode connection resistance
are similar and stable for all the electrodes, the evolution of the ohmic drop
is an indication of the
variation of the electrical resistance of the anode. After 0.25 hours of
electrolysis, the ohmic drop is
assumed to be mainly due to the bulk electrical resistance of the anode and
thus, its increase with x
observed in Figure 7 (from 1.33V for x= 0.3 to 1.69 V for x= 7.2) may be
explained by the larger amount
of insulating iron oxide particles with increasing x (see Figure 4). On the
other hand, the variation of the
ohmic drop with the electrolysis time is considered to reflect the growth of
the oxide scale on the
electrode surface. It clearly appears in Figure 7 that the increase of the
ohmic drop with the electrolysis
time is less marked as x increases with a mean variation of 18, 7, 5 and 3 mV
h-1 for x= 0.3, 1.4, 3.3 and
7.2, respectively. This could reflect the fact that, as x is increased, the
thickness of the oxide scale
decreases or that its conductivity is increased. It was already shown that the
gain of mass, as measured
during TGA experiments, decreases as x is increased, indicating that the
thickness of the oxide scale
varies with x (see Figure 5). However, it will be hereinbelow that a thick
oxide scale is formed at the
surface of the electrode with x = 7.2, suggesting that the conductivity of
this oxide scale is large or that it
is highly porous.
[0048] Figures 8 show BSE cross-section images of the four
(Cu65Ni2oFei5)100-,0, anodes
after 20 hours of electrolysis. The presence of an oxide scale is easily
discernable, which is delaminated
from the bulk alloy for x = 3.3 and 7.2 probably due to the thermal shock when
the electrode is taken out
of the electrolyte. In all cases, the surface scale is composed of three main
layers but their thickness and
nature depend on the electrode composition.
[0049] The schematic representation of these layers determined from
EDX and XRD
analyses after polishing the electrodes for different times but not shown here
is presented for each
electrode in Figures 8.For x = 0 (Figures 8a and 9a), the surface scale
appears dense, which is
supported by the fact that no electrolyte salts was detected into it. The
outermost layer is a Cu20-rich
scale about 200 pm thick containing NiO and FeO, inclusions. The intermediate
layer (about 100 p m in
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thickness) consists of a mixture of Cu20 and NiFe204. This oxide scale
structure results from the outward
diffusion of Cu in Cu oxides and internal oxidation of Fe and Ni with the
subsequent formation of
NiFe204. Near the bulk alloy, a non-continuous layer of FeF2 (about 50 p m in
thickness) is observed,
which is assumed to be mainly formed when the anode was maintained above the
electrolyte for 30
minutes and during the first minutes of electrolysis.
[0050] For x
= 1.4 (Figures 8b and 9b) and x = 3.3 (Figures 8c and 9c), the surface scale
appear less dense than for x = 0.3 with the presence of few pores but no
electrolyte infiltration was
observed into it. Both electrodes present the same layer arrangement with
approximately the same
thickness. The outermost layer (about 120 and about 135 pm thick for x = 1.4
and 3.3, respectively) is
composed of a mixture of NiFe204, FeO,, and Cu20 with NiFe204 as major
constituent. Underneath this
one, a layer (about 80 and about 95 pm thick for x = 1.4 and 3.3,
respectively) containing Cu20 and
NiFe204 is present. Finally, the inner layer (about 50 p m thick) is
constituted of FeF2 inclusions inside the
alloy matrix as observed for x= 0.3. The fact that the outermost layer is
thinner and much poorer in Cu20
for x = 1.4 and 3.3 than for x = 0.3 indicate that the outward diffusion of Cu
in Cu oxides is significantly
slowed down. As discussed before, this is attributed to the more favorable
formation of NiFe204, as
confirmed by the observation of a NiFe204-rich outermost layer, for x = 1.4
and 3.3 due to the presence
of finely dispersed Fe203 inclusions in the Cu-Ni-Fe matrix acting as
nucleation sites for the formation of
Ni Fe204.
[0051] For x=
7.2 (Figures 8d and 9d), the electrode presents the thickest surface scale
with a total thickness of about 440 pm, which is due to the formation of a
thick Cu20-rich outermost layer
(about 340 p m). Moreover, a significant amount of electrolyte was detected
into it. In addition, NiFe204 is
not observed in the outermost layer in contrast to the two other samples
milled under 02 (x = 1.4 and
3.3). Lastly, the intermediate (Cu20 + NiFe204) layer is thinner with a
thickness of about 50 pm
compared to about 80 pm to about 100 pm for the three other electrodes. A
possible explanation is that
for x = 7.2, the Fe203 inclusions formed in the Cu-Ni-Fe matrix during the
consolidation treatment are
present in a too larger amount (Figure 4d), inducing an important chemical
inhomogeneity in the sample
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in addition to produce an inadequate balance between the amounts of NiO and
Fe203 to favor the
formation of a protective NiFe204-rich surface layer as observed for x = 1.4
and 3.3. As a result, this
electrode displays a lower corrosion resistance compared to x= 1.4 and 3.3, as
discussed hereinbelow.
[0052] Figure
10 shows the evolution of the Cu, Ni and Fe contamination (wt. `)/0) in the
produced aluminum as a function of x in (Cu65N12oFe15)100.x0x electrodes after
20 hours of electrolysis.
Two additional compositions (x = 1.0 and 4.5) were evaluated in order to
confirm the tendency of the Al
contamination curves. All the contaminants display the same evolution with a
minimum for x = 1.4 with
the presence in the produced Al of 0.13, 0.08 and 0.03 wt. % Cu, Fe and Ni,
respectively. This
corresponds to an aluminum purity of 99.76 wt. %, which meets the chemical
specification of P1020A
grade Al (purity ?. 99.7%). It can be noted that (Cu65Ni2oFel5)100-x0x
electrodes with x = 1.0 and 3.3 also
give good results with a total impurity level of about 0.3 wt.% compared to
about 0.7 wt.% for x = 0.
Further addition of oxygen (x = 4.5 and 7.2) induces an increase of the Al
contamination with a total
impurity content of about 0.6 wt. c/o. This highlights the fact that oxygen
must be added in appropriate
amount during the mechanical alloying process in order to induce the formation
of finely dispersed Fe203
inclusions in the Cu-Ni-Fe matrix during the subsequent consolidation
treatment (see Figure 4), which is
essential to favor the formation of a protective NiFe204-rich layer at the
surface of the electrode during
the electrolysis process as shown before (Figure 9). This is also in
accordance with the absence of
significant improvement of the corrosion resistance for composite anodes
prepared from a ball-milled
mixture of (Cu65Ni2oFels + x Fe203) or (Cu65Ni2oFei5 + x NiFe204) powders of
micrometric iron oxide size,
i.e. generally of at least 1 urn due to the formation of too large Fe203 or
NiFe204 inclusions in size in
such composite materials (results not shown). In contrast, a significant
improvement of the corrosion
resistance is observed for composite anodes prepared from a ball-milled
mixture of Cu-Ni-Fe alloy +
nanometric iron oxide particles, i.e. generally of at most 100 nm. For
instance, aluminum with a purity of
99.5 wt.% was produced with a composite anode made from a mixture milled for 4
hours of 95.3 wt.%
Cu67 iNi20.6Fe12.3 (pre-milled for 10 h) + 4.7 wt.% Fe70030 (50 nm in size).
[0053]
Electrolyte sampling was performed after 0, 1, 2, 4, 15 and 20 hours of
electrolysis
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for the (Cu65Ni2oFei5)98.601.4 electrode. The evolution of the Cu, Ni and Fe
concentrations in the bath is
plotted as a function of the electrolysis time in Figure 11. The electrolyte
contamination strongly
increases during the first 4 hours of electrolysis but tends to stabilize over
this period. Thus, after 20
hours of electrolysis, steady-state conditions are assumed to be established
with Cu, Fe and Ni
concentrations in the bath of ca. 80 (0.008), 40 (0.004) and 5 (0.0005) ppm
(wt.%), respectively. This
stabilization indicates that equilibrium between the oxide dissolution rate at
the anode and their reduction
rate at the cathode or by Al is reached or that the Cu, Fe and Ni oxides have
reached their saturation
level in the bath. From the amounts of Cu, Ni and Fe impurities in the
electrolyte and in the produced Al
after 20 hours of electrolysis, the wear rate of the (Cu65Ni2oFei5)98.601.4
electrode was calculated
according to equation (2) and is estimated at about 0.8 cm yearl, which is
below the target of 1 cm yearl
specified in the Aluminum industry technology roadmap, by the Aluminum
Association, Aluminum
industry technology roadmap, Washington DC (2003).
[0054] As
people in the art will now be in a position to appreciate, there is provided
an
anode composition and a method of production thereof, for inert anodes of a
high resistance to corrosion,
with an erosion rate of at most 1 cm yearl, during electrolysis of aluminum at
low temperature, i.e. at
about 700 C. There are provided mechanically alloyed Cu-Ni-Fe-O based
materials for inert anodes.
[0055] A Cu-
Ni-Fe alloy with a composition comprising Cu between about 65 and 70%.
Ni-Fe alloy with a composition comprising Cu in the range between 60 and 80
wt. %, Ni in the range
between 20 and 30 wt. % and Fe in the range between 10 and 20 wt. % can be
considered as
appropriate for obtaining an anode with a good corrosion resistance after
subsequent 0 doping in the
range between 1 to 3% by weight. For example, an optimized composition was
about 15% by weight Fe,
about 64% by weight Cu, about 20 % by weight, and about 1.5% by weight oxygen.
[0056]
Oxidization by grinding under oxygen atmosphere after an initial grinding
under
inert atmosphere to allow a proportion of oxygen between about 1 and 3% by
weight is found to be
efficient in increasing the anode corrosion resistance (see Figure 10). In
this 0 concentration range, the
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16
size, dispersion and concentration of Fe203 precipitates in the consolidated
powder are optimized to
promote the formation of a protective NiFe204-rich layer on the anode surface.
[0057] As an
alternative, Cu-Ni-Fe-0 based anodes with dispersed Fe203, precipitates
can also be produced by ball milling Cu-Ni-Fe alloy with nanometric iron oxide
particles, i.e. of a size of at
most 100 nnn (see Figure 12).
[0058] With
anodes of (Cu65Ni2oFe15)10o-x0x with x comprised in the range between about
1 and about 3, aluminum could be produced with a purity of 99.7 %. The rate of
corrosion of the anodes
is very low, at about 8 mm/year, which is well below current industry target
of typically at most 10
mm/year. Moreover, the present anodes have good thermal, and mechanical
stability, and low electric
resistivity. They also have a stable potential and a low overvoltage for the
reaction of oxygen, for
example less than 0.4 V at 0.5 A/cm2.
[0059] The
addition of a small concentration, i.e. at most 5 wt. % and preferably at most
1
wt. %, of rare earth elements (such as Y or Ce for example) to the composition
is expected to further
increase resistance to corrosion (see for instance, works of R. Cueff et al in
Corrosion Science 45 (2003)
1815-10831).
[0060] The
consolidation procedure for producing nanostructured anodes from ball-milled
Cu-Ni-Fe-0 powders can be done through a cold pressing-sintering procedure as
described hereinabove
before. Other techniques characterized by their ability to produce
nanostructured bulk materials or
coatings from ball-milled powders, such cold spray or spark plasma sintering
for example, can also be
used.
[0061] In
order to induce the formation of a protective NiFe204-rich layer at the
surface of
the Cu-Ni-Fe-O electrode before Al electrolysis and then, to prevent the
formation of metal fluorides (e.g.,
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17
FeF2) at the electrode surface during the first minutes of electrolysis, a pre-
treatment of the electrode can
be performed through an air oxidation step (e.g., oxidation under air
atmosphere at 700 C for 3 h).
[0062] Although the present invention has been described hereinabove
by way of
embodiments thereof, it may be modified, without departing from the nature and
teachings of the subject
invention as recited herein.
[0063] References
B. Assouli, M. Pedron, S. Helle, A. Carrere, D. Guay and L. Roué, Light
Metals, 1141 (2009)
S. Helle, M. Pedron, B. Assouli, B. Davis, D. Guay and L. Roué, Corros. Sc.,
52, 3348 (2010)
S. Helle, B. Davis, D. Guay and L. Roué, J. Electrochem. Soc., 157, E173
(2010)
T.R. Beck, Light Metals, 355 (1995)
T.R. Beck, C.M. MacRae and N.C. Wilson, Metal!. Mat. Trans. B, 42, 807 (2011)
US 5,284, 562 (A) Beck et al.
V. de Nora et al., Light Metals, 501 (2007)
V.A. Kovrov, A.P. Khramov, A.A. Redkin and Y.P. Zaikov, ECS Transactions, 16,
7 (2009)
S. Helle, B. Brodu, B. Davis, D. Guay and L. Roué, Corros. Sci., 53, 3248
(2011)
US 6,692,631 Bergsma et al.
US 7,077,945 Bergsma et al.
Aluminum industry technology roadmap, by the Aluminum Association, Aluminum
industry technology
roadmap, Washington DC (2003)
R. Cueff et al, Corrosion Science 45 (2003) 1815-10831
