Note: Descriptions are shown in the official language in which they were submitted.
WO90/10735 1 _ 2 0 3 0 7 8 8 PCT/EPgo/003~
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AN ANODE SUBSTRATE COATED WITH
RARE EARTH OXYCOMPOUNDS
The invention relates to anodes for electrowinning
metals such as aluminum fro~ molten salt electrolytes, of
the type comprising a substrate made of an
electroconductive oxycompound which in use is coated with
a surface coating comprising at least one rare earth
oxycompound, typically including cerium oxyfluoride. The
invention also relates to electrowinning processes using
such anodes.
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BACKGROUND ART ~ :
Materials used as non-consumable anodes in molten
electrolytes must have a good stability in an oxidisin~
atmosphere, good mechanical properties, good electrical
conductivity and be able to operate for prolonged periods
of time under polarising conditions. It is well known that
ceramic materials have better resistance to chemical
corrosion. However, their low electrical conductivity and
difficulties of making mechanical and electrical contact
as well as difficulties in shaping and machining these
materials seriously limit their use.
US Patent 4 6lq 569 describes a method of
electrowinning metals by electrolysis of a melt containing
a dissolved species of the metal to be won usinq an anode
immersed in the melt wherein the anode has a metal, alloy
or cermet substrate and an operative anode surface which
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is a protective surface coating containin~ a compound of a
~etal less noble than the metal to be electrowon, the
protective coating being preserved by maintaining in the
melt a suitable concentration of a species of this less
noble metal. Usually the protective anode coating
comprises a fluorine-containing oxycompound of cerium
(referred to as "cerium oxyfluoride") alone or in
combination with additives such as compounds of tantalum,
niobium, yttrium, lanthanum, praesodymium and other rare
earth elements, this coating being maintained by the
addition of cerium and possibly other elements to the
electrolyte. The electrolyte can be molten cryolite
containing dissolved alumina, i.e. for the production of
aluminum.
This electrowinning method potentially has very
significant advantages. To date, however, there remain
problems with the anode substrate. When the substrate ls a
metal, alloy or cermet, it may be subject to oxidation
leadin~ to a reduced life of the anode, despite the
excellent protective effect of the cerium oxyfluoride
coating which protects the substrate from direct attack by
corrosive electrolyte. When the substrate is a ceramic
oxycompound, conductivity and corrosion are major problems.
A promising solution to these problems has been the
use of a ceramic/metal composite material of at least one
ceramic phase and at least one metallic phase, comprising
mixed o~ides of cerium with aluminum, nickel, iron and/or
copper in the form of a skeleton interwoven with a
continuous metallic network of an alloy or intermetallic
compound of cerium with aluminum, nickel, iron and/or
copper, as described in EP-A-0 257 708. When used as
electrode substrates, these materials have promise,
particularly those based on cerium and aluminum because
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even if they corrode, this does not lead to corrosion
p~oducts that contaminate the electrowon aluminum.
Nevertheless corrosion of the substrate remains a problem.
US Patent 4 374 050 discloses inert electrodes for
aluminum production fabricated from at least two metals or
metal compounds to provide a combination metal compound.
For example, an alloy of two or more metals can be surface
oxidised to form an oxycompound of the metals at the
surface on an unoxidised alloy substrate. US Patent
4 374 761 discloses similar compositions further
comprisin~ a dispersed metal powder in compositions which
may be applied as a preformed o~ide composition on a metal
substrate by cladding or plasma spraying, Such application
techniques, however, are known to involve many drawbacks
and the adhesion is particularly poor. US Patent 4 620 905
describes an oxidised alloy electrode based on tin or
copper with nickel, iron silver, zinc, magnesium, aluminum
and yttrium, either as a cermet or partially oxidised at
its surface. Such partially oxidised alloys suffer serious
disadvantages in that the oxide layers formed are far too
porous to o~ygen, and not sufficiently stable in corrosive
environments. In addition, at high temperatures the
partially oxidised structures continue to oxidize and this
uncontrolled oxidation causes subsequent segregation of
the metal and/or oxide layer. Adherence at the
ceramic-metal interfaces is particularly difficult to
achieve and this very problem has hampered use of such
simple composites. Finally, none of these matesials has
proven satisfactory as substrate for cerium oxyfluoride
coatings of the type discussed.
Improved metal-based substrates are described in
European Patent Applications 88201957.3, 88201851.8,
88201852.6, 88201a53.4 and 882018S4.2 all as yet
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WO90/10735 2 0 ~ ~ 7 8 8 PCT/EP90/003~
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unpublished. These typically include a substrate made of
an alloy of chromium with nickel, cobalt and/or iron. On
the surface of the substrate is a chromium oxide film on
top o~ which is a layer of copper oxide in solid solution
with nickel or manganese, o~tained by oxidising a layer of
nickel/copper or manganese/copper which is applied eg by
electroplating. It was also mentioned that the nickel
oxide in the surface layer may have its electrical
conductivity improved by doping with lithium.
Such composite layers nevertheless remain difficult
to prepare and although they have demonstrated superior
performance over previous anodes, considerable development
is still required to optimize their lifetime and reduce
the production cost.
The aforementioned European Patent Application
88201854.2, mentions further embodiments of ceramic
intermediate layers which in use serve as anchorage for
the in-situ maintained protective coating of cerium
oxyfluoride to the metal substrate, these intermediate
layers including: nickel ferrite; copper oxide and nickel
ferrite; doped, non-stoichiometric and partially
substituted ceramic oxide spinels containing combinations
of divalent nickel, cobalt, magnesium, manganese, copper
and zinc with divalent/trivalent nickel, cobalt, manganese
and/or iron, and optionally dopants selected from Ti4+,
z 4+ Sn4+ Fe4+ Hf4+, Mn4+, Fe , Ni
C 3+ Mn3+ A13+ Cr3+, Fe2+, Ni , Co
M92+, Mn2+, Cu2~, zn2 and Li (see US patent
No. 4 552 630); as well as coatings based on rare earth
oxides and oxyfluorides, in particular pre-applied cerium
oxyfluoride alone or in combination with other components.
To date, very little progress has been made with
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anode substrates made of ceramic electroconductive
oxycompounds. The most widely tested materials in this
category on account of their acceptable conductivity have
been based on tin dioxide. However, it has not yet been
possible to make an adequate electrode substrate based on
tin dioxide despite expedients devised to reduce the
amount of substrate material dissolved in the electrolyte.
See for example EP-A-O 257 709 (E00208) which proposed
doping the oxyfluoride coating with tantalum to render it
more impervious and thereby reduce contamination of the
electrolyte and the electrowon aluminum with tin from the
substrate.
SUMMARY OF THE INvENTION
An object of the invention is to provide electrode
substrates based on electroconductive oxycompounds which
can be produced easily, have excellent conductivity and
perform well as anode substrates when coated with an
oxyfluoride-type coating.
The invention is based on the realization that
sintered copper-n~ckel oxide suitably doped with lithium
oxide to enhance conductivity fulfills the sought-after
requirements of a material for the anode substrate.
According to the invention, an anode for
electrowinning a metal from a molten salt electrolyte,
comprising an electroconductive oxycompound substrate
which in use carries a surface coating comprising at least
one rare earth oxycompound, is characterized in that the
substrate comprises a sintered body composed of a
nickel-copper-lithium oxide solid solution.
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Preferably, the nickel oxide is present in the solid
solution in an amcunt of at least 70 mol%, the copper
oxide is present in an amount of at most 29 mol% and the
iithium oxide is present in an amount of at most 10mol%.
Preferably still, the solid solution contains 70-90 mol%
nickel oxide, 5-29 mol% copper oxide and 1-10 mol% lithium
oxide. The concentration the lithium dopant preferably
ranges from 1 to 10 atom % with an optimum value at about
5 atom %, this usually in combination with about
70-80 mol~ nickel oxide and about 20-25 mol% copper oxide.
It has been shown that a concentration of 1 to 5
atom% lithium increases the conductivity of the (Ni-Cu)O
solid solution by two orders of magnitude, up to about
200(ohm cm) 1 at 1000C. This makes the material an
attractive substrate material for cerium oxyfluoride
coatings for aluminum electrowinning.
A method of preparing an anode substrate according to
the invention comprises mixing powders of nickel oxide,
copper oxide and a compound of lithium, firing at
900~1100C, cooling, grinding, cold pressing and sintering
at 1000-1300C for 30-40 hours. Shapes of lithium doped
(Ni-Cu)O solid solution can thus conveniently be prepared
by mixing powders of Li(,~O3), Li2CO3 or LioH; CuO;
and NiO in the right proportions and firing for example at
900-1050C in air for about 24 hours at a heating rate of
about 100C/hour. After cooling, the material is ground,
cold pressed (eg at 10 tons/cm2) and sintered at
1000-1300C e.g. 1100-1150C for 30-40 hours. The
resulting sintered material shows a density of at least
70% theoretical density, typically 80%, and an electrical
conductivity of about 150 (ohm cm) 1 at 980C compared
to 1 (ohm cm) 1 for the undoped (Ni-Cu)o. A sintered
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WO90/10735 - 7 -- PCT/EP9~00364
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specimen of such composition was tested as an anode
substrate in a neutral cryolite containing l.5% CeF3 and
l.S Ta2O5. A very den~e tantalum-doped cerium
oxyfluoride coating was obtained. No noticeable change in
the substrate composition near the interface was observed.
The process may be optimized to improve dens~f;cation by
hot pressing and/or the addition of sintering aids.
The anode substrate according to the invention can be
used as a massive body supporting the rare earth
oxycompound coating. But it can, if desired, incorporate a
metal or other current collector to assist the supply of
electric current and facilitate connection to the power
supp ly .
It has been observed that using the anode substrate
according to the invention in a cryolite melt containing
dissolved alumina and cerium species produces very dense,
adherent and homogeneous cerium-oxyfluoride coatings. This
is believed to be related to the presence of copper oxide
in the substrate surface and to the surface porosity of
the sintered material. The rare earth oxide coating may be
cerium o~yfluoride alone or preferably may be cerium
oxyfluoride together with at least one compound of
tatalum, niobium, yttrium, lanthanum, praesodymium and
other rare earth elements.
The invention also provides a method of eletrowinning
aluminum from molten cryolite containing alumina wherein
an anode is immersed, the anode having an eletroconductive
oxycompound substrate carying a surface coating comprising
at least one rare earth oxycompound, the surface coating
being maintained by the presence of cerium species in the
molten cryolite, the method comprising passing
electrolysis current between the anode and a cathode to
evolve oxygen and to maintain the surface coating at the
anode and to produce aluminum at the cathode.
The invention will be further illustrated by the
following Example.
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ExamPle
A Li.o5Ni.70Cu.25O sample was prepared using
powder metallurgy techniques: Li(NO3), CuO and NiO
powders were mixed in the right proportions and fired at
1000C in air for 24 hours. The heating rate was
100C/hour. After cooling the specimen was powdered, cold
pressed at 10 tons/cm2 and sintered for 35 hours at
llS0C. The microstructure of the resulting sample showed
a porosity of nearly 20% and CuO precipitates at the grain
boundaries due to the slow cooling rate. A typical SEM-EDX
analysis over a window of about 0.25 mm2 gave nickel
71.0 atom% and copper 28.6 atom~, whereas for individual
grains the composition was: nickel 76.6 atom% and copper
23.2 atom%. This analytical method is not suitable for
detecting the lithium.
An ingot with a surface area of 7.~ cm was
prepared from this sample and exposed for 5 hours in a
neutral cryolitic bath of 900g containing 1.5% of
Ta2O; and 6g (i.e. about 0.7%) CeF3. Using a current
density of 200 mA/cm , a dense tantalum-doped cerium
oxyfluoride coating was formed on the substrate at a rate
ranging from 0.15 to 0.16 g/cm per hour. EDX analysis
revealed that the concentration of nickel and copper did
not significantly change during the cerium oxyfluoride
deposition: 70 and 30 atom% for nickel and copper
respectively for a window analysis; 76 atom% nickel and 23
atom~ copper for a grain analysis. This stability of the
composition of the sample is an indication of the
protective role of the deposit for the cryolite in the
previously mentioned conditions. The relative stability of
the potential during the deposition may be related to the
conductivity of the substrate which is strongly dependant
on the lithium concentration.
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