Apparatus for the electrolytic production of metals

Improved electrolytic cells for producing metals by the electrolytic reduction of a compound dissolved in a molten electrolyte are disclosed. In the improved cells, at least one electrode includes a protective layer comprising an oxide of the cell product metal formed upon an alloy of the cell product metal and a more noble metal. In the case of an aluminum reduction cell, the electrode can comprise an alloy of aluminum with copper, nickel, iron, or combinations thereof, upon which is formed an aluminum oxide protective layer.

BACKGROUND OF THE INVENTION 
A variety of metals having significant industrial uses are not found 
naturally in their elemental forms. Rather, these metals are mined as a 
variety of compounds from which the desirable metal product must be 
extracted. One such metal is aluminum. Commercially, aluminum is produced 
from naturally occurring aluminum compounds by the electrolytic reduction 
of alumina Al.sub.2 O.sub.3. Alumina is obtained from bauxite ore by the 
Bayer process which involves digesting crushed bauxite ore in strong 
caustic soda solution. 
In 1886, Charles Hall in the United States and Paul Heroult in France 
independently developed the currently employed electrolytic process for 
extracting aluminum from alumina. This process, known today as the 
Hall-Heroult process, transformed aluminum from a precious metal into a 
common structural material. The process is still the most widely used 
commercial process for obtaining aluminum metal and is fundamentally the 
same as it was originally disclosed by Hall and Heroult in 1886. 
In the Hall-Heroult process, electric current is passed through molten 
electrolyte containing alumina. An important feature of the Hall-Heroult 
discovery was that molten cryolite, a double salt of aluminum and sodium, 
represented by the chemical formula, Na.sub.3 AlF.sub.6, would dissolve 
alumina and that the dissolved alumina could be electrolytically reduced 
to form molten aluminum metal. 
The electrolytic reduction of metals is often performed in large cells or 
pots. These cells typically have massive carbon cathodes at the base and 
carbon anodes, normally formed in the shape of large blocks, suspended 
above the cell and capable of being lowered into the electrolyte. Direct 
electric current is passed from the anodes through the electrolyte to the 
carbon cathodes. During the reduction of alumina, for example, the carbon 
anodes are consumed in the chemical reaction occurring in the cell. This 
reaction can be represented, as follows: 
EQU 2Al.sub.2 O.sub.3 +3C.fwdarw.4Al+3CO.sub.2. 
This process yields an aluminum product that is very pure, e.g., 99.0% to 
99.8%. The main impurities are traces of iron and silicon. 
Despite its capability to produce high purity aluminum, the Hall-Heroult 
process has always suffered a number of significant problems. The most 
important of these arises from the use of consumable carbon anodes. These 
anodes are expensive to produce, and this cost adds significantly to the 
overall cost of aluminum produced by the Hall-Heroult process. 
Furthermore, it is difficult to maintain uniform anode current loading 
during use since the anodes are consumed, resulting in a continuous change 
in their shape. 
Because of the problems associated with carbon anodes, substantial research 
has been conducted in an effort to find another anode material, in 
particular a material that would result in a non-consumable or inert 
anode. Unlike the carbon anode which is systematically consumed by a 
chemical reaction with the product of the faradaic process occurring at 
the anode, a non-consumable anode would act as a simple electron sink 
sustaining the evolution of pure oxygen. Such an anode is chemically inert 
with respect to the gas product generated by the electrochemical reaction. 
Under such conditions, it is expected that there would be no net 
consumption of the anode, and hence the anode would be non-consumable. 
Another set of problems associated with electrolytic reduction cells arises 
from the lack of a suitable cathode material. Presently, carbon is used as 
the cathode material in these cells. Unfortunately, a product such as 
molten aluminum does not wet carbon. Therefore, in the case of aluminum 
production, it is necessary to maintain a deep pool of molten aluminum on 
the bottom of the cell. This is required because the carbon cathode 
surface must be fully covered in order to prevent contact between the 
molten salt electrolyte and the cathode itself in the presence of molten 
aluminum. Otherwise, the formation of aluminum carbide occurs, and this 
both reduces the productivity of the cell and consumes the carbon cathode. 
The presence of the deep pool, however, creates a new problem. The cell 
currents are generally extremely high, typically on the order of about 100 
kA to about 300 kA. At such currents, electromagnetic forces can cause the 
molten aluminum to develop waves of substantial physical dimension. To 
prevent electrical shorting of the molten aluminum to the anode, allowance 
must be made in the separation of the anode and cathode. This results in 
an excessive voltage drop across the electrolyte and contributes to poor 
energy utilization within the cells. 
Problems such as those discussed above for Hall-Heroult cells also exist 
with other electrolytic cells and processes for the electrolytic 
production of metals from oxide based feed materials. This has in many 
instances, resulted in the metals being produced from more expensive feed 
materials or by use of more complicated and expensive processes than would 
be required if oxide based feed materials could be used. 
As such, a need exists for electrodes for use in electrolytic cells for the 
reduction of oxide-based feed materials that are not consumed under the 
operating conditions of the cell, allow closer anode/cathode spacing, and 
can be shaped to configurations that are thermally and mechanically 
stable. 
SUMMARY OF THE INVENTION 
This invention relates to the discovery that material structures heretofore 
not considered useful in electrodes of cells for the electrolytic 
production of metals from oxide-based feed materials can be employed to 
provide improved electrolytic cells and processes for the electrolytic 
production of metals. In one embodiment, the invention pertains to 
electrode structures useful in Hall-Heroult cells for the electrolytic 
production of aluminum from aluminum oxide. 
The improved electrodes, and particularly anodes of this invention, 
comprise at least an alloy of the product metal and a more noble metal 
upon which is formed an oxide of the product metal as a protective layer. 
Typically, the protective layer will comprise a metal oxide that is the 
same as that used to feed the cell. All surfaces in contact with the 
electrolyte in the cell are formed of the protective material. 
Under the operating conditions of the cell, the protective material is 
rendered insoluble by the saturation conditions at the interface between 
the anode and the electrolyte. The saturation condition can be established 
in a number of ways such as by simple saturation of the bulk electrolyte 
with the material comprising the surface layer of the anode, or by 
generation of gas at the anode to establish saturation conditions in terms 
of the chemical potential of one of the constituents of the material 
comprising the surface layer. Beyond these chemical considerations, the 
materials forming the surface layer upon the electrode must be thermally 
and mechanically stable under the operating conditions of the cell. 
For one embodiment of the invention, electrolyte contained within the cell 
is saturated with the feed material. Since the protective surface layer 
may also be formed of the feed material, this saturation acts to provide 
an additional measure of prevention against consumption of the electrode 
during cell operation. The saturation can be accomplished by running the 
cell for a sufficient period to saturate the electrolyte with materials 
released or discharged from the layer into the electrolyte or, preferably, 
by constituting the electrolyte so as to include a sufficient amount of 
the feed material to saturate the electrolyte prior to cell operation. 
The use of oxides of the product metal (and especially cell feed materials) 
as a protective layer for the electrodes of an electrolytic cell results 
in significant advantages over the use of pre-baked carbon electrodes 
currently employed. For example, once the electrolyte is provided with the 
feed material at saturation levels, the anode becomes effectively 
non-consumable since the materials form a protective surface layer that 
effectively neither co-deposits with, nor is chemically displaced by, the 
metal product to any significant extent. It is noted that, although the 
protective layer may co-deposit, the co-deposited material comprises the 
product metal. Thus, effectively, there is no net co-deposition, as would 
be the case if the co-deposited material differed from that of the desired 
cell product. In the case of an aluminum reduction cell, the use of an 
anode made in accordance with the invention may result in small amounts of 
aluminum being co-deposited into aluminum product. However, this does not 
present a problem, since co-deposited aluminum is not a contaminant with 
respect to the aluminum product. 
Anodes made according to the invention retain their shape thereby 
facilitating the maintenance of uniform current density in the 
electrolytic cell. The result is that problems encountered in maintaining 
proper anode/cathode spacing with consumable carbon electrodes are 
reduced. Thus, the inventive, inert electrodes obviate one of the major 
reasons necessitating the use of anode/cathode spacings greater than 
required which results in inordinate consumption of electrical energy. In 
addition based on the above, the inventive electrodes allow greater 
flexibility in the choice of operating conditions for the electrolytic 
cells. 
The electrodes as described herein will also allow significant reductions 
in capital investment for the production of metals because they eliminate 
the need for expensive pre-baked carbon electrodes as well as the 
expensive baking facilities required to produce these carbon electrodes. 
Instead, the alloys and protective materials described above are readily 
obtainable in monolithic form or can be readily formed upon the surface of 
electrode foundations formed of less expensive, commodity materials. In 
addition, the anodes do not have to be replaced as frequently since they 
are essentially non-consumable. This reduces operating costs, as anode 
changes are labor intensive and result in significant cell down time.

DETAILED DESCRIPTION OF THE INVENTION 
Although the present invention is intended to apply to all cells useful for 
the electrolytic reduction of metals from oxide-based feed materials, 
cells and processes for the production of aluminum are described in detail 
herein for the purpose of illustration. The invention, however, is not 
intended to be limited solely to aluminum production cells and processes. 
A conventional Hall-Heroult cell 10 employing pre-baked carbon anodes is 
illustrated schematically in FIG. 1. This cell has a steel outer shell 12 
with thermal insulation 14 on the inside of shell 12. A carbon cathode 16 
is positioned at the bottom of cell 10 and contains metallic current 
collector bars 18. Carbon anodes 20 are formed from pre-baked carbon 
blocks suspended from steel anode rods 22 which serve to supply electrical 
current to anodes 20. Cell lining 24 is also formed from carbon blocks. 
Molten electrolyte 26 contains dissolved alumina supplied by breaking 
alumina crust 28 and adding fresh alumina. Crust 28 forms on frozen 
electrolyte and helps to minimize heat loss from the top of cell 10. 
Cryolite, Na.sub.3 AlF.sub.6, is commonly employed as the principal 
constituent of the electrolyte since molten cryolite has the capacity to 
dissolve alumina. In addition, certain fluoride salts, such as aluminum 
fluoride, AlF.sub.3, and calcium fluoride, CaF.sub.2, are also present in 
the electrolyte. AlF.sub.3 and CaF.sub.2 decrease the freezing point of 
electrolyte and AlF.sub.3 also improves current efficiency in the cell. 
As electric current is passed from carbon anode 20 through molten 
electrolyte 26 to cathode 16, dissolved alumina is reduced to form molten 
aluminum layer 32 at the bottom of the Hall-Heroult cell and gas 
consisting of carbon dioxide and carbon monoxide is generated at the 
anode. Carbon anode 20 is consumed during this reaction in the approximate 
amount about 1/2 lb. of anode per lb. of aluminum product. 
It is important to prevent molten electrolyte 26 from contacting carbon 
cell lining 24 to prevent cell lining failure caused by the formation of 
intercalation compounds and the formation and dissolution of Al.sub.4 
C.sub.3. To prevent such contact, cell 10 is operated under conditions 
that cause a layer of frozen electrolyte 30 to form between carbon cell 
lining 24 and molten electrolyte 26. Thus, molten electrolyte 26 is 
contained in a shell of frozen electrolyte and supported by a pad of 
molten aluminum 30. Unfortunately, during operation of the Hall-Heroult 
cell, the location of interface between molten and frozen electrolyte 
varies depending upon operating conditions. This adds to the difficulty in 
operating the cell under uniform conditions. As an alternative, cell 
linings having protective layers may be used within the cell. Such linings 
are described in detail in U.S. Pat. No. 4,999,097, the teachings of which 
are incorporated herein by reference. 
Molten aluminum 30 does not wet the carbon cathode 16. Unfortunately, 
electro-deposition of aluminum directly on carbon permits the formation of 
aluminum carbide, Al.sub.4 C.sub.3, which is soluble in the electrolyte. 
Such formation of aluminum carbide and its subsequent dissolution in the 
electrolyte consumes the carbon cathode, and hence, must be prevented. In 
practice, this is accomplished by covering the carbon cathode with a deep 
pool of molten aluminum. In this way, aluminum deposits onto molten 
aluminum rather than onto carbon. Furthermore, any aluminum carbide that 
forms at the interface between the aluminum pool and the carbon cathode 
must diffuse across the deep aluminum pool in order to dissolve in the 
electrolyte. However, there are disadvantages with this arrangement. The 
dimensional instabilities inherent in such a deep cathode pool of aluminum 
through which large electrical currents are passed require excessive 
spacing between the anode and cathode with all attendant disadvantages in 
order to prevent the dimensionally unstable aluminum pool from contacting 
the anode and electrically shorting the cell. 
The present invention results from the discovery that certain alloys, 
capable of being provided with or forming a protective layer comprising an 
oxide of the cell product, can act as inert, non-consumable electrodes in 
electrolytic reduction cells. The protective surface layer materials of 
this invention have properties such that, despite their solubility in the 
electrolyte, there is no net consumption of the protective layer, and 
their presence in the electrolyte does not result in contamination of the 
metal product of the cell. In particular, the protective surface layer can 
preferably comprise an oxide of the ultimate cell product. Thus, for an 
aluminum cell, an Al.sub.2 O.sub.3 productive layer is employed upon an 
alloy of aluminum and a metal more noble than aluminum. 
The electrodes of the present invention comprise a variety of metal alloys 
on which protective films can be formed in situ during operation of the 
electrolytic cells or ex situ prior to cell operation. For example, in the 
latter instance, an oxide film can be produced on an anode by electrolytic 
anodization at room temperature in a citric acid solution. This is not 
intended to be limiting, however, as the art is rich with methods for 
forming oxides on metals. 
The electrodes of the present invention comprise an alloy of the product 
metal with a more noble metal upon which is formed a protective layer 
comprising an oxide of the cell's product metal. In the case of an 
aluminum reduction cell, the electrode comprises an alloy of aluminum with 
a more noble metal (for example, copper, nickel, iron or combinations 
thereof) upon which is formed a thin aluminum oxide protective layer. 
The oxide materials disclosed herein as electrode and cell protective 
layers are typically electrical insulators, i.e., high bandgap materials. 
Thus, they must be present as a relatively thin layer if they are to be 
used upon the electrodes of molten salt electrolysis cells. Otherwise, 
they will impart too great an electrical resistance on the electrodes, 
greatly increasing the amount of electricity needed to operate the cell. 
An effective electrode can be achieved by making an electronically 
conductive portion, i.e., one with a low bandgap, upon which is formed or 
deposited an alloy and an electrode coating material of the type disclosed 
herein. The electronically conductive portion can be a metal, metal alloy, 
electronically conductive inorganic compound or solid solution. In one 
preferred embodiment, an electrode made in accordance with this invention 
can have a multi-layer structure comprising: 1) a foundation of commodity 
material formed into the bulk anode shape, 2) a first layer containing a 
metal alloy of the product metal and a more noble metal and 3) a 
protective layer covering the first layer and comprising an oxide of the 
product metal. 
The foundation of the electrode is chosen from any of a variety of 
materials that are electrically conductive, inexpensive, and easily shaped 
to a desired anode configuration. Preferred foundation materials are 
copper, nickel, iron, or combinations thereof. An anode of this embodiment 
can comprise a copper or nickel foundation upon which a layer of 
aluminum/copper alloy is deposited. An Al.sub.2 O.sub.3 protective layer 
is then formed upon the alloy layer. The protective layer must be kept as 
thin as possible to offer protection from chemical reaction of the anode 
with the electrolyte, while at the same time providing a minimum of 
increase in electrical resistance. 
Alternatively, the base of the electrode material can be formed entirely of 
an alloy of the product metal with a more noble metal. If the 
concentration of aluminum exceeds a critical value, (approximately 4% by 
weight in copper for example), when oxidized, such a structure will form a 
protective layer comprising an oxide of the product metal upon a zone of 
metal. Constructions formed only of the specified alloys and lacking a 
foundation material are particularly desirable for an anode configuration 
that comprises a series of thin plates suspended vertically in the 
electrolyte. Alternatively, designs comprising a monolithic block with 
vertical chimneys to allow central venting of product oxygen gas evolving 
on external surfaces in contact with the electrolyte may be used. 
Sharp compositional differences in electrode materials can result in 
thermal mismatches leading to potential delamination. To prevent such 
delamination, the alloy may be compositionally graded in a manner in which 
the mismatch in lattice parameters between the protective oxide surface 
layer and the underlying metal alloy is minimized. 
The cathode can be constructed similarly; however, it need not be of the 
same specific construction as the anode. Rather, as long as the cathode is 
fabricated to have a construction satisfying the criteria above, it will 
be operable in the cell. 
It should be apparent that the above are merely examples of the wide 
variety of electrode configurations that can be constructed in accordance 
with the invention. Thus, rather than being limiting, the examples are 
intended to be representative of electrodes having a specific class of 
protective surface layers formed upon a specific class of metallic alloys 
to yield a non-consumable electrode. 
The protective surface layer material employed for the anode must also be 
resistant to additional oxidation since oxygen is generated at the anode. 
Thus, the protective surface layer material employed on the anode is 
preferably an oxy-compound with the particularly preferred materials being 
oxides or oxidation products. As used herein, the term oxidation is 
intended to refer to reactions in which the metal forming the protective 
layer undergoes an increase in valence as a result of the chemical 
reaction forming the protective layer. As an example, the reaction to form 
a protective layer of aluminum oxy-fluoride from aluminum metal is an 
oxidation reaction. 
In the case of anodes and cathodes being made according to the present 
invention, it is not necessary that the same material construction be 
employed for both electrodes as long as all materials meet the criteria 
described herein. If the electrode materials are not the same, it is 
desirable to saturate the electrolyte with all materials so that none is 
consumed during operation of the cell. 
The use of electrode constructions satisfying the criteria described herein 
opens up new possibilities for the design of molten salt electrolytic 
cells. One such design, employing a horizontal monopolar anode, is 
schematically illustrated in FIG. 2. Electrolytic cell 40 has a steel 
outer shell 58 with thermal insulation 56 on the inside of the shell 58 
and contains a single anode 42 at the top of cell 40. A protective surface 
layer 41 is present on the surface of the anode 42 at all surfaces of the 
anode which contact the molten electrolyte 45. Anode 42 is connected to a 
supply of electric current by anode rod 44. Molten aluminum 46 is produced 
on the top surface of the cathode located at the bottom of the cell. The 
cathode can be formed from a collector bar 48 embedded in a cathode block 
49 which can be formed to have the same protective material layer 41 as 
the anode. Cell 40 includes a cell lining 52 covered with frozen 
electrolyte 54. 
Another design for a molten salt electrolytic cell employing materials 
meeting the criteria as described herein for the electrodes and cell 
lining is schematically illustrated in FIG. 3. Cell 60 has a series of 
vertically oriented anodes 62 formed having a protective surface from a 
material according to this invention. Cell 60 also contains a plurality of 
vertically oriented cathodes 64 which are preferably also formed in 
accordance with the teachings of this invention. 
Cell lining 66, which is enclosed within a steel outer vessel 68, is also 
formed to have a protective surface layer 65, however, this layer is of a 
different material, and is of the type described in previously 
incorporated U.S. Pat. No. 4,999,097. 
In the standard case where the relative density of liquid metal product is 
greater than that of the molten electrolyte, oxygen gas produced at anodes 
62 rises to the melt surface and liquid metal product 69 falls to the 
bottom of cell 60. Alternatively, in a case where the relative density of 
liquid metal product and molten electrolyte is inverted from the value in 
a present cell, both the oxygen gas and liquid metal product rise to the 
melt surface. Under these conditions, it is desirable to interpose a 
retaining structure or semi-wall 70 between anodes 62 and cathodes 64 to 
prevent the buoyant liquid metal product from forming an electrical short 
between electrodes. The choice of material for semi-wall 70 is subject to 
the same considerations as the choice of material for lining 66. In order 
not to reduce the ability of the electrolyte to dissolve the oxide-based 
feed material, semi-wall 70 and lining 66 should preferably consist of the 
same material. The semi-wall 70 may also include a protective surface 
layer 65 of the type employed on the cell lining 66. 
Still another design for a molten bath electrolytic cell is schematically 
illustrated in FIG. 4. Cell 80 includes a horizontal bipolar electrode 
stack 82. In such a design, each electrode element consists of an anodic 
surface and a cathodic surface having a protective surface layer and 
separated from neighboring elements by electrically insulating spacers. A 
positive feeder electrode 84 and negative feeder electrode 86 are placed 
on the top and bottom of stack 82, respectively. Electrode elements have a 
foundation and a protective surface layer formed from the materials 
described previously. The cell lining 88, enclosed in steel jacket 90, can 
be selected to have the same protective material 65 as that of the cell in 
FIG. 3, or it can comprise a more conventional material. If liquid metal 
product 92 is denser than the molten electrolyte 94, the bipolar stack is 
charged to make the upper surface of each element cathodic and the lower 
surface anodic. By providing a central vent, enhanced circulation of the 
electrolyte can be achieved as a consequence of the gas lift. If the 
liquid metal product is less dense in the electrolyte, a vertical bipolar 
arrangement is preferred. In this case, both the liquid metal product and 
oxygen gas rise to the melt surface. In this case it is necessary to 
introduce a retaining structure or semi-wall to prevent the liquid metal 
product from shorting the cathode to the anode. 
A cross sectional view of one embodiment of an electrode surface is 
represented schematically in FIG. 5. In this embodiment, the electrode 
100, has a base 102, a metallic alloy layer 104 and an oxide layer 106. 
The base 102 is a material that is electrically conductive and readily 
formed into a desired electrode shape. The metallic alloy 104 comprises an 
alloy of the product metal and a more noble metal. In the case of an 
aluminum cell, the alloy 104 preferably comprises an alloy of aluminum 
with copper, nickel, iron or combinations thereof. The protective surface 
layer 106 comprises an oxide of the cell product metal. Thus, for aluminum 
cells, the protective surface layer comprises Al.sub.2 O.sub.3. 
Although the discussion above has largely been limited to electrolytic 
cells and methods for producing aluminum metal from molten salts, the 
materials described herein can also be employed in such cells and methods 
for producing other metals. For example, the criteria employed herein to 
select protective materials for the electrodes of aluminum cells can also 
be applied to select protective materials suitable for the production of 
magnesium, neodymium or other metals from oxide-based feed materials. In 
these cases, the material selected for the electrode must meet the same 
criteria adapted for the specific metal to be produced rather than for 
aluminum. Thus, the electrode will comprise at least an alloy of the 
product metal and a more noble metal upon which is formed an oxide by the 
product metal. 
The invention will now be more particularly pointed out in the examples 
below. 
EXAMPLES 
Example 1 
Aluminum Deposition Using an Aluminum Bronze Anode 
Electrolytic production of aluminum was conducted in a laboratory-scale 
cell of the following design. The anode was a cylinder, 13/16 in. in 
diameter .times.1 in. tall, made of an alloy having a composition of 11.8% 
by weight aluminum, with the balance being copper. An inconel rod, 1/8 in. 
in diameter, was welded to the top of the anode and served as the current 
lead. A sheath of hot-pressed boron nitride covered the vertical and upper 
surfaces of the anode. This was used both to restrict current flow to the 
bottom face of the anode and to protect the anode from exposure to the 
electrolyte at its free surface where it was suspected that conditions are 
highly corrosive. The cathode consisted of a shard of titanium diboride 
plate, 1/4 in. thick, which was covered by a layer of aluminum metal. A 
tungsten rod, 1/8 in. in diameter, contacted the titanium diboride shard 
and served as the current collector. To prevent metal reduction on the 
tungsten rod it was isolated from the electolyte by means of a tube made 
of pyrolytic boron nitride. 
The electrolyte was contained in an aluminum oxide crucible lined with a 
tube of the same material. This had the effect of giving the crucible a 
double wall so as to extend its service life. The electrolyte contained 
cryolite, Na.sub.3 AlF.sub.6, and aluminum fluoride, AlF.sub.3, in 
proportion to give a bath ratio of 1.15, calcium fluoride, CaF.sub.2 in 
the amount of 5% by weight, and aluminum oxide in the amount exceeding its 
saturation value by 4% by weight. 
Electrolysis was conducted for 47 hours. Cell temperature was 970.degree. 
C. Cell current was 4 A. The cell was constantly flushed with a flow of 
argon gas. Oxygen was detected using an oxygen sensor in the argon stream 
exiting the cell. For 31 of the 47 hours, oxygen was detected in the exit 
gas, and during this time the cell voltage measured approximately 3.5 V. 
For the other 16 hours oxygen was not detected in the exit gas, and during 
this the cell voltage was 1.5 V. 
The production of the aluminum was confirmed by weighing the metallic 
product at the bottom of the cell. The composition of this metal was 
confirmed by energy dispersive spectoscopy using a scanning electron 
microscope and found to be predominantly aluminum with a small amount of 
copper (on the order of about 1.7% by weight). The exact amount could not 
be determined as there was uncertainty in the weight of the initial charge 
of aluminum present in the cell at the beginning of the experiment and of 
the final aluminum content of the cell at the conclusion of the 
experiment. Even so, the presence of copper was attributed to the fact 
that for 16 of the 47 hours the cell was in operation, the cell voltage 
was below that expected for oxygen evolution. This oxygen evolution is a 
factor in establishing and maintaining the protective surface layer of 
aluminum oxide. 
To the naked eye, the anode appeared intact and showed no evidence of 
dissolution. However, the anode had undergone a change in its shape and 
color. The anode appeared larger than its original dimension and its 
exterior was more copper colored. The anode was cut open, and the cross 
section showed an inner zone having the characteristic yellow bronze 
color. This was surrounded by an outer zone having a color more 
characteristic of copper metal. This is consistent with the observation 
that for some 16 hours no oxygen evolution could be confirmed. During this 
time it is expected that the anode reaction was the electrodissolution of 
aluminum. This example demonstrates that, provided the conditions in the 
cell maintain the oxide film on the surface, an anode having the 
composition used is capable of supporting the electrolytic production of 
aluminum with the generation of oxygen gas as the accompanying reaction. 
Example 2 
Aluminum Deposition Using an Aluminum Bronze Anode 
Electrolytic production of aluminum was conducted in a laboratory-scale 
cell having a design similar to that of the previous example. However, in 
this example, the anode was made of an alloy consisting of aluminum in the 
amount of 15% by weight, with the balance being copper. No sheath 
protected the anode. The tungsten rod acting as the cathode current 
collector was sheathed with tubing made of aluminum oxide. The electrolyte 
composition was the same as that in Example 1. 
Electrolysis was conducted for a period of 4 hours. Cell temperature was 
970.degree. C. Current was set at 10 A. Cell voltage was measured at 
between 5.0 and 5.6 V. 
In confirmation of oxygen generation at the anode, oxygen was detected in 
the exit gas. The production of aluminum was confirmed by weighing the 
metallic product at the bottom of the cell. The composition of the metal 
product was confirmed by energy dispersive spectroscopy using a scanning 
electron microscope and found to contain 0.45% by weight copper, with the 
balance being aluminum and tungsten which had alloyed with the product 
metal in substantial amounts. The anode remained intact and showed no 
evidence of dissolution. 
Example 3 
Aluminum Deposition Using an Aluminum Bronze Anode - Electrolyte not 
saturated with aluminum oxide 
Electrolytic production of aluminum was conducted in a laboratory-scale 
cell having a design similar to that of the previous examples. The anode 
was made of an alloy consisting of aluminum in the amount of 15% by 
weight, with the balance being copper. No sheath protected the anode. The 
purpose of this test was to learn whether the anode could function in an 
electrolyte not saturated with aluminum oxide. Accordingly, no aluminum 
oxide was employed in the construction of the cell where it would be in 
direct contact with the electrolyte. The tungsten rod acting as cathode 
current collector was sheathed with tubing made of pyrolytic boron 
nitride. The electrolyte was contained in a crucible of pyrolytic boron 
nitride. The composition of the electrolyte was the same as that cited in 
the examples above with the exception of the concentration of aluminum 
oxide was present in the amount 7% by weight. 
During the course of electrolysis the concentration of aluminum oxide 
decreased to a value of approximately 3% by weight. Electrolysis was 
conducted for a period of 2 hours. Cell temperature was 970.degree. C. 
Current was set at 10 A. Cell voltage was measured at between 5.5 and 5.9 
V. 
In confirmation of oxygen generation at the anode, oxygen was detected in 
the exit gas. The production of aluminum was confirmed by weighing the 
metallic product at the bottom of the cell. The composition of this metal 
was confirmed by energy dispersive spectroscopy using a scanning electron 
microscope and found to be in excess of 99.3% by weight aluminum and 0.7% 
by weight copper. The anode remained intact and showed no evidence of 
dissolution. 
Equivalents 
Although the specific features of the invention are included in some 
embodiments and drawings and not in others, it should be noted that each 
feature may be combined with any or all of the other features in 
accordance with the invention. 
Thus, the invention provides an inert, non-consumable electrode for use in 
electrolytic cells for the production of metals from oxide based feed 
materials. 
It should be understood, however, that the foregoing description of the 
invention is intended merely to be illustrative thereof, that the 
illustrative embodiments are presented by way of example only, and that 
other modifications, embodiments, and equivalents may be apparent to those 
skilled in the art without departing from its spirit.