U.S. patent application number 10/810400 was filed with the patent office on 2004-09-16 for low temperature alkali metal electrolysis.
Invention is credited to Bergmann, Oswald Robert, Blank, Howard M., Simmons, Walter John, Thompson, Jeffery S..
Application Number | 20040178080 10/810400 |
Document ID | / |
Family ID | 24138438 |
Filed Date | 2004-09-16 |
United States Patent
Application |
20040178080 |
Kind Code |
A1 |
Thompson, Jeffery S. ; et
al. |
September 16, 2004 |
Low temperature alkali metal electrolysis
Abstract
A low temperature alkali metal electrolysis process is provided.
The process comprises carrying out the electrolysis in the presence
of a co-electrolyte and an alkali metal halide. The co-electrolyte
comprises (1) a nitrogen-containing compound and optionally one ore
more Group IB halides, Group IIIA halides, Group VIII halides; (2)
a Group IIIA halide, a Group VB halide, or combinations of a Group
IIIA halide and a Group VB halide; or (3) water. Also provided is a
low temperature electrolysis process, which comprises carrying out
the process using a cathode that comprises (1) a liquid alkali
metal; (2) an alloy of two or more metals selected from the group
consisting of bismuth, lead, tin, antimony, indium, gallium,
thallium, and cadmium; or (3) an electrically conductive liquid
solvated alkali metal. Further provided is an electrolyte
comprising an alkali metal halide and a co-electrolyte that
comprises (1) a nitrogen-containing compound and optionally one ore
more Group IB halides, Group IIIA halides, Group VIII halides or
(2) a Group IIIA halide, a Group VB halide, or combinations of a
Group IIIA halide and a Group VB halide.
Inventors: |
Thompson, Jeffery S.;
(Wilmington, DE) ; Blank, Howard M.; (Wilmington,
DE) ; Simmons, Walter John; (Martinsburg, WV)
; Bergmann, Oswald Robert; (Wilmington, DE) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
24138438 |
Appl. No.: |
10/810400 |
Filed: |
March 26, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10810400 |
Mar 26, 2004 |
|
|
|
10046485 |
Jan 14, 2002 |
|
|
|
6730210 |
|
|
|
|
10046485 |
Jan 14, 2002 |
|
|
|
09536424 |
Mar 28, 2000 |
|
|
|
6368486 |
|
|
|
|
Current U.S.
Class: |
205/406 ;
205/409 |
Current CPC
Class: |
C25C 3/02 20130101; C25C
1/02 20130101; C25C 7/02 20130101 |
Class at
Publication: |
205/406 ;
205/409 |
International
Class: |
C25C 003/02 |
Claims
What is claimed is:
1. A process for producing an alkali metal comprising electrolyzing
an alkali metal halide in the presence of a co-electrolyte which is
(1) a nitrogen-containing compound and optionally one ore more
Group IB halides, Group IIIA halides, Group VIII halides; (2) a
Group IIIA halide, a Group VB halide, or combinations of a Group
IIIA halide and a Group VB halide; or (3) water.
2. A process according to claim 1 wherein said process is carried
out at a temperature below 300.degree. C.
3. A process according to claim 1, or 2 wherein said nitrogen
compound is one or more compound selected from the group consisting
of imidazolium salts, N-alkylpyridinium salts, tetraalkylammonium
salts, and tetraalkylphosphonium salts.
4. A process according to claim 1, 2, or 3 wherein said nitrogen
compound is one or more compound selected from the group consisting
of 1-ethyl-3-methyl-1H-imidazolium,
1-propyl-3-methyl-1H-imidazolium,
1-butyl-2,3-dimethyl-1H-imidazolium,
1-butyl-2,3,4,5-tetramethylimidzaoli- um,
1,2,3,4,5-pentamethylimidazolium, 1-methylpyridinium, a derivative
of pyridine having alkyl groups in non-nitrogen positions on the
ring, tetramethylammonium, tetramethylphosphonium; said Group IB
halide is copper halide, silver halide, or gold halide; said Group
IIIA halide is aluminum halide, boron halide, gallium halide,
indium halide, or thallium halide; ans said Group VIII halide is
iron halide, cobalt halide, or nickel halide.
5. A process according to claim 4 wherein said imidazolium halide
is 1-ethyl-3-methyl-1 H-imidazolium chloride and said Group IIIA
halide is aluminum chloride.
6. A process according to claim 1, 2, 3, or 5 wherein said
co-electrolyte comprises aluminum chloride and
1-ethyl-3-methyl-1H-imidazolium chloride and said process is
carried out in an electrolysis cell containing an ion exchange
membrane.
7. A process according to claim 1 or 2 wherein said co-electrolyte
comprises aluminum chloride and a fully alkylated imidazolium
chloride and said process is carried out in an electrolysis cell
containing an ion exchange membrane or a diaphragm.
8. A process according to claim 1 or 2 wherein said co-electrolyte
comprises aluminum chloride and tantalum chloride and said process
is carried out in an electrolysis cell containing an ion exchange
membrane.
9. A process according to claim 1 or 2 wherein said co-electrolyte
is water.
10. A process according to claim 9 wherein said process is carried
out in an electrolysis cell whose cathode consists of a liquid, low
melting alloy of two or more metals selected from the group
consisting of Bi, Pb, Sn, Sb, In, Ga, Tl, and Cd to produce an
alkali metal.
11. A process according to claim 10 and wherein the alkali metal is
separated from said alloy by distillation whereby a denuded liquid
cathode alloy is produced and recycled to the said cell.
12. An electrolysis process comprising carrying out said process
using a cathode that comprises (1) a liquid alkali metal; (2) an
alloy of two or more metals selected from the group consisting of
bismuth, lead, tin, antimony, indium, gallium, thallium, and
cadmium; or (3) an electrically conductive liquid solvated alkali
metal.
13. A process according to claim 12 wherein said cathode comprises
a liquid sodium.
14. A process according to claim 12 wherein said cathode comprises
an alloy of two or more metals selected from the group consisting
of bismuth, lead, tin, antimony, indium, gallium, thallium, and
cadmium.
15. A process according to claim 12 wherein said cathode comprises
an electrically conductive liquid solvated sodium and an organic
solvent.
16. A process according to claim 15 wherein said alkali metal is
separated from said organic solvent by distillation whereby a
denuded liquid cathode alloy is produced and recycles to said
electrolysis cell.
17. A process according to claim 1, 2, 3, 5, 10, 11, 12, 13, 14,
15, or 16 wherein said alkali metal is sodium, potassium, or
lithium.
18. A process according to claim 17 wherein said alkali metal is
sodium.
19. A process according to any of claims 17 wherein said has an
impurity less than 400 mg per kg of said sodium.
20. An electrolyte composition comprising an alkali metal halide
and (1) a nitrogen-containing compound and optionally one ore more
Group IB halides, Group IIIA halides, Group VIII halides; or (2) a
Group IIIA halide, a Group VB halide, or combinations of a Group
IIIA halide and a Group VB halide.
Description
FIELD OF THE INVENTION
[0001] The invention relates to an electrolysis process and
electrolytes therefor for producing an alkali metal.
BACKGROUND OF THE INVENTION
[0002] Alkali metals are highly reactive elements and are not found
in elemental form in nature. Typical reducing agents, such as
hydrogen, are not strong enough to reduce the alkali metals from
their compounds to the metallic state. Electrolytic reduction is
necessary and was used historically in the classic experiments
leading to the discovery of the alkali metals in elemental form in
1807 by Sir Humphry Davy, Assistant to Count Rumford/Thompson at
the Royal Institution in London. Electrolytic reduction is used for
industrial production of the alkali metals. The currently used
process, on a worldwide basis, is the so-called "Downs" Process,
which was introduced in the early part of the 20th century for the
production of sodium and lithium from their chlorides.
[0003] The Downs Process uses a molten salt electrolyte consisting
of a mixture of NaCl, CaCl.sub.2 and BaCl.sub.2 in order to reduce
the melting temperature of the electrolyte to slightly below
600.degree. C. This makes the process more practical compared to
using pure NaCl which has a much higher melting point of about
800.degree. C. Nevertheless, operating an electrolytic process at
such temperature is difficult and presents numerous operating
constraints. Because of the high operating temperature of the Down
Process, the cell design uses concentrically cylindrical cathodes,
wire mesh diaphragms, and anodes rather than the much more space
efficient stacked multiple flat electrode and diaphragm element
configuration that is normally used in electrochemical engineering
practice. Furthermore, the high operating temperature would make a
flat wire-mesh steel diaphragm so soft that it would be
mechanically unstable and flap back and forth between anode and
cathode causing partial shorting/arcing and thereby causing holes
to be burned in the diaphragm. Holes in the diaphragm would allow
back mixing of sodium produced at the cathode and chlorine produced
at the anode, thereby causing low current efficiency of the cell.
On the other hand, the concentric cylindrical configuration of the
steel diaphragm between the electrodes avoids this difficulty
because a wire-mesh cylinder is mechanically much stiffer and
mechanically more stable than a flat wire-mesh screen of the same
kind.
[0004] The above-described concentric cylindrical cell design of
the Downs Process, necessitated by the high operating temperature
of about 600.degree. C., also means that the Downs cell has very
poor space efficiency. This translates directly into high capital
and operating cost per unit production.
[0005] The high operating temperature of the Downs cell in
combination with the fact that the molten mixed salt electrolyte
has a freezing temperature only about 20.degree. C. below the cell
operating temperature makes smooth operation of the cells
difficult. Cell `freeze-ups` and other "upsets" are frequent and
result in unusually high operating labor requirements for an
industrial electrolytic process. This in turn is also the reason
why the Downs Process is not amenable to automation. Lithium is
currently produced by a modification of the Downs process.
[0006] Although there is a low temperature electrolytic process
that deposits metallic sodium at the cathode from an NaCl/H.sub.2O
solution, the sodium metal is not pure sodium but a liquid
mercury/sodium amalgam containing a low percentage of sodium,
usually about 0.5% Na. The balance of over 99% is mercury metal.
This process is used to produce aqueous sodium hydroxide solutions
by reacting the dilute sodium amalgam with water. See generally
Sodium, Its Manufacture, Properties and Uses, by Marshall Sitting,
American Chemical Society Monograph Series, Reinhold Publishing
Corp., New York (1956) and Electrochemical Engineering, by C. L.
Mantell, McGraw-Hill Book Co., Inc., New York, Toronto, London
(1960). This process cannot be used to produce metallic sodium
economically because of the problems and the cost of separating
mercury from sodium. For example, separation by distillation is
impractical because mercury has a much lower boiling point
(357.degree. C.) than sodium (880.degree. C.) and it would be too
costly to vaporize 99% mercury to obtain about 1% sodium as the
residue.
[0007] In recent years fundamental physico-chemical studies have
been carried out on electrolytes based on non-aqueous, organic
solvents for alkali metal chlorides for battery applications. See
J. Electrochem. Soc. Vol. 143, No. 7, pages 2262-2266, July 1996.
None of this work resulted in a process for alkali metal
production.
[0008] Therefore, there is an increasing need to develop an
electrolytic process that can be used to produce an alkali metal
more economically. There is also a need to develop a process that
can improve operability such as, for example, making automation
possible.
SUMMARY OF THE INVENTION
[0009] According to the invention a low temperature electrolysis
process is provided, which comprises carrying out the electrolysis
in the presence of a co-electrolyte and an alkali metal halide. The
co-electrolyte comprises (1) a nitrogen-containing compound and
optionally a Group IIIA halide, a Group IB halide, a Group VIII
halide, or combinations of two or more thereof; (2) a Group IIIA
halide, a Group VB halide, or combinations of a Group IIIA halide
and a Group VB halide; or (3) water.
[0010] Also according to the invention an electrolysis process is
provided, which comprises carrying out the process using a cathode
comprising (1) a liquid alkali metal; (2) a liquid low melting
alloy of two or more metals selected from the group consisting of
bismuth, lead, tin, antimony, indium, gallium, thallium, and
cadmium; or (3) a conductive liquid solvated alkali metal.
[0011] Further according to the invention an electrolyte is
provided. The electrolyte comprises an alkali metal halide and a
co-electrolyte that comprises (1) a nitrogen-containing compound
and optionally a Group IIIA halide, a Group IB halide, a Group VIII
halide, or combinations of two or more thereof; (2) a Group IIIA
halide, a Group VB halide, or combinations of a Group IIIA halide
and a Group VB halide; or (3) water.
DETAILED DESCRIPTION OF THE INVENTION
[0012] The electrolysis is carried out at a low temperature. The
term "low temperature" refers to a temperature lower than about
300.degree. C., preferably lower than about 250.degree. C., and
most preferably lower than 200.degree. C. It can be in the range of
from about 20.degree. C. to about 300.degree. C., preferably about
50.degree. C. to 250.degree. C., more preferably about 70.degree.
C. to about 200.degree. C., and most preferably 90.degree. C. to
200.degree. C. The invention electrolysis can produce a
substantially pure alkali metal such as sodium. The term
"substantially pure" refers to an alkali metal containing less than
about 400, preferably less than about 300, more preferably less
than about 200 and most preferably less than 100 mg of impurities
per kg of the alkali metal.
[0013] Any alkali metal halides can be used in the invention. The
term "alkali metal" is referred to lithium, sodium, potassium,
rubidium, cesium, francium, or combinations of two or more thereof.
The presently preferred alkali metal halide is sodium chloride for
it is widely used in an electrolysis to produce sodium.
[0014] According to one embodiment of the invention, a
nitrogen-containing compound, preferably a nitrogen-containing
ionic compound having a cationic moiety and an anionic moiety, can
be used as a co-electrolyte. It can be a solid or liquid.
Presently, a non-aqueous liquid is preferred. Preferably, the
cationic moiety is an organic cation. The cationic moiety includes
one or more imidazolium salts, N-alkylpyridinium salts,
tetraalkylammonium salts, and tetraalkylphosphonium salts. For
example, any imidazolium salts that, when combined with an alkali
metal halide, can reduce the melting point of an alkali metal
halide to a low temperature disclosed above can be used.
[0015] The anion moiety can be any anion as long as it can be used
to lower the melting point of an alkali metal halide. Examples of
suitable anions include one or more chloride, bromide, iodide,
tetrafluoroborate, and hexafluorophosphate. The presently preferred
anion is chloride.
[0016] The nitrogen-containing compound can contain as many
derivatives as different types of substituents. It is presently
preferred that a partially alkylated or a fully alkylated
imidazolium halide such as chloride be used. A fully alkylated
imidazolium halide is one in which all hydrogen on the imidazolium
ring is replaced with a hydrocarbyl radical that can contain 1 to
about 20 carbon atoms and can be an alkyl, alkenyl, or aryl
radical. Generally, the ammonium and phosphonium salts are
peralkylated, the pyridinium salt is N-substituted, and imidazolium
salt is N,N-disubstituted.
[0017] Examples of suitable nitrogen-containing compound include,
but are not limited to, 1-ethyl-3-methyl-1 H-imidazolium,
1-propyl-3-methyl-1 H-imidazolium,
1-butyl-2,3-dimethyl-1H-imidazolium,
1-butyl-2,3,4,5-tetramethylimidzaolium,
1,2,3,4,5-pentamethylimidazolium, 1-methylpyridinium, a derivative
of pyridine having alkyl groups in non-nitrogen positions on the
ring, tetramethylammonium, tetramethylphosphonium, and combinations
of two or more thereof. The presently preferred imidazolium halide
is 1-ethyl-3-methyl-1H-imidazolium chloride,
1-butyl-2,3,4,5-tetramethyl-1H-imidzolium chloride, or
1,2,3,4,5-pentamethyl-1H-imidazolium chloride.
[0018] An imidazolium halide can also be combined with a Group IB
metal halide, a Group IIIA metal halide, a group VIII metal halide,
or combinations of two or more thereof. The term "Group IB", "Group
IIIA", "Group VB", or "Group VIII" used in the invention refers to
the CAS version of the Periodic Table of the Elements, CRC Handbook
of Chemistry & Physics, 67th edition, 1986-1987, CRC Press,
Boca Raton, Fla. Examples of suitable Group IB halides include
copper halide such as copper chloride and copper bromide; silver
halide such as silver chloride; and gold halide such gold chloride,
and combinations of two more thereof. Examples of Group IIIA
halides include aluminum halide such as aluminum chloride and
aluminum bromide; boron halide such as boron chloride; gallium
halide such as gallium chloride; indium halide such as indium
chloride; thallium halide such as thallium chloride; or
combinations of two or more thereof. Examples of suitable Group VB
halides include tantalum halide such as tantalum chloride; vanadium
halide such as vanadium chloride; niobium halide such as niobium
chloride; or combinations of two or more thereof. Examples of Group
VIII halides include one or more of iron halides such iron
chlorides and iron bromides; cobalt halides such as cobalt chloride
and cobalt bromide; and nickel halide such as nickel chloride and
nickel bromide; rhodium halide such as rhodium chloride; and
rhenium halide such as rhodium chloride.
[0019] The co-electrolyte can be present in any quantity so long as
the quantity can reduce the melting temperature of an alkali metal
halide to the low temperature disclosed above. Generally, the molar
ratio of the co-electrolyte to the alkali metal halide can be in
the range of from about 001:1 to about 100:1, preferably about
0.1:1 to about 10:1, and most preferably about 0.5:1 to about 2:1.
If a second metal (Group IB, IIIA, or VIII) halide is present with
an imidazolium salt, the molar ratio of the second metal halide to
the alkali metal halide can be in the same range. For example, an
electrolyte comprising 1-ethyl-3-methyl-1H-imidazolium chloride,
aluminum chloride, and sodium chloride can have a molar ratio of
1-ethyl-3-methyl-1H-imidazolium chloride to aluminum chloride to
sodium chloride about 1:2:1.
[0020] The anode of an electrolysis cell can consist of
electrically conductive carbon, an DSA.RTM. (dimensionally stable
anode), a Group VIII metal oxide, or a Group VIII metal such as
platinum, which is not corroded by the anodically liberated halide
such as, for example, chlorine gas. On the cathode side of the cell
the cathode itself can consist of electrically conductive carbon or
a metal such as a Group VIII metal in contact with a polymeric ion
exchange membrane. The cathode can be provided with physical means
for transporting cathodically produced liquid alkali metal to an
external, heated collection chamber. During operation of the cell,
the liquid alkali metal produced can share in the cathode function
with the cathode. Physical means for transporting liquid alkali
metal away from the cathode can be machined channels or grooves, a
system of holes, or use of porous materials having interconnected
pores permitting molten alkali metal to flow into the collection
vessel. There are numerous other ways of performing this function
which one skilled in the art can devise depending on one's
preference.
[0021] Wishing not to be bound by theory, use of a polymeric ion
exchange membrane can prevent cathodically induced chemical
degradation of the imidazolium-based electrolyte during operation.
The ion exchange membrane can be a material that lets alkali metal
ions pass easily through the membrane but will not let significant
amounts of higher-valent cations pass. Examples of suitable
membrane materials include, but are not limited to, perfluorinated
ion-exchange polymers available under the trade name Nafion.RTM.,
available from E. I. duPont de Nemours & Company, Wilmington,
Del. In order to protect the Nafion.RTM. membrane from degradation
caused by reaction with metallic alkali metal formed on the surface
of the cathode, a second membrane can be used. The second membrane
can be interspersed between the first membrane and the cathode.
Examples of suitable second membranes include acrylic polymers.
Other materials are also suitable. It is preferred to have the
Nafion.RTM. membrane, the acrylic membrane and the cathode in
intimate physical contact with each other. This can be accomplished
in a number of ways. For example, it can be done by painting a
solution of the acrylic polymer onto a carbon cathode followed by
applying a solution of the Nafion.RTM. polymer and drying the
coated cathode/membrane assembly.
[0022] According to the invention, moisture is preferably excluded
because water can react with, and hydrolyze, a halide in the
electrolyte. This can be achieved by constructing the cell in a
gas-tight, hermetically sealed fashion. In order to achieve a high
degree of space efficiency it is advantageous to construct the cell
in the well-known stacked-multiple-flat-plate arrangement. Known
electrochemical engineering practices are used in providing means
for circulating and replenishing the electrolyte, for providing
electrolysis current, for handling cathode product liquid alkali
metal and anode product chlorine gas. The cell is generally
operated in a continuous mode. Because of its low temperature
operation the invention process is well suited for automated
operation.
[0023] Because a fully alkylated imidazolium chloride is resistant
to cathodic reduction, a porous diaphragm instead of a membrane can
be used when it is present in the electrolyte thereby reducing the
operating cost. The diaphragm can consist of a variety of different
materials. Illustrative examples are glass-fiber fabric and
polymeric screens and fabrics. Cell design and construction are
similar to those disclosed above.
[0024] According to the invention, the co-electrolyte can also be a
Group IIIA halide, a Group VB halide, or combinations thereof. For
example, a near-eutectic inorganic salt mixture of a group IIIA
halide and an alkali metal halide or mixtures of a Group IIIA
halide, a Group VB halide, and an alkali metal halide. The molar
ratio of a Group IIIA halide or a Group VB halide to an alkali
metal halide can be in the range of from about 001:1 to about
100:1, preferably about 0.1:1 to about 10:1, and most preferably
about 0.5:1 to about 2:1. For example, a mixture comprising
NaCl/AlCl.sub.3/TaCl.sub.5 (molar ratio of about 20:70:10) can be
used. These mixtures can also be used with an ion exchange membrane
similar to that described above. Operating temperature of an
electrolysis cell using one of these mixtures can be in the range
of about 130.degree. C. to 160.degree. C.
[0025] According to the invention, an electrolysis can also be
carried out using an aqueous solution of an alkali metal halide,
i.e., water can be a co-electrolyte. The weight percent (%) of an
alkali metal halide can be in the range of from about 1 to about
40%, preferably about 10 to about 35%, more preferably about 20 to
about 35% and most preferably about 30%. For example, a solution
containing about 30% NaCl and 70% H.sub.2O can be used with or
without a porous diaphragm.
[0026] When water is used as co-electrolyte, the cathode can
consist of a metal cathode that is a metal alloy having a melting
point below the boiling temperature of the aqueous solution at the
internal operating pressure used in the operation of the cell. If
the cell is operated at ambient atmospheric pressure, the metal
cathode alloy can have a melting temperature below about
105.degree. C. A suitable metal cathode can also include a high
hydrogen overvoltage to favor alkali metal deposition at the
cathode over hydrogen evolution; a degree of solubility of metallic
alkali metal in the metal cathode alloy; and a sufficiently high
boiling temperature of the liquid metal cathode alloy. The boiling
temperature of the alloy is generally substantially above the
boiling temperature of an alkali metal to permit post-electrolysis
separation of alkali metal from the liquid metal cathode alloy via
various distillation methods. According to the invention, it is
preferred that the alloy has (1) a melting temperature below about
105.degree. C., (2) a high hydrogen overvoltage (in the same order
as mercury), (3) a solubility for an alkali metal, and (4) a
boiling temperature substantially above that of the alkali metal.
Examples of suitable liquid metal cathode alloys can be an alloy
derived from two or more metals selected from the group consisting
of bismuth (Bi), lead (Pb), tin (Sn), antimony (Sb), cadmium (Cd),
gallium (Ga), thallium (Tl), and indium (In). Cell design,
construction and operation can be substantially the same as that
disclosed above. An advantage of using a metal alloy in the cathode
is that, because of the higher density of the liquid metal cathode
alloy compared to that of the electrolyte, a horizontal cell design
can be used, analogous to that of the mercury cells for
caustic/chlorine production. Separation of alkali metal from the
alkali metal-containing liquid cathode alloy can be done in an
operation outside the electrolytic cell thereby facilitating the
electrolysis process. For example, various distillation methods can
be used, such as ambient pressure distillation, vacuum distillation
and/or inert carrier gas distillation using nitrogen or other inert
gas species as the carrier gas. Because of its much lower boiling
temperature (much higher vapor pressure), alkali metal vaporizes
out of the liquid alloy and can be subsequently condensed and
collected in a receiver vessel. The "denuded" liquid metal cathode
alloy can be recycled to the cathode compartment of the cell.
Countercurrent heat exchanger techniques can be used to minimize
the energy requirements of the separation or purification step, or
both steps. Other methods of alkali metal separation, such as
solvent extraction, are also feasible.
[0027] In another embodiment of the invention, an alkali metal
electrolysis process using a liquid "solvated alkali metal" cathode
is provided. The cathode can be used in conjunction with an
ion-exchange membrane or a porous diaphragm. The electrolyte can be
the same as that disclosed above in the first embodiment. A liquid
"solvated alkali metal" cathode can have the advantage that the
cell operating temperature can be well below the melting
temperature of alkali metal (98.degree. C.).
[0028] The term "solvated alkali metal" used herein refers to an
alkali metal and one or more organic solvents such as, for example,
ethers and aromatic compounds. An example of a solvated alkali
metal is sodium/naphthalene/ethylene glycol dimethylether having a
weight ratio of about 25:150:50. There are a number of other
suitable solvent systems for alkali metal. The alkali metal content
in the solvated alkali metal can be any weight % as long as that
content can make the solvated alkali metal solution electrically
conductive. Generally, the alkali metal content can be about 5 to
about 80, preferably about 20 to about 70 and most preferably about
15 to about 60% by weight. After electrolysis a part of the alkali
metal-enriched solvated alkali metal solution can be transferred to
a separate vessel in which the solvent can be removed from the
alkali metal by, for example, distillation. Pure alkali metal can
be transferred to alkali metal storage and shipping by conventional
means. The "denuded" solvent can be recycled into the liquid
solvated alkali metal-cathode stream in the electrolysis
circuit.
[0029] The following example is provided to further illustrate the
invention and are not to be construed to unduly limit the scope of
the invention.
[0030] In the example, a procedure for the preparation of sodium
metal in an imidazolium chloride-aluminum chloride-sodium chloride
molten salt bath is disclosed. A membrane-covered cathode is used
for the selective reduction of sodium ions.
[0031] All chemicals were used as received. 1-Ethyl-3-methyl-H
imidazolium chloride, aluminum chloride (99.99%), naphthalene
(99+%), polyacrylic acid (25 wt % solution in water), and
Nafion.RTM. perfluorinate ion-exchange resin (5 wt % solution in a
mixture of lower aliphatic alcohols and water) were purchased from
Aldrich (Milwaukee, Wis. 53201 USA). Sodium chloride, sodium
hydroxide, potassium hydroxide, and tetrahydrofuran (TX-02484-6)
were from EM Science (Gibbstown, N.J. 08027 USA). Tetrahydrofuran
(THF) was deaerated with nitrogen and stored over activated 4A
molecular sieves prior to use. Stainless steel 304 coupons with a
diameter of 1.2 cm were obtained from Metal Samples Co., Inc.
(Munford, Ala. 36268 USA).
[0032] Electrical Equipment. A CV-27 potentiostat from
Bioanalytical Systems (West Lafayette, Ind. 47906-1382 USA) was
used. A C-200H-type cell from the Electrosynthesis Co., Inc. (E.
Amherst, N.Y. 08540) was used to hold electrodes and molten salt
bath. A flat sample holder from EG&-G Princeton Applied
Research (Princeton, N.J. 08540) was used as a cathode. A
10-cm.sup.2 platinum flag electrode from the Electrosynthesis Co.,
Inc. was used as an anode; the same electrode without the
10-cm.sup.2 platinum foil was used as a pseudoreference
electrode.
[0033] Preparation of Membrane. A 1.2.times.1.2 cm piece of carbon
felt (GF-S2 from Electrosynthesis Co., Inc.) was treated with two
applications of polyacrylic acid solution, which had been converted
to the sodium form by addition of 10 ml of 1 N NaOH and 0.5 g
ground NaOH to 10 ml of polyacrylic acid solution. The carbon felt
was air-dried between the applications and then air-dried
overnight. One surface of the resulting derivatized carbon felt was
painted with Nafion.RTM. solution, which was converted to the
sodium form by addition of 0.02 g NaOH to 2 ml of the Nafion.RTM.
resin described above. The carbon felt was air-dried and then
heated at 160.degree. C. for one minute. The membrane was then cut
to the size of the SS304 coupon and dried under vacuum
overnight.
[0034] Preparation of Molten Salt. All manipulations were conducted
in a Vacuum Atmosphere dry box under nitrogen gas. The molten salt
bath containing 1-ethyl-3-methyl-1H-imidazolium chloride, aluminum
chloride, and sodium chloride in a 1:2:1 molar ratio was prepared
according to the literature procedure disclosed in Journal of the
Electrochemical Society, 143, 2262-2266 (1996). The imidazolium
salt and aluminum chloride were mixed first to generate an acidic
bath, which was then neutralized by addition of sodium chloride.
The ternary bath was allowed to mix overnight (about 16 hours)
prior to use.
[0035] Electrolysis was conducted in a Vacuum Atmosphere dry box
under nitrogen gas. The H-type cell was placed in a heating mantle
and buried in sand to a depth of about 2.5 inches (6.35 cm). The
sand bath temperature was maintained at 100-120.degree. C.
throughout the electrolysis. The molten salt was added to the cell
so that when the flat cell holder was placed in the cell, the frits
connecting the chambers of the cell were covered with molten salt.
Solid sodium chloride was added to the bath prior to the start of
electrolysis to maintain the 1:2:1 molar ratio of imidazolium salt:
aluminum chloride: sodium chloride in the molten salt bath.
[0036] The cathode was assembled in the flat cell holder in the
following manner. A Teflon.RTM. washer was placed in the
Tefzel.RTM.-280 specimen holder. The membrane was added next, with
the Nafion.RTM.-coated side facing out of the holder. A SS304
coupon was placed in the holder next. The sample holder plug and
sample holder cap with O-ring were put in place. Attachment of the
electrode mounting rod and working electrode holder allowed the
assembly to be added to the C-200H-type cell. A 1-cm.sup.2 surface
of the coated carbon felt was in contact with the electrolyte.
[0037] The platinum flag electrode was suspended in the
electrolyte. This chamber was flushed with nitrogen into a
methanolic KOH solution to scrub chlorine generated during the
sodium deposition reaction.
[0038] Potentials (relative to the platinum wire electrode) of
-1.95V to -2.1SV were applied for a total of 15 hours over two
days. Applied potential, current, and bath temperature are shown in
Table 1. After the first 10 hours, the applied potential was
discontinued and the bath temperature was reduced to 96.degree. C.
On restart of the electrolysis, the current slowly climbed.
[0039] Sodium formation was demonstrated in the following manner.
After the 15 hours of electrolysis, the membrane was removed from
the flat cell holder and washed briefly with THF to remove the
molten salt. The carbon felt and SS304 coupon, which were cemented
together, were placed in approximately 5 ml of 0.1 M naphthalene
solution in THF. A green color, indicating sodium naphthalide, was
not observed. Approximately 5 ml of water, following the removal of
the membrane, was added. A vigorous evolution of gas was observed
suggesting that the sodium was formed inside the membrane.
Naphthalene cannot penetrate the membrane, but water is easily
transported through both layers. Sodium reacts vigorously with
water to produce sodium hydroxide and hydrogen gas (F. A. Cotton
and G. Wilkinson, "Advanced Inorganic Chemistry," 4th Edition, John
Wiley and Sons: New York, 1980; pp. 257-258).
1 TABLE 1 Temperature Applied Current Time (hour) (.degree. C.)
Potential (V) (mA) 0.00 89 -2.00 -0.234 0.25 91 -2.00 -0.297 0.58
96 -2.00 -0.427 0.83 100 -2.00 -0.600 1.17 103 -2.00 -0.913 1.42
103 -2.00 -1.151 1.67 103 -2.00 -1.270 2.08 103 -2.00 -1.470
-1.95.sup.a -0.88 2.42 106 -1.95 -1.06 4.08 110 -1.95 -1.37 5.67
112 -1.95 -1.39 6.00 112 -2.00 -2.30 -2.00.sup.b 6.83 112 -2.00
-1.70 -2.10.sup.b -3.57 7.08 112 -2.10 -5.14 -2.15.sup.b 8.41 112
-2.15 -7.27 8.58 112 -2.15 -7.32 10.08 112 -2.15.sup.c -7.37 10.38
104 -2.15 -0.645 10.96 105 -2.15 -0.780 11.13 106 -2.15 -1.210
11.71 108 -2.15 -1.910 11.88 105 -2.10.sup.d -1.410 12.21 106 -2.10
-1.510 12.88 106 -2.10 -1.610 -2.15.sup.b -2.41 13.80 105 -2.15
-2.55 14.22 106 -2.15 -3.30 15.22 98 -2.15 -3.71 .sup.aApplied
potential reduced .sup.bApplied potential increased .sup.cPotential
discontinued; bath cooled to 96.degree. C. .sup.dApplied potential
decreased to -2.10 V
* * * * *