U.S. patent application number 11/269811 was filed with the patent office on 2006-05-18 for processes for separating metals from metal salts.
Invention is credited to Jason C. Brady, Michael T. Kelly.
Application Number | 20060102491 11/269811 |
Document ID | / |
Family ID | 36578370 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060102491 |
Kind Code |
A1 |
Kelly; Michael T. ; et
al. |
May 18, 2006 |
Processes for separating metals from metal salts
Abstract
Electrochemical processes and apparatus for obtaining metals
from metal salts, particularly separating alkali metal and borate
ions from alkali metal borate compounds, are disclosed. Aqueous
solutions of metal borates or metal carbonates are converted to
metals by preferred electrochemical processes. These
electrochemical processes also may be integrated into processes for
the production of borohydrides, such as sodium borohydride.
Inventors: |
Kelly; Michael T.;
(Plainsboro, NJ) ; Brady; Jason C.; (Red Bank,
NJ) |
Correspondence
Address: |
DICKSTEIN SHAPIRO MORIN & OSHINSKY LLP
2101 L Street, NW
Washington
DC
20037
US
|
Family ID: |
36578370 |
Appl. No.: |
11/269811 |
Filed: |
November 9, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60626485 |
Nov 10, 2004 |
|
|
|
Current U.S.
Class: |
205/407 |
Current CPC
Class: |
C07F 5/04 20130101; C25B
1/14 20130101; C25B 1/00 20130101; C25C 1/02 20130101 |
Class at
Publication: |
205/407 |
International
Class: |
C25C 3/02 20060101
C25C003/02 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The invention was made with Government support under
Cooperative Agreement No. DE-FC36-04G014008 awarded by the
Department of Energy. The United States Government has certain
rights in this invention.
Claims
1. A process for reducing a metal borate, comprising: providing an
electrolytic cell containing an anode compartment and a cathode
compartment separated by a separator which is permeable to metal
ions and not permeable to water and water vapor; supplying a metal
borate compound to the anode compartment; and applying an electric
potential to the cell.
2. The process of claim 1, wherein the metal borate compound is a
borate salt having the formula zM.sub.nO.xB.sub.2O.sub.3.yH.sub.2O,
wherein z is 0.5 to 5, x is 0.1 to 5, y is 0 to 10, n is 1 or 2;
and M is an alkali metal ion or an alkaline earth metal ion.
3. The process of claim 2, wherein M is selected from the group
consisting of Li.sup.+, Na.sup.+ and K.sup.+.
4. The process of claim 1, wherein the metal borate compound is
supplied as an aqueous solution.
5. The process of claim 1, further comprising supplying a metal or
a metal alloy to the cathode compartment.
6. The process of claim 5, further comprising heating at least the
cathode compartment of the cell to a temperature of about 95 to
about 120.degree. C.
7. The process of claim 1, further comprising reacting hydrogen or
a hydrogen containing gas with the metal produced at the
cathode.
8. The process of claim 1, further comprising passing hydrogen or a
hydrogen containing gas in the anode compartment.
9. The process of claim 8, further comprising supplying hydrogen or
hydrogen containing gas to the anode compartment through a gas
inlet means.
10. The process of claim 9, wherein the gas inlet means is selected
from the group consisting of a pipe, a sparger, a hose and a
hydrogen diffusion material.
11. The process of claim 1, further comprising electrooxidizing
hydrogen at the anode.
12. The process of claim 1, further comprising providing a
supporting electrolyte to the anode compartment.
13. The process of claim 12, wherein the supporting electrolyte
comprises an anion selected from the group consisting of sulfate,
perchlorate, nitrate, and phosphate.
14. The process of claim 12, wherein the supporting electrolyte is
selected from the group consisting of sodium sulfate, sodium
perchlorate, sodium phosphate, and sodium nitrate.
15. The process of claim 1, wherein the separator comprises a
material selected from the group consisting of
lithium-.beta.-aluminum oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, sodium-.beta.-aluminum
oxide, sodium-.beta.''-aluminum oxide,
sodium-.beta./.beta.''-aluminum oxide, potassium-.beta.-aluminum
oxide, potassium-.beta.''-aluminum oxide, and
potassium-.beta./.beta.''-aluminum oxide.
16. The process of claim 1, wherein the separator is a NaSICON
membrane.
17. The process of claim 1, wherein the separator is a LiSICON or a
KSICON membrane.
18. The process of claim 1, wherein the electrical potential is at
least about 1.4 volts.
19. The process of claim 1, wherein the metal borate compound is a
product discharged from a hydrogen generation reaction.
20. The process of claim 19, wherein the product comprises sodium
metaborate and sodium hydroxide.
21. The process of claim 20, wherein the product comprises at least
27% sodium metaborate.
22. The process of claim 1 further comprising providing an alcohol
in the anode compartment to produce an alkyl borate in the anode
compartment.
23. The process of claim 22, wherein the alcohol is represented by
the formula ROH, where R is an alkyl group containing from 1 to 6
carbons.
24. The process of claim 22, further comprising reacting trialkyl
borate with metal hydride to obtain borohydride.
25. A process for reducing a metal carbonate, comprising: providing
an electrolytic cell containing an anode compartment and a cathode
compartment separated by a separator which is permeable to metal
ions and not permeable to water and water vapor; supplying a metal
carbonate compound to the anode compartment; and applying an
electric potential to the cell.
26. The process of claim 25, wherein the metal carbonate compound
is supplied as an aqueous solution.
27. The process of claim 25, further comprising supplying a metal
or a metal alloy to the cathode compartment.
28. The process of claim 27, further comprising heating at least
the cathode compartment of the cell to a temperature of about 95 to
about 150.degree. C.
29. The process of claim 25, further comprising reacting hydrogen
or a hydrogen containing gas with the metal produced at the
cathode.
30. The process of claim 25, further comprising passing hydrogen or
a hydrogen containing gas to the anode compartment through a gas
inlet means.
31. The process of claim 30, wherein the gas inlet means is
selected from the group consisting of a pipe, a sparger, a hose,
and a hydrogen diffusion material.
32. The process of claim 25, further comprising electrooxidizing
hydrogen at the anode.
33. The process of claim 25, wherein the separator comprises a
material selected from the group consisting of
lithium-.beta.-aluminum oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, sodium-.beta.-aluminum
oxide, sodium-.beta.''-aluminum oxide,
sodium-.beta./.beta.''-aluminum oxide, potassium-.beta.-aluminum
oxide, potassium-.beta.''-aluminum oxide, and
potassium-.beta./.beta.''-aluminum oxide.
34. The process of claim 25, wherein the separator is a NaSICON
membrane, a KSICON membrane, or a LiSICON membrane.
35. The process of claim 25, wherein the electric potential is at
least about 3.1 volts.
36. A process for producing a metal and a trialkylborate compound
by reducing a metal borate in an electrolytic cell containing anode
and cathode compartments separated by a separator which is
permeable to metal ions and not permeable to water and water vapor,
comprising: supplying a metal borate compound and at least one
alcohol to the anode compartment; and applying an electric
potential to the cell.
37. The process of claim 36, further comprising forming a boron
species and reacting the boron species with alcohol to form
trialkyl borate.
38. The process of claim 36, further comprising forming boric acid
and reacting the boric acid with alcohol to form trialkyl
borate.
39. The process of claim 38, further comprising maintaining the
cell at a temperature of about 25 to about 300.degree. C.
40. The process of claim 38, wherein the trialkyl borate is
trimethyl borate.
41. The process of claim 36, wherein the metal borate compound is a
borate salt having formula zM.sub.nO.xB.sub.2O.sub.3.yH.sub.2O,
wherein z is 0.5 to 5, x is 0.1 to 5, y is 0 to 10, n is 1 or 2;
and M is an alkali metal ion or an alkaline earth metal ion.
42. The process of claim 41, wherein M is selected from the group
consisting of Li.sup.+, Na.sup.+ and K.sup.+.
43. The process of claim 36, wherein the metal borate compound is
supplied as an aqueous solution.
44. The process of claim 36, further comprising supplying a metal
or a metal alloy to the cathode compartment.
45. The process of claim 36, further comprising passing hydrogen or
a hydrogen containing gas in the anode compartment.
46. The process of claim 45, wherein passing hydrogen or a hydrogen
containing gas further comprises supplying hydrogen or hydrogen
containing gas through a gas inlet means.
47. The process of claim 46, wherein the gas inlet means is
selected from the group consisting of a pipe, a sparger, a hose,
and a hydrogen diffusion material.
48. The process of claim 36, further comprising electrooxidizing
hydrogen at the anode.
49. The process of claim 36, wherein the separator comprises a
material selected from the group consisting of lithium-,-aluminum
oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, sodium-.beta.-aluminum
oxide, sodium-.beta.''-aluminum oxide,
sodium-.beta./.beta.''-aluminum oxide, potassium-.beta.-aluminum
oxide, potassium-.beta.''-aluminum oxide, and
potassium-.beta./.beta.''-aluminum oxide.
50. The process of claim 36, wherein the separator is a NaSICON
membrane, a KSICON membrane, or a LiSICON membrane.
51. The process of claim 36, wherein the electric potential is at
least about 1.4 volts.
52. The process of claim 36, wherein the metal borate compound is
sodium borate and the alcohol is methanol.
53. The process of claim 36, wherein the metal borate compound is a
product from a hydrogen generation reaction.
54. The process of claim 53, wherein the product comprises sodium
metaborate and sodium hydroxide.
55. A process for producing a borohydride, comprising:
electrolyzing an aqueous solution comprising a metal borate and at
least one alcohol in an electrolytic cell to produce a metal and a
trialkyl borate, the electrolytic cell containing anode and cathode
compartments separated by a separator which is permeable to metal
ions and not permeable to water and water vapor; reacting the metal
with hydrogen to produce a metal hydride; and reacting the metal
hydride and the trialkyl borate to form a borohydride.
56. The process of claim 55, wherein the metal borate comprises a
product from a hydrogen generation reaction.
57. The process of claim 55, wherein the metal borate is sodium
borate, the alcohol is methanol, and the borohydride is sodium
borohydride.
58. The process of claim 55, wherein the metal borate is a salt
having formula M.sub.nO.xB.sub.2O.sub.3.yH.sub.2O, wherein z is 0.5
to 5, x is 0.1 to 5, y is 0 to 10, n is 1 or 2; and M is an alkali
metal ion or an alkaline earth metal ion.
59. The process of claim 55, wherein the aqueous solution further
comprises an alkali metal hydroxide.
60. The process of claim 59, wherein the aqueous solution comprises
sodium metaborate and sodium hydroxide.
61. The process of claim 55, further comprising maintaining at
least the cathode compartment of the cell at a temperature of about
95 to about 120.degree. C.
62. The process of claim 55, further comprising passing hydrogen or
a hydrogen containing gas in the anode compartment.
63. The process of claim 62, wherein the step of passing hydrogen
or a hydrogen containing gas further comprises supplying hydrogen
or hydrogen containing gas through a gas inlet means.
64. The process of claim 63, wherein the gas inlet means is
selected from the group consisting of a pipe, a sparger, a hose,
and a hydrogen diffusion material.
65. The process of claim 55, further comprising electrooxidizing
hydrogen at the anode.
66. The process of claim 55, further comprising the step of
providing a supporting electrolyte to the anode compartment.
67. The process of claim 66, wherein the supporting electrolyte
comprises a material selected from the group consisting of sodium
sulfate, sodium perchlorate, sodium phosphate, and sodium
nitrate.
68. The process of claim 55, wherein the separator comprises a
material selected from the group consisting of
lithium-.beta.-aluminum oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, sodium-.beta.-aluminum
oxide, sodium-.beta.''-aluminum oxide,
sodium-.beta./.beta.''-aluminum oxide, potassium-.beta.-aluminum
oxide, potassium-.beta.''-aluminum oxide, and
potassium-.beta./.beta.''-aluminum oxide.
69. The process of claim 55, wherein the separator is a NaSICON
membrane.
70. The process of claim 55, wherein the separator is a LiSICON
membrane or a KSICON membrane.
71. The process of claim 55, further comprising supplying an
electric potential of at least about 1.4 volts to the cell.
72. A process for producing a borohydride, comprising: providing an
electrolytic cell containing anode and cathode compartments
separated by a separator which is permeable to metal ions and not
permeable to water and water vapor; providing an aqueous solution
containing at least an alkali metal borate and at least an alkali
metal hydroxide in the electrolytic cell; subjecting the aqueous
solution to an electrolysis process to produce an alkali metal and
boric acid; reacting the alkali metal with hydrogen to produce an
alkali metal hydride; converting boric acid generated in the anode
compartment to a trialkyl borate; and reacting the alkali metal
hydride and the trialkyl borate to form a borohydride.
73. The process of claim 72, further comprising applying an
electric potential to the cell.
74. The process of claim 73, wherein the electric potential is at
least about 1.4 volts.
75. The process of claim 74, wherein the electrical potential is at
least about 3.25 volts.
76. The process of claim 72, wherein the alkali metal borate is a
salt having formula M.sub.nO.xB.sub.2O.sub.3.yH.sub.2O, wherein z
is 0.5 to 5, x is 0.1 to 5, y is 0 to 10, n is 1 or 2; and M is an
alkali metal ion or an alkaline earth metal ion.
77. The process of claim 72, wherein the aqueous solution comprises
sodium metaborate and sodium hydroxide.
78. The process of claim 72, further comprising maintaining the
cell at a temperature of about 95 to about 120.degree. C.
79. The process of claim 72, further comprising passing hydrogen or
a hydrogen containing gas in the anode compartment.
80. The process of claim 79, wherein passing hydrogen or a hydrogen
containing gas further comprises supplying hydrogen or hydrogen
containing gas through a gas inlet means.
81. The process of claim 80, wherein the gas inlet means is
selected from the group consisting of a pipe, a sparger, a hose,
and a hydrogen diffusion material.
82. The process of claim 72, further comprising electrooxidizing
hydrogen at the anode.
83. The process of claim 72, further comprising the step of
providing a supporting electrolyte to the anode compartment.
84. The process of claim 83, wherein the supporting electrolyte
comprises an anion selected from the group consisting of sulfate,
perchlorate, nitrate, and phosphate.
85. The process of claim 83, wherein the supporting electrolyte
comprises sodium sulfate or sodium nitrate.
86. The process of claim 72, wherein the separator comprises a
material selected from the group consisting of
lithium-.beta.-aluminum oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, sodium-.beta.-aluminum
oxide, sodium-.beta.''-aluminum oxide,
sodium-.beta./.beta.''-aluminum oxide, potassium-.beta.-aluminum
oxide, potassium-.beta.''-aluminum oxide, and
potassium-.beta./.beta.''-aluminum oxide.
87. The process of claim 72, wherein the separator is a NaSICON
membrane, a KSICON membrane, or a LiSICON membrane.
88. A process for producing sodium borohydride, comprising:
electrolyzing an aqueous solution of sodium borate and alcohol to
produce sodium metal and trialkylborate; reacting the sodium metal
with hydrogen to produce sodium hydride; reacting the sodium
hydride and trialkylborate to produce borohydride and sodium
alkoxide; hydrolyzing sodium alkoxide to sodium hydroxide and
methanol; recycling the methanol for production of trialkylborate
from boric acid; and electrolyzing sodium hydroxide to sodium
metal.
Description
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 60/626,485, filed on Nov. 10, 2004, the entire
disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The invention is directed to electrochemical reduction of
metal compounds with applications in elemental metal and metal
borohydride production.
BACKGROUND OF THE INVENTION
[0004] Sodium borohydride is a very versatile chemical and is used
in organic synthesis, waste water treatment, and pulp and paper
bleaching. The high hydrogen content of this compound also makes it
a good candidate for being a hydrogen carrier, and it could play a
major role as an enabler of a hydrogen economy if the cost of
producing this chemical can be reduced.
[0005] Several processes exist for making sodium borohydride, all
of which depend on some form of sodium borate to supply the boron.
Traditionally, the source of boron is the mineral borax. During the
Schlesinger process, which is currently used to supply commercial
sodium borohydride, the sodium and boron contents of the mineral
must be separated. This is achieved by reaction with an acid,
producing boric acid and the sodium salt of the acid. This process
generates large quantities of the sodium salt, typically a
valueless by-product. The sodium needed to make sodium borohydride
is reintroduced from another source, so the process makes no use of
the sodium content of the sodium borate mineral.
[0006] In the manufacture of sodium borohydride, metallic sodium or
sodium hydride is used as a starting material. The largest single
consumer of sodium metal in the United States is the process for
making sodium borohydride. Essentially all sodium in the
marketplace is obtained from energy inefficient electrolysis
processes, such as electrolysis of sodium chloride. As a result,
the market price of sodium is high and this raises the cost of raw
materials for making sodium borohydride. Therefore, it is desirable
to achieve more efficient processes for making sodium.
[0007] U.S. Pat. No. 3,734,842, U.S. Pat. No. 4,904,357, and U.S.
Pat. No. 4,931,154, the disclosures of which are hereby
incorporated by reference herein in their entirety, disclose
electrochemical synthesis of sodium borohydride from aqueous sodium
metaborate solution. Such processes involve conversion of sodium
metaborate and water to form sodium borohydride and oxygen in an
electrical cell, as shown in the following half-cell reactions:
Cathode:
B(OH).sub.4.sup.-+4H.sub.2O+8e.sup.-.fwdarw.BH.sub.4.sup.-+8OH.sup.-
(1a) Anode: 8OH.sup.--.fwdarw.4H.sub.2O+2O.sub.2+8e (1b) However,
none of these processes has been implemented in commercial
practice.
SUMMARY OF THE INVENTION
[0008] The present invention is directed to electrochemical
processes and apparatus for obtaining a metal from a metal
salt.
[0009] In accordance with one aspect of the present invention,
aqueous solutions of metal borates are converted to elemental metal
and borate species by an electrochemical process.
[0010] In accordance with another aspect of the invention, a metal
carbonate is converted to elemental metal by an electrochemical
process.
[0011] In accordance with another aspect of the invention, aqueous
solutions of alkali metal borates and alcohols are converted to
alkali metal and trialkylborates by an electrochemical process.
[0012] In yet another aspect of the invention, product from a
borohydride-based hydrogen generator comprising an aqueous solution
of alkali metal borates and alkali metal hydroxides is converted to
an alkali metal and borate species by an electrochemical process
according to the present invention.
[0013] In another aspect of the invention, the invention provides a
process for the production of sodium borohydride by: (i)
electrolyzing an aqueous solution of sodium borates to produce
sodium metal and boric acid; (ii) reacting the sodium metal with
hydrogen to produce sodium hydride; (iii) converting the boric acid
into trimethylborate; and (iv) reacting the sodium hydride and
trimethylborate to produce sodium borohydride.
[0014] Another aspect of the present invention provides a process
for the production of sodium borohydride by: (i) electrolyzing an
aqueous solution of sodium borates and an alcohol to produce sodium
metal and trialkylborate; (ii) reacting the sodium metal with
hydrogen to produce sodium hydride; and (iii) reacting the sodium
hydride and trialkylborate to produce sodium borohydride.
[0015] In another aspect the present invention provides a process
for the production of sodium borohydride comprising the steps of:
(i) electrolyzing an aqueous solution of sodium borates and
alcohols to produce sodium metal and trialkylborate; (ii) reacting
the sodium metal with hydrogen to produce sodium hydride; (iii)
reacting the sodium hydride and trialkylborate to produce
borohydride and sodium alkoxide, and hydrolyzing the sodium
alkoxide to sodium hydroxide and methanol; (iv) optionally
recycling the alcohol in the process; and (v) electrolyzing the
sodium hydroxide to sodium metal which is combined with the sodium
metal generated by the electrochemical process of the present
invention, and used to make sodium borohydride.
[0016] These and other features and advantages of the invention
will become apparent from the following detailed description that
is provided in connection with the accompanying drawings and
illustrated exemplary embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic view of an exemplary electrolytic cell
for synthesis of an alkali metal from an alkali metal salt.
[0018] FIG. 2 is a schematic view of an exemplary electrolytic cell
in which hydrogen-containing gas is passed into the anode
compartment for synthesis of an alkali metal from an alkali metal
salt.
[0019] FIG. 3 is a flow diagram of a prior art process for
producing sodium borohydride.
[0020] FIG. 4 is a flow diagram of a process for producing sodium
borohydride according to the present invention wherein a sodium
borate compound is converted to boric acid.
[0021] FIG. 5 is a flow diagram of a process for producing sodium
borohydride according to the present invention wherein a sodium
borate compound is converted to trimethylborate.
[0022] FIG. 6 is a flow diagram of a process for producing sodium
borohydride according to the present invention, wherein a sodium
borate compound is converted to trimethylborate and the sodium
methoxide byproduct is converted to sodium metal and methanol.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The present invention relates to electrochemical processes
for obtaining metals from metal salts, and particularly alkali
metals and alkaline earth metals from alkali metal and alkaline
earth metal salts. Alkali metals are the Group I metals, and
preferably are lithium, sodium, and potassium; alkaline earth
metals are the Group II metals, and are preferably calcium and
magnesium. The electrochemical process for obtaining an elemental
metal may further be combined with additional electrochemical
and/or chemical steps to produce metal borohydride compounds.
[0024] FIG. 1 illustrates an exemplary two-compartment cell 100
suitable for the transformation of metal salts to metals. Cell 100
comprises a cathode compartment 104, a cathode 102, an anode
compartment 110, an anode 108, and a membrane 106 that separates
the anode and cathode compartments. The anode and cathode may be
typical electrodes in electrical communication. A pool of molten
metal or metal alloy, particularly molten alkali or alkaline earth
metal or alkali or alkaline earth metal alloy, in electrical
contact with the cathode may act as an auxiliary cathode.
[0025] The anodic and cathodic compartments are separated by an
ion-conducting membrane, which is permeable to metal ions but is
not permeable to water and water vapor. Suitable membrane materials
include, for example, ceramics such as lithium-.beta.-aluminum
oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, lithium analogs of NaSICON
ceramics, LISICONs, and lithium ion conductors with perovskite
structure, sodium-.beta.-aluminum oxide, sodium-.beta.''-aluminum
oxide, sodium-.beta./.beta.''-aluminum oxide, NaSICON ceramics,
potassium-.beta.-aluminum oxide, potassium-.beta.''-aluminum oxide,
potassium-.beta./.beta.''-aluminum oxide, and potassium analogs of
NaSICON ceramics, KSICONs, among others.
[0026] In accordance with one embodiment of the present invention,
an aqueous solution of a metal borate salt represented by the
formula zM.sub.nO.xB.sub.2O.sub.3.yH.sub.2O, wherein z is 1/2 to 5;
x is 0.1 to 5; y is 0 to 10; and M is an alkali metal ion such as
sodium, potassium, or lithium wherein n=2, or an alkaline earth
metal such as calcium or magnesium wherein n=1, and preferably is
sodium, can be converted into a boron compound and elemental metal
through electrolysis. The overall reaction is shown in Equation
(2), where M is selected from the group of alkali metals; the
borate salt: zM.sub.nO.xB.sub.2O.sub.3.yH.sub.2O is shown as
MBO.sub.2 in Equation (2):
4MBO.sub.2+6H.sub.2O.fwdarw.O4M+4B(OH).sub.3+O.sub.2 (2)
[0027] This process is carried out by supplying an aqueous solution
of a borate salt to the anode compartment of an electrolytic cell,
for instance, such as that illustrated by FIG. 1. An alkali or
alkaline earth metal or alloy of alkali metals or alkaline earth
metals is supplied to the cathode compartment, preferably in a
molten state. The reaction may be carried out at room temperature
or at a temperature higher than room temperature, so that the metal
in the cathode compartment is in a liquid or molten state. For
sodium, preferably the temperature in the cathode compartment is
from about 95.degree. C. to about 150.degree. C.
[0028] The anode compartment need not be heated and may be
maintained at ambient temperatures, or the anode compartment may be
maintained at the same elevated temperature as the cathode for ease
and convenience. If the anode compartment is heated to temperatures
greater than the boiling point of the water, the cell may be
pressurized or the solvent in the anolyte may be refluxed and
condensed. The applied voltage may be about 1.4 volts, or greater
than about 1.4 volts, preferably greater than about 3.25 volts.
Water or hydroxide in the anode compartment is oxidized in
accordance with Equations (3a) and (3b).
2H.sub.2O.fwdarw.4H.sup.++O.sub.2+4e.sup.- (3a)
4OH.sup.-.fwdarw.2H.sub.2O+O.sub.2+4e.sup.- (3b)
[0029] Additional reactions may occur in the anolyte solution in
the anode compartment. The hydroxide ion concentration decreases
and the proton concentration increases (and hence solution pH
decreases) as protons are generated in accordance with Equation
(3a) that can react with hydroxide to make water, or hydroxide ions
generated in accordance with Equation (3b) are converted to oxygen
and water. In solution, with lowering pH and increasing H.sup.+
concentration, borate ions may react with protons and water to form
boric acid according to reaction (4).
H.sup.++BO.sub.2.sup.-+H.sub.2O.fwdarw.B(OH).sub.3 (4)
[0030] Depending on conditions including, but not limited to,
current and total charge passed, different borate species may
result in the anode compartment. The borate species formed is
referred to herein as an "enriched boron species" represented by
formula z'M.sub.nO.x'B.sub.2O.sub.3.y'H.sub.2O, wherein z' is 0 to
5; x is 0.1 to 5; y is O to 10; n=1 or 2; and M is an alkali metal
ion or an alkaline earth metal, and z' of the product enriched
boron species is less than z of the starting metal borate salt
and/or x' of the product enriched boron species is greater than x
of the starting metal borate salt. Thus, enriched borate species
such as borax, Na.sub.2B.sub.4O.sub.7 wherein z'=1 and x'=2, may be
formed in the anode compartment.
[0031] Metal ions are transported from the anolyte solution through
the membrane where they are reduced at the cathode to the metal as
shown in Equation (5). 2M.sup.++2e.sup.-.fwdarw.2M (5)
[0032] The overall process provided in Equation (2) can be
summarized by the following individual reactions: Anode Reaction:
2H2O.fwdarw.4H.sup.++O.sub.2+4e.sup.- (3a)
4OH.sup.-.fwdarw.2H.sub.2O+O.sub.2+4e.sup.- (3b) Anolyte Reaction:
H.sup.++BO.sub.2.sup.-+H.sub.2O.fwdarw.B(OH).sub.3 (4) Cathode
Reaction: 2M++2e.sup.-.fwdarw.2M (5)
[0033] In Equation (4), which takes place in the anode compartment,
a strong electrolyte--the borate salt, MBO.sub.2--is being
converted into a weak electrolyte, B(OH).sub.3. Optionally, a
supporting electrolyte may be included in the anode compartment to
maintain ionic conductivity as boric acid is generated. Desirable
supporting electrolytes are water soluble salts, wherein the cation
portion is either the same as the metal cation undergoing reduction
at the cathode or is a cation that will not be transported through
the membrane. The anion portion of the supporting electrolyte
should be relatively difficult to oxidize at the anode as compared
to water, hydroxide anions, or hydrogen. The anion portion should
also have low chemical reactivity with boric acid. One skilled in
the art may readily select appropriate electrolyte systems given
the teachings herein. Suitable anions for electrolytes include, but
are not limited to, sulfate, nitrate, perchlorate, and phosphate.
For example, suitable supporting electrolytes for an NaBO.sub.2
system include, but are not limited to, sodium sulfate,
Na.sub.2SO.sub.4, sodium nitrate, NaNO.sub.3, sodium phosphate,
Na.sub.3PO.sub.4, or sodium perchlorate, NaClO.sub.4, wherein the
cation portion (Na.sup.+) is the same as the cation being reduced
(e.g., Na.sup.+ to sodium metal).
[0034] Alternatively, an aqueous solution of a metal borate salt
can be converted into a boron compound and elemental metal through
electrolysis in the presence of hydrogen gas in the anode
compartment to enable hydrogen-assisted electrolysis as disclosed
in U.S. patent application Ser. No. 10/388,197 entitled
"Hydrogen-Assisted Electrolysis Process," the disclosure of which
is incorporated by reference in its entirety, and which has been
shown to reduce cell voltage and improve electrolytic efficiency
through the preferential electrochemical oxidization of H.sub.2 at
the anode. The overall reaction for hydrogen-assisted electrolysis
is shown in Equation (6), where M is selected from the group of
alkali metals: MBO.sub.2+H.sub.2O+1/2H.sub.2.fwdarw.M+B(OH).sub.3
(6)
[0035] FIG. 2 illustrates an exemplary two-compartment cell 200
suitable for the hydrogen-assisted electrolysis. In FIG. 2,
structures that are the same as shown in FIG. 1 have like
numbering. The cell 200 comprises a cathode compartment 104,
cathode 102, anode compartment 110, anode 108, membrane 106 that
separates the anode and cathode compartments, and a gas inlet means
202 to supply hydrogen gas to the anode compartment. The anode and
cathode may be any suitable electrodes. A pool of molten alkali or
alkaline earth metal or alkali metal alloy or alkaline earth alloy
in electrical contact with the cathode acts as an auxiliary
cathode.
[0036] The process is carried out by supplying an aqueous solution
of a borate salt represented by the formula
zM.sub.nO.xB.sub.2O.sub.3.yH.sub.2O to the anode compartment.
Hydrogen or a hydrogen containing gas is supplied from an external
source to the anode where it is oxidized in accordance with
Equation (7). 1/2H.sub.2.fwdarw.H.sup.++e.sup.- (7)
[0037] The anode compartment may include an optional gas inlet
means for supplying a gas stream comprising hydrogen. Non-limiting
examples of gas inlet means include pipes, spargers, hoses, and
hydrogen gas diffusion materials.
[0038] As previously described, with lowering pH and increasing
H.sup.+ concentration, borate ions may react with protons and water
to form an enriched boron species such as boric add according to
Equation (4), and metal ions are transported from the anolyte
solution through the ion-conducting membrane where they are reduced
at the cathode to the metal as shown in Equation (5). The
ion-conducting membrane may include, for example, ceramics such as
lithium-.beta.-aluminum oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, lithium analogs of NaSICON
ceramics, LISICONs, and lithium ion conductors with perovskite
structure, sodium-.beta.-aluminum oxide, sodium-.beta.''-aluminum
oxide, sodium-.beta./.beta.''-aluminum oxide, NaSICON ceramics,
potassium-.beta.-aluminum oxide, potassium-.beta.''-aluminum oxide,
potassium-.beta./.beta.''-aluminum oxide, and potassium analogs of
NaSICON ceramics, KSICONs.
[0039] Preferably, the cathode compartment is maintained at
temperatures so that the metal is in a liquid molten state. For
sodium, the cathode compartment is preferably maintained at
temperatures from about 95.degree. C. to about 150.degree. C. A
supporting electrolyte, as discussed above, may be included in the
anode compartment to maintain ionic conductivity as the enriched
boron species is generated.
[0040] In accordance with another embodiment of the present
invention, an aqueous solution of a metal borate salt can be
converted into a boron compound and elemental metal through
electrolysis as described above with an applied voltage from about
1.4 volts or greater, preferably greater than about 3.25 volts.
When an alcohol is present in the anolyte solution, the enriched
boron species produced, preferably boric acid, undergoes further
reaction at temperatures from about 25.degree. C. to about
300.degree. C. to form a trialkyl borate, as shown in Equation
(10), wherein R is a straight- or branched-chain or cyclic alkyl
group containing from 1 to 6, preferably from 1 to 4, carbon atoms.
The reaction can be driven to completion by continuously removing
the product by various means such as by distillation.
B(OH).sub.3+3ROH.fwdarw.B(OR).sub.3+3H.sub.2O (10)
[0041] When the alcohol is added to the anode compartment with the
aqueous metal borate solution, the overall reaction is given by
Equation (11), where R is a straight- or branched-chain or cyclic
alkyl group containing from 1 to 6, preferably from 1 to 4, carbon
atoms, and M is chosen from the group of alkali metals:
4MBO.sub.2+12ROH.fwdarw.4M+4B(OR).sub.3+6H.sub.2O+O.sub.2 (11)
[0042] The overall process provided in Equation (2) can be
summarized by the following individual reactions: Anode Reactions:
2H.sub.2O.fwdarw.4H.sup.++O.sub.2+4e.sup.- (3a)
4OH.sup.-.fwdarw.2H.sub.2O+O.sub.2+4e.sup.- (3b) Anolyte Reactions:
H.sup.++BO.sub.2.sup.-+H.sub.2O.fwdarw.B(OH).sub.3 (4)
B(OH).sub.3+3ROH.fwdarw.B(OR).sub.3+3H.sub.2O (10) Cathode
Reaction: 2M++2e.sup.-.fwdarw.2M (5)
[0043] Preferred alcohols are methanol, wherein R is CH.sub.3--,
and n-butanol, where R is CH.sub.3CH.sub.2CH.sub.2CH.sub.2--.
[0044] The anode compartment may include an optional gas inlet
means for supplying a gas stream comprising hydrogen to allow
hydrogen assisted electrolysis. Examples of suitable gas inlet
means include pipes, spargers, hoses, and hydrogen gas diffusion
materials, among others.
[0045] When hydrogen is fed to the anode for hydrogen-assisted
electrolysis, the overall process illustrated in Equation (12)
occurs with an applied voltage from about 1.4 volts or greater,
preferably greater than about 2.25 volts.:
2MBO.sub.2+H.sub.2+6ROH.fwdarw.2M+2B(OR).sub.3+4H.sub.2O (12)
[0046] In the processes depicted in Equations (11) and (12), the
reduction is carried out by supplying an aqueous solution of a
borate salt represented by the formula
zM.sub.nO.xB.sub.2O.sub.3.yH.sub.2O, wherein z is 1/2 to 5; x is
0.1 to 5; y is 0 to 10; and M is an alkali metal ion such as
sodium, potassium, or lithium, and preferably sodium, to the anode
compartment. Hydrogen or a hydrogen containing gas may be supplied
to the anode. Oxidation of water or hydroxide in the anode
compartment occurs in accordance with Equations (3a) and (3b). If
hydrogen is present in the anode compartment, hydrogen is oxidized
at a lower voltage than either water or hydroxide, and is oxidized
preferentially in accordance with Equation (7). With lowering pH
and increasing H.sup.+ concentration, borate ions may react with
protons and water to form an enriched boron species such as boric
acid according to Equation (4).
[0047] The anodic and cathodic compartments are separated by an
ion-conducting membrane that is permeable to metal ions but is not
permeable to water, water vapor, alcohol, and alcohol vapor. Such
membranes include, for example, ceramics such as
lithium-.beta.-aluminum oxide, lithium-.beta.''-aluminum oxide,
lithium-.beta./.beta.''-aluminum oxide, lithium analogs of NaSICON
ceramics, LISICONs, and lithium ion conductors with perovskite
structure, sodium-.beta.-aluminum oxide, sodium-.beta.''-aluminum
oxide, sodium-.beta./.beta.''-aluminum oxide, NaSICON ceramics,
potassium-5-aluminum oxide, potassium-.beta.''-aluminum oxide,
potassium-.beta./.beta.''-aluminum oxide, and potassium analogs of
NaSICON ceramics, KSICONs.
[0048] The metal ions are transported from the anolyte solution
through the membrane where they are reduced at the cathode to the
metal, as shown in Equation (5). Preferably, the cathode
compartment is maintained at temperatures so that the metal is at
least partly in a liquid molten state. For sodium, the cathode
compartment is preferably at temperatures from about 95.degree. C.
to about 150.degree. C. At temperatures greater than the boiling
point of alcohol or water in the anode compartment (e.g., methanol
boils at about 65.degree. C.), the cell may be pressurized to
elevate the boiling point. In some cases, the alcohol, especially
methanol, may form an azeotrope with the trialkylborate, and the
trialkylborate-methanol azeotrope may be distilled from the cell as
it forms.
[0049] In Equation (10), which occurs in the anode compartment, the
weak electrolyte B(OH).sub.3 is being removed from the
electrochemical cell. A supporting electrolyte may be included in
the anode compartment to maintain ionic conductivity as boric acid
is generated. Desirable supporting electrolytes comprise water or
water/alcohol soluble salts, wherein the cation portion is either
the same as the metal cation undergoing reduction at the cathode or
is a cation that will not be transported through the ceramic
membrane. Preferably, the anion portion of the supporting
electrolyte is relatively difficult to oxidize at the anode as
compared to water, hydroxide anions, or hydrogen. Preferably, the
anion portion should also have low chemical reactivity with boric
acid according to the teachings herein. For example, suitable
supporting electrolytes for the NaBO.sub.2 system include, but are
not limited to sodium sulfate, Na.sub.2SO.sub.4, or sodium nitrate,
NaNO.sub.3.
[0050] In another embodiment of the present invention, the aqueous
solution of a borate salt is prepared from the product from a
hydrogen generation apparatus, such as one used to supply a
hydrogen fuel cell and as described in U.S. Pat. No. 6,534,033,
entitled "A System for Hydrogen Generation," and which comprises an
aqueous solution of alkali metal hydroxide and alkali metal borate,
represented by the formula zM.sub.nO.xB.sub.2O.sub.3.yH.sub.2O,
wherein z is 1/2 to 5; x is 0.1 to 5; and y is 0 to 10. Preferably,
the alkali metal ion in both the alkali metal hydroxide and alkali
metal borate is sodium, although other alkali metal ions, such as
potassium, may be utilized. The alkali metals of the alkali metal
hydroxide and alkali metal borate need not be the same. Typically,
the fuel solution that is introduced into a hydrogen generator
comprises from about 15% to 100% by wt. sodium borohydride to about
0 to 15% by weight sodium hydroxide as a stabilizer. The product
comprises sodium metaborate and sodium hydroxide in a molar ratio
corresponding to the fuel concentration, but the percent by weight
of sodium metaborate is from about 27% to 100% by weight as a
result of the higher molecular weight thereof in comparison to
sodium borohydride, and the reduced amount of water present. The
borate product may be a solution, a heterogeneous mixture, a solid,
or a slurry depending on the concentration of the ingredients and
the temperature. The term "about" as used herein refers to .+-.10%
of the stated value.
[0051] The borate product is introduced into the anode compartment
of electrolytic cell 100, and a voltage from about 1.4 volts or
greater, preferably greater than about 3.25 volts, is applied.
Hydrogen or a hydrogen containing gas may be supplied to the anode.
Oxidation of water or hydroxide in the anode compartment occurs in
accordance with Equations (3a) and (3b). If hydrogen is present in
the anode compartment, hydrogen is oxidized at a lower voltage than
either water or hydroxide, and is oxidized preferentially in
accordance with Equation (7). With lowering pH and increasing
H.sup.+ concentration, borate ions react with protons and water to
form boric add according to Equation (4).
[0052] Sodium ions are transported from the anolyte solution
through the membrane to be reduced at the cathode to sodium metal
as shown in Equation (5). Preferably, the cathode compartment is
maintained at temperatures from about 95.degree. C. to about
150.degree. C., to maintain sodium in the liquid molten state.
Boric add is produced in the anode compartment according to
Equation (4). Trialkyl borate, preferably trimethyl borate, can be
generated from the boric acid produced in the anode compartment if
an alcohol, preferably methanol, is added to the anode compartment,
in accordance with Equation (10).
[0053] As the electrolytes NaBO.sub.2 and B(OH).sub.3 are removed
from the electrochemical cell as B(OR).sub.3, it may be desirable
to further include a supporting electrolyte in the anode
compartment to maintain ionic conductivity as boric acid is
generated. One skilled in the art can readily select an appropriate
electrolyte system given the teachings herein and using, for
example, the parameters provided for the other embodiments herein.
Suitable supporting electrolytes for the NaBO.sub.2 system include,
but are not limited to, sodium sulfate, Na.sub.2SO.sub.4, or sodium
nitrate, NaNO.sub.3.
[0054] In another embodiment, an aqueous solution of an alkali
metal carbonate salt represented by formula M.sub.2CO.sub.3 is
converted to an alkali metal. This reduction is carried out by
supplying an aqueous solution of a carbonate salt to the anode
compartment of an electrolytic cell such as the one illustrated by
FIG. 1. An alkali metal or alloy of alkali metals is supplied to
the cathode compartment, preferably in a molten state. The reaction
is carried out at about room temperature or at a temperature higher
than room temperature so that the metal in the cathode compartment
is in a liquid or molten state. For sodium, preferably the
temperature in the cathode compartment is from about 95.degree. C.
to about 150.degree. C.
[0055] The anode compartment need not be heated and may be
maintained at ambient temperatures, or the anode compartment may be
maintained at the same elevated temperature as the cathode for ease
and convenience. If the anode compartment is heated to temperatures
greater than the boiling point of water, the cell may be
pressurized or the solvent in the anolyte may be refluxed and
condensed. The applied voltage may be about 3.1 volts or greater,
preferably greater than about 3.5 volts. Water or hydroxide in the
anode compartment is oxidized in accordance with Equations (3a) and
(3b).
[0056] Alkali metal ions are transported from the anolyte solution
through the membrane where they are reduced at the cathode to the
metal as shown in Equation 5.
[0057] Hydrogen may be present in the anode compartment to enable
hydrogen-assisted electrolysis which will reduce cell voltage and
improve electrolytic efficiency through the preferential
electrochemical oxidization of H.sub.2 at the anode rather than the
oxidation of hydroxide or water as shown in Equations (3a) and
(3b). For a hydrogen assisted process, a solution of an alkali
metal carbonate salt may be supplied to the anode compartment of a
cell such as that illustrated in FIG. 2, with application of a
voltage of about 2 volts or greater.
[0058] In another embodiment of the present invention, the
electrochemical process of the invention is incorporated into a
process for producing sodium borohydride. Today, sodium borohydride
is commercially produced by the so-called Schlesinger process,
which is a multi-step synthetic process, as illustrated in FIG. 3.
The general steps include production of hydrogen by steam methane
reforming at Step 302; electrolysis of sodium chloride to produce
sodium metal in Step 304; preparation of sodium hydride by reaction
of sodium and hydrogen in Step 306; refining of borax to generate
boric acid in Step 308; conversion of boric acid to trimethylborate
in Step 310; and reaction of sodium hydride and trimethylborate to
produce sodium borohydride in Step 312. The electrochemical process
of the present invention can provide a much more efficient
process.
[0059] The raw material input for Step 308 in the Schlesinger
process is generally the mineral borax, which is refined to produce
boric acid through treatment with sulfuric acid. Though this
conversion proceeds with good yield, it results in a substantial
quantity of sodium sulfate as a byproduct. In essence, the
Schlesinger process separates the boron and sodium values present
in borax and discards the sodium from the borax. In order to make
sodium borohydride, sodium must be re-introduced into the
manufacturing process.
[0060] Commercially, sodium metal is prepared by the electrolysis
of sodium chloride despite the fact that more energy is required to
electrolyze sodium chloride than a number of other sodium salts.
Much of the energy inefficiency and related cost of the Schlesinger
process derives from this means of manufacturing sodium metal.
Supplementing Step 304 from the current sodium borohydride method
with the more efficient process for making sodium described in the
present application provides significant improvements in both
energy utilization and cost.
[0061] Reference is now made to FIG. 4, which illustrates an
improved process for the production of sodium borohydride utilizing
the electrochemical process of the present invention, which
utilizes an aqueous solution of a borate salt, which may
illustratively be prepared from the product from a hydrogen
generation reaction but may comprise any borate salt according to
the teachings herein. The process steps in FIG. 4 and FIG. 3 have
like numbering. In Step 402, the aqueous solution of alkali metal
borate, preferably a solution comprising sodium metaborate, and
more preferably a product solution comprising sodium metaborate and
sodium hydroxide, is introduced to the anode compartment and
subjected to the electrochemical process of the present invention
in an electrochemical cell comprising anode and cathode
compartments separated by an ion-conducting membrane. A supporting
electrolyte to enhance ionic conductivity in the anolyte may be
included. The supporting electrolyte is typically not consumed in
the process.
[0062] Aqueous sodium ions are transported from the anode
compartment through the membrane to the cathode chamber and reduced
to sodium metal at the cathode. In the anode compartment, the
borate solution is acidified to form boric acid. Boric acid can be
converted to trimethyl borate through reaction with methanol in a
separate reactor, as shown in Step 310. Alternatively, an alcohol,
preferably methanol, can be introduced to the anode compartment
along with the aqueous solution of alkali metal borate in Step 402
to form a trialkyl borate in situ, which can be removed from the
anode compartment through distillation. This would eliminate Step
310 and this process is illustrated in FIG. 5.
[0063] In Step 306, the sodium metal produced in the cathode
compartment is withdrawn for reaction with hydrogen gas to form
sodium hydride. This step may be the same as in the conventional
Schlesinger process. Sodium borohydride is produced from the
reaction of sodium hydride and trimethyl borate in Step 312.
Additional sodium metal or sodium hydride can be introduced into
the manufacturing process as shown in optional step 304 in FIG.
4.
[0064] Whether trialkyl borate is prepared in situ or not, the
process may be carried out with hydrogen gas supplied to the anode
as shown in optional Step 402a in FIGS. 4 and 5. When hydrogen is
supplied to the anode, the oxidation occurring at the anode is
represented by Equation (7). With no hydrogen, the anode product
will be oxygen instead of water as shown in Equations (3a) and
(3b). In both cases, the rate of boric acid production as a
function of current passed remains the same. Additional sodium
metal or sodium hydride can be introduced into the manufacturing
process, if required to convert all of the boric acid species to a
borohydride compound. Such sodium may be obtained from the
conventional electrolysis of sodium chloride as shown in optional
step 304 or from the hydrogen-assisted processes disclosed in U.S.
patent application Ser. No. 10/388,197 entitled "Hydrogen-Assisted
Electrolysis Process," the disclosure of which is incorporated by
reference herein.
[0065] Steps may be further incorporated in the production of
sodium borohydride to improve process efficiency. The sodium
methoxide byproduct of Step 312 can be converted to sodium
hydroxide and methanol by hydrolysis and separated as in Step 602.
The methanol is recycled to the anode compartment to be
incorporated in the process. The sodium hydroxide can be converted
into sodium metal in Step 604 using, for example, the
hydrogen-assisted processes disclosed in U.S. patent application
Ser. No. 10/388,197 entitled "Hydrogen-Assisted Electrolysis
Process," and further shown in Equation (13).
3NaOH+1.5H.sub.2.fwdarw.+3Na+3H.sub.2O (13)
[0066] Alternatively, sodium can be generated from sodium hydroxide
without employing hydrogen gas at the anode. In this case, the
overall reaction for making sodium from sodium hydroxide is as
shown in Equation (14): 4NaOH.fwdarw.4Na+2H.sub.2O+O.sub.2 (14)
[0067] The following examples further describe and demonstrate
features of the present invention. The examples are given solely
for illustration purposes and are not to be construed as a
limitation of the present invention.
EXAMPLE 1
[0068] A reaction flask was charged with about 200 g of 5.5 weight
percent NaBO.sub.2 aqueous solution. A tube with a NaSICON bottom
was inserted into the solution. The tube contained 1.03 gram of
sodium metal. Embedded into the sodium metal was a nickel wire.
Collectively, the sodium metal and the nickel wire comprised the
cathode. The tube bottom comprised the membrane or separator. The
volume inside the tube defined the cathode compartment and the
volume outside the tube, but inside the reaction flask, comprised
the anode compartment. The aqueous metaborate solution comprised
the anolyte. The anode itself was a nickel wire wrapped around a
nickel plate, the combination of the wire and plate together
comprising the anode.
[0069] The reaction flask was heated to about 115.degree. C. and
pressurized to about 10 psi. Under these conditions, the sodium in
the cathode compartment was molten. A potential of about 5 V was
applied across the anode and the cathode. After 606 mAh of current
passed through the cell, the cell was cooled to room temperature
and the amount of sodium metal that was generated was measured. The
total amount of sodium was 1.35 grams with approximately 0.32 grams
of sodium metal generated by the electrolysis. This represented a
current efficiency of 61%, wherein 61% of the electrons passing
through the cell resulted in conversion of sodium ions from the
sodium metaborate solution into sodium metal. A .sup.11B NMR
spectrum of the aqueous anolyte solution after the electrolysis
confirmed the loss of sodium from the anolyte. The loss of sodium
was determined by the magnitude of shift of the metaborate peak
away from the metaborate form to the boric add form. The loss of
sodium as determined by .sup.11B NMR indicates a current efficiency
of 55%, in expected agreement with the determination from measuring
the sodium yield.
EXAMPLE 2
[0070] A reaction flask was charged with about 150 g of
NaBO.sub.2.4 H.sub.2O (sodium metaborate tetrahydrate). A
Na-.beta.''-alumina tube was inserted into the solution. The tube
contained 0.99 gram of sodium metal. Embedded into the sodium metal
was a nickel wire. Collectively, the sodium metal and the nickel
wire comprised the cathode. The tube bottom comprised the membrane
or separator. The volume inside the tube was the cathode
compartment and the volume outside the tube, but inside the
reaction flask, comprised the anode compartment. The sodium
metaborate comprised the anolyte. The anode itself was a nickel
wire wrapped around a nickel plate, the wire and plate together
comprising the anode.
[0071] The reaction flask was heated to about 135.degree. C. and
pressurized to about 10 psi. Under these conditions, the sodium in
the cathode compartment was molten, and the sodium metaborate
tetrahydrate in the anode compartment was molten. A potential of
about 5 V was applied across the anode and the cathode. After 1000
mAh of current passed through the cell, the cell was cooled to room
temperature and the amount of sodium metal generated measured. The
total amount of sodium was 1.83 grams with approximately 0.84 grams
of sodium metal was generated by the electrolysis. This represented
a current efficiency of 99%, wherein 99% of the electrons passing
through the cell resulted in conversion of sodium ions from the
sodium metaborate solution into sodium metal.
EXAMPLE 3
[0072] A reaction flask was charged with about 200 g of 9.1
weight-% Na.sub.2CO.sub.3 aqueous solution. A tube with a NaSICON
bottom was inserted into the solution. The tube contained about 1
gram of sodium metal. Embedded into the sodium metal was a nickel
wire. Collectively, the sodium metal and the nickel wire comprised
the cathode. The tube bottom comprised the membrane or separator.
The volume inside the tube was the cathode compartment and the
volume outside the tube, but inside the reaction flask comprised
the anode compartment. The aqueous carbonate solution comprised the
anolyte. The anode itself was a nickel wire wrapped around a nickel
plate, the wire and plate together comprising the anode.
[0073] The reaction flask was heated to about 115.degree. C. and
pressurized to about 10 psi. Under these conditions, the sodium in
the cathode compartment was molten. A potential of about 5 V was
applied across the anode and the cathode. After passing 500 mAh of
current through the cell, it was cooled to room temperature. The
amount of sodium metal in the cathode compartment was measured by
hydrolyzing the collected sodium to generate hydrogen gas. The
amount of hydrogen gas captured can be translated into the amount
of sodium hydrolyzed. The total amount of sodium was 1.32 grams, so
approximately 0.32 grams of sodium metal was generated by the
electrolysis. This represented a current efficiency of 75%, wherein
75% of the electrons passing through the cell resulted in
conversion of sodium ions from the sodium carbonate solution into
sodium metal.
EXAMPLE 4
[0074] A reaction flask was charged with about 200 g of 30 weight-%
Na.sub.2CO.sub.3. A Na-.beta.''-alumina tube was inserted into the
solution. The tube contained about 1 gram of sodium metal. Embedded
into the sodium metal was a nickel wire. Collectively, the sodium
metal and the nickel wire comprised the cathode. The tube bottom
comprised the membrane or separator. The volume inside the tube was
the cathode compartment and the volume outside the tube, but inside
the reaction flask, comprised the anode compartment. The sodium
carbonate solution comprised the anolyte. The anode itself was a
nickel wire wrapped around a nickel plate, the wire and plate
together comprising the anode.
[0075] The reaction flask was heated to about 120.degree. C. and
pressurized to about 10 psi. Under these conditions, the sodium in
the cathode compartment was molten, and the sodium carbonate was
fully soluble in the anode compartment solution. A potential of
about 5 V was applied across the anode and the cathode. After
passing 337 mAh of current through the cell, it was cooled to room
temperature. The amount of sodium metal in the cathode compartment
was measured by hydrolyzing the collected sodium to generate
hydrogen gas. The amount of hydrogen gas captured can be translated
into the amount of sodium hydrolyzed. The total amount of sodium
was 1.22 grams, so approximately 0.22 grams of sodium metal was
generated by the electrolysis. This represented a current
efficiency of 77%, wherein 77% of the electrons passing through the
cell resulted in conversion of sodium ions from the sodium
carbonate solution into sodium metal.
[0076] The above description and drawings illustrate preferred
embodiments that achieve the objects, features and advantages of
the present invention. It is not intended that the present
invention be limited to the illustrated embodiments. Any
modification of the present invention that comes within the spirit
and scope of the following claims should be considered part of the
present invention.
* * * * *