U.S. patent application number 12/445375 was filed with the patent office on 2010-11-18 for magnesiothermic som process for production of metals.
Invention is credited to Uday B. Pal.
Application Number | 20100288649 12/445375 |
Document ID | / |
Family ID | 38872080 |
Filed Date | 2010-11-18 |
United States Patent
Application |
20100288649 |
Kind Code |
A1 |
Pal; Uday B. |
November 18, 2010 |
MAGNESIOTHERMIC SOM PROCESS FOR PRODUCTION OF METALS
Abstract
A process and apparatus are provided that allow metals including
metals having stable oxide phases and metals with variable
valencies to be extracted from their respective ores via an
electrolytic process that is environmentally sound and economically
viable. The process for lowering the oxidation state of a metal in
a metal oxide comprises providing an electrolysis chamber housing a
flux containing a highly reactive metal (e.g. Mg) and having a
cathode, an anode, and a solid oxide membrane. A reducing chamber
housing the metal oxide having a higher oxidation state to be
reduced is provided. A solid oxide membrane (SOM) process is used
to generate vapor of the highly reactive metal in the electrolysis
chamber. The vapor of the highly reactive metal is directed to the
reducing chamber, where the vapor of the highly reactive metal
reacts with the metal oxide to be reduced to provide a metal or
metal oxide having a lowest oxidation state and an oxide of the
highly reactive metal (e.g. MgO). In certain embodiments, the oxide
of the highly reactive metal is recycled back to the flux in the
electrolysis chamber.
Inventors: |
Pal; Uday B.; (Dover,
MA) |
Correspondence
Address: |
WILMERHALE/BOSTON
60 STATE STREET
BOSTON
MA
02109
US
|
Family ID: |
38872080 |
Appl. No.: |
12/445375 |
Filed: |
October 11, 2007 |
PCT Filed: |
October 11, 2007 |
PCT NO: |
PCT/US2007/081144 |
371 Date: |
June 10, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60850907 |
Oct 11, 2006 |
|
|
|
Current U.S.
Class: |
205/763 ;
204/233 |
Current CPC
Class: |
Y02P 10/20 20151101;
C22B 5/18 20130101; C22B 21/04 20130101; C22B 34/1263 20130101;
C25C 3/26 20130101; C25C 3/04 20130101; C25C 3/06 20130101; C22B
5/04 20130101; Y02P 10/212 20151101; C22B 26/22 20130101 |
Class at
Publication: |
205/763 ;
204/233 |
International
Class: |
C22B 5/12 20060101
C22B005/12; C25B 15/08 20060101 C25B015/08; C22B 5/18 20060101
C22B005/18 |
Claims
1. A method of lowering the oxidation state of a metal in an oxide
comprising: providing an electrolysis chamber housing a
magnesium-oxide containing flux, the electrolysis chamber
comprising a cathode, an anode, and a solid oxide membrane;
providing a reducing chamber housing the metal oxide to be reduced,
the metal oxide to be reduced having a higher oxidation state;
using a solid oxide membrane (SOM) process to generate Mg vapor in
the electrolysis chamber; and directing the Mg vapor to the
reducing chamber, wherein the Mg vapor reacts with the metal oxide
to be reduced to provide a metal or metal oxide having a lowest
oxidation state and magnesium oxide (MgO).
2. The method of claim 1, comprising isolating the metal or metal
oxide having the lowest oxidation state from the MgO.
3. The method of claim 2, comprising returning the MgO to the
electrolysis chamber via a conduit in communication with the
electrolysis chamber and the reducing chamber.
4. The method of claim 1, comprising subjecting the metal oxide
having the lowest oxidation state to a further reducing step
comprising a second SOM process to generate a metal.
5. The method of claim 4, comprising isolating the metal generated
in the second SOM process.
6. The method of claim 1, wherein the metal provided in the
reducing chamber comprises dissolved oxygen and wherein the metal
provided in the reducing chamber is subjected to a second SOM
process wherein the dissolved oxygen is substantially removed.
7. The method of claim 6, comprising using the metal produced in
the reducing chamber to provide a cathode in the second SOM
process.
8. The method of claim 1, wherein the metal oxide to be reduced
comprises tantalum.
9. The method of claim 1, wherein the metal oxide to be reduced
comprises aluminum.
10. The method of claim 1, wherein the metal oxide to be reduced
comprises titanium.
11. The method of claim 1, wherein using a solid oxide membrane
(SOM) process to generate Mg gas in the electrolysis chamber
includes providing an anode encased in an oxygen-ion-conducting
solid electrolyte.
12. The method of claim 11, wherein the oxygen-ion-conducting solid
electrolyte comprises yttria-stabilized zirconia (YSZ).
13. The method of claim 11, wherein the oxygen-ion-conducting solid
electrolyte is substantially electrochemically stable when the SOM
process is used to generate Mg vapor.
14. The method of claim 1, wherein isolating the metal or metal
oxide having the lowest oxidation state from the MgO comprises a
separation technique selected from the group consisting of
gravimetric sedimentation and selective isolation using a process
comprising dissolution of MgO.
15. A system for use in reducing the oxidation state of a metal in
a metal oxide, comprising: an electrolysis chamber including a
vessel for housing a flux having magnesium oxide (MgO) and an
electrode having a solid oxide membrane (SOM), the electrolysis
chamber constructed and arranged to reduce MgO in the flux having
MgO to produce magnesium vapor; a reduction chamber housing the
metal oxide to be reduced, the metal oxide having a higher
oxidation state, the reduction chamber constructed and arranged to
oxidize magnesium vapor to produce MgO and to reduce the metal
oxide having the higher oxidation state to produce a metal or metal
oxide having a lowest oxidation state; and at least one conduit
between the electrolysis chamber and the reduction chamber for
delivering MgO produced in the reduction chamber to the
electrolysis chamber.
16. The system of claim 15, wherein the electrode having the SOM
comprises an anode encased in an oxygen-ion-conducting solid
electrolyte.
17. The system of claim 16, wherein the vessel for housing the flux
comprises a steel cathode.
18. The system of claim 16, wherein the oxygen-ion-conducting solid
electrolyte comprises yttria-stabilized zirconia (YSZ).
19. The system of claim 18, wherein the oxygen-ion-conducting solid
electrolyte is substantially electrochemically stable when the
electrolysis chamber is used to reduce MgO in the flux having MgO
to produce magnesium vapor.
20. The system of claim 15, comprising separation apparatus for
isolating the metal or metal oxide having the lowest oxidation
state, the separation apparatus selected from the group consisting
of gravimetric sedimentation apparatus and MgO dissolution
apparatus.
21. The system of claim 20, comprising a second electrolysis
chamber housing the metal oxide having the lowest oxidation state
and having a SOM, the second electrolysis chamber constructed and
arranged to reduce at least a portion of the metal oxide having the
lowest oxidation state to produce a metal.
22. The system of claim 20, comprising a second electrolysis
chamber housing the metal produced in the reduction chamber and
having a SOM, wherein the metal produced in the reduction chamber
comprises dissolved oxygen and wherein the second electrolysis
chamber constructed and arranged to substantially remove the
dissolved oxygen.
23. The system of claim 22, wherein the second electrolysis chamber
includes a cathode comprising the metal produced in the reduction
chamber.
24. The system of claim 15, wherein the metal oxide to be reduced
comprises tantalum.
25. The system of claim 15, wherein the metal oxide to be reduced
comprises aluminum.
26. The system of claim 15, wherein the metal oxide to be reduced
comprises titanium.
27. A method of lowering the oxidation state of a metal of a first
metal oxide comprising: providing a first metal oxide and a second
metal oxide, wherein the oxidation potential of the metal of the
second metal oxide is higher than the oxidation potential of the
metal of the first metal oxide; providing an electrolysis chamber
housing a flux comprising the second metal oxide, the electrolysis
chamber comprising a cathode, an anode, and a solid oxide membrane;
providing a reducing chamber housing the first metal oxide to be
reduced, the first metal oxide to be reduced having a high
oxidation state; using a solid oxide membrane (SOM) process at a
temperature sufficient to generate a vapor of the metal of the
second metal oxide in the electrolysis chamber; directing the vapor
of the metal of the second metal oxide to the reducing chamber,
wherein said vapor reacts with the first metal oxide to provide the
metal of the first metal oxide or a metal oxide of the first metal
oxide having a low oxidation state; and regenerating the second
metal oxide.
28. The method of claim 27, wherein the second metal oxide
comprises lithium.
29. The method of claim 27, wherein the second metal oxide
comprises calcium.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) to U.S. Provisional Patent Application No. 60/850,907,
filed on Oct. 11, 2006, entitled Magnesiothermic SOM Process for
Production of Metals, which is incorporated herein by reference in
its entirety.
BACKGROUND
[0002] 1. Technical Field
[0003] This invention relates generally to the production of metals
from metal compounds.
[0004] 2. Discussion of Related Art
[0005] Electrolysis is a common form of electrochemical refining.
In an electrolysis process, the ore is dissolved in an aqueous or
non-aqueous solution or melted in an electrolytic furnace. Once
dissolved or melted, the ore dissociates into ionic species,
forming an electrolyte. The metallic components of the ore to be
extracted become positively charged cations. The remaining
components, typically oxygen, carbonate, sulfate, chloride or
fluoride become negatively charged anions. To extract the metal
from the ore, an electric potential is applied across two
electrodes which are immersed in the electrolyte. The metallic ions
are thereby attracted to the negatively charged cathode, where they
combine with electrons and are deposited as metal. The oxygen,
sulfate, carbonate, chloride or fluoride ions are driven to the
positively charged anode and evolve as waste gases.
[0006] The formation of metals and oxygen from molten metal salts
containing a metal oxide of interest has been described, in which a
cathode is immersed in a molten-salt electrolyte (containing the
metal oxide of interest) separated from an anode by a solid
oxygen-ion-conducting yttria-stabilized zirconia (YSZ) solid
electrolyte. High-energy-content metals such as magnesium, tantalum
and titanium can be synthesized directly from their respective
oxides (dissolved in fluoride-based molten salts). The electrolysis
process is referred to as the solid-oxide-membrane (SOM) process
because the electrolyte is a solid ceramic oxide.
[0007] In the SOM process, the YSZ electrolyte separates a
chemically inert cathode and the molten salt electrolyte from the
anode. The molten salt electrolyte has high ionic conductivity,
high oxide solubility and low viscosity. When the applied
electrical potential between the electrodes exceeds the
dissociation potential of the oxide, oxygen ions are pumped out of
the flux and through the YSZ membrane to the anode where they are
oxidized. The process can be enhanced by the presence of forming
gases at the anode to reduce the oxygen formed at the anode to
carbon dioxide and water or by the use of a carbon-containing
liquid metal anode (such as copper, tin, and silver). The molten
salt electrolyte is referred to, interchangeably, as the flux or
flux containing the metal oxide of interest.
[0008] While electrochemical processes are usually preferred
compared to pyrometallurgical processes for quick energy efficient
extraction and refining of metals, material selection for the
electrolyte and process apparatus prevents broad application. They
are usually restricted to the extraction of metals whose ores form
very stable compounds. Driven by the ever-rising demand for metals
and the increasing scarcity of available mineral resources, there
exists a need for an energy efficient, environmentally benign
process for the refining of ores.
SUMMARY
[0009] A process and apparatus is provided that allow metals
including metals having stable oxide phases and metals with
variable valencies, i.e., metals having more than one oxidation
state, to be extracted from their respective ores via an
electrolytic process that is environmentally sound and economically
viable. The process employs a SOM process to reduce a first metal
oxide source into a highly reactive metal; the highly reactive
metal is then combined with a second metal oxide to reduce the
oxidation state of the second metal oxide and regenerate the first
metal oxide.
[0010] According to one embodiment, a method of lowering the
oxidation state of a metal in a metal oxide comprises providing an
electrolysis chamber housing a magnesium-oxide containing flux. The
electrolysis chamber includes a cathode, an anode, and a solid
oxide membrane. A reducing chamber housing the metal oxide to be
reduced is provided. The metal oxide to be reduced has a first
higher oxidation state. A solid oxide membrane (SOM) process is
used to generate Mg vapor in the electrolysis chamber. Mg vapor is
directed to the reducing chamber, where the Mg vapor reacts with
the metal oxide to be reduced to provide a metal or metal oxide
having a lowest oxidation state and magnesium oxide (MgO).
[0011] In another aspect of the invention, the metal or metal oxide
having the lowest oxidation state is isolated from the MgO.
[0012] In another aspect of the invention, the MgO is returned to
the electrolysis chamber via a conduit in communication with the
electrolysis chamber and the reducing chamber.
[0013] In another aspect of the invention, the metal oxide having
the lowest oxidation state is subjected to a further reducing step
comprising a second SOM process to generate a metal.
[0014] In another aspect of the invention, the metal generated in
the second SOM process is isolated.
[0015] In another aspect of the invention, the metal provided in
the reducing chamber comprises dissolved oxygen and the metal
provided in the reducing chamber is subjected to a second SOM
process in which the dissolved oxygen is substantially removed.
[0016] In another aspect of the invention, the metal produced in
the reducing chamber is used to provide a cathode in the second SOM
process.
[0017] In another aspect of the invention, the metal oxide to be
reduced comprises tantalum.
[0018] In another aspect of the invention, the metal oxide to be
reduced comprises aluminum.
[0019] In another aspect of the invention, the metal oxide to be
reduced comprises titanium.
[0020] In another aspect of the invention, a solid oxide membrane
(SOM) process is used to generate Mg gas in the electrolysis
chamber. The SOM process includes providing an anode encased in an
oxygen-ion-conducting solid electrolyte.
[0021] In another aspect of the invention, the
oxygen-ion-conducting solid electrolyte includes yttria-stabilized
zirconia (YSZ).
[0022] In another aspect of the invention, the
oxygen-ion-conducting solid electrolyte is substantially
electrochemically stable when the SOM process is used to generate
Mg vapor.
[0023] In another aspect of the invention, isolating the metal or
metal oxide having the lowest oxidation state from the MgO includes
a separation technique selected from the group consisting of
gravimetric sedimentation and selective isolation using a process
comprising dissolution of MgO.
[0024] According to another embodiment, a system for use in
reducing the oxidation state of a metal in a metal oxide includes
an electrolysis chamber. The electrolysis chamber includes a vessel
for housing a flux having magnesium oxide (MgO) and an electrode
having a solid oxide membrane (SOM). The electrolysis chamber is
constructed and arranged to reduce MgO in the flux having MgO to
produce magnesium vapor. The system includes a reduction chamber
housing the metal oxide to be reduced where the metal oxide has a
higher oxidation state. The reduction chamber is constructed and
arranged to oxidize magnesium vapor to produce MgO and to reduce
the metal oxide having the higher oxidation state to produce a
metal or metal oxide having a lowest oxidation state. The system
includes at least one conduit between the electrolysis chamber and
the reduction chamber for delivering MgO produced in the reduction
chamber to the electrolysis chamber.
[0025] In another aspect of the invention, the electrode having the
SOM includes an anode encased in an oxygen-ion-conducting solid
electrolyte.
[0026] In another aspect of the invention, the vessel for housing
the flux comprises a steel cathode.
[0027] In another aspect of the invention, the
oxygen-ion-conducting solid electrolyte comprises a
yttria-stabilized zirconia (YSZ).
[0028] In another aspect of the invention, the
oxygen-ion-conducting solid electrolyte is substantially
electrochemically stable when the electrolysis chamber is used to
reduce MgO in the flux having MgO to produce magnesium vapor.
[0029] In another aspect of the invention, separation apparatus for
isolating the metal or metal oxide having the lowest oxidation
state is provided where the separation apparatus selected from the
group consisting of gravimetric sedimentation apparatus and MgO
dissolution apparatus.
[0030] In another aspect of the invention, a second electrolysis
chamber housing the metal oxide having the lowest oxidation state
and having a SOM is provided where the second electrolysis chamber
constructed and arranged to reduce at least a portion of the metal
oxide having the lowest oxidation state to produce a metal.
[0031] In another aspect of the invention, a second electrolysis
chamber housing the metal produced in the reduction chamber and
having a SOM is provided where the metal produced in the reduction
chamber comprises dissolved oxygen and wherein the second
electrolysis chamber constructed and arranged to substantially
remove the dissolved oxygen.
[0032] In another aspect of the invention, the second electrolysis
chamber includes a cathode comprising the metal produced in the
reduction chamber.
[0033] According to another embodiment, a method of lowering the
oxidation state of a metal of a first metal oxide includes
providing a first metal oxide and a second metal oxide, where the
oxidation potential of the metal of the second metal oxide is
higher than the oxidation potential of the metal of the first metal
oxide. An electrolysis chamber housing a flux comprising the second
metal oxide is provided. The electrolysis chamber includes a
cathode, an anode, and a solid oxide membrane. A reducing chamber
housing the first metal oxide to be reduced is provided, where the
first metal oxide to be reduced has a high oxidation state. A solid
oxide membrane (SOM) process is used at a temperature sufficient to
generate a vapor of the metal of the second metal oxide in the
electrolysis chamber. The vapor of the metal of the second metal
oxide is directed to the reducing chamber, where the vapor reacts
with the first metal oxide to provide the metal of the first metal
oxide or a metal oxide of the first metal oxide having a low
oxidation state. The second metal oxide is regenerated.
[0034] In another aspect of the invention, the second metal oxide
comprises lithium.
[0035] In another aspect of the invention, the second metal oxide
comprises calcium
BRIEF DESCRIPTION OF THE DRAWING
[0036] The subject matter is described with reference to the
figures that are described herein, which are presented for the
purpose of illustration only and are not intended to be limiting of
the invention.
[0037] FIG. 1 is a schematic representation of the solid oxide
membrane cell with carbon-consuming liquid metal anodes used for
electrolyzing metal oxides (MeO).
[0038] FIG. 2 is a schematic representation of a solid oxide
membrane system for the production of tantalum using a
magnesiothermic process according to one or more embodiments.
[0039] FIG. 3 is a schematic representation of a solid oxide
membrane system for the production of titanium or lowering the
oxidation state of titanium using a magnesiothermic process
according to one or more embodiments.
[0040] FIG. 4 is a schematic representation of an experimental SOM
setup for synthesizing titanium from TiO-containing flux, according
to one or more embodiments.
DETAILED DESCRIPTION
[0041] In one or more embodiments, the SOM process is employed to
generate magnesium (or other highly reactive metals such as lithium
and calcium) from magnesium oxide (MgO) (or other respective
oxides) and use the generated magnesium (or other highly reactive
metal) to reduce the oxidation state of a metal or metal oxide. The
process is environmentally friendly and energy efficient. By
employing a multi-step process to synthesize metals electrical
energy may be efficiently used to form metal compounds while
significantly reducing environmentally degrading waste products
that would otherwise be produced in alternate processes for the
synthesis of metals from their oxides.
[0042] The energy efficiency of the present process is attributed,
in part, to the combination of two steps, one in which a metal
oxide is reduced to form highly reactive metal and another in which
a metal compound of interest is reduced while the highly reactive
metal is oxidized. In one or more embodiments, it takes
significantly less electrical energy to reduce the highly reactive
metal and use it to produce the desired metal (or metal compound)
from the metal compound of interest than it does to reduce the
metal compound of interest to produce the desired metal (or metal
compound) in an alternate, single-step process. This is because in
the first step of the proposed multi-step process, the energy used
to reduce the metal oxide to form the highly reactive metal is less
than energy that would otherwise be used to directly reduce the
metal of interest in a comparable single-step electrolysis process
(such as a direct SOM electrolysis which is energy intensive). And,
in the second step of the proposed multi-step process, no
substantial amount of energy is needed to reduce the metal of
interest because the highly reactive metal facilitates a chemical
reaction. In sum, less energy is used to reduce a selected amount
of the metal of interest than would be used in a comparable
single-step electrolysis process. The multi-step process is also
environmentally friendly in that the present multi-step process
generates environmentally benign waste products--whereas a
comparable single-step electrolysis process for the production of
the same amount of metal typically generates environmentally
degrading products. In addition, the oxidized form of the highly
reactive metal, generated in the second step of the process, is a
reusable reagent and is recycled back into the first step of the
process. In one or more embodiments, recycling MgO generated in the
second step to produce Mg in the first phase significantly
minimizes the generation of waste Mg. By minimizing waste and
energy consumption, the multi-step process has significant cost
benefits.
[0043] The SOM process has been successfully employed to produce
gaseous magnesium from its oxides. When the dissolved oxide in the
flux is MgO, Mg(g) evolves at the inert cathode (steel) and is
condensed in a separate chamber yielding a high-purity Mg metal.
The SOM process is run at a temperature that forms magnesium vapor,
which is then transferred to another chamber wherein oxides of a
second metal (such as tantalum, titanium, aluminum, etc.) are
reduced by the magnesium vapor. The reactive metals produced by the
SOM process react with the oxides of the second metal to form a
second metal species in a reduced oxidation state. Magnesium
reverts back to its oxide by reducing the oxides of the second
metal. The magnesium oxide that forms as a result of the chemical
reduction of the oxides of tantalum, titanium, aluminum, etc., can
be reused or recycled back into the SOM reactor to continue the
process of magnesiothermic reduction of a metal from its oxide. The
process of magnesiothermic reduction of metals from its oxide
refers to the process by which an SOM process is employed to reduce
magnesium oxide into a magnesium vapor which is then combined with
a selected metal oxide to reduce the oxidation state of that
selected metal oxide and regenerate the magnesium oxide.
[0044] The overall current needed for the multi-step
magnesiothermic reduction is approximately that which is measured
during the SOM electrolysis process whereby the magnesium oxide is
reduced to magnesium. The magnesium is then directed at chemically
producing the less reactive metals (such as tantalum, titanium,
aluminum, etc.) from their oxides while the more reactive magnesium
metal is oxidized, without necessitating an additional current
input. Magnesium metal is essentially cycled between its metallic
and oxide states. Thus, the amount of energy used to generate the
less reactive metal is less than would be required to generate it
directly in a SOM process.
[0045] In the case of certain metal combinations, e.g., aluminum,
it may be possible to directly obtain alloys, e.g.,
aluminum-magnesium alloy since magnesium and aluminum are mutually
soluble.
[0046] The process employs the well proven SOM process for
magnesium production. No new flux is needed. The metal produced can
be easily separated from magnesium oxide by-product from many
industrial processes. The magnesium oxide by-product can be fed
back into the SOM reactor to continue the process. Constant supply
of new magnesium oxide is not needed for the process. The overall
process is energy efficient and environmentally sound.
[0047] Any conventional SOM process suitable for generating
metallic Mg may be used. In one embodiment, the anode is a reactive
anode. One type of anode that has been successfully used in SOM
production of metallic Mg is a liquid metal anode having a high
oxygen solubility, such as liquid copper, tin or silver. The liquid
anode may contain a carbon source. At the liquid anode, the oxygen
ions oxidize, dissolve in the liquid anode and react with the
carbon forming CO(g)/CO.sub.2(g). As an example, the experimental
cell with liquid copper anode can be described as:
C/Cu (l)/Yttria Stabilized Zirconia (YSZ)/ionic flux with dissolved
MgO/Steel.
[0048] The liquid copper (Cu (l)) electrode serves as a medium to
transport oxygen from the YSZ/copper interface to graphite where it
is oxidized. All three liquid metal anodes used (copper, tin and
silver) have low vapor pressure, high oxygen solubility and high
oxygen diffusivity in the temperature range of interest. An
exemplary system for electrolyzing metals such as magnesium is
shown in FIG. 1. When the dissolved oxide in the flux is MgO, Mg(g)
evolves at the inert cathode (steel) and is condensed in a separate
chamber yielding a high-purity Mg metal.
[0049] The anodic and cathodic reactions and the transport of
various species are as shown in FIG. 1. FIG. 1 shows cell 100
having an inert cathode 135, YSZ electrolyte 120, liquid metal
anode 110 and graphite layer 180. The rate of the slowest step
determines the overall metal production rate in the cell. In order
to increase the overall rate, the rate of the slowest step may be
enhanced. The flux 130 is an electron blocker and ionic resistance
of the flux 130 is typically much smaller than that of the YSZ
membrane 120. Adequate stirring of the flux 130 and having
sufficient MgO in the flux 130 ensures that transport in the flux
is rapid and the magnesium is formed in the vapor state. The
temperature is sufficiently high (.gtoreq.1000.degree. C.) so
charge transfer reactions are rapid. Since the oxygen solubility
and diffusivity are high in the liquid anode 110 and the anode is
well stirred by the evolving CO(g)/CO.sub.2(g), oxygen transport in
the liquid anode 110 is also rapid. The free energy change of
carbon oxidation at these temperatures indicates that the carbon
oxidation will occur readily and the product gas will mostly be
CO(g). Quantitative analysis of all these steps is provided in a
published article of the process, A. Roine. "HSC Thermodynamic
Software", Outokumpo Research Oy, Pori, Finland, fifth edition.
2003, the entirety of which is herein incorporated by reference.
Other aspects of magnesium extraction from magnesium oxide by the
SOM process are described in a published article: Rachel DeLucas,
Guosheng Ye, Marko Suput, and Uday Pal, "Modeling of magnesium
extraction from magnesium oxide by SOM process", Advanced
Processing of Metals and Materials Vol. 4: New, Improved and
Existing Technologies: Nonferrous Materials Extraction and
Processing, pages 285-298. TMS, September 2006, the entirety of
which is herein incorporated by reference.
[0050] The individual half-cell reactions can be written as
follows:
At the cathode: Mg.sup.2++2e.sup.-.fwdarw.Mg(g) At YSZ/liquid anode
interface: O.sup.2-=[O].sub.Anode+2e.sup.- At the C/liquid anode
interface: C+[O].sub.Anode.dbd.CO(g) Overall cell reaction can be
given as: Mg.sup.2++C+O.sup.2-.dbd.Mg(g)+CO(g) or
MgO+C.dbd.Mg(g)+CO(g)
[0051] Other electrolysis systems can be used, such as, for
example, systems that employ inert anodes, such as cermets.
Exemplary anodes include Ni--YSZ. A reforming gas can flow over the
anode to improve the efficiency of the SOM process. See, e.g.,
"Emerging SOM Technology for the Green Synthesis of Metals from
Oxides," JOM, October 2001, which is incorporated by reference in
its entirety.
[0052] In one or more embodiments, the SOM process is paired with a
chemical reduction process for reducing the oxidation state of a
second metal. In such embodiments, the magnesium vapor formed in
the SOM process is directed from the SOM cell and into contact with
a metal compound (e.g. a metal oxide to be reduced. The metal
compound may be a metal oxide, metal ore, metal salt, or other form
of metal-containing compound. The metal compound may be housed in a
container or reaction chamber. In the second container, the metal
compound is exposed to the magnesium vapor and undergoes a
reduction reaction. Exemplary metal compounds include metal oxides.
Most metal oxides that are less stable than MgO may be selected for
reduction in this process. The metal oxide may be, for example,
tantalum oxide, aluminum oxide, titanium oxide, or any other
suitable transition metals. Comparatively reactive metals such as
magnesium, once produced by the SOM process detailed above, can
spontaneously chemically revert back to their corresponding oxide
by reducing the oxides of comparatively less reactive metal
compounds. Thus, for the present reduction step, the oxidation
potential of the metal to be reduced will be lower than the
oxidation potential of magnesium, since the magnesium will oxidize
comparatively readily and contribute to the reduction of the metal
compound to be reduced.
[0053] The reduction process performed in the second container may
employ a metal oxide in a form that promotes chemical reaction with
the magnesium. In some embodiments, the metal compound may be a
powder. The powder may be processed to provide high surface area
thereby ensuring that the Mg is able to penetrate the compound. By
way of example the particle size should be sufficiently small to
not interfere with the diffusion process. In one or more
embodiments, the powder is stirred or agitated by introducing Mg
vapor from a lower portion of the container so the Mg vapor
permeates through the powder. The magnesium reduction may take
place at any appropriate temperature. The temperature range for the
reduction reaction stage is selected in accordance with the
requirements for the particular metal oxide chosen, and higher than
the vaporization temperature of Mg. Whereas the SOM electrolysis
process is generally conducted in the temperature range 1050 C-1300
C in order to optimize energy consumption and exploit the high
oxygen-ion conductivity of the SOM membrane, a lower temperature
range will be typically be used for the reduction process performed
in the second container. For example, temperature ranges selected
for performing the reduction process to reduce tantalum oxide
TaO.sub.5 are described in Okabe, T. H. et al., Production of
Tantalum Powder by Magnesiothermic Reduction of Feed Preform,
Materials Transactions, Vol. 44, No. 12 (2003), which is herein
incorporated by reference in its entirety. For other metals oxides
such as titanium oxide and aluminum oxide, the reduction process
will be performed at a suitable temperature as determined by the
thermal balance for the reaction.
[0054] The process performed in the second container reduces the
selected metal compound from a first higher valance-state to a
second lower valance-state and oxidizes the magnesium vapor to form
MgO. Separation of the metal oxide having a lower valance state
from the MgO is then performed using any number of conventional
separation techniques known in the art and detailed below. Then,
the MgO is redirected to electrolysis chamber. The MgO is thus
recycled back to the SOM process to supply the MgO in the flux that
is used to create the Mg vapor.
[0055] In one or more embodiments, the second container is
constructed of a material inert to the selected metal being
produced (e.g. stainless steel). Conduits connect the electrolysis
chamber (e.g. first container) and the reduction chamber (e.g.
second container). A first conduit transports magnesium vapor from
the electrolysis chamber to the reduction chamber. A second conduit
transports MgO from the reduction chamber back to the electrolysis
chamber. Details concerning the transport of Mg vapor are provided
below.
[0056] A schematic of an exemplary system for tantalum production
from its oxide is shown in FIG. 2. A SOM cell 200 was designed to
form magnesium gas and direct the gaseous magnesium from the SOM
cell into a second chamber 250 housing the metal oxide to be
reduced 260.
[0057] The electrolytic cell 200 can utilize up to 33 cm.sup.2 of
the liquid anode area 210 and operate at anodic current densities
as high as 1 A/cm.sup.2. The YSZ solid electrolyte 220 is in the
form of a one-end-closed tube (1.9 cm OD, 1.42 cm ID, 20 cm long)
and contains the liquid anode 210. A high-density graphite rod (0.6
cm OD) 215 is used as a consumable feed in the liquid anode 210.
Liquid copper, tin or silver or other suitable materials can be
used as anodes. The YSZ electrolyte tube is chemically stable when
in contact with the liquid metal anodes, and the flux composition
230 is selected to be inert to the YSZ electrolyte. A steel
crucible 235 holds the MgO-containing ionic flux 230. The steel
crucible 235 can also serve as the cathode. In order to protect the
YSZ tube above the flux from the Mg vapor that is produced along
the wall of the stainless steel container (cathode), an inert gas
such as argon gas may be introduced into the chamber via input 290
as a carrier and diluent. The resultant argon-magnesium gas mixture
passes out of the electrolysis chamber 200 at an exit port 240 to
the lower condensation chamber 250 where a metal oxide 260 to be
reduced is located. Nearly any metal oxide that is less stable than
MgO may be reduced by this process. The exemplary embodiment shows
Ta.sub.2O.sub.5 as the metal oxide 260 to be reduced. As noted
above, the metal oxide may be, for example, tantalum oxide,
aluminum oxide, titanium oxide, or any number of other compounds.
Other embodiments are contemplated, for example, the cell may
include a YSZ membrane and the anode and flux may be located on
opposing sides of the membrane.
[0058] A constant potential is applied to the cell 200 during
electrolysis. The applied electrical potential can be in the range
of about 1-10V, or about 3-5V, and the cell can be operated at
temperatures above the vaporization temperature of magnesium, e.g.,
over 1090.degree. C. In one or more embodiments, the cell is
operated between 1100-1300.degree. C. by running the cell at an
applied electric potential of 3-4 V for an extended period (5-10
hours) with continuous MgO and C feed and collecting a mixture of
tantalum metal and metal oxide in the second chamber 250. The
overall reaction occurring in the second chamber, whereby tantalum
oxide is reduced and magnesium vapor is oxidized, can be written as
follows:
5Mg(g)+Ta.sub.2O.sub.5=5MgO+2Ta(s).
[0059] The resulting reduced metal and MgO can be separated using
conventional separations techniques, such as gravametric
sedimentation or selective dissolution of MgO. The resulting MgO
byproduct is recycled back into the SOM reactor to be dissolved
into the flux 230 to from which magnesium vapor is generated. The
resulting tanatulum metal is isolated and the argon gas is
outputted 295. According to one or more embodiments, constant
supply of new magnesium oxide is not needed to continue the process
of magnesiothermic reduction of tantalum from its oxide. Magnesium
metal may therefore be cycled between its metallic and oxide
states.
[0060] As noted above, any number of metals compound having a lower
oxidation potential than that of magnesium may be produced in the
present process. In one embodiment, Ta.sup.+5 is reduced to
Ta.sup.0 in a single step, however the reduction product is always
or necessarily a zero-valance metal. Reduction of multi-valance
metal compounds poses unique challenges. Titanium compounds provide
one such example. In one aspect, titanium is not an ideal candidate
for the magnesiothermic SOM process for reduction of metals because
certain titanium compounds will not be completely reduced to
titanium metal. In another aspect, titanium is not an ideal
candidate for a single-step SOM process due the presence of
mulit-valance oxides of titanium (Ti.sup.2+, Ti.sup.3+, and
Ti.sup.4+) in the flux that imparts electronic conductivity,
leading to lower current efficiency and SOM degradation More
particularly, it has been found that during electrolysis the higher
valence titanium ions undergo valence reductions at the cathode,
causing intermediate valence states of titanium to exist, which
imparts the electronic conductivity to the MgO-containing flux.
These challenges may be overcome, and effective reduction of
multi-valance metal compounds (such as titanium oxide) may be
achieved, by a two-step process that combines a magnesiothermic SOM
process with an additional SOM process. It is advantageous to
isolate a single titanium oxide from a mixture of different valance
oxides of titanium before producing the titanium metal though an
additional process. By reducing the titanium oxide start up feed to
its lowest oxidation state via reduction by Mg(g) generated from
SOM electrolysis of magnesium oxide in the magnesiothermic SOM
process, the problems of current efficiency and membrane
degradation otherwise found in the single-step SOM process may be
overcome.
[0061] By way of example, titanium (Ti.sup.4+) in the form of
TiO.sub.2 is reduced to yield a titanium having titanium of a lower
valance state Ti.sub.3O. In an exemplary process for the production
of lower-oxidation state of titanium from its higher-oxidation
state, an electrolytic cell and magnesium generation apparatus may
be designed to form magnesium vapor. The magnesium vapor may be
directed from the SOM cell into a second container housing the
titanium oxide (TiO.sub.2) to be reduced. In the present
embodiment, the magnesiothermic SOM process may be used to isolate
titanium in its lowest oxidation state (Ti.sub.3O) from one or more
of its oxides. Then an additional SOM process may be used to
isolate titanium metal from the titanium in its lowest oxide state
(Ti.sub.3O).
[0062] FIG. 3 illustrates apparatus for lowering the oxidation
state of titanium in TiO.sub.2 through SOM MgO electrolysis and
then employing the product for continuous production of titanium
metal. An electrolysis chamber 300 is used to generate magnesium
vapor from MgO in a MgO-containing flux 330 through the SOM process
described above. The apparatus for this electrolytic cell 300 is
substantially the same as that described in the previous examples.
A steel crucible 335 houses the MgO-containing flux 330 and
provides a cathode. A liquid anode 310 is contained in a one-end
closed tube comprising YSZ solid electrolyte 320. In one or more
embodiments, the reduction chamber 350 is used to lower the
oxidation state of titanium in TiO.sub.2 360 to create titanium
oxide having the lowest oxidation state (TiO) according to the
following reaction:
Mg(g)+TiO.sub.2.dbd.MgO+TiO.
In one or more embodiments, the reduction chamber 350 is used to
lower the oxidation state of titanium in TiO.sub.2 360 to create
titanium metal, according to the following reaction:
5Mg(g)+3TiO.sub.2=5MgO+Ti.sub.3O,
where Ti.sub.3O is Titanium metal (Ti.sup.0) having dissolved
oxygen.
[0063] Magnesium vapor generated in the electrolysis chamber is
transported to the reduction chamber via a conduit 340. As
described above, in one aspect, argon gas is introduced as a
carrier gas and diluent and thus a magnesium-argon gas mixture
passes through the conduit 340 and into the reduction chamber 350.
Sufficient amounts of argon gas are introduced to dilute the Mg
vapor and ensure it does not damage the YSZ membrane. Sufficient
dilution is particularly important at higher operating
temperatures. Titanium oxide of a high-valence state [TiO.sub.2] is
housed in the reduction chamber 350 and exposed to the magnesium
vapor. In one aspect, the titanium oxide [TiO.sub.2] is initially
in powder form to increase surface area and facilitate diffusion.
The chemical process produces MgO and titanium oxide of the lowest
oxidation state (TiO) or titanium metal with dissolved oxygen
(Ti.sub.3O).
[0064] The resultant mixture of titanium oxide having the
lowest-oxidation state or titanium metal and MgO collects in the
reduction chamber 350. The reduced titanium oxide and MgO can be
separated using conventional techniques such as gravimetric
sedimentation or selective dissolution of MgO. The inert carrier
gas--in the present embodiment, argon gas--is removed from the
reduction chamber while the magnesium oxide is cycled back to the
electrolysis chamber 300. As described above, the MgO is supplied
to the flux 330 in the SOM reactor 300 to continue the process. The
titanium oxide having the lowest oxidation state (TiO) or titanium
metal having dissolved oxygen ( Ti.sub.3O) may then be supplied to
additional apparatus for the synthesis of titanium.
[0065] One example of apparatus for the synthesis of titanium metal
is shown in FIG. 4. In one or more embodiments, the apparatus may
be a SOM reactor 400 for the synthesis of titanium metal from
titanium oxide having the lowest oxidation state (TiO) using the
SOM process. The SOM reactor 400 includes a steel crucible 435
housing a TiO-containing flux 430, a cathode (Ti) and a
molybdenum/carbon anode having a YSZ membrane casing 420 and liquid
tin 410 inside the YSZ membrane casing. The SOM apparatus 400 also
includes a steel secondary cathode tube for the continuous addition
of TiO 475 and an alumina bisque end cap 470. In this example, the
TiO in the flux is reduced to provide titanium metal. In one or
more embodiments, the additional apparatus may be a SOM reactor for
the synthesis of titanium metal from titanium metal having
dissolved oxygen (Ti.sub.3O). In this example, the (Ti.sub.3O)
serves as the cathode in the SOM electrolysis chamber (having a YSZ
membrane). Through the SOM process, the soluble oxygen is removed
to produce titanium metal. Other examples of apparatus for the
synthesis of titanium from TiO-containing flux or from Ti.sub.3O
are possible and may be envisioned.
[0066] While the present method for production of metals describes
an SOM process using magnesium, other suitable metals (such as
lithium, calcium, etc.) may be used. The electrolysis step,
reduction step, and corresponding apparatus (e.g. solid oxide
membrane) may be selected by a person of ordinary skill in the art,
in accordance with the requirements for the alternate metal. Other
examples of SOM processes for the production of metals that are
consistent with the central aspects of the present disclosure may
also be envisioned by a person of ordinary skill in the art.
[0067] As will be apparent to one of ordinary skill in the art from
reading this disclosure, the present invention can be embodied in
forms other than those specifically disclosed above. The particular
embodiments described above are, therefore, to be considered as
illustrative and not restrictive. In addition, the invention
includes each individual feature, material and method described
herein, and any combination of two or more such features, materials
or methods that are not mutually inconsistent.
* * * * *