U.S. patent application number 11/924808 was filed with the patent office on 2008-05-15 for electrolytic reduction of metal oxides.
Invention is credited to Steve Osborn, Ivan Ratchev, Lazar Strezov.
Application Number | 20080110764 11/924808 |
Document ID | / |
Family ID | 3828435 |
Filed Date | 2008-05-15 |
United States Patent
Application |
20080110764 |
Kind Code |
A1 |
Strezov; Lazar ; et
al. |
May 15, 2008 |
Electrolytic Reduction of Metal Oxides
Abstract
A method of electrolytically reducing a metal oxide (such as
aluminium and magnesium oxides) to produce a metal in an
electrolytic call is disclosed. The method includes
electrolytically reducing the metal oxide in an electrolytic cell
that includes a pool of molten metal, the metal being the metal of
the metal oxide to be reduced, and the molten metal pool forming a
cathode of the cell. The electrolytic cell also includes a pool of
molten electrolyte in contact with the molten metal, the
electrolyte containing alkali and/or alkaline earth halides. The
electrolytic cell also includes an anode extending into the
electrolyte and a body of metal oxide to reduced in contact with
the molten metal and the electrolyte.
Inventors: |
Strezov; Lazar; (Adamstown,
AU) ; Ratchev; Ivan; (Georgetown, AU) ;
Osborn; Steve; (Valentine, AU) |
Correspondence
Address: |
MILES & STOCKBRIDGE PC
1751 PINNACLE DRIVE, SUITE 500
MCLEAN
VA
22102-3833
US
|
Family ID: |
3828435 |
Appl. No.: |
11/924808 |
Filed: |
October 26, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10474746 |
Apr 21, 2004 |
|
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PCT/AU02/00456 |
Apr 10, 2002 |
|
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11924808 |
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Current U.S.
Class: |
205/367 |
Current CPC
Class: |
C25C 3/06 20130101; C25C
3/04 20130101; C25C 3/00 20130101 |
Class at
Publication: |
205/367 |
International
Class: |
C25C 3/00 20060101
C25C003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 10, 2001 |
AU |
PR4439 |
Claims
1. A method of electrolytically reducing a metal oxide to produce a
metal in an electrolytic cell, which method includes
electrolytically reducing the metal oxide in an electrolytic cell
that includes a pool of molten metal, the metal being the metal of
the metal oxide to be reduced, the molten metal pool forming a
cathode of the cell, a pool of molten electrolyte in contact with
the molten metal, the electrolyte containing alkali and/or alkaline
earth halides, an anode extending into the electrolyte, and a body
of metal oxide to be reduced in contact with the molten metal and
the electrolyte.
2. The method defined in claim 1 wherein the metal oxide body has a
geometric shape that maximises contact between (i) the molten
metal, (ii) the metal oxide, and (iii) the electrolyte.
3. The method defined in claim 1 including feeding the metal oxide
body into the electrolytic cell to maintain contact of the metal
oxide and the molten metal.
4. The method defined in claim 1 wherein the metal oxide body
includes one or more of rods, plates and blocks that can be readily
immersed into the electrolyte and brought into contact with the
molten metal.
5. The method defined in claim 1 including maintaining the cell
temperature above the melting points of the electrolyte and the
metal of the metal oxide to be reduced.
6. The method defined in claim 1 including operating the cell at a
potential that is above a decomposition potential of at least one
constituent of the electrolyte so that there are cations of a metal
other than that of the cathode metal oxide in the electrolyte.
7. The method defined in claim 1 wherein the metal oxide is an
aluminium oxide or a magnesium oxide.
8. The method defined in claim 7 wherein the electrolyte is a
CaCl.sub.2-based electrolyte that includes CaO as one of the
constituents.
9. The method defined in claim 8 including maintaining the cell
potential above the decomposition potential for CaO.
10. The method defined in claim 8 including maintaining the cell
potential below the decomposition potential for CaCl.sub.2.
11. The method defined in claim 10 including maintaining the cell
potential less than 3.0V.
12. The method defined in claim 10 including maintaining the cell
potential less than 2.5V.
13. The method defined in claim 10 including maintaining the cell
potential less than 2.0V.
14. The method defined in claim 8 including maintaining the cell
potential to be at least 1.5V.
15. The method defined in claim 1 wherein the cell includes at
least one tap hole for molten metal and the method includes
removing molten metal continuously or periodically via the tap
hole.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
10/474,746 filed Apr. 21, 2004, now abandoned, which claims the
benefit of and priority to International Application No.
PCT/AU02/00456, filed Apr. 10, 2002, all of which are incorporated
herein by reference in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to electrolytic reduction of
metal oxides to produce substantially pure metals.
[0003] In particular, the present invention relates to electrolytic
reduction of aluminum and magnesium oxides using a CaCl.sub.2
electrolyte.
BACKGROUND ART
[0004] The present invention was made during the course of an
on-going research project on the electrolytic reduction of metal
oxides using CaCl.sub.2-based electrolyte being carried out by the
applicant.
[0005] The research project investigated electrolytic reduction of
a range of metal oxides in electrolyte cells based on the use of
using CaCl.sub.2 electrolyte.
[0006] The CaCl.sub.2 electrolyte was a commercially available
source of CaCl.sub.2, namely calcium chloride dihydrate, that
decomposed on heating and produced a very small amount of CaO.
[0007] The applicant operated the electrolytic cells at a potential
above the decomposition potential of CaO and below the
decomposition potential of CaCl.sub.2.
[0008] The applicant found that the cells could electrolytically
reduce a range of metal oxides to metals with very low
concentrations of oxygen.
SUMMARY OF THE INVENTION
[0009] The present invention provides, in broad terms, a method of
electrolytically reducing a metal oxide to produce a metal in an
electrolytic cell, which method includes electrolytically reducing
the metal oxide in an electrolytic cell that includes (a) a pool of
molten metal, the metal being the metal of the metal oxide to be
reduced, the molten metal pool forming a cathode of the cell, (b) a
pool of molten electrolyte in contact with the molten metal, the
electrolyte containing alkali and/or alkaline earth halides, (c) an
anode extending into the electrolyte, and (d) a body of metal oxide
to reduced in contact with the molten metal and the
electrolyte.
[0010] In the above method electrolytic reduction of metal oxide
takes place where there is contact between (i) the molten metal,
(ii) the metal oxide, and (iii) the electrolyte.
[0011] Preferably the metal oxide body has a geometric shape that
maximizes contact between (i) the molten metal, (ii) the metal
oxide, and (iii) the electrolyte.
[0012] Preferably the method includes feeding the metal oxide body
into the electrolytic cell to maintain contact of the metal oxide
and the molten metal.
[0013] The metal oxide body may be in many forms, including rods,
plates, blocks and the like, which can be readily immersed into the
electrolyte and brought into contact with the molten metal.
[0014] Preferably the method includes maintaining the cell
temperature above the melting points of the electrolyte and the
metal of the metal oxide to be reduced.
[0015] Preferably the method includes operating the cell at a
potential that is above a decomposition potential of at least one
constituent of the electrolyte so that there are cations of a metal
other than that of the cathode metal oxide in the electrolyte.
[0016] Preferably the metal oxide is an aluminum oxide or a
magnesium oxide.
[0017] In a situation in which the metal oxide is a aluminum oxide
or magnesium oxide it is preferred that the electrolyte be a
CaCl.sub.2-based electrolyte that includes CaO as one of the
constituents.
[0018] In such a situation it is preferred that the cell potential
be above the decomposition potential for CaO.
[0019] It is also preferred that the cell potential be below the
decomposition potential for CaCl.sub.2.
[0020] It is preferred that the cell potential be less than
3.0V.
[0021] It is preferred particularly that the cell potential be
below 2.5V.
[0022] It is preferred more particularly that the cell potential be
below 2.0V.
[0023] It is preferred that the cell potential be at least
1.5V.
[0024] The CaCl.sub.2-based electrolyte may be a commercially
available source of CaCl.sub.2, such as calcium chloride dihydrate,
that partially decomposes on heating and produces CaO or otherwise
includes CaO.
[0025] Alternatively, or in addition, the CaCl.sub.2-based
electrolyte may include CaCl.sub.2 and CaO that are added
separately or pre-mixed to form the electrolyte.
[0026] At this stage, the applicant does not have a clear
understanding of the electrolytic cell mechanism when the cell is
operated at a potential at which CaCl.sub.2-based electrolyte
partially decomposes. Nevertheless, whilst not wishing to be bound
by the comments in this paragraph, the applicant offers the
following comments by way of an outline of a possible cell
mechanism. The applicant believes that operating the electrolytic
cell above a potential at which CaCl.sub.2-based electrolyte
partially decomposes produces Ca.sup.++ cations that migrate to the
vicinity of the metal oxide in contact with the molten metal
cathode and provide a driving force that facilitates extraction of
O.sup.-- anions produced by electrolytic reduction to metal of
metal oxide in contact with the molten metal cathode. The applicant
also believes that the O.sup.-- anions, once extracted from the
metal oxide, migrate to the anode and react with anode carbon and
produce CO and release electrons that facilitate electrolytic
reduction of metal oxide to metal. The experimental work carried
out by the applicant produced evidence of Ca metal in the
electrolyte. The applicant believes that the Ca metal was the
result of electrodeposition of Ca.sup.++ cations as Ca metal on
electrically conductive sections of the cathode and that at least
part of the Ca metal dissolved in the electrolyte and migrated to
the vicinity of the metal oxide in the cathode and participated in
chemical reduction of oxides.
[0027] It is preferred that the anode be graphite.
[0028] Preferably the cell includes a base and side walls extending
upwardly from the base formed from graphite.
[0029] Preferably the cell includes at least one tap hole for
molten metal in one of the side walls and the method includes
removing molten metal continuously or periodically.
[0030] The above-described method may be started-up in a number of
ways.
[0031] One option is to introduce the (pure) metal and the
electrolyte in solid state into the cell and subsequently heat the
entire system to melt the metal and the electrolyte.
[0032] Another option is to introduce molten metal and molten
electrolyte separately into the cell.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a schematic illustration of an electrolytic cell 5
that can be scaled-up in application of the present invention.
DESCRIPTION OF EXEMPLARY EMBODIMENT
[0034] The following example illustrates an application of the
invention in the process of reducing aluminum oxide (alumina) into
substantially pure aluminum using an electrolytic cell as
illustrated in FIG. 1.
[0035] FIG. 1 is a schematic illustration of an electrolytic cell 5
that can be scaled-up in application of the present invention.
[0036] Whilst the example described below relates to the reduction
of alumina, the basic principle is equally applicable to other
metals, particularly low melting point metals, more particularly
magnesium.
[0037] The electrolytic cell 5 of FIG. 1 includes a graphite
crucible 10 that has a base 21, side walls 31, and a
tapping/discharge opening indicated as 12 in one of the side walls
31.
[0038] The electrolytic cell 5 further includes a bath of molten
CaCl.sub.2 electrolyte 13 in the crucible and a graphite electrode
11 immersed in the molten electrolyte 13. The graphite electrode 11
forms the anode of the cell 5.
[0039] The electrolytic cell 5 further includes a pool 15 of molten
aluminum in a lower section of the crucible 10. The molten aluminum
pool 15 forms the cathode of the cell.
[0040] The electrolytic cell further includes a body 14 that
consists of or incorporates alumina (Al.sub.2O.sub.3) to be reduced
and extends into the electrolyte 13 and contacts the molten
aluminum cathode 15. The alumina is shaped as a rod, sheet or
prismatic body. Alumina body 14 is held in an appropriate manner to
allow controlled movement into and away from the crucible interior
as indicated by the arrow 16.
[0041] The electrolytic cell 5 further includes a suitable power
source 18 connected to the anode 11 and to the molten aluminum
cathode 15.
[0042] The molten aluminum cathode 15 is required in order to
initiate electrolytic reduction of the alumina in the alumina body
14 to aluminum. The electrolytic reduction process is carried out
at an elevated temperature of around 950.degree. C. at which the
CaCl.sub.2 electrolyte is and remains molten. On immersion of the
alumina body 14 into the electrolyte 13 and subsequent contact of
the alumina body 14 with the molten aluminum cathode 15, reduction
of the alumina takes place. Since the process temperatures are
above the melting point of aluminum, the latter will melt into the
bath 15 and the bath level within crucible 10 will tend to
rise.
[0043] In order to maintain optimum reduction conditions, the
alumina body 14 is moved at a rate commensurate with the
melting-off rate of aluminum from the alumina body 14 and the
build-up of aluminum so that immersion of the alumina body 14 in
the molten aluminum is kept at a minimum.
[0044] The process may be operated in a continuous mode by removing
molten aluminum through tap hole 12 and positioning additional
alumina bodies 14 in the electrolyte 13 to replace bodies 14 that
are consumed in the reduction process.
[0045] Many modifications may be made to the embodiment of the
present invention described above without departing from the spirit
and scope of the present invention.
* * * * *