ELECTROLYTIC REDUCTION OF METAL OXIDES

REDUCTION ELECTROLYTIQUE D'OXYDES METALLIQUES

English Abstract

A method of electrolytically reducing a metal oxide (such as aluminium and
magnesium oxides) to produce a metal in an electrolytic cell 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.

French Abstract

L'invention concerne un procédé destiné à réduire, par voie électrolytique, un oxyde métallique (tel que des oxydes d'aluminium et de magnésium) en vue de produire un métal dans une cellule électrolytique. Ce procédé consiste à réduire, par voie électrolytique, cet oxyde métallique dans une cellule électrolytique comprenant une masse de métal en fusion, ce métal étant le métal de l'oxyde métallique à réduire, la masse de métal en fusion formant une cathode de la cellule. Cette cellule électrolytique comprend également une masse d'électrolyte liquide en contact avec le métal en fusion, cet électrolyte contenant des halogénures alcalins et/ou alcalino-terreux. La cellule électrolytique comprend également une anode se prolongeant dans l'électrolyte ainsi qu'un corps d'oxyde métallique à réduire en contact avec le métal en fusion et l'électrolyte.

Claims

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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 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 or claim 2
includes 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 any one of the preceding
claims wherein the metal oxide body includes 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 any one of the preceding
claims includes maintaining the cell temperature above the
melting points of the electrolyte and the metal of the
metal oxide to be reduced.

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6. The method defined in any one of the preceding
claims 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.
7. The method defined in any one of the preceding
claims wherein the metal oxide is an aluminium oxide or a
magnesium oxide.
8. The method defined in claim 7 wherein the
electrolyte is a CaCl2-based electrolyte that includes CaO
as one of the constituents.
9. The method defined in claim 8 includes
maintaining the cell potential above the decomposition
potential for CaO.
10. The method defined in claim 8 or claim 9 includes
maintaining the cell potential below the decomposition
potential for CaCl2.
11. The method defined in claim 10 includes
maintaining the cell potential less than 3.0V.
12. The method defined in claim 10 includes
maintaining the cell potential less than 2.5V.
13. The method defined in claim 10 includes
maintaining the cell potential less than 2.0V.
14. The method defined in any one of claims 8 to 13
includes maintaining the cell potential to be at least
1.5V.

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15. The method defined in any one of the preceding
claims 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

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ELECTROLYTIC REDUCTION OF METAL OXIDES
1. Field of the Invention
The present invention relates to electrolytic
reduction of metal oxides to produce substantially pure
metals.
In particular, the present invention relates to
electrolytic reduction of aluminium and magnesium oxides
using a CaCl2 electrolyte.
2. Background Art
The present invention was made during the course
of an on-going research project on the electrolytic
reduction of metal oxides using CaCl2-based electrolyte
being carried out by the applicant.
The research project investigated electrolytic
reduction of a range of metal oxides in electrolyte cells
based on the use of using CaCl2 electrolyte.
The CaCl2 electrolyte was a commercially
available source of CaClz, namely calcium chloride
dehydrate, that decomposed on heating and produced a very
small amount of CaO.
The applicant operated the electrolytic cells at
a potential above the decomposition potential of Ca0 and
below the decomposition potential of CaCla.
The applicant found that the cells could
electrolytically reduce a range of metal oxides to metals
with very low concentrations of oxygen.
3. Summary of the Invention

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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.
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.
Preferably the metal oxide body has a geometric
shape that maximises contact between (i) the molten metal,
(ii) the metal oxide, and (iii) the electrolyte..
Preferably the method includes feeding the metal
oxide body into the electrolytic cell to maintain contact
of the metal oxide and the molten metal.
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.
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.

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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.
Preferably the metal oxide is an aluminium oxide
or a magnesium oxide.
In a situation in which the metal oxide is a
aluminium oxide or magnesium oxide it is preferred that
the electrolyte be a CaCl2-based electrolyte that includes
Ca0 as one of the constituents.
In such a situation it is preferred that the cell
potential be above the decomposition potential for CaO.
It is also preferred that the cell potential be
below the decomposition potential for CaCl2.
It is preferred that the cell potential be less
than 3.0V.
It is preferred particularly that the cell
potential be below 2.5V.
It is preferred more particularly that the cell
potential be below 2.0V.
It is preferred that the cell potential be at
least 1.5V.
The CaCl2-based electrolyte may be a commercially
available source of CaCl2, such as calcium chloride
dehydrate, that partially decomposes on heating and
produces Ca0 or otherwese includes CaO.

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Alternatively, or in addition, the CaClz-based
electrolyte may include CaCl2 and Ca0 that are added
separately or pre-mised to form the electrolyte.
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 CaCl2-
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 CaClz-based electrolyte
partially decomposes produces Ca++ 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-- anions produced by.electrolytic reduction
to metal of metal oxide in contact with the molten metal
cathode. The applicant also believes that the O'-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++ 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.
It is preferred that the anode be graphite.
Preferably the cell includes a base and side
walls extending upwardly from the base formed from
graphite.

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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.
The above-described method may be started-up in a
number of ways.
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.
Another option is to introduce molten metal and
molten electrolyte separately into the cell.
The following example illustrates an application
of the invention in the process of reducing aluminium
oxide (alumina) into substantially pure aluminium using an
electrolytic cell as illustrated in Figure 1.
4. Description of Exemplary Embodiment
Figure 1 is a schematic illustration of an
electrolytic cell 5 that can be scaled-up in application
of the present invention.
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.
The electrolytic cell 5 of Figure 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.

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The electrolytic cell 5 further includes a bath
of molten CaCl2 electrolyte 13 a.n the crucible and a
graphite electrode 11 immersed in the molten electrolyte
13. The graphite electrode 11 forms the anode of the cell
5.
The electrolytic cell 5 further includes a pool
of molten aluminium in a lower section of the crucible
10 10. The molten aluminium pool 15 forms the cathode of the
cell.
The electrolytic cell further includes a body 14
that consists of or incorporates alumina (A1203) to be
15 reduced and extends into the electrolyte 13 and contacts
the molten aluminium 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.
The electrolytic cell 5 further includes a
suitable power source 18 connected to the anode 11 and to
the molten aluminium cathode 15.
The molten aluminium cathode 15 is required in
order to initiate electrolytic reduction of the alumina in
the alumina body 14 to aluminium. The electrolytic
reduction process is carried out at an elevated
temperature of around 950°C at Which the CaCl2 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 aluminium cathode 15,
reduction of the alumina takes place. Since the process
temperatures are above the melting point of aluminium, the
latter will melt into the bath 15 and the bath level
within crucible 10 will tend to rise.

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In order to maintain optimum reduction
COnd7.tlonS, the alumina body 14 is moved at a rate
commensurate with the melting-off rate of aluminium from
the alumina body 14 and the build-up of aluminium so that
immersion of the alumina body 14 in the molten aluminium
i.s kept at a minimum.
The process may be operated in a continuous mode
by removing molten aluminium 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.
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.

Representative Drawing