U.S. patent application number 10/474745 was filed with the patent office on 2005-06-09 for electrolytic reduction of metal oxides.
Invention is credited to Strezov, Lazar.
Application Number | 20050121333 10/474745 |
Document ID | / |
Family ID | 3828434 |
Filed Date | 2005-06-09 |
United States Patent
Application |
20050121333 |
Kind Code |
A1 |
Strezov, Lazar |
June 9, 2005 |
Electrolytic reduction of metal oxides
Abstract
An electrolytic cell and a method of electrolytically reducing a
metal oxide, such as titania, in a solid state are disclosed. The
electrolytic cell includes (a) a molten electrolyte, (b) a cathode
in contact with the electrolyte, the cathode being formed at least
in part from the metal oxide, and (c) a molten metal anode (such as
silver or copper) in contact with the electrolyte.
Inventors: |
Strezov, Lazar; (Adamstown,
AU) |
Correspondence
Address: |
MILES & STOCKBRIDGE PC
1751 PINNACLE DRIVE
SUITE 500
MCLEAN
VA
22102-3833
US
|
Family ID: |
3828434 |
Appl. No.: |
10/474745 |
Filed: |
April 27, 2004 |
PCT Filed: |
April 10, 2002 |
PCT NO: |
PCT/AU02/00457 |
Current U.S.
Class: |
205/398 ;
75/10.62 |
Current CPC
Class: |
C25C 3/28 20130101; C25C
3/00 20130101 |
Class at
Publication: |
205/398 ;
075/010.62 |
International
Class: |
C22B 004/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 10, 2001 |
AU |
PR 4438 |
Claims
1. An electrolytic cell for electrolytic reduction of a metal oxide
in a solid state, which electrolytic cell includes (a) a molten
electrolyte, (b) a cathode in contact with the electrolyte, the
cathode being formed at least in part from the metal oxide, and (c)
a molten metal anode in contact with the electrolyte.
2. The electrolytic cell defined in claim 1 wherein the metal of
the molten metal anode has a relatively high saturation level for
oxygen at the operating temperature of the cell.
3. The electrolytic cell defined in claim 1 wherein the metal of
the molten metal anode is chosen such that its melting point is
within the operating temperature ranges of the electrolyte.
4. The electrolytic cell defined in claim 1 wherein the melting
point of the metal of the molten metal anode is higher than the
melting point of the electrolyte and lower than the vaporisation
and/or decomposition temperature of the electrolyte.
5. The electrolytic cell defined in claim 1 wherein the metal of
the molten metal anode has a very low solubility in the molten
electrolyte at the cell operating temperatures,
6. The electrolytic cell defined in claim 1 wherein the metal of
the molten metal anode is silver or copper.
7. The electrolytic cell defined in claim 1 further including a
means for removing oxygen that has diffused into the molten metal
anode from the cell.
8. The electrolytic cell defined in claim 7 wherein the cell oxygen
removal means includes a duct that communicates with the molten
metal anode and a device to create a partial pressure reduction
between the molten metal anode and a head of molten metal within
the duct.
9. A method of electrolytically reducing a metal oxide in a solid
state in an electrolytic cell, which electrolytic cell includes (a)
a molten electrolyte, (b) a cathode in contact with the
electrolyte, the cathode being formed at least in part from the
metal oxide, and (c) a molten metal anode in contact with the
electrolyte, which method includes applying a cell potential across
the anode and the cathode.
10. The method defined in claim 9 including maintaining the cell
temperature above the melting points of the electrolyte and the
metal of the metal anode.
11. The method defined in claim 9 including applying a cell
potential 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.
12. The method defined in claim 9 wherein the metal oxide is a
titanium oxide.
13. The method defined in claim 9 wherein the metal oxide is
titania.
14. The method defined in claim 9 wherein the electrolyte is a
CaCl.sub.2-based electrolyte that includes CaO as one of the
constituents.
15. The method defined in claim 14 including maintaining the cell
potential above the decomposition potential for CaO.
16. The method defined in claim 14 including maintaining the cell
potential below the decomposition potential for CaCl.sub.2.
17. The method defined in claim 14 including maintaining the cell
potential below 3.0V.
18. The method defined in claim 14 including maintaining the cell
potential below 2.5V.
19. The method defined in claim 14 including maintaining the cell
potential below 2.0V.
20. The method defined in including maintaining the cell potential
at least 1.5V.
Description
1. FIELD OF THE INVENTION
[0001] The present invention relates to electrolytic reduction of
metal oxides.
2. BACKGROUND OF AND PRIOR ART TO THE INVENTION
[0002] The present invention was made during the course of an
on-going research project on the electrolytic reduction of titania
(TiO.sub.2) carried out by the applicant.
[0003] During the course of the research project the applicant
carried out experimental work on an electrolytic cell that included
a graphite crucible that formed an anode of the cell, a pool of
molten CaCl.sub.2-based electrolyte in the crucible, and a cathode
that included solid titania.
[0004] The CaCl.sub.2-based 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.
[0005] The applicant operated the electrolytic cell at a potential
above the decomposition potential of CaO and below the
decomposition potential of CaCl.sub.2.
[0006] The applicant found that the cell could electrolytically
reduce titania to titanium with very low concentrations of
oxygen.
[0007] The applicant does not have a clear understanding of the
electrolytic cell mechanism at this stage. 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
experimental electrolytic cell above a potential at which the
CaCl.sub.2-based electrolyte partially decomposed had the result of
producing Ca.sup.++ cations that migrated to the vicinity of the
titania in the cathode and provided a driving force that
facilitated extraction of O.sup.-- anions produced by electrolytic
reduction of titania to titanium in the cathode. The applicant also
believes that the O.sup.-- anions, once extracted from the titania,
migrated to the anode and reacted with anode carbon and produced CO
and released electrons that facilitated electrolytic reduction of
titania to titanium in the cathode. In addition, the applicant
believes that carbon in the anode reacted with Ca.sup.++ cations
and produced a complex calcium carbide. The experimental worked
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 titania in the cathode and participated in
chemical reduction of oxides.
[0008] However, notwithstanding that the cell could
electrolytically reduce titania to titanium with very low
concentrations of oxygen, the applicant also found that there were
relatively significant amounts of carbon transferred from the anode
to the electrolyte and to the titanium produced at the cathode
under a wide range of cell operating conditions. Carbon in the
titanium is an undesirable contaminant. In addition, carbon
transfer was responsible for low energy efficiency of the cell.
Both problems are significant barriers to commercialisation of
electrolytic reduction technology.
[0009] The applicant carried out experimental work to identify the
mechanism for carbon transfer and to determine how to minimise
carbon transfer and/or to minimise the adverse effects of carbon
transfer.
3. SUMMARY OF INVENTION
[0010] Broadly, the invention resides in replacing the carbon anode
with a molten metal anode.
[0011] According to the present invention there is provided an
electrolytic cell for electrolytic reduction of a metal oxide in a
solid state, which electrolytic cell includes (a) a molten
electrolyte, (b) a cathode formed at least in part from the metal
oxide in contact with the electrolyte, and (c) a molten metal anode
in contact with the electrolye.
[0012] Preferably the metal of the molten metal anode has a
relatively high saturation level for oxygen at the operating
temperature of the cell.
[0013] Preferably the metal is chosen such that its melting point
is within the operating temperature ranges of the electrolyte.
[0014] Preferably the melting point of the metal of the molten
metal anode is higher than the melting point of the electrolyte and
lower than the vaporisation and/or decomposition temperature of the
electrolyte in order to prevent electrolyte consumption and removal
through vaporisation.
[0015] Preferably the metal of the molten metal anode has a very
low solubility in the molten electrolyte at the cell operating
temperatures, as high solubility is detrimental because the anode
metal will deplete and deposit on the cathode. The latter might not
be a serious problem where there is low solubility and reactability
of the metal with the cathode metal at the operating
temperature.
[0016] Preferably the metal of the molten metal anode is silver or
copper.
[0017] The solubility of oxygen in the Ag--O system at 1000.degree.
C. is around 0.32% by weight. Ag has a melting point of 960.degree.
C., which is about 300 to 100.degree. C. above the melting point of
alkali and alkaline earth halides that provide suitable
electrolytes.
[0018] The solubility of oxygen in the Cu--O system at 1100.degree.
C. is 0.39% by weight. The melting point of copper is 1083.degree.
C., which is well below the boiling points of the above mentioned
electrolytes.
[0019] Preferably the electrolytic cell further includes a means
for removing oxygen that has diffused into the molten metal anode
from the cell.
[0020] Such an "oxygen scavenging pump" means can have a number of
different forms.
[0021] One option includes a duct that communicates with the molten
metal anode and a device to create a partial pressure reduction
between the molten metal anode and a head of molten metal within
the duct.
[0022] An advantage of an "oxygen scavenging pump" means is that
the amount of the molten metal anode required can be minimised,
since the saturation wt % limits of oxygen within the molten anode
metal are no longer the sole determining parameter of oxygen uptake
by the anode.
[0023] For example, in order to reduce log of titania to pure
titanium, 1 kg Ag would be required in the absence of an oxygen
scavenging pump means to remove substantially all of the oxygen
from the molten metal anode. Continuous removal of oxygen from the
molten metal anode facilitated by the means allows the process to
be performed continuously, as compared with batch processing.
[0024] According to the present invention there is also provided a
method of electrolytically reducing a metal oxide in a solid state
in an electrolytic cell, which electrolytic cell includes (a) a
molten electrolyte, (b) a cathode in contact with the electrolyte,
the cathode being formed at least in part from the metal oxide, and
(c) a molten metal anode in contact with the electrolye, which
method includes applying a cell potential across the anode and the
cathode.
[0025] Preferably the method includes maintaining the cell
temperature above the melting points of the electrolyte and the
metal of the metal anode.
[0026] Preferably the method includes maintaining the cell
temperature below the vaporisation and/or decomposition
temperatures of the electrolyte.
[0027] Preferably the method includes applying a cell potential
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.
[0028] Preferably the metal oxide is a titanium oxide.
[0029] It is preferred that the metal oxide be titania.
[0030] In a situation in which the metal oxide is titania it is
preferred that the electrolyte be a CaCl.sub.2-based electrolyte
that includes CaO as one of the constituents.
[0031] In such a situation it is preferred that the method includes
maintaining the cell potential above the decomposition potential
for CaO.
[0032] It is also preferred that the method includes maintaining
the cell potential below the decomposition potential for
CaCl.sub.2.
[0033] It is preferred that the method includes maintaining the
cell potential less than 3.0V.
[0034] It is preferred particularly that the method includes
maintaining the cell potential below 2.5V.
[0035] It is preferred more particularly that the method includes
maintaining the cell potential below 2.0V.
[0036] It is preferred that the method includes maintaining the
cell potential at least 1.5V.
[0037] The following example illustrates an application of the
invention in the process of reducing titania into substantially
pure titanium using an electrolytic cell constructed in accordance
with the present invention and as illustrated schematically in FIG.
1.
4. DESCRIPTION OF EXEMPLARY EMBODIMENT
[0038] FIG. 1 is a schematic illustration of a electrolytic cell
that can be scaled-up in application of the present invention.
[0039] Whilst the example described below relates to the
electrolytic reduction of titania, the basic principle is equally
applicable to other metal oxides, in particular oxides of Si, Ge or
alloys containing these metals.
[0040] With reference to the figure, the electrolytic cell 5
includes a graphite-free crucible 10 made of a suitable refractory
material that is essentially inert as regards reaction with the
electrolyte and electrode materials described below at cell
operating temperatures of between 1000.degree. C. and 1200.degree.
C.
[0041] The electrolytic cell further includes a pool 18 of molten
CaCl.sub.2 electrolyte within the crucible 10.
[0042] The electrolytic cell 5 further includes a pool 14 of molten
silver or copper (within the crucible 10. The molten Ag or Cu forms
the anode 14 of the cell. In view of the different densities, the
molten metal anode 14 is below the molten electrolyte pool 18.
[0043] The electrolytic cell 5 further includes a titania plate 12
positioned within a cage 12b. The cage 12b (and therefore the plate
12) is suspended into the crucible 10 by means of a lead 12a. This
assembly forms the cathode 20 of the cell.
[0044] The electrolytic cell 5 further includes a power source 16
and electrical connections between the power source 16 and the
anode 14 and the cathode 20. The connections include electrical
leads 17 and 12a and a suitable high-temperature resistant plate
member 15, preferably of stainless steel, that provides electric
connection between the interior of crucible 10 (and thus anode 14)
and the lead 17.
[0045] In use, power source 16 provides for constant potential
(voltage) settings thereby allowing the cell 5 to draw the amount
of charge required during the electrolytic refining of the metal
oxide body at a selectable potential.
[0046] The electrolytic cell 5 further includes type B
thermocouples contained in heat-resistant, inert sheaths (not
illustrated) for monitoring temperature in the molten metal anode
14 and the molten electrolyte 18.
[0047] The electrolytic cell 5 further includes a refractory tube
20 that connects the interior of the crucible 10, below the molten
metal anode bath level (a), with a device for imparting a negative
pressure differential between anode bath 14 and the head (b) of
molten Ag suctioned into the tube 20. The pressure differential
need only be slight to provide a driving force for diffusion and
transport of oxygen that is dissolved into the metal anode bath 14
into the tube 20 which is preferably vented to atmosphere.
[0048] In use, the above-described electrolytic cell 5 is
positioned in a suitable furnace to maintain the electrolyte and
the anode metal in their respective molten states. The atmosphere
around the crucible 10 is preferred to be an inert gas, such as
argon, that does not react with the molten electrolyte.
[0049] Once the cell reaches its operating temperature, about 1150
to 1200.degree. C., depending on the anode metal employed, a
constant voltage of around 2.5-3 V is applied between the cell
electrodes 12 and 14, the cell potential being above the
decomposition potential of CaO in the electrolyte but below the
decomposition potential of CaCl.sub.2, whereby reduction of the
titania in the cathode is carried out as described above.
[0050] The oxygen that passes into the electrolyte 18 is
subsequently transported to the metal bath anode 14 where it
dissolves. The dissolved oxygen then diffuses through the molten
anode bath 14 under the pressure differential imparted through duct
20 and is released as O.sub.2 into the surrounding atmosphere.
[0051] As will be noted, this transport mechanism is effective for
as long as oxygen in the molten metal anode is below the saturation
level.
[0052] By way of example, it is noted that other shapes and
configurations of the titania cathode 20 are equally employable,
bearing in mind the need to ensure proper electric contact between
the power source 16 and the titania to be reduced within the
cell.
* * * * *