U.S. patent application number 12/961068 was filed with the patent office on 2011-05-26 for reduction of metal oxides in an electrolytic cell.
This patent application is currently assigned to Metalysis Limited. Invention is credited to Steve Osborn, Ivan Ratchev, LES STREZOV.
Application Number | 20110120881 12/961068 |
Document ID | / |
Family ID | 3829995 |
Filed Date | 2011-05-26 |
United States Patent
Application |
20110120881 |
Kind Code |
A1 |
STREZOV; LES ; et
al. |
May 26, 2011 |
REDUCTION OF METAL OXIDES IN AN ELECTROLYTIC CELL
Abstract
A method of reducing a titanium oxide in a solid state in an
electrolytic cell which includes an anode, a cathode formed at
least in part from the titanium oxide, and a molten electrolyte
which includes cations of a metal that is capable of chemically
reducing the cathode titanium oxide, which method includes
operating the cell at a potential that is above a potential at
which cations of the metal that is capable of chemically reducing
the cathode titanium oxide deposit as the metal on the cathode,
whereby the metal chemically reduces the cathode titanium oxide,
and which method is characterised by refreshing the electrolyte
and/or changing the cell potential in later stages of the operation
of the cell as required having regard to the reactions occurring in
the cell and the concentration of oxygen in the titanium oxide in
the cell in order to produce high purity titanium.
Inventors: |
STREZOV; LES; (Adamstown,
AU) ; Ratchev; Ivan; (Georgetown, AU) ;
Osborn; Steve; (Valentine, AU) |
Assignee: |
Metalysis Limited
|
Family ID: |
3829995 |
Appl. No.: |
12/961068 |
Filed: |
December 6, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10482055 |
May 4, 2004 |
7918985 |
|
|
PCT/AU02/00843 |
Jun 28, 2002 |
|
|
|
12961068 |
|
|
|
|
Current U.S.
Class: |
205/560 |
Current CPC
Class: |
C22B 34/129 20130101;
C22B 5/00 20130101 |
Class at
Publication: |
205/560 |
International
Class: |
C25C 1/00 20060101
C25C001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 2001 |
AU |
PR 6029 |
Claims
1-9. (canceled)
10. A method of reducing a titanium oxide in a solid state in an
electrolytic cell which includes an anode, a cathode formed at
least in-part from the titanium oxide, and a molten electrolyte
which includes cations of a metal that is capable of chemically
reducing the cathode titanium oxide, said method includes:
operating the cell at a potential that is above a potential at
which cations of the metal that is capable of chemically reducing
the cathode titanium oxide deposit as the metal on the cathode,
whereby the metal chemically reduces the cathode titanium oxide;
and operating the cell at constant current and refreshing the
electrolyte in later stages of the operation of the cell as
required having regard to reactions occurring in the cell and a
concentration of oxygen in the titanium oxide in the cell in order
to produce high purity titanium (.alpha.Ti).
11. The method defined in claim 10, wherein the metal deposited on
the cathode is soluble in the electrolyte and can dissolve in the
electrolyte and thereby migrate to the vicinity of the cathode
titanium oxide.
12. The method defined in claim 10, wherein the electrolyte is a
CaCl.sub.2-based electrolyte that includes CaO as one of a
plurality of constituents of the electrolyte.
13. The method defined in claim 12, wherein the cell potential is
above a decomposition potential of CaO, at which potential Ca metal
can deposit on the cathode.
14. The method defined in claim 12, wherein the cell potential is
below the decomposition potential of CaCl.sub.2.
15. The method defined in claim 12, wherein a graphite anode is
employed, and said cell is operated at a temperature in the range
of 600-1100.degree. C., with the cell potential being between 1.3
and 3.5V.
16. The method defined in claim 12, wherein the CaCl.sub.2-based
electrolyte is a commercially available source of CaCl.sub.2 that
partially decomposes on heating and produces CaO or otherwise
includes CaO.
17. The method defined in claim 12, wherein the CaCl.sub.2-based
electrolyte includes CaCl.sub.2 and CaO that are added separately
or pre-mixed to form the electrolyte.
18. The method defined in claim 10, wherein the anode is graphite
or an inert anode.
19. A method of reducing titanium oxide in a solid state in an
electrolytic cell in order to produce high purity titanium
(.alpha.Ti), the electrolytic cell including an anode, a cathode
formed at least in-part from the titanium oxide, and a molten
electrolyte which includes cations of a metal that is capable of
chemically reducing the cathode titanium oxide, the method
comprising: operating the cell at a potential that is above a
potential at which cations of the metal that is capable of
chemically reducing the cathode titanium oxide deposit as the metal
on the cathode, whereby the metal chemically reduces the cathode
titanium oxide; operating the cell at a constant current; and
refreshing the electrolyte in the cell based on a concentration of
oxygen present in the titanium oxide at different stages of the
titanium oxide reduction.
20. The method defined in claim 19, wherein the metal deposited on
the cathode is soluble in the electrolyte and can dissolve in the
electrolyte and thereby migrate to the vicinity of the cathode
titanium oxide.
21. The method defined in claim 19, wherein the electrolyte is a
CaCl.sub.2-based electrolyte that includes CaO as one of a
plurality of constituents of the electrolyte.
22. The method defined in claim 21, wherein the cell potential is
above a decomposition potential of CaO, at which potential Ca metal
can deposit on the cathode.
23. The method defined in claim 21, wherein the cell potential is
below the decomposition potential of CaCl.sub.2.
24. The method defined in claim 21, wherein a graphite anode is
employed, and said cell is operated at a temperature in the range
of 600-1100.degree. C., with the cell potential being between 1.3
and 3.5V.
25. The method defined in claim 21, wherein the CaCl.sub.2-based
electrolyte is a commercially available source of CaCl.sub.2 that
partially decomposes on heating and produces CaO or otherwise
includes CaO.
26. The method defined in claim 21, wherein the CaCl.sub.2-based
electrolyte includes CaCl.sub.2 and CaO that are added separately
or pre-mixed to form the electrolyte.
27. The method defined in claim 19, wherein the anode is graphite
or an inert anode.
28. A method of operating an electrolytic cell in order to produce
high purity titanium (.alpha.Ti) by reducing titanium oxide in a
solid state in the electrolytic cell, the electrolytic cell
including an anode, a cathode formed at least in-part from the
titanium oxide, and a molten electrolyte which includes cations of
a metal that is capable of chemically reducing the cathode titanium
oxide, the method comprising: operating the operating the cell at a
potential that is above a potential at which cations of the metal
that is capable of chemically reducing the cathode titanium oxide
deposit as the metal on the cathode, whereby the metal chemically
reduces the cathode titanium oxide; operating the cell at a
constant current; and refreshing the electrolyte and/or changing
the electrolyte composition in the electrolytic cell in order to
remove oxygen from partially reduced titanium oxide at later stages
of the reduction process.
29. The method defined in claim 28, wherein the high purity
titanium is obtained without processing the titanium oxide outside
of the electrolytic cell.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 10/482,055, filed May 4, 2004, currently pending, which is a
U.S. National Stage Application of International Application No.
PCT/AU02/00843, filed Jun. 28, 2002, which claims priority to
Australian Application No. PR 6029, filed Jun. 26, 2001, all of
which are hereby incorporated by reference herein in their
entireties.
FIELD OF THE INVENTION
[0002] The present invention relates to reduction of metal oxides
in an electrolytic cell.
[0003] 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.
[0004] 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.
BACKGROUND OF THE INVENTION
[0005] One objective of the experimental work was to reproduce the
results reported in International application PCT/GB99/01781
(Publication no. WO99/64638) in the name of Cambridge University
Technical Services Limited and in technical papers published by the
inventors.
[0006] The Cambridge International application discloses two
potential applications of a "discovery" in the field of
metallurgical electrochemistry.
[0007] One application is the direct production of a metal from a
metal oxide.
[0008] In the context of this application, the "discovery" is the
realisation that an electrolytic cell can be used to ionise oxygen
contained in a metal oxide so that the oxygen dissolves in an
electrolyte. The Cambridge International application discloses that
when a suitable potential is applied to an electrolytic cell with a
metal oxide as a cathode, a reaction occurs whereby oxygen is
ionised and is subsequently able to dissolve in the electrolyte of
the cell.
[0009] European patent application 9995507.1 derived from the
Cambridge International application has been allowed by the
European Patent Office.
[0010] The allowed claims of the European patent application inter
alia define a method of electrolytically reducing a metal oxide
(such as titania) that includes operating an electrolytic cell at a
potential that is lower than the deposition potential of cations in
the electrolyte.
[0011] The Cambridge European patent application does not define
what is meant by deposition potential and does not include any
specific examples that provide values of the deposition potential
for particular cations.
[0012] However, submissions dated 2 Oct. 2001 to the European
Patent Office by the Cambridge patent attorneys, which pre-dated
the lodgement of the claims that were ultimately allowed, indicate
that they believe that the decomposition potential of an
electrolyte is the deposition potential of a cation in the
electrolyte.
[0013] Specifically, page 5 of the submissions state that:
[0014] "The second advantage described above is achieved in part
through carrying out the claimed invention below the decomposition
potential of the electrolyte. If higher potentials are used then,
as noted in Dl and D2, the cation in the electrolyte deposits on
the metal or semi-metal compound. In the example of Dl, this leads
to calcium deposition and therefore consumption of this reactive
metal . . . . During operation of the method, the electrolytic
cation is not deposited on the cathode"
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a graph illustrating the variation of potential
with the concentration of oxygen in titanium.
BRIEF DESCRIPTION OF THE INVENTION
[0016] Contrary to the findings of Cambridge, the experimental work
carried out by the applicant has established that it is essential
that the electrolytic cell be operated at a potential that is above
the potential at which Ca.sup.++ cations in the electrolyte can
deposit as Ca metal on the cathode.
[0017] Specifically, as a consequence of the experimental work, the
applicant has invented a method of reducing a metal oxide such as
titanium oxides in a solid state in an electrolytic cell which
includes an anode, a cathode formed at least in part from the metal
oxide, and a molten electrolyte which includes cations of a metal
that is capable of chemically reducing the cathode metal oxide,
which method includes a step of operating the cell at a potential
that is above a potential at which cations of the metal that is
capable of chemically reducing the cathode metal oxide deposit as
the metal on the cathode, whereby the metal chemically reduces the
cathode metal oxide.
[0018] The above method is described in Australian provisional
application PS3049 in the name of the applicant lodged on 20 Jun.
2002, and the disclosure in the patent specification lodged with
the application is incorporated herein by cross-reference.
[0019] In addition to the above, the experimental work (and
associated theoretical analysis work) carried out by the applicant
has determined a number of important factors that play a role in
the actual reduction process.
DETAILED DESCRIPTION OF THE INVENTION
[0020] The relevant experimental data indicates that (i) Cl.sub.2
gas is removed at the anode of the electrolytic cell at potentials
well below the theoretical decomposition potential of the
electrolyte CaCl.sub.2, (ii) Ca.sub.XTi.sub.YO.sub.Z, is present at
the cathode during some stages of the electrolysis, and (iii) CaO
is formed in the molten electrolyte bath.
[0021] In view of the above, the applicant has concluded that a
number of steps are involved in the method of reducing titanium
oxides and that some of these steps are represented by reactions
(1) to (8) mentioned below. Reactions (1) to (8) relate to
reduction of titanium oxides using an electrolytic cell with
CaCl.sub.2 (containing O anions) as the electrolyte and a graphite
anode, with their standard potentials at 950.degree. C.
CaCl.sub.2+3TiO.sub.2=CaTiO.sub.3+Cl.sub.2(g)+Ti.sub.2O.sub.3
(1)
E.degree..sub.950C=-1.45 V
CaCl.sub.2+2TiO.sub.2=CaTiO.sub.3+Cl.sub.2(g)+TiO (2)
E.degree..sub.950C=-1.63 V
CaCl.sub.2+0.5TiO.sub.2=CaO+Cl.sub.2(g)+0.5Ti (3)
E.degree..sub.950C=-2.4 V
CaTiO.sub.3+C=CaO+TiO+CO(g) (4)
E.degree..sub.950C=-0.86 V
CaTiO.sub.3+2C=CaO+Ti+2CO(g) (5)
E.degree..sub.950C=-0.96 V
Ti.sub.2O.sub.3+C=2TiO+CO(g) (6)
E.degree..sub.950C=-0.58 V
TiO+C=Ti+CO(g) (7)
E.degree..sub.950C=-1.07 V
[O]Ti+C=CO(gas) (8)
[0022] Reactions (1) to (8) are not an exhaustive list, of the
possible reaction and other reactions can take place. Specifically,
the applicant suspects that other reactions, involving titanium
suboxides, represented by the formula Ti.sub.nO.sub.2n-1, and
calcium titanates, represented by the formula CaTi.sub.nO.sub.3n+1,
can take place.
[0023] The potential of reaction (8) in particular varies with the
concentration of oxygen in titanium. The graph of FIG. 1
illustrates the variation of potential with concentration of oxygen
in titanium in a cell operating at 950.degree. C. The graph was
prepared by the applicant using published data.
[0024] It is clear from the graph that reaction (8) requires higher
potentials at lower concentrations of oxygen and thus there is
increased resistance to oxygen removal as the oxygen concentration
decreases.
[0025] The solubility of different titanium oxides in CaCl.sub.2 is
not taken into consideration in the calculation of the potentials
for reactions (1) to (8). The significance of this is that some of
reactions (1) to (8) may take place at potentials that are higher
or lower than the potentials stated above at the stated temperature
of 950.degree. C.
[0026] For example, reduced activity of TiO will reduce the value
of the potentials of reactions (2), (4) and (6) (i.e. make the
potentials more positive) and at the same time will increase the
potential of reaction (7) (i.e. make it more negative).
[0027] In view of the above, the applicant has realised that it is
likely to be extremely difficult to reduce titanium oxide in an
electrolytic cell to titanium (.alpha.Ti) of high purity, i.e. low
concentration of oxygen (no more than 100 ppm oxygen) in a single
stage operation.
[0028] Specifically, the applicant has realised that it is
necessary to refresh the electrolyte and/or to change cell
potential in a later stage or in later stages of the operation of
the electrolytic cell in order to reduce titanium oxide in an
electrolytic cell to .alpha. titanium of high purity, ie low
concentration of oxygen.
[0029] According to the present invention there is provided a
method of reducing a titanium oxide in a solid state in an
electrolytic cell which includes an anode, a cathode formed at
least in part from the titanium oxide, and a molten electrolyte
which includes cations of a metal that is capable of chemically
reducing the cathode titanium oxide, which method includes
operating the cell at a potential that is above a potential at
which cations of the metal that is capable of chemically reducing
the cathode titanium oxide deposit as the metal on the cathode,
whereby the metal chemically reduces the cathode titanium oxide,
and which method is characterised by refreshing the electrolyte
and/or changing the cell potential in later stages of the operation
of the cell as required having regard to the reactions occurring in
the cell and the concentration of oxygen in the titanium oxides in
the cell in order to produce high purity titanium (.alpha.Ti).
[0030] The term "high purity" is understood to mean that the
concentration of oxygen is no more than 100 ppm in the
titanium.
[0031] In effect, the present invention is concerned with selecting
the operating conditions of the cell, including cell potential
and/or electrolyte composition, during various stages of the
operation in the cell having regard to the reactions that take
place in the cell. The applicant envisages at this stage that
commercial operations will be at constant currant and that it may
not be possible to achieve voltages required to remove oxygen to
very low levels because of composition changes in the electrolyte.
In these circumstances, refreshing and or changing the electrolyte
composition is important in order to produce a high purity .alpha.
titanium.
[0032] The above-described method makes it possible to produce
titanium of high purity with respect to oxygen in an electrolytic
cell and without refining or otherwise processing the titanium
outside the electrolytic cell.
[0033] The method may include refreshing the electrolyte by adding
new electrolyte to the existing electrolyte or otherwise adjusting
the composition of the electrolyte.
[0034] In addition, the method may include carrying out the method
in a series of electrolytic cell and successively transferring the
partially reduced titanium oxide to each of the cells in the
series.
[0035] The composition of the electrolyte in each cell may be
selected having regard to the reactions occurring in the cell and
the concentration of oxygen in the titanium oxide in the cell.
[0036] The cell potential may be changed at different stages in the
method on a continuous or a step-change basis.
[0037] Preferably the metal deposited on the cathode is soluble in
the electrolyte and can dissolve in the electrolyte and thereby
migrate to the vicinity of the cathode titanium oxide.
[0038] It is preferred that the electrolyte be a CaCl.sub.2-based
electrolyte that includes CaO as one of the constituents of the
electrolyte.
[0039] In such a situation it is preferred that the cell potential
be above the potential at which Ca metal can deposit on the
cathode, i.e. the decomposition potential of CaO.
[0040] The decomposition potential of CaO can vary over a
considerable range depending on factors such as the composition of
the anode, the electrolyte temperature and electrolyte
composition.
[0041] In a cell containing CaO saturated CaCl.sub.2 at 1373K
(1100.degree. C.) and a graphite anode this would require a minimum
cell potential of 1.34V.
[0042] It is also preferred that the cell potential be below the
decomposition potential of CaCl.sub.2.
[0043] In a cell containing CaO saturated CaCl.sub.2 at 1373K
(1100.degree. C.) and a graphite anode this would require that the
cell potential be less than 3.5V.
[0044] The decomposition potential of CaCl.sub.2 can vary over a
considerable range depending on factors such as the composition of
the anode, the electrolyte temperature and electrolyte
composition.
[0045] For example, a salt containing 80% CaCl.sub.2 and 20% KCl at
a temperature of 900K (657.degree. C.), decomposes to Ca (metal)
and Cl.sub.2 (gas) above 3.4V and a salt containing 100% CaCl.sub.2
at 1373K (1100.degree. C.) decomposes at 3.0V.
[0046] In general terms, in a cell containing CaO--CaCl.sub.2 salt
(not saturated) at a temperature in the range of 600-1100.degree.
C. and a graphite anode it is preferred that the cell potential be
between 1.3 and 3.5V.
[0047] 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.
[0048] 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.
[0049] It is preferred that the anode be graphite or an inert
anode.
[0050] The cell may be of the type disclosed in the drawings of the
patent specification lodged with Australian provisional application
PS3049.
* * * * *