U.S. patent application number 10/939001 was filed with the patent office on 2005-05-05 for minimising carbon transfer in an electrolytic cell.
This patent application is currently assigned to BHP Billiton Innovation Pty. Ltd.. Invention is credited to Bliznyukov, Sergey Alexander, Osborn, Steve, Ratchev, Ivan, Strezov, Les.
Application Number | 20050092129 10/939001 |
Document ID | / |
Family ID | 3834768 |
Filed Date | 2005-05-05 |
United States Patent
Application |
20050092129 |
Kind Code |
A1 |
Strezov, Les ; et
al. |
May 5, 2005 |
Minimising carbon transfer in an electrolytic cell
Abstract
An electrolytic cell for reducing a metal oxide, such as
titania, in a solid state is disclosed. The electrolytic cell
includes an anode formed from carbon and a cathode formed at least
in part from the metal oxide. The electrolytic cell also includes a
membrane that is permeable to oxygen anions and is impermeable to
carbon in ionic and non-ionic forms positioned between the cathode
and the anode to thereby prevent migration of carbon to the
cathode.
Inventors: |
Strezov, Les; (Adamstown,
AU) ; Ratchev, Ivan; (Georgetown, AU) ;
Osborn, Steve; (Valentine, AU) ; Bliznyukov, Sergey
Alexander; (Jesmond, AU) |
Correspondence
Address: |
BRINKS HOFER GILSON & LIONE
P.O. BOX 10395
CHICAGO
IL
60610
US
|
Assignee: |
BHP Billiton Innovation Pty.
Ltd.
|
Family ID: |
3834768 |
Appl. No.: |
10/939001 |
Filed: |
September 10, 2004 |
Current U.S.
Class: |
75/10.23 ;
204/282 |
Current CPC
Class: |
C22B 34/129 20130101;
C25C 7/005 20130101 |
Class at
Publication: |
075/010.23 ;
204/282 |
International
Class: |
C25C 003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 13, 2002 |
AU |
PS 1170 |
Mar 13, 2003 |
WO |
PCT/AU03/00305 |
Claims
1. An electrolytic cell for reducing a metal oxide in a solid
state, which electrolytic cell includes an anode formed from
carbon, a cathode formed at least in part from the metal oxide, and
a membrane that is permeable to oxygen anions and is impermeable to
carbon in ionic and non-ionic forms positioned between the cathode
and the anode to thereby prevent migration of carbon to the
cathode.
2. The cell defined in claim 1 wherein the anode is formed from
graphite.
3. The cell defined in claim 1 wherein the membrane is formed from
a solid electrolyte.
4. The cell defined in claim 3 wherein the solid electrolyte is
yttria stabilised zirconia.
5. The cell defined in claim 1 wherein the cathode also includes an
electrical conductor.
6. A method of reducing a metal oxide in a solid state using an
electrolytic cell that includes an anode formed from carbon, a
cathode formed at least in part from the metal oxide, and a
membrane that is permeable to oxygen anions and is impermeable to
carbon in ionic and non-ionic forms positioned between the cathode
and the anode to thereby prevent migration of carbon to the
cathode, which method includes operating the cell at a potential
that electrolytically reduces the metal oxide.
7. The method defined in claim 6 includes operating the cell at a
potential that is above a decomposition potential of at least one
of the constituents of the electrolyte so that there are cations of
a metal other than that of the metal oxide in the electrolyte.
8. The method defined in claim 6 wherein the metal oxide is a
titanium oxide, such as titania and the electrolyte is a
CaCl.sub.2-based electrolyte that includes CaO as one of
constituents.
9. The method defined in claim 8 includes operating the cell at a
potential that is above the decomposition potential for CaO.
10. The method defined in claim 8 includes operating the cell at a
potential that is below the decomposition potential for
CaCl.sub.2.
11. The method defined in claim 6 wherein the cell potential is
less than or equal to 3.0 V.
12. The method defined in claim 11 wherein the cell potential is
below 2.5 V.
13. The method defined in claim 12 wherein the cell potential is
below 2.0 V.
14. The method defined in claim 6 wherein the cell potential is
above 1.5 V.
Description
[0001] This application claims priority to PCT Application No.
PCT/AU03/00305 filed on Mar. 13, 2003 and published in English as
PCT WO 03/076692 on Sep. 18, 2003 and to Australian Application PS
1170 filed Mar. 13, 2002, the entire contents of both are
incorporated herein by reference.
[0002] The present invention relates to to reduction of metal
oxides in a solid state in an electrolytic cell.
[0003] The present invention was made during the course of an
on-going research project on solid state reduction of titania
(TiO2) carried out by the applicant.
[0004] During the course of the research project the applicant
carried out experimental work on the reduction of titania using 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 range of cathodes that included solid
titania.
[0005] The CaCl.sub.2-based electrolyte was a commercially
available source of CaCl.sub.2, namely calcium chloride dihydrate,
that partially decomposed on heating and produced CaO.
[0006] The applicant operated the electrolytic cell at a potential
above the decomposition potential of CaO and below the
decomposition potential of CaCl.sub.2.
[0007] The applicant found that the cell could electrochemically
reduce titania to titanium with very low concentrations of
oxygen.
[0008] 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 and the following
paragraphs, the applicant offers the following comments by way of
an outline of a possible cell mechanism.
[0009] The experimental work carried out by the applicant produced
evidence of Ca metal dissolved in the electrolyte. The applicant
believes that, at least during the early stages of operation of the
cell, the Ca metal was the result of electrodeposition of Ca.sup.++
cations as Ca metal on electrically conductive sections of the
cathode.
[0010] The experimental work was carried out using a
CaCl.sub.2-based electrolyte at a cell potential below the
decomposition potential of CaCl.sub.2. The applicant believes that
the initial deposition of Ca metal on the cathode was due to the
presence of Ca.sup.++ cations and O.sup.-- anions derived from CaO
in the electrolyte. The decomposition potential of CaO is less than
the decomposition potential of CaCl.sub.2. In this cell mechanism
the cell operation is dependent, at least during the early stages
of cell operation, on decomposition of CaO, with Ca.sup.++ cations
migrating to the cathode and depositing as Ca metal and O.sup.--
anions migrating to the anode and forming CO and/or CO.sub.2 (in a
situation in which the anode is a graphite anode).
[0011] The applicant believes that the Ca metal that deposited on
electrically conductive sections of the cathode was deposited
predominantly as a separate phase in the early stages of cell
operation and thereafter dissolved in the electrolyte and migrated
to the vicinity of the titania in the cathode and participated in
chemical reduction of titania.
[0012] The applicant also believes that at later stages of the cell
operation part of the Ca metal that deposited on the cathode was
deposited directly on partially deoxidised titanium and thereafter
participated in chemical reduction of titanium.
[0013] 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/or CO.sub.2 (and in some instances
CaO) and released electrons that facilitated electrolytic
deposition of Ca metal on the cathode.
[0014] However, notwithstanding that the cell could
electrochemically 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.
[0015] Carbon in the titanium is an undesirable contaminant. In
addition, carbon transfer was partially responsible for low energy
efficiency of the cell. Both problems are significant barriers to
commercialisation of electrolytic reduction technology.
[0016] 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.
[0017] The experimental work indicated that the mechanism of carbon
transfer is electrochemical rather than erosion and that one way of
minimising carbon transfer and therefore contamination of titanium
produced at the cathode by electrochemical reduction of titania at
the cathode is to position a membrane that is permeable to oxygen
anions and is impermeable to carbon in ionic and non-ionic forms
between the cathode and the anode and thereby prevent migration of
carbon to the cathode.
[0018] Accordingly, the present invention provides an electrolytic
cell for reducing a metal oxide in a solid state, which
electrolytic cell includes an anode formed from carbon, a cathode
formed at least in part from the metal oxide, and a membrane that
is permeable to oxygen anions and is impermeable to carbon in ionic
and non-ionic forms positioned between the cathode and the anode to
thereby prevent migration of carbon to the cathode.
[0019] Preferably, the anode is formed from graphite.
[0020] The membrane may be formed from any suitable material.
[0021] Preferably, the membrane is formed from a solid
electrolyte.
[0022] One suitable solid electrolyte tested by the applicant is
yttria stabilised zirconia.
[0023] Preferably, the cathode also includes an electrical
conductor.
[0024] The present invention also provides a method of reducing a
metal oxide in a solid state using the above-described electrolytic
cell.
[0025] Preferably, the method includes a step of operating the cell
at a potential that is above a decomposition potential of at least
one of the constituents of the electrolyte so that there are
cations of a metal other than that of the metal oxide in the
electrolyte.
[0026] In a situation in which the metal oxide is a titanium oxide,
such as titania, it is preferred that the electrolyte be a
CaCl.sub.2-based electrolyte that includes CaO as one of
constituents.
[0027] In such a situation it is preferred that the cell potential
be above the decomposition potential for CaO.
[0028] It is also preferred that the cell potential be below the
decomposition potential for CaCl.sub.2.
[0029] It is preferred that the cell potential be less than or
equal to 3.0 V.
[0030] It is preferred particularly that the cell potential be
below 2.5 V.
[0031] It is preferred more particularly that the cell potential be
below 2.0 V.
[0032] It is preferred that the cell potential be above 1.5 V.
[0033] 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.
[0034] 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.
[0035] The present invention is described further with reference to
the following Example that relates to experimental work on the
above-described electrolytic cell.
[0036] As indicated above, the cell included a high density
graphite crucible that formed the anode of the cell, a pool of
molten CaCl.sub.2 electrolyte in the crucible, and a cathode that
included solid titania. In the initial experimental set-up the
solid titania was in the form of titania pellets connected to a
lower end of a Kanthal or stainless steel electrically conductive
wire.
[0037] As indicated above, experimental work on the cell identified
carbon transfer as a significant issue in terms of contamination of
cathode titanium and causing low energy efficiency of the cell. In
addition, as indicated above, the experimental work established
that carbon transfer was caused by an electrochemical reaction at
the anode.
[0038] Thereafter the applicant carried out experimental work to
investigate whether it was possible to prevent migration of carbon
from the anode to the cathode.
[0039] One experiment investigated the impact of a solid ionic
barrier on carbon migration.
[0040] The ionic barrier was in the form of a yttria stabilised
zirconia membrane positioned between the anode and the cathode,
thereby dividing the cell into an outer anode chamber and an inner
cathode chamber.
[0041] FIG. 1 is a schematic of the cell set-up for the experiment.
With reference to the Figure, the cell included a graphite crucible
3 that formed the anode, a pool 19 of molten CaCl.sub.2 electrolyte
in the crucible, titania pellets 5 and an electrically conductive
wire 7 that formed the cathode immersed in the electrolyte, and a
yttria stabilised zirconia membrane 9 immersed in the electrolyte
between the anode and the cathode. The cell was located in a
resistance furnace 11 heated to a temperature to maintain the
electrolyte in a molten state. The experimental set-up also
included gas monitoring, cleaning, and analysis equipment. The cell
was operated at an applied potential of 3V for a period of 35
hours, during which time there was continuous monitoring of the
off-gas from the furnace. At the conclusion of the experiment, the
cell was cooled and the solidified electrolyte, the membrane, the
anode and the cathode were analysed.
[0042] FIG. 2 is a summary of the results of the experiment.
[0043] FIG. 2 shows measured voltage, current, CO and CO.sub.2
composition of the off-gas for the experiment.
[0044] Visual and analytical examination of the cathode and the
cathode chamber indicated that there was no carbon on the cathode
and in the cathode chamber.
[0045] In addition, the visual and analytical examination of the
cathode indicated that titania was reduced to titanium. It follows
from this finding that the yttria stabilised zirconia membrane did
not restrict migration of O.sup.-- anions from the cathode to the
anode.
[0046] Many modifications may be made to the present invention as
described above without departing from the spirit and scope of the
invention.
[0047] By way of example, whilst the above description of the
invention focuses on reduction of titania, the invention is not so
limited and extends to electrolytic reduction of other titanium
oxides and to oxides of other metals and alloys.
[0048] Examples of other potentially important meals are aluminium,
silicon, germanium, hafnium, magnesium, and molybdenum.
[0049] Furthermore, whilst the above description focuses on
CaCl.sub.2-based electrolyte, the invention is not so limited and
extends to any other suitable electrolytes.
[0050] Generally, suitable electrolytes will be salts and oxides
that are soluble in salts. One example of a potentially suitable
electrolyte is BaCl.sub.2.
* * * * *