U.S. patent application number 11/662426 was filed with the patent office on 2008-08-14 for electro-deoxidation method, apparatus and product.
This patent application is currently assigned to BRITISH TITANIUM PLC.. Invention is credited to Greg Doughty, Derek John Fray, Carsten Schwandt.
Application Number | 20080190777 11/662426 |
Document ID | / |
Family ID | 36046771 |
Filed Date | 2008-08-14 |
United States Patent
Application |
20080190777 |
Kind Code |
A1 |
Fray; Derek John ; et
al. |
August 14, 2008 |
Electro-Deoxidation Method, Apparatus and Product
Abstract
The subject invention concerns an electro-decomposition process
wherein a cathode comprising a metal compound is contacted with a
fused salt electrolyte in an electrochemical cell. The metal
compound is a compound between a metal and another substance, and a
voltage is applied between the cathode and an anode such that the
substance is removed from the metal compound. In the improved
method, the applied voltage increases with time, either
continuously or stepwise, up to a predetermined maximum voltage. In
addition, or in the alternative, the fused salt composition is
selected so as to maintain a predetermined concentration of the
substance in the fused salt during electro-decomposition.
Inventors: |
Fray; Derek John;
(Cambridge, GB) ; Schwandt; Carsten; (Cambridge,
GB) ; Doughty; Greg; (Cambridgeshire, GB) |
Correspondence
Address: |
SALIWANCHIK LLOYD & SALIWANCHIK;A PROFESSIONAL ASSOCIATION
PO BOX 142950
GAINESVILLE
FL
32614-2950
US
|
Assignee: |
BRITISH TITANIUM PLC.
London
GB
CAMBRIDGE ENTERPRISE LIMITED
Cambridge
GB
|
Family ID: |
36046771 |
Appl. No.: |
11/662426 |
Filed: |
September 9, 2005 |
PCT Filed: |
September 9, 2005 |
PCT NO: |
PCT/GB2005/003497 |
371 Date: |
February 29, 2008 |
Current U.S.
Class: |
205/401 ;
204/243.1; 205/367 |
Current CPC
Class: |
C25C 5/04 20130101; C22B
34/129 20130101; C25C 3/28 20130101; C22B 34/1263 20130101; C25C
3/00 20130101 |
Class at
Publication: |
205/401 ;
205/367; 204/243.1 |
International
Class: |
C25C 3/28 20060101
C25C003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 9, 2004 |
GB |
0419991.5 |
Oct 8, 2004 |
GB |
0422416.8 |
Claims
1. A method for removing a substance from a solid compound
comprising the substance and a metal, comprising the steps of: a)
contacting a cathode comprising the solid compound with a
fused-salt electrolyte; b) contacting an anode with the
electrolyte; and c) applying a voltage between the cathode and the
anode which increases with time.
2. The method according to claim 1, in which the voltage increases
substantially continuously with time.
3. The method according to claim 1, in which the voltage increases
stepwise with time.
4. The method according to claim 1, in which the electrolyte
comprises a cation and the voltage is applied such that the
potential at the cathode increases to a potential which is less
than a potential for continuous deposition of the cation from the
electrolyte at the cathode.
5. The method according to claim 1, in which the voltage is applied
such that the effective potential between the cathode and the anode
increases to a potential which is less than a potential sufficient
to cause continuous decomposition of the electrolyte.
6. The method according to claim 1, in which the voltage is applied
such that the potential at the cathode increases from a potential
which is insufficient to cause removal of the substance from the
compound.
7. The method according to claim 1, in which the voltage is applied
such that the potential at the cathode increases from a potential
of approximately 0 V.
8. The method according to claim 1, in which, after the increasing
voltage has been applied, a substantially constant voltage is
applied between the cathode and the anode.
9. The method according to claim 8, in which the electrolyte
comprises a cation and the substantially-constant voltage is such
that the potential at the cathode is less than a potential for
continuous deposition of the cation from the electrolyte at the
cathode.
10. The method according to claim 8, in which the
substantially-constant voltage is insufficient to cause continuous
decomposition of the electrolyte.
11. The method according to claim 1, in which, after the increasing
voltage has been applied, a voltage is applied between the cathode
and the anode such that the potential at the cathode is
substantially constant.
12. The method according to claim 11, in which the electrolyte
comprises a cation and the substantially-constant potential is less
than a potential for continuous deposition of the cation from the
electrolyte at the cathode.
13. The method according to claim 11, in which the
substantially-constant potential is insufficient to cause
continuous decomposition of the electrolyte.
14. The method according to claim 1, in which the potential at the
cathode is monitored using a reference electrode.
15. The method according to claim 1, in which the reference
electrode comprises a carbon electrode.
16. The method according to claim 1, in which the fused-salt
electrolyte comprises a second salt in solution in a first salt,
the first and second salts containing different anions, and in
which the increasing voltage increases at a rate sufficiently slow
to favour discharge of anions from the second salt rather than
anions from the first salt at the anode.
17. The method according to claim 16, in which the substance
comprises oxygen, the solid compound comprises an oxide and the
electrolyte comprises calcium chloride containing calcium oxide in
solution therein, and in which the increasing voltage increases at
a rate sufficiently slow to favour discharge of oxygen anions
rather than chlorine anions at the anode.
18. The method according claim 1, in which the increasing voltage
increases at a rate sufficiently slow to suppress precipitation of
the substance removed from the solid metal compound, or of a
material comprising the substance removed from the solid metal
compound, in the region of the cathode.
19. The method according to claim 18, in which the substance
comprises oxygen, the solid compound comprises an oxide and the
electrolyte comprises calcium chloride containing calcium oxide in
solution therein, and in which the increasing voltage increases at
a rate sufficiently slow to suppress precipitation of calcium oxide
at the cathode.
20. The method according to claim 1, in which removal of the
substance from the solid compound to produce the metal involves the
formation of an intermediate compound comprising the substance and
the metal, and in which the increasing voltage increases at a rate
sufficiently slow to favour formation of the intermediate compound
before formation of the metal.
21. The method according to claim 1, in which the voltage increases
at a rate of less than 0.2 mV/s, and preferably at a rate of less
than 0.1 mV/s.
22. The method according to claim 1, in which: the fused-salt
electrolyte comprises a second salt in solution in a first salt,
the second salt but not the first salt comprising an anion which is
the same as the substance; and the concentration of the second salt
in solution in the first salt is greater than zero, and preferably
greater than 1 mol %, during the application of the voltage.
23. The method according to claim 22, in which the concentration of
the second salt in solution in the first salt during the
application of the voltage is sufficient to enable transport of the
substance through the electrolyte from the cathode to the
anode.
24. The method according to claim 22, in which the removal of the
substance from the solid compound to produce the metal involves the
formation of an intermediate compound comprising a cation and/or an
anion of the second salt, and the fused-salt electrolyte contains a
sufficient quantity of the second salt both to enable formation of
the intermediate compound and to retain the concentration of the
second salt in solution in the first salt greater than zero, and
preferably greater than 1 mol %, during the application of the
voltage.
25. The method according to claim 24, in which the fused-salt
electrolyte contains a sufficient quantity of the first salt to
retain the sufficient quantity of the second salt in solution.
26. The method according to claim 22, in which the substance
comprises oxygen, the compound comprises an oxide, and the anion in
the second salt comprises oxygen.
27. The method according to claim 26, in which the first salt is
calcium chloride and the second salt is calcium oxide.
28. The method according to claim 26, in which the compound
comprises a titanium oxide.
29. The method according to claim 1, in which the electrolyte
comprises calcium oxide; and, the voltage applied between the
cathode and the anode is such that the potential at the cathode is
insufficient to cause continuous decomposition of the calcium
oxide.
30. The method according to claim 29, in which the potential at the
cathode, as measured with respect to a carbon reference electrode,
is less than about (1541-116 log C) mV, where C is the normalised
concentration of calcium oxide in the fused-salt electrolyte.
31. The method according to claim 29, in which the potential at the
cathode, as measured with respect to a carbon reference electrode,
is less than 1.7 V, and preferably less than 1.6 V.
32. The method according to claim 29, in which the metal compound
comprises a titanium oxide, the anode is a carbon anode, and the
voltage applied between the cathode and the anode is such that the
potential at the cathode, as measured with respect to a carbon
reference electrode, is more than 1.1 V, and preferably more than
1.3 V.
33. The method according to claim 1, in which the solid metal
compound comprises a titanium oxide, and the voltage applied
between the cathode and the anode is such that the potential at the
cathode is more than 1.1 V, and preferably more than 1.3 V.
34. A method for removing a substance from a solid compound
comprising the substance and a metal, comprising the steps of: a)
contacting a cathode comprising the solid compound with a
fused-salt electrolyte comprising a second salt in solution in a
first salt, the second salt but not the first salt comprising an
anion which is the same as the substance; b) contacting an anode
with the electrolyte; and c) applying a voltage between the cathode
and the anode such that the potential at the cathode is less than a
potential for deposition of cations from the electrolyte; the
concentration of the second salt in solution in the first salt
being greater than zero, and preferably greater than 1 mol %,
during the application of the voltage.
35. The method according to claim 34, in which the concentration of
the second salt in solution in the first salt during the
application of the voltage is sufficient to enable transport of the
substance through the electrolyte from the cathode to the
anode.
36. The method according to claim 34, in which the removal of the
substance from the solid compound to produce the metal involves the
formation of an intermediate compound comprising a cation and/or an
anion of the second salt, and the fused-salt electrolyte contains a
sufficient quantity of the second salt both to enable formation of
the intermediate compound and to retain the concentration of the
second salt in solution in the first salt greater than zero, and
preferably greater than 1 mol %, during the application of the
voltage.
37. The method according to claim 36, in which the fused-salt
electrolyte comprises a sufficient quantity of the first salt to
retain the sufficient quantity of the second salt in solution.
38. The method according to claim 34, in which the substance
comprises oxygen, the compound comprises an oxide, and the second
salt comprises an oxide.
39. The method according to claim 38, in which the first salt is
calcium chloride and the second salt is calcium oxide.
40. The method according to claim 38, in which the compound
comprises a titanium oxide.
41. A method for removing a substance from a solid metal compound
comprising the substance and a metal, comprising the steps of: a)
contacting a cathode comprising the solid metal compound with a
fused-salt electrolyte, the electrolyte comprising a corresponding
salt which contains the substance as an anion; b) contacting an
anode with the electrolyte; and c) applying a voltage between the
cathode and the anode which is insufficient to cause continuous
decomposition of the corresponding salt.
42. The method according to claim 41, in which the solid metal
compound comprises a metal oxide and the corresponding salt
comprises an oxide.
43. The method according to claim 42, in which the corresponding
salt is calcium oxide, the anode is a carbon anode and the
potential at the cathode, as measured with respect to a carbon
reference electrode, is less than about (1541-116 log C) mV, where
C is the normalised concentration of calcium oxide in the
fused-salt electrolyte.
44. The method according to claim 42, in which the corresponding
salt is calcium oxide, the anode is a carbon anode and the
potential at the cathode, as measured with respect to a carbon
reference electrode, is less than 1.7 V, and preferably less than
1.6 V.
45. The method according to claim 41, in which the metal oxide
comprises a titanium oxide, the anode is a carbon anode, and the
voltage applied between the cathode and the anode is such that the
potential at the cathode, as measured with respect to a carbon
reference electrode, is more than 1.1 V, and preferably more than
1.3 V.
46. A method for removing oxygen from a solid titanium oxide,
comprising the steps of: a) contacting a cathode comprising the
solid titanium oxide with a fused-salt electrolyte; b) contacting a
carbon anode with the electrolyte; and c) applying a voltage
between the cathode and the anode such that the potential at the
cathode, as measured with respect to a carbon reference electrode,
is more than 1.1 V, and preferably more than 1.3 V.
47. A method for removing a substance from a solid compound
comprising the substance and a metal, comprising the steps of: a)
contacting a cathode comprising the solid compound with a
fused-salt electrolyte; b) contacting an anode with the
electrolyte; and c) applying a voltage between the cathode and the
anode such that the potential at the cathode increases with
time.
48. The method according to claim 47, in which the potential at the
cathode increases continuously with time.
49. The method according to claim 47, in which the potential at the
cathode increases stepwise with time.
50. (canceled)
51. A method for forming an alloy comprising two or more metal
components, comprising the steps of forming a precursor material
containing mixed solid compounds of each of the metal components
with a substance or substances, and processing the precursor
material to remove the substance or substances as defined in: i) a
method for removing a substance from a solid compound comprising
the substance and a metal, comprising the steps of: a) contacting a
cathode comprising the solid compound with a fused-salt
electrolyte; b) contacting an anode with the electrolyte; and c)
applying a voltage between the cathode and the anode which
increases with time; or ii) a method for removing a substance from
a solid compound comprising the substance and a metal, comprising
the steps of: a) contacting a cathode comprising the solid compound
with a fused-salt electrolyte comprising a second salt in solution
in a first salt, the second salt but not the first salt comprising
an anion which is the same as the substance; b) contacting an anode
with the electrolyte; and c) applying a voltage between the cathode
and the anode such that the potential at the cathode is less than a
potential for deposition of cations from the electrolyte; the
concentration of the second salt in solution in the first salt
being greater than zero, and preferably greater than 1 mol %,
during the application of the voltage; or iii) a method for
removing a substance from a solid metal compound comprising the
substance and a metal, comprising the steps of: a) contacting a
cathode comprising the solid metal compound with a fused-salt
electrolyte, the electrolyte comprising a corresponding salt which
contains the substance as an anion; b) contacting an anode with the
electrolyte; and c) applying a voltage between the cathode and the
anode which is insufficient to cause continuous decomposition of
the corresponding salt; or iv) a method for removing a substance
from a solid metal compound comprising the substance and a metal,
comprising the steps of: a) contacting a cathode comprising the
solid metal compound with a fused-salt electrolyte, the electrolyte
comprising a corresponding salt which contains the substance as an
anion; b) contacting an anode with the electrolyte; and c) applying
a voltage between the cathode and the anode which is insufficient
to cause continuous decomposition of the corresponding salt; or v)
a method for removing oxygen from a solid titanium oxide,
comprising the steps of: a) contacting a cathode comprising the
solid titanium oxide with a fused-salt electrolyte; b) contacting a
carbon anode with the electrolyte; and c) applying a voltage
between the cathode and the anode such that the potential at the
cathode, as measured with respect to a carbon reference electrode,
is more than 1.1 V, and preferably more than 1.3 V; or vi) a method
for removing a substance from a solid compound comprising the
substance and a metal, comprising the steps of: a) contacting a
cathode comprising the solid compound with a fused-salt
electrolyte; b) contacting an anode with the electrolyte; and c)
applying a voltage between the cathode and the anode such that the
potential at the cathode increases with time.
52. A metal or alloy formed using the method of claim 51.
53. An apparatus for carrying out a method selected from: i) a
method for removing a substance from a solid compound comprising
the substance and a metal, comprising the steps of: a) contacting a
cathode comprising the solid compound with a fused-salt
electrolyte; b) contacting an anode with the electrolyte; and c)
applying a voltage between the cathode and the anode which
increases with time; or ii) a method for removing a substance from
a solid compound comprising the substance and a metal, comprising
the steps of: a) contacting a cathode comprising the solid compound
with a fused-salt electrolyte comprising a second salt in solution
in a first salt, the second salt but not the first salt comprising
an anion which is the same as the substance; b) contacting an anode
with the electrolyte; and c) applying a voltage between the cathode
and the anode such that the potential at the cathode is less than a
potential for deposition of cations from the electrolyte; the
concentration of the second salt in solution in the first salt
being greater than zero, and preferably greater than 1 mol %,
during the application of the voltage; or iii) a method for
removing a substance from a solid metal compound comprising the
substance and a metal, comprising the steps of: a) contacting a
cathode comprising the solid metal compound with a fused-salt
electrolyte, the electrolyte comprising a corresponding salt which
contains the substance as an anion; b) contacting an anode with the
electrolyte; and c) applying a voltage between the cathode and the
anode which is insufficient to cause continuous decomposition of
the corresponding salt; or iv) a method for removing a substance
from a solid metal compound comprising the substance and a metal,
comprising the steps of: a) contacting a cathode comprising the
solid metal compound with a fused-salt electrolyte, the electrolyte
comprising a corresponding salt which contains the substance as an
anion; b) contacting an anode with the electrolyte; and c) applying
a voltage between the cathode and the anode which is insufficient
to cause continuous decomposition of the corresponding salt; or v)
a method for removing oxygen from a solid titanium oxide,
comprising the steps of: a) contacting a cathode comprising the
solid titanium oxide with a fused-salt electrolyte; b) contacting a
carbon anode with the electrolyte; and c) applying a voltage
between the cathode and the anode such that the potential at the
cathode, as measured with respect to a carbon reference electrode,
is more than 1.1 V, and preferably more than 1.3 V; or vi) a method
for removing a substance from a solid compound comprising the
substance and a metal, comprising the steps of: a) contacting a
cathode comprising the solid compound with a fused-salt
electrolyte; b) contacting an anode with the electrolyte; and, c)
applying a voltage between the cathode and the anode such that the
potential at the cathode increases with time; or vii) a method for
forming an alloy comprising two or more metal components,
comprising the steps of forming a precursor material containing
mixed solid compounds of each of the metal components with a
substance or substances, and processing the precursor material to
remove the substance or substances as defined in method (i), (ii),
(iii), (iv), (v), or (vi).
54. An apparatus for removing a substance from a solid compound
comprising the substance and a metal, comprising; a) a cathode
assembly for holding the solid compound; b) an anode; c) a
reference electrode of carbon; d) a receptacle for holding a
fused-salt electrolyte in contact with the cathode assembly, the
anode and the reference electrode; e) a voltage source for applying
a voltage between the cathode and the anode; and f) a means for
monitoring the voltage between the cathode and the reference
electrode and controlling the voltage source in response to the
monitored voltage.
Description
[0001] The invention relates to a method and an apparatus for
removing a substance from a solid compound by an electrolytic
process, and in particular to improvements to the FFC, or
electro-decomposition, process.
BACKGROUND OF THE INVENTION
[0002] The FFC process, also termed electro-decomposition or
electro-reduction, is described in International Patent Application
No. WO/99/01781 and in subsequent publications by the inventors
named in the International patent application (D. Fray, G. Chen and
T. Farthing), such as "Nature", vol. 407, 361-364 (21 Sep. 2000).
These documents are incorporated herein by reference in their
entirety.
[0003] As is known from the prior art, the FFC process is able to
remove substances from solid compounds between those substances and
metals or semi-metals. For example, as is known from the published
literature, it is possible to apply electro-decomposition to remove
oxygen from a metal compound such as titanium dioxide or from a
semi-metal compound such as silica. For the sake of clarity, the
term "metal" will be used throughout this document to encompass
both metals and semi-metals, and should be construed as such.
[0004] Electro-decomposition is an electrolytic process carried out
in a fused-salt electrolyte. In order to remove a substance from a
solid compound comprising the substance and a metal, a cathode
comprising the solid compound is contacted with the fused salt. An
anode is also contacted with the fused-salt and a voltage applied
between the cathode and the anode such that the substance is
transferred into, or dissolves in, the electrolyte.
Electro-decomposition may thus advantageously enable extraction of
the metal from the metal compound. Electro-decomposition of a
mixture of solid compounds of two or more metals, or of a mixture
of one or more metals and one or more metal compounds, may
advantageously enable fabrication of an alloy comprising the
metals.
[0005] The present inventors have refined this process and the
present invention relates to the resulting improvements to the
process.
SUMMARY OF THE INVENTION
[0006] In its various aspects the invention provides methods,
apparatus, metals and alloys as defined in the appended independent
claims. Preferred or advantageous features of the invention are
defined in dependent subclaims.
[0007] In a first aspect, the invention provides a method for
removing a substance from a solid compound comprising the substance
and a metal, in which a fused-salt electrolyte is contacted with a
cathode comprising the solid compound and with an anode, and a
voltage is applied between the cathode and the anode that increases
with time, or is ramped. The voltage may increase substantially
continuously (either linearly or non-linearly) or stepwise or in
any combination of these.
[0008] In an alternative embodiment, a voltage is applied between
the cathode and the anode such that the potential at the cathode
increases with time, or is ramped. (A cathodic potential is a
negative quantity, and so reference to the potential at the cathode
increasing means that the numerical value of the potential
increases, the cathodic potential becoming more negative). The
cathode potential may increase substantially continuously (either
linearly or non-linearly) or stepwise or in any combination of
these.
[0009] If it is desired that the increasing potential should start
from an initial level at which no electro-decomposition occurs,
then the increasing potential may start at 0 V or may start from
some other potential. For example, the thermodynamics of a reaction
involved in the removal of a substance from a metal compound may
determine a minimum cathode potential below which an
electro-decomposition reaction cannot proceed. The increasing
potential may therefore, in a preferred embodiment, start from a
cathode potential less than or equal to this minimum
electro-decomposition potential. An increasing cell voltage applied
between the cathode and the anode may advantageously rise from a
corresponding voltage level.
[0010] It is also preferable that the increasing potential should
not exceed a maximum level. In an electro-decomposition process, it
is desirable that cations from the fused-salt electrolyte should
not be continuously discharged or deposited at the cathode, which
may eventually consume or disadvantageously change the composition
of the electrolyte and risk contaminating the desired metal product
at the cathode. Consequently, it is preferable that the increasing
potential does not exceed a potential for continuous discharge or
deposition of a cation from the electrolyte, namely the cation
deposition potential. An increasing applied voltage should
therefore preferably not exceed a level corresponding to this
potential at the cathode.
[0011] In order to achieve the desired aim of not consuming or
changing the composition of the electrolyte unnecessarily or
excessively, in a further embodiment it is preferable that the
increasing potential does not cause the cathode potential to rise
to a level which is sufficient to cause continuous decomposition of
the electrolyte. An increasing voltage applied to the cell should
therefore preferably not exceed a corresponding level.
[0012] In a preferred embodiment, when the increasing potential or
voltage has risen to its respective maximum level, the potential or
voltage may be maintained at that level for a further period of
time in order to allow the electro-decomposition to proceed.
[0013] The potential at the cathode may be controlled, monitored or
measured by, for example, the use of a reference electrode which
contacts the fused salt. In a preferred embodiment, the reference
electrode may be implemented as a so-called pseudo-reference
electrode in which an electrode of a suitable material is contacted
with the electrolyte and the potential between the cathode and the
electrode monitored. In this document, the term "reference
electrode" has been used to encompass both reference and
pseudo-reference electrodes, and should be construed accordingly.
Suitable materials for a pseudo-reference electrode in a fused-salt
electrolyte may include carbon, refractory metals or tin oxide.
[0014] Advantageously, little or no current may flow between the
cathode and the reference electrode, so that little or no
polarisation occurs at the reference electrode.
[0015] There may be an advantage in using the same material for the
anode and for the reference electrode in a cell. In a preferred
embodiment, measurement of the voltage between the cathode and the
reference electrode can then be subtracted from the voltage between
the cathode and the anode in order to give an indication of
polarisation at the anode.
[0016] Although, in the embodiments described above, the increasing
potential or voltage may be followed by the application of a
potential or voltage at the maximum level of the increasing
potential or voltage, the increasing potential or voltage may in an
alternative embodiment be followed by the application of a
different potential or voltage.
[0017] In a further alternative embodiment, the potential or
voltage applied after the increasing potential or voltage may be
variable, rather than constant. However, as described below there
may in general be a preferred range for the potential to achieve a
maximum electro-decomposition effect and so in a preferred
embodiment a substantially constant cathode potential within this
range, or a cell voltage for generating a cathode potential in this
range, should be applied.
[0018] During the application of the increasing potential or
voltage, the following advantages may preferably be achieved.
[0019] First, if the fused-salt electrolyte comprises a mixture of
salts, such as a second salt in solution in a first salt, the salts
containing different anion species, the application of an
increasing potential or voltage may enable control of the anion
deposited or evolved at the anode. For example, if the first salt
is a chloride and the second salt is an oxide, for environmental or
other reasons it may be preferable for the reaction at the anode to
involve oxygen discharge rather than chlorine discharge. Chlorine
discharge generally requires a higher potential at the anode and so
oxygen discharge would normally be favoured, as long as the
activity or concentration of oxygen in the region of the anode is
sufficient to avoid excessive polarisation of the oxygen discharge
reaction. Applying an increasing potential or voltage at the
initial stage of an electro-decomposition reaction may
advantageously prevent excessive reduction in the oxygen
concentration or activity in the region of the anode by controlling
the initial rate of oxygen discharge, and thereby encourage oxygen
discharge rather than chlorine discharge during
electro-decomposition. Similar considerations may apply to other
mixtures of salts, as the skilled person would readily
appreciate.
[0020] Consequently, in a preferred embodiment the increasing
potential or voltage increases at a rate sufficiently slow to
favour deposition or discharge of a predetermined or desired
species of anion at the anode.
[0021] Second, electro-decomposition of a solid metal compound
involves the transfer of the substance removed from the compound
into the fused-salt electrolyte. If the substance is removed from
the solid compound at a rate faster than the substance can dissolve
or disperse in the electrolyte, then the presence of the substance,
or of a material comprising the substance, in the electrolyte in
the region of the cathode may reduce the rate of the
electro-decomposition reaction, or even prevent further reaction.
For example, if the local concentration of the substance or a
material comprising the substance at or in the region of the
cathode reaches saturation, it may precipitate or solidify in the
region of the cathode, and prevent or restrict further access of
the fused-salt electrolyte to the solid metal compound and so
prevent or restrict further electro-decomposition. Even if the
local concentration does not reach saturation, an increase in
concentration of the substance in the region of the cathode may
still slow electro-decomposition by restricting access of the
electrolyte to the solid compound. For example, the solid compound
is commonly in the form of a porous pellet or other precursor and
so the rate of transport or diffusion of the electrolyte into the
pores and the substance out of the pores may be limited. The
concentration of the substance in the metal compound is generally
highest at the initial stages of electro-decomposition and
therefore the rate of its removal from the metal compound is likely
to be highest in the initial stages. Consequently, the application
of an increasing potential or voltage starting from a low level may
advantageously control or limit the rate of electro-decomposition
in the early stages of the reaction and reduce any tendency for the
concentration of the removed substance or for undissolved material
to build up at the cathode.
[0022] Consequently, in a preferred embodiment the increasing
potential or voltage increases at a rate sufficiently slow to
control or limit the local concentration of the substance or of a
material comprising the substance, or to suppress the precipitation
of the substance or of the material containing the substance, in
the region of the cathode.
[0023] Third, it has been found that in many cases, application of
the electro-decomposition process to remove a substance from a
solid compound to produce metal does not proceed at the same rate
at all parts of the solid compound at the cathode. Commonly, for
example, such uneven reaction may cause the formation of metallic
layer encasing a partially-reduced core of the solid compound at
the cathode, which may disadvantageously reduce the rate of
completion of electro-decomposition of the core. In such cases, it
may be advantageous to control or limit the rate of
electro-decomposition in order to allow a more even reduction of
the solid metal compound throughout the cathode structure.
[0024] Consequently, in a preferred embodiment the increasing
potential or voltage increases at a rate sufficiently slow to
favour an even progression of the reduction throughout the solid
metal compound at the cathode.
[0025] Fourth, it has been found that in many cases
electro-decomposition involves the formation of one or more
intermediate compounds comprising the substance and the metal. In
such cases, it may be advantageous to control the progress of the
electro-decomposition reaction through the formation of any
intermediate compounds, for example to avoid the formation of
fully-reduced metal at one portion of the cathode while reduction
has only proceeded as far as an intermediate compound at another
portion of the cathode.
[0026] Consequently, in a preferred embodiment, the increasing
potential or voltage increases at a rate sufficiently slow to
control the formation of intermediate compounds, for example to
allow formation of an intermediate compound throughout a
predetermined proportion of the bulk of the solid compound at the
cathode before further reduction proceeds.
[0027] In order to achieve these effects, the applied potential or
voltage may be increased at a rate of less than 1 V/s or 100 mV/s,
preferably at a rate of less than 50 mV/s or 10 mV/s, particularly
preferably at a rate of less than 1 mV/s and in a preferred
embodiment at a rate of less than 0.5 mV/s or 0.2 mV/s. If the
applied potential or voltage is not increased linearly, the average
rate of increase may advantageously not exceed these preferred ramp
rates.
[0028] A second aspect of the invention provides a method for
electro-decomposition of a solid compound in a fused-salt
electrolyte, in which the electrolyte comprises a second salt in
solution in a first salt, the second salt but not the first salt
comprising an anion which is the same as the substance to be
removed from the solid compound. The concentration of the second
salt in solution in the first salt is advantageously greater than
zero during electro-decomposition, and is preferably sufficient to
enable transport of the substance through the electrolyte from the
cathode to the anode throughout electro-decomposition.
[0029] Advantageously, the concentration of the second salt in
solution in the first salt is greater than 0.1 mol %, preferably
greater than 0.5 mol % and particularly preferably greater than 1
mol % or greater than 2 mol %, during the application of the
electro-decomposition voltage.
[0030] It is believed that it is advantageous to maintain a
predetermined minimum concentration of the second salt in the first
salt in order to maintain a pre-determined minimum concentration of
the substance (which is the same as the anion in the second salt)
in the electrolyte throughout the electro-decomposition process.
Although it is not an essential feature of electro-decomposition,
it is preferred that the anion discharged at the anode should be
the same as or should comprise the substance removed from the solid
metal compound. Under preferred conditions, this enables
electro-decomposition to be carried out without excessive or
unnecessary consumption of the fused-salt electrolyte and
preferably with minimum, or limited, variation of the composition
of the fused-salt electrolyte. Clearly, if the substance dissolved
at the cathode is different from the anion discharged at the anode
at any stage during electro-decomposition, the composition of the
fused-salt electrolyte must change, and it may be desirable to
limit or control any such effects, for example if it is desired to
recycle or reuse the electrolyte.
[0031] In order to ensure as far as possible that the anion
discharged at the anode is the same as or comprises the substance
dissolved at the cathode, it is desirable to set up a flux of the
substance or anion from the cathode to the anode, which requires
that the electrolyte between the cathode and the anode preferably
always comprises a pre-determined minimum concentration of the
substance or anion. Consequently, it is preferred that when a
fused-salt electrolyte comprises a second salt in solution in a
first salt, or in other words a mixture of first and second salts,
and only one of the salts contains an anion corresponding to the
substance being dissolved at the cathode, then the concentration of
that salt should be maintained above a pre-determined minimum
level.
[0032] A fused-salt electrolyte comprising first and second salts
may comprise further components, such as other salts. For example
an electrolyte for electro-decomposition of a metal oxide may
comprise a salt such as calcium oxide as the second salt for
providing a predetermined oxide concentration in the melt, in
solution in, or mixed with, a first salt comprising a eutectic
mixture of calcium chloride and lithium chloride. Similarly, the
electrolyte may comprise more than one component for the second
salt; for example an electrolyte for electro-decomposition of an
oxide may comprise more than one oxide in the melt, such as calcium
oxide and barium oxide.
[0033] In some cases, an electro-decomposition process may involve
a reaction between a component of the electrolyte and the solid
metal compound, such as a chemical or electrochemical reaction. A
proportion of the second salt or its constituent ions may then be
consumed at the cathode by this reaction. For example, such a
reaction may involve the formation or reaction of an intermediate
compound or compounds, formed as an intermediate stage in the
electro-decomposition process. In such a case, it is preferable
that a sufficient quantity of the second salt should initially be
present in the electrolyte to sustain a pre-determined minimum
concentration of the second salt in solution in the first salt even
when a portion of the second salt has been consumed or reacted at
the cathode. Consumption or reaction of the second salt at the
cathode is generally temporary; this can be seen by considering the
case where electro-decomposition achieves complete reduction of the
solid compound to the metal, in which case any of the second salt
which was consumed or reacted at the cathode during
electro-decomposition must have been released back into the fused
salt later in the electro-decomposition process.
[0034] The pre-determined minimum concentration of the second salt,
or of anions corresponding to the dissolved substance, to be
maintained in solution may vary depending on the metal compound
being reduced and the composition of the fused-salt electrolyte,
but in the case of a fused-salt electrolyte comprising calcium
chloride as the first salt and calcium oxide as the second salt
being used to reduce a metal oxide, it is understood that a calcium
oxide concentration of 5 mol % is generally sufficient. In
preferred embodiments, lower concentrations such as concentrations
of less than 2 mol % or less than 1 mol % may suffice.
[0035] The minimum pre-determined concentration may also depend on
the rate of electro-decomposition and the rate at which the
substance is dissolved at the cathode. It may therefore be
advantageous to control the electro-decomposition voltage, and
therefore the driving force for electro-decomposition, as described
above as well as controlling the electrolyte composition.
[0036] In the embodiment discussed above in which a portion of one
component of the fused-salt electrolyte (usually the second salt)
is consumed or reacts at the cathode, the quantity of the second
salt in the electrolyte may reduce during electro-decomposition.
The quantity of the second salt removed from the electrolyte will
generally be related to the quantity of the solid metal compound
present at the cathode. In such a case, it is advantageous if the
initial quantity of the second salt in the electrolyte is
sufficient, taking into account the quantity of the solid metal
compound at the cathode, to maintain a pre-determined minimum
concentration of the second salt in the electrolyte throughout
electro-decomposition.
[0037] It is further preferred that the electrolyte contains a
sufficient quantity of the first salt to keep the sufficient
quantity of the second salt in solution in the electrolyte at all
times, bearing in mind that the second salt may have a solubility
limit for mixing with the first salt.
[0038] In addition, if the transfer of the substance from the solid
compound into the electrolyte may affect the solubility of the
second salt in the first salt, then this may also affect the
quantity of the first salt desired in the electrolyte. For example,
if the substance is the same as the anion in the second salt, then
the electrolyte should preferably contain a sufficient quantity of
the first salt to accommodate transfer of the substance into the
melt without the solubility limit of the second salt in the first
salt being approached or exceeded.
[0039] These considerations can readily be extended by the skilled
person to cases in which the electrolyte contains more complex
mixtures of salts, such as the example described above in which the
first salt comprises a eutectic or other mixture of salts.
[0040] As a particular example, during electro-decomposition of
titanium dioxide in an electrolyte comprising calcium chloride as
the first salt and calcium oxide as the second salt, it is found
that for each mole of titanium dioxide at the cathode,
approximately 0.67 moles of calcium oxide is consumed in the
formation of intermediate compounds such as calcium titanates at
the cathode. It is therefore preferred that in this
electro-decomposition process, for each mole of titanium dioxide at
the cathode, the electrolyte should contain more than 0.67 moles of
calcium oxide. For example, in a preferred embodiment, for each
mole of titanium dioxide at the cathode, one mole of calcium oxide
may be present in the electrolyte.
[0041] In this preferred embodiment, electro-decomposition may
typically be carried out at about 900 C, at which temperature the
solubility limit for calcium oxide in calcium chloride is about 20
mol %. Consequently, for each mole of titanium dioxide present, and
therefore for each mole of calcium oxide present at the start of
the process, the electrolyte must contain at least about 4 moles of
calcium chloride to ensure that the calcium oxide remains in
solution at all times. It may be preferred, however, for the
electrolyte to contain more calcium chloride than this in order to
allow for any increase in the oxygen content of the melt caused by
the removal of oxygen from the titanium dioxide, including any
local increase in oxygen content in the region of the cathode
caused by the removal of oxygen from the titanium dioxide, which
might otherwise encourage calcium oxide precipitation in the region
of the cathode.
[0042] In a further aspect, the invention provides a method for
electro-decomposition in which a voltage is applied within a
pre-determined voltage range in order to enhance the effectiveness
of the electro-decomposition process. As described above, the
thermodynamics of the reaction in which the substance is removed
from the metal compound to form the metal determines a minimum
cathode potential. A preferred upper limit arises because it is
advantageous for the cathode potential to be less than a potential
for continuous discharge or deposition of cations from the
electrolyte at the cathode. However, it has been found that within
this range of cathode potential there is a range for which
electro-decomposition is enhanced.
[0043] For example, when the solid metal compound for
electro-decomposition is an oxide it has been found that a
fused-salt electrolyte comprising calcium oxide may advantageously
be used. In this embodiment the potential at the cathode, measured
with respect to a carbon reference electrode, is advantageously
less than 1.7 V, preferably less than 1.6 V and is particularly
preferably less than about (1541-116 log C) mV, where C is the
normalised concentration (i.e. the actual concentration divided by
the saturation concentration) of calcium oxide in the fused-salt
electrolyte.
[0044] In an embodiment in which the metal oxide comprises a
titanium oxide, a voltage applied between the cathode and the anode
is preferably such that the potential at the cathode, measured with
respect to a carbon reference electrode, is more than 1.1 V, and
preferably more than 1.3 V. Thus, the preferred cathode potential
range for reduction of a titanium oxide in a fused-salt electrolyte
comprising calcium oxide is between 1.3 V and 1.7 V, and
particularly preferably is between 1.4 V and 1.6 V, or the
potential is about 1.5 V, measured with respect to a carbon
reference electrode.
[0045] In all of these embodiments, if a different reference
electrode is used, then the same principles apply to the
enhancement of the electro-decomposition process, except that a
voltage offset may need to be introduced in order to account for
the difference in potential measured using the reference electrode
rather than a carbon reference electrode as described above.
Determining or evaluating the necessary voltage offset in each case
would be well within the competency of the skilled person.
[0046] Thus, in order for a method, apparatus or product to fall
within this aspect of the invention, it should be noted that a
carbon reference electrode does not have to be used in the method
or apparatus or in the manufacture of the product. The reference to
the carbon reference electrode in relation to this aspect of the
invention is only to define precisely the cathode potentials
involved. Any appropriate method may be used in practice to measure
or to generate the cathode potential, as the skilled person would
appreciate.
[0047] In a further aspect, the invention provides a method and a
corresponding apparatus in which a carbon or refractory metal or
tin oxide reference electrode is used to monitor the cathode
potential. In this method, the cathode, an anode and the reference
electrode are contacted with the fused-salt electrolyte and an
electro-decomposition voltage applied between the cathode and the
anode. Measurement of the voltage between the cathode and the
reference electrode can then be used to monitor the cathode
potential and, in turn, used to control the electro-decomposition
voltage in order to achieve a pre-determined desired cathode
potential at each stage of the electro-decomposition process.
[0048] The foregoing discussion describes various aspects of the
electro-decomposition process. These aspects may advantageously be
used in combination. For example, an increasing voltage may
initially be applied between the cathode and the anode, for example
so as to generate an increasing cathode potential, terminating at a
voltage within the preferred range for continuous
electro-decomposition. A substantially-constant voltage may then be
applied at that level, or a voltage applied so as to maintain a
substantially-constant cathode potential at the preferred level.
The fused-salt-electrolyte composition during this procedure may
advantageously be predetermined such that a sufficient
concentration of anions corresponding to the substance to be
removed from the solid compound is maintained in the electrolyte at
all times, so as to ensure a sufficient flux of the substance from
the cathode to the anode. A carbon or other reference electrode may
advantageously be contacted with the electrolyte to monitor the
cathode potential during the process.
[0049] In alternative embodiments, other combinations of the
various aspects of the invention may advantageously be used, as the
skilled person would appreciate.
[0050] The various aspects of the invention may advantageously be
applied to the electro-decomposition of a wide range of solid metal
compounds, including compounds of titanium, silicon, germanium,
zirconium, hafnium, samarium, uranium, aluminium, magnesium,
neodymium, molybdenum, chromium, niobium, boron, scandium,
vanadium, manganese, iron, cobalt, nickel, copper, gallium,
yttrium, tantalum, tungsten, rhenium, lead, cerium and plutonium.
Aspects of the invention may find particular application in
reducing oxides of these metals, for forming the metals and for
forming alloys comprising these metals.
[0051] In general, the fused salt electrolyte for
electro-decomposition may advantageously contain a chloride of
calcium, strontium, barium, lithium or a rare earth metal. These
salts are particularly efficacious for electro-decomposition of
oxides as these salts dissolve their own oxides; that is, for
example, calcium oxide is soluble in calcium chloride, and so on.
Thus the first salt in the foregoing description may comprise one
of the chlorides mentioned above and the second salt may comprise
the corresponding oxide. Alternatively the second salt may comprise
a different cation from the first salt, such as barium oxide mixed
with calcium chloride. In further embodiments, more complex
mixtures of salts may be used. For example an electrolyte
containing calcium chloride may also contain sodium chloride or
potassium chloride, for instance to modify the melting point of the
electrolyte. A suitable oxide may then be included in the salt to
enable oxygen transfer to the anode, where an oxide is to be
electro-decomposed.
SPECIFIC EMBODIMENTS AND BEST MODE OF THE INVENTION
[0052] Specific embodiments of the invention will now be described
by way of example, with reference to the accompanying drawings, in
which:
[0053] FIG. 1 is a diagram of an apparatus for carrying out an
electro-decomposition process according to an embodiment of the
invention;
[0054] FIG. 2 is a diagram of a cathode structure comprising a
titanium dioxide pellet suspended on a nickel wire, for use with
the apparatus of FIG. 1;
[0055] FIG. 3 is a diagram of an electrode arrangement for carrying
out pre-electrolysis of a fused-salt electrolyte, according to a
further embodiment of the invention; and
[0056] FIG. 4 is a diagram of an electrode arrangement for use with
the apparatus of FIG. 1, and incorporating a reference
electrode.
[0057] FIG. 1 is a schematic diagram of an apparatus 2 for carrying
out an electro-decomposition process. The apparatus comprises an
alumina crucible 4 containing a fused-salt electrolyte 6. A cathode
8, an anode 10 and a shielded thermocouple 12 are immersed in the
electrolyte. The anode and the cathode are connected to a
potentiostatically-controlled voltage source 14. The crucible is
positioned on a ceramic insulator 16 within an Inconel.RTM. reactor
18 of height 65 cm and inner diameter 13.4 cm. An upper end of the
reactor is provided with water cooling 20 and closed by a
stainless-steel cover 22 sealed with an O-ring 24. The cover is
provided with electrical feedthroughs for the power supply and for
a lead for the thermocouple, as well as with a gas inlet 26 and gas
outlet 28 which enable control of the atmosphere within the
reactor. The reactor is externally heated inside a vertical-tube
furnace (not shown); in an alternative embodiment, the reactor may
be internally heated.
[0058] In the apparatus of FIG. 1, the anode is a graphite rod
connected by a nickel wire to the power supply, or voltage source;
the graphite rod and the nickel wire are each threaded into blind
bores at opposite ends of a stainless-steel connector.
EXAMPLE 1
TiO.sub.2
[0059] In a first Example, in the apparatus of FIG. 1 the cathode
comprises a disk or pellet 32 of titanium dioxide, prepared as
described below, connected to the power source by means of a nickel
wire 30. The titania disk has a hole drilled through its centre;
the nickel wire is then threaded through the hole and twisted back
on itself to secure the disk at the end of the wire, as illustrated
in FIG. 2.
[0060] The titania disk is formed from a commercial powder (Alfa
Aesar) specified as rutile, 99.5% pure, average particle size
between 1 and 2 .mu.m. The powder is dried in an oven for several
days at temperatures around 100.degree. C. Then 1.0% by mass of a
PVB/PVA mixture (poly vinyl butyral-co-vinyl alcohol-co-vinyl
acetate, approximately 80 wt % butyral) and 0.5% by mass of PEG
(poly(ethylene glycol), average molecular mass 200) are added. The
components are mixed by wet milling using alumina balls in
iso-propanol for 24 hours. The powder is dried at around
100.degree. C. and passed through a vibrating 53 .mu.m stainless
steel sieve. The treated titanium dioxide powder is then pressed
into disks weighing between 1 g and 8 g which are prepared by,
firstly, uniaxial pressing at about 50 MPa to form a green body
and, second, isostatic pressing at 175 MPa in order to further
densify the green body. Disks are then sintered at 1050 C for 150
min in air. Sintering temperatures of between 1000 C and 1250 C, or
preferably between 1000 C and 1100 C, are found to be particularly
effective in producing disks or other electro-decomposition
precursors of titania that have sufficient mechanical strength to
be used in electro-decomposition without damage, and open porosity
of about 30%, which advantageously enables intimate access of the
fused-salt electrolyte to the interior of the disk or other
precursor during electro-decomposition.
[0061] It is particularly important that the sintering temperature
for the pellets is at least as high, and preferably higher than,
the electro-decomposition temperature (electrolyte temperature) to
be used. This advantageously avoids further sintering of the pellet
on immersion into the fused-salt electrolyte and thus enhances
control of the pellet properties. Consequently, the sintering
temperatures and ranges described above are particularly effective
for electro-decomposition at 900 C.
[0062] Clearly, the preparation procedure may be modified by the
skilled person in order to prepare electro-decomposition precursor
materials of compounds other than titanium dioxide, for example if
metals other than titanium are to be extracted, or to incorporate
mixtures of compounds if alloys are to be prepared.
[0063] The fused-salt electrolyte in the embodiment is as follows.
1.95 mol of anhydrous calcium chloride (Fluka, >97% purity)
(corresponding to 216 g) and 0.05 mol of anhydrous calcium oxide
(formed by calcining CaCO.sub.3, Aldrich, >99% purity)
(corresponding to 2.8 g) are melted to form the electrolyte, giving
a nominal calcium oxide concentration of 2.5 mol %. This gives a
molten-salt depth of between 4 and 5 cm in the apparatus of FIG.
1.
[0064] Alternative embodiments may vary from the apparatus of FIG.
1 in a number of ways in accordance with known
electro-decomposition processes as published in patent application
WO 99/01781 and elsewhere. For example the cathode may comprise the
solid compound in any form of electro-decomposition precursor, such
as pellets or other artefacts contained in a metal basket or other
cathode assembly, or a precursor of a predetermined shape for
forming an electro-decomposition product of a predetermined shape.
The crucible may be of a conducting material and used as the anode
or as the cathode, in the latter case contacting the solid metal
compound. The composition of the fused-salt electrolyte may be
selected according to, for example, the metal compound to be
reduced. The electrolyte may comprise a single salt or a mixture of
any number of salts. For example it may comprise a eutectic mixture
of salts if it is desired to obtain a low-melting-point
electrolyte. Advantageously, salts or mixtures of salts may
comprise salts of the Group 1 or Group 2 metals, including halides
and oxides of these metals. The electrolyte temperature, anode
materials, and other parameters of the electro-decomposition
process may also be varied. For example, higher temperatures might
be used to accelerate diffusion rates and so accelerate
electro-decomposition, but may also disadvantageously affect
corrosion of the apparatus.
[0065] In the embodiment described above, before
electro-decomposition begins a thermal drying and pre-electrolysis
procedure is performed as follows, in order to remove any remaining
water from the electrolyte. The thermal drying procedure comprises
the following heating sequence; ramp at 2 C/min to 150 C and hold
for at least 5 hours, ramp at 2 C/min to 300 C and hold for 5
hours, and ramp to target temperature of 900 C. Pre-electrolysis is
then performed as follows. Three graphite electrodes are immersed
into the molten-salt electrolyte and are applied, as illustrated in
FIG. 3, as working, counter and pseudo-reference electrodes 34, 36,
38. A voltage source 40 is applied between the working electrode
(cathode) and the counter electrode (anode) and the cathodic
polarisation of the working electrode against the reference
electrode monitored and used to control the voltage applied by the
voltage source. During pre-electrolysis, the applied voltage is
controlled so that the cathodic polarisation of the working
electrode versus the reference electrode is first ramped from 0 V
to 1 V at a ramp rate of 0.087 mV/s and then maintained at a
constant potential of 1 V. Pre-electrolysis is continued until a
small and constant background current is encountered.
[0066] In a preferred embodiment, electro-deoxidation is then
performed using the apparatus illustrated in FIG. 1 but
additionally incorporating a graphite pseudo-reference electrode
42, immersed in the electrolyte and connected as illustrated in
FIG. 4 so that the voltage applied by the voltage source can be
controlled in response to the cathodic polarisation of the cathode,
measured with respect to the reference electrode.
[0067] In each of a set of experiments using the apparatus of FIG.
1, a titanium dioxide pellet of approximately 4 g weight, 4 mm
thickness and 30% open porosity was connected to a nickel wire to
form the cathode. A graphite rod was employed as the anode and the
thermally dried and pre-electrolysed calcium chloride/calcium oxide
melt described above was used as the electrolyte. The anode-cathode
separation was about 4 cm. During reduction the voltage source 14
was controlled so that the cathodic polarisation of the cathode
against the pseudo-reference electrode was first ramped from 0 V to
a maximum voltage level, and then the maximum voltage level
maintained in a constant manner for an additional period of 16
hours. A number of experiments were carried out in which the ramp
rate was 0.087 mV/s and the maximum voltage levels were 0.9, 1.0,
1.1, 1.3, 1.4, 1.5, 1.6, 1.7 and 1.9 V respectively, measured
between the cathode and the reference electrode. The durations of
the individual experiments were thus between about 18.9 and 22.1
hours.
[0068] In each experiment, an atmosphere of dry argon and a
reaction temperature of 900 C were applied. After each experiment,
the electro-reduced pellet was removed from the electrolyte, rinsed
with water, acid-leached and dried, before being broken up and
inspected.
[0069] The pellets obtained from reductions performed at maximum
voltage levels of 0.9 and 1.0 V were violet inside and dark yellow
outside, suggesting the presence of CaTi.sub.2O.sub.4 and Tio
respectively. This suggests that only partial reduction of the
titanium dioxide was achieved. The pellets processed at 1.1, 1.3
and 1.4 V maximum voltage levels exhibited a grey metallic outer
scale around a partially-reduced core, the thickness of the
metallic scale being greater for the higher voltage levels. The
pellets made at 1.5 and 1.6 V maximum voltages were metallic grey
throughout their entire volumes, indicating complete conversion
into titanium metal. The pellets processed at 1.7 and 1.9 V maximum
voltages were reduced to metal only at the surface, over a
partially-reduced core.
[0070] In other experiments, faster and slower ramp rates
(including 0.174 and 0.044 mV/s) were used, terminating at the same
potentials as described above, but a ramp rate of 0.087 mV/s was
found to be most effective in accelerating electro-decomposition in
the embodiment.
[0071] During electro-decomposition, it was observed that the cell
voltage applied between the anode and the cathode was approximately
1 V greater than the voltage measured between the cathode and the
pseudo-reference electrode. This suggests that polarisation of the
reaction at the anode consumes about 1 V of the applied cell
voltage.
[0072] The fact that the formation of titanium metal is not
observed below an effective voltage (measured against the
pseudo-reference electrode) of 1.1 V is in good agreement with
thermodynamic expectations, as described below. It is believed that
the electro-deoxidation of TiO.sub.2 to Ti metal may involve the
reactions 1 to 8 below. Reactions 1 to 5 describe the progressive
reduction of titanium and the associated formation of calcium
titanates. The calcium titanates CaTiO.sub.3 and CaTi.sub.2O.sub.4
are observed in the electro-decomposition of TiO.sub.2 and it is
believed that these decompose through reactions 6, 7 and 8.
Reaction 6 is a chemical reaction and reaction 7 is an
electrochemical reaction, both converting CaTiO.sub.3 to
CaTi.sub.2O.sub.4. Reaction 8 is an electrochemical reaction
converting CaTi.sub.2O.sub.4 to TiO, which is then believed to
reduce according to reaction 5. It is assumed that CO is evolved at
the anode; in practice, this may depend on the anode material and
reaction kinetics at the anode. It is believed that an equilibrium
between C and CO occurs at a surface of the graphite
pseudo-reference electrode and therefore that it is appropriate to
refer to this reaction to calculate expected cathodic polarisations
E relative to the reference electrode, as listed below.
5TiO.sub.2+CaO+C.dbd.Ti.sub.4O.sub.7+CaTiO.sub.3+CO E=+266 mV
(1)
4Ti.sub.4O.sub.7+CaO+C=5Ti.sub.3O.sub.5+CaTiO.sub.3+CO E=+137 mV
(2)
3Ti.sub.3O.sub.5+CaO+C=4Ti.sub.2O.sub.3+CaTiO.sub.3+CO E=+33 mV
(3)
2Ti.sub.2O.sub.3+CaO+C=3TiO+CaTiO.sub.3+CO E=-342 mV (4)
TiO+C.dbd.Ti+CO E=-1114 mV (5)
CaTiO.sub.3+TiO.dbd.CaTi.sub.2O.sub.4 (6)
2CaTiO.sub.3+C.dbd.CaTi.sub.2O.sub.4+CaO+CO (7)
CaTi.sub.2O.sub.4+C=2TiO+CaO+CO (8)
[0073] The potentials E listed above are as calculated using
standard software (HSC Chemistry, Version 4.1, Outokumpu Research
Oy, Pori, Finland). The calculations are based on the free-enthalpy
change in each reaction, at a temperature of 900 C and assuming
that all compounds are present at unit activity (clearly this may
not be the case but actual activity differences are expected to
have only a limited effect on the calculated potentials). Following
common conventions, a negative potential E corresponds to a
positive enthalpy change.
[0074] If CO.sub.2 rather than CO were to be formed (i.e. to be the
species determining the reference electrode potential) at the
reference electrode, all of the potentials would become more
negative by 90 mV.
[0075] It can clearly be seen that the expected potential required
to prepare Ti metal, as indicated by equation 5, is approximately
1.1 V, which shows good agreement with the experiment described
above.
[0076] The preferred potential for reduction of the entire pellet
is between 1.4 V and 1.7 V, as evidenced by the successful
reduction to Ti in the experiments carried out at 1.5 V and 1.6 V.
The requirement for a potential in excess of 1.1 V indicates that
there may be some polarisation of the reaction at the cathode. The
less-successful results at potentials of 1.7 V and above indicate
that these potentials are less preferred.
EXAMPLE 2
Cr.sub.2O.sub.3
[0077] This example illustrates the reduction of Cr.sub.2O.sub.3 to
Cr metal. Chromium sesquioxide, Cr.sub.2O.sub.3, disks were formed
from a commercial powder of particle size less than 3 .mu.m
(Elementis Pigments). The powder was dried, mixed with a binder and
sieved in the same way as the titania powder described above. The
treated powder was then pressed into disks weighing 2.7 g which
were prepared by uniaxial pressing at about 100 Mpa to form a green
body. The subsequent isostatic pressing step used for preparing
titania disks was found to be dispensable as the uniaxially-pressed
disks were sufficiently robust. The Cr.sub.2O.sub.3 disks were then
sintered at 1300 C in air for 150 min. Sintering temperatures of
between 1100 and 1500 C are found to be particular effective in
producing disks or other electro-decomposition precursors of
chromia that have sufficient mechanical strength to be used in
electro-decomposition without damage, while maintaining a
substantial degree of open porosity that enables intimate access of
the fused-salt electrolyte to the interior of the disk or other
precursor during electro-decomposition.
[0078] Using the apparatus of FIG. 1, a porous chromium sesquioxide
pellet (disk) of approximately 2.7 g weight and 3 mm thickness was
connected to nickel wire to form the cathode. A graphite rod was
employed as the anode and the thermally dried and pre-electrolysed
calcium chloride/calcium oxide melt described above was used as the
electrolyte. The anode-cathode separation was 4 cm, and a carbon
reference electrode was provided. During reduction the voltage
source 14 was controlled so that the cathodic polarisation of the
cathode against the carbon reference electrode was first ramped
from 0 V to a maximum voltage level, and then the maximum voltage
maintained in a constant manner. The ramp rate was 0.087 mV/s, the
maximum voltage level was 1.0 V measured between the cathode and
the reference electrode, and the dwell time at the maximum voltage
level was 8 h, rendering the duration of the experiment about 11.2
h.
[0079] In the experiment, an atmosphere of dry argon and a reaction
temperature of 900 C were applied. After the experiment, the
electro-reduced pellet was removed from the electrolyte, rinsed
with water, leached with semi-concentrated acetic acid and dried,
before being broken up and inspected.
[0080] The processed specimen was metallic throughout its entire
volume. X-ray diffraction analysis proved the exclusive presence of
chromium metal, and the quantitative determination of oxygen
content yielded a numerical value of approximately 2800 ppm.
[0081] The fact that the formation of chromium metal is observed at
a relatively low effective voltage (measured against the
pseudo-reference electrode) of 1.0V is in accordance with the
thermodynamics, as illustrated in equations 9 to 11. The electrode
potentials calculated for each reaction based on free-enthalpy
changes at 900 C are given.
Cr.sub.2O.sub.3+3C=2Cr+3CO E=-322 mV (9)
3Cr.sub.2O.sub.3+2CaO+3C=2Cr+2CaCr.sub.2O.sub.4+3CO E=-117 mV
(10)
CaCr.sub.2O.sub.4+3C=2Cr+CaO+3CO E=-424 mV (11)
[0082] The temporary presence of calcium chromites during
electro-reduction of chromium sesquioxide is observed on analysis
of incompletely-reduced samples.
[0083] The preferred potential for reduction of chromium
sesquioxide is between 0.5 V (i.e. a little greater than the
potential of -424 mV for reaction 11), as measured with respect to
a carbon reference electrode, and the potential at which
substantial electronic conduction through the electrolyte occurs.
The potential (as measured against a carbon reference electrode) is
therefore preferably between 0.5 V and 1.7 V and particularly
preferably between 0.6 V and 1.3 V, or between 0.65 V and 1.0
V.
[0084] The rapid and successful preparation of Cr through
electro-decomposition of Cr.sub.2O.sub.3 clearly demonstrates the
strength of the process. Through the choice of a potential which is
sufficiently high to reduce the Cr.sub.2O.sub.3 but low enough to
avoid unnecessary or excessive damage to or consumption of the
electrolyte, decomposition of the CaO dissolved in the CaCl.sub.2
is substantially precluded, and the problems arising from
electronic transference in the electrolyte are avoided. This may
advantageously improve process control and increase current
efficiency in a significant manner.
[0085] Background Understanding
[0086] Although the reaction mechanisms involved in the
electro-decomposition process are not yet fully understood, the
inventors' current understanding of the mechanism of
electro-decomposition in the embodiments described above is as
follows. Since this understanding is not yet complete or
definitive, while the following comments may guide the skilled
person they should not be considered to limit the present invention
beyond the definitions set out in the claims.
[0087] Reactions 1 to 8 illustrate that the reduction of TiO.sub.2
to Ti is believed to involve the formation of a sequence of
intermediate compounds, including calcium titanates and titanium
oxides containing titanium in various oxidation states, and the
corresponding potentials show that increasing potential values are
required to reduce the titanium to progressively lower oxidation
states. Thus, it can be seen that the ramped potential applied in
the experiments above may advantageously encourage the formation of
the intermediate compounds in sequence throughout the bulk of the
metal compound at the cathode. The ramped voltage may also help to
reduce any tendency for full conversion to metal to occur most
rapidly at the surface of the metal compound, and so may ensure
that the electro-decomposition reaction proceeds more evenly
throughout the bulk of the titania pellet, reducing the likelihood
of creating a metallic surface surrounding an unreduced or
partially-reduced core.
[0088] The inventors believe that a further factor in increasing
the rate of electro-decomposition may concern the formation of
calcium oxide at the cathode, as oxygen is removed from the cathode
and dissolved in the electrolyte. Calcium oxide has limited
solubility in calcium chloride (approximately 20 mol % at 900 C)
and a limited rate of dissolution, and if oxygen is removed from
the cathode too rapidly, these limits may be exceeded and cause
precipitation of calcium oxide in the region of the cathode. Since
the cathode is a porous body of titanium dioxide, if calcium oxide
precipitates in the pores of the body, further reaction may become
extremely slow or even stall completely. The inventors term this
phenomenon "oxide quenching". It is believed that a further
advantage of the application of an increasing potential or voltage
starting from a low level at the beginning of electro-decomposition
may be to control the rate of reaction at the cathode to a rate at
which dissolved oxygen can be transported away from the cathode
region before calcium oxide precipitation occurs.
[0089] In cases where the solid compound reacts with the
electrolyte, as in the reactions of TiO.sub.2 and Cr.sub.2O.sub.3
with CaO described above, it is believed that the compounds formed
in such reactions generally decompose as electro-reduction proceeds
to produce the desired metal. This may lead to an effective
increase in the rate of removal of the substance from the solid
compound; in the example of TiO.sub.2 this would involve the
decomposition of calcium titanates releasing CaO back into the
electrolyte. This process may increase the risk of precipitation of
the substance or of a material comprising the substance (e.g. CaO)
in the region of the cathode, and it is thought that this may
advantageously be controlled by applying an increasing voltage
rising from a low level at the start of electro-decomposition,
and/or by controlling the electro-decomposition voltage to a
suitable level throughout electro-decomposition so as not to drive
the process too fast. Suitable design of the solid-compound
precursor, for example to increase porosity and reduce precursor
size, may also assist in accelerating diffusion and dissolution of
the substance.
[0090] The specific oxide-ion conductivity of an electrolyte is
given by the following expression.
.sigma. .sub.o2-=2 F c .sub.o2-u .sub.o2- (12)
[0091] where .sigma., F, c, u are specific conductivity, Faraday
constant, molar concentration and electrical mobility,
respectively, and the subscript denotes the ion species concerned.
Thus, increasing the oxide concentration in the electrolyte by
adding calcium oxide to calcium chloride is believed to increase
the specific oxide-ion conductivity of the electrolyte. Although it
is known that electro-decomposition of metal oxides can proceed at
low oxygen-ion concentrations, for example by discharging chlorine
from a calcium chloride electrolyte at the anode, it has been
observed by the inventors that the presence of calcium oxide
dissolved in calcium chloride tends to accelerate the rate of the
reduction process for metal oxides. This points to the possible
occurrence of a transport limitation in the electrolyte for the
transport of oxygen dissolved at the cathode, through the
electrolyte, to the anode under conditions when very little or no
calcium oxide is present in the electrolyte.
[0092] In order to benefit from the accelerating effect of the
dissolved calcium oxide, it is believed that the calcium oxide
should not be reduced to too low a concentration in the electrolyte
during electro-decomposition, through chemical or electrochemical
reactions. During electro-decomposition of titanium dioxide,
dissolved calcium oxide in the electrolyte may react with the
titanium dioxide or other titanium oxides at the cathode to form
calcium titanates, as in reactions 1 to 4 for example. For this
reason, it is believed to be advantageous to ensure that the
electrolyte contains a sufficient amount of calcium oxide to ensure
that a sufficient concentration of calcium oxide will still be
present in the electrolyte to achieve oxygen transport to the anode
after the reaction of calcium oxide with the cathode material has
removed some of the calcium oxide from the electrolyte. In this
preferred embodiment, maintenance of an adequate calcium oxide
concentration in the melt may not only accelerate oxygen transport
to the anode but also reduce any polarisation of the
oxygen-discharge reaction at the anode and thereby suppress
chlorine discharge from the calcium chloride.
[0093] It is important to note that reaction of the electrolyte
with the metal compound in this way does not involve continuous
electrochemical decomposition of the salt or deposition of metal
from the salt. As clearly shown in reactions 1 to 4, the reaction
is between CaO and the metal compounds at the cathode, and
therefore stops when one of these reagents is exhausted. As
described above, it is believed to be preferable that a
concentration of oxygen ions is maintained in the electrolyte
throughout the electro-decomposition and so it is preferred that
the amounts of the solid metal compound and CaO present are
predetermined so that the metal compound is the reagent exhausted
by the reaction between them.
[0094] It is believed that the concentration of dissolved calcium
oxide in the calcium chloride, and hence the concentration of oxide
ions, may disadvantageously decrease if the effective potential
across the electrolyte, i.e. the cell voltage minus external losses
and electrode polarisation, were to reach or exceed the
decomposition potential of the dissolved calcium oxide. Assuming
that the cathode is essentially inert, this potential as measured
against a carbon reference electrode may be calculated. For an
electrolyte temperature of 900 C, unit activities of the chemical
components, and under the assumption that CO is formed at the
reference electrode, the calculated decomposition potential is
-1541 mV and becomes more negative by 116 mV for each order of
magnitude by which the calcium oxide concentration falls. (As noted
above, the inventors' experiments indicate that significant
polarisation occurs at a working carbon anode during
electro-decomposition and that the effective potential between the
cathode and the anode is typically at least about 1 V higher than
the potential between the cathode and the reference electrode. Of
course, the applied cell voltage required to achieve this effective
potential would be higher still, to account for IR losses in the
cell.) During electro-decomposition, the inventors believe that
this decomposition potential, as measured between the cathode and
the carbon reference electrode, should advantageously not be
exceeded because if such a high voltage is applied then calcium
oxide decomposition may reduce the oxide concentration in the
electrolyte and the rate of electro-deoxidation may
disadvantageously be reduced by any resulting oxide-ion transport
limitation in the electrolyte. This is expected to be the case even
though the driving force for electro-deoxidation may be high
(corresponding to the high applied voltage) and even if there is a
considerable amount of oxygen left in the cathode. In effect, it
may be proposed that calcium oxide electrolysis (decomposition) and
electro-deoxidation are opposing processes.
[0095] It is, however, believed that some discharge of calcium ions
from the electrolyte may occur at the cathode even at voltages
below the decomposition voltage for any of the salts in the
electrolyte. Calcium, in the form of Ca.sup.0 or Ca.sup.1+, has
some solubility in calcium chloride. Consequently, when a voltage
below the calcium oxide decomposition voltage is applied to a cell,
the inventors believe that some reaction of calcium ions may occur
at the cathode, converting Ca.sup.2+ to Ca.sup.1+ or Ca.sup.0 in
solution in the electrolyte until a corresponding activity for the
dissolved calcium species in the melt is reached. It is important
to note, however, that this is not a continuous process and
therefore does not constitute decomposition of the calcium oxide.
It is a self-limiting process which only proceeds until a
calcium-activity level corresponding to the applied cathode
potential is reached in the salt, and then stops. This process does
not involve continuous discharge or deposition of calcium. However,
the generation of Ca.sup.0 or Ca.sup.1+ in solution in the melt may
have an important impact on electro-decomposition in that these
species are understood to increase the electronic conductivity of
the electrolyte. Clearly, this is a disadvantageous effect, as any
electronic conduction does not contribute to the
electro-decomposition process and reduces the electrical efficiency
of the process. Consequently, the inventors believe that it may be
desirable to operate the electro-decomposition process at as low a
voltage as possible in order to reduce or limit the concentration
of Ca.sup.0 or Ca.sup.+ in the electrolyte. An adequate voltage is
required to achieve electro-decomposition, as illustrated by the
examples discussed above, but excessive voltages may also
advantageously be avoided.
[0096] Application of Background Understanding
[0097] Following this explanation of the inventors' understanding
of the mechanisms involved in electro-decomposition, it is possible
to apply these concepts to the various aspects of the
invention.
[0098] First, the ramp rate, or rate of increase of the voltage
initially applied during electro-decomposition, should preferably
be selected so as to encourage an even progression of the reaction
throughout the bulk of the metal compound at the cathode, and so as
to avoid the initial stages of the reaction progressing too quickly
to allow dissolution or dispersal of the substance removed from the
metal compound in the electrolyte. Otherwise, the reaction may be
expected to proceed most rapidly in its early stages, when the
maximum amount of the substance is present in the solid
compound.
[0099] In the embodiments described above, ramp rates of 0.087 mV/s
and faster and slower ramp rates (including 0.174 and 0.044 mV/s)
were used, although 0.087 mV/s was found to be more effective in
accelerating electro-decomposition of TiO.sub.2 under the
conditions in the embodiments. However, in view of the discussion
above the skilled person would appreciate that the optimum rate of
voltage increase is likely to depend on factors such as the rate at
which the removed substance can dissolve in the electrolyte and be
transported away from the cathode, which is a function of the
materials involved and the cathode geometry (in that a small,
highly-porous cathode would allow more rapid transport) and would
involve consideration of the reactions involved at the cathode,
which may vary for different metal compounds and different
electrolytes. The skilled person may thus apply the teaching in the
present patent application to different electro-decomposition
processes and parameters, including for example the
electro-reduction of different materials or different structures
(e.g. using feed materials of different dimensions and shapes,
porosity and particle size), the use of different electrolytes at
different temperatures, and the design of different reactor
geometries, without inventive input.
[0100] The starting level for the increasing voltage may
advantageously be selected depending on the thermodynamics of the
reaction at the cathode. The increase in voltage may start from 0 V
but if no reaction can occur at the cathode until a higher voltage
level (because a certain minimum applied voltage, or a
corresponding minimum cathode potential, is required to drive the
electro-decomposition reaction) then any voltage or potential below
that minimum voltage or potential may be selected as the initial
level for the increasing voltage or potential.
[0101] The maximum voltage level of the increasing voltage may
advantageously be selected so as to maximise the driving force for
electro-decomposition while not exceeding a level at which
excessive consumption of or damage to the electrolyte may occur. In
addition, if the substance removed from the solid compound has low
solubility in the electrolyte, it may be important not to drive the
electro-decomposition reaction too rapidly, as this may cause
precipitation of the substance or a material comprising the
substance at the cathode. In that case, the maximum voltage level
should be selected so as to apply a reduced driving force for the
reaction, so as to avoid the rate of reaction being limited by
kinetic effects such as the rate of dissolution of the
substance.
[0102] Second, the voltage level selected for continuous
application during electro-decomposition, either from the start of
the reaction or following a ramped, increasing voltage, may
similarly be selected so as to drive the reaction as rapidly as
possible without excessively or unnecessarily consuming or damaging
the electrolyte and without introducing kinetic limitations such as
may be caused by the precipitation of material at the cathode.
[0103] Third, it may be desirable for the electrolyte to contain a
sufficient concentration of anions of the same species as the
substance to be removed from the solid compound, if it is envisaged
that that substance will be transported through the electrolyte
from the cathode for discharge at the anode. If the electrolyte
comprises a mixture of salts in which the salt containing this
anion species is present in relatively low concentration, then it
may be desirable to maintain a sufficient concentration of this
salt at all times during electro-decomposition in order to avoid
introducing anion-transport limitations. If, as in the case of the
reduction of titanium dioxide in an electrolyte of calcium chloride
and calcium oxide, a reaction between the salt and the solid
compound occurs, then this should be taken into account when
selecting the quantity of the metal compound and the quantity of
the corresponding salt in an electro-decomposition reactor. A
sufficient quantity of the electrolyte should then preferably be
provided to keep the corresponding salt in solution.
[0104] Fourth, it may be important to monitor the cathode potential
accurately, particularly in an experimental situation where the
full details of the reaction may not be understood, for example by
using a reference electrode or pseudo-reference electrode as
described above. Otherwise, polarization effects at the anode,
which may be very significant, may not be accurately accounted
for.
[0105] Although these aspects of the invention and the factors
involved in enhancing the electro-decomposition process have been
exemplified with reference to the reduction of titanium dioxide in
a mixture of calcium chloride and calcium oxide, the various
aspects of the invention may equally be applicable to other metal
compounds and other electrolytes, and other parameters of the
process such as the operating temperature may be varied, as the
skilled person would appreciate. For example, other salts may be
used as the electrolyte, including mixtures of salts such as
eutectic mixtures, and including mixtures of two or more salts
which may comprise the same or different cations and/or anions. The
use of different electrolytes would typically change the
electrolyte or salt decomposition potential or potentials, and so
may alter parameters such as the preferred range of the applied
voltage during electro-decomposition, but such modifications to the
embodiments described herein could easily be made by the skilled
person, using his common general knowledge in the field.
* * * * *