U.S. patent application number 13/001563 was filed with the patent office on 2011-05-05 for method of determining the extent of a metal oxide reduction.
This patent application is currently assigned to UNIVERSITY OF LEEDS. Invention is credited to Animesh Jha.
Application Number | 20110100831 13/001563 |
Document ID | / |
Family ID | 39707900 |
Filed Date | 2011-05-05 |
United States Patent
Application |
20110100831 |
Kind Code |
A1 |
Jha; Animesh |
May 5, 2011 |
METHOD OF DETERMINING THE EXTENT OF A METAL OXIDE REDUCTION
Abstract
The present invention relates to a method for determining the
extent of electrochemical extraction of a metal (M) from a metal
(M) oxide caused by a voltage applied between a cathode comprising
(or consisting essentially of) or in contact with the metal (M)
oxide and an inert metal alloy anodein an oxygen-dissolving molten
electrolyte.
Inventors: |
Jha; Animesh; (Yorkshire,
GB) |
Assignee: |
UNIVERSITY OF LEEDS
Leeds, Yorkshire
GB
|
Family ID: |
39707900 |
Appl. No.: |
13/001563 |
Filed: |
June 26, 2009 |
PCT Filed: |
June 26, 2009 |
PCT NO: |
PCT/GB2009/050739 |
371 Date: |
December 27, 2010 |
Current U.S.
Class: |
205/336 |
Current CPC
Class: |
C25C 3/32 20130101; C25C
3/00 20130101; C25C 3/34 20130101; C25C 3/26 20130101 |
Class at
Publication: |
205/336 |
International
Class: |
C25C 3/00 20060101
C25C003/00; C25C 3/28 20060101 C25C003/28; C25C 3/26 20060101
C25C003/26; C25C 3/34 20060101 C25C003/34 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 2, 2008 |
GB |
0812102.2 |
Claims
1. A method for determining the extent of electrochemical
extraction of a metal (M) from a metal (M) oxide caused by a
voltage applied between a cathode comprising or in contact with the
metal (M) oxide and an inert metal alloy anode in an
oxygen-dissolving molten electrolyte, the method comprising: (a)
measuring the current flow between the cathode and the inert metal
alloy anode over a temporal range; (b) relating a characteristic of
the current flow between the cathode and the inert metal alloy
anode over the temporal range to the extent of electrochemical
extraction of the metal (M) from the metal (M) oxide.
2. A method as claimed in claim 1 wherein the current flow between
the cathode and the inert metal alloy anode over the temporal range
includes a point of inflection.
3. A method, as claimed in claim 1 wherein in step (b), the
characteristic of the current flow between the cathode and the
inert metal alloy anode over the temporal range is a quantitative
characteristic.
4. A method as claimed in claim 3 wherein the quantitative
characteristic is beyond a point of inflection of the current flow
over the temporal range.
5. A method as claimed in claim 3 wherein the quantitative
characteristic of the current flow beyond a point of inflection of
the current flow over the temporal range is the measured current
and step (b) is: relating the measured current to the rate of
extraction of the metal (M) from the metal (M) oxide.
6. A method as claimed in claim 4 wherein the quantitative
characteristic is a threshold current beyond a point of inflection
of the current flow over the temporal range.
7. A method as claimed in claim 1 wherein step (b) comprises:
relating the characteristic of the current flow between the cathode
and the inert metal alloy anode over the temporal range to an
extent of extraction beyond which is the onset of the formation of
undesirable by-products or the onset of corrosive conditions.
8. A method as claimed in claim 1 wherein step (b) comprises:
relating the characteristic of the current flow between the cathode
and the inert metal alloy anode over the temporal range to a target
rate of extraction (%) of metal (M) from metal (M) oxide or a
target oxygen content of metal (M).
9. A method as claimed in claim 1 wherein the temporal range is
less than 8 hours.
10. A method as claimed in claim 1 wherein the electrochemical
extraction is carried out in the presence of an alkali metal
(M.sup.a) oxide.
11. A method as claimed in claim 1 wherein the metal (NI) is one or
more metals selected from the group consisting of Ti, Nb, Ta, U,
Th, Cr, Fe, steel and Zr.
12. A method as claimed in claim 1 wherein the metal (M) is Ti.
13. A method as claimed in claim 1 wherein the metal (M) oxide
optionally in admixture with an alkali metal (M.sup.a) oxide or an
alkali metal (M.sup.a) salt decomposable into the alkali metal
(M.sup.a) oxide is the cathode.
14. A method as claimed in claim 1 wherein the molten electrolyte
contains CaCl.sub.2.
15. A method as claimed in claim 1 wherein the anode is composed of
an Al-E-Cu based alloy comprising an intermetallic phase of
formula: wherein: E denotes one or more metallic elements; x is an
integer in the range 1 to 5; y is an integer being 1 or 2; and z is
an integer being 1 or 2.
16. A method as claimed in claim 15 wherein E is one or more
metallic elements selected from the group consisting of Ru, Zr, Cr,
Nb, V, Co, Ta, Fe, Ni, La and Mn.
17. A method as claimed in claim 1 wherein the metal (A) oxide is
TiO.sub.2, the molten electrolyte contains CaCl.sub.2 and the
electrochemical extraction is carried out in the presence of an
alkali metal (M.sup.a) oxide.
18. The method of claim 1 wherein the cathode consists essentially
of a metal (M) oxide or is in contact with the metal (M) oxide.
Description
[0001] The present invention relates to a method for determining
the extent of electrochemical extraction of a metal (M) from a
metal (M) oxide caused by a voltage applied between a cathode
comprising (or consisting essentially of) or in contact with the
metal (M) oxide and an inert metal alloy anode in an
oxygen-dissolving molten electrolyte.
[0002] The extent to which electrochemical extraction of a metal
(M) from a metal (M) oxide in an oxygen-dissolving molten
electrolyte extraction has occurred is frequently measured after
the completion of the reaction. This may be by (for example)
microstructural analysis (eg X-ray diffraction) or a weight loss
technique. Neither of these techniques is equipped to provide an
instant assessment of the extent to which extraction has occurred
whilst extraction is ongoing.
[0003] The conditions under which is carried out the
electrochemical extraction of a metal (M) from a metal (M) oxide in
an oxygen-dissolving molten electrolyte are harsh. Furthermore the
extraction process may lead to the formation of aggressive or
corrosive by-products typically under highly reducing conditions
near to completion of the extraction process. These factors may
preclude the use in situ of sensitive measuring equipment.
[0004] The present invention is based on the recognition that a
current vs time plot usefully and reproducably characteristises the
progress of an electrochemical extraction in an oxygen-dissolving
molten electrolyte. In particular, the current vs time plot may be
used to determine instantly and accurately the extent of extraction
whilst extraction is ongoing.
[0005] Viewed from a first aspect the present invention provides a
method for determining the extent of electrochemical extraction of
a metal (M) from a metal (M) oxide caused by a voltage applied
between a cathode comprising (or consisting essentially of) or in
contact with the metal (M) oxide and an inert metal alloy anode in
an oxygen-dissolving molten electrolyte, the method comprising:
[0006] (a) measuring the current flow between the cathode and the
inert metal alloy anode over a temporal range;
[0007] (b) relating a characteristic of the current flow between
the cathode and the inert metal alloy anode over the temporal range
to the extent of electrochemical extraction of the metal (M) from
the metal (M) oxide.
[0008] By allowing the extent of electrochemical extraction to be
determined instantly, the present invention makes it possible to
exercise hitherto unattainable levels of control over
electrochemical extraction in a molten oxygen-dissolving
electrolyte.
[0009] Step (a) may be carried out discretely at intervals or
continuously (eg to produce a current vs time plot). Preferably
step (a) is carried out continuously.
[0010] Preferably the current flow between the cathode and the
inert metal alloy anode over the temporal range includes a point of
inflection. The point of inflection is followed by a steep rise in
current promoted by the use of the inert alloy anode which is
usefully exploited in quantitative measurements.
[0011] Preferably the current flow between the cathode and the
inert metal alloy anode over the temporal range is substantially as
illustrated in FIG. 2 hereinafter.
[0012] In step (b), the characteristic of the current flow between
the cathode and the inert metal alloy anode over the temporal range
may be a qualitative characteristic.
[0013] The qualitative characteristic of the current flow between
the cathode and the inert metal alloy anode over the temporal range
may be a point of inflection. The qualitative characteristic of the
current flow between the cathode and the inert metal alloy anode
over the temporal range may be a current flow beyond a point of
inflection (eg by a predetermined amount). The qualitative
characteristic of the current flow between the cathode and the
inert metal alloy anode over the temporal range may be that the
current flow is increasing (eg by a predetermined amount).
[0014] In step (b), the characteristic of the current flow between
the cathode and the inert metal alloy anode over the temporal range
may be a quantitative characteristic.
[0015] Preferably the quantitative characteristic is beyond a point
of inflection of the current flow over the temporal range.
[0016] The use of a quantitative characteristic of the current flow
beyond the point of inflection advantageously exploits a steep rise
in current promoted by the use of the inert alloy anode. The
quantitative characteristic is highly time-dependent beyond the
point of inflection and accurate measurement of the current flow
leads to precise determination of the extent of electrochemical
extraction of the metal (M) from the metal (M) oxide. For example,
a rate of extraction (%) of metal (M) from metal (M) oxide may be
determined instantly and with high precision.
[0017] Preferably in step (b) the quantitative characteristic of
the current flow is the measured current beyond a point of
inflection of the current flow over the temporal range.
Particularly preferably step (b) is: relating the measured current
to the rate of extraction (%) of the metal (M) from the metal (M)
oxide. Alternatively step (b) may be: relating the measured current
to the oxygen content of metal (M).
[0018] Preferably the quantitative characteristic of the current
flow is a threshold current beyond a point of inflection of the
current flow over the temporal range.
[0019] Particularly preferably the method further comprises:
[0020] (c) ceasing electrochemical reduction in response to the
attainment of the threshold current.
[0021] Particularly preferably step (b) comprises: relating the
threshold current to an extent of extraction beyond which is the
onset of the formation of undesirable by-products (eg gaseous
by-products which may be noxious or environmentally undesirable
such as chlorine).
[0022] Particularly preferably step (b) comprises: relating the
threshold current to an extent of extraction beyond which is the
onset of corrosive conditions (eg anode corrosive conditions).
[0023] Particularly preferably step (b) comprises: relating the
threshold current to a target rate of extraction (%) of metal (M)
from metal (M) oxide.
[0024] The target rate of extraction is typically 99% or more,
preferably 99.5% or more.
[0025] Particularly preferably step (b) comprises: relating the
threshold current to a target oxygen content of metal (M). The
target oxygen content is typically less than 2500 ppm O.sub.2 by
weight of metal (M), preferably less than 1500 ppm O.sub.2 by
weight of metal (M).
[0026] In step (b), the characteristic of the current flow between
the cathode and the inert metal alloy anode over the temporal range
may be related to the extent of electrochemical extraction of the
metal (M) from the metal (M) oxide qualitatively.
[0027] The qualitative extent of electrochemical extraction of the
metal (M) from the metal (M) oxide may be an extent of extraction
beyond which is the onset of the formation of undesirable
by-products or the onset of corrosive conditions.
[0028] In step (b), the characteristic of the current flow between
the cathode and the inert metal alloy anode over the temporal range
may be related to the extent of electrochemical extraction of the
metal (M) from the metal (M) oxide quantitatively.
[0029] The quantitative extent of electrochemical extraction of the
metal (M) from the metal (M) oxide may be a rate of extraction (%)
of metal (M) from metal (M) oxide, the oxygen content of metal (M),
a target rate of extraction (%) of metal (M) from metal (M) oxide
or a target oxygen content of metal (M).
[0030] In a preferred embodiment, the metal (M) oxide is TiO.sub.2,
the molten electrolyte contains CaCl.sub.2 and the electrochemical
extraction is carried out in the presence of an alkali metal
(M.sup.a) oxide.
[0031] The method of the invention is carried out at an elevated
temperature typically in the range 600-1000.degree. C., preferably
850-1000.degree. C. (eg about 900.degree. C.).
[0032] In the method of the invention, the temporal range may be
less than 20 hours, preferably less than 10 hours (eg 8 hours),
particularly preferably less than 4 hours.
[0033] The voltage is typically less than the discharge potential
of metals in the molten electrolyte. For example, the voltage may
be less than 3.5V (eg about 3.1V).
[0034] In a preferred embodiment, the method of the invention is
carried out in an oxygen deficient atmosphere (eg an inert
atmosphere such as argon).
[0035] The electrochemical cell of which the cathode and one or
more inert metal alloy anodes are a part may be calibrated
straightforwardly to obtain a relationship between a characteristic
of the current flow between the cathode and the inert metal alloy
anode and the extent of electrochemical extraction of the metal (M)
from the metal (M) oxide. For calibration purposes, the extent of
electrochemical extraction may be measured by conventional
techniques such as microstructural analysis or by measuring weight
loss.
[0036] In a preferred embodiment, the electrochemical extraction is
carried out in the presence of an alkali metal (M.sup.a) oxide. The
alkali metal (M.sup.a) oxide may be a caesium, rubidium, lithium,
sodium or potassium oxide. Preferably the alkali metal (M.sup.a)
oxide is lithium, sodium or potassium oxide. Particularly
preferably the alkali metal (M.sup.a) oxide is potassium oxide.
[0037] The alkali metal (M.sup.a) oxide may be an additive or may
be formed in situ by decomposition of a decomposable alkali metal
(M.sup.a) salt into the alkali metal (M.sup.a) oxide. Preferably
the alkali metal (M.sup.a) oxide forms the alkali metal (M.sup.a)
metallate (M) phase from a reaction of the alkali metal (M.sup.a)
oxide with a metal (M'') metallate (M) phase. Particularly
preferably the metal (M'') metallate (M) phase is a solid phase.
Particularly preferably the metal (M'') metallate (M) phase is a
perovskite (or perovskite-type) phase. Preferably M'' is an
alkaline earth metal, particularly preferably Ca, Sr or Ba, most
preferably Ca.
[0038] Preferably the diffusivity of oxygen in the alkali metal
(M.sup.a) metallate (M) phase is higher than the diffusivity of
oxygen in the metal (M'') metallate (M) phase.
[0039] Preferably the alkali metal (M.sup.a) metallate (M) phase is
a liquid. Preferably the alkali metal (M.sup.a) metallate (M) phase
is a transitional phase.
[0040] In a preferred embodiment, the alkali metal (M.sup.a) oxide
is an additive. Preferably the alkali metal (M.sup.a) oxide is in
admixture with the metal (M) oxide in (or in contact with) the
cathode.
[0041] The alkali metal (M.sup.a) oxide and metal (M) oxide may
form a self-supporting mixture (eg a pellet, slab, sheet, wire,
foil, basket or tube). The self-supporting mixture may be the
cathode or may be contactable with the cathode.
[0042] The alkali metal (M.sup.a) oxide may be present in the
self-supporting mixture in an amount in excess of a trace amount,
preferably in excess of 5 wt %, particularly preferably in excess
of 10 wt %, more preferably in excess of 20 wt %. Preferably the
alkali metal (M.sup.a) oxide is present in the self-supporting
mixture in an amount in the range 10-70 wt %, particularly
preferably 20-50 wt %.
[0043] In a preferred embodiment, the alkali metal (M.sup.a) oxide
is formed in situ by decomposition of a decomposable alkali metal
(M.sup.a) salt. The decomposable alkali metal (M.sup.a) salt may be
thermally decomposable.
[0044] Preferably the decomposable alkali metal (M.sup.a) salt is
in admixture with the metal (M) oxide in (or in contact with) the
cathode. Particularly preferably the mixture of decomposable alkali
metal (M.sup.a) salt and metal (M) oxide is a self-supporting
mixture (eg a pellet, slab, sheet, wire, basket, foil or tube).
[0045] The decomposable alkali metal (M.sup.a) salt may be present
in the self-supporting mixture in an amount in excess of a trace
amount, preferably in excess of 5 wt %, particularly preferably in
excess of 10 wt %, more preferably in excess of 20 wt %. Preferably
the decomposable alkali metal (M.sup.a) salt is present in the
self-supporting mixture in an amount in the range 10-70 wt %,
particularly preferably 20-50 wt %.
[0046] Preferably the decomposable alkali metal (M.sup.a) salt is
decomposable into one or more gaseous species. The gaseous species
may be selected from the group consisting of water and carbon
dioxide. Decomposition of the alkali metal (M.sup.a) salt into one
or more gaseous species may advantageously promote electrochemical
reduction by forming porosity within the cathode. Continuous
formation of pores permits fast transport of molten electrolyte
species (eg CaO and CaCl.sub.2) which accelerates chemical
reduction.
[0047] The decomposable alkali metal (M.sup.a) salt may be an
alkali metal (M.sup.a) halide, carbonate, bicarbonate, hydrogen
sulphide, hydrogen sulphate, nitrate, chlorate or sulphate.
Preferably the decomposable alkali metal (M.sup.a) salt is an
alkali metal (M.sup.a) bicarbonate.
[0048] The decomposable alkali metal (M.sup.a) salt may be a
caesium, rubidium, lithium, sodium or potassium salt. Preferably
the decomposable alkali metal (M.sup.a) salt is a lithium, sodium
or potassium salt. Particularly preferably the decomposable alkali
metal (M.sup.a) salt is a potassium salt.
[0049] In a preferred embodiment, the decomposable alkali metal
(M.sup.a) salt may be present with an amount of endogenous
hydroxide ions.
[0050] In a preferred embodiment, the decomposable alkali metal
(M.sup.a) salt may be present with an amount of exogenous hydroxide
ions. Preferably the exogenous hydroxide ions are provided by an
alkaline additive. The alkaline additive may be an alkali metal
hydroxide (such as lithium, sodium or potassium hydroxide), an
alkali metal hydride (such as lithium, sodium or potassium hydride)
or an alkaline earth metal hydroxide. The alkaline additive may be
added to the oxygen-dissolving molten electrolyte.
[0051] The metal (M) may be a reactive metal element, semi-metal
element, metal alloy or metalloid element.
[0052] In a preferred embodiment, the metal (M) forms a solid
perovskite (or perovskite-type) phase in the molten electrolyte.
The solid perovskite phase may be an alkaline earth metal (eg Ca)
metallate (M) phase.
[0053] The metal (M) may be one or more metals selected from the
group consisting of group HA metals, group IIIA metals, group IVA
metals, group B transition metals, rare earth metals and alloys
thereof. Preferably the metal (M) is one or more metals selected
from the group consisting of Mg, Al, Si, Ge, group IVB transition
metals, group VB transition metals, group VIB transition metals,
group VIIB transition metals, group VIIIB transition metals,
lanthanides, actinides and alloys thereof. Particularly preferably
the metal (M) is one or more metals selected from the group
consisting of group IVB transition metals, group VB transition
metals, group VIB transition metals, group VIIIB transition metals,
actinides and alloys thereof. Especially preferably the metal (M)
is one or more metals selected from the group consisting of Ti, Nb,
Ta, U, Th, Cr, Fe, steel and Zr. More especially preferred is one
or more metals selected from the group consisting of Ti, Nb, Ta and
Zr. Most preferred is Ti.
[0054] During electrochemical extraction, Ti advantageously forms
sub-oxides (eg Magneli phases, TiO and Ti metal) which contribute
to a sharp rise in current beyond a point of inflection.
[0055] The alkali metal (M.sup.a) metallate (M) phase may be
M.sup.a.sub.2MO.sub.3 or M.sup.a.sub.4MO.sub.4. Preferred is
M.sup.a.sub.4MO.sub.4. For example, where M is titanium, the
preferred phase is M.sup.a.sub.4TiO.sub.4.
[0056] The metal (M) oxide may be the cathode or the metal (M)
oxide in admixture with either the alkali metal (M.sup.a) oxide or
the alkali metal (M.sup.a) salt decomposable into the alkali metal
(M.sup.a) oxide may be the cathode. Preferably the metal (M) oxide
in admixture with either the alkali metal (M.sup.a) oxide or the
alkali metal (M.sup.a) salt decomposable into the alkali metal
(M.sup.a) oxide is the cathode.
[0057] Alternatively the metal (M) oxide may be in contact with a
cathode. In this embodiment, the metal (M) oxide may be in
admixture with the alkali metal (M.sup.a) oxide or the alkali metal
(M.sup.a) salt decomposable into the alkali metal (M.sup.a) oxide.
Alternatively the metal (M) oxide may be in the electrolyte in
contact with the cathode. The alkali metal (M.sup.a) oxide or the
alkali metal (M.sup.a) salt decomposable into the alkali metal
(M.sup.a) oxide may be in the electrolyte in contact with the
cathode. The cathode may be a metal substrate such as steel which
may be in the form of a cathodic bath, crucible or basket.
[0058] The oxygen-dissolving molten electrolyte may be (or contain)
a compound of an alkali metal (eg Li, K or Cs), alkaline earth
metal (eg Mg, Ca, Sr or Ba), Zn, Al or Y (or a mixture thereof).
Preferably the oxygen-dissolving molten electrolyte contains a
compound of Ca.
[0059] The oxygen-dissolving molten electrolyte may be (or contain)
a hydrogen phosphate, dihydrogen phosphate or halide. Preferred is
a halide (eg a chloride or fluoride), particularly preferably a
chloride. The oxygen-dissolving molten electrolyte may be
CaCl.sub.2-containing or cryolite.
[0060] Preferably the molten electrolyte contains (eg consists
essentially of) CaCl.sub.2. Particularly preferably the molten
electrolyte contains CaCl.sub.2 and an alkali metal halide
(preferably a chloride). Preferred is a mixture of CaCl.sub.2 and
KCl or of CaCl.sub.2 and LiCl.
[0061] Preferred is an inert metal alloy anode which is
substantially unreactive with oxygen. Preferred is an inert metal
alloy anode which is substantially insoluble in the molten
electrolyte.
[0062] Preferably the anode is composed of an Al-E-Cu based alloy
comprising an intermetallic phase of formula:
Al.sub.xE.sub.yCu.sub.z
wherein:
[0063] E denotes one or more metallic elements;
[0064] x is an integer in the range 1 to 5;
[0065] y is an integer being 1 or 2; and
[0066] z is an integer being 1 or 2.
[0067] The Al-E-Cu based alloy may be substantially monophasic or
multiphasic. Preferably the intermetallic phase is present in the
Al-E-Cu based alloy in an amount of 50 wt % or more (eg in the
range 50 to 99 wt %). Preferably the Al-E-Cu based alloy further
comprises an ordered high-temperature intermetallic phase of E with
aluminium, particularly preferably Al.sub.3E. Other intermetallic
phases may be present.
[0068] In a preferred embodiment, the Al-E-Cu based alloy is
substantially free of CuAl.sub.2. This is advantageous because
CuAl.sub.2 has a tendency to melt at the elevated temperatures
which are deployed typically in the method of the invention.
Preferably CuAl.sub.2 is complexed.
[0069] In a preferred embodiment, the Al-E-Cu based alloy falls
other than on the E poor side of the tie line joining Al.sub.3E and
ECu.sub.4 (eg on the E rich side of the tie line joining Al.sub.3E
and ECu.sub.4).
[0070] In a preferred embodiment, the Al-E-Cu based alloy comprises
an intermetallic phase falling on or near to the tie line joining
Al.sub.3E and ECu.sub.4.
[0071] In a preferred embodiment, the Al-E-Cu based alloy falls
other than on the E poor side of the tie line joining Al.sub.3E and
AlECu.sub.2 (eg on the E rich side of the tie line joining
Al.sub.3E and AlECu.sub.2).
[0072] In a preferred embodiment, the Al-E-Cu based alloy comprises
an intermetallic phase falling on or near to the tie line joining
Al.sub.3E and AlECu.sub.2.
[0073] In a preferred embodiment, the Al-E-Cu based alloy falls
other than on the E poor side of the .zeta., Al.sub.5E.sub.2Cu,
EAlCu.sub.2 and .beta.-ECu.sub.4 phase tie line (wherein E is a
phase falling between Al.sub.3Ti and Al.sub.2Ti with 3 at % or less
of Cu (eg 2-3 at % Cu)).
[0074] In a preferred embodiment, the Al-E-Cu based alloy comprises
an intermetallic phase falling on or near to the Al.sub.5E.sub.2Cu,
EAlCu.sub.2 and .beta.-ECu.sub.4 phase tie line.
[0075] Preferably the intermetallic phase is Al.sub.5E.sub.2Cu.
Particularly preferably the Al-E-Cu based alloy further comprises
Al.sub.3E.
[0076] Preferably the intermetallic phase is EAlCu.sub.2.
Particularly preferably the Al-E-Cu based alloy further comprises
.beta.-ECu.sub.4.
[0077] The anode may be composed of a homogenous, partially
homogenous or non-homogeneous Al-E-Cu based alloy.
[0078] Typically E has a potential in the anode which is lower than
it would be in the molten electrode.
[0079] In a preferred embodiment, the anode develops a passivating
layer. Preferably the passivating layer withstands oxidation in
anodic conditions.
[0080] In a preferred embodiment, E is a single metallic element.
The single metallic element is preferably Ti.
[0081] In an alternative preferred embodiment, E is a plurality (eg
two, three, four, five, six or seven) of metallic elements. In this
embodiment, a first metallic element is preferably Ti. Typically
the first metallic element of the plurality of metallic elements is
present in a substantially higher amount than the other metallic
elements of the plurality of metallic elements. Each of the other
metallic elements may be present in a trace amount. Each of the
other metallic elements may be a dopant. Each of the other metallic
elements may substitute Al, Cu or the first metallic element. The
presence of the other metallic elements may improve the
high-temperature stability of the alloy (eg from 1200.degree. C. to
1400.degree. C.).
[0082] In a preferred embodiment, E is a pair of metallic elements.
In this embodiment, a first metallic element is preferably Ti.
Typically the first metallic element of the pair of metallic
elements is present in a substantially higher amount than a second
metallic element of the pair of metallic elements (eg in a weight
ratio of about 9:1). The second metallic element may be present in
a trace amount. The second metallic element may be a dopant. The
second metallic element may substitute Al, Cu or the first metallic
element. The presence of a second metallic element may improve the
high-temperature stability of the alloy (eg from 1200.degree. C. to
1400.degree. C.).
[0083] Preferably the pair of metallic elements has similar atomic
radii. Preferably the atomic radius of the second metallic element
is similar to the atomic radius of Cu. Preferably the atomic radius
of the second metallic element is similar to the atomic radius of
Al.
[0084] In a preferred embodiment, E is one or more of the group
consisting of group B transition metal elements (eg first row group
B transition metal elements) and lanthanide elements. Preferably E
is one or more group IVB, VB, VIIB, VIIB or VIIIB transition metal
elements, particularly preferably one or more group IVB, VIIB or
VIIIB transition metal elements.
[0085] In a preferred embodiment, E is one or more metallic
elements of valency II, III, IV or V, preferably II, III or IV.
[0086] In a preferred embodiment, E is one or more metallic
elements selected from the group consisting of Ru, Ti, Zr, Cr, Nb,
V, Co, Ta, Fe, Ni, La and Mn. In a particularly preferred
embodiment, E is one or more metallic elements selected from the
group consisting of Ti, Fe, Cr and Ni.
[0087] Preferably E is or includes a metallic element capable of
reducing the tendency of CuAl.sub.2 towards grain boundary
segregation at an elevated temperature. In this embodiment, the
metallic element capable of reducing the tendency of CuAl.sub.2
towards grain boundary segregation at an elevated temperature may
be the second metallic element of a plurality (eg a pair) of
metallic elements. Particularly preferably E is or includes a
metallic element capable of forming a complex with CuAl.sub.2.
Preferred metallic elements for this purpose are selected from the
group consisting of Fe, Ni and Cr, particularly preferably Ni and
Fe, especially preferably Ni.
[0088] Preferably E is or includes a metallic element capable of
reducing the tendency of the first metallic element or Cu to
dissolve in molten extractant. In this embodiment, the metallic
element may be the second metallic element of a plurality (eg a
pair) of metallic elements. Preferred metallic elements for this
purpose are selected from the group consisting of Fe, Ni, Co, Mn
and Cr, particularly preferably the group consisting of Fe and Ni
(optionally together with Cr).
[0089] Preferably E is or includes a metallic element capable of
promoting the passivation of the surface of the anode in the
presence of a molten electrolyte. For this purpose, the metallic
element may form or stabilise an oxide film. In this embodiment,
the metallic element may be the second metallic element of a
plurality (eg a pair) of metallic elements. Preferred metallic
elements for this purpose are selected from the group consisting of
Ru, Fe, Ni and Cr. Particularly preferably E is Ti, Fe, Ni and Cr
in which the formation of a combination of oxides such as iron
oxides, chromium oxides, nickel oxides and alumina advantageously
promotes passivation.
[0090] Preferably E is or includes a metallic element selected from
the group consisting of Zr, Nb and V. Particularly preferred is V
or Nb. These second metallic elements are advantageously strong
intermetallic formers. In this embodiment, the metallic element is
the second metallic element of a plurality (eg a pair) of metallic
elements.
[0091] Preferably E is or includes a metallic element capable of
forming an ordered high-temperature intermetallic phase with
aluminium metal. Particularly preferably E is or includes a
metallic element capable of forming Al.sub.3E.
[0092] Preferably E is or includes Ti. A titanium containing alloy
typically has electrical resistivity in the range 3 to 15 .mu.ohm
cm at room temperature.
[0093] Preferably the intermetallic phase is Al.sub.5Ti.sub.2Cu.
Particularly preferably the Al--Ti--Cu based alloy further
comprises Al.sub.3Ti.
[0094] Preferably the intermetallic phase is TiAlCu.sub.2.
Particularly preferably the Al--Ti--Cu based alloy further
comprises .beta.-TiCu.sub.4.
[0095] In a preferred embodiment, E is or includes Ti and a second
metallic element selected from the group consisting of Fe, Cr, Ni,
V, La, Nb and Zr, preferably the group consisting of Fe, Cr and Ni.
The second metallic element advantageously serves to enhance
high-temperature stability of the Al--Ti--Cu phases.
[0096] The anode may be composed of an Al-E-Cu based alloy
obtainable by processing a mixture of 35 atomic % Al or more
(preferably 50 atomic % Al or more), 35 atomic % E or more (wherein
E is a first metallic element as hereinbefore defined) and a
balance of Cu and optionally E' (wherein E' is one or more of the
additional metallic elements hereinbefore defined).
[0097] In a preferred embodiment, the anode is composed of an
Al-E-Cu based alloy obtainable by processing a mixture of (65+x)
atomic % Al, (20+y) atomic % E (wherein E is a first metallic
element as hereinbefore defined) and (15-x-y) atomic % Cu,
optionally together with z atomic % of E' (wherein E' is one or
more of the additional metallic elements hereinbefore defined)
wherein E' substitutes Cu, Al or E.
[0098] In this embodiment, the alloy may be obtainable by casting,
preferably in an oxygen deficient atmosphere (eg an inert
atmosphere). For example, a mixture may be melted in an argon-arc
furnace under an atmosphere of argon gas and then solidified in an
argon atmosphere. Alternatively in this embodiment, the alloy may
be obtainable by flux-assisted melting, vacuum arc or vacuum
melting using a resistance furnace. Contamination by O, C, N, S or
P should be minimised.
[0099] In a preferred embodiment, the anode is at least as
conducting at elevated temperature (eg at 900.degree. C.) as a
carbon electrode. Preferably the anode is more conducting at
elevated temperature (eg at 900.degree. C.) than a carbon
electrode.
[0100] It has been recognised that the dissociation of molten
CaCl.sub.2 electrolyte into chlorine substantially coincides with
the attainment of a desirable level of metal (M) extraction from a
metal (M) oxide.
[0101] Viewed from a further aspect the present invention provides
a method for determining the extent of electrochemical extraction
of a metal (M) from a metal (M) oxide caused by a voltage applied
between a cathode comprising (or consisting essentially of) or in
contact with the metal (M) oxide and an inert metal alloy anode in
molten CaCl.sub.2, the method comprising:
[0102] (A) measuring the evolution of chlorine over a temporal
range;
[0103] (B) relating the onset or level of chlorine evolution over
the temporal range to the extent of electrochemical extraction of
the metal (M) from the metal (M) oxide.
[0104] By allowing the extent of electrochemical extraction to be
determined instantly and whilst the reaction is ongoing, the
present invention makes it possible to exercise hitherto
unattainable levels of control over electrochemical extraction in
molten CaCl.sub.2.
[0105] Step (A) may be carried out discretely at intervals or
continuously (eg to produce a chlorine evolution vs time plot).
Preferably step (A) is carried out continuously.
[0106] Particularly preferably step (B) is: relating the onset or
level of chlorine evolution to the rate of extraction (%) of the
metal (M) from the metal (M) oxide. Alternatively step (B) may be:
relating the onset or level of chlorine evolution to the oxygen
content of metal (M).
[0107] Preferably the method further comprises:
[0108] (C) ceasing electrochemical reduction in response to the
attainment of a threshold level of chlorine evolution.
[0109] Particularly preferably step (B) comprises: relating the
threshold level of chlorine evolution to a target rate of
extraction (%) of metal (M) from metal (M) oxide.
[0110] The target rate of extraction is typically 99% or more,
preferably 99.5% or more.
[0111] Particularly preferably step (B) comprises: relating the
threshold level of chlorine evolution to a target oxygen content of
metal (M). The target oxygen content is typically less than 2500
ppm O.sub.2 by weight of metal (M), preferably less than 1500 ppm
O.sub.2 by weight of metal (M).
[0112] The present invention will now be described in a
non-limitative sense with reference to the Examples and
accompanying Figures in which:
[0113] FIGS. 1a and 1B: XRD of a TiO.sub.2+KHCO.sub.3 pellet
roasted for 1 hour and electrolysed for 0.5 hours (see FIG. 1a) and
1 hour (see FIG. 1b) in a molten bath of CaCl.sub.2--LiCl showing
phases of Ti (ICDD 5-682), CaTiO.sub.3 (ICDD 42-423),
CaTi.sub.2O.sub.4 (ICDD 11-29) and TiO (ICDD 8-117); and
[0114] FIG. 2: Current vs time graph measured according to the
method of the invention at an applied voltage of 3.1V.
EXAMPLE 1
Method
[0115] Pellets were prepared by mixing 1-2 g of TiO.sub.2 with
0.2-0.5 g of KHCO.sub.3 at different weight ratios. In each case,
the mixture was heat treated for 1 hour at 1073K and pressed in a
die at a pressure of 3643 atm. A hole was drilled in the pellet
with a 2 mm drill bit. The pellet was suspended in a steel
electrode which acted as a cathode with a molybdenum wire. An
Al--Ti--Cu intermetallic anode was suspended on a steel electrode
with a molybdenum wire. The two electrodes were connected to a
power supply which was set to a constant voltage of 3.1 V.
[0116] Molten electrolytic mixtures of KCl--CaCl.sub.2 and
LiCl--CaCl.sub.2 were prepared by taking 180 gms of CaCl.sub.2 with
20 gms of KCl and LiCl respectively. In each case, the mixture was
transferred into a zircon crucible which was lowered into a furnace
maintained at 320.degree. C. The mixture was heat treated for 24
hours and then transferred into an alumina crucible and heated to
800.degree. C. at 0.5.degree. C. per minute after which the
temperature was raised to 920.degree. C. at a rate of 2.degree. C.
per minute. During heating, argon gas was passed into the furnace
at 500 ml min.sup.-1. Once the electrolyte was fully molten, the
temperature of the furnace was lowered to 900.degree. C. The two
electrodes were lowered into the furnace and a potential of 3.1V
was applied using an Agilent 6651A DC power supply. The experiments
were carried out for a period of 8-24 hours.
[0117] Pellets were removed at intervals of 30 and 60 minutes of
electrolysis and washed in water for 24 hours. The pellets were
finely ground using a mortar and pestle for X-ray powder
diffraction analysis. The diffraction was carried out using Cu--Ka
as target at a scanning rate of 0.02.degree. sec.sup.-1.
Results
[0118] An increase in internal porosity was achieved readily in
situ by the presence of KHCO.sub.3 in the TiO.sub.2 pellet. As
KHCO.sub.3 decomposes, it produces potassium oxide, carbon dioxide
and water. The liberated gaseous mixture of CO.sub.2 and H.sub.2O
increases the porosity in the pellet which enhances the contact
surface area between CaCl.sub.2 and TiO.sub.2 and facilitates rapid
cathodic dissociation of TiO.sub.2.
[0119] Besides pore formation, a much more significant reaction
takes place between K.sub.2O and CaTiO.sub.3. K.sup.+ ions diffuse
into the perovskite lattice which breaks the structure by forming
more stable liquid potassium titanates as shown in equation [1]
(ascertained from an equilibrium calculation performed using
FACTSAGE see C. Bale et al., FACTSAGE (Ecole Polytechnique CRCT,
Montreal, Quebec Canada)). The calcium oxide formed in this
reaction is dissolved in the molten salt bath until it reaches
saturation:
CaTiO.sub.3+2K.sub.2O=K.sub.4TiO.sub.4+CaO .DELTA.G=-334349.6 J
mole.sup.-1 at T=900.degree. C. [1]
[0120] As the diffusivity of O.sup.2- ions in the liquid phase is
faster than in solid CaTiO.sub.3, the reduction of K.sub.4TiO.sub.4
to the Magneli phases through to Ti metal occurs rapidly as no
major reorganisation of crystalline TiO.sub.2 is required. The
Magneli phases (Ti.sub.4O.sub.7, Ti.sub.3O.sub.5) all have a
distorted rutile structure with a larger number of oxygen vacant
sites. From a phase equilibrium analysis, it was established that
the K.sub.4TiO.sub.4 liquid phase can be in equilibrium with the
Magneli phase and continue to shift the equilibrium with the
progression of reduction to the metallic phase. The formation of
liquid phase increases the reaction kinetics which is evident as Ti
metal was observed within the first half an hour of electrolysis.
The K.sup.+ ions produced from the decomposition of potassium
titanate reacts with the molten electrolyte and forms
KCaCl.sub.3.
[0121] By controlling the volume of the liquid phase of potassium
titanate, the loss of Ti in the molten salt can be prevented. If
the liquid phase drains out from the solid pellet into the
CaCl.sub.2 bath, TiO.sub.2 is then irreversibly lost into the
CaCl.sub.2 bath.
[0122] FIGS. 1a and 1B are the XRD pattern of the pellet at 0.5
hours (see FIG. 1a) and 1 hour (see FIG. 1b) of electrolysis.
Phases of Ti (ICDD 5-682), CaTiO.sub.3 (ICDD 42-423),
CaTi.sub.2O.sub.4 (ICDD 11-29) and TiO (ICDD 8-117) are present. A
comparison of FIGS. 1a and 1b shows that the perovskite peak is
suppressed as perovskite is decomposed. After 20 hours of
electrolysis, the XRD pattern (FIG. 6) shows that titanium metal is
present.
EXAMPLE 2
[0123] A number of experiments were carried out to change the ratio
of potassium bicarbonate in the pellet in the range 10-50 wt %.
Experiments were also conducted on the two different types of
molten salt containing CaCl.sub.2-KCl and CaCl.sub.2--LiCl mixtures
at 900.degree. C. with a constant voltage of 3.1 V. Both processes
yielded complete reduction of TiO.sub.2 pellet to Ti metal. The
residual concentration of oxygen dissolved in the Ti metal was
determined by X-ray diffraction analysis (see M. Dechamps et al.,
Scripta Metallurgica 11 (11), 941 (1977)) and was found to be 1350
ppm by weight.
[0124] A first experiment was carried out with a pellet containing
20 wt % potassium bicarbonate in a CaCl.sub.2-KCl bath for 8 hours.
A Ti metal layer with a thickness of 500 .mu.m was formed beyond
which there is a high concentration of calcium, titanium, potassium
and chlorine.
[0125] When the concentration of potassium bicarbonate was
increased from 20 wt % to 50 wt % and electrolysis was performed
for 20 hours, it was found that a uniform microstructure of Ti
metal was formed across the cross section of the pellet with the
majority of the area being metallised. Furthermore when the salt
bath was replaced by LiCl--CaCl.sub.2 and 50 wt % of potassium
bicarbonate was mixed with TiO.sub.2 and electrolysed for 20 hours,
it also led to full metallisation. The reduction in the two molten
salts proves that the formation of K.sub.4TiO.sub.4 liquid phase is
important for increasing reaction kinetics and is independent of
the molten salt used. During electrolysis, all the experiments
showed an increase in the current with the inert metallic anode
which is in sharp contrast with previous observations (see C.
Schwandt and D. J. Fray, Electrochimica Acta 51 (1), 66 (2005); M.
Ma et al., Journal of Alloys and Compounds 420 (1-2), 37 (2006);
and R. O, Suzuki et al, Metallurgical and Materials Transactions
B-Process Metallurgy and Materials Processing Science 34 (3), 287
(2003)).
Current-Time Analysis
[0126] FIG. 2 displays the current-time plot for the reaction of
Example 2. Although a smooth curve was observed, there was
oscillation in the current with a variation of .+-.0.1 amps during
electrolysis. It can be seen from FIG. 2 that there was a decrease
in the current for the first half hour to a point of inflection
after which the current increased rapidly. Beyond two hours, there
was a slow increase in current which plateaus at around 4.0
amps.
[0127] The large initial current is due to the use of the inert
anode which has high conductivity and decreases cell resistance.
The initial decrease in current in FIG. 2 is due to the formation
of a perovskite phase (verified from the X-ray diffraction
analysis). The XRD data for half hour electrolysis showed the
presence of CaTiO.sub.3, CaTi.sub.2O.sub.4, Ti.sub.3O.sub.5, TiO
and Ti metal phases. After 4 hours, almost 95% of TiO.sub.2 was
reduced to Ti. In previous experiments (Alexander et al, Acta
Materialia 54 (11), 2933 (2006) and Schwandt [supra]), no titanium
metal had been observed in the first 30 minutes of the process.
[0128] From the Ti--O phase diagram, it is known that
Ti.sub.3O.sub.5 can never be in equilibrium with Ti from which it
is concluded that (at an early stage) two simultaneous reactions
occur. The first reaction is the formation of CaTiO.sub.3,
CaTi.sub.2O.sub.4 and Ti.sub.3O.sub.5 which dominates the phase
constitution. The second reaction is the decomposition of
K.sub.4TiO.sub.4 to form TiO and Ti metal. Since the Magneli phases
are more electrochemically conducting and the Ti metal is formed in
the first hour of electrolysis, an increase in current is eminent
which is what is seen in FIG. 2. The diffraction pattern after one
hour of electrolysis showed small peaks of CaTiO.sub.3 and
predominant peaks of Ti, CaTi.sub.2O.sub.4 and Ti.sub.3O.sub.5. XRD
does not show the presence of the potassium titanate phase because
it is a transitional liquid phase during electrolysis.
[0129] The amount of Ti metal produced is verified by
microstructural analysis and by measuring the weight loss after
electrolysis (as previously demonstrated by G. Z. Chen et al,
Metallurgical and Materials Transactions B-Process Metallurgy and
Materials Processing Science 35 (2), 223 (2004) in the case of
electro-reduction of Cr.sub.2O.sub.3 in molten CaCl.sub.2). After
electrolysis of 1 g of TiO.sub.2 pellet for 20 hours, the pellet
was washed in water for 24 hours and the weight of the pellet was
measured again and was found to be 0.605 g. The theoretical amount
of Ti produced from 1 g of TiO.sub.2 is 0.6 g which is within the
error of experimental observation thus verifying complete
metallisation.
[0130] These measurements may be used to calibrate the
electrochemical cell to obtain a relationship between current
measured temporally and the rate of conversion of TiO.sub.2 to Ti
metal. In this way, the current vs time plot of a new sample of
TiO.sub.2 can be exploited to determine the rate of conversion
(whilst the extraction is ongoing) or a desired end-point of the
electrochemical reduction (typically Ti metal at about 99.5%) to a
high degree of accuracy.
[0131] Similarly the electrochemical cell may be calibrated to
obtain a relationship between current measured temporally and the
onset of corrosive conditions or conditions under which chlorine is
evolved from the molten electrolyte. In this way, the current vs
time plot of a new sample of TiO.sub.2 can be exploited to
determine when electrochemical reduction should be ceased to
prevent corrosion and/or production of chlorine.
* * * * *