U.S. patent application number 12/865510 was filed with the patent office on 2011-04-14 for process.
This patent application is currently assigned to University of Leeds. Invention is credited to Animesh Jha, Abhishek Lahiri, Xiaobing Yang.
Application Number | 20110083969 12/865510 |
Document ID | / |
Family ID | 39186659 |
Filed Date | 2011-04-14 |
United States Patent
Application |
20110083969 |
Kind Code |
A1 |
Lahiri; Abhishek ; et
al. |
April 14, 2011 |
PROCESS
Abstract
The present invention relates to a process for electrochemical
extraction of a metal (M) from a metal (M) oxide, to a conducting
electrode and to an electrolytic cell comprising the conducting
electrode.
Inventors: |
Lahiri; Abhishek;
(Bangalore, IN) ; Yang; Xiaobing; (Leeds, GB)
; Jha; Animesh; (Leeds, GB) |
Assignee: |
University of Leeds
Leeds
GB
|
Family ID: |
39186659 |
Appl. No.: |
12/865510 |
Filed: |
January 26, 2009 |
PCT Filed: |
January 26, 2009 |
PCT NO: |
PCT/GB2009/000233 |
371 Date: |
December 8, 2010 |
Current U.S.
Class: |
205/560 ;
204/242; 204/291 |
Current CPC
Class: |
C22B 34/129 20130101;
C25C 3/00 20130101; C25C 3/26 20130101; C25C 3/28 20130101 |
Class at
Publication: |
205/560 ;
204/291; 204/242 |
International
Class: |
C25C 1/22 20060101
C25C001/22; C25B 11/04 20060101 C25B011/04; C25B 9/00 20060101
C25B009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2008 |
GB |
0801791.5 |
Apr 28, 2008 |
GB |
0807687.9 |
Jul 2, 2008 |
GB |
0812098.2 |
Claims
1.-27. (canceled)
28. A process for electrochemical extraction of a metal (M) from a
metal (M) oxide comprising: applying a voltage between a cathode
comprising or in contact with the metal (M) oxide and an anode in
an oxygen-dissolving molten electrolyte in the presence of an
alkali metal (M.sup.a) oxide whereby to form an alkali metal
(M.sup.a) metallate (M) phase, wherein 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.
29. A process as claimed in claim 28, wherein the alkali metal
(M.sup.a) oxide is potassium oxide.
30. A process as claimed in claim 28, wherein the metal (M'')
metallate (M) phase is a perovskite (or perovskite-type) phase.
31. A process as claimed in claim 28, wherein 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.
32. A process as claimed in claim 28, wherein the alkali metal
(M.sup.a) metallate (M) phase is a liquid.
33. A process as claimed in claim 28, wherein the alkali metal
(M.sup.a) metallate (M) phase is M.sup.a.sub.4MO.sub.4.
34. A process as claimed in claim 28, wherein the alkali metal
(M.sup.a) oxide is in admixture with the metal (M) oxide in or in
contact with the cathode.
35. A process as claimed in claim 28, further comprising: mixing
the alkali metal (M.sup.a) oxide and the metal (M) oxide and
forming the mixture of alkali metal (M.sup.a) oxide and metal (M)
oxide into a self-supporting mixture.
36. A process as claimed in claim 28, wherein the alkali metal
(M.sup.a) oxide is formed in situ by decomposition of a
decomposable alkali metal (M.sup.a) salt, wherein the decomposable
alkali metal (M.sup.a) salt is in admixture with the metal (M)
oxide in or in contact with the cathode.
37. A process as claimed in claim 36, further comprising: mixing
the decomposable alkali metal (M.sup.a) salt and the metal (M)
oxide and forming the mixture of decomposable alkali metal
(M.sup.a) salt and metal (M) oxide into a self-supporting
mixture.
38. A process as claimed in claim 36, wherein the decomposable
alkali metal (M.sup.a) salt is an alkali metal (M.sup.a)
bicarbonate.
39. A process as claimed in claim 36, wherein the metal (M) is one
or more metals selected from the group consisting of Ti, Nb, Ta, U,
Th, Cr, Fe, steel and Zr.
40. A process as claimed in claim 28, wherein the metal (M) is one
or more metals selected from the group consisting of Ti, Nb, Ta and
Zr.
41. A process as claimed in claim 28, comprising: applying a
voltage between a cathode comprising TiO.sub.2 in admixture with an
alkali metal (M.sup.a) salt decomposable into the alkali metal
(M.sup.a) oxide and an anode in an oxygen-dissolving molten
CaCl.sub.2-containing electrolyte whereby to form a liquid alkali
metal (M.sup.a) titanate phase.
42. A conducting electrode comprising a metal (M) oxide and either
an alkali metal (M.sup.a) oxide capable of forming an alkali metal
(M.sup.a) metallate (M) phase or an alkali metal (M.sup.a) salt
decomposable into an alkali metal (M.sup.a) oxide capable of
forming an alkali metal (M.sup.a) metallate (M) phase.
43. An electrolytic cell comprising a cathode which comprises or is
in contact with a metal (M) oxide and one or more inert anodes in
contact with a fusible or fused oxygen-dissolving electrolyte in
the presence of an alkali metal (M.sup.a) oxide.
44. An electrolytic cell as claimed in claim 43, comprising a
single inert anode, wherein the cathode is a cathodic basket in
which is carried the metal (M) oxide.
45. An electrolytic cell as claimed in claim 43, wherein the
cathode is a cathodic vessel which is adapted to facilitate in use
continuous flow of the fused oxygen-dissolving electrolyte between
a feeder end into which the fused oxygen-dissolving electrolyte is
feedable and a discharge end from which the fused electrolyte is
dischargeable, wherein the electrolytic cell comprises a plurality
of inert anodes housed in the cathodic vessel between the feeder
end and the discharge end.
46. An electrolytic cell as claimed in claim 43, comprising a
plurality of inert anodes housed in a vessel which contains the
fused oxygen-dissolving electrolyte, wherein a mixture of the
alkali metal (M.sup.a) oxide and metal (M) oxide in contact with a
cathode is present in the form of a plurality of self-supporting
elements conveyable in use through the fused oxygen-dissolving
electrolyte.
47. An electrolytic cell as claimed in claim 43, comprising a
plurality of inert anodes housed in a vessel which contains the
fused oxygen-dissolving electrolyte, wherein the alkali metal
(M.sup.a) oxide and metal (M) oxide are present in the
oxygen-dissolving electrolyte in contact with a plurality of
cathodic elements conveyable in use through the fused
oxygen-dissolving electrolyte.
48. An electrolytic cell as claimed in claim 43, wherein the
cathode is a metal crucible containing the alkali metal (M.sup.a)
oxide and metal (M) oxide in molten admixture, wherein the metal
crucible is suspended in the fused oxygen-dissolving electrolyte.
Description
[0001] The present invention relates to a process for
electrochemical extraction of a metal (M) from a metal (M) oxide,
to a conducting electrode and to an electrolytic cell comprising
the conducting electrode.
[0002] In recent years, the direct electrochemical reduction of
TiO.sub.2 to Ti metal in molten CaCl.sub.2 has stimulated
significant scientific and industrial interest (see for example G.
Z. Chen, et al, Nature 407 (6802), 361 (2000); R. O, Suzuki and K.
Ono, in Molten Salts Xiii, edited by P. C. Trulove, H. C. DeLong,
R. A. Mantz et al. (2002), Vol. 2002, pp. 810; T. H. Okabe et al,
Journal of Alloys and Compounds 364 (1-2), 156 (2004); S. L. Wang
and Y. J. Li, Journal of Electroanalytical Chemistry 571 (1), 37
(2004); and T. Nohira et al, Nature Materials 2 (6), 397 (2003)).
However the formation of a stable perovskite phase in the
intermediate stage of reduction hinders the diffusion of O.sup.2-
ions forming a layered structure in the pellet which slows the
overall kinetics (D. T. L. Alexander et al, Acta Materialia 54
(11), 2933 (2006) and K. Jiang et al., Angewandte
Chemie-International Edition 45 (3), 428 (2006)).
[0003] The conventional FFC process is of the type disclosed in
WO-A-99/64638 for the formation of Ti metal from a TiO.sub.2 pellet
cathode using a carbon rod anode in a molten bath of CaCl.sub.2 at
900.degree. C. at a constant voltage of 3.1V in an argon
atmosphere. The FFC process involves several intermediate steps one
of which includes the formation of stable perovskite phases
(Alexander [supra] and C. Schwandt and D. J. Fray, Electrochimica
Acta 51 (1), 66 (2005)). The formation of perovskite not only
reduces the diffusion of O.sup.2- ions but also due to larger grain
size reduces the pore diffusion of CaCl.sub.2 in the pellet.
Although Jiang [supra] and R. Lilia Centeno-Sanchez et al, Journal
of materials science 42, 7494 (2007) showed that an increase in
porosity could be achieved by adding carbon and polyethylene
precursors to the pellet or by directly starting from perovskite,
the process still takes more than 24 hours to produce Ti with 3000
ppm by weight of oxygen. It is evident that diffusion of O.sup.2-
ions is one of the limiting steps in the overall reduction of
oxides in the FFC process. The low rate of reduction hinders the
process being scaled-up and full metallisation is difficult to
attain even with a small pellet. These limitations are a barrier to
a continuous process and render the FFC process solely a batch
process.
[0004] The present invention is based on the recognition that the
presence of an alkali metal oxide (or a salt from which an alkali
metal oxide can be derived) serves to increase the rate of
electrochemical reduction of a metal oxide in an oxygen-dissolving
molten electrolyte.
[0005] Viewed from a first aspect the present invention provides a
process for electrochemical extraction of a metal (M) from a metal
(M) oxide comprising:
[0006] applying a voltage between a cathode comprising (or
consisting essentially of) or in contact with the metal (M) oxide
and an anode in an oxygen-dissolving molten electrolyte in the
presence of an alkali metal (M.sup.a) oxide whereby to form an
alkali metal (M.sup.a) metallate (M) phase.
[0007] In accordance with the process of the invention, alkali
metal (M.sup.a) ions improve the diffusivity of oxygen by forming
the alkali metal (M.sup.a) metallate (M) phase. By way of example,
where the alkali (M.sup.a) metal oxide is potassium oxide and the
metal (M) oxide is TiO.sub.2, TiO.sub.2 is reduced to nearly 100%
Ti metal with 1350 ppm of oxygen in less than 20 hours. The
presence of a potassium titanate (K.sub.4TiO.sub.4) liquid phase
provides an efficient O.sup.2- ion transport medium which
substantially shortens the Ti production time. This opens up the
possibility of continuous Ti production at lower cost and therefore
the more widespread exploitation of Ti in consumer products.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] In a preferred embodiment, the alkali metal (M.sup.a) oxide
is an additive. The alkali metal (M.sup.a) oxide may be added (e.g.
in the form of a powder) to the oxygen-dissolving molten
electrolyte.
[0013] Preferably the alkali metal (M.sup.a) oxide is in admixture
with the metal (M) oxide in or in contact with the cathode. The
mixture of alkali metal (M.sup.a) oxide and metal (M) oxide may be
solid or liquid (eg molten).
[0014] Preferably the process of the invention further comprises:
mixing the alkali metal (M.sup.a) oxide and the metal (M) oxide.
Particularly preferably the process of the invention further
comprises: forming the mixture of alkali metal (M.sup.a) oxide and
metal (M) oxide into a self-supporting mixture (eg a pellet, slab,
sheet, wire, foil, basket or tube). The forming step may be
pressing. The self-supporting mixture may be the cathode or may be
contactable with the cathode. Preferably the self-supporting
mixture is a pellet. The mixing step may be followed by heat
treating the mixture.
[0015] 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 %.
[0016] 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. The decomposable alkali metal (M.sup.a)
salt may be added (e.g. in the form of a powder) to the
oxygen-dissolving molten electrolyte
[0017] Preferably the decomposable alkali metal (M.sup.a) salt is
in admixture with the metal (M) oxide in or in contact with the
cathode.
[0018] Preferably the process of the invention further comprises:
mixing the decomposable alkali metal (M.sup.a) salt and the metal
(M) oxide. Particularly preferably the process of the invention
further comprises: forming the mixture of decomposable alkali metal
(M.sup.a) salt and metal (M) oxide into a self-supporting mixture
(eg a pellet, slab, sheet, wire, basket, foil or tube). The forming
step may be pressing. The self-supporting mixture may be the
cathode or may be contactable with the cathode. Preferably the
self-supporting mixture is a pellet. The mixing step may be
followed by heat treating the mixture.
[0019] 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 %.
[0020] 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 (e.g. CaO and CaCl.sub.2) which accelerates chemical
reduction.
[0021] 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.
[0022] 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, more preferably KCl.
[0023] The metal (M) may be a reactive metal element, semi-metal
element, metal alloy or metalloid element.
[0024] In a preferred embodiment, the metal (M) forms a solid
perovskite (or perovskite-type) phase in the oxygen-dissolving
molten electrolyte. The solid perovskite phase may be an alkaline
earth metal (e.g. Ca) metallate (M) phase.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] Alternatively the metal (M) oxide may be in contact with a
cathode. In this embodiment, the metal (M) oxide may be
self-supporting (e.g. in the form of a pellet) and the cathode may
be a bath, crucible or basket (e.g. a perforated basket).
Alternatively 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. The mixture may
be a self-supporting mixture (e.g. in the form of a pellet or a
perforated basket) or a molten mixture. Where the mixture is a
molten mixture, the cathode is preferably a crucible. The crucible
may be composed of a metal such as titanium or a titanium alloy and
this embodiment advantageously prevents contamination of the
oxygen-dissolving molten electrolyte.
[0029] Alternatively the metal (M) oxide may be in the
oxygen-dissolving molten 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
oxygen-dissolving molten 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, basket or one or more
pellets.
[0030] The oxygen-dissolving molten electrolyte may be (or contain)
a compound of an alkaline earth metal (e.g. Ca, Sr or Ba), Li, Cs
or Y (or a mixture thereof). Preferably the oxygen-dissolving
molten electrolyte is a compound of Ca. The oxygen-dissolving
molten electrolyte may be (or contain) a halide. 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.
[0031] The anode may be carbon (e.g. graphite).
[0032] Typically in the process of the invention the anode is an
inert anode. Preferred is an anode which is substantially
unreactive with oxygen. Preferred is an anode which is
substantially insoluble in the molten electrolyte.
[0033] Typically the inert anode is a non-carbon anode. Preferred
is an inert metal alloy anode. An inert metal alloy anode
advantageously provides effective current efficiency.
[0034] 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:
[0035] E denotes one or more metallic elements;
[0036] x is an integer in the range 1 to 5;
[0037] y is an integer being 1 or 2; and
[0038] z is an integer being 1 or 2.
[0039] 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.
[0040] 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 process of the invention.
Preferably CuAl.sub.2 is complexed.
[0041] 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 (e.g. on the E rich side of the tie line joining
Al.sub.3E and ECu.sub.4).
[0042] 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.
[0043] 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).
[0044] 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.
[0045] In a preferred embodiment, the Al-E-Cu based alloy falls
other than on the E poor side of the .xi., Al.sub.5E.sub.2Cu,
EAlCu.sub.2 and .beta.-ECu.sub.4 phase tie line (wherein .xi. is a
phase falling between Al.sub.3Ti and Al.sub.2Ti with 3 at % or less
of Cu (e.g. 2-3 at % Cu)).
[0046] In a preferred embodiment, the Al-E-Cu based alloy comprises
an intermetallic phase falling on or near to the .xi.,
Al.sub.5E.sub.2Cu, EAlCu.sub.2 and .beta.-ECu.sub.4 phase tie
line.
[0047] Preferably the intermetallic phase is Al.sub.5E.sub.2Cu.
Particularly preferably the Al-E-Cu based alloy further comprises
Al.sub.3E.
[0048] Preferably the intermetallic phase is EAlCu.sub.2.
Particularly preferably the Al-E-Cu based alloy further comprises
.beta.-ECu.sub.4
[0049] The anode may be composed of a homogenous, partially
homogenous or non-homogeneous Al-E-Cu based alloy.
[0050] Typically E has a potential in the anode which is lower than
it would be in the molten electrode.
[0051] In a preferred embodiment, the anode develops a passivating
layer. Preferably the passivating layer withstands oxidation in
anodic conditions.
[0052] In a preferred embodiment, E is a single metallic element.
The single metallic element is preferably Ti.
[0053] 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.).
[0054] 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 (e.g. from 1200.degree. C.
to 1400.degree. C.).
[0055] 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.
[0056] In a preferred embodiment, E is one or more of the group
consisting of group B transition metal elements (e.g. 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.
[0057] In a preferred embodiment, E is one or more metallic
elements of valency II, III, IV or V, preferably II, III or IV.
[0058] 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.
[0059] 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 (e.g. 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.
[0060] 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).
[0061] Preferably E is or includes a metallic element capable of
promoting the passivation of the surface of the anode in the
presence of a oxygen-dissolving 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 (e.g. 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.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] Preferably the intermetallic phase is Al.sub.5Ti.sub.2Cu.
Particularly preferably the Al--Ti--Cu based alloy further
comprises Al.sub.3Ti.
[0066] Preferably the intermetallic phase is TiAlCu.sub.2.
Particularly preferably the Al--Ti--Cu based alloy further
comprises .beta.-TiCu.sub.4
[0067] 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.
[0068] 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).
[0069] 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.
[0070] 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.
[0071] In a preferred embodiment, the anode is at least as
conducting at elevated temperature (e.g. at 900.degree. C.) as a
carbon electrode. Preferably the anode is more conducting at
elevated temperature (e.g. at 900.degree. C.) than a carbon
electrode.
[0072] In a preferred embodiment, the decomposable alkali metal
(M.sup.a) salt may be present with an amount of endogenous
hydroxide ions. A hydroxide ion decomposes at the cathode into an
oxide ion (which moves to the anode) and a proton. At the cathode,
this leads to the formation of occluded hydrogen in the metal (M)
which may react with oxygen (for example in subsequent steps such
as remelting) to advantageously lower the oxygen content of the
metal (M) (e.g. to a level as low as 1100 ppm). A hydrogenated
metal (M) (e.g. hydrogenated uranium) produced in this embodiment
is useful. For example, a hydrogenated metal (M) may be a useful
hydrogen storage material. The hydrogen may be removed by (for
example) plasma melting.
[0073] 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.
[0074] The process of the invention may be carried out at an
elevated temperature typically in the range 600-1000.degree. C.,
preferably 850-1000.degree. C. (e.g. about 900.degree. C.).
[0075] The process of the invention for a discrete batch of metal
(M) oxide may be carried out to substantially complete conversion
over a period of less than 20 hours, preferably less than 10 hours
(e.g. 8 hours), particularly preferably less than 4 hours. This
advantageously minimises energy input and therefore costs.
[0076] The voltage is typically less than the discharge potential
of metals in the oxygen-dissolving molten electrolyte. For example,
the voltage may be less than 3.5V (eg about 3.0V).
[0077] In a preferred embodiment, the process of the invention is
carried out in an oxygen deficient atmosphere (eg an inert
atmosphere such as argon).
[0078] The process of the invention typically achieves a rate of
metal (M) extraction of 99% or more, preferably 99.9% or more.
[0079] The process of the invention typically produces metal (M)
with an oxygen content of less than 2500 ppm O.sub.2 by weight,
preferably less than 1500 ppm O.sub.2 weight.
[0080] In a preferred embodiment, the process of the invention
comprises:
[0081] applying a voltage between a cathode comprising TiO.sub.2 in
admixture with an alkali metal (M.sup.a) salt decomposable into the
alkali metal (M.sup.a) oxide and an anode in an oxygen-dissolving
molten CaCl.sub.2-containing electrolyte whereby to form a liquid
alkali metal (M.sup.a) titanate phase.
[0082] In a preferred embodiment, the process of the invention
further comprises:
[0083] measuring the current flow between the cathode and the inert
metal alloy anode over a temporal range;
[0084] 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.
[0085] Viewed from a further aspect the present invention provides
a conducting electrode comprising (or consisting essentially of) a
metal (M) oxide and either an alkali metal (M.sup.a) oxide capable
of forming an alkali metal (M.sup.a) metallate (M) phase or an
alkali metal (M.sup.a) salt decomposable into an alkali metal
(M.sup.a) oxide capable of forming an alkali metal (M.sup.a)
metallate (M) phase.
[0086] The conducting electrode may (in use) be a cathode as
hereinbefore defined.
[0087] In a preferred embodiment, the conducting electrode
comprises a metal (M) oxide and an alkali metal (M.sup.a) salt
decomposable into an alkali metal (M.sup.a) oxide capable of
forming an alkali metal (M.sup.a) metallate (M) phase.
[0088] Viewed from a still further aspect the present invention
provides the use of a conducting electrode or cathode as
hereinbefore defined in an electrolytic cell.
[0089] Viewed from an even still yet further aspect the present
invention provides an electrolytic cell comprising a cathode which
comprises or is in contact with a metal (M) oxide and one or more
inert anodes in contact with a fusible or fused oxygen-dissolving
electrolyte in the presence of an alkali metal (M.sup.a) oxide.
[0090] The (or each) inert anode may be as hereinbefore defined.
The fused oxygen-dissolving electrolyte may be an oxygen-dissolving
molten electrolyte as hereinbefore defined. The cathode may be as
hereinbefore defined.
[0091] Generally the electrolytic cell is operated in an inert
atmosphere (eg an argon atmosphere). Preferably the fusible or
fused oxygen-dissolving electrolyte comprises CaCl.sub.2.
[0092] In a first preferred embodiment, the electrolytic cell
comprises a single inert anode. The alkali metal (M.sup.a) oxide
(eg K.sub.2O) may be present in the fused oxygen-dissolving
electrolyte. Preferably the cathode is a cathodic basket (eg a
perforated basket) or crucible in which is carried the metal (M)
oxide (eg in the form of a pellet).
[0093] The electrolytic cell may be a continuous cell.
[0094] In a second preferred embodiment, the cathode is a cathodic
vessel which is adapted to facilitate in use continuous flow of the
fused oxygen-dissolving electrolyte between a feeder end into which
the fused oxygen-dissolving electrolyte is feedable and a discharge
end from which the fused electrolyte is dischargeable, wherein the
electrolytic cell comprises a plurality of inert anodes housed in
the cathodic vessel between the feeder end and the discharge
end.
[0095] Particularly preferably the electrolytic cell further
comprises: a cathodic separation vessel downstream from the
discharge end, wherein the cathodic separation vessel houses an
inert anode.
[0096] Preferably the cathodic separation vessel houses a chlorine
meter.
[0097] Preferably the cathodic separation vessel houses an oxygen
meter.
[0098] Preferably the cathodic separation vessel comprises a
reference electrode to assist in the measurement of current flow
between the cathodic separation vessel and the inert anode. The
current flow may be used to determine the extent of electrochemical
reduction of the metal (M) oxide.
[0099] In the second preferred embodiment, the metal (M) oxide may
be present in the fused oxygen-dissolving electrolyte (eg in the
form of a suspended powder or a pellet). The alkali metal (M.sup.a)
oxide (e.g. K.sub.2O) may be present in the fused oxygen-dissolving
electrolyte.
[0100] In a third preferred embodiment, the electrolytic cell
comprises a plurality of inert anodes housed in a vessel which
contains the fused oxygen-dissolving electrolyte, wherein a mixture
of the alkali metal (M.sup.a) oxide and metal (M) oxide in contact
with a cathode is present in the form of a plurality of
self-supporting elements conveyable in use through the fused
oxygen-dissolving electrolyte.
[0101] Each self-supporting element may be a pellet or a basket
(e.g. a perforated basket). The self-supporting elements may be
mounted on a conveyor. The self-supporting elements may be
dismountably mounted on a conveyor. The self-supporting elements
may be conveyed in and out of the fused oxygen-dissolving
electrolyte. The self-supporting elements may be circulatory (e.g.
recirculatory).
[0102] In a fourth preferred embodiment, the electrolytic cell
comprises a plurality of inert anodes housed in a vessel which
contains the fused oxygen-dissolving electrolyte, wherein the
alkali metal (M.sup.a) oxide and metal (M) oxide are present in the
fused oxygen-dissolving electrolyte (e.g. in the form a suspension)
in contact with a plurality of cathodic elements conveyable in use
through the fused oxygen-dissolving electrolyte.
[0103] Each cathodic element may be a pellet. The cathodic elements
may be mounted on a conveyor. The cathodic elements may be
dismountably mounted on a conveyor. The cathodic elements may be
conveyed in and out of the fused oxygen-dissolving electrolyte. The
cathodic elements may be circulatory (eg recirculatory).
[0104] In a fifth preferred embodiment, the cathode is a metal
crucible containing the alkali metal (M.sup.a) oxide and metal (M)
oxide in molten admixture, wherein the metal crucible is suspended
in the fused oxygen-dissolving electrolyte. The metal crucible may
be composed of titanium metal or a titanium metal alloy. The fifth
embodiment advantageously prevents contamination of the fused
oxygen-dissolving electrolyte by the molten admixture.
[0105] The present invention will now be described in a
non-limitative sense with reference to the Examples and
accompanying Figures in which:
[0106] FIG. 1: K--Ti--O phase diagram at 1173K plotted using
FACTSAGE thermodynamic software (C. Bale et al., FACTSAGE (Ecole
Polytechnique CRCT, Montreal, Quebec Canada));
[0107] FIG. 2: Elemental map of the cross section of a partially
reacted TiO.sub.2 pellet after treatment according to an embodiment
of the process of the invention;
[0108] FIG. 3: Current vs time graph for the process according to
the invention at an applied voltage of 3.1V;
[0109] FIG. 4a: Low magnification image of the cross section of a
Ti pellet fully metallised in a LiCl--CaCl.sub.2 molten bath;
[0110] FIG. 4b: High magnification image of Ti metal obtained from
the inner region of the pellet seen in FIG. 3;
[0111] FIGS. 5a and 5B: XRD of a TiO.sub.2+KHCO.sub.3 pellet
roasted for 1 hour and electrolysed for 0.5 hours (see FIG. 5a) and
1 hour (see FIG. 5b) 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);
[0112] FIG. 6: XRD of titanium metal formed after 20 hours of
electrolysis;
[0113] FIG. 7: A schematic illustration of a first embodiment of
the electrolytic cell according to the invention;
[0114] FIG. 8: A schematic illustration of a second embodiment of
the electrolytic cell according to the invention;
[0115] FIG. 9: A schematic illustration of a third embodiment of
the electrolytic cell according to the invention;
[0116] FIG. 10: A schematic illustration of a fourth embodiment of
the electrolytic cell according to the invention; and
[0117] FIG. 11: A schematic illustration of a fifth embodiment of
the electrolytic cell according to the invention
EXAMPLE 1
Method
[0118] 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 3.1V.
[0119] 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.
[0120] 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--K.alpha. as target at a scanning rate of 0.02.degree.
sec.sup.-1.
Results
[0121] 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.
[0122] 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 Bale [supra]). The calcium oxide formed in this
reaction is dissolved in the molten salt bath until it reaches
saturation:
CaTiO.sub.3+2K.sub.2O.dbd.K.sub.4TiO.sub.4.sup.+CaO
.DELTA.G=-334349.6 J mole.sup.-1 at T=900.degree. C. [1]
[0123] 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 (see FIG. 1). 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.
[0124] 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.
[0125] FIGS. 5a and 5B are the XRD pattern of the pellet at 0.5
hours (see FIGS. 5a) and 1 hour (see FIG. 5b) 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. 5a and 5b 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
[0126] 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 3.1V. 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.
[0127] A first experiment was carried out with a pellet containing
20 wt % potassium bicarbonate in a CaCl.sub.2--KCl bath for 8
hours. The elemental map in FIG. 2 demonstrates the formation of Ti
metal layer with a thickness of 500 .mu.m beyond which there is a
high concentration of calcium, titanium, potassium and chlorine.
From the elemental map in FIG. 2, it is possible to observe
discrete regions of KCl within the CaCl.sub.2 layer which can occur
via reaction [2] at 900.degree. C.:
K.sub.2O+CaCl.sub.2=CaO+KCl .DELTA.G=-346968 J mole.sup.-1 [2]
[0128] 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 (C. Schwandt
[supra]; 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)).
[0129] FIG. 3 displays the current-time plot for the reaction at
3V. Although a smooth curve is observed, there was oscillation in
the current with a variation of .+-.0.1 amps during electrolysis.
It can be seen from FIG. 3 that there is a decrease in the current
for the first half hour of the process after which the current
increases. Beyond two hours, there is a slow increase in current
which plateaus at around 4.0 amps. The large initial current is due
to the use of the inert anode which has high conductivity compared
to a carbon anode and therefore decreases the cell resistance. The
initial decrease in current in FIG. 3 is due to the formation of a
perovskite phase (verified from the X-ray diffraction analysis).
The 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 is reduced to Ti. It
is important to note that in previous experiments (Alexander
[supra] and Schwandt [supra]), no titanium metal has been observed
in the first 30 minutes of the process.
[0130] 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 found in FIG. 3. 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. It
must be noted that the XRD does not show the presence of the
potassium titanate phase because it is a transitional liquid phase
during electrolysis.
[0131] The amount of Ti metal produced is not only shown by
microstructural analysis but also 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. FIG. 3 shows a low magnification image of the cross
section of a fully metallised TiO.sub.2 pellet which was reduced in
a CaCl.sub.2--LiCl molten electrolyte. As evident from FIG. 4a, the
layered structure as seen in FIG. 2 is absent throughout the cross
section. The inset in FIG. 4a reveals that it has a metallic grey
colour with a large number of cracks on the surface. There was 20%
shrinkage in the pellet from its starting thickness of 5 mm to a
final thickness of 4 mm. The corresponding high magnification image
of the inner region (within the hole) is shown in FIG. 4b. The
microstructure has a distinctive Ti metal morphology obtained from
the electro-reduction process and compares well with literature
data (Schwandt et al [supra]). The EDX from this region confirms Ti
having K.sup..alpha., K.sup..beta. and L.sup..alpha. peaks. In the
EDX spectrum shown in FIG. 4b, an oxygen peak at 1350 ppm
concentration in the reduced Ti metal is not anticipated. The
designated oxygen peak is Ti L.sup..alpha. and not O
K.sup..alpha..
EXAMPLE 3
[0132] FIG. 7 is a schematic illustration of a first embodiment of
the electrolytic cell according to the invention designated
generally by reference numeral 1. The electrolytic cell 1 comprises
an inert alloy anode 2 and a cathodic basket 3 in a molten
electrolyte 6 of CaCl.sub.2 containing K.sub.2O. Inside the
cathodic basket 3 is a TiO.sub.2 pellet 4 around which is formed a
perovskite layer 5. The cell 1 operates at an applied voltage of
about 3.1V
EXAMPLE 4
[0133] FIG. 8 is a schematic illustration of a second embodiment of
the electrolytic cell according to the invention designated
generally by reference numeral 11. The electrolytic cell 11 is
deployed for continuous metal production.
[0134] The electrolytic cell 11 comprises four inert alloy anodes
12a-d. Inert alloy anodes 12a-c are mounted in a cathodic vessel 13
containing a molten electrolyte 16 of CaCl.sub.2. The molten
electrolyte 16 is fed continuously into the cathodic vessel 13
together with TiO.sub.2 powder and K.sub.2O into the feed end 20
and a controlled flow of molten electrolyte 16 from the feed end 20
to a discharge end 21 is achieved by a slope in the cathodic vessel
13.
[0135] During the continuous flow of molten electrolyte 16,
TiO.sub.2 is reduced to titanium sub-oxide. In accordance with the
invention, this is only made feasible by the presence of K.sub.2O.
At the discharge end 21, there is a discharge port 22 through which
titanium sub-oxide is discharged into a cathodic separation vessel
31 which houses inert alloy anode 12d and completes the reduction
of titanium suboxide to titanium metal. Titanium metal is
discharged from the discharge outlet 30 and the molten electrolyte
is recycled to the cathodic vessel 13. To determine the end point
of the process, the separation vessel 31 is fitted with a reference
electrode to facilitate the measurement of a current vs time
plot.
EXAMPLE 5
[0136] FIG. 9 is a schematic illustration of a third embodiment of
the electrolytic cell according to the invention designated
generally by reference numeral 21. The electrolytic cell 21 is
deployed for continuous metal production.
[0137] The electrolytic cell 21 comprises three inert alloy anodes
22a-c housed in a vessel 23 containing a molten electrolyte 26 of
CaCl.sub.2. In contact with a cathode 29 is a plurality of baskets
30 each composed of a self-supporting mixture of TiO.sub.2 and
K.sub.2O. Each basket 30 is mounted on a conveyor which circulates
the baskets 30 in and out of the molten electrolyte 26 in the
direction X.
EXAMPLE 6
[0138] FIG. 10 is a schematic illustration of a fourth embodiment
of the electrolytic cell according to the invention designated
generally by reference numeral 221. The electrolytic cell 221 is
deployed for continuous metal production.
[0139] The electrolytic cell 221 comprises three inert alloy anodes
222a-c housed in a vessel 223 containing a molten electrolyte 226
of CaCl.sub.2. TiO.sub.2 and K.sub.2O is added to the molten
electrolyte 226 to form a suspension. A plurality of cathodic
pellets 230 is mounted on a conveyor which circulates the pellets
230 in and out of the molten electrolyte 226 in the direction
X.
EXAMPLE 7
[0140] FIG. 7 is a schematic illustration of a fifth embodiment of
the electrolytic cell according to the invention designated
generally by reference numeral 1. The electrolytic cell 1 comprises
an inert alloy anode 2 and a cathodic crucible 3 made of titanium
or titanium alloy. The cathodic vessel 3 is suspended in a molten
electrolyte 6 of CaCl.sub.2. Inside the cathodic crucible 3 is a
molten mixture 4 of TiO.sub.2 and K.sub.2O. At surface A, TiO.sub.2
is reduced to titanium metal and oxide ions are transported from
surface B through the molten electrolyte 6 to the inert alloy anode
2.
* * * * *