U.S. patent application number 15/321439 was filed with the patent office on 2017-06-08 for method and apparatus for electrolytic reduction of a feedstock comprising oxygen and a first metal.
The applicant listed for this patent is METALYSIS LIMITED. Invention is credited to GREG DOUGHTY.
Application Number | 20170159193 15/321439 |
Document ID | / |
Family ID | 51410211 |
Filed Date | 2017-06-08 |
United States Patent
Application |
20170159193 |
Kind Code |
A1 |
DOUGHTY; GREG |
June 8, 2017 |
METHOD AND APPARATUS FOR ELECTROLYTIC REDUCTION OF A FEEDSTOCK
COMPRISING OXYGEN AND A FIRST METAL
Abstract
A method of electrolytic reduction of a feedstock comprising
oxygen and a first metal comprises the steps of, arranging the
feedstock in contact with a cathode and a molten salt within an
electrolysis cell, arranging an anode in contact with the molten
salt within the electrolysis cell, the anode comprising a molten
second metal and applying a potential between the anode and the
cathode such that oxygen is removed from the feedstock to form a
reduced feedstock. The oxygen removed from the feedstock reacts
with the molten second metal to form an oxide comprising the second
metal. The second metal is aluminium. The reduced feedstock may
comprise a proportion of aluminium.
Inventors: |
DOUGHTY; GREG; (ROTHERHAM,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
METALYSIS LIMITED |
ROTHERHAM |
|
GB |
|
|
Family ID: |
51410211 |
Appl. No.: |
15/321439 |
Filed: |
June 25, 2015 |
PCT Filed: |
June 25, 2015 |
PCT NO: |
PCT/GB2015/051851 |
371 Date: |
December 22, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 14/00 20130101;
C25C 3/26 20130101; C25C 3/36 20130101; C25C 7/025 20130101; C25C
5/04 20130101 |
International
Class: |
C25C 3/36 20060101
C25C003/36; C25C 7/02 20060101 C25C007/02; C22C 14/00 20060101
C22C014/00; C25C 5/04 20060101 C25C005/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 26, 2014 |
GB |
1411433.4 |
Claims
1. A method of electrolytic reduction of a feedstock, the feedstock
comprising oxygen and a first metal, the method comprising the
steps of, arranging the feedstock in contact with a cathode and a
molten salt within an electrolysis cell, arranging an anode in
contact with the molten salt within the electrolysis cell, the
anode comprising a molten second metal, the second metal being
aluminium, and applying a potential between the anode and the
cathode such that oxygen is removed from the feedstock to form a
reduced feedstock, the oxygen removed from the feedstock reacting
with the molten second metal to form an oxide comprising the second
metal.
2. The method according to claim 1, in which a proportion of the
second metal is deposited at the cathode when the potential is
applied such that the reduced feedstock comprises the first metal
and a proportion of the second metal.
3. The method according to claim 2, in which the reduced feedstock
is a metallic alloy comprising the first metal and between 0.01
percent by weight (wt %) and 5 wt % of the second metal, for
example, the reduced feedstock may comprise between 0.01 wt % and
3.0 wt % of the second metal, or between 0.05 wt % and 2.0 wt %, or
between 0.10 wt % and 1.50 wt %, or between 0.50 wt % and 1.0 wt %
of the second metal.
4. The method according to claim 2, in which controlling the length
of time for which a potential is applied between the anode and the
cathode determines the proportion of the second metal in the
reduced feedstock.
5. The method according to claim 1, in which the feedstock is a
compound comprising oxygen and the first metal, for example an
oxide of the first metal.
6. The method according to claim 1, in which the feedstock contains
oxides of more than one different metal, and/or in which the first
metal is an alloy.
7. The method according to claim 1, in which the feedstock is a
metallate compound, a metallate compound being a compound of the
first metal, oxygen and at least one reactive metal, the reactive
metal being a metal selected from the group consisting of calcium,
lithium, sodium and potassium.
8. The method according to claim 1, in which the second metal is
commercially pure aluminium metal, or in which the second metal is
an aluminium alloy, for example an alloy of eutectic
composition.
9. The method according to claim 1, in which the first metal is, or
is an alloy of, any metal selected from the group consisting of
silicon, scandium, titanium, vanadium, chromium, manganese, iron,
cobalt, nickel, aluminium, germanium, yttrium, zirconium, niobium,
molybdenum, hafnium, tantalum, tungsten, lanthanum, cerium,
praseodymium, neodymium, samarium, actinium, thorium, protactinium,
uranium, neptunium and plutonium.
10. The method according to claim 1, in which the molten salt is at
a temperature at which the second metal is molten, but below 1000
degrees centigrade when the potential is applied between the
cathode and the anode, or less than 850 degrees centigrade,
preferably or less than 800, or 750, or 700 degrees centigrade.
11. The method according to claim 1, in which the molten salt is a
lithium bearing salt or a calcium bearing salt, or a salt
comprising lithium chloride or calcium chloride.
12. The method according to claim 1, comprising a further step of
reducing the oxide comprising the second metal to recover the
second metal.
13. The method according to claim 1, in which the feedstock
comprises a titanium oxide and the anode comprises molten
aluminium.
14. The method according to claim 1, in which the reduced feedstock
is a titanium alloy comprising between 0.01 percent by weight (wt
%) and 5 wt % of aluminium, for example, the reduced feedstock may
comprise between 0.01 wt % and 3.0 wt % aluminium, or between 0.05
wt % and 2.0 wt %, or between 0.10 wt % and 1.50 wt %, or between
0.50 wt % and 1.0 wt % aluminium.
15. The method according to claim 1, in which the feedstock
comprises a calcium titanate or a lithium titanate and the second
metal is aluminium.
16. The method according to claim 1, in which the feedstock is in
the form of powder or particles having an average particle size of
less than 3 mm.
17. The method according claim 1, in which the reduced feedstock is
a metal powder.
18. The method according to claim 1, in which substantially no
gases are evolved at the anode during electrolysis.
19. The method according to claim 1, in which there is no carbon in
contact with the molten salt within the electrolysis cell.
20. The method according to claim 1, in which the reduced feedstock
comprises less than 100 ppm carbon, for example less than 50 ppm,
or less than 25 ppm carbon.
21. An apparatus for producing metal by electrolytic reduction of a
feedstock comprising oxygen and a first metal, the apparatus
comprising a cathode and an anode arranged in contact with a molten
salt in which the cathode is in contact with the feedstock and the
anode comprises a molten metal, the molten metal being
aluminium.
22. The apparatus according to claim 21, comprising a power source
connected to the cathode and the anode.
23. The apparatus according to claim 22, in which there is no
carbon in contact with the molten salt.
Description
[0001] The invention relates to a method and apparatus for
electrolytic reduction of a feedstock comprising an oxygen and a
first metal, in particular to the production of metal by the
reduction of a metal oxide.
BACKGROUND
[0002] The present invention concerns a method for the electrolytic
reduction of a feedstock comprising oxygen and a first metal. As is
known from the prior art, electrolytic processes may be used, for
example, to reduce metal compounds or semi-metal compounds to
metals, semi-metals, or partially-reduced compounds, or to reduce
mixtures of metal compounds to form alloys. In order to avoid
repetition, unless otherwise indicated the term metal will be used
in this document to encompass all such products, such as metals,
semi-metals, alloys, intermetallics. The skilled person will
appreciate that the term metal may, where appropriate, also include
partially reduced products.
[0003] In recent years, there has been great interest in the direct
production of metal by direct reduction of a solid metal oxide
feedstock. One such direct reduction process is the Cambridge
FFC.RTM. electro-decomposition process, as described in WO
99/64638. In the FFC process, a solid compound, for example a metal
oxide, is arranged in contact with a cathode in an electrolysis
cell comprising a fused salt. A potential is applied between the
cathode and an anode of the cell such that the compound is reduced.
In the FFC process, the potential that produces the solid compound
is lower than a deposition potential for a cation from the fused
salt.
[0004] Other reduction processes for reducing feedstock in the form
of a cathodically connected solid metal compound have been
proposed, such as the Polar.RTM. process described in WO 03/076690
and the process described in WO 03/048399.
[0005] Typical implementations of direct reduction processes
conventionally use carbon-based anode materials. During the
reduction process the carbon-based anode materials are consumed and
the anodic product is an oxide of carbon, for example gaseous
carbon monoxide or carbon dioxide. The presence of carbon in the
process leads to a number of issues that reduce the efficiency of
the process and lead to contamination of the metal produced by
reduction at the cathode. For many products it may be desirable to
eliminate carbon from the system altogether.
[0006] Numerous attempts have been made to identify so-called inert
anodes that are not consumed during electrolysis and evolve oxygen
gas as an anodic product. Of conventional, readily-available
materials, tin oxide has shown some limited success. A more exotic
oxygen-evolving anode material based on calcium ruthenate has been
proposed, but the material has limited mechanical strength, suffers
from degradation during handling, and is expensive.
[0007] Platinum has been used as an anode in LiCl-based salts for
the reduction of uranium oxide and other metal oxides, but the
process conditions need to be very carefully controlled to avoid
degradation of the anode and this too is expensive. Platinum anodes
are not an economically viable solution for an industrial scale
metal production process.
[0008] While an oxygen-evolving anode for use in the FFC process
may be desirable, the actual implementation of a commercially
viable material appears to be difficult to achieve. Furthermore,
additional engineering difficulties may be created in the use of an
oxygen-evolving anode, due to the highly corrosive nature of oxygen
at the high temperatures involved in direct electrolytic reduction
processes.
[0009] An alternative anode system is proposed in WO 02/083993 in
which the anode in an electrolysis cell was formed from molten
silver or molten copper. In the method disclosed in WO 02/083993
oxygen removed from a metal oxide at the cathode is transported
through the electrolyte and dissolves in the metal anode. The
dissolved oxygen is then continuously removed by locally reducing
oxygen partial pressure over a portion of the metal anode. This
alternative anode system has limited use. The removal of oxygen is
dependent on the rate at which the oxygen can diffuse into the
molten silver or copper anode material. Furthermore, the rate is
also dependent on the continuous removal of oxygen by locally
reducing partial pressure over a portion of the anode.
[0010] Thus, this process does not appear to be a commercially
viable method of producing metal.
SUMMARY OF THE INVENTION
[0011] The invention provides a method and apparatus for,
electrolytic reduction of a feedstock comprising oxygen and a first
metal, as defined in the appended independent claims. Preferred
and/or advantageous features of the invention are set out in
various dependent sub-claims.
[0012] In a first aspect a method of electrolytic reduction of a
feedstock may be provided, the feedstock comprising oxygen and a
first metal, for example being a compound comprising oxygen and a
first metal. The method comprises the steps of arranging the
feedstock in contact with a cathode and a molten salt within an
electrolysis cell, arranging an anode in contact with the molten
salt within the electrolysis cell, and applying a potential between
the anode and the cathode such that oxygen is removed from the
feedstock to form a reduced feedstock. The anode comprises a molten
metal, which is preferably a different metal to the first metal
comprised in the feedstock. The molten metal may be referred to as
a second metal. The second metal is either aluminium or tin. While
the second metal is not molten at room temperature it is molten at
the temperature of electrolysis within the cell, when the potential
is applied between the anode and the cathode. Oxygen removed from
the feedstock is transported through the salt to the anode where it
reacts with the molten metal of the anode to form an oxide
comprising the molten anode metal and oxygen.
[0013] A key difference between the invention described in this
aspect and the prior art disclosure of WO 02/083993 is that the
molten anode metal of the present invention is consumed during the
electrolysis process. In other words, the molten anode metal is a
metal that readily oxidises on contact with an oxygen species in
order to form an oxide comprising the second metal and oxygen.
[0014] Oxides formed at the anode during electrolysis may be in the
form of particles which may sink into the molten metal exposing
more molten metal for oxidation. The oxide formed at the anode may
form particles that disperse into the molten salt and expose more
molten metal for subsequent oxidation. The oxide formed at the
anode may form as a liquid phase dissolved within the metal. The
oxide can form rapidly at the surface of the molten anode, and can
disperse away from the surface of the molten anode. Thus, formation
of the oxide does not provide a significant kinetic inhibition on
the oxidation reaction. By contrast the dissolution of oxygen into
the molten metal anode of WO 02/083993 is dependent on solubility
of oxygen in the molten metal anode, the diffusion of oxygen into
the molten anode, and the transport of oxygen out of the anode
under a reduced partial pressure.
[0015] Since the molten metal anode does not evolve oxygen gas, in
contrast to inert anodes, the potential for oxidation of the cell
materials of construction is removed. For example, when employing
"standard" inert anodes, exotic materials would need to be selected
for construction of the cell that are able to withstand oxygen at
elevated temperatures.
[0016] The use of a carbon anode would result in CO and CO.sub.2
evolution. Both CO and CO.sub.2 are oxidising agents, but to a
lesser extent than oxygen, and can attack the materials of
construction. This may result in corrosion products entering the
melt and consequently the product.
[0017] It is preferred that the second metal at the anode is at a
temperature close to, and just above, its melting point during
operation of the apparatus in order to reduce losses of the anode
material by excessive vaporisation.
[0018] During operation of apparatus, a proportion of the second
metal from the anode is preferably deposited at the cathode, where
it may deposit on or interact with the reduced feedstock. Thus, the
reduced feedstock may comprise both the first metal, i.e. the metal
of the metal oxide in the feedstock, and additionally a proportion
of the second metal.
[0019] The reduced feedstock may therefore comprise the first metal
doped, or alloyed, with a proportion of the second metal. Doping,
or alloying, of the first metal with a proportion of the second
metal may introduce advantageous physical or electrical properties
to the reduced feedstock. For example, a reduced feedstock
comprising the first metal doped with a proportion of the second
metal may exhibit a higher dielectric constant than a reduced
feedstock comprising only the first metal. Other benefits of doping
or alloying of the first metal with the second metal may include
increased tensile strength, increased capacitance, increased
electrical conductivity, reduced electrical conductivity, increased
melting point, or reduced melting point. It may be advantageous to
reduce feedstocks that contain a proportion of the second metal,
for example aluminium, with the aim of forming metal alloys that
comprise a proportion of the second metal. For example, if an
operator wished to make a Ti-6Al-4V alloy, a feedstock may be
prepared comprising a mixture of TiO.sub.2, V.sub.2O.sub.5 and
Al.sub.2O.sub.3. Aluminium contamination of the product would not
be a problem in this circumstance. Indeed, the alumina content may
be varied to reflect additional aluminium alloying originating from
the anode.
[0020] The reduced feedstock may be a metallic alloy containing the
second metal in various proportions. Preferably, the reduced
feedstock is a metallic alloy comprising the first metal and
between and between 0.01 percent by weight (wt %) and 5 wt % of the
second metal. For example, the reduced feedstock may comprise
between 0.01 wt % and 3.0 wt % of the second metal, or between 0.05
wt % and 2.0 wt %, or between 0.10 wt % and 1.50 wt %, or between
0.50 wt % and 1.0 wt % of the second metal. The present invention
may be a convenient way of alloying a first metal with a low
proportion of a second metal, the second metal being aluminium or
tin.
[0021] Preferably, the proportion of the second metal comprised in
the reduced feedstock may be controlled. Particularly preferably,
controlling the length of time for which a potential is applied
between the anode and the cathode determines the proportion of the
second metal in the reduced feedstock.
[0022] The first metal is a different metal or alloy to the second
metal. Preferably the first metal is, or is an alloy of, any metal
selected from the list consisting of silicon, scandium, titanium,
vanadium, chromium, manganese, iron, cobalt, nickel, germanium,
yttrium, zirconium, niobium, molybdenum, uranium, actinides,
hafnium, tantalum, tungsten, lanthanum, cerium, praseodymium,
neodymium, samarium, actinium, thorium protactinium, uranium,
neptunium and plutonium.
[0023] The skilled person will be able to select a feedstock
comprising any first metal listed above and an anode comprising
aluminium or tin.
[0024] The feedstock may be in the form of powder or particles or
may be in the form of preformed shapes or granules formed from a
powdered compound comprising oxygen and a first metal. In a
preferred embodiment, the feedstock is in the form of powder or
particles having an average particle size of less than 5 mm, for
example less than 3 mm, or less than 2 mm.
[0025] The feedstock may preferably be an oxide of the first metal,
for example titanium dioxide. The feedstock may contain oxides of
more than one different metal. The feedstock may comprise complex
oxides having multiple metallic species. The first metal may be an
alloy. For example, the feedstock may be an oxide comprising an
alloy of titanium and another metal. Alternatively, the feedstock
may be a metallate compound, a metallate compound being a compound
of the first metal, oxygen and at least one reactive metal, the
reactive metal preferably being a group 1 or group 2 metal, for
example a metal selected from the list consisting of calcium,
lithium, sodium and potassium. The feedstock may be a metallate
comprising titanium as the first metal, for example a calcium
titanate such as CaTiO.sub.3 or a lithium titanate such as
Li.sub.2TiO.sub.3.
[0026] The second metal, i.e. the anode metal, may be commercially
pure aluminium metal. Alternatively, the second metal may be an
alloy of aluminium with one or more other elements, for example an
alloy of eutectic composition. It may be desirable to have an alloy
of eutectic composition in order to lower the melting point of the
anode metal and thereby operate the process at a more favourable
lower temperature.
[0027] The second metal, i.e. the anode metal, may be commercially
pure tin metal. Alternatively, the second metal may be an alloy of
tin with one or more other elements, for example an alloy of
eutectic composition.
[0028] It may be desirable that the molten salt is at a temperature
below 1000.degree. C. when the potential is applied between the
cathode and the anode. It may be particularly preferable to have
the temperature of the molten salt during the process as low as
possible in order to minimise the vapour pressure above the molten
anode and thus the loss of the molten anode material. Thus, it may
be preferable that the molten salt is maintained at a temperature
of lower than 850.degree. C., for example lower than 800.degree. C.
or 750.degree. C. or 700.degree. C., during electrolysis. So that
the second metal comprising the anode is molten during the process,
the molten salt must be maintained at a temperature greater than or
equal to the melting point of the second metal. For example, when
the anode metal is commercially pure aluminium metal, the molten
salt should be maintained at a temperature greater than 660.degree.
C. When the anode metal is commercially pure tin metal, the molten
salt should be maintained at a temperature greater than 232.degree.
C.
[0029] Any salt suitable for use in the electrolysis process may be
used. Commonly used salts in the FFC process include calcium
chloride containing salts. The molten salt may be a calcium
containing salt, preferably a salt comprising calcium chloride. Due
to the desirability of low temperature operation, it may be
particularly desirable that the molten salt is a lithium-bearing
salt, for example preferably a salt comprising lithium chloride.
The salt may comprise lithium chloride and lithium oxide.
[0030] Fresh salts may contain residual carbonates and these
carbonates may deposit carbon on the cathode, thereby increasing
the carbon content of the product. Thus, it may be advantageous to
pre-electrolyse the salt to remove residual carbonates prior to
reduction of tantalate. Once used, salt is preferably re-used for
multiple reductions. The use of a pre-electrolysed salt or a used
salt may result in the salt having lower carbonate content and may
help to produce tantalum with very low carbon content.
[0031] The second metal in the anode is consumed during the process
due to the formation of an oxide between the second metal and
oxygen. The method may advantageously comprise the further step of
reducing the oxide formed at the anode, i.e. the oxide comprising
the second metal and oxygen, in order to recover and re-use the
second metal. The step of further reducing the oxide may take place
after the electrolysis reaction has completed. For example, the
oxide formed may be taken and reduced by carbothermic reduction or
by standard FFC reduction. The recovered second metal may be
returned to the anode.
[0032] The step of reducing the oxide comprising the second metal
and oxygen may involve a system in which molten material at the
anode is constantly pumped from the anode to a separate cell or
chamber where it is reduced to recover the second metal, which is
then transferred back to the anode. Such a system may allow a
reduction cell to be operated for a long period of time, or a
continuous period of time, as the anode material is constantly
replenished as it is being consumed.
[0033] In preferred embodiments the feedstock may comprise a
titanium oxide and the anode comprises molten aluminium. The
reduced feedstock product may be titanium doped with aluminium.
Titanium doped with a proportion of aluminium may possess different
physical properties to pure titanium metal.
[0034] For example, doping titanium with aluminium may improve its
strength. The reduced feedstock may be a titanium alloy comprising
between 0.01 percent by weight (wt %) and 5 wt % of aluminium. For
example, the reduced feedstock may comprise between 0.01 wt % and
3.0 wt % aluminium, or between 0.05 wt % and 2.0 wt %, or between
0.10 wt % and 1.50 wt %, or between 0.50 wt % and 1.0 wt %
aluminium.
[0035] In a preferred embodiment, the feedstock comprises a lithium
titanate and the second metal is aluminium. In a particularly
preferred embodiment, the feedstock comprises a calcium titanate,
and the second metal is aluminium.
[0036] The use of an aluminium anode may provide a particular
advantage over traditional carbon anodes when it comes to energy
consumption. Due to the overpotential of aluminium being lower than
that of carbon, a cell employing an aluminium anode may achieve
reduction of its feedstock at a lower voltage than one using a
carbon anode. For example, a cell using an aluminium anode may be
run at a voltage of 1.5V to 2V, compared to 3V to 3.5V for similar
reductions carried out using a carbon anode. This reduction in
operating voltage may have significant beneficial cost
implications.
[0037] In other preferred embodiments the feedstock may comprise a
titanium oxide and the anode comprises molten tin. The reduced
feedstock product may be titanium doped with tin. The reduced
feedstock may be a titanium alloy comprising between 0.01 percent
by weight (wt %) and 5 wt % of tin. For example, the reduced
feedstock may comprise between 0.01 wt % and 3.0 wt % tin, or
between 0.05 wt % and 2.0 wt %, or between 0.10 wt % and 1.50 wt %,
or between 0.50 wt % and 1.0 wt % tin.
[0038] In a preferred embodiment, the feedstock comprises a lithium
titanate and the second metal is tin. In a particularly preferred
embodiment, the feedstock comprises a calcium titanate, and the
second metal is tin.
[0039] The reaction of the oxygen removed from the feedstock with
the anode material to form an oxide means that there is no
evolution of oxygen within the cell. This may have significant
engineering benefits, as the necessity to deal with high
temperature oxygen off gases is negated.
[0040] As there is no carbon required for the electrolysis reaction
to proceed, the product of the process, i.e. the reduced feedstock,
has little to no carbon contamination. Although carbon
contamination may not be an issue in the direct electrolytic
reduction of some metals, for other applications and metals any
level of carbon contamination is undesirable. The use of this
method allows a direct reduction of an oxide material to metal at a
commercially viable rate while eliminating carbon contamination.
Furthermore, although the anode material is consumed during the
electrolysis, it is possible to recover the oxide resulting from
this consumption, reduce this oxide, and re-use the anode
material.
[0041] Preferably, there is no carbon in contact with the molten
salt within the electrolysis cell during the reduction process.
Particularly preferably, the reduced feedstock produced by this
process may comprise less than 100 ppm carbon, for example less
than 50 ppm, or less than 25 ppm carbon.
[0042] The method may be used to reclaim metallic material such as
metallic powder that has become contaminated with oxygen. For
example, the feedstock may be metallic powder that has been heated
in the presence of oxygen and thus contaminated with oxygen. Such
powder may be formed, for example, as a waste product of a 3D
printing process such as selective laser sintering or selective
laser melting. Powder that is not incorporated into a product in
such processes may be heated to a high temperature and cooled
again, thereby picking up unwanted oxygen. The method may then be
conveniently used to reclaim the contaminated powder.
[0043] In a second aspect, an apparatus for producing metal by
electrolytic reduction of a feedstock comprising oxygen and a first
metal comprises a cathode and an anode arranged in contact with a
molten salt, the cathode being in contact with the feedstock and
the anode comprising a molten metal. The molten metal is either
aluminium or tin.
[0044] The apparatus may also comprise a power source connected to
the cathode and the anode. This power source is capable of applying
a potential between the cathode and the anode such that, in use,
oxygen is removed from the feedstock.
SPECIFIC EMBODIMENTS OF THE INVENTION
[0045] Specific embodiments of the invention will now be described
with reference to the figures, in which
[0046] FIG. 1 is schematic diagram illustrating an apparatus
according to one or more aspects of the invention; and
[0047] FIG. 2 is a schematic diagram of a second embodiment of an
apparatus according to one or more aspects of the invention.
[0048] FIG. 1 illustrates an electrolysis apparatus 10 for
electrolytic reduction of an oxygen bearing feedstock such as an
oxide feedstock. The apparatus 10 comprises a crucible 20
containing a molten salt 30. A cathode 40 comprising a pellet of
metal oxide 50 is arranged in the molten salt 30. An anode 60 is
also arranged in the molten salt. The anode comprises a crucible 61
containing a molten metal 62, and an anode connecting rod 63
arranged in contact with the molten salt 62 at one end and coupled
to a power supply at the other. The anode connecting rod 63 is
sheathed with an insulating sheath 64 so that the connecting rod 63
does not contact the molten salt 30.
[0049] The crucible 20 may be made from any suitable insulating
refractory material. It is an aim of the invention to avoid
contamination with carbon, therefore the crucible is not made from
a carbon material. A suitable crucible material may be alumina. The
metal oxide 50 may be any suitable metal oxide. A number of metal
oxides have been reduced using direct electrolytic processes such
as the FFC process and are known in the prior art. The metal oxide
50 may be, for example, a pellet of titanium dioxide or tantalum
pentoxide. The crucible 61 containing the molten metal 62 may be
any suitable material, but again alumina may be a preferred
material. The anode lead rod 63 may be shielded by any suitable
insulating material 64, and alumina may be a suitable refractory
material for this purpose.
[0050] The molten metal 62 is either aluminium or tin, both of
which are liquid in the molten salt at the temperature of
operation. The molten metal 62 must be capable of reacting with
oxygen ions removed from the metal oxide to create an oxide of the
molten metal species. The molten salt 30 may be any suitable molten
salt used for electrolytic reduction. For example, the salt may be
a chloride salt, for example, a calcium chloride salt comprising a
portion of calcium oxide. Preferred embodiments of the invention
may use a lithium based salt such as lithium chloride or lithium
chloride comprising a proportion of lithium oxide. The anode 60 and
cathode 40 are connected to a power supply to enable a potential to
be applied between the cathode 40 and its associated metal oxide 50
on the one hand and the anode 60 and its associated molten metal 62
on the other.
[0051] The arrangement of the apparatus illustrated in FIG. 1
assumes that the molten metal 62 is more dense than the molten salt
30. This arrangement may be suitable, for example, where the salt
is a lithium chloride salt and the molten metal is molten
aluminium. In some circumstances, however, the molten metal may be
less dense than the molten salt used for the reduction. In such a
case an apparatus arrangement as illustrated in FIG. 2 may be
appropriate.
[0052] FIG. 2 illustrates an alternative apparatus for producing
metal by electrolytic reduction of an oxide feedstock. The
apparatus 110 comprises a crucible 120 containing a molten salt
130, a cathode 140 comprises a pellet of metal oxide 150 and the
cathode 140 and the pellet of metal oxide 150 are arranged in
contact with the molten salt 130. An anode 160 is also arranged in
contact with the molten salt 130 and comprises a metallic anode
connecting rod 163 sheathed by an insulating material 164. One end
of the anode 160 is coupled to a power supply and the other end of
the anode is in contact with a molten salt 162 contained within a
crucible 161. The crucible 161 is inverted so as to retain the
molten metal 162 which is less dense than the molten salt 130. This
arrangement may be appropriate, for example, where the molten metal
is a liquid aluminium-magnesium alloy and the molten salt is
calcium chloride.
[0053] The skilled person would be able to consult data charts to
determine whether a particular molten metal is more or less dense
than a particular molten salt in a combination used in an
electrolysis reduction process. Thus, it is straightforward to
determine whether or not an apparatus according to that illustrated
in FIG. 1 or an apparatus according to that illustrated in FIG. 2
is most appropriate for conducting the reduction.
[0054] Although the illustrations of apparatus shown in FIGS. 1 and
2 show arrangements where a feedstock pellet is attached to a
cathode, it is clear that other configurations are within the scope
of the invention, for example, an oxide feedstock may be in the
form of grains or powder and may be simply retained on the surface
of a cathodic plate in an electrolysis cell.
[0055] The method of operating the apparatus will now be described
in general terms with reference to FIG. 1. A cathode 40 comprising
a metal oxide 50 and an anode 60 comprising a molten metal 62 are
arranged in contact with a molten salt 30 within an electrolysis
chamber 20 of an electrolysis cell 10. The oxide 50 comprises an
oxide of a first metal. The molten metal is aluminium, which is
capable of being oxidised. A potential is applied between the anode
and the cathode such that oxygen is removed from the metal oxide
50. This oxygen is transported from the metal oxide 50 towards the
anode where it reacts with the molten aluminium 62 forming
aluminium oxide. The oxygen is therefore removed from the oxide 50
and retained within a second oxide of the molten anode metal.
[0056] The parameters for operating such an electrolysis cell such
that oxygen is removed are known through such processes as the FFC
process. Preferably the potential is such that oxygen is removed
from the metal oxide 50 and transported to the molten metal 62 of
the anode without any substantial breakdown of the molten salt 30.
As a result of the process the metal oxide 50 is converted to metal
and the molten metal 62 is converted, as least in part, to a metal
oxide. The metal product of the reduction can then be removed from
the electrolysis cell.
[0057] The inventors have carried out a number of specific
experiments based on this general method, and these are described
below. The metal product produced in the examples was analysed
using a number of techniques. The following techniques were
used.
[0058] Carbon analysis was performed using an Eltra CS800
analyser.
[0059] Oxygen analysis was performed using an Eltra ON900
analyser.
[0060] Surface area was measured using a Micromeritics Tristar
surface area analyser.
[0061] Particle size was measured using a Malvern Hydro 2000MU
particle size determinator.
Experiment 1
[0062] Aluminium used as the anode material was 99.5% Al shot
supplied by Acros Organics. A feedstock pellet of mixed titanium
oxide, niobium oxide, zirconium oxide and tantalum oxide was
prepared by wet mixing powders of the four oxides, before drying,
pressing into a pellet and sintering for 2 hours at 1000.degree.
C.
[0063] A 28 gram feedstock pellet of mixed oxides 50 was connected
to a tantalum rod 40 and used as a cathode. 150 grams of aluminium
62 was contained in an alumina crucible 61 and connected to a power
supply via a tantalum connecting rod 63 sheathed in a dense alumina
tube 64. This construction was used as an anode 60. One kilogram of
calcium chloride 30 was used as an electrolyte and contained within
a large alumina crucible 20. The anode and pellet were arranged
within the molten salt 30 and the temperature of the salt was
raised to approximately 830.degree. C.
[0064] The cell was operated in constant current mode. A constant
current of 4 amps was applied between the anode and cathode for a
period of 23.4 hours. During this time the potential between the
anode and the cathode remained at roughly 1.5 volts.
[0065] There were no gases evolved at the anode during
electrolysis. This was due to the formation of aluminium oxide in
the molten aluminium anode 62. A total charge of 336680 coulombs
was passed during the electrolysis reaction.
[0066] After a period of 23.4 hours the cathode and cathode pellet
were removed and the cathode pellet 50 had been discovered to have
reduced to a metal alloy. Analysis showed that the metal alloy was
contaminated with aluminium. Oxygen analysis of the reduced product
provided an average value of 2289 ppm, a carbon content of 82 ppm
and aluminium content of 4560 ppm.
[0067] Aluminium oxide is a solid at the temperatures of reduction.
Aluminium oxide formed at the surface is likely to become entrapped
within the molten aluminium in the alumina crucible and, therefore,
free more molten aluminium for reaction with further oxygen
ions.
Experiment 2
[0068] In order to demonstrate the drop in carbon content provided
by the method of the present invention, Experiment 1 was repeated
using a carbon anode instead of a molten aluminium anode.
[0069] A feedstock pellet of mixed titanium oxide, niobium oxide,
zirconium oxide and tantalum oxide was prepared by wet mixing
powders of the four oxides, before drying, pressing into a pellet
and sintering for 2 hours at 1000.degree. C.
[0070] A 28 gram feedstock pellet of mixed oxides was connected to
a tantalum rod and used as a cathode. A carbon anode was connected
to a power supply via a tantalum connecting rod sheathed in a dense
alumina tube. One kilogram of calcium chloride was used as an
electrolyte and contained within a large alumina crucible. The
anode and pellet were arranged within the molten salt and the
temperature of the salt was raised to approximately 830.degree.
C.
[0071] The cell was operated in constant current mode. A constant
current of 4 amps was applied between the anode and cathode for a
period of 18 hours. During this time the potential between the
anode and the cathode remained at roughly 1.5 volts.
[0072] A total charge of 259039 coulombs was passed during the
electrolysis reaction.
[0073] After a period of 18 hours the cathode and cathode pellet
were removed and the cathode pellet 50 was discovered to have
reduced to a metal alloy. Oxygen analysis of the reduced product
provided an average oxygen value of 4039 ppm, and a carbon content
of 3373 ppm. No aluminium was detected in the reduced metal
alloy.
[0074] This showed that the use of a carbon anode resulted in the
reduced feedstock having a carbon content of 3373 ppm--much higher
than the 82 ppm carbon content produced in the same reduced
feedstock when using an aluminium anode.
Experiment 3
[0075] A 45 gram pellet of tantalum pentoxide 50 was connected to a
tantalum rod 40 and used as a cathode. 150 grams of aluminium 62
was contained in an alumina crucible 61 and connected to a power
supply via a tantalum connecting rod 63 sheathed in a dense alumina
tube 64. This construction was used as an anode 60. 1.6 kilogram of
calcium chloride 30 was used as an electrolyte and contained within
a large alumina crucible 20. The anode and pellet were arranged
within the molten salt 30 and the temperature of the salt was
raised to approximately 830.degree. C.
[0076] The cell was operated in constant current mode. A constant
current of 4 amps was applied between the anode and cathode for a
period of 20 hours. During this time the potential between the
anode and the cathode remained at roughly 1.5-2.5 volts.
[0077] There were no gases evolved at the anode during
electrolysis. This was due to the formation of aluminium oxide in
the molten aluminium anode 62. A total charge of 289391 coulombs
was passed during the electrolysis reaction.
[0078] After reduction, the resulting metallic tantalum product was
sieved and analysed. It was found that the courser material
retained by a 500 pm sieve contained 5590 ppm 0, 20 ppm C, and had
a surface area of 3.4464 m.sup.2/g. The fine material that passed
through the sieve contained 5873 ppm 0, 87 ppm C, and had a surface
area of 1.3953 m.sup.2/g. The product contained between 1.32 and
2,01 wt % aluminium.
Experiment 4
[0079] In a further example, a 28 g pellet was manufactured from a
sample of Iluka NR95 natural rutile powder. The powder was sieved
to select a fraction consisting of particles having a particle size
range of 150-212 microns. The pellet was reduced in calcium
chloride using an molten aluminium anode. EDX analysis of the
reduced product showed an aluminium content of 1.3 wt. %.
* * * * *