U.S. patent application number 15/855241 was filed with the patent office on 2018-05-03 for method and apparatus for producing metal by electrolytic reduction.
The applicant listed for this patent is METALYSIS LIMITED. Invention is credited to GREG DOUGHTY.
Application Number | 20180119299 15/855241 |
Document ID | / |
Family ID | 47682582 |
Filed Date | 2018-05-03 |
United States Patent
Application |
20180119299 |
Kind Code |
A1 |
DOUGHTY; GREG |
May 3, 2018 |
METHOD AND APPARATUS FOR PRODUCING METAL BY ELECTROLYTIC
REDUCTION
Abstract
A method is provided for producing metal by electrolytic
reduction of a feedstock comprising an oxide of 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. The
anode comprises a second metal, which at the temperature of
electrolysis within the cell is a molten metal. The second metal is
a different metal to the first metal. Oxygen removed from the
feedstock during electrolysis reacts with the molten second metal
to form an oxide comprising the second metal. Thus, oxygen is not
evolved as a gas at the molten anode.
Inventors: |
DOUGHTY; GREG; (ROTHERHAM,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
METALYSIS LIMITED |
Rotherham |
|
GB |
|
|
Family ID: |
47682582 |
Appl. No.: |
15/855241 |
Filed: |
December 27, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
14655012 |
Jun 23, 2015 |
|
|
|
PCT/EP2013/077855 |
Dec 20, 2013 |
|
|
|
15855241 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25C 3/26 20130101; C25C
7/025 20130101; C25C 3/00 20130101 |
International
Class: |
C25C 3/26 20060101
C25C003/26; C25C 7/02 20060101 C25C007/02; C25C 3/00 20060101
C25C003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 24, 2012 |
GB |
1223375.5 |
Claims
1. An apparatus for producing metal by electrolytic reduction of a
feedstock comprising an oxide of a first metal and oxygen, 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 capable of forming an oxide.
2. The apparatus according to claim 1, in which the molten metal
is, or is an alloy of, any metal selected from zinc, tellurium,
bismuth, lead, or magnesium.
3. The apparatus according to claim 1, in which there is no carbon
in contact with the molten salt.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 14/655,012; filed Jun. 23, 2015, which is the National Stage of
International Application No. PCT/EP2013/077855, filed Dec. 20,
2013, each of which is hereby incorporated by reference herein in
its entirety, including any figures, tables, nucleic acid
sequences, amino acid sequences, or drawings.
[0002] The invention relates to a method and apparatus for
producing metal by electrolytic reduction of a feedstock comprising
an oxide of a first metal.
BACKGROUND
[0003] The present invention concerns a method for the production
of metal by reduction of a feedstock comprising an oxide of a
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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] Thus, this process does not appear to be a commercially
viable method of producing metal.
SUMMARY OF THE INVENTION
[0012] The invention provides a method and apparatus for producing
metal by electrolytic reduction of a feedstock comprising a
metallic oxide as defined in the appended independent claims.
Preferred and/or advantageous features of the invention are set out
in various dependent sub-claims.
[0013] In the first aspect a method for producing metal by
electrolytic reduction of a feedstock comprising an oxide of a
first metal and oxygen may comprise 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. The anode comprises a molten metal, which is a different
metal to the first metal comprised in the feedstock. The molten
metal may be referred to as a second metal. While the second metal
may not be 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.
[0014] The feedstock may be in the form of powder or particles of
an oxide or may be in the form of preformed shapes or granules
formed from a powdered metallic oxide. The feedstock may comprise
more than one oxide, i.e. oxides of more than one metallic species.
The feedstock may comprise complex oxides having multiple metallic
species. The feedstock may simply comprise a metal oxide such as
titanium dioxide or tantalum pentoxide.
[0015] 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 must
be a metal that readily oxidises on contact with an oxygen species
in order to form an oxide comprising the second metal and
oxygen.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] During operation of apparatus, a proportion of the second
metal from the anode is likely to deposit 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.
[0021] It may be desirable that the method comprises a further step
of separating the second metal from the reduced feedstock to
provide a product that comprises the first metal but not the second
metal. Such separations may conveniently be carried out by thermal
processes such as thermal distillation. For example, if the boiling
point of the first metal is considerably higher than the boiling
point of the second metal, then the reduced product comprising the
first metal and the second metal may be heated in order to
evaporate the second metal. The evaporated second metal may be
condensed to recover the second metal and replenish the anode
material.
[0022] The second metal may be removed from the first metal by a
process such as treatment in an acid wash. The appropriateness of
this method will depend on the relative properties of the first
metal and the second metal, and whether the second metal is
susceptible to dissolution in certain solutions, for example acid
solutions, and the first metal is not.
[0023] If the second metal is to be separated from the first metal,
it is desirable that the second metal is a metal that does not form
a highly stable alloy or intermetallic with the first metal. If the
first metal and the second metal do form an alloy or intermetallic,
it is preferred that the alloy or intermetallic is not stable above
the boiling point of the second metal, allowing the second metal to
be removed by thermal treatment. Such information may be readily
obtained by the skilled person on consulting phase diagrams. For
example, if the feedstock comprises titanium oxide and the molten
anode is formed from molten zinc, then the reduced feedstock will
comprise titanium with a proportion of zinc. Zinc does form an
alloy with titanium at low zinc concentrations and can also form
intermetallic compounds. However, since zinc has a boiling point of
905.degree. C., and the alloys and intermetallics are not stable at
this temperature, the zinc can be removed from the reduced
feedstock by heating the reduced feedstock above 905.degree. C. and
vaporising the zinc. By using an apparatus in which the second
metal is a metal that can be easily removed, such as zinc, the
contamination of the reduced product at the cathode may be
described as transient contamination.
[0024] The second metal, i.e. the anode metal, may be a
commercially pure metal. Alternatively, the second metal may be an
alloy of two or more 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.
[0025] Preferably, the second metal has a melting point of less
than 1000.degree. C., such that it is molten at temperatures under
which the electrolysis process is likely to be performed, and a
boiling point of less than 1500.degree. C. to enable the second
metal to be removed from the first metal by thermal treatment. It
may be particularly preferred if the melting point is less than
600.degree. C. and the boiling point is less than 1000.degree.
C.
[0026] The second metal may preferably be a metal or alloy of any
metal selected from the list consisting of zinc, tellurium,
bismuth, lead, and magnesium.
[0027] It is particularly preferred that the second metal is zinc
or a zinc alloy. Zinc is a relatively low cost material and is
relatively harmless in comparison to many other metals.
[0028] 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, and samarium.
[0029] The skilled person will be able to select a feedstock
comprising any first metal listed above and an anode comprising any
second metal listed above.
[0030] 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. or 650.degree. C., during
electrolysis.
[0031] Any salt suitable for use in the electrolysis process may be
used. Commonly used salts in the FFC process include calcium
chloride containing salts. 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.
[0032] 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.
[0033] 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.
[0034] It is particularly preferred that the anode comprises molten
zinc. Zinc melts at around 420.degree. C. and boils at 905.degree.
C. and, advantageously, is a metal that does not react strongly
with many commercially desirable metals such as titanium and
tantalum. The low boiling point of zinc means that any zinc
contamination of the reduced product may be dealt with by heat
treatment of the reduced product to evaporate any zinc.
[0035] Zinc oxide produced at the anode can be easily converted
back to zinc by reaction with carbon.
[0036] A further particularly preferred anode material may be
tellurium. A still further preferred anode material may be
magnesium, although there are hazards associated with this metal
due to its high reactivity.
[0037] In preferred embodiments the feedstock may comprise a
tantalum oxide and the anode comprises molten zinc, the reduced
product being tantalum metal contaminated with zinc. The
contamination of the reduced product with zinc may be corrected by
heat treating the reduced product leaving tantalum metal.
[0038] In preferred embodiments the feedstock may comprise a
titanium oxide and the anode comprises molten zinc. The product
will thus be titanium.
[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 simple to recover the oxide resulting from this
consumption, reduce this oxide, and re-use the anode material.
[0041] In a second aspect, an apparatus for producing metal by
electrolytic reduction of feedstock comprising a metal oxide of a
first metal and oxygen 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 a metal capable of forming an oxide.
[0042] Preferably, the molten metal is, or is an alloy of, any
metal selected from the list consisting of zinc, tellurium,
bismuth, lead, indium, and magnesium.
SPECIFIC EMBODIMENTS OF THE INVENTION
[0043] Specific embodiments of the invention will now be described
with reference to the figures, in which
[0044] FIG. 1 is schematic diagram illustrating an apparatus
according to one or more aspects of the invention; and
[0045] FIG. 2 is a schematic diagram of a second embodiment of an
apparatus according to one or more aspects of the invention.
[0046] FIG. 1 illustrates an electrolysis apparatus 10 for
producing metal by electrolytic reduction of 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.
[0047] 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.
[0048] The molten metal 62 is any suitable metal that is liquid in
the molten salt at the temperature of operation. To be a suitable
molten metal, 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. A particularly preferable molten metal may be
zinc. 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.
[0049] 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 zinc. 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.
[0050] 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 liquid magnesium and the molten salt is calcium chloride.
[0051] 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.
[0052] 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.
[0053] 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 a second metal
different from the first metal and 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 metal 62 forming an oxide of the molten
metal 62 and oxygen. The oxygen is therefore removed from the oxide
50 and retained within a second oxide of the molten metal.
[0054] 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.
[0055] 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.
[0056] Carbon analysis was performed using an Eltra CS800
analyser.
[0057] Oxygen analysis was performed using an Eltra ON900
analyser.
[0058] Surface area was measured using a Micromeritics Tristar
surface area analyser.
[0059] Particle size was measured using a Malvern Hydro 2000MU
particle size determinator.
Experiment 1
[0060] Zinc used as the anode material was AnalaR Normapur.RTM.
pellets supplied by VWR International Limited. Tantalum oxide was
99.99% purity and pressed and sintered to around 45% porosity. The
powder supplier was F&X electrochemicals.
[0061] An 11 gram pellet of tantalum pentoxide 50 was connected to
a tantalum rod 40 and used as a cathode. 250 grams of zinc 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 800.degree. C.
[0062] The cell was operated in constant current mode. A constant
current of 2 amps was applied between the anode and cathode for a
period of 8 hours. During this time the potential between the anode
and the cathode remained at roughly 1.5 volts.
[0063] There were no gases evolved at the anode during
electrolysis. This was due to the formation of zinc oxide in the
molten zinc anode 62. A total charge of 57700 coulombs was passed
during the electrolysis reaction.
[0064] After a period of 8 hours the cathode and cathode pellet
were removed and the cathode pellet 50 had been discovered to have
reduced to tantalum metal. Analysis showed that the metal was
contaminated with zinc. Oxygen analysis of the reduced product
provided an average value of 2326 ppm, a carbon content of 723 ppm
and the product had a surface area of 0.3697 meters squared per
gram. Typical carbon contents of tantalum reduced in calcium
chloride at this temperature using carbon anodes in the same
experimental arrangement are 2000-3000 ppm. Considerable zinc
dusting was observed in the cold parts of the reactor.
[0065] In order to remove the zinc contamination from the tantalum,
the reduced product was placed in an alumina crucible and heated to
950.degree. C. for 30 minutes under an argon atmosphere. After
cooling the product was again examined in an SEM, it was found that
the contaminating zinc had been removed from the reduced product
leaving a tantalum powder.
[0066] It is believed that the overall reaction was
Ta.sub.2O.sub.5+5Zn=2Ta+5ZnO. Thus, for a 46 gram Ta.sub.2O.sub.5
pellet, 34.03 grams of zinc should theoretically be consumed. At
the cathode the reaction may be
Ta.sub.2O.sub.5+5e.sup.-=2Ta=50.sup.2-. The O.sup.2- may be
transported through the molten electrolyte to the molten zinc
anode. The reaction at the molten zinc anode may be
5Zn+50.sup.2-=5ZnO. Zinc oxide is a solid at the temperatures of
reduction. Zinc oxide formed at the surface is likely to become
entrapped within the molten zinc in the alumina crucible and,
therefore, free more molten zinc for reaction with further oxygen
ions.
Experiment 2
[0067] Lithium chloride used in this experiment was standard
lithium chloride 99% purity from Leverton Clarke. In a cell
configuration as illustrated in FIG. 1, a 45 g pellet 50 of
tantalum pentoxide was reduced in a lithium chloride salt for a
period of 25 hours at 750.degree. C. The cell was operated at a
constant current of 4 amps. The product was analysed and found to
have oxygen content of 2404 ppm, carbon content of 104 ppm and a
surface area of 0.3135 meters squared per gram. Less zinc dusting
in the cold parts of the reactor was evident compared to the
experiment performed at 800.degree. C.
[0068] The reduced product contained some zinc contamination. This
contamination could be removed by employing the heating process
described in experiment 1 above.
Experiment 3
[0069] A 45 g pellet of tantalum pentoxide was reduced in a lithium
chloride molten salt using a molten zinc anode at a temperature of
650.degree. C. A constant current of 4 amps was applied for a
period of 30 hours and the Product contained 1619 ppm oxygen, 121
ppm carbon and a surface area of 0.6453 m.sup.2/g. No gas evolution
during electrolysis was measured by mass spectrometry. Even less
zinc dusting in the cold parts of the reactor was evident compared
to the experiment performed at 800.degree. C. In contrast, tantalum
oxide reduced at 650.degree. C. in lithium chloride contained 1346
ppm carbon.
[0070] The reduced product contained some zinc contamination. This
contamination could be removed by employing the heating process
described in experiment 1 above.
Experiment 4
[0071] A 45 g pellet of tantalum pentoxide was reduced in a lithium
chloride molten salt using a 200 g molten zinc anode at a
temperature of 650.degree. C. A constant current of 4 amps was
applied for a period of 24 hours and the reduced product contained
2450 ppm oxygen, 9 ppm carbon and had a surface area of 0.6453
m.sup.2/g. ICP-MS analysis of the product showed a Fe content of 93
ppm, which was the approximate level in the starting oxide. In
contrast, tantalum pentoxide reduced in the same set-up but with
carbon anodes that generate anodic gases typically contain 500-1000
ppm iron contamination originating from the metal components of the
reactor that react with the anodic gases.
Experiment 5
[0072] A 28 g pellet of mixed titanium oxide, niobium oxide,
zirconium oxide and tantalum oxide was prepared by wet mixing the
powders, drying, pressing and sintering at 1000.degree. C. for 2
hours. This was reduced in lithium chloride using a zinc anode at
650.degree. C. by passing 295000 C of charge to produce an alloy
Ti-23Nb-0.7Ta-2Zr containing 37000 ppm oxygen and 232 ppm carbon.
No gases were evolved during electrolysis.
* * * * *