GB2534332A

GB2534332A - Method and apparatus for producing metallic tantalum by electrolytic reduction of a feedstock

Info

Publication number: GB2534332A
Application number: GB1411431.8A
Authority: GB
Inventors: Doughty Greg; Mellor Ian
Original assignee: Metalysis Ltd
Current assignee: Metalysis Ltd
Priority date: 2014-06-26
Filing date: 2014-06-26
Publication date: 2016-07-27
Also published as: GB201411431D0

Abstract

A method for producing metallic tantalum by electrolytic reduction of a feedstock comprising oxygen and tantalum comprising the steps of: arranging the feedstock 50 in contact with a cathode 40 and a molten salt 30 within an electrolysis cell 10, arranging a molten aluminium anode 60,62 in contact with the molten salt within the electrolysis cell, and applying a potential between the anode and the cathode to form metallic tantalum comprising between 0.01 and 5 wt% of aluminium. The oxygen removed from the feedstock reacts with the molten aluminium at the anode to form an oxide comprising aluminium rather than being liberated as a gas. The feedstock is preferably a powder and is an oxide of tantalum, or tantalum comprising a high concentration of oxygen. The aluminium may be commercially pure, or a eutectic aluminium alloy. The molten salt may be lithium or calcium chloride. The tantalum formed may be capacitor grade. Also claimed is a method of manufacturing an aluminium-doped tantalum capacitor, and apparatus 10 comprising a molten aluminium anode and a cathode in contact with a molten salt.

Description

Intellectual Property Office Application No. GII1411431.8 RTM Date:28 April 2016 The following terms are registered trade marks and should be read as such wherever they occur in this document: Cambridge FFC -Page 1 (not found on website) Polar-Page 1 Eltra -Page 11 Micromeritics Tristar -Page 11 Malvern -Page 11 Acros -Page 12 F/X -Page 12 Intellectual Property Office is an operating name of the Patent Office www.gov.uk /ipo Method and Apparatus for Producing Metallic Tantalum by Electrolytic Reduction of a feedstock The invention relates to a method and apparatus for producing metallic tantalum by electrolytic reduction of a feedstock comprising oxygen and tantalum.

Background

The present invention concerns a method for the production of metallic tantalum by reduction of a feedstock comprising oxygen and tantalum. 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.

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® 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.

Other reduction processes for reducing feedstock in the form of a cathodically connected solid metal compound have been proposed, such as the Polar® process described in WO 03/076690 and the process described in WO 03/048399.

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.

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.

Platinum has been used as an anode in LiCI-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.

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.

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.

Thus, this process does not appear to be a commercially viable method of producing metal.

Summary of the Invention

The invention provides a method and apparatus for producing metallic tantalum by electrolytic reduction of a feedstock, the feedstock being a compound comprising oxygen and tantalum, as defined in the appended independent claims. Preferred and/or advantageous features of the invention are set out in various dependent sub-claims.

In the first aspect a method for producing metallic tantalum by electrolytic reduction of a feedstock, the feedstock comprising oxygen and tantalum, preferably a compound comprising oxygen and tantalum. 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 metallic tantalum. The anode comprises molten aluminium. While aluminium 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. In other words, the potential is applied between the anode and the cathode when the molten salt is at a temperature at which the aluminium is molten. 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 aluminium and oxygen.

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 readily oxidises on contact with an oxygen species in order to form an oxide comprising aluminium and oxygen.

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 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.

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.

The use of a carbon anode would result in CO and CO2 evolution. Both CO and CO2 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.

It is preferred that the aluminium 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.

During operation of apparatus, a proportion of the aluminium from the anode is deposited at the cathode, where it deposits on and interacts with the metallic tantalum. The metallic tantalum thus comprises tantalum doped, or alloyed, with a proportion of aluminium from the anode. Doping, or alloying, tantalum with aluminium may give the metallic tantalum improved physical or electrical properties compared to un-doped tantalum. For example, aluminium-doped tantalum may exhibit a higher capacitance than pure tantalum metal.

The metallic tantalum may comprise aluminium in various proportions.

Preferably, the metallic tantalum may comprise tantalum and between 0.01 percent by weight (wt%) and 5 wt% of aluminium. For example, the metallic tantalum may comprise between 0.01 wt% and 3.0 wt% Al, 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 aluminium. The present invention may be a convenient way of alloying tantalum with a low proportion of aluminium.

Preferably, the proportion of aluminium comprised in the metallic tantalum 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 aluminium in the metallic tantalum.

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 tantalum. In a preferred embodiment, the feedstock is in the form of powder or particles having an average particle size of less than 5mm, for example less than 3mm, or less than 2mm.

The feedstock may preferably be an oxide of tantalum, for example tantalum pentoxide or tantalum dioxide.

The aluminium comprising the anode metal may be commercially pure aluminium metal. Alternatively, the anode 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.

It may be desirable that the molten salt is at a temperature below 1000°C when the potential is applied between the cathode and the anode. So that the SS ahiminium annrip is molten during the process, the molten salt must be maintained at a temperature greater than or equal to the melting point of aluminium. For example, when the anode metal is commercially pure aluminium metal, the molten salt should be maintained at a temperature greater than 660°C.

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. Alternatively, 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.

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.

The aluminium in the anode is consumed during the process due to the formation of an oxide between the aluminium and oxygen. The method may advantageously comprise the further step of reducing the aluminium oxide formed at the anode in order to recover and re-use the aluminium. 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 aluminium may be returned to the anode.

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.

As there is no carbon required for the electrolysis reaction to proceed, the product of the process, i.e. the metallic tantalum, has little to no carbon contamination. Carbon contamination is undesirable in tantalum for use in capacitors, as carbon contamination reduces the capacitance of metallic tantalum. The use of this method allows a direct reduction of an oxide material to metal at a commercially viable rate while eliminating carbon contamination.

Preferably, there is no carbon in contact with the molten salt within the electrolysis cell during the reduction process. Particularly preferably, the metallic tantalum produced by this process may comprise less than 100ppm carbon, for example less than 50ppm, or less than 25ppm carbon.

Preferably the metallic tantalum produced by this process may comprise a capacitor-grade tantalum alloy, i.e. a high-purity alloy which, when sintered into a capacitor, exhibits high capacitance and a high breakdown voltage.

The method may be used to reclaim tantalum powder that has become contaminated with oxygen. For example, the feedstock may be tantalum powder having a tantalum oxide coating, or an oxygen content of over 5,000 ppm or over 10,000 ppm. The method may then be conveniently used to reclaim the tantalum powder and dope the tantalum powder with Al.

In a further aspect a method of forming a capacitor may be provided. The method comprises the steps of forming a metallic tantalum powder using any method described above, pressing the tantalum powder to a density of between 5 and 6 g/cm3 and coupling the pressed powder to an anode lead, thereby forming a tantalum anode. A dielectric layer is then formed on the tantalum anode to form a capacitor. Preferably the capacitor has a composition comprising tantalum with 0.01 -3 wt % aluminium and the dielectric layer comprises both tantalum and aluminium oxides.

An electrochemical junction may form at the connection between the anode lead and the pressed powder due to compositional variations. This may not be desirable. Thus, the method may comprise the steps of forming a metallic tantalum powder using any method described above, taking a first portion of the tantalum powder and forming a tantalum wire suitable for use as an anode lead, taking a second portion of the tantalum powder and pressing the second portion of the tantalum powder to a density of between 5 and 6 g/cm3, and coupling the pressed powder formed from the second portion of tantalum powder to the anode lead formed from the first portion of tantalum powder, thereby forming a tantalum anode in which the anode lead and anode body are formed from tantalum having the same composition. A dielectric layer is then formed on the tantalum anode to form a capacitor. Preferably the capacitor has a composition comprising tantalum with 0.01 -3 wt % aluminium and the dielectric layer comprises both tantalum and aluminium oxides.

A capacitor formed using any method described herein may also be provided. For example, a capacitor may be provided in which the anode body and the anode lead are formed from tantalum of the same composition.

A capacitor formed from a metallic powder as described herein may have a capacitance of between 9-13.5 kCV/g for a powder with a specific surface area of about 0.3m2/g and a capacitance of between 310-565 kCV/g for a powder with a specific surface area of about 10m2/g.

In a further aspect, an apparatus is provided for producing metallic tantalum by electrolytic reduction of a feedstock, the feedstock comprising oxygen and tantalum. The apparatus 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 molten aluminium.

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 Specific embodiments of the invention will now be described with reference to the figures, in which Figure 1 is schematic diagram illustrating an apparatus according to one or more aspects of the invention; and Figure 2 is a schematic diagram of a second embodiment of an apparatus according to one or more aspects of the invention.

Figure 1 illustrates an exemplary electrolysis apparatus 10 for producing metallic tantalum by 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 tantalum 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 molten aluminium 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.

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 it is preferred that the crucible is not made from a carbon material. A suitable crucible material may be alumina. The tantalum oxide 50 may be any suitable tantalum oxide. The tantalum oxide 50 may be, for example, a pellet of tantalum pentoxide. The crucible 61 containing the molten aluminium 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.

The molten aluminium 62 is liquid in the molten salt at the temperature of operation. The molten aluminium 62 is capable of reacting with oxygen ions removed from the tantalum oxide to create an aluminium oxide. 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 tantalum oxide 50 on the one hand and the anode 60 and its associated molten aluminium 62 on the other.

The arrangement of the apparatus illustrated in Figure 1 assumes that the molten aluminium 62 is more dense than the molten salt 30. This arrangement is suitable, for example, where the salt is a lithium chloride salt and the molten aluminium is pure aluminium. In some circumstances, however, the molten aluminium may be less dense than the molten salt used for the reduction. In such a case an apparatus arrangement as illustrated in Figure 2 may be appropriate.

Figure 2 illustrates an alternative apparatus for producing metallic tantalum 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 tantalum oxide 150 and the cathode 140 and the pellet of tantalum 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 anode 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.

Although the illustrations of apparatus shown in Figures 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, a 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.

The method of operating the apparatus will now be described in general terms with reference to Figure 1. A cathode 40 comprising a tantalum oxide 50, and an anode 60 comprising molten aluminium 62, are arranged in contact with a molten salt 30 within an electrolysis chamber 20 of an electrolysis cell 10. A potential is applied between the anode and the cathode such that oxygen is removed from the tantalum oxide 50. This oxygen is transported from the tantalum oxide 50 towards the anode where it reacts with the molten aluminium 62 forming aluminium oxide. The oxygen is therefore removed from the tantalum oxide 50 and retained as an oxide at the anode.

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 tantalum oxide 50 and transported to the molten aluminium 62 of the anode without any substantial breakdown of the molten salt 30. As a result of the process the tantalum oxide 50 is converted to tantalum metal that is doped or alloyed with a proportion of aluminium, and the molten aluminium 62 is converted, at least in part, to aluminium oxide. The metallic tantalum product of the reduction can then be removed from the electrolysis cell.

The inventors have carried specific experiments based on this general method, and one of these is described below. The metallic tantalum product produced in the examples was analysed using a number of techniques. The following techniques were used.

Carbon analysis was performed using an Eltra CS800 analyser.

Oxygen analysis was performed using an Eltra ON900 analyser.

Surface area was measured using a Micromeritics Tristar surface area analyser.

Particle size was measured using a Malvern Hydro 2000MU particle size determinator.

Experiment 1 The aluminium used as the anode material was 99.5% Al shot supplied by Acros Organics. The tantalum oxide used as the feedstock was of 99.99% purity, and was pressed and sintered to around 45% porosity. The powder supplier was F&X electrochemicals.

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°C.

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.

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.

After reduction, the resulting metallic tantalum product was sieved and analysed. It was found that the coarser material retained by a 500pm sieve contained 5590ppm 0, 20ppm C, and had a surface area of 3.4464m2/g. The fine material that passed through the sieve contained 5873ppm 0, 87ppm C, and had a surface area of 1.3953m2/g. The product contained between 1.32 and 2,01wP/0 aluminium.

A capacitor may be formed from the coarser material, which has carbon levels of 20 ppm, using the following exemplary method. A first portion of the tantalum powder may be selected and made into a tantalum wire using a "AF drowine, nrne-nee A second portion of the tantalum powder may be pressed to a density of 5.5 g/cm3 onto the wire to form a tantalum anode. The tantalum anode may then be heat treated at a temperature of between 1000 and 1600°C for 10 minutes under vacuum. A di-electric layer (of Ta205 with a portion of A1203) may then be formed on the anode by electrolysis using a current of 150mA/g in a phosphoric acid solution at 85°C between 10 and 100V, thereby forming the capacitor.

Claims

Claims 1. A method for producing metallic tantalum by electrolytic reduction of a feedstock, the feedstock comprising oxygen and tantalum, 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 molten aluminium, and applying a potential between the anode and the cathode such that oxygen is removed from the feedstock to form metallic tantalum comprising between 0.01 and 5 wt% of aluminium, the oxygen removed from the feedstock reacting with the molten aluminium at the anode to form an oxide comprising aluminium.
2. A method according to claim 1 in which the metallic tantalum comprises tantalum and between 0.02 and 3 wt% of aluminium, for example 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 aluminium.
3. A method according to claim 1 or 2 in which controlling the length of time for which a potential is applied between the anode and the cathode determines the proportion of aluminium in the metallic tantalum.
4. A method according to any preceding claim in which the feedstock is a compound comprising oxygen and tantalum, for example an oxide of tantalum, or in which the feedstock is tantalum comprising a high concentration of oxygen.
5. A method according to any preceding claim in which the aluminium is commercially pure aluminium metal, or in which the second metal is an aluminium alloy, for example an alloy of eutectic composition.
6. A method according to any preceding claim in which the molten salt is at a temperature at which the aluminium is molten, but below 1000 degrees centigrade when the potential is applied between the cathode and the anode, preferably less than 850 degrees centigrade, preferably less than 800, or 750, or 700 degrees centigrade.
7. A method according to any preceding claim in which the molten salt is a lithium bearing salt or a calcium bearing salt, preferably a salt comprising lithium chloride or calcium chloride.
8. A method according to any preceding claim in which the metallic tantalum is a capacitor-grade tantalum alloy.
9. A method according to any preceding claim in which the metallic tantalum comprises less than 100ppm carbon, for example less than 5Oppm, or less than 25ppm carbon.
10. A method according to any preceding claim in which the feedstock is in the form of powder or particles having an average particle size of less than 3mm.
11. A method according to any preceding claim in which the metallic tantalum is a metal powder.
12. A method according to any preceding claim in which substantially no gases are evolved at the anode during electrolysis.
13. A method according to any preceding claim in which there is no carbon in contact with the molten salt within the electrolysis cell.
14. A method of manufacturing an aluminium-doped tantalum capacitor, comprising producing a powder according to the method of claims 1 to 13, and the further step of: providing an electrode in contact with the metallic tantalum; and sintering the metallic tantalum to form a capacitor.
15. A method of manufacturing an aluminium-doped tantalum capacitor, comprising steps of producing a metallic tantalum powder according to the method of any of claims 1 to 13, pressing the tantalum powder to a density of between 5 and 6 g/cm3, coupling the pressed powder to an anode lead, thereby forming a tantalum anode, and forming a dielectric layer on the tantalum anode to form a capacitor.
16. A method of manufacturing an aluminium-doped tantalum capacitor, comprising steps of producing a metallic tantalum powder according to the method of any of claims 1 to 13, taking a first portion of the tantalum powder and forming a tantalum wire suitable for use as an anode lead, taking a second portion of the tantalum powder and pressing the second portion of the tantalum powder to a density of between 5 and 6 g/cm3 forming an anode body and coupling the anode body to the anode lead formed from the first portion of tantalum powder, thereby forming a tantalum anode in which the anode lead and anode body are formed from tantalum having the same composition, and forming a dielectric layer on the tantalum anode to form a capacitor.
17. A method according to claim 14, 15, or 16 in which the capacitor has a composition comprising tantalum with 0.01 -3 wt % aluminium and the dielectric layer comprises both tantalum and aluminium oxides.
18. A capacitor formed by a method according to any of claims 14 to 17 having an anode body and an anode lead formed from metallic tantalum having the same composition.
19. An apparatus for producing tantalum metal by electrolytic reduction of a feedstock, the feedstock being a compound comprising oxygen and tantalum, 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 molten aluminium.
20. An apparatus according to claim 19, comprising a power source connected to the cathode and the anode.
21. An apparatus according to claim 19 or 20 in which there is no carbon in contact with the molten salt.