METHOD OF PRODUCING METALLIC TANTALUM

Abstract

A method of producing metallic tantalum comprises the steps of providing a precursor comprising a tantalate of a first metal, arranging the precursor material in contact with a molten salt in an electrolytic cell, the electrolysis cell further comprising an anode and a cathode arranged in contact with the molten salt, and applying a potential between the anode and the cathode such that the precursor material is reduced to tantalum. The first metal is an alkali metal or an alkaline earth metal. The anode does not comprise a carbon material, which prevents contamination of the tantalum and improves current efficiency of the process.

Claims

1. A method of producing metallic tantalum comprising the steps of, providing a precursor material, the precursor material comprising a tantalate of a first metal, in which the first metal is an alkali metal or an alkaline earth metal, arranging the precursor material in contact with a molten salt in an electrolytic cell, the electrolysis cell further comprising an anode and a cathode arranged in contact with the molten salt, and applying a potential between the anode and the cathode such that the precursor material is reduced to tantalum, in which the anode does not comprise a carbon material.

2. The method according to claim 1, in which the precursor material is arranged in contact with the cathode and in which the anode comprises a molten second metal, the second metal being different to the first-metal and having a melting point that is sufficiently low enough for the second metal to be in the molten state during reduction of the precursor material, in which oxygen released from the precursor material when the potential is applied between the anode and the cathode reacts with the molten second metal at the anode.

3. The method according to claim 2, in which the second metal is a commercially pure metal, or in which the second metal is an alloy.

4. The method according to claim 2, in which the second metal has a melting point of less than 1000 degrees centigrade and a boiling point of less than 1750 degrees centigrade.

5. The method according to claim 2, in which the second metal is, or is an alloy of, any metal selected from the group consisting of zinc, tellurium, bismuth, lead, magnesium, tin, and aluminium.

6. The method according to claim 1, in which a proportion of the second metal is deposited at the cathode when the potential is applied between the anode and the cathode, such that the metallic tantalum comprises a proportion of the second metal, for example between 0.1 wt % and 5 wt % of the second metal, for example between 0.5 wt % and 2 wt %.

7. The method according claim 6, comprising the further step of separating the second metal from the metallic tantalum to provide a product that comprises less than 0.1 wt % of the second metal.

8. The method according to claim 2, in which oxygen removed from the precursor material during reduction reacts with the molten second metal at the anode to form an oxide between the oxygen and the second metal.

9. The method according to claim 8, in which substantially all of the oxygen removed from the precursor material reacts with the molten second metal at the anode to form an oxide between the oxygen and the second metal. cm 10. The method according to claim 1, in which the anode is a solid, non-carbon, oxygen-evolving anode.

11. The method according to claim 1, in which the first metal is calcium and the precursor material comprises a calcium tantalate, or in which the first metal is lithium and the precursor material comprises a lithium tantalate.

12. The method according claim 1, in which the precursor material comprises the most thermodynamically stable tantalate formable between tantalum and the metal.

13. The method according to claim 1, in which the precursor material comprises a calcium tantalate having the chemical formula Ca.sub.3(CaTa.sub.2)O.sub.9.

14. The method according to claim 1, in which the precursor material consists of the tantalate of the first metal.

15. The method according to claim 1, in which the precursor material is a mixture of the tantalate of the first metal and tantalum oxide, or a mixture of the tantalate of the first metal and metallic tantalum, or a mixture of the tantalate of the first metal, tantalum oxide and metallic tantalum.

16. The method according to claim 1, in which the salt comprises a salt of the first metal, or a halide salt of the first metal, or a chloride salt of the first metal.

17. The method according to claim 1, in which the salt comprises CaCl.sub.2.

18. The method according to claim 1, in which the precursor material is in the form of powder, agglomerates, or granules or in the form of a porous pellet or shaped preform.

19. The method according to claim 1, comprising the further step of forming the tantalate of the first metal.

20. The method according to claim 19, in which the tantalate of the first metal is formed by reaction between a tantalum oxide and the metal or a compound comprising the metal.

21. The method according to claim 19, in which the tantalate of the first metal is a calcium tantalate and is formed by reaction between a tantalum oxide, preferably Ta.sub.2O.sub.5, and calcium, the calcium preferably being in the form of CaCO.sub.3.

22. The method according to claim 19, in which the tantalate of the first metal is formed by a chemical co-precipitation or a sol-gel reaction.

23. The method according to claim 20, in which the tantalum oxide has a predetermined average particle size or is processed to produce a predetermined average particle size prior to forming the metal tantalate.

24. The method according to claim 1, further comprising the step of processing the precursor material prior to arrangement in the cell to provide a predetermined precursor average particle size or predetermined precursor particle size distribution.

25. The method according to claim 1, in which the precursor material has an average particle size of between 0.1 and 100 micron.

26. The method according to claim 1, in which the precursor material has an average particle size of about 0.5 to 10 micron.

27. The method according to claim 1, in which the molten salt is not in contact with a carbon material during reduction of the precursor material

28. The method according to claim 1, in which the metallic tantalum has a carbon content of lower than 250 ppm, or lower than 200 ppm, or lower than 150 ppm, or lower than 100 ppm.

29. The method according to claim 1, comprising the further step of forming the metallic tantalum into a capacitor.

30. A capacitor formed by a method comprising the steps of, providing a precursor material, the precursor material comprising a tantalate of a first metal, in which the first metal is an alkali metal or an alkaline earth metal, arranging the precursor material in contact with a molten salt in an electrolytic cell, the electrolysis cell further comprising an anode and a cathode arranged in contact with the molten salt, and applying a potential between the anode and the cathode such that the precursor material is reduced to tantalum, in which the anode does not comprise a carbon material, wherein the capacitor comprises an anode body and an anode lead formed from metallic tantalum having the same composition.

Description

[0073] Specific examples and embodiments of the invention will now be described with reference to the figures in which;

[0074] FIGS. 1 and 2 are SEM images of a partially reduced tantalum pentoxide powder, illustrating intermediate calcium tantalates of diverse size and shape;

[0075] FIG. 3 is a schematic diagram of an electrolytic cell suitable for performing a reduction of a metal tantalate by the FFC process using a consumable molten metal anode,

[0076] FIG. 4 is an SEM image of Ca.sub.3(CaTa.sub.2)O.sub.9 powder, and

[0077] FIG. 5 is an SEM image of Ca.sub.2Ta.sub.2O.sub.7 powder.

[0078] FIG. 1 is an SEM image illustrating a tantalum pentoxide powder that has been partially reduced in a calcium chloride salt using the FFC process. The initial pentoxide powder had been sieved through a 25 micrometers mesh prior to reduction. The partially reduced powder as illustrated in FIG. 1 consists of varying particles of calcium tantalate phases. As can be seen, these tantalates vary considerably in size and morphology. As some of the tantalate phases have different growth rates to the others the resulting partially reduced material has a non-homogeneous structure containing some very large particles. If the reduction was allowed to proceed to its ultimate conclusion, i.e. the production of tantalum metal, then the Ta powder formed will also have a non-homogeneous structure.

[0079] The inventors realised that, as the reaction pathway between tantalum pentoxide and tantalum metal during electrolysis in a molten salt is not easily controllable, there may be significant benefits to be had from producing a precursor material for reduction directly from, or comprising, one of the intermediate tantalates. The particle size and particle morphology of the intermediate tantalate could then be controlled in order to improve the control over the properties of the reduced tantalum.

[0080] Preferably, the intermediate tantalate will be the final tantalate in the reaction pathway, which in the case of reduction of tantalum pentoxide to tantalum metal appears to be the O.sub.9 tantalate.

[0081] FIG. 3 illustrates an electrolysis apparatus 10 for producing metallic tantalum by electrolytic reduction of a precursor material or feedstock. The apparatus 10 comprises a crucible 20 containing a molten salt 30. A cathode 40 comprising a pellet formed from the precursor material 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.

[0082] 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. Neither is any component of the apparatus that may contact the molten salt formed from a carbon material. A suitable crucible material may be alumina. The precursor material 50 is a metal tantalate, the metal being a group 1 metal or a group 2 metal. 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.

[0083] 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. A further preferred molten metal may be aluminium. 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 precursor material 50 on the one hand and the anode 60 and its associated molten metal 62 on the other.

[0084] Although the illustration of apparatus shown in FIG. 3 shows an arrangement 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 metal tantalate 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.

[0085] The method of will now be described in general terms with reference to FIG. 3. A potential is applied between the anode and the cathode such that oxygen is removed from the precursor material 50. This oxygen is transported from the precursor material 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.

[0086] The parameters for operating such an electrolysis cell such that oxygen is removed from an oxygen-bearing non-metallic feedstock are known through such processes as the FFC process. Preferably the potential is such that oxygen is removed from the precursor material 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 precursor material 50 is converted to metal and the molten metal 62 is converted, as least in part, to a metal oxide. The metallic tantalum product of the reduction can then be removed from the electrolysis cell.

[0087] The inventors have carried out 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.

[0088] Carbon analysis was performed using an Eltra CS800 analyser.

[0089] Oxygen analysis was performed using an Eltra ON900 analyser.

[0090] Surface area was measured using a Micromeritics Tristar surface area analyser.

[0091] One precursor material used for reduction in a calcium chloride salt was Ca.sub.3(CaTa.sub.2)O.sub.9. This tantalate was produced by calcination according to the following method.

[0092] The starting materials for the calcination were Ta.sub.2O.sub.5 with a primary crystallite size of about 0.3 micron and CaCO.sub.3 powder. D.sub.50 for the Ta oxide powder is 9 microns, due to aggregation of the particles. The Ta powder was then sieved at 25 micrometers. The CaCO.sub.3 powder was sieved at 106 micrometers.

[0093] These materials were mixed in a proportion of Ta.sub.2O.sub.5 to CaCO.sub.3=1.1244 and mixed in a turbular mixer for one hour. This proportion of tantalum pentoxide to calcium carbonate is slightly lower than the molar proportion required to form the O.sub.9 tantalate to prevent excess calcium oxide remaining in the tantalate powder. Calcination was performed at a temperature of 1200 C. for two hours, resulting in the formation of the Ca.sub.3(CaTa.sub.2)O.sub.9 tantalate powder.

[0094] A further precursor material used for reduction in a calcium chloride salt was Ca.sub.2Ta.sub.2O.sub.7. To make Ca.sub.2Ta.sub.2O.sub.7, tantalum pentoxide and calcium carbonate were mixed in a proportion of Ta.sub.2O.sub.5 to CaCO.sub.3=2.2075 and mixed in a turbular mixer for one hour. Calcination was performed at a temperature of 1200 C. for two hours, resulting in the formation of the Ca.sub.2Ta.sub.2O.sub.7 tantalate powder.

[0095] The tantalate powder was formed into pellets for reduction. The tantalate powder was then passed through a 106 micron sieve and pellets were pressed from the powder at a pressure of 20 bar (approximately 210.sup.6 Pascal). After pressing the pellets were sintered at a temperature of 1100 C. for six hours.

[0096] FIG. 4 illustrates the microstructure of the sintered Ca.sub.3(CaTa.sub.2)O.sub.9pellet. FIG. 5 illustrates the microstructure of the sintered Ca.sub.2Ta.sub.2O.sub.7 pellet. The uniformity of structure and fine scale particle size can be clearly seen in comparison with those formed during the reduction of tantalum pentoxide to tantalum (illustrated in FIGS. 1 and 2).

[0097] The porosity of the pellet formed and the particle size of the tantalate within the pellet may be controlled by varying the sintering temperature and/or time in order to have some control over the pellet properties prior to reduction.

EXAMPLES

[0098] Each of the examples was produced using the following conditions.

[0099] With reference to FIG. 3, tantalate pellets 35 were mounted onto a cathode 30 of an FFC cell 5 and reduced at a temperature of 650 C. using the FFC process. The salt 20 used in the electrolytic cell 5 was primarily lithium chloride containing 0.1-1.0 wt % lithium oxide. The anode 40 of the cell comprised either molten zinc or, for comparison reductions, carbon.

Example 1

[0100] A 38 g pellet of Ca.sub.3(CaTa.sub.2)O.sub.9was reduced to metal using a carbon anode. 277330 Coulombs were passed at a current of 3.5 Amps. The tantalum produced was recovered, analysed and found to have a surface area of 5.1 m.sup.2/g, an oxygen content of 17000 ppm, and a carbon content of 5719 ppm.

Example 2

[0101] A 20 g pellet of Ca.sub.3(CaTa.sub.2)O.sub.9 was reduced to metal using a molten zinc anode. 42458 Coulombs were passed at a current of 2 Amps. The tantalum produced was recovered, analysed and found to have a surface area of 5.6 m.sup.2/g, an oxygen content of 21000 ppm, and a carbon content of 493 ppm. The carbon content can be seen to be considerably lower than the comparative example produced using a carbon anode (Example 1).

Example 3

[0102] A 38 g pellet of Ca.sub.2Ta.sub.2O.sub.7was reduced to metal using a carbon anode. 270389 Coulombs were passed at a current of 3.5 Amps. The tantalum produced was recovered, analysed and found to have a surface area of 11.04 m.sup.2/g, an oxygen content of 34000 ppm, and a carbon content of 1817 ppm.

Example 4

[0103] A 38 g pellet of Ca.sub.2Ta.sub.2O.sub.7was reduced to metal using a molten zinc anode. 271492 Coulombs were passed at a current of 3.5 Amps. The tantalum produced was recovered, analysed and found to have a surface area of 6.74 m.sup.2/g, an oxygen content of 13000 ppm and a carbon content of 651 ppm. The carbon content can be seen to be considerably lower than the comparative example produced using a carbon anode (Example 3).

Example 5

[0104] A 20 g pellet of Ca.sub.2Ta.sub.2O.sub.7was reduced to metal using a molten zinc anode in a molten salt that had undergone a pre-electrolysis routine at a temperature of 650 C. 46218 Coulombs were passed at a current of 2 Amps. The tantalum produced had a surface area of 5.01 m.sup.2/g, an oxygen content of 14000 ppm and a carbon content of 386 ppm. Although this example is almost identical to example 4, the carbon content is even lower. This may be attributed to the pre-electrolysis of the salt removing residual carbonates from the salt, thereby further lowering carbon contamination. To produce tantalum with very low carbon levels it may be advantageous to electrolyse the salt to remove carbon compounds prior to introducing the tantalate.

[0105] The reductions described above allow tantalum powder to be formed having a BET surface area that is predictable and controllable. For example, to lower the BET surface area of tantalum powder the tantalate particle size in the precursor material could be increased by, for example, sintering the powder for a longer period of time or calcining the powder for an extended period of time to grow the tantalate particles. Likewise, a tantalum powder with an increased BET surface area could be produced by lowering the particle size of the starting tantalate.

[0106] Calcium and oxygen are released from the precursor material during the electrolytic reduction and the tantalum powder formed appeared to have a finer grain size and increased surface area compared with the starting tantalate.

[0107] 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.

[0108] A capacitor may be formed from any tantalum powder described above using the following exemplary method. A first portion of the tantalum powder may be selected and made into a tantalum wire using a drawing process. A second portion of the tantalum powder may be pressed to a density of 5.5 g/cm.sup.3 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 dielectric layer (of Ta.sub.2O.sub.5 with a portion of Al.sub.2O.sub.3) may then be formed on the anode by electrolysis using a current of 150 mA/g in a phosphoric acid solution at 85 C. between 10 and 100V, thereby forming the capacitor.