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.

Claims

1. A method for producing metal by electrolytic reduction of a feedstock comprising an oxide of 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 different to the first metal, and applying a potential between the anode and the cathode such that oxygen is removed from the feedstock, the oxygen removed from the feedstock reacting with the molten second metal to form an oxide comprising the second metal such that substantially no gases are evolved at the anode during electrolysis.

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, comprising the further step of separating the second metal from the first metal to provide a product that comprises the first metal but not the second metal.

4. The method according to claim 3, in which the second metal is separated from the first metal by thermal treatment, such as thermal distillation.

5. The method according to claim 3, in which the second metal is removed from the first metal by treatment using an acid wash.

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 second metal is an alloy, for example an alloy of eutectic composition.

8. The method according to claim 1, 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.

9. The method according to claim 1, in which the first metal is, or is an alloy of, any metal selected from silicon, scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, germanium, yttrium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, lanthanum, cerium, praseodymium, neodymium, samarium, actinium, thorium, protactinium, uranium, neptunium, or plutonium.

10. The method according to claim 1, in which the second metal is, or is an alloy of, any metal selected from zinc, tellurium, bismuth, lead, or magnesium.

11. The method according to claim 1, in which the molten salt is at a temperature below 1000 degrees centigrade when the potential is applied between the cathode and the anode.

12. The method according to claim 1, in which the molten salt is at a temperature below 850 degrees centigrade when the potential is applied between the cathode and the anode.

13. The method according to claim 1, in which the molten salt is at a temperature below 750 degrees centigrade when the potential is applied between the cathode and the anode.

14. The method according to claim 1, in which the molten salt is at a temperature below 650 degrees centigrade when the potential is applied between the cathode and the anode.

15. The method according to claim 1, in which the molten salt is a lithium bearing salt.

16. The method according to claim 15, in which the lithium bearing salt comprises lithium chloride.

17. The method according to claim 1, comprising a further step of reducing the oxide comprising the second metal to recover the second metal.

18. The method according to claim 17, in which the oxide comprising the second metal is transferred from the anode to a separate cell or chamber and reduced to recover the second metal, which is transferred back to the anode.

19. The method according to claim 1, in which the feedstock comprises a tantalum oxide and the anode comprises molten zinc.

20. The method according to claim 1, in which the feedstock comprises a titanium oxide and the anode comprises molten zinc.

21. The method according to claim 1, in which there is no carbon in contact with the molten salt within the electrolysis cell.

Description

SPECIFIC EMBODIMENTS OF THE INVENTION

(1) Specific embodiments of the invention will now be described with reference to the figures, in which

(2) FIG. 1 is schematic diagram illustrating an apparatus according to one or more aspects of the invention; and

(3) FIG. 2 is a schematic diagram of a second embodiment of an apparatus according to one or more aspects of the invention.

(4) 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.

(5) 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.

(6) 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.

(7) 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.

(8) 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.

(9) 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.

(10) 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.

(11) 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. 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.

(12) 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.

(13) Carbon analysis was performed using an Eltra CS800 analyser.

(14) Oxygen analysis was performed using an Eltra ON900 analyser.

(15) Surface area was measured using a Micromeritics Tristar surface area analyser.

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

Experiment 1

(17) Zinc used as the anode material was AnalaR Normapur 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.

(18) 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 C.

(19) 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.

(20) 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.

(21) 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.

(22) In order to remove the zinc contamination from the tantalum, the reduced product was placed in an alumina crucible and heated to 950 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.

(23) 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

(24) 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 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 C.

(25) The reduced product contained some zinc contamination. This contamination could be removed by employing the heating process described in experiment 1 above.

Experiment 3

(26) 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 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 C. In contrast, tantalum oxide reduced at 650 C. in lithium chloride contained 1346 ppm carbon.

(27) The reduced product contained some zinc contamination. This contamination could be removed by employing the heating process described in experiment 1 above.

Experiment 4

(28) 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 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

(29) 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 C. for 2 hours. This was reduced in lithium chloride using a zinc anode at 650 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.