Method and apparatus for electrolytic reduction of a feedstock comprising oxygen and a first metal

Abstract

A method of electrolytic reduction of a feedstock comprising oxygen and a first metal comprises the steps of, arranging the feedstock in contact with a cathode and a molten salt within an electrolysis cell, arranging an anode in contact with the molten salt within the electrolysis cell, the anode comprising a molten second metal and applying a potential between the anode and the cathode such that oxygen is removed from the feedstock to form a reduced feedstock. The oxygen removed from the feedstock reacts with the molten second metal to form an oxide comprising the second metal. The second metal is aluminium. The reduced feedstock may comprise a proportion of aluminium.

Claims

1. A method of electrolytic reduction of a feedstock, the feedstock comprising oxygen and a first metal, the method comprising the steps of, arranging the feedstock in contact with a cathode and a molten salt within an electrolysis cell, arranging an anode in contact with the molten salt within the electrolysis cell, the anode comprising a molten second metal, the second metal being aluminium, and applying a potential between the anode and the cathode such that oxygen is removed from the feedstock to form a reduced feedstock, the oxygen removed from the feedstock reacting with the molten second metal to form an oxide comprising the second metal, 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.

2. The method according to claim 1, in which the reduced feedstock is a metallic alloy comprising the first metal and between 0.01 percent by weight (wt %) and 5 wt % of the second metal, for example, the reduced feedstock may comprise between 0.01 wt % and 3.0 wt % of the second metal, or between 0.05 wt % and 2.0 wt %, or between 0.10 wt % and 1.50 wt %, or between 0.50 wt % and 1.0 wt % of the second metal.

3. The method according to claim 1, in which the length of time for which a potential is applied between the anode and the cathode is controlled to determine the proportion of the second metal in the reduced feedstock.

4. The method according to claim 1, in which the feedstock is a compound comprising oxygen and the first metal, for example an oxide of the first metal.

5. 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.

6. The method according to claim 1, in which the feedstock is a metallate compound, a metallate compound being a compound of the first metal, oxygen and at least one reactive metal, the reactive metal being a metal selected from the group consisting of calcium, lithium, sodium and potassium.

7. The method according to claim 1, in which the second metal is commercially pure aluminium metal, or in which the second metal is an aluminium alloy, for example an alloy of eutectic composition.

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

9. The method according to claim 1, in which the molten salt is at a temperature at which the second metal is molten, but below 1000 degrees centigrade when the potential is applied between the cathode and the anode, or less than 850 degrees centigrade, or less than 800, or 750, or 700 degrees centigrade.

10. The method according to claim 1, in which the molten salt is a lithium bearing salt or a calcium bearing salt, or a salt comprising lithium chloride or calcium chloride.

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

12. The method according to claim 1, in which the reduced feedstock is a titanium alloy comprising between 0.01 percent by weight (wt %) and 5 wt % of aluminium, for example, the reduced feedstock may comprise between 0.01 wt % and 3.0 wt % aluminium, or between 0.05 wt % and 2.0 wt %, or between 0.10 wt % and 1.50 wt %, or between 0.50 wt % and 1.0 wt % aluminium.

13. The method according to claim 1, in which the feedstock comprises a calcium titanate or a lithium titanate and the second metal is aluminium; or in which the feedstock is in the form of powder or particles having an average particle size of less than 3 mm; or in which the reduced feedstock is a metal powder.

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

15. The method according to claim 1, in which the reduced feedstock comprises less than 100 ppm carbon, for example less than 50 ppm, or less than 25 ppm carbon.

16. An apparatus for producing metal by electrolytic reduction of a feedstock comprising oxygen and a first metal, the apparatus comprising a cathode and an anode arranged in contact with a molten salt in which the cathode is in contact with the feedstock and the anode comprises a molten metal, the molten metal being aluminium.

17. The apparatus according to claim 16, comprising a power source connected to the cathode and the anode.

18. The apparatus according to claim 17, in which there is no carbon in contact with the molten salt.

19. The method according to claim 1, in which substantially no gases are evolved at the anode during electrolysis.

20. A method of electrolytic reduction of a feedstock, the feedstock comprising oxygen and a first metal and the feedstock additionally comprising aluminium or aluminium oxide, the method comprising the steps of, arranging the feedstock in contact with a cathode and a molten salt within an electrolysis cell, arranging an anode in contact with the molten salt within the electrolysis cell, the anode comprising a molten second metal, the second metal being aluminium, and applying a potential between the anode and the cathode such that oxygen is removed from the feedstock to form a reduced feedstock, the oxygen removed from the feedstock reacting with the molten second metal to form an oxide comprising the second metal, 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.

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 electrolytic reduction of an oxygen bearing feedstock such as an oxide feedstock. The apparatus 10 comprises a crucible 20 containing a molten salt 30. A cathode 40 comprising a pellet of metal oxide 50 is arranged in the molten salt 30. An anode 60 is also arranged in the molten salt. The anode comprises a crucible 61 containing a molten metal 62, and an anode connecting rod 63 arranged in contact with the molten salt 62 at one end and coupled to a power supply at the other. The anode connecting rod 63 is sheathed with an insulating sheath 64 so that the connecting rod 63 does not contact the molten salt 30.

(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 either aluminium or tin, both of which are liquid in the molten salt at the temperature of operation. The molten metal 62 must be capable of reacting with oxygen ions removed from the metal oxide to create an oxide of the molten metal species. The molten salt 30 may be any suitable molten salt used for electrolytic reduction. For example, the salt may be a chloride salt, for example, a calcium chloride salt comprising a portion of calcium oxide. Preferred embodiments of the invention may use a lithium based salt such as lithium chloride or lithium chloride comprising a proportion of lithium oxide. The anode 60 and cathode 40 are connected to a power supply to enable a potential to be applied between the cathode 40 and its associated metal oxide 50 on the one hand and the anode 60 and its associated molten metal 62 on the other.

(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 aluminium. In some circumstances, however, the molten metal may be less dense than the molten salt used for the reduction. In such a case an apparatus arrangement as illustrated in FIG. 2 may be appropriate.

(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 a liquid aluminium-magnesium alloy 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 aluminium, which is capable of being oxidised. A potential is applied between the anode and the cathode such that oxygen is removed from the metal oxide 50. This oxygen is transported from the metal oxide 50 towards the anode where it reacts with the molten aluminium 62 forming aluminium oxide. The oxygen is therefore removed from the oxide 50 and retained within a second oxide of the molten anode metal.

(12) 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.

(13) 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.

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

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

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

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

(18) Experiment 1

(19) Aluminium used as the anode material was 99.5% Al shot supplied by Acros Organics. A feedstock pellet of mixed titanium oxide, niobium oxide, zirconium oxide and tantalum oxide was prepared by wet mixing powders of the four oxides, before drying, pressing into a pellet and sintering for 2 hours at 1000° C.

(20) A 28 gram feedstock pellet of mixed oxides 50 was connected to a tantalum rod 40 and used as a cathode. 150 grams of aluminium 62 was contained in an alumina crucible 61 and connected to a power supply via a tantalum connecting rod 63 sheathed in a dense alumina tube 64. This construction was used as an anode 60. One kilogram of calcium chloride 30 was used as an electrolyte and contained within a large alumina crucible 20. The anode and pellet were arranged within the molten salt 30 and the temperature of the salt was raised to approximately 830° C.

(21) The cell was operated in constant current mode. A constant current of 4 amps was applied between the anode and cathode for a period of 23.4 hours.

(22) During this time the potential between the anode and the cathode remained at roughly 1.5 volts.

(23) There were no gases evolved at the anode during electrolysis. This was due to the formation of aluminium oxide in the molten aluminium anode 62. A total charge of 336680 coulombs was passed during the electrolysis reaction.

(24) After a period of 23.4 hours the cathode and cathode pellet were removed and the cathode pellet 50 had been discovered to have reduced to a metal alloy. Analysis showed that the metal alloy was contaminated with aluminium. Oxygen analysis of the reduced product provided an average value of 2289 ppm, a carbon content of 82 ppm and aluminium content of 4560 ppm.

(25) Aluminium oxide is a solid at the temperatures of reduction. Aluminium oxide formed at the surface is likely to become entrapped within the molten aluminium in the alumina crucible and, therefore, free more molten aluminium for reaction with further oxygen ions.

(26) Experiment 2

(27) In order to demonstrate the drop in carbon content provided by the method of the present invention, Experiment 1 was repeated using a carbon anode instead of a molten aluminium anode.

(28) A feedstock pellet of mixed titanium oxide, niobium oxide, zirconium oxide and tantalum oxide was prepared by wet mixing powders of the four oxides, before drying, pressing into a pellet and sintering for 2 hours at 1000° C.

(29) A 28 gram feedstock pellet of mixed oxides was connected to a tantalum rod and used as a cathode. A carbon anode was connected to a power supply via a tantalum connecting rod sheathed in a dense alumina tube. One kilogram of calcium chloride was used as an electrolyte and contained within a large alumina crucible. The anode and pellet were arranged within the molten salt and the temperature of the salt was raised to approximately 830° C.

(30) The cell was operated in constant current mode. A constant current of 4 amps was applied between the anode and cathode for a period of 18 hours. During this time the potential between the anode and the cathode remained at roughly 1.5 volts.

(31) A total charge of 259039 coulombs was passed during the electrolysis reaction.

(32) After a period of 18 hours the cathode and cathode pellet were removed and the cathode pellet 50 was discovered to have reduced to a metal alloy. Oxygen analysis of the reduced product provided an average oxygen value of 4039 ppm, and a carbon content of 3373 ppm. No aluminium was detected in the reduced metal alloy.

(33) This showed that the use of a carbon anode resulted in the reduced feedstock having a carbon content of 3373 ppm—much higher than the 82 ppm carbon content produced in the same reduced feedstock when using an aluminium anode.

(34) Experiment 3

(35) 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.

(36) 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.

(37) 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.

(38) After reduction, the resulting metallic tantalum product was sieved and analysed. It was found that the courser material retained by a 500 μm sieve contained 5590 ppm O, 20 ppm C, and had a surface area of 3.4464 m.sup.2/g. The fine material that passed through the sieve contained 5873 ppm O, 87 ppm C, and had a surface area of 1.3953 m.sup.2/g. The product contained between 1.32 and 2.01 wt % aluminium.

(39) Experiment 4.

(40) In a further example, a 28 g pellet was manufactured from a sample of Iluka NR95 natural rutile powder. The powder was sieved to select a fraction consisting of particles having a particle size range of 150-212 microns. The pellet was reduced in calcium chloride using an molten aluminium anode. EDX analysis of the reduced product showed an aluminium content of 1.3 wt. %.