Electrolytic production of powder

Abstract

A method of producing metallic powder comprises steps of arranging a volume of feedstock comprising a plurality of non-metallic particles within an electrolysis cell, causing a molten salt to flow through the volume of feedstock, and applying a potential between a cathode and an anode such that the feedstock is reduced to metal. In preferred embodiments the feedstock is a plurality of discrete powder particles and these particles are reduced to a corresponding plurality of discrete metallic particles. In advantageous embodiments, the feedstock may be sand.

Claims

1. A method for producing metallic powder comprising the steps of: arranging a cathode and an anode in contact with a molten salt within an electrolysis cell, arranging a volume of feedstock comprising a plurality of non-metallic particles within the electrolysis cell, in which the volume of feedstock is arranged on an upper surface of the cathode and a lower surface of the anode is vertically spaced from the feedstock and the upper surface of the cathode, and in which the D90 particle size of the feedstock is no more than 100% greater than the D10 particle size of the feedstock and in which the particles making up the feedstock have an average particle diameter of less than 5 mm, and in which the feedstock has an average crystallite size that is greater than 10% of the average particle size, causing the molten salt to flow through the volume of feedstock, and applying a potential between the cathode and the anode such that the feedstock is reduced to metal.

2. The method according to claim 1, in which the D10 particle size for the feedstock is greater than 60 microns and the D90 particle size for the feedstock is lower than 3 mm.

3. The method according to claim 1, in which the feedstock is a bulk feedstock that has not been settled or compacted.

4. The method according to claim 1, in which the feedstock has a voidage of greater than 43%.

5. The method according to claim 1, in which the particles making up the feedstock are porous.

6. The method according to claim 1, in which the particles making up the feedstock have a density of between 3.5 g/cm.sup.3 and 7.5 g/cm.sup.3.

7. The method according to claim 1, in which the feedstock comprises a first set of particles having a composition in which a first metallic element forms the greater proportion by mass, and a second set of particles in which a second metallic element forms the greater proportion by mass, the feedstock being reduced under conditions such that there is no alloying between the first set of particles and the second set of particles.

8. The method according to claim 1, in which the feedstock comprises one or more naturally occurring minerals.

9. The method according to claim 8, in which the one or more minerals is one or more of rutile, ilmenite, anatase, leucoxene, scheelite, cassiterite, monazite, lanthanum, zircon, cobaltite, chromite, bertrandite, beryl, uranite, pitchblende, quartz, molybdenite or stibnite.

10. The method according to claim 1, in which the feedstock comprises a synthetic mineral.

11. The method according to claim 1, in which the feedstock comprises a first non-metallic particle having a first composition and a second non-metallic particle having a second composition, in which the feedstock is reduced under conditions such that the first non-metallic particle is reduced to a first metallic particle having a first metallic composition and the second non-metallic particle is reduced to a second metallic particle having a second metallic composition.

12. The method according to claim 1, in which the feedstock comprises more than 94% wt of TiO.sub.2.

13. The method according to claim 1, in which the feedstock particles have an average diameter and the feedstock is loaded onto the upper surface of the cathode to a feedstock depth of between 10 and 500 times the average diameter of the feedstock particles.

14. The method according to claim 1, in which the feedstock particles comprise crystallites having an average crystallite diameter and the feedstock is loaded onto the upper surface of the cathode to a feedstock depth of between 10 and 500 times the average diameter of the feedstock crystallites.

15. The method according to claim 1, in which the upper surface of the cathode comprises a mesh having a mesh size smaller than the D10 particle size of the feedstock.

16. The method according to claim 1, in which the cathode comprises a retaining barrier allowing the feedstock to be supported on its upper surface to a depth of greater than 5 mm.

17. The method according to claim 16, in which the retaining barrier is a peripheral barrier.

18. The method according to claim 1, in which the feedstock is reduced with substantially no sintering between particles such that a powder can be recovered having an average diameter lower than an average diameter of the particles making up the feedstock.

19. The method according to claim 1, in which the reduced feedstock forms a friable mass of metallic particles that may be broken up to form the metallic powder, substantially each of the particles forming the metallic powder corresponding to one non-metallic particle in the feedstock.

20. The method according to claim 1, in which the feedstock consists of free-flowing discrete particles of non-metallic material.

21. The method according to claim 20, in which the free-flowing discrete particles have an average size (D50) of between 100 and 250 microns as measured by laser diffraction.

22. The method according to claim 1, in which the feedstock comprises synthetic rutile.

23. A method for producing metallic powder comprising the steps of: arranging a cathode and an anode in contact with a molten salt within an electrolysis cell, arranging a volume of feedstock comprising a plurality of non-metallic particles within the electrolysis cell, in which the particles making up the feedstock are crystalline and have an average crystallite size of greater than 10 micrometers, causing the molten salt to flow through the volume of feedstock, and applying a potential between the cathode and the anode such that the feedstock is reduced to metal.

24. A method for producing metallic powder comprising the steps of: arranging a cathode and an anode in contact with a molten salt within an electrolysis cell, arranging a volume of feedstock comprising a plurality of non-metallic particles within the electrolysis cell, in which the feedstock has an average crystallite size that is greater than 10% of the average particle size, causing the molten salt to flow through the volume of feedstock, and applying a potential between the cathode and the anode such that the feedstock is reduced to metal.

25. The method according to claim 24, in which the feedstock comprises a synthetic mineral.

26. The method according to claim 24, in which the feedstock comprises more than 94% wt of TiO.sub.2.

27. The method according to claim 24, in which the feedstock is reduced with substantially no sintering between particles such that a powder can be recovered having an average diameter lower than an average diameter of the particles making up the feedstock.

28. The method according to claim 24, in which the reduced feedstock forms a friable mass of metallic particles that may be broken up to form the metallic powder, substantially each of the particles forming the metallic powder corresponding to one non-metallic particle in the feedstock.

29. The method according to claim 24, in which the feedstock consists of free-flowing discrete particles of non-metallic material.

30. The method according to claim 24, in which the feedstock comprises one or more naturally occurring minerals.

31. The method according to claim 30, in which the one or more minerals is one or more of rutile, ilmenite, anatase, leucoxene, scheelite, cassiterite, monazite, lanthanum, zircon, cobaltite, chromite, bertrandite, beryl, uranite, pitchblende, quartz, molybdenite or stibnite.

32. The method according to claim 24, in which the feedstock comprises synthetic rutile.

33. A method for producing metallic powder comprising the steps of: arranging a cathode and an anode in contact with a molten salt within an electrolysis cell, arranging a volume of feedstock comprising a plurality of non-metallic particles within the electrolysis cell, in which the volume of feedstock is arranged on an upper surface of the cathode and a lower surface of the anode is vertically spaced from the feedstock and the upper surface of the cathode, and in which the D90 particle size of the feedstock is no more than 100% greater than the D10 particle size of the feedstock and in which the particles making up the feedstock have an average particle diameter of less than 5 mm, and in which the particles making up the feedstock are substantially free from porosity, causing the molten salt to flow through the volume of feedstock, and applying a potential between the cathode and the anode such that the feedstock is reduced to metal.

34. A method for producing metallic powder comprising the steps of: arranging a cathode and an anode in contact with a molten salt within an electrolysis cell, arranging a volume of feedstock comprising a plurality of non-metallic particles within the electrolysis cell, in which the volume of feedstock is arranged on an upper surface of the cathode and a lower surface of the anode is vertically spaced from the feedstock and the upper surface of the cathode, and in which the D90 particle size of the feedstock is no more than 100% greater than the D10 particle size of the feedstock and in which the particles making up the feedstock have an average particle diameter of less than 5 mm, and in which the particles making up the feedstock are crystalline and have an average crystallite size of greater than 10 micrometers, causing the molten salt to flow through the volume of feedstock, and applying a potential between the cathode and the anode such that the feedstock is reduced to metal.

Description

SPECIFIC EMBODIMENT OF THE INVENTION

(1) A specific embodiment of the invention will now be described with reference the accompanying drawings, in which;

(2) FIG. 1 is a schematic diagram illustrating an electrolysis apparatus arranged for performing a method according to an embodiment of the invention,

(3) FIG. 2A is a schematic cross-sectional view illustrating additional detail of the cathode structure of the electrolysis apparatus of FIG. 1,

(4) FIG. 2B is a plan view of the cathode illustrated in FIG. 2A,

(5) FIGS. 3 and 4 are SEM (scanning electron micrography) micrographs illustrating particles of a rutile sand feedstock,

(6) FIGS. 5 and 6 are SEM micrographs illustrating metallic powder particles resulting from the reduction of a rutile sand feedstock using a method according to an embodiment of the invention,

(7) FIG. 7 is a SEM micrograph illustrating particles of a synthetic rutile feedstock, and

(8) FIG. 8 is an SEM micrograph illustrating titanium particles resulting from the reduction of a synthetic rutile feedstock.

(9) FIG. 1 illustrates an electrolysis apparatus 10 configured for use in performing a reduction method embodying the invention. The apparatus comprises a stainless steel cathode 20 and a carbon anode 30 situated within a housing 40 of an electrolysis cell. The anode 30 is disposed above, and spatially separated from, the cathode 20. The housing 40 contains 500 kg of a calcium chloride based molten salt electrolyte 50, the electrolyte comprising CaCl.sub.2 and 0.4 wt % CaO, and both the anode 30 and the cathode 20 are arranged in contact with the molten salt 50. Both the anode 30 and the cathode 40 are coupled to a power supply 60 so that a potential can be applied between the cathode and the anode.

(10) The cathode 20 and the anode 30 are both substantially horizontally oriented, with an upper surface of the cathode 20 facing towards a lower surface of the anode 30.

(11) The cathode 20 incorporates a rim 70 that extends upwards from a perimeter of the cathode and acts as a retaining barrier for a feedstock 90 supported on an upper surface of the cathode. The rim 70 is integral with, and formed from the same material as, the cathode. In other embodiments, the rim may be formed from a different material to the cathode, for example from an electrically insulating material.

(12) The structure of the cathode may be seen in more detail in FIG. 2A and FIG. 2B. The rim 70 is in the form of a hoop having a diameter of 30 cm. A first supporting cross-member 75 extends across a diameter of the rim. The cathode also comprises a mesh-supporting member 71, which is in the form of a hoop having the same diameter as the rim 70. The mesh-supporting member has a second supporting cross-member 76 of the same dimensions as the supporting cross-member 75 on the rim 70. A mesh 80 is supported by being sandwiched between the rim 70 and the mesh-supporting member 71 (the mesh 80 is shown as the dotted line in FIG. 2A). The mesh 80 comprises a stainless steel cloth of mesh-size 100 that is held in tension by the rim 70 and the mesh-supporting member. The cross-member 75 is disposed against a lower surface of the mesh 80 and acts to support the mesh. An upper surface of the mesh 80 acts as the upper surface of the cathode.

(13) The stainless steel cloth forming the mesh 80 is fabricated from 30 micrometer thick wires of 304 grade stainless steel that have been woven to form a cloth having square holes with a 150 micrometer opening. The mesh 80, cross-member 75 and rim 70 that form the cathode are all electrically conductive. In other embodiments, the mesh may be the only electrically conductive component of the cathode.

Example 1

(14) A method embodying the invention will be illustrated with an example in which the feedstock to be reduced is a natural conventionally beneficiated rutile sand. Rutile is a naturally occurring mineral containing a high proportion (perhaps 94-96 wt %) of TiO.sub.2. Rutile sand also contains many other elements and particles or grains of other non-rutile minerals. The skilled person will be aware of the compositions of typical rutile sands.

(15) The rutile sand used in this specific example comprises grains of material having an average particle diameter as measured by laser diffraction (using a Malvern Mastersizer Hydro 2000MU) of about 200 micrometers and a bulk density of about 2.3 g/cm.sup.3. The density of individual grains forming the sand may be in the range from about 4 g/cm.sup.3 to about 7 g/cm.sup.3, depending on the composition and crystal structure of each individual grain. FIG. 3 is a SEM micrograph illustrating the individual particles in the feedstock. The particles are mainly angular and predominantly TiO.sub.2.

(16) The SEM micrograph of FIG. 4 illustrates a polished section of some of the individual grains. The majority of the particles are imaged having a light grey colour 400 and are grains that are substantially TiO.sub.2 (although there will be many impurity elements and each grain will have a slightly different composition). One of the grains is imaged as a lighter grey 410. This is a particle of zircon. Another grain has a darker grey colouring 420 and this is a grain with a high concentration of silicon indicating it is probably quartz.

(17) About 3 kg of the feedstock 90, consisting of natural rutile sand, was arranged on the upper surface of the cathode 20 and in contact with the molten salt 50 (which consisted of CaCl.sub.2 and 0.4 wt % CaO). Thus, the rutile sand 90 was supported by the mesh 80 of the cathode and retained at a depth of approximately 2 cm by the cathode-rim 70. The bed depth of the rutile is approximately 100 times the average particle diameter of the rutile sand particles.

(18) The molten salt was maintained at a temperature of about 1000 C. and a potential was applied between the anode and the cathode. Thermal currents and gas lift effect generated by the buoyancy of the gases (which are predominantly CO and CO.sub.2) generated at the anode cause the molten salt to circulate within the cell and generate flow through the bed of rutile supported on the cathode. The cell was operated in constant current mode, at a current of 400 A, for 52 hours. After this time, the cell was cooled and the cathode removed and washed to free salt from the reduced feedstock.

(19) The reduced feedstock was removed from the cathode as a friable lump or cake of metallic powder particles that could be separated using light manual pressure. The lumps of material were tumbled in a barrelling tumbler containing alumina balls, and the material separated out into individual powder particles. These powder particles were then dried.

(20) FIGS. 5 and 6 are SEM micrographs illustrating individual powder grains from the reduced sand. It can be seen that the metallic particles of the powder correspond in size and shape to the grains that formed the sand (the average particle size of the reduced material is slightly lower than the average particle size of the feedstock). Analysis revealed that the compositional differences between individual grains forming the feedstock were maintained in the individual grains forming the reduced powder. This suggests that each individual grain has been reduced individually to metal within the bed and that alloying between grains of different composition has not occurred.

Example 2

(21) FIG. 7 is an SEM image showing synthetic rutile particles formed by treating ilmenite (by leaching as described above) to remove unwanted elements. The particles are slightly porous when compared with natural rutile. A feedstock was prepared by sieving synthetic rutile particles and selecting the fraction falling between meshes of 63 microns and 212 microns.

(22) 1129 grams of the synthetic rutile feedstock was loaded onto the upper surface of a cathode and reduced as described above in relation to Example 1, except that the temperature of the salt was maintained at 980 degrees centigrade and the reduction proceeded for 50 hours. After reduction a powder was extracted and washed as described above.

(23) FIG. 8 illustrates a titanium powder particle from the resulting powder. It can be seen that the general size and shape of the metallic particle is of the same order as the feedstock particles, but the metallic particle is more porous and has a slightly nodular shape.

Example 3

(24) The following experiments were carried out to investigate the effect of different particle size ranges on progress of reduction. A rutile sand material was sourced from ABSCO Materials that comprised greater than 95% TiO2 and had a particle size range defined as a maximum of 4% of material retained on a 180 micron sieve. This material was taken by the applicant and sieved (using Retch brand sieves) into three fractions. The fractions were (1) particles having diameter less than 150 microns (i.e. particles that passed through a sieve having a mesh size of 150 microns), (2) particles having a diameter between 150 microns and 212 microns (i.e. particles that pass through a sieve of 212 micron mesh size but are retained by a sieve having 150 micron mesh size), and (3) particles having a diameter greater than 212 microns (i.e. particles that are retained by a sieve having a mesh size of 212 microns). Each of these three size fractions was used as a free-flowing particulate feedstock for reduction to metal. Particle size distribution was measured for each fraction using laser diffraction (Malvern Mastersizer Hydro 4000MU). These results are shown in table 1 below.

(25) The reduction of each feedstock was carried out substantially as described above in relation to Example 1. Reduction was performed in a molten salt consisting of CaCl.sub.2 with 0.6 wt % CaO held at a temperature of 950 C. Reduction was performed at a constant current of 400 A for a period of 68 hours. The distance between the cathode and the anode was set as 5 cm.

(26) The bulk density and bed porosity for each feedstock were calculated, and the results are given in table 1 below. For these calculations it was assumed that the grains had the same density as TiO.sub.2.

(27) TABLE-US-00001 TABLE 1 Parameters of three rutile feedstocks having different particle sizes. Sieve Bulk Bed Feed- fraction density porosity stock (m) D10 (m) D50 (m) D90 (m) (g/cm.sup.3) (%) (1) <150 108 156 225 2.30 45.6 (2) 150-212 121 180 267 2.38 43.7 (3) >212 205 280 382 2.44 42.3

(28) After reduction for 68 hours, feedstock number 2 (150-212 micron size fraction) and feedstock number 3 (>212 micron size fraction) had reduced to discrete particles of titanium. Oxygen analysis on the titanium powder product of these reductions (using Eltra ON-900) showed that oxygen had been reduced to levels of between 3000 and 4500 ppm.

(29) Feedstock number 1 (size fraction <150 micron), however, did not fully reduce, and did not form discrete particles of titanium. A metallic crust had formed on the top and bottom of the feedstock bed and the centre of the bed had converted to calcium titanates. This suggests that there was insufficient salt flow through the bed of feedstock 1. This may be attributable to the small size of the interstices between particles in feedstock 1, as compared with relatively larger interstices between particles in feedstock number 2 and number 3.