WO2013170310A1

WO2013170310A1 - Drained cathode electrolysis cell for production of rare earth metals

Info

Publication number: WO2013170310A1
Application number: PCT/AU2013/000515
Authority: WO
Inventors: Jeffery KENIRY; Anthony Rudland Kjar
Original assignee: Lynas Services Pty Ltd
Priority date: 2012-05-16
Filing date: 2013-05-15
Publication date: 2013-11-21
Also published as: WO2013170310A8

Abstract

An electrolytic cell for production of rare earth metals is described. The electrolytic cell includes a cell housing; a cathodic structure configured to define a channel, the channel having one end elevated in respect to an opposing lower end; one or more anodes having a lower anodic surface suspended within the cell housing, wherein the anodes are configured in longitudinal alignment above the cathodic structure in an arrangement whereby respective lower surfaces of the anodes are spaced from and substantially parallel to said upper surface of the cathodic structure along the length of the channel; and a sump for receiving molten rare earth metals from the channel, wherein the sump is spaced apart and isolated from the cathodic structure and the one or more anodes.

Description

Drained cathode electrolysis cell for production of rare earth metals

Field The present disclosure relates generally to electrolytic cells, in particular electrolytic cells adapted to produce rare earth metals, such as neodymium, praseodymium, lanthanum, cerium and mixtures thereof, by an electrolysis process in a molten fluoride or chloride salt bath. Background

Electrolytic cells for production of aluminium in a molten fluoride or chloride salt bath are well known and many of their design features address important considerations. In particular, it is important to maintain a stable and low anode-cathode distance (ACD) as an energy saving measure in a highly energy intensive process. Maintaining a constant ACD may prove difficult where molten aluminium pools on the surface of the cathode and is under hydrodynamic forces imposed by strong magnetic fields.

Accordingly, in some cell configurations, the cathodes may be suspended above the cell floor onto which the molten aluminium pools. In other configurations, the cathodes may be provided with channels into which the molten aluminium may collect, thereby draining the molten aluminium from the cathode surface as soon as it forms to maintain a constant ACD.

It is also important that the electrolytic cell is configured to liberate carbon dioxide gas, which evolves at the anode surface during the electrolysis, from the interelectrode space to substantially prevent 'back reaction' with the aluminium metal as it forms on the cathode surface, thereby reducing the efficiency of the electrolysis process.

Neodymium and praseodymium, mixtures thereof, and other rare earth metals, are also currently made commercially by an electrolysis process in a molten mixed fluoride salt bath. In contrast to the electrolytic production of aluminium, the anodes and cathodes are disposed in a vertical orientation and the molten metal is collected into a receiving vessel on the floor of the cell. The interelectrode space is not affected by the molten metal accumulation, but it is nevertheless subject to change by the continuous electrolytic consumption of the carbon anode surfaces. The cathodes are typically comprised of an inert metal, such as molybdenum or tungsten. As the anodes are consumed, there is no effective means to keep anode-cathode separation distance uniform. As the major part of the process heat is delivered by the ohmic resistance of the electrode spacing, the process temperature is highly variable and generally controlled by reduction in current supplied to the cell. This is impractical in larger scale operations where a number of cells would be connected in electrical series. Furthermore, deterioration in current throughout the electrolysis process is also undesirable since it decreases the production capacity of the cell. Most importantly, failure to closely control the process temperature reduces the process yield, or Faraday efficiency, and results in the formation of insoluble sludge which settles on the floor of the cell. Consequently, the electrolysis has to be periodically halted to remove the sludge from the cell, thereby inhibiting continuous electrolysis.

Poor control of the process temperature also increases the vapour emissions from the cell, which are harmful to the working atmosphere and the environment if they are not contained.

Additionally, as the anodes are consumed, their displaced volume in the electrolyte decreases and the electrolyte level in the cell falls. This reduces the working area of the anode immersed in the electrolyte, to the detriment of process efficiency including power consumption and increased possibility of 'anode effects' generating highly polluting gases.

Moreover, the product rare earth metal is reactive with carbon at the process temperature. Carbon is a highly undesirable impurity for certain rare earth metal product applications. Decreasing the possibility of contact between fugitive carbon in the cell and the metal and/or the residence time of product metal in the cell are desirable design attributes that are not apparent in the current commercial cell designs. This particular problem is not a factor in the design of electrolytic cells for aluminium production because aluminium does not react with carbon under these conditions.

Additionally, in current electrolytic cell designs for rare earth metals, it is difficult to maintain the product rare earth metals in a molten state because the operating temperatures are preferably only 10-30 °C above the freezing point of the product rare earth metals. This problem is not an issue and is not addressed in electrolytic cells for electrolytic production of aluminium because the process temperature is about 300 °C above the freezing point of aluminium. Current commercial activities for electrolytic production of rare earth metals are small in scale, labour intensive and operated in a semi-batch manner. Several deficiencies prevent the process from being scaled up to allow higher productivity, continuous electrolysis, and high standards of environmental performance, occupational health and safety to be achieved.

Firstly, the electrolysis cells generally operate in a limited current range of 5-10 kiloamperes, commensurate with low production capacity. There is poor control of a rare earth oxide feed material to the cell, resulting in the accumulation of insoluble sludges that require frequent cell clean-out thereby hindering continuous electrolysis. Additionally, feed material is delivered to the cell manually, without a known reference to the current oxide concentration in the cell. The existing technology uses vertical electrode arrangements. Such arrangements are not amenable to achieving a high Faraday efficiency. For example, gas bubbles which evolve and rise from the anode surface are likely to be entrained in the electrolyte flows and make contact with the product metal forming on the cathode plates, thereby reducing the process yield consequent to back-oxidation of the product metal.

Keller in US Patent No. 5,810,993 describes a method of producing neodymium in an electrolytic cell designed to operate without the occurrence of anode effects, therefore avoiding the generation and release of highly polluting perfluorinated carbon (PFC) gases. In this invention, the objectives are achieved firstly by providing a multitude of anode plates such that the anodic current density remains well below that at which the anode effect may occur, and secondly by physically separating the vertical cathodes from the vertical anodes using an inert barrier material which remains porous to neodymium ions, such that a higher concentration of dissolved neodymium oxide can be maintained in the anode region than in the cathode region. The disclosed invention has a number of deficiencies and impracticalities however. There is no demonstration in the cited examples that the barrier material (boron nitride) is indeed permeable to neodymium ions as would be required for a continuous electrolysis process. Further, the proposed anode design is complex and the wear rate of the anode plates may be expected to be highly non-uniform and wasteful. The compartmental separation of the anodic and cathodic zones further results in a large interelectrode separation distance, and a resulting inefficient energy consumption. Further, the invention proposes use of carbon as the inert cathode material, while it is well known that carbon will react with and contaminate the product metal.

There is therefore a need for alternative or improved electrolytic cells and processes for producing rare earth metals.

The above references to the background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the electrolytic cell as disclosed herein.

Summary of the Disclosure

In a first aspect there is disclosed an electrolytic cell for production of rare earth metals comprising:

a cell housing;

a cathodic structure having an upper cathodic surface configured to define a channel, the channel having one end elevated in respect to an opposing end;

one or more anodes having a lower anodic surface suspended within the cell housing, wherein the anodes are configured in longitudinal alignment above the cathodic structure in an arrangement whereby respective lower surfaces of the anodes are spaced from and substantially parallel to said upper surface of the cathodic structure; and,

a sump for receiving molten rare earth metals from the channel, wherein the sump is spaced apart and isolated from the cathodic structure and the one or more anodes.

In a second aspect there is disclosed an electrolytic cell for production of rare earth metals comprising:

a cell housing;

an inclined cathodic structure having an upper cathodic surface configured to define a V-shaped channel;

one or more anodes suspended within the cell housing in longitudinal parallel alignment above the inclined cathodic structure, wherein a lower surface of the or each anode is shaped and sized to complement the V-shaped channel; and,

a sump for receiving molten rare earth metals from the V-shaped channel, wherein the sump is spaced apart and isolated from the inclined cathodic structure and the one or more anodes. In a third aspect there is disclosed an electrolytic cell for production of rare earth metals comprising:

a cell housing;

5 an inclined cathodic structure having an upper cathodic surface configured to define a channel along which molten rare earth metals produced on the upper cathodic surface can drain;

one or more consumable anodes suspended within the cell housing in longitudinal parallel alignment above the inclined cathodic structure, wherein an upper l o surface of the or each consumable anode is provided with means to conductively adjoin a lower surface of a subsequent consumable anode; and,

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In a fourth aspect there is disclosed an electrolytic cell for production of rare earth metals comprising:

a cell housing;

an inclined cathodic structure having an upper cathodic surface configured to 20 define a channel along which molten rare earth metals produced on the upper cathodic surface can drain;

one or more consumable anodes suspended within the cell housing in longitudinal parallel alignment above the inclined cathodic structure;

a means to control a distance between the anodes and the upper cathodic 25 surface in response to anode consumption; and,

30 In a further aspect there is disclosed a system for electrolytically producing rare earth metals comprising:

an electrolytic cell in accordance with any one of the first, second, third or fourth aspects as defined above;

a feed material comprising one or more rare earth metal compounds capable of 35 undergoing electrolysis to produce rare earth metals;

a molten electrolyte in which the feed material is soluble; and,

a source of direct current configured to pass a current between an anode and a cathodic structure of the electrolytic cell to electrolyse the feed material and thereby produce molten rare earth metal product on the cathodic structure.

In another aspect there is disclosed a process for electrolytically producing rare earth metals comprising:

providing an electrolytic cell in accordance with any one of the first, second, third or fourth aspects as defined above;

charging the electrolytic cell with a feed material comprising one or more rare earth metal compounds capable of undergoing electrolysis to produce rare earth metals and an electrolyte bath comprising molten electrolyte in which the feed material is soluble;

passing a direct current between at least one consumable anode and a cathode in the electrolytic cell to electrolyse the feed material and thereby produce molten rare earth metal product on the cathode; and,

isolating the molten rare earth metal product in the sump spaced apart from the cathode and the at least one consumable anode.

providing an electrolytic cell in accordance with the fourth aspect;

translating the or each consumable anode toward the cathode in response to a rate of anode consumption to maintain a constant anode-cathode distance in the electrolytic cell.

Embodiments disclosed allow improved control capability for anode-cathode distance (ACD) and consequently process temperature, improved control of electrolyte height in the electrolytic cell and anode immersion, better mixing of the electrolyte to enhance dissolution of the oxide, and higher Faraday efficiency by limiting opportunity for back reaction of anode gas with produced metal. A further advantage of embodiments of the disclosure is that there is increased anodic surface area per footprint area of the cell, leading to increased cell capacity.

Brief Description of the Figures

5

Notwithstanding any other forms which may fall within the scope of the disclosure as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: l o Figure 1 is cross-sectional view of an electrode arrangement within an

electrolytic cell in accordance with one specific embodiment;

Figure 2 is a side view of the electrolytic cell in accordance with the specific embodiment referred to in Figure 1 ;

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Figure 3 is a side view of an electrode arrangement within an electrolytic cell in accordance with another specific embodiment;

Figure 4 is a cross-sectional view of an electrode arrangement within an

20 electrolytic cell in accordance with a further embodiment; and

Figure 5 is a side view of the electrolytic cell in accordance with the further embodiment referred to in Figure 4.

25 Detailed Description of Specific Embodiments

The description broadly relates to an electrolytic cell arranged to produce rare earth metals by an electrolysis process in a molten electrolytic salt bath.

30 The rare earth metals produced in the electrolytic cell disclosed herein include those rare earth metals having a melting point less than 1 100 °C. Exemplary rare earth metals include, but are not limited to, Ce. La, Nd, Pr, Sm, Eu, and alloys thereof including didymium and mischmetal.

35 The molten electrolytic salt bath behaves as a solvent for the feed material. The electrolyte for use in the molten electrolytic salt bath may comprise halide salts, in particular fluoride salts. Examples of 'fluoride salts' include, but are not limited to, metal fluoride salts including rare earth metal fluorides such as LaF₃, CeF₃, NdF₃ and PrF₃, alkali metal fluorides such as LiF, KF, and alkaline earth metal fluorides such as CaF₂, BaF₂. Selection of a feed material for the electrolysis process will depend on the desired rare earth metal product and the composition of the electrolyte. Where the electrolyte is composed of fluoride salts, the feed material that is subjected to the electrolysis process may comprise oxides of the rare earth metals. The term 'rare earth metal oxide' broadly refers to any oxide or any precursors of such oxides of a rare earth metal, including rare earth metal hydroxides, carbonates or oxalates. Rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium. Scandium and yttrium are considered rare earth metals since they tend to occur in the same ore deposits as the lanthanides and exhibit similar chemical properties. The lanthanides include lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Suitable examples of feed material for electrolytic production of neodymium or praseodymium include neodymium oxide (Nd₂0₃) or praseodymium oxide (Pr₆On). Where an alloy, such as didymium, is the desired product the feed material may comprise two or more oxides of rare earth metals (e.g. Nd₂0₃ and Pr₆On) in the desired stoichiometric ratio of the desired alloy. Mischmetal may be prepared from oxides of several rare earth metals, such as Ce, La, Nd, Pr, wherein the ratio of rare earth metals in the mischmetal corresponds to the ratio of rare earth metal oxides in the feed material.

Alternatively, where the electrolyte is composed of chloride salts, the feed material may comprise chloride salts of the rare earth metals.

In one embodiment the electrolyte comprises one or more rare earth metal fluorides and lithium fluoride. The one or more rare earth metal fluorides may be present in the electrolyte in a range of about 70-95 wt% with the balance as lithium fluoride.

Optionally, the electrolyte may further comprise up to 20 wt% calcium fluoride and/or barium fluoride. It will be appreciated by persons skilled in the art that the operating temperature of the electrolytic cell will depend on the target rare earth metal product or rare earth metal alloy, the composition of the electrolyte, and consequently the respective freezing points of the rare earth metal, alloy and electrolyte. In one embodiment, the operating temperature of the electrolytic cell may be in the range of 5 - 50 °C above the freezing point of the electrolyte, and preferably 10 - 20 °C above the melting point of the electrolyte. The composition of the electrolyte is selected so that the liquidus of the electrolyte may be in a range of 5 - 50 °C above the freezing point of the metal. In some embodiments, where the target rare earth metal product is mischmetal (a mixture of cerium, lanthanum, neodymium and praseodymium), the freezing point is variable depending on the composition of the mischmetal and the relative ratios of the rare earth metals therein, but nonetheless is around 800 °C. In these embodiments, the electrolyte may include barium or calcium fluorides as described above to achieve an electrolyte liquidus in the range of 5 - 50 °C above the freezing point of the mischmetal.

In other embodiments, where the freezing points of the rare earth metal alloys or mixtures are 800 °C or lower, the electrolyte may optionally comprise chloride salts.

Referring to Figures 1 to 5, where like numerals refer to like parts throughout, there are shown embodiments of an electrolytic cell 10 for production of rare earth metals. The cell 10 includes a housing 12, a cathodic structure 14, a sump 16, and a plurality of anodes 18.

The housing 12 is formed from anti-corrosive materials which are inert in view of the electrolyte composition and operating conditions, as has been described in the preceding paragraphs. In particular, the anti-corrosive materials used to internally line the housing 12 should be resistant to forming an alloy with the rare earth metals produced therein. In one embodiment the housing 12 may be lined internally with refractory materials. Suitable refractory materials include, but are not limited to, carbon, silicon carbide, silicon nitride, boron nitride, or certain stainless steels such as will be well known to those skilled in the art. The cathodic structure 14 includes an upper cathodic surface 20 configured to define an inclined channel 22. The upper cathodic surface 20 may be formed from an electrically conductive material with sufficient resistive heat properties to ensure free flow of the molten rare earth metals at temperatures marginally greater than their melting points. Such materials should be resistant to forming alloys with the rare earth metals produced on the cathodic surface 20. Suitable materials include, but are not limited to, refractory metals such as tungsten, molybdenum , or tantalum.

The inclined channel 22 is configured to receive molten rare earth metals produced on the upper cathodic surface 20. Generally, the inclined channel 22 extends longitudinally with respect to the housing 12, preferably along a centre line thereof. It will be appreciated, however, that the cathodic structure 14 may be provided with more than one inclined channel 22 disposed in parallel alignment with one another. The channel 22 may be inclined from the horizontal at an angle β of up to about 10°.

In one embodiment, as shown in Figure 1 , the channel 22 has a V-shape cross- section. It will be appreciated, however, that in alternative embodiments, the cross- section of the channel 22 may take other forms, such as a U-shape or a rectangular cross-section.

In some embodiments, such as shown in Figure 4, the channel 22 may be defined by opposing cathode surfaces 24 which downwardly incline towards one another at an angle a from the horizontal. Angle a may be in a range of from about 5° to about 45°, thereby forming a sharp V-shaped channel 22. In other embodiments, angle a may be in a range of from about 5° to about 20°, thereby forming a shallow V-shaped channel 22. The upper cathodic surface 20 may take various configurations to encourage draining of the molten rare earth metal from the upper cathodic surface 20 into the channel 22. In Figure 1 , the channel 22 is disposed between the opposing cathode surfaces 24 of the upper cathodic structure 20. In this particular embodiment, the opposing cathode surfaces 24 downwardly incline towards the channel 22 at angle a from the horizontal. Angle a may be in a range of from about 5° to about 45°, preferably from about 5° to about 20°.

In other embodiments, as shown in Figure 3, the upper cathodic surface 20 may comprise a plurality of pairs of opposing cathode surfaces 24 downwardly inclined towards one another. In this particular embodiment the pairs of opposing cathode surfaces 24 define a series of parallel V-shaped depressions 25 in the upper cathodic surface 20 which are laterally disposed with respect to the channel 22. Said V-shaped depressions 25 may be inclined towards the channel 22 so that, in use, molten rare earth metal formed on the upper cathodic surface 20 may drain into the channel 22 and thence into the sump 16. The upper cathodic surface 20 is supported by a body 26 of refractory and insulating material. The body 26 may house a plurality of electrical current delivery means 27 in the form of electrical current conduits, thereby ensuring substantially uniform current distribution throughout the upper cathodic surface 20. The sump 16 is configured relative to the channel 22 to receive, in use, molten rare earth metal from the lower end of the channel 22.

The housing 12 may also be provided with an upright plate 17 disposed proximal to the anode 18 adjacent to the opposing end 32 of the channel 22 and the sump 16. In use, the upright plate 17 is arranged to separate an active electrolysis zone (i.e. the cathodic structure 20 and the anodes 18) from a metal storage zone (i.e. sump 16) and to provide an obstacle to electrolyte flow in the vicinity of the sump 16 and thereby minimise contamination of molten rare earth metal product accumulated in the sump 16 with fugitive anodic material, such as carbon, or contact with (and therefore back- reaction with) gas which evolves at the anodes 18. In this way, the sump 16 and its contents are spaced apart and isolated from the cathodic structure 20 and the one or more anodes 18.

The upright plate 17 is formed from similar refractory materials as the inner linings of the housing 12 as described previously.

The arrangement of the sump 16 and the upright plate 17 allows for continuous removal of molten rare earth metal product from the upper cathodic surface 20 which provides several advantages. Continuous removal of the molten metal away from the active electrolysis zone into a quiescent storage sump that is isolated from the cathodic structure 20 and the anodes 18 reduces the possibility of undesirable reactions of the metal with fugitive anode carbon dust that may be present in the cell. Moreover, in prior art electrolytic cells where a pool of molten rare earth metal product is allowed to form, particularly on the floor of the cell or at a cathodic surface, it is common for the molten rare earth metal product to become contaminated with 'sludge' which comprises undissolved and partially molten rare earth feed material, reaction intermediates, and byproducts. In the electrolytic cell disclosed herein, in the absence of molten rare earth metal product, the sludge remains in contact with the cathodic surface and is thereby provided with an opportunity for re-dissolution in the molten electrolyte. The sump 16 may be provided with a heater to maintain a temperature above the melting point of the molten rare earth metal. The sump 16 may also be provided with a port (not shown) from which molten rare earth metal may be tapped as required. The sump 16 may be formed from inert metals similar to those used for the upper cathodic surface 20 as described previously.

The anodes 18 are suspended within the cell housing 12 above the cathodic structure 14 in parallel alignment therewith. In the form as illustrated, the anodes 18 comprise blocks of consumable anodic material having an upper surface 36, a lower surface 38, opposing elongate sides 40 and opposing ends 42. Suitable examples of consumable anodic material include, but are not limited to, carbon-based materials in particular high purity carbon, electrode grade graphite, calcined petroleum coke-coal tar pitch formulations. Such formulations will be well known to those skilled in electrolytic production of rare earth metals and other metals such as aluminium. The anodes 18 are configured in adjacent alignment with one another whereby opposing elongate sides 40 of adjacent anodes 18 are longitudinally aligned with one another. Respective opposing ends 42 of adjacent anodes 18 face one another. It will be appreciated by persons skilled in the art that spacing between facing opposing ends 42 of adjacent anodes 18 is as narrow as possible.

The blocks of anodic material are correspondingly sized so that, in the arrangement as described above, an effective length of the adjacently disposed anodes 18 is substantially the same as or marginally shorter than the length of the upper cathodic surface 20. Further, the blocks of anodic material are correspondingly sized so that the width of the anode 18 (i.e. distance between opposing elongate sides 40) is substantially the same as the width of the upper cathodic surface 20. Alternatively, as shown in Figure 1 , a pair of blocks of anodic material may be correspondingly sized and configures so that an effective width of the pair of anodes may be substantially the same as the width of the upper cathodic surface 20.

Alternatively, a single anode 18 having a similar length and width as the upper cathodic surface 20 may be employed in the electrolytic cell 10 as disclosed herein. The distance between the lower surfaces 38 of the anodes 18 and the upper cathodic surface 20 of the cathodic structure 14 is defined as the anode-cathode distance (ACD). The lower surfaces 38 of the anodes 18 are spaced from and substantially parallel to said upper surface 20 of the cathodic structure 14 such that the ACD is substantially constant over the area of the upper cathodic surface 20.

It will be appreciated therefore, with respect to the embodiment illustrated in Figures 4 and 5, that the lower surfaces 38 of the anodes 18 may be downwardly inclined from the horizontal at an angle γ of up to about 10° to complement the angle of inclination a of the upper cathodic surface 10. The complementary anode angles may be formed and maintained by the electrolytic consumption of the anode material, such that pre- shaping of the new anode surfaces is not necessary. Advantageously, the arrangement whereby both the lower surfaces 38 of the anodes 18 and the upper cathodic surface 20 are inclined, as shown in Figure 5, rather than horizontally disposed, provides an increased electrode surface area relative to the area occupied by the housing 12 of the electrolytic cell 10. Under most operating conditions the ACD in the electrolytic cell, as disclosed herein, may be between about 30 mm to about 200 mm, although an ACD of between about 50 mm to about 100 mm is preferred. The person skilled in the art may readily determine an appropriate ACD depending on the desired heat generation in the electrolyte zone, electrolyte flows for optimum solubility of the feed material, and optimisation of the process yield (Faraday efficiency).

The anode is consumed during electrolysis and consequently the ACD may increase as electrolysis progresses. The electrolysis cell 10 disclosed herein may be provided with a means to control the ACD, in particular to maintain a substantially constant ACD. Said means may comprise a vertical positioning device in operative

communication with the one or more anodes 18. In use, the vertical positioning device may downwardly translate the one or more anodes 18 in response to a rate at which the anode is consumed so that the ACD may remain substantially constant. The rate of anode consumption may be calculated by reference to current flow. Alternatively, the vertical positioning device may downwardly translate the one or more anodes 18 in response to variation in cell resistance from a predetermined value. Downwardly translating the one or more anodes 18 towards the upper cathodic surface 20 to maintain a constant ACD also has the concomitant effect of displacing the electrolyte in the housing 12 so as to maintain a substantially constant electrolyte bath height in the housing 12 throughout the electrolysis process. In some embodiments, the consumable anodes 18 may be replenished at appropriate intervals by fixing one or more subsequent anodes 18a to the upper surface 36 of the or each consumable anode 18, as illustrated in Figures 4 and 5. The upper surface 36 of the or each consumable anode 18 is provided with means to conductively adjoin the lower surface 38a of the one or more subsequent anodes 18a. Suitable means to conductively adjoin the upper surface 36 of the anode 18 with the lower surface 38a of the subsequent anode 18a may include, but is not limited to, conductive pins or plates formed from metal or other electrically conductive materials including electrically conductive glues. It will be appreciated that the lower surface 38 of the one or more subsequent anodes 18a will be shaped and sized to correspond with the upper surface 36 of the anode 18 to which subsequent anode 18a is conductively adjoined.

In one embodiment, the lower surfaces 38 of the anodes 18 are shaped and sized to complement the shape and size of the upper cathodic surface 20. In the form as illustrated in Figure 3, the lower surface 38 defines a shallow inverted V-shape formed from opposing sides 44a, 44b which downwardly taper towards one another at an angle γ from the horizontal. Angle γ corresponds to angle a which may be in a range of from about 5° to about 20°. In the form as illustrated in Figure 5, the opposing sides 44a, 44b define a sharper inverted V-shape and angle γ may be in a range of from about 5° to about 45°, corresponding to angle a of the upper cathodic surface 20. A sharper inverted V-shaped lower surface 38 results in a larger anodic surface area in comparison to a shallow inverted V-shaped lower surface 38, thereby resulting in a more effective release of bubbles at the anodic surface 38. Consequently, there is improved bubble driven mixing of the electrolyte with the oxide feed in the electrolytic cell 10 in the vicinity of the one or more anodes 18 having a lower surface 38 defining a sharper inverted V-shape.

In another embodiment, as shown in Figure 1 , the respective lower surfaces 38 of a pair of anodes 18 downwardly incline towards one another to complement the angle of inclination a of the corresponding upper cathodic surface 20.

The anode configuration in relation to the upper cathodic surface 20 has several advantages. Firstly, the anode-cathode distance (ACD) and the process temperature may be better controlled when the cathode is provided as a dimensionally stable surface without metal accumulation theron. Secondly, the electrolytic shaping of the anode under surfaces by the imprinting of the cathode shape allows the anode gases to be released in a more controlled manner, providing reduced potential for back- oxidation of molten rare earth metal product with the anode gases.

In some embodiments, as shown in Figure 5, the housing 12 of the electrolytic cell 10 may be provided with a sealing plate 46 disposed at an upper portion of the housing 12. The sealing plate 46 limits ingress of air into the electrolytic cell 10, thus avoiding oxidation at the anodic surface 38.

In use, the electrolysis process may be performed by charging the molten electrolyte to the electrolytic cell 10 as described herein. An alternating current may be supplied between the cathodic structure 14 and the one or more anodes 18 and the resistance of the electrodes 14, 18 raises the operating temperature of the electrolytic cell 10 to a predetermined temperature. Alternatively, the cell may be preheated to the operating temperature using externally applied heating means such as electrical elements of fuel burners. The feed material is then charged to the electrolytic cell 10 and dissolves in the molten electrolyte. A direct current in a range of 5-100 kiloamperes is supplied to the anodes 18, whereupon electrolysis of the dissolved feed material commences. In the electrolytic reaction the feed material is reduced to molten rare earth metal(s) on the upper cathodic surface of the cathodic structure 14. The molten rare earth metal(s) subsequently drain along the channel 22 into the sump 16, which is tapped as required. Feed material may be semi-continuously charged to the electrolytic cell 10, particularly in areas of high electrolyte to promote dissolution of feed material.

It will be appreciated that the electrolysis process may be performed under an inert or low oxygen atmosphere within the electrolytic cell 10. The inert atmosphere may be established and maintained by supplying an inert gas or gas mixtures to the electrolytic cell 10 to exclude air therefrom and thereby prevent undesirable reactions with the molten electrolyte and/or the electrodes 14, 18. Suitable examples of inert gases include, but are not limited to, helium, argon, and nitrogen.

Numerous variations and modifications will suggest themselves to persons skilled in the relevant art, in addition to those already described, without departing from the basic inventive concepts. All such variations and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the preceding description.

For example, it will be appreciated that the cathodic structure 14 may comprise more than one inclined channel 22 disposed in parallel alignment with one another, as illustrated in the Figures. In this alternative embodiment the electrolytic cell 10 may be provided with a single sump 16 extending along a collective breadth of the channels 22 or more than one sump 16, each sump 16 being disposed adjacent to respective lower ends of the channels 22. In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word "comprise" and variations such as "comprises" or "comprising" are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the apparatus and method as disclosed herein.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. An electrolytic cell for production of rare earth metals comprising:

a cell housing;

5 a cathodic structure having an upper cathodic surface configured to define a channel, the channel having one end elevated in respect to an opposing lower end; one or more anodes having a lower anodic surface suspended within the cell housing, wherein the anodes are configured in longitudinal alignment above the cathodic structure in an arrangement whereby respective lower surfaces of the anodes l o are spaced from and substantially parallel to said upper surface of the cathodic

structure along the length of the channel; and,

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2. An electrolytic cell for production of rare earth metals comprising:

a cell housing;

20 one or more anodes suspended within the cell housing in longitudinal parallel alignment above the inclined cathodic structure, wherein a lower surface of the or each anode is shaped and sized to complement the V-shaped channel; and,

a sump for receiving molten rare earth metals from the V-shaped channel, wherein the sump is spaced apart and isolated from the cathodic structure and the one 25 or more anodes. .

3. An electrolytic cell for production of rare earth metals comprising:

a cell housing;

an inclined cathodic structure having an upper cathodic surface configured to 30 define a channel along which molten rare earth metals produced on the upper cathodic surface can drain;

one or more consumable anodes suspended within the cell housing in longitudinal parallel alignment above the inclined cathodic structure, wherein an upper surface of the or each consumable anode is provided with means to conductively 35 adjoin a lower surface of a subsequent consumable anode; and,

a sump for receiving molten rare earth metals from the channel, wherein the sump is spaced apart and isolated from the cathodic structure and the one or more consumable anodes.

4. An electrolytic cell for production of rare earth metals comprising:

a cell housing;

an inclined cathodic structure having an upper cathodic surface configured to define a channel along which molten rare earth metals produced on the upper cathodic surface can drain;

a means to control a distance between the anodes and the upper cathodic surface in response to anode consumption; and,

a sump for receiving molten rare earth metals from the channel, wherein the sump is spaced apart and isolated from the inclined cathodic structure and the one or more anodes. .

5. The electrolytic cell as defined in claim 4, wherein the means to control the distance between the one or more anodes and the opposing sides of the cathodic structure comprises a vertical positioning apparatus in operative communication with the one or more anodes.

6. The electrolytic cell as defined in claim 5, wherein the vertical positioning

apparatus translates the one or more anodes towards the cathodic structure in response to a rate at which the anodes are consumed.

7. The electrolytic cell as defined in claim 6, wherein the rate at which the one or more anodes are consumed is determined by reference to current flow.

8. The electrolytic cell as defined in claim 6, wherein the rate at which the one or more anodes are consumed is determined by variation in cell resistance from a pre-determined value.

9. The electrolytic cell as defined in any one of the preceding claims, wherein the channel comprises a pair of opposing longitudinal surfaces which downwardly incline towards one another at an angle in a range of about 5° to about 45° from the horizontal.

10. The electrolytic cell as defined in claim 9, wherein the opposing longitudinal surfaces downwardly incline towards one another at an angle in a range of about 5° to about 20° from the horizontal.

1 1. The electrolytic cell as defined in any one of the preceding claims, wherein the channel or the V-shaped channel is inclined from the horizontal at an angle of

5 up to about 10°.

12. The electrolytic cell as defined in any one of the preceding claims, wherein the anodes are configured in adjacent alignment with one another whereby opposing elongate sides of adjacent anodes are longitudinally aligned with one l o another.

13. The electrolytic cell as defined in any one of the preceding claims, wherein lower surfaces of the anodes are sized and shaped by electrolytic consumption to complement the shape and size of the channel.

15

14. The electrolytic cell as defined in claim 13, wherein the respective lower

surfaces of the one or more anodes define a V-shape.

15. The electrolytic cell as defined in claim 14, wherein the lower surfaces taper 20 towards one another at an angle in a range of about 5° to about 45° from the horizontal.

16. The electrolytic cell as defined in any one of the preceding claims further

comprising an upright plate disposed between the lower end of the channel and 25 the sump for isolating the sump from the cathodic structure and the one or more anodes.

17. A system for electrolytically producing rare earth metals comprising:

an electrolytic cell in accordance with any of the preceding claims; 30 a feed material comprising one or more rare earth metal compounds capable of undergoing electrolysis to produce rare earth metals;

a molten electrolyte in which the feed material is soluble; and, a source of direct current configured to pass a current between an anode and a cathodic structure of the electrolytic cell to electrolyse the feed material 35 and thereby produce molten rare earth metal product on the cathodic structure.

18. A process for electrolytically producing rare earth metals comprising:

providing an electrolytic cell according to any one of claims 1 to 16; charging the electrolytic cell with a feed material comprising one or more rare earth metal compounds capable of undergoing electrolysis to produce rare earth metals and an electrolyte bath comprising molten electrolyte in which the feed material is soluble;

passing a direct current between at least one anode or consumable anode and a cathode in the electrolytic cell to electrolyse the feed material and thereby produce molten rare earth metal product on the cathode;

collecting and draining molten rare earth metal product in the channel or V-shaped channel; and,

isolating the molten rare earth metal product in the sump spaced apart from the cathode and the at least one consumable anode..

19. A process for electrolytically producing rare earth metals comprising:

translating the or each consumable anode toward the cathode in response to a rate of anode consumption to maintain a constant cathode-anode distance in the electrolytic cell.