Patent Application
20150159286
The present disclosure relates generally to electrolytic cells, in particular electrolytic cells adapted to produce rare earth metals, such as neodymium, praseodymium, cerium, lanthanum and mixtures thereof, by an electrolysis process in a molten fluoride or chloride electrolyte bath.
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 U.S. Pat. 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.
In a first aspect there is disclosed an electrolytic cell for production of rare earth metals comprising:
In a second aspect there is disclosed an electrolytic cell for production of rare earth metals comprising:
In one embodiment said displacement device controls the height of the electrolyte bath contained in the cell housing in response to anode consumption and a volume of rare earth metal product contained in the cell housing.
In a third aspect there is disclosed an electrolytic cell for production of rare earth metals comprising:
In a further aspect there is disclosed a system for electrolytically producing rare earth metals comprising:
In another aspect there is disclosed a process for electrolytically producing rare earth metals comprising:
In one embodiment, the step of displacing is performed in response to a rate of anode consumption and/or a change in a volume of rare earth metal product contained in the electrolytic cell.
In a still further aspect there is disclosed a process for electrolytically producing rare earth metals comprising:
Embodiments disclosed allow improved control capability for anode-cathode distance (ACD) and consequently process temperature, improved control of electrolyte bath height in the electrolytic cell and anode immersion, better mixing of the electrolyte to enhance dissolution of the feed material, and higher Faraday efficiency by limiting opportunity for back reaction of anode gas with produced metal.
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:
The description broadly relates to an electrolytic cell arranged to produce rare earth metals by an electrolysis process in a molten electrolytic salt bath.
The rare earth metals produced in the electrolytic cell disclosed herein include those rare earth metals having a melting point less than 1100° C. Exemplary rare earth metals include, but are not limited to, Ce, La, Nd, Pr, Sm, Eu, and alloys thereof including didymium and mischmetal. The electrolytic cell disclosed herein is also suitable for the production of alloys of rare earth metals with iron.
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 (Nd2O3) or praseodymium oxide (Pr6O11). 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. Nd2O3 and Pr6O11) 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 one or more rare earth metal chloride and lithium chloride salts.
Referring to
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 inclined floor 14 has one or more inclined channels 22 disposed therein along which molten rare earth metals produced in the electrolytic cell 10 can drain. In one embodiment, the one or more inclined channels 22 are inclined from the horizontal at an angle α of up to about 10°.
In the embodiment shown in
In some forms of the invention the floor 14 may be provided with more than one inclined channel 22, as shown in
The floor 14, or an upper surface of the floor 14, may be formed from anti-corrosive materials similar to or the same as those materials selected for the lining of the cell housing 12. All surfaces having direct contact with the rare earth metal product, including the channel(s) 22 and the sump 16 should be resistant to forming alloys with the rare earth metals produced in the electrolytic bath. Suitable lining materials for the channel(s) 22 and the sump 16 include, but are not limited to, metals such as tungsten, molybdenum, or tantalum.
The sump 16 is configured to receive, in use, molten rare earth metal produced on the one or more cathodes 18 which collects in the channel and drains towards the lower end 26 of the channel 22. The sump 16 is spaced apart and isolated from the one or more cathodes 18 and the one or more anodes 20.
The sump 16 may be provided with a heater to maintain a temperature above the liquidus 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 housing 12.
The arrangement allows for continuous removal of molten rare earth metal product from the floor 14 of the cell 10 which prevents pooling of the molten rare earth metal product and consequently provides several advantages. 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 10 disclosed herein, in the absence of molten rare earth metal product, the sludge remains in contact with the molten electrolyte and is thereby provided with an opportunity for re-dissolution in the molten electrolyte.
The molten rare earth metal product collected in the sump 16 is spaced apart from and isolated from the one or more cathodes 18 and the one or more anodes 20. Consequently, the molten rare earth metal is protected from reaction and/or contamination with fugitive carbon arising from the one or more anodes 20, and back reactions with off gases from the one or more anodes 20.
The one or more cathodes 18 are suspended in the electrolyte bath 11 contained within the cell housing 12 above the channel 22 in substantially vertical alignment therewith. In the form as illustrated, the cathodes 18 comprise plates of cathodic material having an upper surface 28 and opposing elongate surfaces 30, with the underside 24 being disposed above the channel 22 in so that molten rare earth metal produced on the opposing surfaces 30 may fall under gravity directly into the underlying channel 22. The opposing surfaces 30 of the cathodes 18 are supported by an inert refractory filler material 32 which further avoids the formation of an inactive electrolyte zone in the cell 10.
The cathodes 18 are configured in adjacent alignment with one another whereby opposing elongate surfaces 30 of adjacent cathodes 18 are respectively longitudinally aligned with one another and respective opposing end surfaces of adjacent cathodes 18 face one another. It will be appreciated by persons skilled in the art that spacing between facing opposing end surfaces of adjacent cathodes 18 is as narrow as possible.
The plates of cathodic material are correspondingly sized so that, in the arrangement as described above, an effective length of the adjacently disposed cathodes 18 is substantially the same as or marginally shorter than the length of the channel 22.
Alternatively, a single cathode 18 having a similar length as the channel 22 may be employed in the electrolytic cell 10 as disclosed herein.
The opposing elongate surfaces 30 of the cathodes 18 are downwardly and outwardly inclined at an angle from the vertical, whereby a cross-sectional shape of the cathode 18 is substantially triangular. The opposing elongate surfaces 30 may be inclined from the vertical by angle β of up to about 45°, and preferably from 2° to 10°.
The angle of inclination is selected on the basis of optimised bubble-driven flow of electrolyte to achieve good mixing with feed material, and maintenance of high Faraday yield. The desired angle β may be determined by computational modelling for the specific cell geometry.
In embodiments where a single rare earth metal or an alloy of rare earth metals is the desired electrolytic product, the cathodes 18 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 in the electrolytic bath. Suitable materials include, but are not limited to, metals such as tungsten, molybdenum, or tantalum.
In alternative embodiments where an alloy of iron with one or more rare earth metals is desired, the cathode 18 may be formed from iron. It will be appreciated by persons skilled in the art that in these particular embodiments, the cathode 18 will be consumed during the electrolytic process for production of the iron-rare earth metal alloy.
In the embodiment shown in
The proximal elongate surfaces 36b of the anodes 18 face respective opposing elongate surfaces 30 of the cathodes 18. Both surfaces 36b and 30 are inclined from the vertical by corresponding angle β′ such that the said surfaces 36b and 30 are spaced apart in parallel alignment with one another so as to define a substantially constant anode-cathode distance therebetween.
The anodes 20 are configured in adjacent alignment with one another whereby opposing elongate surfaces 36a, 36b of adjacent anodes 20 are respectively longitudinally aligned with one another and respective opposing ends 38 of adjacent anodes 20 face one another. It will be appreciated by persons skilled in the art that spacing between facing opposing ends 38 of adjacent anodes 20 is as narrow as possible.
The plates of anodic material are correspondingly sized so that, in the arrangement as described above, an effective length of the adjacently disposed anodes 20 is substantially the same as or marginally shorter than the length of the channel 22.
Alternatively, a single pair of anodes 20 having a similar length as the channel 22 may be employed in the electrolytic cell 10 as disclosed herein.
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 are consumed as the electrolysis process progresses and the angle of inclination β of proximal elongate side 36b remains substantially constant. Gas bubbles released from the anode 20 are therefore retained close to the proximal elongate surface 36b as the gas bubbles rise to the electrolyte surface, by virtue of the inclined profile of proximal elongate surface 36b, as illustrated in
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 device 40 operatively associated with the one or more anodes 20 to control the ACD, in particular to maintain a substantially constant ACD. Said device 40 may comprise a horizontal positioning apparatus in operative communication with the one or more anodes 20. In use, the horizontal positioning apparatus may laterally translate the one or more anodes 20 toward the cathode 18 in response to a rate at which the anode 20 is consumed so that the ACD may remain substantially constant. The rate of anode consumption may be determined by reference to current flow. Alternatively, the horizontal positioning apparatus may translate the one or more anodes 20 in response to variation in cell resistance from a predetermined value.
Consequent to anode consumption, the volume occupied by the anodes 20 in the electrolytic cell 10 decreases thereby lowering the height of the electrolyte bath in the housing 12. Similarly, the intermittent cell operations such as the replacement of spent anodes with new anodes, and the removal of rare earth metal product from the cell, will result in substantial and undesirable variation in the height of the electrolyte bath and the electrode immersion depth. The electrolysis cell 10 disclosed herein may be provided with a displacement device 42 to control the height of the electrolyte bath in the housing 12, in particular to maintain a substantially constant height of the electrolyte bath in the housing 12. The displacement device 42 may comprise an inert body which is suspended in the housing 12 and positionable in a vertical direction. In use, the inert body may be downwardly or upwardly translated in response to specific cell operation so that the height of the electrolyte bath may remain substantially constant. The inert body may take any suitable form, for example a bar as illustrated in the Figures.
The displacement device 42 may formed from similar refractory materials as the inner linings of the housing 12 as described previously.
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 cathodes 18 and the anodes 20 and the resistance of the electrodes 18, 20 raises the operating temperature of the electrolytic cell 10 to a predetermined temperature. 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 20, 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 opposing elongate surfaces 30 of the cathode 18. The molten rare earth metal(s) subsequently fall into the channel 22 and drain along the channel 22 into the sump 16, which is tapped as required. Feed material may be regularly charged to the electrolytic cell 10 into areas of high electrolyte flow, at a rate corresponding more or less to the consumption rate. It will be appreciated by those familiar with the art that the feed rate may be finely controlled to achieve a target cell resistance corresponding to the desired concentration of feed in the electrolyte.
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 18, 20. 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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012902017 | May 2012 | AU | national |
| 2013204396 | Apr 2013 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2013/000500 | 5/15/2013 | WO | 00 |
