A METHOD OF ELECTROCHEMICAL PRODUCTION OF RARE EARTH ALLOYS AND METALS COMPRISING A COMPOSITE ANODE

Description

[0001] A method of electrochemical production of rare earth alloys and metals comprising a composite anode, and a system thereof.

[0002] The work leading to this invention has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 603564.

TECHNICAL FIELD

[0003] The present invention is related to a method and an electrochemical cell thereof providing electrochemical production of rare earth (RE) alloys and metals and especially to a method wherein raw material used in the process is supplied through a rare-earth-oxide-carbon composite anode.

BACKGROUND

[0004] Rare earth (RE) materials are a strategic commodity today, and rare earth elements are important ingredients in most electronic circuitry used in our daily life. China is today the most prominent supplier of rare earth elements, rare earth alloys and rare earth metals. A common method when producing rare earth metals like Nd, Pr, La, Ce as well as some alloys with Fe, for example Dy-Fe, can be produced by electrolysis from molten fluoride based electrolytes using raw materials comprising rare earth oxides. This is the dominant technology in China used in industrial level production of rare earth elements and alloys.

[0005] The report by D. K. Dysinger and J. E. Murphy, "Electrowinning of Neodymium From a Molten Oxide-Fluoride Electrolyte," United States Department of the Interior, Report of Investigations 9504, disclose some of these techniques.

[0006] An electrolytic process may be using a vertical set-up cell comprising graphite anodes, and molybdenum and iron as non-consumable or consumable cathode materials, respectively. Tungsten may also be used instead of molybdenum. The electrolyte may comprise an equimolar REF₃-LiF mixture, and the raw material used in the electrolysis is RE₂O₃. The raw material can be placed as a batch in the electrolyte, or continuously or semi-continuously added at the top of the electrolyte.

[0007] There are some technical challenges operating such a cell. It is necessary to have a good balance between the feeding rate of raw material and oxide consumption. It is necessary that the amount of dissolved oxides match the supply of electrolytic current. For example, if the oxide concentration becomes too low the fluoride electrolyte itself will start to decompose. On the other hand, if the oxide concentration is too high, some of the oxides will settle at the bottom of the cell as sludge instead.

[0008] Therefore, it is a need of an improved electrochemical process when producing rare earth elements, rare earth alloys and rare earth metals.

OBJECT OF THE INVENTION

[0009] In particular, it may be seen as an object of the present invention to provide an electrochemical production method comprising a composite anode-supplying raw material for the production.

[0010] It is a further object of the present invention to provide an alternative to the prior art.

SUMMARY

[0011] Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a method of producing rare earth elements or rare earth alloys in a molten salt electrochemical process, comprising:

• manufacturing a solid composite anode with one or multiple oxygen containing compounds of one or more rare earth elements, then
• arranging an electrolysis cell with the solid composite anode and a cathode facilitating deposition of rare earth elements, wherein a molten salt electrolyte in the cell comprises chloride compounds,
• wherein the electrochemical process results in that the oxygen in the oxide containing rare earth compound(s) are released as oxygen containing gas species and the rare earth element(s) in the anode is(are) electrochemically dissolved as rare earth metal ion(s) in the electrolyte,
• collecting rare earth element(s) or rare earth alloy(s) from the cathode.

[0012] The present invention is defined only by the appended claims.

[0013] A further aspect which does not fall within the scope of the present invention comprises a composite anode comprising carbon compound(s) mixed with rare earth oxide(s) in amounts such that the molar ratio between carbon and rare earth oxide(s) yields stoichiometric amounts according to a specific electrochemical reaction at a specific operating temperature.

[0014] A further aspect which does also not fall within the scope of the present invention comprises an electrochemical electrolysis cell comprising at least one composite anode.

FIGURES

[0015] The method according to the present invention will now be described in more detail with reference to the accompanying figure. The figure illustrates examples of embodiments of the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

[0016] Figure 1 illustrates an example of embodiment of the present invention.

DETAILED DESCRIPTION

[0017] Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the present examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference sign in the claims with respect to elements indicated in the figures shall also not be construed as limiting to the scope of the invention.

[0018] According to an aspect of the present invention, production of rare earth elements or alloys containing one or more rare earth elements comprises using a molten salt electrochemical process, more specifically molten chlorides with known low solubility of oxide containing rare earth compounds are used.

[0019] According to an example of embodiment of the present invention, the problem of low oxide solution and sludge formation can be mitigated if the raw material (RE₂O₃) can be supplied through a rare-earth-oxide-carbon composite electrode being a consumable anode.

[0020] If for example RE oxide is mixed with a carbon source, acting as a binder, and formed into a suitable bar or cylinder, and then heated or baked and used as an anode, the expected anode reaction is that during polarisation carbon will react with the oxygen atoms from RE₂O₃ forming carbon oxide or/and carbon dioxide, and RE ions.

[0021] Then the RE ions are electrochemically released and will recombine with halide ions, thus dissolving in the electrolyte as a RE halide complex. For neodymium in a chloride melt, the reactions can be noted stoichiometric in the following way when using a Nd₂O₃ - composite. The same reaction scheme is valid for other rare earths, substituting Nd with another rare earth element(s).

Anode:

[0022] 

        Nd₂O₃ + 3 C = 2 Nd³⁺ (dissolved) + 3 CO (g) + 6 e-     (I)

or

        Nd₂O₃ + 3/2 C = 2 Nd³⁺ (dissolved) + 3/2 CO₂ (g) + 6 e-     (II)

Electrolyte:

[0023] 

        Nd³⁺ (dissolved) + 3 Cl^- = NdCl₆^3-     (III)

Cathode:

[0024] 

        NdCl₆^3- + 3 e^- = Nd + 6 Cl^-     (IV)

with an overall cell reaction:

        Nd₂O₃ + 3 C = 2 Nd + 3 CO (g)     (V)

or

        Nd₂O₃ + 3/2 C = 2 Nd + 3/2 CO₂ (g)     (VI)

with a theoretical standard potential E° = -1.48 and -1.55 volt for reaction (V) and (VI), respectively at 850 °C. The theoretical standard potential for other rare earths varies only slightly from the potential of the reaction with Nd, as all RE elements have standard potential in the same range at given temperatures, as known to a person skilled in the art.

[0025] Obtaining expected results in examples of embodiments of the present invention comprises manufacturing an anode in such a way that the rare earth metal ion(s) is (are) dissolved in a molten salt electrolyte while the oxygen in the oxide containing rare earth compound(s) is (are) released as oxygen containing gas species, most commonly CO or CO₂, as a result of an electrochemical process. The dissolved rare earth element(s) is (are) deposited at the cathode, either in a pure form or as an alloy or as an alloy with the cathode material.

[0026] The advantage of the present invention is that rare earth elements, or alloys containing rare earth elements, can be produced in an electrolyte with low solubility of the oxide.

Example:

[0027] The composite anode can be made by multiple procedures: For example, the carbon compound(s) is (are) mixed with the rare earth oxide(s) in amounts such that the molar ratio between carbon and rare earth oxide(s) yields stoichiometric amounts according to the electrochemical reaction at the operating temperature. An example of method according to the present invention is mixing a carbon based binder, e.g. coal tar pitch, petroleum tar pitch or a synthetic binder, either in solid or liquid form, with the rare earth oxide(s) to yield the stoichiometric composition (equation (V) or (VI) above), when baked or heated to operating temperature. Some of the pitch may be substituted with another carbon source or another carbon containing source, e.g. graphite, carbon black, carbides or oxy-carbides of the RE(s). It is advantageous that intimate mixing is achieved avoiding solid particles falling off the anode during use.

[0028] Electrochemical characterisation, polarisation and gas evolution: When the anode is polarised, e.g. at 150 mV vs an Ag/AgCl reference electrode, in the molten equimolar NaCl-KCl mixture at 860 °C, the Nd₂O₃ and carbon reacts, evolving CO and/or CO₂ at the anode. Using a gold coated see-through furnace and a quartz container for the electrolysis cell, the gas evolution that occur during the electrolysis can be visually observed. The gas bubbles observed were small and evenly distributed.

[0029] Any other methods of manufacturing the anode providing same operational characteristics as disclosed above is within the scope of the present invention. Especially, baked anodes (REO-C anodes) made from a mix of graphite powder, oxide and various amounts of pitch are proven to provide good results with respect to criteria like sufficient electrical conductivity, mechanical stability during electrolysis, even gas distribution, small gas bubbles and enough RE ions that is released during electrolysis.

[0030] Consumable anodes may be manufactured externally and be placed inside an electrochemical cell according to the present invention when production starts. It is also within the scope of the present invention to arrange the step of baking an anode when the anode is placed inside the electrochemical cell, and baking the anode there before or during the production.

[0031] An aspect of the present invention comprises manufacturing an anode which may comprise mixing a binder with the rare earth compounds of the anode, followed by forming the anode into a desired shape by pressing or vibro-forming and baking the anode, either in-situ in the cell or in a separate baking furnace forming a solid composite anode.

[0032] Figure 1 illustrates the principle layout of an example of an electrochemical cell being able to support respective method steps of examples of embodiments of the present invention.

[0033] A vessel 10 defines the outer walls of an electrochemical cell according to the present invention. In the example illustrated in Figure 1, two manufactured anodes 11, for example manufactured according to the example disclosed above, is located inside the vessel 10 and are partly submerged into an electrolyte 13 comprising chloride compounds. A cathode 12 manufactured as known in prior art is located in between the two anodes 11. During operation of the cell an electric power source (not illustrated) supplies current to the anodes 11 and the cathode 12. The electrochemical reaction dissolves RE and oxygen containing species 14 as disclosed above. Liquid RE/RE alloy products 15 are collected by the cathode, and below the cathode there is a compartment 16 receiving the liquid RE/RE alloy products 15. A tubing or channel 17 is removing collected liquid RE/RE alloy products from the inside of the compartment 15.

[0034] The method according to the present invention comprises steps of:

• manufacturing a solid composite anode with one or multiple oxygen containing compounds of one or more rare earth elements, then
• arranging an electrolysis cell with the solid composite anode and a cathode facilitating deposition of rare earth elements, wherein a molten salt electrolyte in the cell comprises chloride compounds,
• wherein the electrochemical process results in that the oxygen in the oxide containing rare earth compound(s) are released as oxygen containing gas species and the rare earth element(s) in the anode is(are) electrochemically dissolved as rare earth metal ion(s) in the electrolyte,
• collecting rare earth element(s) or rare earth alloy(s) from the cathode.

[0035] Further, the step of manufacturing the anode may comprise adding at least one or multiple reducing agents participating in the anode reaction when passing direct current through the electrolysis cell.

[0036] Further, the at least one or multiple reducing agents may comprise at least one or multiple carbon allotropes.

[0037] Further, the at least one or multiple reducing agents may comprise at least one or multiple carbon compounds.

[0038] Further, the at least one or multiple reducing agents may comprise a mixture of at least one or multiple carbon allotropes and at least one or multiple carbon compounds.

[0039] Further, the step of manufacturing the anode may comprise mixing a binder with the rare earth compounds of the anode, followed by forming the anode into a desired shape by pressing or vibro-forming and baking the anode, either in-situ in the cell or in a separate baking furnace forming a solid composite anode.

[0040] Further, the electrolyte may comprise a composition of molten halides with low or no solubility of the oxide containing rare earth compounds of the anode.

[0041] Further, the oxygen containing gas species are carbon oxide or carbon dioxide unless other reducing agents in the anode participates in the reaction.

[0042] Further, if sulfur or a sulfur compound is present in the anode and participating in the reaction, the oxygen containing gas species may contain COS or SO₂.

[0043] Further, the deposition of the rare earth elements on the cathode is either in pure form, or as an alloy, or as an alloy with the cathode material.

Claims

1. A method of producing rare earth elements or rare earth alloys in a molten salt electrochemical process, comprising:

- manufacturing a solid composite anode with one or multiple oxygen containing compounds of one or more rare earth elements, then

- arranging an electrolysis cell with the solid composite anode and a cathode facilitating deposition of rare earth elements, wherein a molten salt electrolyte in the cell comprises chloride compounds,

- wherein the electrochemical process results in that the oxygen in the oxide containing rare earth compound(s) are released as oxygen containing gas species and the rare earth element(s) in the anode is(are) electrochemically dissolved as rare earth metal ion(s) in the electrolyte,

- collecting rare earth element(s) or rare earth alloy(s) from the cathode.

2. The method according to claim 1, wherein the step of manufacturing the anode comprises adding at least one or multiple reducing agents participating in the anode reaction when passing direct current through the electrolysis cell.

3. The method according to claim 2, wherein the at least one or multiple reducing agents comprises at least one or multiple carbon allotropes.

4. The method according to claim 2, wherein the at least one or multiple reducing agents comprises at least one or multiple carbon compounds.

5. The method according to claim 2, wherein the at least one or multiple reducing agents comprises a mixture of at least one or multiple carbon allotropes and at least one or multiple carbon compounds.

6. The method according to any claim 1-5, wherein the step of manufacturing the anode comprises mixing a binder with the rare earth compounds of the anode, followed by forming the anode into a desired shape by pressing or vibro-forming and baking the anode, either in-situ in the cell or in a separate baking furnace forming a solid composite anode.

7. The method according to claim 1, wherein the electrolyte comprises a composition of molten halides with low or no solubility of the oxide containing rare earth compounds of the anode.

8. The method according to claim 1, wherein the oxygen containing gas species released at the anode are carbon oxide or carbon dioxide.

9. The method according to claim 8, wherein the oxygen containing gas species released at the anode are carbon oxide or carbon dioxide and other oxygen containing species from other reducing agents that participates in the anode reaction, e.g. sulphur forming COS and/or SO₂ oxygen containing gas species.

10. The method according to claim 1, wherein the deposition of the rare earth elements on the cathode is either in pure form, or as an alloy, or as an alloy with the cathode material.

Ansprüche

1. Verfahren zum Herstellen von Seltenerdelementen oder Seltenerdlegierungen in einem elektrochemischen Salzschmelzenvorgang, umfassend:

- Fertigen einer festen Verbundanode mit einer oder mehreren sauerstoffhaltigen Verbindungen eines oder mehrerer Seltenerdelemente, dann

- Anordnen einer Elektrolysezelle mit der festen Verbundanode und einer Kathode zum Erleichtern einer Abscheidung von Seltenerdelementen, wobei ein Salzschmelzenelektrolyt in der Zelle Chloridverbindungen umfasst,

- wobei der elektrochemische Vorgang dazu führt, dass der Sauerstoff in der/n oxidhaltigen Seltenerdverbindung(en) als sauerstoffhaltige Gasspezies freigesetzt werden und das/die Seltenerdelement(e) in der Anode als Seltenerdmetallion(en) in dem Elektrolyten elektrochemisch gelöst wird/werden,

- Sammeln von Seltenerdelement(en) oder Seltenerdlegierung(en) von der Kathode.

2. Verfahren nach Anspruch 1, wobei der Schritt des Fertigens der Anode ein Hinzufügen von wenigstens einem oder mehreren Reduktionsmitteln umfasst, die an der Anodenreaktion teilnehmen, wenn Gleichstrom durch die Elektrolysezelle geleitet wird.

3. Verfahren nach Anspruch 2, wobei das wenigstens eine oder die wenigstens mehreren Reduktionsmittel wenigstens ein oder mehrere Kohlenstoffallotrope umfassen.

4. Verfahren nach Anspruch 2, wobei das wenigstens eine oder die wenigstens mehreren Reduktionsmittel wenigstens ein oder mehrere Kohlenstoffverbindungen umfassen.

5. Verfahren nach Anspruch 2, wobei das wenigstens eine oder die wenigstens mehreren Reduktionsmittel eine Mischung aus wenigstens einem oder wenigstens mehreren Kohlenstoffallotropen und wenigstens einem oder wenigstens mehreren Kohlenstoffverbindungen umfassen.

6. Verfahren nach einem der Ansprüche 1-5, wobei der Schritt des Fertigens der Anode ein Mischen eines Bindemittels mit den Seltenerdverbindungen der Anode, gefolgt von einem Ausbilden der Anode in eine gewünschte Form durch Pressen oder Vibroformen und Brennen der Anode, entweder in situ in der Zelle oder in einem separaten Brennofen, das eine feste Verbundanode ausbildet, umfasst.

7. Verfahren nach Anspruch 1, wobei der Elektrolyt eine Zusammensetzung von geschmolzenen Halogeniden mit geringer oder keiner Löslichkeit der sauerstoffhaltigen Seltenerdverbindungen der Anode umfasst.

8. Verfahren nach Anspruch 1, wobei die an der Anode freigesetzten sauerstoffhaltigen Gasspezies Kohlenstoffoxid oder Kohlendioxid sind.

9. Verfahren nach Anspruch 8, wobei die an der Anode freigesetzten sauerstoffhaltigen Gasspezies Kohlenstoffoxid oder Kohlendioxid und andere sauerstoffhaltige Spezies von anderen Reduktionsmitteln sind, die an der Anodenreaktion teilnehmen, z. B. schwefelausbildendes COS und/oder sauerstoffhaltige SO₂-Gasspezies.

10. Verfahren nach Anspruch 1, wobei die Abscheidung der Seltenerdelemente auf der Kathode entweder in reiner Form oder als eine Legierung oder als eine Legierung mit dem Kathodenmaterial erfolgt.

Revendications

1. Procédé de fabrication d'éléments de terres rares ou d'alliages de terres rares dans un procédé électrochimique à sel fondu, consistant à :

- fabriquer une anode composite solide avec un ou plusieurs composés contenant un ou plusieurs éléments de terres rares contenant de l'oxygène, puis

- agencer une cellule d'électrolyse avec l'anode composite solide et une cathode facilitant le dépôt d'éléments de terres rares, un électrolyte de sel fondu dans la cellule comprenant des composés de chlorure,

- le processus électrochimique ayant pour résultat que l'oxygène dans le ou les composé(s) de terres rares contenant de l'oxyde est ou sont libéré(s) sous forme d'espèces gazeuses contenant de l'oxygène et que le ou les élément(s) de terres rares dans l'anode sont dissous électrochimiquement sous forme d'ion(s) de terres rares dans l'électrolyte,

- recueillir un ou des élément(s) de terres rares ou un ou des alliage(s) de terres rares de la cathode.

2. Procédé selon la revendication 1, dans lequel l'étape de fabrication de l'anode consiste à ajouter au moins un ou plusieurs agent(s) réducteur(s) participant à la réaction de l'anode lors du passage de courant continu à travers la cellule d'électrolyse.

3. Procédé selon la revendication 2, dans lequel le ou les agents réducteurs comprennent au moins un ou plusieurs allotropes de carbone.

4. Procédé selon la revendication 2, dans lequel le ou les agents réducteurs comprennent au moins un ou plusieurs composé(s) de carbone.

5. Procédé selon la revendication 2, dans lequel le ou les agent(s) réducteur(s) comprennent un mélange d'au moins un ou plusieurs allotropes de carbone et au moins un ou plusieurs composés de carbone.

6. Procédé selon l'une quelconque des revendications 1 à 5, dans lequel l'étape de fabrication de l'anode consiste à mélanger un liant avec les composés de terres rares de l'anode, puis à former l'anode en une forme souhaitée par compression ou vibro-formage et cuisson de l'anode, soit in situ dans la cellule, soit dans un four de cuisson séparé formant une anode composite solide.

7. Procédé selon la revendication 1, dans lequel l'électrolyte comprend une composition d'halogénures fondus ayant une solubilité faible ou nulle de l'oxyde contenant des composés de terres rares de l'anode.

8. Procédé selon la revendication 1, dans lequel les espèces gazeuses contenant de l'oxygène libérées à l'anode sont l'oxyde de carbone ou le dioxyde de carbone.

9. Procédé selon la revendication 8, dans lequel les espèces gazeuses contenant de l'oxygène libérées à l'anode sont de l'oxyde de carbone ou du dioxyde de carbone et d'autres espèces contenant de l'oxygène d'autres agents réducteurs qui participent à la réaction de l'anode, par exemple des espèces gazeuses contenant du COS formant du soufre et/ou de l'oxygène SO₂.

10. Procédé selon la revendication 1, dans lequel le dépôt des éléments de terres rares sur la cathode est soit sous forme pure, soit sous forme d'un alliage, soit sous forme d'un alliage avec le matériau de cathode.

Drawing