PREPARATION OF RARE EARTH METALS WITH DOUBLE SALTS

Abstract

Disclosed are embodiments of a method for producing a rare earth metal from a metallothermic reaction of a reductant metal and a double salt of the rare earth metal. The double salt of rare earth metal can be prepared from an alkali halide and a salt of the rare earth metal at room temperature over a time period of 2 hours of less without the requirement to use hydrofluoric acid.

Claims

1. A method of producing rare earth (RE) metal, comprising: reacting an alkali-halide double salt of the RE metal with a reductant metal to produce RE metal, a halide of the reductant metal, and alkali halide.

2. The method of claim 1, wherein the alkali-halide double salt of the RE metal comprises an alkali selected from the group consisting of lithium, sodium, potassium, and combinations thereof.

3. The method of claim 1, wherein the alkali-halide double salt comprises a halide selected from the group consisting of fluorine, chlorine, bromine, and combinations thereof.

4. The method of claim 1, wherein the alkali halide double salt of the RE metal has the form of A.sub.xRE.sub.yX.sub.z in which A is selected from the group consisting of Li, Na, K, and combinations thereof and X is selected from the group consisting of F, Cl, Br, and combinations thereof.

5. The method of claim 4, wherein x is in a range from 0.15 to 4, y is in a range from 0.5 to 3, and z is in a range from 4 to 10.

6. The method of claim 1, wherein the RE metal is selected from a group consisting of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and combinations thereof.

7. The method of claim 1, further comprising: mixing a first aqueous solution comprising an RE salt and a second aqueous solution comprising an alkali-halide salt to form a mixture; separating the alkali-halide double salt of the RE metal that precipitates from the mixture.

8. The method of claim 7, wherein the RE salt is selected from a group consisting of chlorides, nitrates, acetates, oxalates, carbonates, phosphates, sulphates, and combinations thereof.

9. The method of claim 7, wherein the mixing and separating takes place at room temperature.

10. The method of claim 1, wherein the RE metal gravimetrically separates from the halide of the reductant metal and alkali halide.

11. The method of claim 1, wherein the reductant metal is selected from a group consisting of calcium, aluminum, magnesium, lanthanum, and combinations thereof.

12. A method of producing rare earth (RE) metal, comprising: electrolytically reacting an alkali-halide double salt of the RE metal with at least one of an RE oxide or an RE halide to produce RE metal under flux in an electrolytic cell.

13. The method according to claim 12, wherein the alkali halide double salt of the RE metal has the form of A.sub.xRE.sub.yX.sub.z in which A is selected from the group consisting of Li, Na, K, and combinations thereof and X is selected from the group consisting of F, Cl, Br, and combinations thereof.

14. The method according to claim 12, wherein the RE metal is selected from a group consisting of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and combinations thereof.

15. The method according to claim 12, further comprising utilizing a consumable anode during the electrolytically reacting.

16. The method according to claim 12, further comprising utilizing a non-consumable anode during the electrolytically reacting.

17. A method of extracting rare earth (RE) metal, comprising: mixing a first aqueous solution comprising an RE salt and a second aqueous solution comprising an alkali-halide salt to form a mixture; precipitating an alkali-halide double salt of the RE metal having the form of A.sub.xRE.sub.yX.sub.z, in which A is an alkali metal and X is a halide; separating the alkali-halide double salt of the RE metal that precipitates from the mixture; and electrolytically or metallothermically reacting the alkali-halide double salt of the RE metal to obtain the RE metal.

18. The method of claim 17, wherein x is in a range from 0.15 to 4, y is in a range from 0.5 to 3, and z is in a range from 4 to 10.

19. The method of claim 17, wherein A is selected from a group consisting of Li, Na, K, and combinations thereof, wherein X is selected from a group consisting of F, Cl, Br, and combinations thereof, and wherein the reductant metal is selected from a group consisting of calcium, aluminum, magnesium, lanthanum, and combinations thereof.

20. The method of claim 17, wherein the RE salt is selected from a group consisting of chlorides, nitrates, acetates, oxalates, carbonates, phosphates, sulphates, and combinations thereof.

21. The method of claim 17, wherein the RE metal is selected from a group consisting of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and combinations thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:

[0035] FIG. 1 is an x-ray diffraction (XRD) pattern of sodium-rare earth-fluoride double salt Na.sub.0.76Nd.sub.1.75F.sub.6.Math.XH.sub.2O, according to an exemplary embodiment;

[0036] FIG. 2 is a thermogravimetry/differential scanning calorimetry (TG-DSC) plot of mass changes of the sodium-rare earth-fluoride double salt as a function of temperature, according to an exemplary embodiment;

[0037] FIG. 3 is an XRD pattern of neodymium metal isolated from the sodium-rare earth-fluoride double salt along with a neodymium metal XRD reference pattern, according to an exemplary embodiment; and

[0038] FIG. 4 is a TG-DSC plot of the neodymium metal isolated from the sodium-rare earth-fluoride double salt, including an inset picture of Nd metal from the TG-DSC analysis, according to an exemplary embodiment.

[0039] While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

[0040] According to embodiments of the presently disclosed methods, rare earth (RE) metal is produced from a double salt of the RE metal in a metallothermic or electrolytic reaction. In particular, a salt of RE metal is converted to an alkali-halide (A-X) double salt of the rare earth metal (A-RE-X) using A-X salt under specific conditions. In one or more embodiments, the RE metal is at least one of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium. In one or more embodiments, the alkali metal (A) is one or more of Li, Na, or K. In one or more embodiments, the halide (X) is one or more of F, Cl, or Br.

[0041] According to a first aspect, the double salt A-RE-X is reacted with a metal reductant, such as calcium (Ca), aluminum (Al), magnesium (Mg), or lanthanum (La), in a metallothermic reaction. In one or more embodiments, the double salt A-RE-X and reductant metal are reacted in a sealed vessel. In one or more embodiments, the reaction takes place in a refractory tube, such as a quartz tube. For example, the tube may include the double salt A-RE-X and calcium and be sealed under argon gas. In one or more embodiments, the tube may be heated using plasma induction. In another example, the reaction may take place in a vacuum furnace, e.g., at a vacuum of 10-1 Torr. In still one or more other embodiments, the heating takes place in a normal atmosphere (i.e., in air at standard pressure), especially where the double salt A-RE-X is a molten salt and/or the metallothermic reaction produces slag that may protect the reactants during reduction.

[0042] In one or more embodiments, the RE metal is converted to a double salt of the form A.sub.xRE.sub.yX.sub.z in which x is in a range from 0.15 to 4, y is in a range from 0.5 to 3, and z is in a range from 4 to 10. The inclusion of the alkali metal (including Na, Li, and/or K) in the compound helps in reducing the melting point of the compound. In one or more embodiments, metallothermic reduction can be used to produce RE metal according to Equation 2 in which RM represents the reductant metal.

[00002]$\begin{array}{cc}{A}_x{\mathrm{RE}}_y{X}_z+\mathrm{wRM}.fwdarw.{\mathrm{yRE}}^0+\mathrm{xAX}+{\mathrm{wRMX}}_{\left(2\mathrm{or}3\right)}+H\left(\mathrm{heat}\right)& \left(2\right)\end{array}$

[0043] According to a second aspect, the low melting point double salt A.sub.xRE.sub.yX.sub.z as described above is reacted with at least one of an RE oxide (RE.sub.2O.sub.3) or an RE halide (REX.sub.3) in an electrolytic cell in the presence of a molten salt flux. In one or more embodiments, the electrolytic cell includes consumable anodes, such as carbon (in particular, graphite) anodes. In one or more other embodiments, the anodes are not consumable, such as anodes formed from metal alloys. In such embodiments, the reduced melting point double salt compound can be used in an electrolytic cell containing a molten salt to reduce RE.sub.2O.sub.3 or REX.sub.3 to metal using consumable electrode according to Equation 3 or non-consumable electrode according to Equation 4.

[00003]$\begin{array}{cc}2C\left(\mathrm{anode}\right)+{\mathrm{RE}}_2{O}_3\left(\mathrm{or}2{\mathrm{REX}}_3\right)+\left\{A_x{\mathrm{RE}}_y{X}_z+\mathrm{flux}\right\}.fwdarw.2{\mathrm{RE}}^0+\left(\mathrm{CO}+{\mathrm{CO}}_2\right)\left(\mathrm{or}C-X\mathrm{compounds}\right)+\left\{A_x{\mathrm{RE}}_y{X}_z+\mathrm{flux}\right\}& \left(3\right)\end{array}$$\begin{array}{cc}2{\mathrm{RE}}_2{O}_3\left(\mathrm{or}4{\mathrm{REX}}_3\right)+\left\{A_x{\mathrm{RE}}_y{X}_z+\mathrm{flux}\right\}.fwdarw.4{\mathrm{RE}}^0+3O_2\left(\mathrm{or}6X_2\right)+\left\{A_x{\mathrm{RE}}_y{X}_z+\mathrm{flux}\right\}& \left(4\right)\end{array}$

[0044] According to the proposed processes, the A-RE-X double salt can be prepared from a wide range of RE starting salts, such as chlorides, nitrates, acetates, oxalates, carbonates, phosphates, and sulphates. That is, non-limiting examples of the salt of the rare earth metal that is converted to an A-X double salt include RE chlorides, RE nitrates, RE acetates, RE oxalates, RE carbonates, RE phosphates, or RE sulphates, among other possibilities.

[0045] The RE starting salt is dissolved in a first aqueous solution, and the A-X salt is dissolved in a second aqueous solution. In one or more embodiments, the first aqueous solution is added to the second aqueous solution, and in one or more embodiments, the second aqueous solution is added to the first aqueous solution. In one or more embodiments, the aqueous solution having the lower concentration of salt is added to the aqueous solution having the higher concentration of salt. In one or more embodiments, the RE salt aqueous solution and the A-X salt aqueous solution are mixed at room temperature. In one or more embodiments, one aqueous salt solution is added dropwise to the other aqueous salt solution. In one or more embodiments, the aqueous solution of RE salt is mixed with the aqueous solution of A-X salt at a ratio of 1 part RE salt to at least 4 parts A-X salt. The solutions may be mixed using constant stirring over a time period of 0.5 hours to 5 hours, in particular about 1 hour to about 2 hours.

[0046] Over time, the A-RE-X double salt will precipitate from solution, and the A-RE-X double salt can be separated (e.g., by decantation). Thereafter, the A-RE-X double salt may be dried and may then be used in the metallothermic process described above in Equation 2. Double salts produced according to the present disclosure may have a variety of forms, such as AREX.sub.4, A.sub.3REX.sub.6, and A.sub.2REX.sub.5, among others, that are suitable for use in the metallothermic process.

[0047] Applicant expects that the presence of alkali metal in the rare earth halide can reduce the temperature of the reaction (metallothermic or electrolytic) in addition to lowering the carbon footprint and usage of toxic acids (such as HF). Specifically, the presently disclosed process is expected to have a lower carbon footprint than existing metallothermic and electrolytic processes in which rare earth oxides are produced from rare earth chlorides, nitrates, oxalates, carbonates, or phosphates and in which rare earth fluorides are produced from HF. Further, the disclosed process does not involve the usage of harmful HF for the synthesis of fluorides. Additionally, the synthesis of A-RE-X, such as Na-RE-F, can be carried out at room temperature with relatively short reaction times, such as 1 hour or less.

EXAMPLES

Example 1-Synthesis of Sodium Rare-Earth Fluoride from Rare-Earth Acetate

[0048] A first solution of neodymium (III) acetate was dissolved in water. A second solution of sodium fluoride was dissolved in water. In this example, the first solution was gently added to the second solution under vigorous stirring (however, as discussed above, the second solution may instead be added to the first solution). A purple precipitate started to form. The reaction was allowed to proceed at room temperature while continuing to stir. The contents were carefully decanted, and the precipitate was allowed to dry in a hot air oven. The x-ray diffraction (XRD) pattern for the dried precipitate is provided in FIG. 1. The XRD pattern is consistent with the presence of a major phase of Na.sub.0.76Nd.sub.1.75F.sub.6.Math.XH.sub.2O as found in literature.

[0049] The thermogravimetry-differential scanning calorimetry (TG-DSC) plot of the synthesized NaNdF is provided in FIG. 2. As can be seen from FIG. 2, the synthesized NaNdF demonstrated an endothermic melting peak at 750 C. The net mass loss was 1.45% at 480 C. and 2.4% at 950 C. These characteristics are consistent with rare earth fluorides synthesized using standard synthetic protocols (e.g., using HF and NH.sub.4HF.sub.2).

Example 2-Synthesis of Rare-Earth Metal Using Calciothermic Reduction

[0050] From the NaNdF double salt prepared in Example 1, Na.sub.0.76Nd.sub.1.75F.sub.6 was packed with pure calcium in a quartz tube, and the contents were sealed under argon. Using plasma induction, the contents were melted for 1 hour. Gravity separated the reaction products with flux (NaF and CaF.sub.2) floating to the top of the tube where it could be removed as slag upon solidification. The remaining neodymium metal was subjected to XRD spectroscopy, and the resulting XRD pattern is provided in FIG. 3. As can be seen, the experimentally determined peaks correspond to the reference XRD pattern for neodymium metal.

[0051] The recovered neodymium metal was subjected to TG-DSC testing with the results provided in FIG. 4. As can be seen in FIG. 4, the recovered neodymium metal exhibited a melting point of about 1016 C., which is consistent with the melting temperature of neodymium metal found in literature. The ball of neodymium metal produced in Example 2 can be seen in the figure inset of FIG. 4.

[0052] The recovered neodymium metal was also analyzed to determine the presence of carbon, nitrogen, oxygen, and sulfur impurities. The estimated concentration of such impurities was 14545 ppm C, 493 ppm N, 82394 ppm O, and <1 ppm S. Thus, each individual impurity was present at a level of 1000 ppm or less, and collectively, the impurities were together at a level of 2000 ppm or less.

[0053] All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

[0054] The use of the terms a and an and the and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms comprising, having, including, and containing are to be construed as open-ended terms (i.e., meaning including, but not limited to,) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., such as) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0055] Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.