Electrolytic system and method for electrolytic separations

Abstract

Described herein is an electrochemical cell for recovering a target metal from a molten salt. The system comprises a tubular shroud electrode surrounding a liquid metal separation electrode; and a counter electrode spaced from the shroud electrode, as shown in FIGS. 1 and 2. The electrodes contact a molten salt solution comprising target metal ions (e.g., Cs) dissolved in a molten salt (e.g., LiCl—KCl). The system is operated by applying a voltage to the electrodes to electrochemically reduce a metal ion (e.g., Li+) from the molten salt to its metallic form at the shroud electrode to form a first alloy, thereby increasing the relative concentration of the target metal in the vicinity of the separation electrode. Ions of the target metal are reduced at the separation electrode to form the second alloy containing the target metal(s).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application Ser. No. 63 / 789,797 filed on Apr. 16, 2025, which is incorporated herein by reference in its entirety.STATEMENT OF GOVERNMENT INTEREST

[0002] The United States Government has rights in this invention pursuant to Contract No. DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory.FIELD OF THE INVENTION

[0003] This invention relates to electrochemical separation of metals. More particularly, this invention relates to a system and method for separation of metals from a molten salt solution.BACKGROUND

[0004] Several methods for removing stable alkali and alkaline earth metals from LiCl—KCl pyroprocessing salts have been identified in the literature. Electrochemical deposition of alkali and alkaline earth metals into liquid metal electrodes has been demonstrated for reactive electrode materials such as Pb, Zn, Bi, and Cd. Separation of Sr and Ba into those metals has been demonstrated, although some Li and K was also deposited into the liquid metal electrodes. In comparison, Cs deposition is less well studied, and only minimal Cs recovery has been demonstrated electrochemically. Similarly, reactive boron electrodes have been used to recover Cs from NaCl—KCl by electrochemically incorporating the Cs into the boron structure. In that case, the Cs concentration in the salt was reduced from 1.72 percent by weight (wt %) to 0.09 wt % and no further Cs was recovered without significant salt degradation.

[0005] Electroactive separation diaphragms have been used in separation cells to selectively separate similarly-behaved species from molten salts solution. Such separation cells consists of multiple compartments separated by thin nickel metal diaphragms that selectively alloy with a component in the first compartment, allow that species to move through the nickel, and then oxidize into the second compartment where it can be removed electrochemically. The nickel is impermeable to the salt and prevents the two compartments from mixing. These diaphragms operate similarly to the single reactive electrodes described above, meaning that significant co-deposition of the base salt components (e.g., LiCl, KCl) is expected if used for separating out stable species (e.g., Cs, Sr, Ba).

[0006] Non-electrochemical methods for separating stable components from molten salts include vaporization and fractional crystallization. Vaporization uses heat and vacuum to vaporize the stable chlorides, which collect in a separate chamber. Fractional crystallization uses controlled heating and cooling of the salts to freeze a pure LiCl—KCl crystal on a cold finger or cold plate while leaving behind a molten salt heel that contains concentrated stable chloride species.

[0007] In other molten salt operations, liquid metal reactive electrodes have been used to separate actinide and lanthanide species from nuclear fuel recycling salts. The most common example is the use of a liquid cadmium cathode in pyroprocessing of used nuclear fuel to collect uranium and transuranic (i.e., Pu, Am, Np) species during electrorefining. Actinide and lanthanide elements are much easier to deposit than alkali and alkaline earth species. The work in those reports did not target separation of alkali or alkaline earth species from the molten salt.

[0008] There is an ongoing need for new methods and systems for electrochemically separating metals from molten salt electrolyte solutions. The methods and systems described herein address this need.SUMMARY

[0009] The system described herein comprises a reactive tubular-shaped electrode (also referred to herein as a “shroud-type” electrode or a “shroud” electrode or a “depletion electrode”), which surrounds another reactive separation electrode, which are used to separate metals from a molten salt solution. During operation, the reactive shroud electrode alloys with components that the operator does not want to recover from the solution (e.g., lithium and potassium) to locally increase the concentration of solution components the operator wants to recover (e.g. cesium and strontium). As used herein, the term “reactive” as applied to an electrode means an electrode that electrochemically reduces a metal ion component in the electrolyte to its metallic form, which then alloys with the electrode. Reactive electrodes can be solid electrodes, such as the shroud electrode, or they can be a liquid metal electrode. After sufficient alloying has occurred at the reactive shroud electrode, the operator activates the nested separation electrode to deposit the solution components of interest into the separation electrode. The nested separation electrode can be removed after separation is complete while the shroud remains in the solution. The components alloyed into the shroud (e.g., lithium) will dissolve back into the molten salt electrolyte over time when the shroud is not in operation.

[0010] The system described herein uses a salt-permeable shroud electrode constructed of a tube of a solid reactive metal, such as aluminum 6061 or brass 464, which selectively alloys with a metal (e.g., lithium) generated metal ions from the molten salt electrolyte (e.g., LiCl—KCl). The shroud is salt-permeable and does not impede the mixing of the multiple components in the electrolyte solution. Additionally, the shroud holds the base salt species (e.g., Li) in reserve so that the concentration of the metal species to be recovered increases and can be recovered using the separate liquid metal reactive electrode. In contrast, the previously described diaphragm method targets the species of interest.

[0011] As noted above, the purpose of this shroud-style reactive electrode design is to selectively deplete undesired solution components at one electrode in order to enable deposition of desired solution components at a second electrode. Reactive electrodes function by alloying the deposited metal (e.g., a component of the molten salt used as the solvent for the electrodeposition) into the reactive electrode material. Configuring the reactive electrode as a tube allows it to surround the separation electrode and to selectively deplete electrolyte components such as lithium and / or potassium near the separation electrode. This locally increases the concentration of solution components targeted for recovery, which are then deposited at the separation electrode nested within the tubular shroud electrode.

[0012] In the past, the use of a single reactive electrode was previously proposed to recover stable fission product ions such as cesium, strontium and barium from molten salts used during pyroprocessing of used nuclear fuel. Reactive electrodes also strongly alloy with electrolyte salt constituents like lithium and potassium, degrading the process salt without effectively removing the stable ions from the salt. This results in unwanted residual lithium and potassium in the reactive electrode and ineffective separation of the stable fission product ions from pyroprocessing salts. Lithium and potassium in the pyroprocessing salts exist in significantly higher quantities than the stable ions. This means that multiple reactive electrode separations would be necessary to remove a significant quantity of the stable ions which will degrade the process salt which is not acceptable.

[0013] During pyroprocessing of nuclear fuel, alkali metal and alkaline earth fission products will build up in the molten salt over time and must be periodically removed. In this example, removal of these components would be performed using a reactive shroud made of aluminum surrounding a liquid metal reactive electrode. The reactive shroud electrode will selectively deplete the molten salt ions, such as lithium and potassium near the liquid metal separation electrode and locally increase the concentration of fission product ions such as barium, strontium, and cesium near the liquid metal electrode. The liquid metal reactive electrode would be used to remove the barium, strontium, and cesium from the salt. After separation, the liquid metal reactive electrode is removed from the salt for conversion to a durable waste form. The aluminum reactive shroud is left behind in the molten salt and the alloyed lithium will dissolve back into the salt over time.

[0014] Use of the reactive shroud electrode in a system for separating cesium from a LiCl / KCl molten salt solution resulted in the separation of cesium from pyroprocessing salts into a liquid metal electrode. Tests using liquid metal reactive electrodes without the reactive shroud electrode failed to separate any cesium from the pyroprocessing salt.

[0015] The systems and methods described herein increases the recovery of stable fission products such as Sr, Ba, and Cs that accumulate during reprocessing of used nuclear fuel. These fission products are more stable than the base LiCl—KCl salt, have high decay heat, which changes the salt properties, and are radiotoxic. These species accumulate in the pyroprocessing salt over time and must eventually be removed to ensure that the pyroprocessing products are not contaminated with them. The systems and methods achieve this goal efficiently by using simple to construct and operate electrode designs.

[0016] The initial testing has demonstrated improved separation of stable CsCl from a LiCl—KCl pyroprocessing salt. The shroud increased the recovered Cs content from 0.2 atom % (at %) without the use of the shroud to 3.4 at % after using the shroud. Additional tests in the same salt without the use of the shroud showed an increase in Cs content up to 17.5 at % in parts of a liquid metal electrode. This indicates that locally increasing the Cs content leads to more effective separation of the stable components.

[0017] The following non-limiting embodiments are provided to illustrate certain aspects and features of the methods described herein.

[0018] Embodiment 1 is an electrochemical cell for electrochemically recovering a target metal from a molten salt electrolyte; the electrochemical cell comprising:

[0019] a molten salt-permeable tubular-shaped first electrode (i.e., a shroud electrode);

[0020] a liquid metal second electrode (i.e., a separation electrode) contained within a ceramic crucible situated within and surrounded by the tubular-shaped first electrode; and

[0021] a counter electrode spaced from the first electrode;

[0022] the first and second electrodes and the counter electrode being in contact with a molten salt electrolyte solution comprising ions of the target metal dissolved in a molten salt medium;

[0023] wherein the first electrode is composed of a material capable of forming a first alloy with a metal of the molten salt medium (e.g., Li); and the liquid metal of the second electrode comprises a material that is capable forming a second alloy with the target metal (e.g., Cs); wherein in use, the molten salt medium is maintained at a temperature above the melting point of the liquid metal of the second electrode; the first and second electrodes and the counter electrode are placed in circuit with an active DC power source to electrochemically reduce a metal ion from the molten salt medium to its metallic form at the first electrode thereby forming the first alloy and increasing the relative concentration of the target metal in the vicinity of the second electrode; and ions of the target metal are reduced at the second electrode to form the second alloy.

[0024] Embodiment 2 is the electrochemical cell of embodiment 1, further comprising a reference electrode spaced from the first electrode and the counter electrode, to provide a reference point for measuring the potential of the electrodes in the electrochemical cell.

[0025] Embodiment 3 is the electrochemical cell of embodiment 1 or embodiment 2, wherein the first electrode is composed of at least one material selected from the group consisting of aluminum, an aluminum alloy, copper, a copper alloy, nickel, and nickel alloy.

[0026] Embodiment 4 is the electrochemical cell of any one of embodiments 1 to 3, wherein the first electrode is composed of aluminum or an aluminum alloy.

[0027] Embodiment 4 is the electrochemical cell of any one of embodiments 1 to 3, wherein the liquid metal of the second electrode comprises bismuth, antimony, lead, tin or a combination thereof.

[0028] Embodiment 5 is the electrochemical cell of any one of embodiments 1 to 4, wherein the target metal comprises cesium, strontium, barium, or a combination of two or more thereof.

[0029] Embodiment 6 is the electrochemical cell of any one of embodiments 1 to 5, wherein the molten salt medium comprises a lithium salt, a potassium salt, or a combination thereof.

[0030] Embodiment 7 is the electrochemical cell of any one of embodiments 1 to 6, wherein the molten salt medium comprises lithium chloride, a potassium chloride, or a combination thereof.

[0031] Embodiment 8 is the electrochemical cell of any one of embodiments 1 to 7, wherein the molten salt medium is maintained at a temperature of about 200 °C to about 900 °C.

[0032] Embodiment 9 is the electrochemical cell of any one of embodiments 1 to 7, wherein the first electrode is composed of aluminum or an aluminum alloy; the molten salt medium comprises lithium chloride, potassium chloride, or a combination thereof; and the first alloy comprises a lithium aluminum alloy, a potassium aluminum alloy, or a lithium potassium aluminum alloy.

[0033] Embodiment 8 is the electrochemical cell of any one of embodiments 1 to 9, wherein the liquid metal of the second electrode comprises bismuth, antimony, lead, tin or a combination thereof; the target metal comprises cesium, strontium, barium, or a combination of two or more thereof; and the second alloy comprises an alloy of cesium, strontium and / or barium with antimony lead, tin and / or bismuth.

[0034] Embodiment 9 is a method for recovering a target metal from a molten salt solution, the method comprising:

[0035] (a) providing an electrochemical cell comprising the steps of:

[0036] (i) a molten salt-permeable tubular-shaped first electrode;

[0037] (ii) a liquid metal second electrode contained within a ceramic crucible situated within and surrounded by the tubular-shaped first electrode; and

[0038] (iii) a counter electrode spaced from the first electrode;

[0039] the first and second electrodes and the counter electrode being in contact with a molten salt electrolyte solution comprising ions of the target metal dissolved in a molten salt medium;

[0040] wherein the first electrode is composed of a material capable of forming a first alloy with a metal of the molten salt medium; and the liquid metal of the second electrode comprises a material that is capable forming a second alloy with the target metal; and the cell is maintained at a temperature sufficient to keep the molten salt medium and the liquid metal of the second electrode in a liquid state;

[0041] (b) placing the first electrode, the second electrodes and the counter electrode in circuit with an active DC power source to electrochemically reduce a metal ion from the molten salt medium to its metallic form at the first electrode thereby forming the first alloy and increasing the relative concentration of the target metal in the vicinity of the second electrode; and ions of the target metal are reduced at the second electrode to form the second alloy;

[0042] (c) deactivating the DC power source;

[0043] (d) removing the second electrode from the molten salt medium; and

[0044] (e) recovering the target metal from the second alloy.

[0045] Embodiment 10 is the method of embodiment 9, wherein the electrochemical cell further comprises a reference electrode spaced from the first electrode and the counter electrode, to provide a reference point for measuring the potential of the electrodes in the electrochemical cell.

[0046] Embodiment 11 is the method of embodiment 9 or embodiment 10, wherein after step (d), the first electrode is left within the molten salt medium at a temperature sufficient to maintain the molten salt medium in a liquid state, so that the metal from the molten salt medium in the first alloy will redissolve in the molten salt medium.

[0047] Embodiment 12 is the method of any one of embodiments 9 to 11, wherein the first electrode is composed of aluminum or an aluminum alloy; the molten salt medium comprises lithium chloride, potassium chloride, or a combination thereof; and the first alloy comprises a lithium aluminum alloy, a potassium aluminum alloy, or a lithium potassium aluminum alloy.

[0048] Embodiment 13 is the method of any one of embodiments 9 to 12, wherein the liquid metal of the second electrode comprises bismuth, antimony, lead, tin, or a combination thereof; the target metal comprises cesium, strontium, barium, or a combination of two or more thereof; and the second alloy comprises an alloy of cesium, strontium and / or barium with antimony, lead, tin, and / or bismuth.

[0049] Embodiment 14 is the method of any one of embodiments 9 to 13, wherein the molten salt medium is maintained at a temperature of about 200 °C to about 900 °C.

[0050] Embodiment 15 is the method of any one of embodiments 9 to 14, wherein the DC power source provides a voltage of about 0 to about 10 V.BRIEF DESCRIPTION OF THE DRAWINGS

[0051] FIG. 1 schematically illustrates a system for electrochemical separation as described herein.

[0052] FIG. 2 illustrates details of the system of FIG. 1 in a tope view (A) and a perspective side view (B).

[0053] FIG. 3 provides a plot of standard potentials compared to the equilibrium potentials of active metals in liquid bismuth at 5 and 10 mol % active metals in bismuth at 500 °C.

[0054] FIG. 4 provides equilibrium potentials for active metals and for 5 and 10 mol % active metals in bismuth at 500 °C.

[0055] FIG. 5 provides a schematic illustration of a typical sectioned electrode prepared for SEM analysis.

[0056] FIG. 6 provides plots of cell currents during initial electrodeposition tests at 0.65 V vs. Li+ / Li0.

[0057] FIG. 7 provides plots of cell current during electrodeposition to the lithium depletion electrode (top) and to the Bi—Sb electrode (bottom).

[0058] FIG. 8 provides plots of cell current during electrodeposition at 0.1 V vs. Li+ / Li0.

[0059] FIG. 9 shows a backscattered electron scanning electron micrographic (SEM-BSE) photomicrograph of the surface of a bismuth electrode that was cooled in the salt after electrodeposition, with marked locations of EDS analyses.

[0060] FIG. 10 shows BSE-SEM photomicrograph of a Bi—Sb electrode which was allowed to cool in the salt after electrodeposition without a lithium depletion electrode (A) and the same image with marked locations for EDS analysis (B).

[0061] FIG. 11 shows a BSE-SEM photomicrograph of the Bi—Sb electrode from a lithium depletion electrode test (A), which was removed from the salt after electrodeposition of cesium to mitigate back reactions, and the same image with numbers locations of Eds spot analyses (B).

[0062] FIG. 12 shows a high magnification image of the Sb-rich region (A) and the same image with numbered locations of EDS spot analyses (B).DETAILED DESCRIPTION

[0063] Described herein is an electrochemical cell for electrochemically recovering a target metal from a molten salt electrolyte. The electrochemical cell comprises a tubular shroud electrode surrounding a liquid metal separation electrode; and a counter electrode spaced from the shroud electrode, as shown in FIGS. 1 and 2. The electrodes contact a molten salt electrolyte solution comprising target metal ions (e.g., Cs+) dissolved in a molten salt (e.g., LiCl—KCl). The electrochemical cell is operated by applying a voltage to the electrodes to electrochemically reduce a metal ion (e.g., Li+) from the molten salt to its metallic form at the shroud electrode to form a first alloy, thereby increasing the relative concentration of the target metal in the vicinity of the separation electrode. Ions of the target metal are reduced at the separation electrode to form the second alloy therewith containing the target metal(s).

[0064] Referring now to the figures, FIG. 1 schematically illustrates an electrochemical cell for electrochemically separating metals as described herein. Cell 100 includes tubular-shaped, Molten salt permeable shroud electrode 102 surrounding a liquid metal separation electrode 104 comprising a first liquid metal 106 contained within ceramic crucible 108. FIG. 2, Panel A, provides a top plan view of shroud electrode 102 surrounding separation electrode 104, and illustrates liquid metal 106 within crucible 108, which could not be shown in FIG. 1. Shroud electrode 102 includes conductive lead 110 in contact therewith, and liquid metal electrode 104 comprises conductive lead 112 in contact with the first liquid metal 106. In FIG. 1, electrode 102 is shown as transparent to illustrate the nested arrangement of electrode 104 within electrode 102. FIG. 2, Panel B shows a solid perspective view of shroud electrode 102 with leads 110 and 112.

[0065] Reference electrode 114, which provides a reference for measuring the operating potential difference of the electrodes, is spaced from electrode 102, and comprises a second molten metal (not shown) contained within second ceramic crucible 116, with conductive lead 118 contacting the molten metal of reference electrode 114. Counter electrode 120 (e.g., a graphite rod) is spaced from electrodes 102 and 104. Shroud electrode 102, separation electrode 104, and counter electrode 120 all contact molten salt electrolyte 130, with separation electrode 104 being submersed in electrolyte 130 such that first liquid metal 106 contacts electrolyte 130 at the surface thereof exposed to the electrolyte at the open end of crucible 108. The molten metal of reference electrode 114 contacts the molten metal thereof through a capillary hole 117 in crucible 116. The cell illustrated in FIG. 1 also includes a second working electrode 140, comprising crucible 142 containing a third liquid metal 144 and conductive lead 146 contacting liquid metal 144. The second working electrode 140 is present only to perform a comparison electrochemical separation using a working electrode that is not surrounded by a shroud electrode, and is not necessary for the system and method described herein. The entire collection of electrodes 102, 104, 114, 120, and 140, and the molten salt electrolyte 130 are housed within heated ceramic crucible 150.

[0066] Tests with two liquid metals were used to assess the efficiency of removing cesium and other active metals from molten LiCl—KCl salt by electrodeposition. Like cesium chloride, alkaline earths chloride compounds are more stable than the LiCl—KCl eutectic base salt, as illustrated in the plot of standard potentials compared to the equilibrium potentials of active metals in liquid bismuth at 5 and 10 mol % active metals in bismuth at 500 °C shown in FIG. 3. FIG. 3 shows the deposition potentials of pure alkali metals and alkaline earth metals and for those metals dissolved in bismuth at 5 or 10 mol %. Dissolution into bismuth changes the deposition potentials of strontium and barium more than it changes those of lithium and potassium, such that the deposition potentials of strontium and barium are more positive than those of lithium and potassium. This means strontium and barium can be deposited without co-depositing lithium and potassium. The deposition potential of cesium is expected to be more negative.Typical Materials of Construction.

[0067] The shroud electrode comprises a material that alloys with the metal to be depleted from the molten salt medium (e.g., Li, K, etc.). In the case of lithium depletion, suitable metals include aluminum, copper, nickel, and alloys-thereof.

[0068] The molten salt medium typically include alkali metal chloride salts (e.g., LiCl—KCl eutectic at 500 °C, LiCl at 650-700 °C, NaCl—KCl at 700 °C). Other anions (e.g., phosphates, hydroxides, fluorides, etc.) can be used in place of chloride, provided they do not form insoluble compounds with the molten salt medium, or metal ions dissolved in the molten salt medium.

[0069] The crucibles for the molten salt medium typically are formed from boron nitride. Other suitable ceramics include, but are not limited to boron-nitride / aluminum-nitride composite, aluminum nitride, and oxide ceramics such as alumina, magnesia, beryllia, or zirconia.

[0070] The electrical leads typically comprise tungsten, stainless steel, molybdenum, tantalum, and nickel.

[0071] The counter electrode typically comprises graphite (e.g., a graphite rod) attached to stainless steel. Other materials for forming the counter electrode include inert oxides, metals typically comprised of but not limited to platinum, rhodium, stainless steel, nickel, and nickel chrome.

[0072] The liquid metals for the working separation electrode and the reference electrode typically comprise metals (including metalloid elements) with a melting point below the operating temperature of the molten salt medium (e.g., LiCl—KCl). The liquid metal should interact with the metal of interest strongly enough to shift the reduction potential of the metal (e.g., Cs, Sr, and Ba) more positive than or as close as possible to Li and K. The metals should maximize recovery of Cs, Ba, and Sr and minimize co-deposition of Li and K. Bismuth and a 50:50 mol % Bi—Sb alloy are two suitable examples for recovering Cs, Ba, and Sr. Alternatively, Sb alone can be used. Other materials include Sn, Pb, and alloys of Bi, Sn, Pb, Sb, or any combination thereof.Observations.

[0073] The removal of cesium from a LiCl—KCl salt mixture by electrochemical deposition into a liquid metal was demonstrated to occur and process conditions necessary for the effective removal of cesium (and other active metals) from a surrogate pyroprocessing salt by electrodeposition in liquid metals were determined. The back reaction between deposited cesium and LiCl / KCl is thermodynamically favorable and was suspected to be the cause of the low cesium concentrations detected in initial tests relative to that expected based on the measured deposition currents. Electrodes used in subsequent tests were promptly removed from the salt after deposition rather than cooling in the salt to mitigate the effects of back reaction. Further, the local depletion of lithium near the liquid metal electrode done prior to and during deposition increased the amount of cesium deposited into the liquid metal electrodes.

[0074] The distributions of cesium and co-deposited lithium and potassium in the metal cathodes were measured by using SEM / EDS to provide qualitative insights into the deposition behavior. Cesium deposited in bismuth electrodes appears to form locally concentrated inclusions in the bismuth. It is inferred from the oxygen content in electrodes that deposited lithium is distributed fairly uniformly throughout the bulk of the bismuth electrode. The mechanisms that lead to the observed segregation of localized concentrated cesium and the inferred uniform lithium distributions across the liquid bismuth cathodes should be elucidated in further testing. Cesium was observed to deposit preferentially into antimony-rich phases of the Bi—Sb alloy electrode. The preferential deposition of cesium into the antimony indicates that deposition into pure antimony at 650 °C can be effective, as well. These tests were conducted at 500 °C to match electrorefining operations, but the method could be applied separately.

[0075] The target metal can be recovered from the liquid metal separation electrode by electrochemically re-dissolving the target metals in another media and electrochemically reducing them to a usable metallic form, by isolating the target metals in a form that is insoluble in the liquid metal and the molten salt medium through gaseous sparging (e.g., using oxygen gas to isolate Cs as Cs2O), or by distilling the target metal away from the reactive separation electrode under vacuum.EXAMPLESMethods

[0076] Mixtures of LiCl (ACS Reagent, 99.9%), KCl (ACS Reagent, 99.9%), and CsCl (anhydrous, 99.9%) were used for each test and each reagent was baked out overnight at 300 °C to remove adsorbed water before use. Appropriate amounts of each salt were added to an alumina crucible to generate approximately 100 g of the LiCl—KCl eutectic with 10 wt % CsCl. The working separation electrodes consisted of either bismuth or equimolar amounts of bismuth and antimony, contained in boron nitride crucibles. All tests were conducted at 500 °C. The liquid metals were melted in the boron nitride crucibles prior to the deposition tests. A 65 mol % lithium in bismuth alloy was used as the reference electrode, which was contained in a boron nitride crucible with a 1-mm hole in the side to provide salt contact. The open circuit potential of the reference electrode is known to be 0.716 V vs. Li+ / Li0. Tungsten rods served as the electrical leads for the working and reference electrodes. A graphite rod attached to an 18-8 stainless steel rod served as the counter electrode for all tests.

[0077] A schematic of the electrochemical system is shown in FIG. 1. During lithium depletion tests, a lithium depletion shroud electrode was placed around the working electrode containing the liquid metal, as depicted in FIG. 1. This electrode also had a 1-mm diameter tungsten electrical lead contacting the liquid metal. Working electrodes with (electrode 104) and without the surrounding lithium depletion electrode (electrode 140) are depicted in FIG. 1, but only one working electrode was used in a test.

[0078] Deposition parameter values were chosen based on the thermochemical properties of the liquid metals used to alloy with the constituent alkali metals. In FIG. 4, the deposition potentials for pure lithium, potassium and cesium are compared to the equilibrium potentials for depositing those species into liquid bismuth at 5 mol % (open squares) and 10 mol % (filled squares) of each alkali metal. Note that electromotive force measurements of Rb+ in bismuth are used to represent Cs+ in bismuth in FIG. 4 due to a lack of available measurements for Cs+ in bismuth. Electrodepositions of Cs into liquid bismuth and into the liquid Bi—Sb alloy were conducted at a constant voltage of 0.65 V vs. Li+ / Li0 in initial tests (indicated as Deposition Potential A) and at 0.1 V vs. Li+ / Li0 in subsequent tests (indicated as Deposition Potential B).

[0079] The liquid metal electrodes were allowed to cool in the salt after deposition in the initial tests. Two methods were used to improve separation of cesium from the molten salt in subsequent tests. The first method was to use a lithium depletion electrode surrounding the liquid metal electrode. Lithium will alloy with this electrode material without generating pure lithium metal, thereby depleting lithium from salt near the liquid metal electrode to decrease the amount of lithium co-deposited into the liquid metal. Deposition tests were conducted with bismuth and Bi—Sb liquid metal electrodes surrounded by a lithium depletion electrode.

[0080] The second method was to immediately remove the liquid metal electrode from the salt at the conclusion of deposition. Deposited cesium metal will chemically reduce the base salt constituent lithium and potassium metal if left in contact with the melt for extended periods of time. Tests were conducted in which cesium was deposited into bismuth and 50:50 mol % Bi:Sb alloy with immediate removal of the electrode from the salt after deposition.

[0081] Characterization of the liquid metal electrodes after deposition testing was done using scanning electron microscopy with associated energy-dispersive X-ray spectroscopy (SEM / EDS). The entire electrode was embedded in epoxy, sectioned vertically perpendicular to the salt metal interface and dry-polished to 1200 grit in an inert atmosphere glovebox prior to characterization. FIG. 5 shows a schematic of a cross-sectioned separation electrode 504 (including crucible 508 lead 512, solidified salt 531 and solidified molten metal 507) surrounded by epoxy 560 as prepared for SEM analysis with a black box 570 indicating a typical analysis region. Surface analyses were performed on materials generated in initial demonstration tests rather than bulk analysis of the salt or alloy because of the greater sensitivity of SEM / EDS and the capacity to measure local concentrations at the metal / salt interface.Deposition with Bi and Bi—Sb Alloy without a Lithium Depletion Electrode

[0082] Initial tests were performed to assess electrodepositing cesium into liquid bismuth or 50:50 Bi—Sb alloy cathodes. An electrode with 2.236 g bismuth was submerged into 100 g of eutectic LiCl—KCl containing 10 wt % CsCl. A constant voltage of 0.65 V vs. Li+ / Li0 (Deposition Potential A in FIG. 4) was applied to the cathode and 258 coulombs of charge were passed, equivalent to deposition of twice the initial cesium content in the salt. The cathode was left in the salt to cool after deposition. In a separate test, a 50:50 mol % Bi—Sb alloy a mixture of 1.357 g of bismuth and 0.783 g of antimony was submerged into a fresh mixture of 100 g of eutectic LiCl—KCl containing 10 wt. % CsCl and was subjected to a constant voltage of 0.65 V vs. Li+ / Li0. A total of 247 coulombs of charge was passed, equivalent to 115 C / gelectrode. The cathode was left in the salt to cool after deposition. FIG. 6 shows the currents measured at the imposed voltage during each deposition test over time. Currents were approximately 1.52 A in the test with the bismuth electrode and approximately 1.64 A in the test with the Bi—Sb alloy electrode. The stability of the currents over time indicates secondary phase nucleation did not occur at either electrode-electrolyte interface during deposition. The greater cathodic (more negative) current achieved in the test with the Bi—Sb electrode indicates the affinity for alkali metals to alloy is higher with antimony than with bismuth due to the more electronegative nature of antimony. The large currents achieved by both electrodes suggests significant total amounts of metal were deposited during the course of each test. Based upon the thermodynamic differences shown in FIG. 4, significant co-deposition of lithium and potassium is expected to have occurred. The cell currents during initial electrodeposition tests at 0.65 V vs. Li+ / Li0 are shown in FIG. 6.Deposition Tests with Lithium Depletion Electrode

[0083] In the lithium depletion electrode electrodeposition test, a 50:50 mol % Bi—Sb alloy consisting of a mixture of 1.247 g bismuth and 0.726 g antimony was submerged into 100 g of eutectic LiCl—KCl containing 10 wt % CsCl. A constant voltage of 0.1 V vs. Li+ / Li0 was applied to the lithium depletion electrode to deposit lithium and to the alloy electrode to deposit cesium. A total of 113 coulombs of charge was passed to the lithium depletion electrode first and then 225 coulombs of charge were passed to the Bi—Sb alloy (115 C / gelectrode). The cathode and depletion electrodes were both promptly removed from the salt after the test.

[0084] FIG. 7 shows the currents measured during deposition to the lithium depletion electrode (top) and then during deposition to the Bi—Sb alloy (bottom). The cathodic current during the deposition into the lithium depletion electrode was initially smooth and as high as about −0.65 A, but quickly decreased to near −0.1 A with very noisy behavior in which the current oscillated between near 0 A and −0.15 A. This is attributed to saturation of the lithium depletion electrode and the slow diffusion of deposited lithium into the depletion electrode. During deposition to the Bi—Sb alloy, the current became steady at ~−0.15 A and was lower than that measured for the depletion electrode due to the successful depletion of electroactive ions near the liquid metal electrode. After about 650 s, the current decreased to about −0.05 A indicating a phase transition occurred in the Bi—Sb, likely the nucleation of an additional phase. The current remained steady, indicating that the nucleated phase does not block the surface of the electrode and is likely not a floating solid. In FIG. 7, the cell current during electrodeposition to the lithium depletion electrode is shown in the top plot, and to the Bi—Sb electrode in the bottom plot.

[0085] The deposition of alkali metals into Bi—Sb in the test without the depletion electrode occurred with much greater current (approximately −1.64 A in FIG. 6) than in the test with the depletion electrode (approximately −0.15 A in FIG. 7). It is likely that the depletion electrode successfully decreased the lithium concentration in the salt local to the Bi—Sb electrode and the amount of lithium that was deposited in the Bi—Sb electrode. Depletion of lithium in the salt concentrates both CsCl and KCl in the vicinity of the deposition. However, the difference between the equilibrium potentials of potassium and cesium is much smaller than that between lithium and cesium. Because the current was low (~−0.15 A), the deposition was likely of some species present in low concentration in the salt. The concentration of cesium was approximately five-times lower than the concentration of potassium. Significantly less codeposition of potassium than lithium had been observed during deposition tests with barium and strontium in other reported processes.Deposition Tests with Bismuth and Bi—Sb Alloy with Rapid Removal

[0086] Back reaction (oxidation) of cesium with LiCl into CsCl and lithium metal is expected to have occurred when liquid metal electrodes with active metal deposits were allowed to cool in the salt after polarization was stopped. Therefore, additional tests were run in which the alloy electrode was quickly removed from the salt after deposition to minimize the effect of back reactions. These tests were performed in the salts remaining after the lithium depletion electrode tests with bismuth and the Bi—Sb electrodes had been completed. For deposition into bismuth, an electrode with 1.828 g of bismuth was submerged into the salt bath that had less than the original 100 g of eutectic LiCl—KCl containing 10 wt % CsCl. Constant voltage was applied to the cathode equivalent to 0.1 V vs. Li+ / Li0. A total of 211 coulombs of charge were passed. The electrode was removed from the salt immediately after deposition to minimize the extent of back reaction.

[0087] For deposition into a 50:50 mol % Bi—Sb alloy, a mixture of 1.249 g of bismuth and 0.727 g antimony was submerged into the salt bath that had less than the original 100 g of eutectic LiCl—KCl containing 10 wt % CsCl. Constant voltage was applied to the cathode equivalent to 0.1 V vs. Li+ / Li0 and 200 coulombs of charge were passed. The electrode was removed from the salt prior to cooling.

[0088] As seen in FIG. 8, the deposition into bismuth generated a higher cathodic (more negative) current than deposition into the Bi—Sb alloy, at about 0.8 A. Because of the high current attained, the targeted amount of charge passed occurred within a much shorter test duration, and the test was stopped. The different durations are attributed to differences in salt compositions:salt in the test with the Bi—Sb alloy had been depleted of lithium, potassium and cesium by deposition on both the lithium depletion electrode and the liquid Bi—Sb alloy, whereas the salt in the test with bismuth was only depleted of lithium due to the electrical short. This indicates the current measured in the test with the Bi—Sb alloy was due primarily to deposition of cesium. That current is significantly lower than was measured in initial tests without lithium depletion (see FIG. 6).Analysis of Electrodeposits.

[0089] Alloy electrodes from each test were embedded in epoxy, then cross sectioned perpendicular to the salt-alloy interface and dry-polished in an argon atmosphere glovebox in preparation for scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and related techniques. The sectioned and polished electrodes were transported from the glovebox to an SEM inside a container under argon atmosphere. It is expected that these samples contained deposited lithium metal and were highly reactive to moisture in the air. The samples were briefly exposed to air during transfer from the container to the SEM chamber, which was quickly evacuated. Methods to mitigate air exposure are being evaluated. SEM / EDS analysis was performed on the samples to examine the distribution of active metals near the surface of the metal electrode in contact with the salt. Secondary electron imaging (SEI) was used to evaluate texture and back-scattered electron imaging (BSE) was used to highlight compositional differences: heavier elements generate higher contrast than lighter elements. Although EDS does not detect lithium, the contrast of phases in SEM-BSE images is sensitive to its presence. Compositional EDS analyses were performed as spot analyses, small area analyses, and to generate X-ray maps of the rastered image. Analyses of the solidified electrode surfaces are more sensitive to alkali metal deposition than analysis of the depleted salt.Analyses of Electrodeposits from Deposition Tests with Bi and Bi—Sb AlloyBismuth Electrode

[0090] SEM-BSE was used to examine the electrodeposits from deposition tests with Bi and Bi—Sb alloy. The image (FIG. 9) includes regions of the bismuth electrode across the lower third of the image (light grey), salt on the right side of the image (dark grey), and regions of bismuth surrounded by a darker material (possibly salt) in the upper left of the image. It is possible that the darker contrast of the phases in the top two-thirds of the image are due to lithium being deposited in (alloyed with) the bismuth. Liquid bismuth that was suspended in the salt during the test would not have been polarized and deposition would not have occurred into that material. In either case, the image indicates a single static interface between molten bismuth and molten salt was not maintained as the electrode was cooled.

[0091] EDS spot analyses were performed at the numbered locations shown in FIG. 9. These analyses show that there are regions near the surface of the bismuth electrode that contain detectable amounts of cesium and potassium.

[0092] Results of EDS analyses are given in Table 1. Locations 1, 2, and 3 are below the interface of the salt and bismuth electrode and cesium and potassium are not detected. Spot analyses at locations 5, 6, and 7 show high concentrations of bismuth and spot 4 has a significant concentration of potassium. In the photomicrograph, materials at spots 4, 5, 6 and 7 all appear to have a different morphology than the electrode material at spots 1, 2, and 3. It is unclear if this morphology is of a salt-filled porous bismuth structure or indicates lithium deposited in the bismuth. Locations 8 and 9 have detectable amounts of cesium and potassium, but the concentrations are lower than expected based on the measured deposition currents. Cesium is expected to deposit at the surface of the electrode contacting the salt, and the current passed during the deposition test was sufficient to deposit twice the amount of cesium present in the salt. It is suspected that deposited cesium and potassium reacted with LiCl in the salt to reform CsCl and KCl and lithium metal while the electrode was immersed in the salt but not polarized. Those salt phases are more thermodynamically stable than the metallic deposits when the electrode is not polarized. This may explain the morphology at spots 4, 5, 6, and 7, as well as the lower-than-expected cesium content.TABLE 1Normalized concentration of elements (at. %) determinedby EDS analysis at locations indicated in FIG. 9.LocationKCsBi10.00.199.920.00.0100.030.10.099.981.10.398.690.20.399.5416.01.083.050.00.0100.062.00.197.970.00.599.5Bi—Sb Electrode

[0093] FIG. 10 shows (A) an SEM-BSE photomicrograph of the Bi—Sb electrode that was cooled in the salt after electrodeposition. The photomicrograph contains several dark grey regions in the center of the image that were found to be rich in potassium and have detectable amounts of cesium. There is a light grey region at the bottom and left side of the image adjacent to the dark grey regions that is rich in bismuth. There is a medium grey region between the potassium-rich dark grey regions and along the right side of the image rich in antimony. These light and medium grey regions have an irregular morphology which may be an artifact of the coarseness of the sample surface. It is unclear if the segregation of these phases occurred upon cooling the electrode or if the potassium rich region formed during deposition. De-alloying may occur at low temperatures, which could explain how the observed cesium and potassium-rich inclusion phases formed.

[0094] FIG. 10, panel B, shows the locations of EDS spot analyses performed to measure the compositions of the potassium-rich dark grey regions (locations 3, 4, 5 and 6) in the center of the image or in the surrounding bismuth-rich regions (locations 1, 8, and 10) and the antimony-rich regions (locations 2, 7 and 9). The measured elemental concentrations of potassium, antimony, cesium, and bismuth in each of these areas are shown in Table 2. Note the reported results of EDS spot analyses are affected by the surface roughness of the analyzed regions, but indicate constituents that are present. Results are presented to the nearest 0.1 at % to facilitate comparisons, but are only qualitative.TABLE 2Normalized concentrations of elements (at %) determinedby EDS analysis at locations indicated in FIG. 10.LocationKSbCsBi10.244.70.055.020.396.70.03.0387.43.63.85.2482.55.73.58.3591.23.84.20.8685.44.83.56.370.396.60.03.180.878.80.020.491.064.00.035.0101.17.00.091.8

[0095] The measured concentrations given in Table 2 indicate trace amounts of cesium are present in the potassium-rich dark grey regions seen in the center of the image shown in FIG. 10, but not in Bi-rich or Sb-rich domains. Segregation of electrodeposited species into distinct regions (K and Cs) could occur during electrodeposition or coalesce upon cooling. The high potassium concentration relative to cesium (compared to the ~5:1 ratio in the salt) could indicate preferential deposition of potassium, or that back reaction of cesium occurred preferentially.Analysis of Electrodeposits from Deposition Tests with Lithium Depletion Electrode

[0096] FIG. 11 provides a BSE photomicrograph showing the top edge of the Bi—Sb alloy recovered from the test with the depletion electrode (A), and the same image with numbered locations of EDS spot analyses. The field of view includes several distinct regions. There are regions of salt at the top and down both sides of the image, and a bright Bi—Sb alloy region in the bottom center. There are distinct grey streaks of an antimony-rich phase throughout the Bi—Sb alloy and a prominent cap of an antimony-rich phase sitting atop the Bi—Sb alloy. The antimony-rich cap has two distinct regions: the medium grey colored phase contains trace amounts of cesium, and the darker grey phase does not contain cesium.

[0097] EDS X-ray maps of the region in the micrograph in FIG. 11 for bismuth, antimony, cesium, potassium, chlorine and oxygen were made. An antimony-rich cap was clearly seen in the antimony map and is devoid of potassium, bismuth and chlorine. There were trace amounts of cesium appearing in the top portion of the antimony rich cap. The oxygen was detected in the outer layer of the cap, likely due to Li2O formed by reaction of deposited lithium metal and moisture from exposure to air during transfer of the specimen from the container into the SEM.

[0098] The measured elemental concentrations of oxygen, chlorine, potassium, antimony, cesium, and bismuth in each of these identified spots are shown in Table 3. Spot 1 is located in the salt and contains cesium and chlorine, indicating the material is salt. Spots 2 and 3 are within the two different regions of the antimony-rich cap. Spot 2 is in the lighter colored region showing measurable cesium concentration and oxygen content (indicative of Li2O). Spot 3 is the darker grey region in the cap and shows no cesium content in that phase, but a similar oxygen content. The composition of spot 4 is similar to that of spot 3 indicating it is also an antimony-rich phase with possible lithium deposition. Spots 5 and 6 are located deeper into the bulk of the Bi—Sb electrode and contain no detectable cesium and only trace amounts of chlorine, potassium and oxygen.TABLE 3Normalized concentration of elements (at %) in the Bi—Sb alloy electrode after the lithium depletion test, determined by EDS analysis at locations indicated in FIG. 11.LocationOClKSbCsBi138.532.621.60.37.00.0215.72.80.679.11.80.1317.82.42.677.00.00.2417.90.90.180.60.00.553.11.50.072.60.022.866.84.90.011.60.076.7

[0099] EDS X-ray maps of the region in the micrograph in FIG. 11 for the elements: bismuth, antimony, cesium, potassium, chlorine and oxygen were obtained. The cap was largely antimony with cesium and oxygen being found in an outer portion of the cap. No potassium or chlorine were found within the cap. The presence of cesium and oxygen (indicative of lithium) in the outer region of the cap indicates that deposition of these active metals is preferred into antimony rich phases rather than bismuth rich phases.

[0100] FIG. 12 shows a high magnification image of the Sb-rich region (A) and the same image with numbered locations of EDS spot analyses (B). The concentrations of oxygen, chlorine, potassium, antimony, cesium, and bismuth measured in each of the numbered EDS spot analysis areas are shown in Table 4. Spots 1 and 2 from the amorphous salt region adjacent to the left side of the antimony rich cap are confirmed to be salt by the high concentrations of chlorine, potassium, and cesium that are detected. Spots 3 and 5 in the lighter grey outer layer of the cap are primarily antimony with measurable cesium and oxygen contents, consistent with the EDS maps. Spot 4 in the darker grey inner region of the antimony-rich cap does not contain cesium or bismuth, but has a significant oxygen content, possibly indicating lithium is present in this area.TABLE 4Normalized concentrations of elements (at %) in the Bi—Sb alloy electrode after the lithium depletion test, determined by EDS analysis at locations indicated in FIG. 12LocationOClKSbCsBi147.635.21.40.315.40.0219.143.636.00.01.40.0317.05.50.775.90.70.3420.32.60.276.90.00.0517.05.10.276.80.90.0

[0101] Any 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.

[0102] The use of the terms “a” and “an” and “the” and similar referents in the context of describing materials or methods (especially in the context of the following claims) are 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. The terms “consisting of” and “consists of” are to be construed as closed terms, which limit any compositions or methods to the specified components or steps, respectively, that are listed in a given claim or portion of the specification. In addition, and because of its open nature, the term “comprising” broadly encompasses compositions and methods that “consist essentially of” or “consist of” specified components or steps, in addition to compositions and methods that include other components or steps beyond those listed in the given claim or portion of the specification. 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 numerical values obtained by measurement (e.g., weight, concentration, physical dimensions, removal rates, flow rates, and the like) are not to be construed as absolutely precise numbers, and should be considered to encompass values within the known limits of the measurement techniques commonly used in the art, regardless of whether or not the term “about” is explicitly stated. 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 certain aspects of the materials or methods described herein and does not pose a limitation on the scope of the claims unless otherwise stated. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the claims.

[0103] Preferred embodiments are described herein, including the best mode known to the inventors for carrying out the claimed 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 claimed invention to be practiced otherwise than as specifically described herein. Accordingly, the claimed 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 claimed invention unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

1. An electrochemical cell for electrochemically recovering a target metal from a molten salt electrolyte; the electrochemical cell comprising:a molten salt-permeable tubular-shaped first electrode;a liquid metal second electrode contained within a ceramic crucible situated within and surrounded by the tubular-shaped first electrode; anda counter electrode spaced from the first electrode;the first and second electrodes and the counter electrode being in contact with a molten salt electrolyte solution comprising ions of the target metal dissolved in a molten salt medium;wherein the first electrode is composed of a material capable of forming a first alloy with a metal of the molten salt medium; and the liquid metal of the second electrode comprises a material that is capable forming a second alloy with the target metal;wherein in use, the molten salt medium is maintained at a temperature above the melting point of the liquid metal of the second electrode; the first and second electrodes and the counter electrode are placed in circuit with an active DC power source to electrochemically reduce a metal ion from the molten salt medium to its metallic form at the first electrode thereby forming the first alloy and increasing the relative concentration of the target metal in the vicinity of the second electrode; and ions of the target metal are reduced at the second electrode to form the second alloy.

2. The electrochemical cell of claim 1, further comprising a reference electrode spaced from the first electrode and the counter electrode, to provide a reference point for measuring the potential of the electrodes in the electrochemical cell.

3. The electrochemical cell of claim 1, wherein the first electrode is composed of at least one material selected from the group consisting of aluminum, an aluminum alloy, copper, a copper alloy, nickel, and a nickel alloy4. The electrochemical cell of claim 1, wherein the first electrode is composed of aluminum or an aluminum alloy.

5. The electrochemical cell of claim 1, wherein the liquid metal of the second electrode comprises bismuth, antimony, or a combination thereof.

6. The electrochemical cell of claim 1, wherein the target metal comprises cesium, strontium, barium, or a combination of two or more thereof.

7. The electrochemical cell of claim 1, wherein the molten salt medium comprises a lithium salt, a potassium salt, or a combination thereof.

8. The electrochemical cell of claim 1, wherein the molten salt medium comprises lithium chloride, a potassium chloride, or a combination thereof.

9. The electrochemical cell of claim 1, wherein the molten salt medium is maintained at a temperature of about 200 °C to about 900 °C.

10. The electrochemical cell of claim 1, wherein the first electrode is composed of aluminum or an aluminum alloy; the molten salt medium comprises lithium chloride, potassium chloride, or a combination thereof; and the first alloy comprises a lithium aluminum alloy, a potassium aluminum alloy, or a lithium potassium aluminum alloy.

11. The electrochemical cell of claim 10, wherein the liquid metal of the second electrode comprises bismuth, antimony, or a combination thereof; the target metal comprises cesium, strontium, barium, or a combination of two or more thereof; and the second alloy comprises an alloy of cesium, strontium and / or barium with antimony, lead, tin, and / or bismuth.

12. The electrochemical cell of claim 1, wherein the liquid metal of the second electrode comprises bismuth, antimony, lead, tin, or a combination thereof; the target metal comprises cesium, strontium, barium, or a combination of two or more thereof; and the second alloy comprises an alloy of cesium, strontium and / or barium with antimony, lead, tin, and / or bismuth.

13. A method for recovering a target metal from a molten salt solution, the method comprising:(a) providing an electrochemical cell comprising the steps of:(i) a molten salt-permeable tubular-shaped first electrode;(ii) a liquid metal second electrode contained within a ceramic crucible situated within and surrounded by the tubular-shaped first electrode; and(iii) a counter electrode spaced from the first electrode;the first and second electrodes and the counter electrode being in contact with a molten salt electrolyte solution comprising ions of the target metal dissolved in a molten salt medium;wherein the first electrode is composed of a material capable of forming a first alloy with a metal of the molten salt medium; and the liquid metal of the second electrode comprises a material that is capable forming a second alloy with the target metal; and the cell is maintained at a temperature sufficient to keep the molten salt medium and the liquid metal of the second electrode in a liquid state;(b) placing the first electrode, the second electrodes and the counter electrode in circuit with an active DC power source to electrochemically reduce a metal ion from the molten salt medium to its metallic form at the first electrode thereby forming the first alloy and increasing the relative concentration of the target metal in the vicinity of the second electrode; and ions of the target metal are reduced at the second electrode to form the second alloy;(c) deactivating the DC power source;(d) removing the second electrode from the molten salt medium; and(e) recovering the target metal from the second alloy.

14. The method of claim 13, wherein the electrochemical cell further comprises a reference electrode spaced from the first electrode and the counter electrode, to provide a reference point for measuring the potential of the electrodes in the electrochemical cell.

15. The method of claim 13, wherein after step (d), the first electrode is left within the molten salt medium at the temperature sufficient to maintain the molten salt medium in a liquid state, so that the metal from the molten salt medium in the first alloy will redissolve in the molten salt medium.

16. The method of claim 13, wherein the first electrode is composed of aluminum or an aluminum alloy; the molten salt medium comprises lithium chloride, potassium chloride, or a combination thereof; and the first alloy comprises a lithium aluminum alloy, a potassium aluminum alloy, or a lithium potassium aluminum alloy.

17. The method of claim 13, wherein the liquid metal of the second electrode comprises bismuth, antimony, lead, tin, or a combination thereof; the target metal comprises cesium, strontium, barium, or a combination of two or more thereof; and the second alloy comprises an alloy of cesium, strontium and / or barium with antimony, lead, tin, and / or bismuth.

18. The method of claim 13, wherein the molten salt medium is maintained at a temperature of about 200 °C to about 900 °C.

19. The method of claim 13, wherein the DC power source provides a voltage of about 0 to about 10V.

20. The method of claim 13, wherein:the first electrode is composed of aluminum or an aluminum alloy; the molten salt medium comprises lithium chloride, potassium chloride, or a combination thereof; and the first alloy comprises a lithium aluminum alloy, a potassium aluminum alloy, or a lithium potassium aluminum alloy;the liquid metal of the second electrode comprises bismuth, antimony, or a combination thereof; the target metal comprises cesium, strontium, barium, or a combination of two or more thereof; and the second alloy comprises an alloy of cesium, strontium and / or barium with antimony and / or bismuth; andthe molten salt medium is maintained at a temperature of about 200 °C. to about 900 °C; and the DC power source provides a voltage of about 0 to about 10 V.

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