WO2015006331A1

WO2015006331A1 - Clean, efficient metal electrolysis via som anodes

Info

Publication number: WO2015006331A1
Application number: PCT/US2014/045762
Authority: WO
Inventors: Uday B. Pal; Salvador BARRIGA; Stephen Joseph DEREZINSKI; Matthew Earlam
Original assignee: POWELL, Adam, Clayton, IV
Priority date: 2013-07-08
Filing date: 2014-07-08
Publication date: 2015-01-15
Also published as: US20160376719A1

Abstract

In some aspects, the invention relates to apparatuses for recovering a metal comprising providing a sealed container for holding a molten electrolyte, the container having an interior surface; a liner disposed along at least a portion of the interior container surface; a cathode disposed to be in electrical contact with the molten electrolyte when the molten electrolyte is disposed in the container; a solid oxygen ion-conducting membrane disposed to be in ion-conducting contact with the electrolyte when the molten electrolyte is disposed in the container; an anode in contact with the solid oxygen ion-conducting membrane, the solid oxygen ion-conducting membrane electrically separating the anode from the molten electrolyte; and a power source for generating an electric potential between the anode and the cathode.

Description

CLEAN, EFFICIENT METAL ELECTROLYSIS VIA SOM ANODES

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. provisional patent application Ser. No. 61/843,798, filed July 8, 2013, entitled "Clean, Efficient Aluminum

Electrolysis via SOM Anodes", the disclosure of which is hereby incorporated by reference in its entirety for all purposes.

[0002] All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. The patent and scientific literature referred to herein establishes knowledge that is available to those skilled in the art. The issued patents, applications, and other publications that are cited herein are hereby incorporated by reference to the same extent as if each was specifically and individually indicated to be incorporated by reference. In the case of inconsistencies, the present disclosure will prevail.

GOVERNMENT SUPPORT

[0003] This invention was made with government support under Grant No. 1026639 awarded by the National Science Foundation and Award No. DE-AR0000412 awarded by the Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

[0004] The invention relates to apparatuses and methods for production of metals from metal oxides.

BACKGROUND OF THE INVENTION

[0005] The Hall-Heroult cell revolutionized aluminum production in 1886 (U.S. Patent No. 400,664; herein incorporated by reference in its entirety) by reducing aluminum oxide dissolved in a molten salt, with a consumable carbon anode that reacts with the oxygen to form carbon dioxide. However, the Hall-Heroult molten salt electrolysis remains limited in cost and energy efficiency for several reasons. Production of the required carbon anode is costly, as well as carbon- and energy-intensive. In addition, liquid cryolite is extremely corrosive, such that containment thereof in air requires a "frozen sidewall" of cryolite, which rapidly removes thermal energy from the cell. Moreover, emission of hot carbon dioxide from the anode contains HF, which renders heat recovery from exiting gases impractical, and perfluorocarbons, powerful greenhouse gases with global warming potential (GWP) of 6000-9000 times that of C0₂.

[0006] Use of a solid electrolyte, such as stabilized zirconia, between the molten salt and anode removes the anode requirement of chemical stability in contact with a molten salt. For reactive metals such as aluminum, magnesium, calcium, sodium, potassium and rare earth metals, the solid electrolyte improves current efficiency considerably by presenting a solid barrier between the metal produced at the cathode and oxidizing gases produced at the anode, preventing back-reaction (see, for example, U.S. Patent Nos. 5,976,345 and 6,299,742; each herein incorporated by reference in its entirety). The process comprises a solid oxygen ion- conducting membrane (SOM) typically consisting of zirconia stabilized by yttria (YSZ) or other low valence oxide-stabilized zirconia, for example, magnesia- or calcia-stabilized zirconia (MSZ or CSZ, respectively) in contact with the molten salt electrolyte bath in which the metal oxide is dissolved, an anode in ion-conducting contact with the solid oxygen ion-conducting membrane, and a power supply for establishing a potential between the cathode and anode.

[0007] Due to the SOM process, consumable carbon anodes are not required and volatile contamination of anode gases is mitigated. However, heat loss and crust formation remain an issue. Thus, there remains a need for more efficient methods and systems to produce metals. This invention addresses these needs.

BRIEF SUMMARY OF THE INVENTION

[0008] In one aspect, an apparatus for recovering a metal is provided comprising: (a) a sealed container for holding a molten electrolyte, the container having an interior surface; (b) a liner disposed along at least a portion of the interior container surface; (c) a cathode disposed to be in electrical contact with the molten electrolyte when the molten electrolyte is disposed in the container; (d) a solid oxygen ion-conducting membrane disposed to be in ion-conducting contact with the electrolyte when the molten electrolyte is disposed in the container; (e) an anode in contact with the solid oxygen ion-conducting membrane, the solid oxygen ion-conducting membrane electrically separating the anode from the molten electrolyte; and (f) a power source for generating an electric potential between the anode and the cathode.

[0009] In another aspect, a method for recovering a metal is provided comprising: (a) providing a sealed container for holding a molten electrolyte, the container having an interior surface; (b) providing a liner disposed along at least a portion of the interior container surface; (c) providing a cathode disposed to be in electrical contact with the molten electrolyte when the molten electrolyte is disposed in the container; (d) providing a solid oxygen ion-conducting membrane disposed to be in ion-conducting contact with the electrolyte when the molten electrolyte is disposed in the container; (e) providing an anode in contact with the solid oxygen ion-conducting membrane, the solid oxygen ion-conducting membrane electrically separating the anode from the molten electrolyte; (f) dissolving at least a portion of an oxide of the metal into the electrolyte; (g) establishing a non-oxidizing environment within the container; and (h) generating an electric potential between the anode and the cathode, whereby the oxide of the metal is reduced to form metal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The following figures are illustrative only and are not intended to be limiting.

[0011] Figure 1. An illustrative embodiment of an electrolytic cell configuration and method according to an embodiment of the invention.

[0012] Figure 2. An illustrative embodiment of an electrolytic cell configuration and method according to an embodiment of the invention.

[0013] Figure 3. An illustrative embodiment of an electrolytic cell configuration and method according to an embodiment of the invention.

[0014] Figure 4. An illustrative embodiment of an electrolytic cell configuration and method according to an embodiment of the invention.

DETAILED DESCRIPTION

[0015] As used herein and in the appended claims, the singular forms "a," "an," and "the" include plural references unless the content clearly dictates otherwise.

[0016] The term "about" is used herein to mean approximately, in the region of, roughly, or around. When the term "about" is used in conjunction with a numerical range, it modifies that range by extending the boundaries above and below the numerical values set forth. The term "about" is used herein to modify a numerical value above and below the stated value by a variance of 20%.

[0017] The Hall-Heroult process electrical energy efficiency is below 55% ("U.S.

Energy Requirements for Aluminum Production," U.S. Department of Energy, EERE Report, Feb. 2007; herein incorporated by reference in its entirety), and generates about 0.9 kg C0₂- equivalent (C0₂e) direct greenhouse gas (GHG) emissions per kg of aluminum produced, leaving much room for improvement. Its molten cryolite salt bath loses heat rapidly through the frozen cryolite cell "side-wall". HF in off-gases makes heat exchanger energy recovery impossible, let alone capture for by-product sale or sequestration. Moreover, production of carbon anodes adds considerable energy use and expense. The industry has spent the past 125 years trying new innovations.

[0018] The aluminum industry has tried various approaches to non-consumable inert anodes to avoid the high cost of carbon anodes. Approaches have included cermets, generally of doped nickel ferrites with base/noble metal alloys, transition metals which form conducting surface oxides, composites which combine these two, aluminum bronzes generally with copper, and porous graphite with fuel gas flow to the outer surface. See, e.g., US Patents 4,397,729; 4,960,494; 5,254,232; 5,865,980; 6,030,518; 6,113,758; 6,126,799; 6,217,739; 6,248,227;

6,284,562; 6,322,969; 6,372,009; 6,372,119; 6,416,649; 6,423,195; 6,423,204; 6,521,115;

6,758,991; 6,821,312; 6,878,247; 7,033,469; 7,235,161; 7,431,812; 7,507,322; and 8,366,891; and Sankar Namboothiri, Mark P. Taylor, John J. J. Chen, Margaret M. Hyland and Mark Cooksey, "Aluminum production options with a focus on the use of a hydrogen anode: a review," Asia-Pacific Journal of Chemical Engineering 2(5):442-447 (2007); each herein incorporated by reference in its entirety.

[0019] Aluminum trichloride electrolysis with non-consumable multipolar carbon electrodes as also been attempted. Producing anhydrous A1C1₃ without HC1 evolution, and operating a chlorine compressor, add considerably more cost than the savings in electrical energy and anode production. Aluminum trichloride is also acutely toxic, with health hazard 3 out of a maximum of 4 on the NFPA diamond scale (See, e.g.,

http://www.sciencelab.com/msds.php?msdsId=9922851, herein incorporated by reference in its entirety). This approach has never proven economically viable.

[0020] Low-temperature aluminum trichloride electrolysis with ionic liquids loses less energy than a molten salt cell; however anhydrous AICI₃ production cost and toxicity make this unworkable.

[0021] Drained cathodes such as slanted TiB₂ plate cathodes drain off liquid aluminum into wells as it is produced. This reduces anode-cathode distance (ACD) and resistance heat generation, as well as magneto-hydrodynamics (MHD) interactions of the high magnetic field with the otherwise thick liquid aluminum pad. The slant also improves productivity per unit area. However, the carbon anode, frozen sidewall, and anode gas issues remain.

[0022] Today's aluminum plants are modular, but must locate near low cost electric power sources such as dams. Carbothermic reduction plants may potentially have lower energy cost location flexibility to provide liquid metal at customer facilities. However, carbothermic reduction of aluminum requires very high temperature well above 2100 °C, leading to significant materials and energy management challenges.

[0023] With the exception of carbothermic reduction, all of the above can retrofit into existing plants. However, for the reasons mentioned above, none of these technologies is in widespread use.

[0024] Development of the solid oxide membrane (SOM) electrolysis process has provided an alternative method for refinement of metal oxides (see, e.g, U.S. Patent Nos.

5,976,345, and 6,299,742; each herein incorporated by reference in its entirety). The process as applied to metal production consists of a metal cathode, a molten salt electrolyte bath that dissolves the metal oxide that is in electrical contact with the cathode, a solid electrolyte oxygen ion conducting membrane (SOM) typically consisting of zirconia stabilized by yttria (YSZ) or other low valence oxide-stabilized zirconia, for example, magnesia- or calcia-stabilized zirconia (MSZ or CSZ, respectively) in ion-conducting contact with the molten salt bath, an anode in ion- conducting contact with the SOM, and a power source for establishing a potential between the cathode and anode. Metal cations are reduced to metal at the cathode, and oxygen ions migrate through the membrane to the anode where they are oxidized to produce oxygen gas or other oxides. The SOM blocks back-reaction between anode and cathode products. It also blocks ion cycling, which is the tendency for subvalent cations to be re-oxidized at the anode, by removing the electronic connection between the anode and the metal ion containing molten salt because the SOM conducts only oxide ions, not electrons (see, U.S. Patent Nos. 5,976,345, and

6,299,742; each herein incorporated by reference in its entirety); however the process runs at high temperatures, typically 1000 - 1300 °C in order to maintain high ionic conductivity of the SOM.

[0025] The SOM electrolysis can proceed with high- and low-density electrolytes, and in some embodiments combines several desired characteristics of metal oxide reduction technology including, for example, 1) low-cost, low-toxicity metal oxide feedstock to avoid costs of producing, dehydrating and handling toxic metal chlorides; 2) insulated molten salt containment to reduce heat loss from the frozen sidewall; 3) separation between anode gas and molten salt for reduced emission of HF contaminated gases and heat recovery from the voluminous anode gas; 4) low anode-cathode distance for low resistance and excess heat production; 5) scale-down ability to mini-mills and on-site sale of liquid metal to customers; 6) ability to retrofit existing metal smelters; 7) use of clean, inexpensive fuel (e.g., natural gas) with high energy content and half the CHG emissions of expensive carbon anodes, optionally with uncontaminated carbon dioxide capture for sequestration or sale as a by-product; and 8) optional oxygen-producing inert anode without contamination of the oxygen by-product by HF, fluorine or other toxic gases. Moreover, anodes that switch between fueled and inert operation can take advantage of fluctuating electricity costs due to intermittent renewable energy sources or low demand periods (such as overnight).

[0026] The zirconia solid electrolyte represents a fundamental departure from other molten salt electrolysis anodes and is advantageous over other molten salt electrolysis anodes for several reasons including, e.g., blocking back-reaction between dissolved metal in salt and oxygen or carbon dioxide anode product, presenting a solid barrier that separates the anode gases from the molten salt, prevents the anode effect (eliminating periodic outages and carbon tetrafluoride emissions), enables multiple inert anode materials such as perovskites and liquid metals, prevents carbon contamination of the product, and blocks other anions, resulting in high purity oxygen by-product or combustion oxidant.

[0027] In addition, advantages particular to primary production of metal, e.g., aluminum, include, e.g., eliminating carbon anode costs, clean emissions with no anode effect from SOM selectivity, anode flexibility and energy efficiency, reduction of thermal loss, simpler metal collection, simplified feeding and improved electrode geometry.

[0028] Eliminating Carbon Anode Costs. The consumption of carbon anodes in traditional methods is not only expensive, but also requires an additional production line located next to most metal smelter operations. Its operation involves handling carcinogenic materials that can contaminate ground water. Eliminating the anode plant will significantly reduce capital expenditures, operating cost, energy use and several types of emissions.

[0029] Clean Emissions and No Anode Effect from SOM Selectivity. SOM selectivity delivers anode product gas with no HF, perflurocarbons (no anode effect) and no fluorine or oxyfluorides, thus making it suitable for gas heat exchange and raw material preheating. Using this heat for partial aluminum hydroxide calcining (>180 - 200 °C; Hollingbery, L.A. and Hull, T Richard, "The Structure and Thermal Decomposition of Hydromagnesite and Huntite - A review," Thermochimica Acta 509: 1-11 (2010); herein incorporated by reference in its entirety) can reduce upstream energy use even further. Avoided costs include, e.g., no emissions control expense or energy use and/or lower permitting costs and delays.

[0030] Anode Flexibility and Energy Efficiency. The zirconia solid electrolyte tubes can accommodate either liquid silver or perovskite inert anodes that produce pure oxygen byproduct or fueled (e.g., natural gas) anodes emitting only water and carbon dioxide. Hot swapping anode assemblies are also contemplated, as is exchanging current collectors while leaving zirconia tubes in place to change between inert and fueled (e.g., natural gas) operation for GHG and cost savings.

[0031] Reducing Thermal Loss. The molten salt (cryolite) bath used in the Hall-Heroult cell requires a "frozen sidewall" of cryolite for containment since it dissolves all refractory oxides and accelerates oxidation of most metals. This frozen sidewall withdraws an enormous amount of thermal energy from the process, which increases energy consumption dramatically. With SOM technology, oxygen originates inside the SOM tube(s), and the bath produces little to no gas. Thus, a metal container can contain the salts without a frozen side wall, there is no oxidant (e.g., oxygen, carbon dioxide or water) outside of the tube(s) that would oxidize the metal container, and anode gas originates in the SOM and does not pick up corrosive volatiles (e.g., HF) so it is easy to seal and inert the container using very little inert gas, e.g., argon or nitrogen, preventing oxidation of a metal container. Such a configuration permits advantageous operation in a sealed, e.g., welded, steel vessel surrounded by thermal insulation, eliminating much of the thermal losses.

[0032] Improved Electrode Geometry. Vertical plate or rod cathodes with rows of zirconia anode assemblies between them result in low anode-cathode distance similar to drained cathodes, and provide much more electrode surface area per unit cross-section area for a given cell size. This both reduces energy losses and reduces building size for a given production volume.

[0033] Simpler Metal Collection. When the molten salt is denser than the metal, the metal floats on top of the cell. This enables tapping of the cell at a side collection well. The new cell geometry is without a bottom entry, making it less likely to leak.

[0034] No MHD-induced Short Circuiting. Separating metal from the electrolysis section removes the MHD effect of the large magnetic field on the liquid metal pad. For example, current flow between the SOM tube(s) and the cathode does not significantly involve the metal. In conventional Hall cells, all current is between the metal and the carbon anode above, resulting in interaction with the magnetic field and waves on the metal surface can short circuit to the carbon anode.

[0035] Simplified Feeding. Today's technology uses a frozen cryolite crust over the top to reduce heat loss. However, feeding metal oxide raw material, e.g. alumina, requires a complex system which breaks this crust, resulting in mechanical disruption and thermal energy loss. SOM technology will not have a frozen crust, so illustratively a simple and robust radiation-shielded augur system with counter-current inert gas flow can feed the metal oxide into the all-liquid salt bath in a well-insulated containment system.

[0036] In one aspect, an apparatus for recovering a metal is provided comprising: (a) a sealed container for holding a molten electrolyte, the container having an interior surface; (b) a liner disposed along at least a portion of the interior container surface; (c) a cathode disposed to be in electrical contact with the molten electrolyte when the molten electrolyte is disposed in the container; (d) a solid oxygen ion-conducting membrane disposed to be in ion-conducting contact with the electrolyte when the molten electrolyte is disposed in the container; (e) an anode in contact with the solid oxygen ion-conducting membrane, the solid oxygen ion-conducting membrane electrically separating the anode from the molten electrolyte; and (f) a power source for generating an electric potential between the anode and the cathode. Electrical separation of the anode and the molten electrolyte prevents direct electrical contact between the anode and the molten electrolyte.

[0037] In another aspect, a method for recovering a metal is provided comprising: (a) providing a sealed container for holding a molten electrolyte, the container having an interior surface; (b) providing a liner disposed along at least a portion of the interior container surface; (c) providing a cathode disposed to be in electrical contact with the molten electrolyte when the molten electrolyte is disposed in the container; (d) providing a solid oxygen ion-conducting membrane disposed to be in ion-conducting contact with the electrolyte when the molten electrolyte is disposed in the container; (e) providing an anode in contact with the solid oxygen ion-conducting membrane, the solid oxygen ion-conducting membrane electrically separating the anode from the molten electrolyte; (f) dissolving at least a portion of an oxide of the metal into the electrolyte; (g) establishing a non-oxidizing environment within the container; and (h) generating an electric potential between the anode and the cathode, whereby the oxide of the metal is reduced to form metal. Electrical separation of the anode and the molten electrolyte prevents direct electrical contact between the anode and the molten electrolyte.

[0038] In some embodiments, the container comprises steel.

[0039] In some embodiments, the container is electrically isolated from the cathode.

[0040] In some embodiments, the interior surface includes a floor and the floor comprises carbon.

[0041] In some embodiments, the liner extends from the floor upward along the interior surface of the container. In some embodiments, the liner comprises boron nitride, S1₃N₄, fused alumina or zirconia. [0042] In some embodiments, during generation of the electric potential, the metal is recovered from an oxide of the metal dissolved in the molten electrolyte, the metal collects on the floor of the container, and the liner extends to a level that prevents contact between the metal-molten electrolyte interface and the interior surface of the container.

[0043] In some embodiments, during generation of the electric potential, the metal is recovered from an oxide of the metal dissolved in the molten electrolyte and the metal collects on a top surface of the molten electrolyte when the electrolyte is disposed in the container, the liner extends from a first level below the metal-molten electrolyte interface to a second level above the metal-molten electrolyte interface, and the liner prevents contact between the metal- molten electrolyte interface and the interior surface of the container.

[0044] In some embodiments, a side wall of the container and the liner define a passage between the interior of the container and a well external to the container.

[0045] In some embodiments, a partition is disposed inside the container, the partition extending from a third level below the metal-molten electrolyte interface to a fourth level above the top surface of the metal, the third level being above a bottom surface of the container, and the partition preventing recovered metal from collecting on top of a portion of the molten electrolyte.

[0046] In some embodiments, a sheath is disposed around at least a portion of the solid oxygen ion-conducting membrane, the sheath extends from a third level below the metal-molten electrolyte interface to a fourth level above the top surface of the metal, and the sheath prevents contact between the metal being recovered and the solid oxygen ion-conducting membrane. In some embodiments, the sheath comprises boron nitride, S1₃N₄, fused alumina or zirconia.

[0047] In some embodiments, the solid oxygen ion-conducting membrane and electrical sheath define an annular space between the membrane and sheath. In some embodiments, a gas inlet in communication with the annular space is provided.

[0048] In some embodiments, the container is not electrically isolated from the cathode.

[0049] In some embodiments, the interior surface includes a floor and the floor comprises carbon.

[0050] In some embodiments, the liner extends from the floor upward along the interior surface of the container to a level that prevents contact between molten electrolyte and the interior surface of the container. [0051] In some embodiments, a non-oxidizing environment is established. In some embodiments, an inert gas is added to the container. In some embodiments, the inert gas is dry- scrubbed with alumina after exiting the container.

[0052] In some embodiments, at least a portion of the metal floats on the molten electrolyte.

[0053] In some embodiments, a tapping well is provided on the sealed container.

[0054] In some embodiments, a sheath is disposed around the at least a portion of the oxide ion-conducting membrane. In some embodiments, an inert gas enters the sheath-oxide ion- conducting membrane annulus. In some embodiments, the inert gas comprises a noble gas. In some embodiments, the inert gas comprises argon or nitrogen. In some embodiments, the inert gas comprises argon. In some embodiments, the inert gas comprises nitrogen.

[0055] In some embodiments, at least a portion of the container is thermally insulated.

[0056] A schematic embodiment is shown in Figure 1. An anode (100) is shown in contact with a solid oxygen ion-conducting electrolyte (105) and a current collector (110) that conducts electrons. The molten salt (115) is contained within an enclosed container (120) that is thermally insulated (125) and further contains one or a plurality of cathodes (130). Metal oxide, e.g., alumina, is fed into the container (135) and reduced to liquid aluminum metal (140), which settles toward the bottom of the container. Oxygen ions migrate from the molten salt through the solid electrolyte to the liquid metal anode, where they form dissolved oxygen atoms. The oxygen atoms diffuse through the liquid metal anode to the gas phase where they form oxygen gas which flows away from the anode (See, e.g., U.S. Patent No. 8,658,007; herein incorporated by reference in its entirety). A second type of anode (145) in contact with a second solid oxygen ion-conducting electrolyte (150) and fuel (e.g., natural gas) inlet (155) can be provided in addition to or in place of anode (100). The second anode (145) enables use of less electrical energy and generation of water and carbon dioxide (See, e.g., U.S. Patent Publication No.

2013/0186769; herein incorporated by reference in its entirety). An inlet (160) and outlet (165) for an inert gas such as argon is also provided.

[0057] Another schematic embodiment is shown in Figure 2. An anode (200) is shown in contact with a solid oxygen ion-conducting electrolyte (205) and a current collector (210) that conducts electrons. The molten salt (215) is contained within a sealed container (220) that is thermally insulated (225) and further contains one or a plurality of cathodes (230). Metal oxide, e.g., alumina, is fed into the container (235) and reduced to liquid aluminum metal (240), which is separated from the alumina feed via a partition (280) and removed via a tap (270) on the side of the container. Oxygen ions migrate from the molten salt through the solid electrolyte to the liquid metal anode, where they form dissolved oxygen atoms. The oxygen atoms diffuse through the liquid metal anode to the gas phase where they form oxygen gas which flows away from the anode. A second type of anode (245) in contact with a second solid oxygen ion-conducting electrolyte (250) and fuel (e.g., natural gas) inlet (255) can be provided in addition to or in place of anode (200). The second anode (245) enables use of less electrical energy and generation of water and carbon dioxide. An inlet (260) and outlet (265) for an inert gas such as argon is also provided. The inlet (260) is disposed between the annulus of the SOM (205) and an electrically insulating sheath (275).

[0058] Another embodiment of a sealed (e.g. welded) steel cell (320) is shown in Figure 3. Sealing the vessel creates a controlled environment which can be made inert by injecting argon or helium, nitrogen, or other gas (360). This prevents rapid steel corrosion by molten salt catalysis of steel oxidation. It also prevents molten salt catalysis of TiB₂ corrosion, enabling that cathode (330) material to protrude upward out of the salt (315). An anode (300) is shown in contact with a solid oxygen ion-conducting electrolyte (305) and a current collector (310) that conducts electrons. The molten salt (315) is contained within an enclosed container (320) that is thermally insulated (325) and further contains a plurality of cathodes (330). Metal oxide, e.g., alumina, is fed into the container (335) and reduced to liquid aluminum metal (340), which settles toward the bottom of the container. Oxygen ions migrate from the molten salt through the solid electrolyte to the liquid metal anode, where they form dissolved oxygen atoms. The oxygen atoms diffuse through the liquid metal anode to the gas phase where they form oxygen gas which flows away from the anode. A second type of anode (345) in contact with a second solid oxygen ion-conducting electrolyte (350) and fuel (e.g., natural gas) inlet (355) can be provided in addition to or in place of anode (300). The second anode (345) enables use of less electrical energy and generation of water and carbon dioxide. An inlet (360) and outlet (365) for an inert gas such as argon is also provided.

[0059] In some embodiments, the injection site for the inert gas is submerged in the bath, in order to create gas lift stirring and promote oxide circulation. The cathode material is advantageously compatible with liquid metal and molten salt and has high electronic

conductivity. In some embodiments, the cathode comprises TiB₂. To reduce the cost of TiB₂ for cathodes, closed-end TiB₂ tubes can be used, and filled with a high-conductivity liquid metal such as aluminum, copper, tin or bismuth (See, e.g., U.S. Patent No. 4,612,103; herein incorporated by reference in its entirety). [0060] The liquid aluminum product (340), which can act as a cathode, rests on a low- cost carbon floor (385) to prevent it from contacting the steel (320). At the aluminum-salt bath interface level, a liner material (390) prevents aluminum reaction with the steel vessel, and also salt catalysis of aluminum carbide (A1₄C₃) formation. Because the aluminum-salt bath interface level rises and falls with production and withdrawal of liquid aluminum, the liner extends vertically over the full extent of the rising and falling of the interface. In some embodiments, it is advantageous that the liner extend over the entire vertical interior surface of the vessel.

[0061] The liner can be one of several materials stable in contact with steel, not soluble in liquid metal, and either not soluble or slow-dissolving in the molten salt bath. Exemplary materials include stabilized zirconia, such as yttria-stabilized zirconia similar to that used in the zirconia tubes, or low-cost calcia-stabilized zirconia; oxide materials such as alumina, e.g. fuse- cast alumina; other compounds such as boron nitride, TiB₂, SiC, and/or Si₃N₄.

[0062] The liner materials can be applied by several methods including: insertion of plates, tiles, or bricks; or thermal spray to create an adhering layer of material on the steel and carbon. The liners need not create a perfect seal between the steel vessel and liquid metal or molten salt bath. The inert environment makes the steel vessel stable in the molten salt, and the liner need only slow down metal-steel interaction kinetics sufficiently to prevent liquid metal from breaking out of the steel vessel, and to prevent steel from contaminating the liquid metal beyond the product composition specification.

[0063] The steel vessel creates opportunities for thermal manipulation. For example one can apply heating (including fuel-fired heaters) directly to the steel exterior during start-up or accidental cooling due to power failure. If the cell gets too hot, removal of insulation or blowing cooling fluid through insulation can help to cool it. This can potentially reduce or eliminate thermal runaway problems during a voltage/temperature increase event caused by metal oxide (e.g., alumina) depletion.

[0064] Traditional cells such as Hall-Heroult cells do not have this advantage. Cell heat loss through the side walls is fixed by the need to maintain precise crust geometry. If too much heat is extracted, the crust grows, freezing the bath from the sides and reducing current until the cell shuts down. If too little heat is extracted, the crust shrinks, leading to liquid bath-metal interface contact with carbon lining and rapid dissolution of the carbon. In the instant invention, the cell can operate over a much wider range of temperature and heat flux without

compromising containment or hindering cell operation. [0065] In addition, in some embodiments the apparatus and method provides one or more of the following benefits: high productivity per unit cross-section area due to the high surface area of vertical cathodes and zirconia-encased anodes; insulation of steel to reduce vessel thermal loss well below that of a conventional cell; opportunity for direct external heating or cooling of the steel vessel; simplified feeding due to lack of frozen salt crust; in situ dry scrubbing of the inert gas (illustratively argon, but could also be other gases) by counter-current flow of argon out through the metal oxide feeder.

[0066] Another embodiment of a sealed cell (420), high-density salt bath is shown in Figure 4, with the electrolysis cell configuration with a high-density molten salt (415) bath, i.e. denser than liquid metal (e.g., aluminum) (440) at the cell operating temperature. It also uses a liner (490) from the same candidate material list and application method to prevent aluminum- steel contact and to prevent molten salt catalysis of aluminum carbide (A1₄C₃) dissolution. This configuration also uses a gas to create an inert or reducing environment and prevent corrosion of the steel vessel in the molten salt. In some embodiments, the injection site for the inert or reducing gas is submerged in the bath, in order to create gas lift stirring and promote oxide circulation. An anode (400) is shown disposed in contact with a solid oxygen ion-conducting electrolyte (405) and a current collector (410) that conducts electrons. The molten salt (415) is contained within a sealed container (420) that is thermally insulated (425) and further contains a plurality of cathodes (430). Metal oxide, e.g., alumina, is fed into the container (435) and reduced to liquid aluminum metal (440), which is separated from the alumina feed via a partition (480) and removed via a tap (470) on the side of the container. Oxygen ions migrate from the molten salt through the solid electrolyte to the liquid metal anode, where they form dissolved oxygen atoms. The oxygen atoms diffuse through the liquid metal anode to the gas phase where they form oxygen gas which flows away from the anode. A second type of anode (445) in contact with a second solid oxygen ion-conducting electrolyte (450) and fuel (e.g., natural gas) inlet (455) can be provided in addition to or in place of anode (400). The second anode (445) enables use of less electrical energy generation of water and carbon dioxide. An inlet (460) and outlet (465) for an inert gas such as argon is also provided. The inlet (460) is disposed between the annulus of the SOM (405) and an electrically insulating sheath (475).

[0067] If the floating liquid metal is connected to the cathodes, then this configuration requires sheathing around zirconia tubes to separate the liquid metal (e.g., aluminum) from the tubes, in order to prevent short-circuiting and damage to the zirconia. In some embodiments, inert or reducing gas bleeding into the sheath-zirconia annulus can prevent molten electrolyte or metal from entering that annulus. Thus, the annular region allows for a non-electrical insulator to be used as the sheath. That sheath can be made of one of the materials in the liner list, though an electronic insulator such as zirconia, alumina or boron nitride is preferred for preventing shortcircuiting rather than a conductor such as TiB₂. That sheath can be a separate tubular part surrounding the zirconia tube, or it can be a coating attached to the tube. In some embodiments, insulating sheaths are placed around the SOM and inert gas inlet into the zirconia-sheath annulus prevents short-circuiting in the floating metal configuration. The sheath and inert gas block electrical connection between the cathode and the zirconia, and reduction of the zirconia. The sheath can be boron nitride, silicon nitride, zirconia with low ionic conductivity. The sheath can also be ion-conducting zirconia or electron-conducting metal; however if the sheath is conducting, it is advantageous that the SOM does not contact the sheath. Thus, non-conducting spacers can be inserted between the sheath and the SOM.

[0068] In some embodiments, this configuration uses a dam, tube, or similar partition to create an opening through the floating liquid metal and permit direct feeding of metal oxide into the molten salt bath. This constraint material preferably has minimal reaction with the metal and molten salt, such that zirconia, TiB₂, boron nitride, or similar materials will have long lifetime in this function.

[0069] In some embodiments, larger metal oxide (e.g., alumina) pellets are fed into the liquid metal (e.g., aluminum), when the metal oxide has higher density than liquid metal, such that as long as surface tension does not support the metal oxide, it will sink through the metal into the salt and dissolve therein.

[0070] Floating metal configurations enable simplified tapping using a separate chamber, or seal pot, holding liquid metal, which is lined with carbon, boron nitride, TiB₂ or similar material not soluble in the metal. An opening in the vessel can allow liquid metal to flow out of the vessel and into the seal pot.

[0071] In some embodiments, the floating metal configuration adds greater flexibility in metal oxide feed morphology: pellets of sintered metal oxide fines which fall to the bottom slowly dissolve in the molten salt, where in the other configurations or the conventional Hall- Heroult cell they would sink through the liquid metal and accumulate as sludge; less

contamination of metal product if a zirconia tube breaks and releases the liquid metal anode, which will likely have higher density than the salt bath as the main candidates are silver, copper and tin; and opportunity for simplified tapping using the liquid metal seal pot with top tapping.

[0072] A sealed container, e.g. steel, can be used to produce metal and is enabled by creation of a non-oxidizing environment such as a reducing or inert environment. The sealed container prevents ingress of air and/or gas that would react with the liquid metal or corrode the interior of the container. The non-oxidizing environment prevents or minimizes oxidation of the steel container catalyzed by the molten electrolyte. The non-oxidizing environment exists because oxygen containing components (oxygen gas, carbon dioxide or water vapor) are positioned on the inside of the zirconia tube, and the applied electrical potential drives oxygen into the zirconia tube from the molten salt outside of it. Such configurations enable insulation to reduce thermal energy loss and/or allow the process to run at high salt superheat (above its melting point), which avoids the complication of crust management in conventional systems. Thus, feeding metal oxide is simplified because there is no crust to break in order to mix the metal oxide into the molten electrolyte. Embodiments where liquid metal is disposed on top of the molten electrolyte salt are also enabled by the non-oxidizing environment, as the non- oxidizing environment prevents oxidation of the floating metal.

[0073] A non-oxidizing environment can be established by any number of ways including, e.g., providing an inert gas and, optionally, electrochemically pumping oxygen by establishing an anode-cathode potential, or by bubbling inert gas below the molten electrolyte and/or the liquid metal. Bubbling inert gas below the motel electrolyte and/or liquid metal advantageously provides mixing to assist the metal oxide reach the oxide-ion conducting membrane.

[0074] The inert gas may comprise a noble gas, e.g., argon, or nitrogen. Other inert gases include SF₆ and CF₄. In some embodiments, the inert gas comprises a noble gas or nitrogen. In some embodiments, the inert gas comprises argon or nitrogen. In some

embodiments, the inert gas comprises argon. In some embodiments, the inert gas comprises nitrogen.

[0075] Dry scrubbing the inert gas is also an optional process. Thus, any HF present (e.g., in the inert gas or otherwise) can be absorbed by the metal oxide, e.g., alumina, and converted to metal fluoride (e.g., aluminum trifluoride) and water. In some embodiments, the inert gas exit bleed (265) is controlled and carries HF to the metal oxide (235) to form metal fluoride. By feeding the resulting metal fluoride back into the molten electrolyte, such embodiments preserve fluoride contents in the system. Metal oxide particles known to avoid sludge problems can also be used for the scrubbing process.

[0076] In some embodiments wherein metal is disposed on top of the molten electrolyte salt, a side tapping well is added on the container. The well enables removal of metal from the apparatus. In some embodiments wherein metal is disposed on top of the molten electrolyte salt, a partition or dam is provided to separate the metal from a portion of the top surface of the molten salt electrolyte, such that the metal oxide can be fed directly into the salt.

[0077] The metal produced is not limited to aluminum. In some embodiments, the metal is any metal that has a density less than the molten salt electrolyte at the operating temperature of the cell. Thus, in some embodiments, the metal will float on top of the electrolyte. In some embodiments, the metal comprises aluminum, magnesium, lithium, beryllium, silicon, sodium, potassium or calcium. In some embodiments, the metal comprises aluminum, lithium, beryllium, silicon, sodium, or potassium. In some embodiments, the metal comprises aluminum, lithium, beryllium, sodium, or potassium. In some embodiments, the metal comprises aluminum, lithium, sodium, or potassium. In some embodiments, the metal comprises aluminum. It is understood that the metal oxide comprises an oxide of the metal to be produced.

[0078] In some embodiments employing a fueled anode, the fuel comprises methane, syngas, hydrogen, or other hydrocarbons. In some embodiments, the fuel tube comprises a conductive metal, such as nickel, molybdenum or cobalt, and can be attached to, and form part of, the current collector. In some embodiments, the fuel delivery tube is a stable oxide, such as aluminum oxide, mullite, or magnesium oxide, such that it is stable in both oxygen and fuel gas, and the device can operate in either oxygen production or fueled modes depending on the flow rate of fuel.

[0079] In some embodiments, the solid electrolyte comprises zirconia doped with yttria, calcia, magnesia, scandia, other rare earth oxide, or other additives that stabilize its cubic phase and enhance its conductivity; or ceria doped with oxides to increase its ion, e.g oxygen, conductivity; or any other oxygen ion-conducting solid electrolyte. In some embodiments, the solid electrolyte comprises zirconia doped with yttria, calcia, magnesia, scandia, or other rare earth oxide; or ceria doped with oxides to increase its oxygen ion conductivity. In some embodiments, the solid electrolyte comprises zirconia doped with yttria, calcia, magnesia, scandia, or other rare earth oxide. In some embodiments, the solid electrolyte comprises zirconia doped with yttria, calcia, magnesia, or scandia. In some embodiments, the solid electrolyte comprises ceria doped with oxides.

[0080] The molten electrolyte composition may be comprised of several components. Preferred molten electrolyte systems are selected based on several criteria:

[0081] Cation oxide free energy. All salt cation species ideally have oxide free energies of formation more negative than that of the metal, such that the process does not reduce spectator cations along with the product. Representative cation species include magnesium, sodium, cerium, lanthanum, calcium, strontium, barium, lithium, potassium and ytterbium. Thus, in some embodiments, the molten electrolyte comprises cations of magnesium, sodium, cerium, lanthanum, calcium, strontium, barium, lithium, potassium or ytterbium. In some embodiments, the molten electrolyte comprises cations of magnesium, sodium, cerium, and lanthanum.

[0082] Low volatility. Preferably the salt exhibits very low vapor pressure and evaporation rate in the process temperature range. Combining thermo-gravimetric analysis (TGA) with differential scanning calorimetry (DSC) or differential thermal analysis (DTA) experiments can efficiently evaluate the salt for this criterion. In some embodiments, fluoride salts are preferable over chlorides, and the volatility of lithium fluoride makes it less attractive.

[0083] Low melting point. A preferred range of about 900-1200° C provides balance between energy efficiency and material flexibility at low temperature, and good oxide ion conductivity in stabilized zirconia at high temperature. The salt must be liquid at the operating temperature.

[0084] High ionic conductivity supports high current density without significant transport limitation. By way of example, a salt with low ionic conductivity inhibits mass transfer to the zirconia and the cathode; at the zirconia oxygen ions are depleted in the boundary layer, reducing the current, and at the cathode the target metal ions are depleted in the boundary layer, leading to reduction and co-deposition of salt cations, reducing purity.

[0085] Low viscosity. Viscosity inhibits mass transport to electrodes. Salts with high fluoride/oxide ratio have had sufficiently high ionic conductivity and low viscosity to support up to about 2 A/cm² anode and cathode current density.

[0086] Target oxide solubility. The salt must dissolve the metal oxide to at least about 3- 5 wt % in order to achieve preferred ionic current densities at the anode and cathode. DSC or DTA experiments at various compositions can efficiently characterize oxide solubility.

[0087] Low metal solubility and electronic conductivity. If the salt is a good electronic conductor, then it effectively becomes an extended cathode and can reduce the zirconia. This is impacted in part by solubility of the reduced metal in the salt, e.g. calcium metal is soluble in many salts. Methods for reducing metal solubility inline are also contemplated (See, e.g., U.S. Patent Publication No. 2013/0152734; herein incorporated by reference in its entirety).

[0088] Zirconia stability. Salt corrosion of the zirconia solid electrolyte is preferably very slow. Ideally, the salt preferably satisfies two criteria: salt optical basicity and yttria (or other stabilizing oxide) chemical potential are both close to those values in the zirconia. To evaluate stability, zirconia is immersed in the salt at the process temperature for several hundred hours, then sectioned and characterized.

[0089] Fluoride salts are particularly preferred as they offer advantageous combinations of properties due to their low volatility and viscosity, high oxide solubility and ionic

conductivity, and low basicity.

[0090] Preferred molten salts comprise LiF, NaF, CaF₂, MgF₂, SrF₂, and A1F₃. These have melting points around 700 °C with relatively low vapor pressure. The optimal molten salt will have maximum metal oxide solubility and minimum metal solubility.

[0091] In some embodiments, a liner is disposed between the container and the metal. Exemplary materials for the liner include carbon, boron nitride, titanium diboride, SiC, Si₃N₄, fused alumina or zirconia.

[0092] In some embodiments, a second liner is disposed along at least a portion of an interior surface of the container. In some embodiments, liner prevents contact between the container and the molten salt electrolyte. The liner and/or insulating sheath may be applied, e.g., as a pre-fabricated sheath or sheath components, including tiles or bricks, or via thermal spraying onto the desired surface such as plasma spraying.

[0093] The anode may be an oxygen-generating inert anode, or a fueled anode. In some embodiments, the fuel comprises natural gas. In some embodiments, liquid silver anodes are preferred. Porous perovskite conductors of solid oxide fuel cell (SOFC) technology, such as La 0.8 Sr 0.2 Mn0₃ (LSM), are potential candidates for this role. Other potential anode materials comprise antimony, bismuth, copper, gallium, indium, and tin.

[0094] It will be recognized that one or more features of any embodiments disclosed herein may be combined and/or rearranged within the scope of the invention to produce further embodiments that are also within the scope of the invention.

[0095] Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are also intended to be within the scope of the present invention.

EXAMPLES

[0096] The examples provided below facilitate a more complete understanding of the invention. The following examples illustrate exemplary modes of making and practicing the invention. However, the scope of the invention is not limited to specific embodiments disclosed in such examples, which are illustrative only, since alternative methods can be utilized to obtain similar results.

[0097] A salt comprising aluminum fluoride and aluminum oxide was placed into a stainless steel crucible and heated in an argon environment to 105° C, then 400° C, and finally to 1000° C. While at temperature, the salt was stirred using a stainless steel mixer to ensure homogeneity and expedite the dissolution of aluminum oxide. The crucible and salt were then cooled and removed from the furnace.

[0098] After cooling, a stainless steel block containing two boron nitride cups (one to insulate the anode and another to collect aluminum metal) was placed into the crucible on top of the salt block in the crucible. A threaded rod was inserted into the block in order to facilitate the raising and lowering of the block. A YSZ SOM, zirconium diboride (ZrB₂) cathode, platinum reference electrode, and a thermocouple were also inserted into the crucible. A silver anode and an oxygen-stable current collector (following the design in U.S. Patent Application 13/600,761 publication 2013/0192998; herein incorporated by reference in its entirety) were inserted inside the YSZ SOM. Both the cathode and reference electrode were attached to stainless steel rods with boron nitride sheaths in between in order to insulate them at the salt/argon interface.

[0099] The cage was placed in the furnace and heated to 970° C. After the salt was molten, the stainless steel block was lowered to the bottom of the crucible. The electrodes were lowered simultaneously; the anode into one boron nitride cup in the stainless steel block, the cathode to right above the other boron nitride cup, and the reference electrode to just under the salt surface. The current collector was lowered into the liquid silver anode.

[00100] At this point AC impedance measurements were taken between the electrodes, and chronoamperaometry measurements were made at several voltages below the reduction potential of aluminum. Following these measurements electrolysis was started at 5 volts for 203 minutes producing 2.78 liters of oxygen gas at the anode.

[00101] Upon cooling to room temperature, the aluminum metal product was recovered from the top surface of the salt attached to the boron nitride sheath on the ZrB₂ cathode.

[00102] This experiment demonstrated the ability of the non-oxidizing

environment to allow production of aluminum on the top surface of the salt, without oxidizing as it would have in the presence of air or other oxidizing gas. [00103] As will be apparent to one of ordinary skill in the art from a reading of this disclosure, further embodiments of the present invention can be presented in forms other than those specifically disclosed above. The particular embodiments described above are, therefore, to be considered as illustrative and not restrictive. Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific embodiments described herein. Although the invention has been described and illustrated in the foregoing illustrative embodiments, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of implementation of the invention can be made without departing from the spirit and scope of the invention, which is limited only by the claims that follow. Features of the disclosed embodiments can be combined and rearranged in various ways within the scope and spirit of the invention. The scope of the invention is as set forth in the appended claims and equivalents thereof, rather than being limited to the examples contained in the foregoing description.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for recovering a metal, comprising:

(a) a sealed container for holding a molten electrolyte, the container having an

interior surface;

(b) a liner disposed along at least a portion of the interior container surface;

(c) a cathode disposed to be in electrical contact with the molten electrolyte when the molten electrolyte is disposed in the container;

(d) a solid oxygen ion-conducting membrane disposed to be in ion-conducting

contact with the electrolyte when the molten electrolyte is disposed in the container;

(e) an anode in contact with the solid oxygen ion-conducting membrane, the solid oxygen ion-conducting membrane electrically separating the anode from the molten electrolyte; and

(f) a power source for generating an electric potential between the anode and the cathode.

2. The apparatus of claim 1, wherein the container comprises steel.

3. The apparatus of claim 1, wherein the container is electrically isolated from the cathode.

4. The apparatus of claim 1, wherein the interior surface of the container includes a floor and the floor comprises carbon.

5. The apparatus of claim 4, wherein the liner extends from the floor upward along the interior surface of the container.

6. The apparatus of claim 5, wherein, during generation of the electric potential, the metal is recovered from an oxide of the metal dissolved in the molten electrolyte and the metal collects on the floor of the container and wherein the liner extends to a level that prevents contact between the metal being recovered and the interior surface of the container.

7. The apparatus of any of claims 1-6, wherein the liner comprises carbon, boron nitride, titanium diboride, SiC, S1₃N₄, fused alumina or zirconia.

8. The apparatus of claim 1, wherein, during generation of the electric potential, the metal is recovered from an oxide of the metal dissolved in the molten electrolyte and the metal collects on a top surface of the molten electrolyte when the electrolyte is disposed in the container and wherein the liner extends from a first level below the metal-molten electrolyte interface to a second level above the metal-molten electrolyte interface, and the liner prevents contact between the metal-molten electrolyte interface and the interior surface of the container.

9. The apparatus of claim 8, wherein a side wall of the container and the liner define a

passage between the interior of the container and a well external to the container.

10. The apparatus of claim 9, further comprising a partition disposed inside the container, the partition extending from a third level below the metal-molten electrolyte interface to a fourth level above the top surface of the metal, the third level being above a bottom surface of the container, and the partition preventing recovered metal from collecting on top of a portion of the molten electrolyte.

11. The apparatus of any of claims 8-10, further comprising a sheath disposed around at least a portion of the solid oxygen ion-conducting membrane, the sheath extending from a third level below the metal-molten electrolyte interface to a fourth level above the top surface of the metal, and the sheath preventing contact between the metal being recovered and the solid oxygen ion-conducting membrane.

12. The apparatus of claim 11, wherein the sheath comprises boron nitride, S1₃N₄, fused alumina or zirconia.

13. The apparatus of claim 11, the solid oxygen ion-conducting membrane and sheath

defining an annular space between the membrane and sheath, the apparatus further comprising a gas inlet in communication with the annular space.

14. The apparatus of claim 11 , wherein the container is not electrically isolated from the cathode.

15 The apparatus of claim 14, wherein the interior surface includes a floor and the floor comprises carbon.

16. The apparatus of claim 15, wherein the liner extends from the floor upward along the interior surface of the container to a level that prevents contact between molten electrolyte and the interior surface of the container.

17. The apparatus of claim 16, wherein the liner comprises boron nitride, S1₃N₄, fused

alumina or zirconia.

18. A method for recovering a metal, comprising:

(a) providing a sealed container for holding a molten electrolyte, the container

having an interior surface;

(b) providing a liner disposed along at least a portion of the interior container

surface;

(c) providing a cathode disposed to be in electrical contact with the molten

electrolyte when the molten electrolyte is disposed in the container;

(d) providing a solid oxygen ion-conducting membrane disposed to be in ion- conducting contact with the electrolyte when the molten electrolyte is disposed in the container;

(e) providing an anode in contact with the solid oxygen ion-conducting membrane, the solid oxygen ion-conducting membrane electrically separating the anode from the molten electrolyte;

(f) dissolving at least a portion of an oxide of the metal into the electrolyte;

(g) establishing a non-oxidizing environment within the container; and

(h) generating an electric potential between the anode and the cathode, whereby the oxide of the metal is reduced to form metal.

19. The method of claim 18, wherein the container comprises steel.

20. The method of claim 18, wherein the container is electrically isolated from the cathode.

21 The method of claim 18, wherein the interior surface includes a floor and the floor comprises carbon.

22. The method of claim 21, wherein the liner extends from the floor upward along the

interior surface of the container.

23. The method of claim 22, wherein, during generation of the electric potential, the metal is recovered from metal oxide dissolved in the molten electrolyte and the metal collects on the floor of the container and wherein the liner extends to a level that prevents contact between the metal being recovered and the interior surface of the container.

24. The method of any of claims 18-23, wherein the liner comprises carbon, boron nitride, titanium diboride, SiC, S1₃N₄, fused alumina or zirconia.

25. The method of claim 18, wherein, during generation of the electric potential, the metal is recovered from a metal oxide dissolved in the molten electrolyte and the metal collects on a top surface of the molten electrolyte when the electrolyte is disposed in the container and wherein the liner extends from a first level below the metal-molten electrolyte interface to a second level above the metal-molten electrolyte interface, and the liner prevents contact between the metal-molten electrolyte interface and the interior surface of the container.

26. The method of claim 25, wherein a side wall of the container and the liner define a

27. The method of claim 26, further comprising providing a partition disposed inside the container, the partition extending from a third level below the metal-molten electrolyte interface to a fourth level above the top surface of the metal, the third level being above a bottom surface of the container, and the partition preventing recovered metal from collecting on top of a portion of the molten electrolyte.

28. The method of any of claims 25-27, further comprising providing a sheath disposed

around at least a portion of the solid oxygen ion-conducting membrane, the sheath extending from a third level below the metal-molten electrolyte interface to a fourth level above the top surface of the metal, and the sheath preventing contact between the metal being recovered and the solid oxygen ion-conducting membrane.

29 The method of claim 28, wherein the sheath comprises boron nitride, Si₃N₄, fused alumina or zirconia.

30. The method of claim 28, the solid oxygen ion-conducting membrane and sheath defining an annular space between the membrane and sheath, the method further comprising providing a gas inlet in communication with the annular space.

31. The method of claim 18, wherein the container is not electrically isolated from the

cathode.

32. The method of claim 31 , wherein the interior surface includes a floor and the floor

comprises carbon.

33. The method of claim 33, wherein the liner extends from the floor upward along the interior surface of the container to a level that prevents contact between molten electrolyte and the interior surface of the container.

34. The method of claim 34, wherein the liner comprises boron nitride, Si₃N₄, fused alumina or zirconia.

35. The method of claim 18, wherein the metal oxide is fed directly into the molten

electrolyte.

36. The method of claim 18, wherein the metal comprises aluminum, magnesium, lithium, beryllium, silicon, sodium, potassium or calcium.

37. The method of claim 36, wherein the metal comprises aluminum, lithium, beryllium, silicon, sodium, or potassium.

38. The method of claim 37, wherein the metal comprises aluminum.