US20060102491A1

US20060102491A1 - Processes for separating metals from metal salts

Info

Publication number: US20060102491A1
Application number: US11/269,811
Authority: US
Inventors: Michael Kelly; Jason Brady
Original assignee: Millennium Cell Inc
Current assignee: Millennium Cell Inc
Priority date: 2004-11-10
Filing date: 2005-11-09
Publication date: 2006-05-18
Also published as: WO2006062673A3; WO2006062673A2

Abstract

Electrochemical processes and apparatus for obtaining metals from metal salts, particularly separating alkali metal and borate ions from alkali metal borate compounds, are disclosed. Aqueous solutions of metal borates or metal carbonates are converted to metals by preferred electrochemical processes. These electrochemical processes also may be integrated into processes for the production of borohydrides, such as sodium borohydride.

Description

This application claims the benefit of U.S. Provisional Application Ser. No. 60/626,485, filed on Nov. 10, 2004, the entire disclosure of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was made with Government support under Cooperative Agreement No. DE-FC36-04G014008 awarded by the Department of Energy. The United States Government has certain rights in this invention.

FIELD OF THE INVENTION

The invention is directed to electrochemical reduction of metal compounds with applications in elemental metal and metal borohydride production.

BACKGROUND OF THE INVENTION

Sodium borohydride is a very versatile chemical and is used in organic synthesis, waste water treatment, and pulp and paper bleaching. The high hydrogen content of this compound also makes it a good candidate for being a hydrogen carrier, and it could play a major role as an enabler of a hydrogen economy if the cost of producing this chemical can be reduced.
Several processes exist for making sodium borohydride, all of which depend on some form of sodium borate to supply the boron. Traditionally, the source of boron is the mineral borax. During the Schlesinger process, which is currently used to supply commercial sodium borohydride, the sodium and boron contents of the mineral must be separated. This is achieved by reaction with an acid, producing boric acid and the sodium salt of the acid. This process generates large quantities of the sodium salt, typically a valueless by-product. The sodium needed to make sodium borohydride is reintroduced from another source, so the process makes no use of the sodium content of the sodium borate mineral.
In the manufacture of sodium borohydride, metallic sodium or sodium hydride is used as a starting material. The largest single consumer of sodium metal in the United States is the process for making sodium borohydride. Essentially all sodium in the marketplace is obtained from energy inefficient electrolysis processes, such as electrolysis of sodium chloride. As a result, the market price of sodium is high and this raises the cost of raw materials for making sodium borohydride. Therefore, it is desirable to achieve more efficient processes for making sodium.
U.S. Pat. No. 3,734,842, U.S. Pat. No. 4,904,357, and U.S. Pat. No. 4,931,154, the disclosures of which are hereby incorporated by reference herein in their entirety, disclose electrochemical synthesis of sodium borohydride from aqueous sodium metaborate solution. Such processes involve conversion of sodium metaborate and water to form sodium borohydride and oxygen in an electrical cell, as shown in the following half-cell reactions:
Cathode: B(OH)₄ ⁻+4H₂O+8e⁻→BH₄ ⁻+8OH⁻ (1a)
Anode: 8OH⁻⁻→4H₂O+2O₂+8e (1b)
However, none of these processes has been implemented in commercial practice.

SUMMARY OF THE INVENTION

The present invention is directed to electrochemical processes and apparatus for obtaining a metal from a metal salt.
In accordance with one aspect of the present invention, aqueous solutions of metal borates are converted to elemental metal and borate species by an electrochemical process.
In accordance with another aspect of the invention, a metal carbonate is converted to elemental metal by an electrochemical process.
In accordance with another aspect of the invention, aqueous solutions of alkali metal borates and alcohols are converted to alkali metal and trialkylborates by an electrochemical process.
In yet another aspect of the invention, product from a borohydride-based hydrogen generator comprising an aqueous solution of alkali metal borates and alkali metal hydroxides is converted to an alkali metal and borate species by an electrochemical process according to the present invention.
In another aspect of the invention, the invention provides a process for the production of sodium borohydride by: (i) electrolyzing an aqueous solution of sodium borates to produce sodium metal and boric acid; (ii) reacting the sodium metal with hydrogen to produce sodium hydride; (iii) converting the boric acid into trimethylborate; and (iv) reacting the sodium hydride and trimethylborate to produce sodium borohydride.
Another aspect of the present invention provides a process for the production of sodium borohydride by: (i) electrolyzing an aqueous solution of sodium borates and an alcohol to produce sodium metal and trialkylborate; (ii) reacting the sodium metal with hydrogen to produce sodium hydride; and (iii) reacting the sodium hydride and trialkylborate to produce sodium borohydride.
In another aspect the present invention provides a process for the production of sodium borohydride comprising the steps of: (i) electrolyzing an aqueous solution of sodium borates and alcohols to produce sodium metal and trialkylborate; (ii) reacting the sodium metal with hydrogen to produce sodium hydride; (iii) reacting the sodium hydride and trialkylborate to produce borohydride and sodium alkoxide, and hydrolyzing the sodium alkoxide to sodium hydroxide and methanol; (iv) optionally recycling the alcohol in the process; and (v) electrolyzing the sodium hydroxide to sodium metal which is combined with the sodium metal generated by the electrochemical process of the present invention, and used to make sodium borohydride.
These and other features and advantages of the invention will become apparent from the following detailed description that is provided in connection with the accompanying drawings and illustrated exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an exemplary electrolytic cell for synthesis of an alkali metal from an alkali metal salt.
FIG. 2 is a schematic view of an exemplary electrolytic cell in which hydrogen-containing gas is passed into the anode compartment for synthesis of an alkali metal from an alkali metal salt.
FIG. 3 is a flow diagram of a prior art process for producing sodium borohydride.
FIG. 4 is a flow diagram of a process for producing sodium borohydride according to the present invention wherein a sodium borate compound is converted to boric acid.
FIG. 5 is a flow diagram of a process for producing sodium borohydride according to the present invention wherein a sodium borate compound is converted to trimethylborate.
FIG. 6 is a flow diagram of a process for producing sodium borohydride according to the present invention, wherein a sodium borate compound is converted to trimethylborate and the sodium methoxide byproduct is converted to sodium metal and methanol.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to electrochemical processes for obtaining metals from metal salts, and particularly alkali metals and alkaline earth metals from alkali metal and alkaline earth metal salts. Alkali metals are the Group I metals, and preferably are lithium, sodium, and potassium; alkaline earth metals are the Group II metals, and are preferably calcium and magnesium. The electrochemical process for obtaining an elemental metal may further be combined with additional electrochemical and/or chemical steps to produce metal borohydride compounds.
FIG. 1 illustrates an exemplary two-compartment cell 100 suitable for the transformation of metal salts to metals. Cell 100 comprises a cathode compartment 104, a cathode 102, an anode compartment 110, an anode 108, and a membrane 106 that separates the anode and cathode compartments. The anode and cathode may be typical electrodes in electrical communication. A pool of molten metal or metal alloy, particularly molten alkali or alkaline earth metal or alkali or alkaline earth metal alloy, in electrical contact with the cathode may act as an auxiliary cathode.
The anodic and cathodic compartments are separated by an ion-conducting membrane, which is permeable to metal ions but is not permeable to water and water vapor. Suitable membrane materials include, for example, ceramics such as lithium-β-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, lithium analogs of NaSICON ceramics, LISICONs, and lithium ion conductors with perovskite structure, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, NaSICON ceramics, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, potassium-β/β″-aluminum oxide, and potassium analogs of NaSICON ceramics, KSICONs, among others.
In accordance with one embodiment of the present invention, an aqueous solution of a metal borate salt represented by the formula zM_nO.xB₂O₃.yH₂O, wherein z is ½ to 5; x is 0.1 to 5; y is 0 to 10; and M is an alkali metal ion such as sodium, potassium, or lithium wherein n=2, or an alkaline earth metal such as calcium or magnesium wherein n=1, and preferably is sodium, can be converted into a boron compound and elemental metal through electrolysis. The overall reaction is shown in Equation (2), where M is selected from the group of alkali metals; the borate salt: zM_nO.xB₂O₃.yH₂O is shown as MBO₂in Equation (2):
4MBO₂+6H₂O→O4M+4B(OH)₃+O₂ (2)
This process is carried out by supplying an aqueous solution of a borate salt to the anode compartment of an electrolytic cell, for instance, such as that illustrated by FIG. 1. An alkali or alkaline earth metal or alloy of alkali metals or alkaline earth metals is supplied to the cathode compartment, preferably in a molten state. The reaction may be carried out at room temperature or at a temperature higher than room temperature, so that the metal in the cathode compartment is in a liquid or molten state. For sodium, preferably the temperature in the cathode compartment is from about 95° C. to about 150° C.
The anode compartment need not be heated and may be maintained at ambient temperatures, or the anode compartment may be maintained at the same elevated temperature as the cathode for ease and convenience. If the anode compartment is heated to temperatures greater than the boiling point of the water, the cell may be pressurized or the solvent in the anolyte may be refluxed and condensed. The applied voltage may be about 1.4 volts, or greater than about 1.4 volts, preferably greater than about 3.25 volts. Water or hydroxide in the anode compartment is oxidized in accordance with Equations (3a) and (3b).
2H₂O→4H⁺+O₂+4e⁻ (3a)
4OH⁻→2H₂O+O₂+4e⁻ (3b)
Additional reactions may occur in the anolyte solution in the anode compartment. The hydroxide ion concentration decreases and the proton concentration increases (and hence solution pH decreases) as protons are generated in accordance with Equation (3a) that can react with hydroxide to make water, or hydroxide ions generated in accordance with Equation (3b) are converted to oxygen and water. In solution, with lowering pH and increasing H⁺ concentration, borate ions may react with protons and water to form boric acid according to reaction (4).
H⁺+BO₂ ⁻+H₂O→B(OH)₃ (4)
Depending on conditions including, but not limited to, current and total charge passed, different borate species may result in the anode compartment. The borate species formed is referred to herein as an “enriched boron species” represented by formula z′M_nO.x′B₂O₃.y′H₂O, wherein z′ is 0 to 5; x is 0.1 to 5; y is O to 10; n=1 or 2; and M is an alkali metal ion or an alkaline earth metal, and z′ of the product enriched boron species is less than z of the starting metal borate salt and/or x′ of the product enriched boron species is greater than x of the starting metal borate salt. Thus, enriched borate species such as borax, Na₂B₄O₇wherein z′=1 and x′=2, may be formed in the anode compartment.
Metal ions are transported from the anolyte solution through the membrane where they are reduced at the cathode to the metal as shown in Equation (5).
2M⁺+2e⁻→2M (5)
The overall process provided in Equation (2) can be summarized by the following individual reactions:
Anode Reaction: 2H2O→4H⁺+O₂+4e⁻ (3a)
4OH⁻→2H₂O+O₂+4e⁻ (3b)
Anolyte Reaction: H⁺+BO₂ ⁻+H₂O→B(OH)₃ (4)
Cathode Reaction: 2M++2e⁻→2M (5)
In Equation (4), which takes place in the anode compartment, a strong electrolyte—the borate salt, MBO₂—is being converted into a weak electrolyte, B(OH)₃. Optionally, a supporting electrolyte may be included in the anode compartment to maintain ionic conductivity as boric acid is generated. Desirable supporting electrolytes are water soluble salts, wherein the cation portion is either the same as the metal cation undergoing reduction at the cathode or is a cation that will not be transported through the membrane. The anion portion of the supporting electrolyte should be relatively difficult to oxidize at the anode as compared to water, hydroxide anions, or hydrogen. The anion portion should also have low chemical reactivity with boric acid. One skilled in the art may readily select appropriate electrolyte systems given the teachings herein. Suitable anions for electrolytes include, but are not limited to, sulfate, nitrate, perchlorate, and phosphate. For example, suitable supporting electrolytes for an NaBO₂system include, but are not limited to, sodium sulfate, Na₂SO₄, sodium nitrate, NaNO₃, sodium phosphate, Na₃PO₄, or sodium perchlorate, NaClO₄, wherein the cation portion (Na⁺) is the same as the cation being reduced (e.g., Na⁺ to sodium metal).
Alternatively, an aqueous solution of a metal borate salt can be converted into a boron compound and elemental metal through electrolysis in the presence of hydrogen gas in the anode compartment to enable hydrogen-assisted electrolysis as disclosed in U.S. patent application Ser. No. 10/388,197 entitled “Hydrogen-Assisted Electrolysis Process,” the disclosure of which is incorporated by reference in its entirety, and which has been shown to reduce cell voltage and improve electrolytic efficiency through the preferential electrochemical oxidization of H₂at the anode. The overall reaction for hydrogen-assisted electrolysis is shown in Equation (6), where M is selected from the group of alkali metals:
MBO₂+H₂O+½H₂→M+B(OH)₃ (6)
FIG. 2 illustrates an exemplary two-compartment cell 200 suitable for the hydrogen-assisted electrolysis. In FIG. 2, structures that are the same as shown in FIG. 1 have like numbering. The cell 200 comprises a cathode compartment 104, cathode 102, anode compartment 110, anode 108, membrane 106 that separates the anode and cathode compartments, and a gas inlet means 202 to supply hydrogen gas to the anode compartment. The anode and cathode may be any suitable electrodes. A pool of molten alkali or alkaline earth metal or alkali metal alloy or alkaline earth alloy in electrical contact with the cathode acts as an auxiliary cathode.
The process is carried out by supplying an aqueous solution of a borate salt represented by the formula zM_nO.xB₂O₃.yH₂O to the anode compartment. Hydrogen or a hydrogen containing gas is supplied from an external source to the anode where it is oxidized in accordance with Equation (7).
½H₂→H⁺+e⁻ (7)
The anode compartment may include an optional gas inlet means for supplying a gas stream comprising hydrogen. Non-limiting examples of gas inlet means include pipes, spargers, hoses, and hydrogen gas diffusion materials.
As previously described, with lowering pH and increasing H⁺ concentration, borate ions may react with protons and water to form an enriched boron species such as boric add according to Equation (4), and metal ions are transported from the anolyte solution through the ion-conducting membrane where they are reduced at the cathode to the metal as shown in Equation (5). The ion-conducting membrane may include, for example, ceramics such as lithium-β-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, lithium analogs of NaSICON ceramics, LISICONs, and lithium ion conductors with perovskite structure, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, NaSICON ceramics, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, potassium-β/β″-aluminum oxide, and potassium analogs of NaSICON ceramics, KSICONs.
Preferably, the cathode compartment is maintained at temperatures so that the metal is in a liquid molten state. For sodium, the cathode compartment is preferably maintained at temperatures from about 95° C. to about 150° C. A supporting electrolyte, as discussed above, may be included in the anode compartment to maintain ionic conductivity as the enriched boron species is generated.
In accordance with another embodiment of the present invention, an aqueous solution of a metal borate salt can be converted into a boron compound and elemental metal through electrolysis as described above with an applied voltage from about 1.4 volts or greater, preferably greater than about 3.25 volts. When an alcohol is present in the anolyte solution, the enriched boron species produced, preferably boric acid, undergoes further reaction at temperatures from about 25° C. to about 300° C. to form a trialkyl borate, as shown in Equation (10), wherein R is a straight- or branched-chain or cyclic alkyl group containing from 1 to 6, preferably from 1 to 4, carbon atoms. The reaction can be driven to completion by continuously removing the product by various means such as by distillation.
B(OH)₃+3ROH→B(OR)₃+3H₂O (10)
When the alcohol is added to the anode compartment with the aqueous metal borate solution, the overall reaction is given by Equation (11), where R is a straight- or branched-chain or cyclic alkyl group containing from 1 to 6, preferably from 1 to 4, carbon atoms, and M is chosen from the group of alkali metals:
4MBO₂+12ROH→4M+4B(OR)₃+6H₂O+O₂ (11)
The overall process provided in Equation (2) can be summarized by the following individual reactions:
Anode Reactions: 2H₂O→4H⁺+O₂+4e⁻ (3a)
4OH⁻→2H₂O+O₂+4e⁻ (3b)
Anolyte Reactions: H⁺+BO₂ ⁻+H₂O→B(OH)₃ (4)
B(OH)₃+3ROH→B(OR)₃+3H₂O (10)
Cathode Reaction: 2M++2e⁻→2M (5)
Preferred alcohols are methanol, wherein R is CH₃—, and n-butanol, where R is CH₃CH₂CH₂CH₂—.
The anode compartment may include an optional gas inlet means for supplying a gas stream comprising hydrogen to allow hydrogen assisted electrolysis. Examples of suitable gas inlet means include pipes, spargers, hoses, and hydrogen gas diffusion materials, among others.
When hydrogen is fed to the anode for hydrogen-assisted electrolysis, the overall process illustrated in Equation (12) occurs with an applied voltage from about 1.4 volts or greater, preferably greater than about 2.25 volts.:
2MBO₂+H₂+6ROH→2M+2B(OR)₃+4H₂O (12)
In the processes depicted in Equations (11) and (12), the reduction is carried out by supplying an aqueous solution of a borate salt represented by the formula zM_nO.xB₂O₃.yH₂O, wherein z is ½ to 5; x is 0.1 to 5; y is 0 to 10; and M is an alkali metal ion such as sodium, potassium, or lithium, and preferably sodium, to the anode compartment. Hydrogen or a hydrogen containing gas may be supplied to the anode. Oxidation of water or hydroxide in the anode compartment occurs in accordance with Equations (3a) and (3b). If hydrogen is present in the anode compartment, hydrogen is oxidized at a lower voltage than either water or hydroxide, and is oxidized preferentially in accordance with Equation (7). With lowering pH and increasing H⁺ concentration, borate ions may react with protons and water to form an enriched boron species such as boric acid according to Equation (4).
The anodic and cathodic compartments are separated by an ion-conducting membrane that is permeable to metal ions but is not permeable to water, water vapor, alcohol, and alcohol vapor. Such membranes include, for example, ceramics such as lithium-β-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, lithium analogs of NaSICON ceramics, LISICONs, and lithium ion conductors with perovskite structure, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, NaSICON ceramics, potassium-5-aluminum oxide, potassium-β″-aluminum oxide, potassium-β/β″-aluminum oxide, and potassium analogs of NaSICON ceramics, KSICONs.
The metal ions are transported from the anolyte solution through the membrane where they are reduced at the cathode to the metal, as shown in Equation (5). Preferably, the cathode compartment is maintained at temperatures so that the metal is at least partly in a liquid molten state. For sodium, the cathode compartment is preferably at temperatures from about 95° C. to about 150° C. At temperatures greater than the boiling point of alcohol or water in the anode compartment (e.g., methanol boils at about 65° C.), the cell may be pressurized to elevate the boiling point. In some cases, the alcohol, especially methanol, may form an azeotrope with the trialkylborate, and the trialkylborate-methanol azeotrope may be distilled from the cell as it forms.
In Equation (10), which occurs in the anode compartment, the weak electrolyte B(OH)₃is being removed from the electrochemical cell. A supporting electrolyte may be included in the anode compartment to maintain ionic conductivity as boric acid is generated. Desirable supporting electrolytes comprise water or water/alcohol soluble salts, wherein the cation portion is either the same as the metal cation undergoing reduction at the cathode or is a cation that will not be transported through the ceramic membrane. Preferably, the anion portion of the supporting electrolyte is relatively difficult to oxidize at the anode as compared to water, hydroxide anions, or hydrogen. Preferably, the anion portion should also have low chemical reactivity with boric acid according to the teachings herein. For example, suitable supporting electrolytes for the NaBO₂system include, but are not limited to sodium sulfate, Na₂SO₄, or sodium nitrate, NaNO₃.
In another embodiment of the present invention, the aqueous solution of a borate salt is prepared from the product from a hydrogen generation apparatus, such as one used to supply a hydrogen fuel cell and as described in U.S. Pat. No. 6,534,033, entitled “A System for Hydrogen Generation,” and which comprises an aqueous solution of alkali metal hydroxide and alkali metal borate, represented by the formula zM_nO.xB₂O₃.yH₂O, wherein z is ½ to 5; x is 0.1 to 5; and y is 0 to 10. Preferably, the alkali metal ion in both the alkali metal hydroxide and alkali metal borate is sodium, although other alkali metal ions, such as potassium, may be utilized. The alkali metals of the alkali metal hydroxide and alkali metal borate need not be the same. Typically, the fuel solution that is introduced into a hydrogen generator comprises from about 15% to 100% by wt. sodium borohydride to about 0 to 15% by weight sodium hydroxide as a stabilizer. The product comprises sodium metaborate and sodium hydroxide in a molar ratio corresponding to the fuel concentration, but the percent by weight of sodium metaborate is from about 27% to 100% by weight as a result of the higher molecular weight thereof in comparison to sodium borohydride, and the reduced amount of water present. The borate product may be a solution, a heterogeneous mixture, a solid, or a slurry depending on the concentration of the ingredients and the temperature. The term “about” as used herein refers to ±10% of the stated value.
The borate product is introduced into the anode compartment of electrolytic cell 100, and a voltage from about 1.4 volts or greater, preferably greater than about 3.25 volts, is applied. Hydrogen or a hydrogen containing gas may be supplied to the anode. Oxidation of water or hydroxide in the anode compartment occurs in accordance with Equations (3a) and (3b). If hydrogen is present in the anode compartment, hydrogen is oxidized at a lower voltage than either water or hydroxide, and is oxidized preferentially in accordance with Equation (7). With lowering pH and increasing H⁺ concentration, borate ions react with protons and water to form boric add according to Equation (4).
Sodium ions are transported from the anolyte solution through the membrane to be reduced at the cathode to sodium metal as shown in Equation (5). Preferably, the cathode compartment is maintained at temperatures from about 95° C. to about 150° C., to maintain sodium in the liquid molten state. Boric add is produced in the anode compartment according to Equation (4). Trialkyl borate, preferably trimethyl borate, can be generated from the boric acid produced in the anode compartment if an alcohol, preferably methanol, is added to the anode compartment, in accordance with Equation (10).
As the electrolytes NaBO₂and B(OH)₃are removed from the electrochemical cell as B(OR)₃, it may be desirable to further include a supporting electrolyte in the anode compartment to maintain ionic conductivity as boric acid is generated. One skilled in the art can readily select an appropriate electrolyte system given the teachings herein and using, for example, the parameters provided for the other embodiments herein. Suitable supporting electrolytes for the NaBO₂system include, but are not limited to, sodium sulfate, Na₂SO₄, or sodium nitrate, NaNO₃.
In another embodiment, an aqueous solution of an alkali metal carbonate salt represented by formula M₂CO₃is converted to an alkali metal. This reduction is carried out by supplying an aqueous solution of a carbonate salt to the anode compartment of an electrolytic cell such as the one illustrated by FIG. 1. An alkali metal or alloy of alkali metals is supplied to the cathode compartment, preferably in a molten state. The reaction is carried out at about room temperature or at a temperature higher than room temperature so that the metal in the cathode compartment is in a liquid or molten state. For sodium, preferably the temperature in the cathode compartment is from about 95° C. to about 150° C.
The anode compartment need not be heated and may be maintained at ambient temperatures, or the anode compartment may be maintained at the same elevated temperature as the cathode for ease and convenience. If the anode compartment is heated to temperatures greater than the boiling point of water, the cell may be pressurized or the solvent in the anolyte may be refluxed and condensed. The applied voltage may be about 3.1 volts or greater, preferably greater than about 3.5 volts. Water or hydroxide in the anode compartment is oxidized in accordance with Equations (3a) and (3b).
Alkali metal ions are transported from the anolyte solution through the membrane where they are reduced at the cathode to the metal as shown in Equation 5.
Hydrogen may be present in the anode compartment to enable hydrogen-assisted electrolysis which will reduce cell voltage and improve electrolytic efficiency through the preferential electrochemical oxidization of H₂at the anode rather than the oxidation of hydroxide or water as shown in Equations (3a) and (3b). For a hydrogen assisted process, a solution of an alkali metal carbonate salt may be supplied to the anode compartment of a cell such as that illustrated in FIG. 2, with application of a voltage of about 2 volts or greater.
In another embodiment of the present invention, the electrochemical process of the invention is incorporated into a process for producing sodium borohydride. Today, sodium borohydride is commercially produced by the so-called Schlesinger process, which is a multi-step synthetic process, as illustrated in FIG. 3. The general steps include production of hydrogen by steam methane reforming at Step 302; electrolysis of sodium chloride to produce sodium metal in Step 304; preparation of sodium hydride by reaction of sodium and hydrogen in Step 306; refining of borax to generate boric acid in Step 308; conversion of boric acid to trimethylborate in Step 310; and reaction of sodium hydride and trimethylborate to produce sodium borohydride in Step 312. The electrochemical process of the present invention can provide a much more efficient process.
The raw material input for Step 308 in the Schlesinger process is generally the mineral borax, which is refined to produce boric acid through treatment with sulfuric acid. Though this conversion proceeds with good yield, it results in a substantial quantity of sodium sulfate as a byproduct. In essence, the Schlesinger process separates the boron and sodium values present in borax and discards the sodium from the borax. In order to make sodium borohydride, sodium must be re-introduced into the manufacturing process.
Commercially, sodium metal is prepared by the electrolysis of sodium chloride despite the fact that more energy is required to electrolyze sodium chloride than a number of other sodium salts. Much of the energy inefficiency and related cost of the Schlesinger process derives from this means of manufacturing sodium metal. Supplementing Step 304 from the current sodium borohydride method with the more efficient process for making sodium described in the present application provides significant improvements in both energy utilization and cost.
Reference is now made to FIG. 4, which illustrates an improved process for the production of sodium borohydride utilizing the electrochemical process of the present invention, which utilizes an aqueous solution of a borate salt, which may illustratively be prepared from the product from a hydrogen generation reaction but may comprise any borate salt according to the teachings herein. The process steps in FIG. 4 and FIG. 3 have like numbering. In Step 402, the aqueous solution of alkali metal borate, preferably a solution comprising sodium metaborate, and more preferably a product solution comprising sodium metaborate and sodium hydroxide, is introduced to the anode compartment and subjected to the electrochemical process of the present invention in an electrochemical cell comprising anode and cathode compartments separated by an ion-conducting membrane. A supporting electrolyte to enhance ionic conductivity in the anolyte may be included. The supporting electrolyte is typically not consumed in the process.
Aqueous sodium ions are transported from the anode compartment through the membrane to the cathode chamber and reduced to sodium metal at the cathode. In the anode compartment, the borate solution is acidified to form boric acid. Boric acid can be converted to trimethyl borate through reaction with methanol in a separate reactor, as shown in Step 310. Alternatively, an alcohol, preferably methanol, can be introduced to the anode compartment along with the aqueous solution of alkali metal borate in Step 402 to form a trialkyl borate in situ, which can be removed from the anode compartment through distillation. This would eliminate Step 310 and this process is illustrated in FIG. 5.
In Step 306, the sodium metal produced in the cathode compartment is withdrawn for reaction with hydrogen gas to form sodium hydride. This step may be the same as in the conventional Schlesinger process. Sodium borohydride is produced from the reaction of sodium hydride and trimethyl borate in Step 312. Additional sodium metal or sodium hydride can be introduced into the manufacturing process as shown in optional step 304 in FIG. 4.
Whether trialkyl borate is prepared in situ or not, the process may be carried out with hydrogen gas supplied to the anode as shown in optional Step 402 a in FIGS. 4 and 5. When hydrogen is supplied to the anode, the oxidation occurring at the anode is represented by Equation (7). With no hydrogen, the anode product will be oxygen instead of water as shown in Equations (3a) and (3b). In both cases, the rate of boric acid production as a function of current passed remains the same. Additional sodium metal or sodium hydride can be introduced into the manufacturing process, if required to convert all of the boric acid species to a borohydride compound. Such sodium may be obtained from the conventional electrolysis of sodium chloride as shown in optional step 304 or from the hydrogen-assisted processes disclosed in U.S. patent application Ser. No. 10/388,197 entitled “Hydrogen-Assisted Electrolysis Process,” the disclosure of which is incorporated by reference herein.
Steps may be further incorporated in the production of sodium borohydride to improve process efficiency. The sodium methoxide byproduct of Step 312 can be converted to sodium hydroxide and methanol by hydrolysis and separated as in Step 602. The methanol is recycled to the anode compartment to be incorporated in the process. The sodium hydroxide can be converted into sodium metal in Step 604 using, for example, the hydrogen-assisted processes disclosed in U.S. patent application Ser. No. 10/388,197 entitled “Hydrogen-Assisted Electrolysis Process,” and further shown in Equation (13).
3NaOH+1.5H₂→+3Na+3H₂O (13)
Alternatively, sodium can be generated from sodium hydroxide without employing hydrogen gas at the anode. In this case, the overall reaction for making sodium from sodium hydroxide is as shown in Equation (14):
4NaOH→4Na+2H₂O+O₂ (14)
The following examples further describe and demonstrate features of the present invention. The examples are given solely for illustration purposes and are not to be construed as a limitation of the present invention.

EXAMPLE 1

A reaction flask was charged with about 200 g of 5.5 weight percent NaBO₂aqueous solution. A tube with a NaSICON bottom was inserted into the solution. The tube contained 1.03 gram of sodium metal. Embedded into the sodium metal was a nickel wire. Collectively, the sodium metal and the nickel wire comprised the cathode. The tube bottom comprised the membrane or separator. The volume inside the tube defined the cathode compartment and the volume outside the tube, but inside the reaction flask, comprised the anode compartment. The aqueous metaborate solution comprised the anolyte. The anode itself was a nickel wire wrapped around a nickel plate, the combination of the wire and plate together comprising the anode.
The reaction flask was heated to about 115° C. and pressurized to about 10 psi. Under these conditions, the sodium in the cathode compartment was molten. A potential of about 5 V was applied across the anode and the cathode. After 606 mAh of current passed through the cell, the cell was cooled to room temperature and the amount of sodium metal that was generated was measured. The total amount of sodium was 1.35 grams with approximately 0.32 grams of sodium metal generated by the electrolysis. This represented a current efficiency of 61%, wherein 61% of the electrons passing through the cell resulted in conversion of sodium ions from the sodium metaborate solution into sodium metal. A ¹¹B NMR spectrum of the aqueous anolyte solution after the electrolysis confirmed the loss of sodium from the anolyte. The loss of sodium was determined by the magnitude of shift of the metaborate peak away from the metaborate form to the boric add form. The loss of sodium as determined by ¹¹B NMR indicates a current efficiency of 55%, in expected agreement with the determination from measuring the sodium yield.

EXAMPLE 2

A reaction flask was charged with about 150 g of NaBO₂.4 H₂O (sodium metaborate tetrahydrate). A Na-β″-alumina tube was inserted into the solution. The tube contained 0.99 gram of sodium metal. Embedded into the sodium metal was a nickel wire. Collectively, the sodium metal and the nickel wire comprised the cathode. The tube bottom comprised the membrane or separator. The volume inside the tube was the cathode compartment and the volume outside the tube, but inside the reaction flask, comprised the anode compartment. The sodium metaborate comprised the anolyte. The anode itself was a nickel wire wrapped around a nickel plate, the wire and plate together comprising the anode.
The reaction flask was heated to about 135° C. and pressurized to about 10 psi. Under these conditions, the sodium in the cathode compartment was molten, and the sodium metaborate tetrahydrate in the anode compartment was molten. A potential of about 5 V was applied across the anode and the cathode. After 1000 mAh of current passed through the cell, the cell was cooled to room temperature and the amount of sodium metal generated measured. The total amount of sodium was 1.83 grams with approximately 0.84 grams of sodium metal was generated by the electrolysis. This represented a current efficiency of 99%, wherein 99% of the electrons passing through the cell resulted in conversion of sodium ions from the sodium metaborate solution into sodium metal.

EXAMPLE 3

A reaction flask was charged with about 200 g of 9.1 weight-% Na₂CO₃aqueous solution. A tube with a NaSICON bottom was inserted into the solution. The tube contained about 1 gram of sodium metal. Embedded into the sodium metal was a nickel wire. Collectively, the sodium metal and the nickel wire comprised the cathode. The tube bottom comprised the membrane or separator. The volume inside the tube was the cathode compartment and the volume outside the tube, but inside the reaction flask comprised the anode compartment. The aqueous carbonate solution comprised the anolyte. The anode itself was a nickel wire wrapped around a nickel plate, the wire and plate together comprising the anode.
The reaction flask was heated to about 115° C. and pressurized to about 10 psi. Under these conditions, the sodium in the cathode compartment was molten. A potential of about 5 V was applied across the anode and the cathode. After passing 500 mAh of current through the cell, it was cooled to room temperature. The amount of sodium metal in the cathode compartment was measured by hydrolyzing the collected sodium to generate hydrogen gas. The amount of hydrogen gas captured can be translated into the amount of sodium hydrolyzed. The total amount of sodium was 1.32 grams, so approximately 0.32 grams of sodium metal was generated by the electrolysis. This represented a current efficiency of 75%, wherein 75% of the electrons passing through the cell resulted in conversion of sodium ions from the sodium carbonate solution into sodium metal.

EXAMPLE 4

A reaction flask was charged with about 200 g of 30 weight-% Na₂CO₃. A Na-β″-alumina tube was inserted into the solution. The tube contained about 1 gram of sodium metal. Embedded into the sodium metal was a nickel wire. Collectively, the sodium metal and the nickel wire comprised the cathode. The tube bottom comprised the membrane or separator. The volume inside the tube was the cathode compartment and the volume outside the tube, but inside the reaction flask, comprised the anode compartment. The sodium carbonate solution comprised the anolyte. The anode itself was a nickel wire wrapped around a nickel plate, the wire and plate together comprising the anode.
The reaction flask was heated to about 120° C. and pressurized to about 10 psi. Under these conditions, the sodium in the cathode compartment was molten, and the sodium carbonate was fully soluble in the anode compartment solution. A potential of about 5 V was applied across the anode and the cathode. After passing 337 mAh of current through the cell, it was cooled to room temperature. The amount of sodium metal in the cathode compartment was measured by hydrolyzing the collected sodium to generate hydrogen gas. The amount of hydrogen gas captured can be translated into the amount of sodium hydrolyzed. The total amount of sodium was 1.22 grams, so approximately 0.22 grams of sodium metal was generated by the electrolysis. This represented a current efficiency of 77%, wherein 77% of the electrons passing through the cell resulted in conversion of sodium ions from the sodium carbonate solution into sodium metal.
The above description and drawings illustrate preferred embodiments that achieve the objects, features and advantages of the present invention. It is not intended that the present invention be limited to the illustrated embodiments. Any modification of the present invention that comes within the spirit and scope of the following claims should be considered part of the present invention.

Claims

1. A process for reducing a metal borate, comprising:

providing an electrolytic cell containing an anode compartment and a cathode compartment separated by a separator which is permeable to metal ions and not permeable to water and water vapor;

supplying a metal borate compound to the anode compartment; and

applying an electric potential to the cell.

2. The process of claim 1, wherein the metal borate compound is a borate salt having the formula zM_nO.xB₂O₃.yH₂O, wherein z is 0.5 to 5, x is 0.1 to 5, y is 0 to 10, n is 1 or 2; and M is an alkali metal ion or an alkaline earth metal ion.

3. The process of claim 2, wherein M is selected from the group consisting of Li⁺, Na⁺ and K⁺.

4. The process of claim 1, wherein the metal borate compound is supplied as an aqueous solution.

5. The process of claim 1, further comprising supplying a metal or a metal alloy to the cathode compartment.

6. The process of claim 5, further comprising heating at least the cathode compartment of the cell to a temperature of about 95 to about 120° C.

7. The process of claim 1, further comprising reacting hydrogen or a hydrogen containing gas with the metal produced at the cathode.

8. The process of claim 1, further comprising passing hydrogen or a hydrogen containing gas in the anode compartment.

9. The process of claim 8, further comprising supplying hydrogen or hydrogen containing gas to the anode compartment through a gas inlet means.

10. The process of claim 9, wherein the gas inlet means is selected from the group consisting of a pipe, a sparger, a hose and a hydrogen diffusion material.

11. The process of claim 1, further comprising electrooxidizing hydrogen at the anode.

12. The process of claim 1, further comprising providing a supporting electrolyte to the anode compartment.

13. The process of claim 12, wherein the supporting electrolyte comprises an anion selected from the group consisting of sulfate, perchlorate, nitrate, and phosphate.

14. The process of claim 12, wherein the supporting electrolyte is selected from the group consisting of sodium sulfate, sodium perchlorate, sodium phosphate, and sodium nitrate.

15. The process of claim 1, wherein the separator comprises a material selected from the group consisting of lithium-β-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, and potassium-β/β″-aluminum oxide.

16. The process of claim 1, wherein the separator is a NaSICON membrane.

17. The process of claim 1, wherein the separator is a LiSICON or a KSICON membrane.

18. The process of claim 1, wherein the electrical potential is at least about 1.4 volts.

19. The process of claim 1, wherein the metal borate compound is a product discharged from a hydrogen generation reaction.

20. The process of claim 19, wherein the product comprises sodium metaborate and sodium hydroxide.

21. The process of claim 20, wherein the product comprises at least 27% sodium metaborate.

22. The process of claim 1 further comprising providing an alcohol in the anode compartment to produce an alkyl borate in the anode compartment.

23. The process of claim 22, wherein the alcohol is represented by the formula ROH, where R is an alkyl group containing from 1 to 6 carbons.

24. The process of claim 22, further comprising reacting trialkyl borate with metal hydride to obtain borohydride.

25. A process for reducing a metal carbonate, comprising:

supplying a metal carbonate compound to the anode compartment; and

applying an electric potential to the cell.

26. The process of claim 25, wherein the metal carbonate compound is supplied as an aqueous solution.

27. The process of claim 25, further comprising supplying a metal or a metal alloy to the cathode compartment.

28. The process of claim 27, further comprising heating at least the cathode compartment of the cell to a temperature of about 95 to about 150° C.

29. The process of claim 25, further comprising reacting hydrogen or a hydrogen containing gas with the metal produced at the cathode.

30. The process of claim 25, further comprising passing hydrogen or a hydrogen containing gas to the anode compartment through a gas inlet means.

31. The process of claim 30, wherein the gas inlet means is selected from the group consisting of a pipe, a sparger, a hose, and a hydrogen diffusion material.

32. The process of claim 25, further comprising electrooxidizing hydrogen at the anode.

33. The process of claim 25, wherein the separator comprises a material selected from the group consisting of lithium-β-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, and potassium-β/β″-aluminum oxide.

34. The process of claim 25, wherein the separator is a NaSICON membrane, a KSICON membrane, or a LiSICON membrane.

35. The process of claim 25, wherein the electric potential is at least about 3.1 volts.

36. A process for producing a metal and a trialkylborate compound by reducing a metal borate in an electrolytic cell containing anode and cathode compartments separated by a separator which is permeable to metal ions and not permeable to water and water vapor, comprising:

supplying a metal borate compound and at least one alcohol to the anode compartment; and

applying an electric potential to the cell.

37. The process of claim 36, further comprising forming a boron species and reacting the boron species with alcohol to form trialkyl borate.

38. The process of claim 36, further comprising forming boric acid and reacting the boric acid with alcohol to form trialkyl borate.

39. The process of claim 38, further comprising maintaining the cell at a temperature of about 25 to about 300° C.

40. The process of claim 38, wherein the trialkyl borate is trimethyl borate.

41. The process of claim 36, wherein the metal borate compound is a borate salt having formula zM_nO.xB₂O₃.yH₂O, wherein z is 0.5 to 5, x is 0.1 to 5, y is 0 to 10, n is 1 or 2; and M is an alkali metal ion or an alkaline earth metal ion.

42. The process of claim 41, wherein M is selected from the group consisting of Li⁺, Na⁺ and K⁺.

43. The process of claim 36, wherein the metal borate compound is supplied as an aqueous solution.

44. The process of claim 36, further comprising supplying a metal or a metal alloy to the cathode compartment.

45. The process of claim 36, further comprising passing hydrogen or a hydrogen containing gas in the anode compartment.

46. The process of claim 45, wherein passing hydrogen or a hydrogen containing gas further comprises supplying hydrogen or hydrogen containing gas through a gas inlet means.

47. The process of claim 46, wherein the gas inlet means is selected from the group consisting of a pipe, a sparger, a hose, and a hydrogen diffusion material.

48. The process of claim 36, further comprising electrooxidizing hydrogen at the anode.

49. The process of claim 36, wherein the separator comprises a material selected from the group consisting of lithium-,-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, and potassium-β/β″-aluminum oxide.

50. The process of claim 36, wherein the separator is a NaSICON membrane, a KSICON membrane, or a LiSICON membrane.

51. The process of claim 36, wherein the electric potential is at least about 1.4 volts.

52. The process of claim 36, wherein the metal borate compound is sodium borate and the alcohol is methanol.

53. The process of claim 36, wherein the metal borate compound is a product from a hydrogen generation reaction.

54. The process of claim 53, wherein the product comprises sodium metaborate and sodium hydroxide.

55. A process for producing a borohydride, comprising:

electrolyzing an aqueous solution comprising a metal borate and at least one alcohol in an electrolytic cell to produce a metal and a trialkyl borate, the electrolytic cell containing anode and cathode compartments separated by a separator which is permeable to metal ions and not permeable to water and water vapor;

reacting the metal with hydrogen to produce a metal hydride; and

reacting the metal hydride and the trialkyl borate to form a borohydride.

56. The process of claim 55, wherein the metal borate comprises a product from a hydrogen generation reaction.

57. The process of claim 55, wherein the metal borate is sodium borate, the alcohol is methanol, and the borohydride is sodium borohydride.

58. The process of claim 55, wherein the metal borate is a salt having formula M_nO.xB₂O₃.yH₂O, wherein z is 0.5 to 5, x is 0.1 to 5, y is 0 to 10, n is 1 or 2; and M is an alkali metal ion or an alkaline earth metal ion.

59. The process of claim 55, wherein the aqueous solution further comprises an alkali metal hydroxide.

60. The process of claim 59, wherein the aqueous solution comprises sodium metaborate and sodium hydroxide.

61. The process of claim 55, further comprising maintaining at least the cathode compartment of the cell at a temperature of about 95 to about 120° C.

62. The process of claim 55, further comprising passing hydrogen or a hydrogen containing gas in the anode compartment.

63. The process of claim 62, wherein the step of passing hydrogen or a hydrogen containing gas further comprises supplying hydrogen or hydrogen containing gas through a gas inlet means.

64. The process of claim 63, wherein the gas inlet means is selected from the group consisting of a pipe, a sparger, a hose, and a hydrogen diffusion material.

65. The process of claim 55, further comprising electrooxidizing hydrogen at the anode.

66. The process of claim 55, further comprising the step of providing a supporting electrolyte to the anode compartment.

67. The process of claim 66, wherein the supporting electrolyte comprises a material selected from the group consisting of sodium sulfate, sodium perchlorate, sodium phosphate, and sodium nitrate.

68. The process of claim 55, wherein the separator comprises a material selected from the group consisting of lithium-β-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, and potassium-β/β″-aluminum oxide.

69. The process of claim 55, wherein the separator is a NaSICON membrane.

70. The process of claim 55, wherein the separator is a LiSICON membrane or a KSICON membrane.

71. The process of claim 55, further comprising supplying an electric potential of at least about 1.4 volts to the cell.

72. A process for producing a borohydride, comprising:

providing an electrolytic cell containing anode and cathode compartments separated by a separator which is permeable to metal ions and not permeable to water and water vapor;

providing an aqueous solution containing at least an alkali metal borate and at least an alkali metal hydroxide in the electrolytic cell;

subjecting the aqueous solution to an electrolysis process to produce an alkali metal and boric acid;

reacting the alkali metal with hydrogen to produce an alkali metal hydride;

converting boric acid generated in the anode compartment to a trialkyl borate; and

reacting the alkali metal hydride and the trialkyl borate to form a borohydride.

73. The process of claim 72, further comprising applying an electric potential to the cell.

74. The process of claim 73, wherein the electric potential is at least about 1.4 volts.

75. The process of claim 74, wherein the electrical potential is at least about 3.25 volts.

76. The process of claim 72, wherein the alkali metal borate is a salt having formula M_nO.xB₂O₃.yH₂O, wherein z is 0.5 to 5, x is 0.1 to 5, y is 0 to 10, n is 1 or 2; and M is an alkali metal ion or an alkaline earth metal ion.

77. The process of claim 72, wherein the aqueous solution comprises sodium metaborate and sodium hydroxide.

78. The process of claim 72, further comprising maintaining the cell at a temperature of about 95 to about 120° C.

79. The process of claim 72, further comprising passing hydrogen or a hydrogen containing gas in the anode compartment.

80. The process of claim 79, wherein passing hydrogen or a hydrogen containing gas further comprises supplying hydrogen or hydrogen containing gas through a gas inlet means.

81. The process of claim 80, wherein the gas inlet means is selected from the group consisting of a pipe, a sparger, a hose, and a hydrogen diffusion material.

82. The process of claim 72, further comprising electrooxidizing hydrogen at the anode.

83. The process of claim 72, further comprising the step of providing a supporting electrolyte to the anode compartment.

84. The process of claim 83, wherein the supporting electrolyte comprises an anion selected from the group consisting of sulfate, perchlorate, nitrate, and phosphate.

85. The process of claim 83, wherein the supporting electrolyte comprises sodium sulfate or sodium nitrate.

86. The process of claim 72, wherein the separator comprises a material selected from the group consisting of lithium-β-aluminum oxide, lithium-β″-aluminum oxide, lithium-β/β″-aluminum oxide, sodium-β-aluminum oxide, sodium-β″-aluminum oxide, sodium-β/β″-aluminum oxide, potassium-β-aluminum oxide, potassium-β″-aluminum oxide, and potassium-β/β″-aluminum oxide.

87. The process of claim 72, wherein the separator is a NaSICON membrane, a KSICON membrane, or a LiSICON membrane.

88. A process for producing sodium borohydride, comprising:

electrolyzing an aqueous solution of sodium borate and alcohol to produce sodium metal and trialkylborate;

reacting the sodium metal with hydrogen to produce sodium hydride;

reacting the sodium hydride and trialkylborate to produce borohydride and sodium alkoxide;

hydrolyzing sodium alkoxide to sodium hydroxide and methanol;

recycling the methanol for production of trialkylborate from boric acid; and

electrolyzing sodium hydroxide to sodium metal.