Processes for separating metals from metal salts

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.

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 n O.xB 2 O 3 .yH 2 O, 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: 
 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 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 n O.xB 2 O 3 .yH 2 O, 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 n O.xB 2 O 3 .yH 2 O, 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 n O.xB 2 O 3 .yH 2 O, 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.