Electrolytic process to produce aluminum hydroxide

Abstract

Methods and apparatus for separating aqueous solution of alkali aluminate into alkali hydroxide and aluminate hydroxide are disclosed. These methods are enabled by the use of alkali ion conductive membranes in electrolytic cells that are chemically stable and alkali ion selective. The alkali ion conductive membrane includes a chemically stable ionic-selective cation membrane.

1 . A method for producing and recovering aluminum hydroxide from an aqueous solution containing alkali aluminate, the method comprising:
 obtaining an electrolytic cell comprising an alkali ion conductive membrane configured to selectively transport alkali ions, the membrane separating an anolyte compartment configured with an anode and a catholyte compartment configured with a cathode;   feeding an anolyte solution comprising an alkali aluminate (MAl(OH) 4 , wherein M is an alkali metal) into the anolyte compartment;   feeding an aqueous catholyte solution into the catholyte compartment;   applying an electric current to the electrolytic cell thereby:
 i. producing hydrogen ions at the anode in the anolyte compartment to facilitate the reaction: H + +MAl(OH) 4 →Al(OH) 3 +H 2 O+M + ; 
 ii. causing alkali ions (M + ) to pass through the alkali ion conductive membrane from the anolyte compartment to the catholyte compartment; and 
 iii. decomposing water in the presence of alkali ions in the catholyte compartment according to the following reaction: M + +H 2 O+e − →MOH+½H 2 ; and 
   removing anolyte solution containing aluminum hydroxide from the anolyte compartment.   
     
     
         2 . A method for producing and recovering aluminum hydroxide according to  claim 1 , further comprising:
 precipitating aluminum hydroxide in the anolyte solution removed from the anolyte compartment; and   separating precipitated aluminum hydroxide from the anolyte solution to yield a supernate stream.   
     
     
         3 . A method for producing and recovering aluminum hydroxide according to  claim 2 , further comprising recycling the supernate stream back to the anolyte compartment to further produce and recover aluminum hydroxide. 
     
     
         4 . A method for producing and recovering aluminum hydroxide according to  claim 2 , further comprising converting the precipitated aluminum hydroxide into alumina by heating. 
     
     
         5 . A method for producing and recovering aluminum hydroxide according to  claim 1 , further comprising removing alkali hydroxide from the catholyte compartment. 
     
     
         6 . A method for producing and recovering aluminum hydroxide according to  claim 1 , wherein the alkali ion conductive membrane comprises a chemically stable ionic-selective ceramic membrane selective to transfer M +  ions. 
     
     
         7 . A method for producing and recovering aluminum hydroxide according to  claim 6 , wherein the cation-conductive ceramic membrane comprises a solid MSICON (Metal Super Ion CONducting) material, where M is Na, K, or Li. 
     
     
         8 . A method for producing and recovering aluminum hydroxide according to  claim 1 , wherein the alkali ion conductive membrane comprises a layered composite comprising a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane. 
     
     
         9 . A method for producing and recovering aluminum hydroxide according to  claim 8 , wherein the cation-conductive ceramic membrane comprises a solid MSICON (Metal Super Ion CONducting) material, where M is Na, K, or Li. 
     
     
         10 . A method for producing and recovering aluminum hydroxide according to  claim 1  further comprising maintaining the anolyte solution at a temperature of at least 40° C. 
     
     
         11 . An apparatus for producing and recovering aluminum hydroxide from an aqueous solution containing alkali aluminate comprising:
 an electrolytic cell comprising an alkali ion conductive membrane configured to selectively transport alkali ions, the membrane separating an anolyte compartment configured with an anode and a catholyte compartment configured with a cathode;   an anolyte feed stream connected to the anolyte compartment for feeding an anolyte solution into the anolyte compartment comprising an alkali aluminate (MAl(OH) 4 , wherein M is an alkali metal);   a catholyte feed stream connected to the catholyte compartment for feeding an aqueous catholyte solution into the catholyte compartment;   a source of electric potential connected to the cathode and anode to thereby:
 i. producing hydrogen ions at the anode in the anolyte compartment to facilitate the reaction: H + +MAl(OH) 4 →Al(OH) 3 +H 2 O+M + ; 
 ii. causing alkali ions (M + ) to pass through the alkali ion conductive membrane from the anolyte compartment to the catholyte compartment; and 
 iii. decomposing water in the presence of alkali ions in the catholyte compartment according to the following reaction: M + +H 2 O+e − →MOH+½H 2 ; and 
   an anolyte exit stream removing anolyte solution containing aluminum hydroxide from the anolyte compartment;   a separator connected to the anolyte exit stream for precipitating and recovering aluminum hydroxide from a supernate; and   a supernate stream connected to the separator to receive the supernate and deliver at least a portion of the supernate to the anolyte feed stream.   
     
     
         12 . An apparatus for producing and recovering aluminum hydroxide according to  claim 11 , further comprising a catholyte exit stream removing alkali hydroxide from the catholyte compartment. 
     
     
         13 . An apparatus for producing and recovering aluminum hydroxide according to  claim 11 , wherein the alkali ion conductive membrane comprises a chemically stable ionic-selective ceramic membrane selective to transfer M +  ions. 
     
     
         14 . An apparatus for producing and recovering aluminum hydroxide according to  claim 13 , wherein the cation-conductive ceramic membrane comprises a solid MSICON (Metal Super Ion CONducting) material, where M is Na, K, or Li. 
     
     
         15 . An apparatus for producing and recovering aluminum hydroxide according to  claim 11 , wherein the alkali ion conductive membrane comprises a layered composite comprising a chemically stable ionic-selective polymer and a cation-conductive ceramic membrane. 
     
     
         16 . An apparatus for producing and recovering aluminum hydroxide according to  claim 15 , wherein the cation-conductive ceramic membrane comprises a solid MSICON (Metal Super Ion CONducting) material, where M is Na, K, or Li. 
     
     
         17 . An apparatus for producing and recovering aluminum hydroxide according to  claim 11 , further comprising a temperature control unit to maintain the anolyte solution at a temperature of at least 40° C. 
     
     
         18 . An apparatus for producing and recovering aluminum hydroxide according to  claim 11 , further comprising an oxygen vent to recover oxygen gas produced in the anolyte compartment. 
     
     
         19 . An apparatus for producing and recovering aluminum hydroxide according to  claim 11 , further comprising a hydrogen vent to recover hydrogen gas produced in the catholyte compartment. 
     
     
         20 . An apparatus for producing and recovering aluminum hydroxide according to  claim 11 , wherein the alkali metal M is sodium.