Electrochemical Ion Separation in Molten Salts

Abstract

A purification method that uses ion-selective ceramics to electrochemically filter waste products from a molten salt. The electrochemical method uses ion-conducting ceramics that are selective for the molten salt cations desired in the final purified melt, and selective against any contaminant ions. The method can be integrated into a slightly modified version of the electrochemical framework currently used in pyroprocessing of nuclear wastes.

We claim: 
     
         1 . An electrochemical cell for electrochemical ion separation, comprising:
 an ion-selective ceramic membrane having selectivity for at least one molten salt cation and selectivity against transport of at least one contaminant cation,   a molten salt electrolyte,   a cathode immersed in a purified molten salt electrolyte on the cathode side of the ceramic membrane,   an anode immersed in a contaminated molten salt electrolyte comprising the at least one molten salt cation and the at least one contaminant cation on the anode side of the ceramic membrane, and   an electrical circuit for applying an electrical bias across the anode and cathode, thereby causing the at least one molten salt cation to transport across the ceramic membrane but substantially excluding the at least one contaminant cation from transporting across the ceramic membrane and thereby concentrating the at least one contaminant ion in the contaminated molten salt electrolyte on the anode side of the ceramic membrane.   
     
     
         2 . The electrochemical cell of  claim 1 , wherein the ion-selective ceramic membrane is selective by size exclusion or valence-charge exclusion of the at least one contaminant cation. 
     
     
         3 . The electrochemical cell of  claim 1 , wherein the ion-selective ceramic membrane comprises Na 1+x Zr 2 Si x P 3−x O 12 . 
     
     
         4 . The electrochemical cell of  claim 1 , wherein the Na 1+x Zr 2 Si x P 3−x O 12  comprises cation-substituted Na 1+x Zr 2 Si x P 3−x O 12 . 
     
     
         5 . The electrochemical cell of  claim 4 , wherein the cation-substituted Na 1+x− Zr 2 Si x P 3−x O 12  comprises KZr 2 (PO 4 ) 3 . 
     
     
         6 . The electrochemical cell of  claim 1 , wherein the ion-selective ceramic membrane comprises a garnet-structured lithium lanthanum tantalate ceramic. 
     
     
         7 . The electrochemical cell of  claim 6 , wherein the garnet-structured lithium lanthanum tantalate ceramic comprises Li 6 BaLa 2 Ta 2 O 12 . 
     
     
         8 . The electrochemical cell of  claim 1 , wherein the molten salt electrolyte comprises LiCl—KCl. 
     
     
         9 . The electrochemical cell of  claim 8 , wherein the molten salt electrolyte comprises a eutectic of LiCl—KCl. 
     
     
         10 . The electrochemical cell of  claim 8 , wherein chloride ions in the molten salt electrolyte are oxidized to form chlorine gas at the anode and the chlorine gas is reduced to chloride ions at the cathode. 
     
     
         11 . The electrochemical cell of  claim 1 , wherein the at least one contaminant cation comprises a fission product. 
     
     
         12 . The electrochemical cell of  claim 1 , wherein the at least one contaminant cation comprises a monovalent cation. 
     
     
         13 . The electrochemical cell of  claim 12 , wherein the monovalent cation comprises Cs + . 
     
     
         14 . The electrochemical cell of  claim 1 , wherein the at least one contaminant cation comprises a divalent cation. 
     
     
         15 . The electrochemical cell of  claim 14 , wherein the divalent cation comprises Cu 2+  or Sr 2+ . 
     
     
         16 . The electrochemical cell of  claim 1 , wherein the anode comprises copper. 
     
     
         17 . The electrochemical cell of  claim 1 , wherein the cathode comprises platinum, carbon, tungsten, copper, or steel. 
     
     
         18 . The electrochemical cell of  claim 1 , wherein the ion-selective ceramic membrane comprises a planar configuration. 
     
     
         19 . The electrochemical cell of  claim 1 , wherein the ion-selective ceramic membrane comprises a tubular configuration. 
     
     
         20 . A method to purify KCl—LiCl—CuCl 2  molten salt, comprising:
 adding lithium formate and/or potassium formate to a KCl—LiCl—CuCl 2  molten salt to precipitate copper metal according to the reaction:
   2LiCOOH+CuCl 2 →2LiCl+Cu(s)++2CO 2  +H 2 , and/or
 
   2KCOOH+CuCl 2 →2KCl+Cu(s)++2CO 2 +H 2 .