Electrochemical process to recycle aqueous alkali chemicals using ceramic ion conducting solid membranes

Abstract

A method for producing an alkali metal hydroxide, comprises providing an electrolytic cell that includes at least one membrane having ceramic material configured to selectively transport alkali metal ions. The method includes introducing a first solution comprising an alkali metal hydroxide solution into a catholyte compartment such that said first solution is in communication with the membrane and a cathode. A second solution comprising at least one alkali metal salt and one or more monovalent, divalent, or multivalent metal salts is introduced into an anolyte compartment such that said second solution is in communication with the membrane and an anode. The method includes applying an electric potential to the electrolytic cell such that alkali metal ions pass through the membrane and are available to undertake a chemical reaction with hydroxyl ions in the catholyte compartment to form alkali metal hydroxide.

1 . A method for producing an alkali metal hydroxide, comprising:
 providing an electrolytic cell comprising at least one membrane comprising ceramic material configured to selectively transport the alkali metal ions, the membrane positioned between an anolyte compartment configured with an anode and a catholyte compartment configured with a cathode;   introducing a first solution comprising an alkali metal hydroxide solution into the catholyte compartment of the electrolytic cell such that said first solution is in communication with the membrane and the cathode;   introducing a second solution comprising at least one alkali metal salt and one or more monovalent, divalent, or multivalent metal salts into the anolyte compartment of the electrolytic cell such that said second solution is in communication with the membrane and the anode; and   applying an electric potential to the electrolytic cell such that alkali metal ions pass through the membrane and are available to undertake a chemical reaction with hydroxyl ions in the catholyte compartment to form alkali metal hydroxide.   
     
     
         2 . The method of  claim 1 , wherein introducing a first solution into the catholyte compartment and introducing a second solution into the anolyte compartment comprise a continuous operation. 
     
     
         3 . The method of  claim 1 , wherein introducing a first solution into the catholyte compartment and introducing a second solution into the anolyte compartment comprise a batch operation. 
     
     
         4 . The method of  claim 1 , wherein the alkali metal comprises sodium. 
     
     
         5 . The method of  claim 4 , wherein introducing a first solution into the catholyte compartment comprises introducing sodium hydroxide as an aqueous solution wherein the concentration of sodium hydroxide is between about 1% by weight and about 50% by weight of the solution. 
     
     
         6 . The method of  claim 5 , further comprising maintaining the concentration of sodium hydroxide in the catholyte compartment between about 10% and about 20% by weight. 
     
     
         7 . The method of  claim 4 , further comprising maintaining the concentration of the sodium salt in the anolyte compartment between about 1% and about 50% by weight of the second solution. 
     
     
         8 . The method of  claim 7 , further comprising maintaining the concentration of sodium in the anolyte compartment between about 5% and about 20% by weight. 
     
     
         9 . The method of  claim 4 , wherein the ceramic membrane comprises a NaSICON material. 
     
     
         10 . The method of  claim 4 , wherein the ceramic membrane comprises a NaSICON material having the formula Na 1+x Zr 2 Si x P 3−x O 12  where 0≦x≦3. 
     
     
         11 . The method of  claim 4 , wherein the ceramic membrane comprises a NaSICON material having the formula, M 1+x M I   2 Si x P 3−x O 12  where 0≦x≦3, where M is selected from the group consisting of Li, Cs, Na, K, or Ag, or mixture thereof, and where M I  is selected from the group consisting of Zr, Ge, Y, Ti, Sn, Y or Hf, or mixtures thereof. 
     
     
         12 . The method of  claim 4 , wherein the ceramic membrane comprises a NaSICON material having the formula Na 5 RESi 4 O 12  where RE is Y, Nd, Dy, or Sm, or any mixture thereof. 
     
     
         13 . The method of  claim 4 , wherein the ceramic membrane comprises a non-stoichiometric sodium-deficient NaSICON material having the formula (Na 5 RESi 4 O 12 ) 1−δ (RE 2 O 3 .2SiO 2 ) δ , where RE is Nd, Dy, or Sm, or any mixture thereof and where δ is the measure of deviation from stoichiometry. 
     
     
         14 . The method of  claim 4 , wherein the second solution introduced into the anolyte compartment comprises a sodium salt selected from the group consisting of: sodium hydroxide, sodium chloride, sodium carbonate, sodium bicarbonate, sodium sulfate, sodium chlorate, sodium phosphate, sodium perchlorate, sodium nitrite, sodium fluoride, sodium oxalate, sodium organic salts and any combination thereof. 
     
     
         15 . The method of  claim 1 , wherein the second solution comprises one or more monovalent, divalent, or multivalent metal salts selected from Na, K, Cs, Ca, Sr, Ba, Al, and mixtures thereof. 
     
     
         16 . The method of  claim 1 , wherein the second solution comprises one or more non-alkali, radioactive metal salts and wherein the alkali metal hydroxide formed in the catholyte compartment is substantially non-radioactive. 
     
     
         17 . The method of  claim 1 , wherein the membrane operates at a current density of between about 20 mA/cm 2  and about 200 mA/cm 2 . 
     
     
         18 . The method of  claim 1 , wherein the sodium-ion conducting ceramic membrane operates at a current density greater than 100 mA/cm 2 . 
     
     
         19 . The method of  claim 1 , wherein the electrolytic cell comprises a plurality of membranes, each configured to selectively transport sodium ions, and at least one bipolar electrode positioned between a pair of said membranes such that the electrolytic cell comprises a plurality of anolyte compartments and a plurality of catholyte compartments. 
     
     
         20 . The method of  claim 19 , wherein alkali metal hydroxide solution is simultaneously received from the plurality of catholyte compartments. 
     
     
         21 . The method of  claim 20 , wherein sodium hydroxide is received from a first catholyte compartment and introduced into a second catholyte compartment to increase the concentration of the sodium hydroxide in a sodium hydroxide solution in successive catholyte compartments. 
     
     
         22 . The method of  claim 1 , wherein the ceramic membrane comprises a material having the formula Na 1+z L z Zr 2−z P 3 O 12  where 0≦z≦2.0, and where L is selected from the group consisting of Cr, Yb, Er, Dy, Sc, Fe, In, or Y, or mixtures thereof; 
     
     
         23 . The method of  claim 1 , wherein the ceramic membrane comprises a material having the formula M II   5 RESi 4 O 12 , where M II  may be Li, Na, K or Ag, or mixtures thereof, and where RE is Y or any rare earth element.