Method for concentrating dilute sulfuric acid and an apparatus for concentrating dilute sulfuric acid

Abstract

A method for concentrating dilute sulfuric acid involves feeding dilute sulfuric acid through at least two electrolytic cells depolarized by sulfur dioxide, and through a cation conductive membrane separating the cathode and anode sides in the electrolytic cell. Also provided is an apparatus for concentrating sulfuric acid, wherein at least a first electrolytic cell is in fluid communication with a second electrolytic cell, and wherein a cation conductive membrane separates the cathode and anode sides in the electrolytic cells.

1 . A method for concentrating dilute sulfuric acid, comprises feeding sulfuric acid into a first of at least two electrolytic cells which are depolarized by sulfur dioxide, and wherein a cation conductive membrane separates the cathode and anode sides in the electrolytic cells. 
     
     
         2 . The method according to  claim 1 , wherein the concentration of sulfuric acid is carried out by feeding the sulfuric acid sequentially through 2 to 5 electrolytic cells that are installed in a cascading fashion and depolarized by sulfur dioxide. 
     
     
         3 . The method according to  claim 1 , wherein dilute sulfuric acid and sulfur dioxide are fed to the anode side of a first electrolytic cell comprising a membrane, so that the following reaction takes place on the anode side:
   SO 2 (aq)+2(1+x)H 2 O→H 2 SO 4 (aq)+2(H 2 O) x H + (aq)+2e −   (9)
   where x=1 to 4,   in which case the formed complex ions pass through the membrane of the electrolytic cell to the cathode side, and the following reaction takes place at the cathode:
   2(H 2 O) x H + (aq)+2e − →H 2 (g)+2x H 2 O   (10)
 
   where x=1 to 4,   and the concentrated mixture of sulfuric acid and water obtained from the anode side of the first electrolytic cell is fed to the anode side of the second electrolytic cell together with the non-reacted and/or added sulfur dioxide, and the above described reactions take place in said second electrolytic cell, and the further concentrated sulfuric acid formed as a reaction product from the anode side of the second electrolytic cell is recovered or fed to the anode side of the a successive electrolytic cell.   
     
     
         4 . The method according to  claim 1 , wherein the cation conductive membrane separating the cathode and anode in the electrolytic cell sides is a functionally fixed electrolyte that serves both as an insulator and a proton conductor, and prevents the gases from flowing from one side of the membrane to the other. 
     
     
         5 . The method according to  claim 4 , wherein the membrane comprises a sulfonic acid membrane. 
     
     
         6 . The method according to  claim 1 , wherein the concentration of the sulfuric acid fed to the anode side of the first electrolytic cell is 78 wt % or less. 
     
     
         7 . The method according to  claim 1 , wherein the concentration of the sulfuric acid fed to the cathode side of the first electrolytic cell is 78 wt % or less. 
     
     
         8 . The method according to  claim 1 , wherein the sulfuric acid is concentrated at a temperature of 10 to 120° C., preferably at a temperature of 20 to 100° C. 
     
     
         9 . The method according to  claim 1 , wherein the sulfuric acid is concentrated at a pressure of 0.5 to 7 bar, preferably at a pressure of 1 to 5 bar. 
     
     
         10 . The method according to  claim 1 , wherein the concentrated sulfuric acid is recovered and lead to a sulfuric acid plant. 
     
     
         11 . An apparatus for concentrating dilute sulfuric acid, comprises at least a first electrolytic cell depolarized by sulfur dioxide and a second electrolytic cell depolarized by sulfur dioxide, wherein the second electrolytic cell is configured for receiving sulfuric acid concentrated in the first electrolytic cell from the first electrolytic cell, and which apparatus comprises a cation conductive membrane for separating the cathode and anode sides in the electrolytic cells. 
     
     
         12 . The concentration apparatus according to  claim 11 , wherein the apparatus comprises a total of 2 to 5 electrolytic cells that are installed in a cascading fashion and depolarized by sulfur dioxide. 
     
     
         13 . The concentration apparatus according to  claim 11 , wherein the first electrolytic cell comprises an anode side and a cathode side, and that the second electrolytic cell comprises an anode side and a cathode side,
 and that the apparatus comprises a first set of feed elements for feeding dilute sulfuric acid and sulfur dioxide to the anode side of the first electrolytic cell comprising the membrane, in order to generate the following reaction on the anode side:
   SO 2 (aq)+2(1+x)H 2 O→H 2 SO 4 (aq)+2(H 2 O) x H + (aq)+2e −   (9)
 
   where x=1 to 4,   in which case the formed complex ions pass through the membrane of the electrolytic cell to the cathode side, and the following reaction takes place at the cathode:
   2(H 2 O) x H + (aq)+2e − →H 2 (g)+2x H 2 O   (10)
 
   where x=1 to 4,   and a second set of feed elements for feeding the concentrated mixture of sulfuric acid and water obtained from the anode side of the first electrolytic cell, together with non-reacted and/or added sulfur dioxide, to the anode side of the second electrolytic cell, in which second electrolytic cell the above described reactions take place, and a third set of feed elements for feeding the further concentrated sulfuric acid formed as a reaction product on the anode side of the second electrolytic cell to recovery means or further to the anode side of a third electrolytic cell.   
     
     
         14 . The concentration apparatus according to  claim 11 , wherein the membrane in the first electrolytic cell and/or the membrane in the second electrolytic cell is a cation conductive, functionally fixed electrolyte separating the anode side and the cathode side, arranged to serve both as an insulator and a proton conductor, and to prevent the gases from flowing from one side of the membrane to the other. 
     
     
         15 . The concentration apparatus according to  claim 14 , wherein the membrane comprises a sulfonic acid membrane. 
     
     
         16 . The concentration apparatus according to  claim 11 , wherein the apparatus comprises means for adding sulfur dioxide to the concentrated mixture of sulfuric acid and water obtained from the anode side of the first electrolytic cell, which mixture is fed together with non-reacted and/or added sulfur dioxide to the anode side of the second electrolytic cell. 
     
     
         17 . The concentration apparatus according to  claim 11 , wherein the apparatus comprises means for removing hydrogen from the cathode side of the electrolytic cells, and means for the recovery of hydrogen. 
     
     
         18 . The concentration apparatus according to  claim 11 , wherein the apparatus comprises means for recycling catholyte from the cathode side of one electrolytic cell to the cathode side of another electrolytic cell, and/or means for recycling the catholyte of the last electrolytic cell to the anode side of the first electrolytic cell. 
     
     
         19 . The method according to  claim 6 , wherein the concentration of sulfuric acid fed to the anode side of the first electrolytic cell is 1 to 25 wt %. 
     
     
         20 . The method according to  claim 7 , wherein the concentration of sulfuric acid fed to the cathode side of the first electrolytic cell is 1 to 35 wt %.  
     
     
         21 . The method according to  claim 8 , wherein the method is carried out at a temperature of 20 to 100° C. 
     
     
         22 . The method according to  claim 9 , wherein the method is carried out at a pressure of 1 to 5 bar.