Production of ultra pure salt

Abstract

This invention relates to the methods and installations for producing ultra pure sodium chloride salt crystals primarily for use in saturating depleted brine resulting from the electrolytic decomposition of saturated brine in chlor alkali membrane cells for the production of chlorine, caustic soda and hydrogen. More particularly, this invention related to the production of ultra pure sodium chloride salt crystals by processing primary treated brine by first acidifying the primary treated brine, then stripping the carbonic acid produced by acidification as carbon dioxide, and then returning the brine to a pH of about 6 or higher which is sufficient for processing it in evaporation equipment where the ultra pure salt crystals are produced.

1 . The method for producing ultra pure salt, containing below 500 parts per billion (ppb) of calcium and other multivalent metal ions, which comprises: 
 acidifying primary treated brine from pH 9.5 to 12 to ph 2 to 4, thereby converting the calcium, barium, strontium, and other reactive multivalent metal compounds to soluble chlorides, such acidification also converting excess sodium carbonate and hydroxides to salt, water, and dissolved carbon dioxide,    removing the carbon dioxide by stripping,    neutralizing the stripped material to a pH of at least 6 but not exceeding 8, thereby preventing metals corrosion, at pH less than 6, and carbon dioxide uptake from the surroundings, at pH above 8,    concentrating the neutralized stripped brine, and washing the crystals so produced, to produce ultra pure salt crystals in a conventional brine evaporation plant,    feeding the ultra pure salt to a membrane chlor alkali electrolytic cell plant to re-saturate depleted recycle brine, said plant including a secondary brine treating system, thereby substantially decreasing the ion exchange regeneration frequency requrements of said secondary brine treating system, whereby acidification, neutralization and resin costs are reduced.    
   
   
       2 . The method of  claim 1  wherein the ultra pure salt is fed to the membrane chlor alkali electrolytic cell plant to re-saturate acidified, chlorinated, depleted recycle brine thereby permitting the elimination of the membrane plant secondary brine treating system.  
   
   
       3 . The method of  claim 1  wherein the ultra pure salt is fed to the membrane chlor alkali electrolytic cell plant to re-saturate acidified, chlorinated, depleted recycle brine, and wherein the acidification acid is first employed to acid wash product salt, when the washing systems within the plant do not operate at sufficiently high wash ratios, thereby permitting the elimination of the membrane plant secondary brine treating system.  
   
   
       4 . The method of  claim 1  wherein the ultra pure salt fed to the membrane chlor alkali electrolytic cell plant contains 5 to 200 ppb calcium and other multivalent metal ions.  
   
   
       5 . The method of  claim 1  wherein the ultra pure salt fed to the membrane chlor alkali electrolytic cell plant is produced by evaporation of the stripped brine.  
   
   
       6 . The method of  claim 1  wherein the acidified brine is stripped by an inert gas prior to evaporation.  
   
   
       7 . The method of  claim 1  wherein the acidified brine is stripped by air stripping prior to evaporation.  
   
   
       8 . The method of  claim 1  wherein the acidified brine is stripped by vacuum stripping with nitrogen prior to evaporation.  
   
   
       9 . The method of  claim 1  wherein the acidified brine is stripped by vacuum steam stripping prior to evaporation.  
   
   
       10 . The method of  claim 1  wherein dilute or concentrated aqueous hydrochloric acid is employed in acidifying the primary treated brine.  
   
   
       11 . The method of  claim 1  wherein anhydrous hydrochloric acid is employed in acidifying the primary treated brine.  
   
   
       12 . The method of  claim 10  wherein the hydrochloric acid is introduced into the primary treated brine through a pipeline reactor.  
   
   
       13 . The method of  claim 11  wherein the hydrochloric acid is introduced into the primary treated brine through a pipeline reactor.  
   
   
       14 . The method of  claim 1  wherein dilute or concentrated liquid sodium hydroxide is employed to neutralize the stripped material.  
   
   
       15 . The method of  claim 1  wherein the stripped material is introduced into a water scrubber to reduce emissions of brine droplets.  
   
   
       16 . The method of  claim 1  wherein evaporator condensate is employed in washing the crystals.  
   
   
       17 . The method of  claim 1  wherein the primary treated brine has a pH of about 10 and is acidified to a pH of about 3.