Method for making anhydrous sodium perchlorate

Abstract

The invention concerns a method for making anhydrous sodium perchlorate whereby an aqueous sodium perchlorate solution, directly derived from electrolysis of an aqueous sodium chlorate solution, is subjected to vacuum evaporation. The invention also concerns anhydrous sodium perchlorate crystals which preserve good pourability and methods for making them.

1 . Process for the manufacture of anhydrous sodium perchlorate, according to which an aqueous sodium perchlorate solution, originating directly from an electrolysis stage or from a succession of individual electrolytic stages on an aqueous chlorate solution of the said metal, is subjected to vacuum evaporation at a temperature of between 52 and 75° C., preferably of between 60 and 70° C.  
     
     
         2 . Process according to  claim 1 , characterized in that the absolute pressure is between 1500 and 7000 Pa.  
     
     
         3 . Process according to  claim 1  or  2 , characterized in that the aqueous sodium perchlorate solution is chosen so that the anhydrous sodium perchlorate can be isolated directly by crystallization.  
     
     
         4 . Process according to  claim 3 , characterized in that the composition of the aqueous sodium perchlorate solution lies within the region of the NaClO 4 —NaClO 3 —H 2 O ternary diagram delimited by the points: 
 A: 58 g of NaClO 3 , 270 g of NaClO 4    
 B: 87 g of NaClO 3 , 295 g of NaClO 4    
 C: 280 g of NaClO 4    
 D: 300 g of NaClO 4  per 100 g of water.  
 
     
     
         5 . Process according to any one of  claims 1  to  4 , characterized in that, on conclusion of the evaporation stage, the suspension of the anhydrous sodium perchlorate crystals is filtered and then the crystals are optionally washed with water.  
     
     
         6 . Process according to  claim 5 , characterized in that the temperature of the aqueous wash liquor is between 55 and 80° C., preferably between 55 and 65° C.  
     
     
         7 . Process according to  claim 5  or  6 , characterized in that the filtered anhydrous sodium perchlorate crystals are subsequently subjected to a drying stage until a residual water content of less than 0.1% by weight and preferably of less than or equal to 0.05% by weight is obtained.  
     
     
         8 . Process according to  claim 5  or  6 , characterized in that a sufficient amount of finely divided silica is added to the filtered anhydrous sodium perchlorate crystals.  
     
     
         9 . Process according to  claim 8 , characterized in that the amount of silica added is between 0.05 and 0.5% by weight with respect to the anhydrous sodium perchlorate crystals.  
     
     
         10 . Process according to  claim 8  or  9 , characterized in that the silica is a hydrophobic silica.  
     
     
         11 . Process according to any one of  claims 8  to  10 , characterized in that the specific surface area of the silica is between 100 and 300 m 2 /g.  
     
     
         12 . Anhydrous sodium perchlorate crystals which are capable of being obtained according to any one of  claims 7  to  11 .