Method for obtaining sodium carbonate crystals

Abstract

Method for producing sodium carbonate, according to which an aqueous sodium chloride solution ( 5 ) is electrolyzed in a membrane-type cell ( 1 ) from which an aqueous sodium hydroxide solution ( 9 ) is collected, and carbonated by direct contact with carbon dioxide ( 15 ) to form a slurry of crystals of anhydrous sodium carbonate ( 16 ).

1 . A method for producing sodium carbonate, comprising
 electrolyzing an aqueous sodium chloride solution in a membrane-type cell ( 1 ) with an ion permselective membrane in order to produce hydrogen ( 8 ), chlorine ( 7 ) and an aqueous solution ( 9 ) comprising sodium hydroxide, and   submitting the aqueous solution ( 9 ) comprising sodium hydroxide to a carbonation ( 2 ) wherein the carbonation ( 2 ) is carried out by directly contacting carbon dioxide ( 15 ) with the aqueous solution ( 9 ) comprising sodium hydroxide in a gas liquid contactor under conditions such as to cause the conversion of the aqueous solution ( 9 ) into an aqueous slurry ( 16 ) of anhydrous sodium carbonate crystals.   
   
   
       2 . The method according to  claim 1 , wherein the temperature inside the gas-liquid contactor is above 100° C. 
   
   
       3 . The method according to  claim 1 , wherein the direct contacting of the carbon dioxide ( 15 ) with the aqueous sodium hydroxide solution ( 9 ) in the gas liquid contactor is carried out by circulating said solution ( 9 ) in countercurrent to a gas containing the carbon dioxide in a carbonation tower consisting of a stack of at least two superimposed segments separated by a partition perforated with at least two openings, the segments comprising at least one transverse wall for causing circulation of the solution in said segment. 
   
   
       4 . The method according to  claim 1 , wherein the aqueous solution ( 9 ) comprising sodium hydroxide is essentially free of carbonate ions, bicarbonate ions or both, when directly contacted with the carbon dioxide. 
   
   
       5 . The method according to  claim 1 , wherein a chlorine derivative ( 39 ) is produced in a chlorine derivative production unit ( 13 ) from the chlorine ( 7 ) and wherein the carbonation ( 2 ) is effected at least partly using carbon dioxide ( 15 ) produced in the chlorine derivative production unit ( 13 ). 
   
   
       6 . The method according to  claim 1 , wherein the slurry ( 16 ) is evaporated in order to produce sodium carbonate crystals, which are separated, and a mother liquor. 
   
   
       7 . The method according to  claim 6 , wherein, to evaporate the slurry ( 16 ), said slurry ( 16 ) is treated in an evaporator-crystallizer ( 3 ). 
   
   
       8 . The method according to  claim 7 , wherein the evaporator-crystallizer ( 3 ) comprises a multistage evaporator or a mechanical vapour recompression evaporator. 
   
   
       9 . The method according to  claim 1 , wherein the electrolysis in the cell ( 1 ) is regulated so that the aqueous sodium hydroxide solution ( 9 ) comprises about 32% by weight of sodium hydroxide. 
   
   
       10 . The method according to  claim 7 , wherein the operating conditions in the evaporator-crystallizer ( 3 ) are regulated so that the sodium carbonate crystals resulting from the evaporation of the slurry ( 16 ) are crystals of sodium carbonate monohydrate. 
   
   
       11 . The method according to  claim 1 , wherein at least part of the carbon dioxide ( 15 ) used is obtained by attacking limestone ( 14 ) with an aqueous hydrochloric acid solution ( 12 ) obtained by absorbing, in water ( 11 ), hydrogen chloride obtained by reacting ( 10 ) chlorine ( 7 ) with hydrogen ( 8 ) produced in the membrane-type cell ( 1 ). 
   
   
       12 . The method according to  claim 1 , wherein at least part of the chlorine ( 7 ) is utilized in a vinyl chloride monomer production plant producing hydrochloric acid, at least a part of the produced hydrochloric acid being reacted with limestone ( 14 ) to produce at least a part of the carbon dioxide ( 15 ) used. 
   
   
       13 . The method according to  claim 1 , wherein at least part of the carbon dioxide ( 15  used is a flue gas from a thermal installation for cogeneration of heat and power. 
   
   
       14 . The method according to  claim 1 , wherein the membrane-type cell ( 1 ) comprises at least one anode chamber, and wherein a dilute brine ( 29 ) is collected from the membrane-type cell ( 1 ) and recycled in the anode chamber of the cell ( 1 ), after having purified and concentrated it with sodium chloride. 
   
   
       15 . The method according to  claim 14 , wherein the dilute brine ( 29 ) is concentrated by circulating it in a rock salt deposit ( 22 ).