Process for the recovery of anhydrous hydrogen fluoride from aqueous solutions of fluosilicic acid and hydrogen fluoride

A process for recovering anhydrous hydrogen fluoride from a dilute solution comprising hydrogen fluoride, fluosilicic acid and water, wherein the percentage composition of said solution is such that anhydrous hydrogen fluoride cannot be recovered therefrom by distillation, said process comprising the steps of:

• (a) introducing said dilute solution into a dilute compartment of at least one unit cell of an electrodialysis assembly comprising one or more unit cells positioned between an anolyte compartment containing an anode immersed in an anolyte, and a catholyte compartment containing a cathode immersed in a catholyte, each of said unit cells comprising a concentrate compartment and a dilute compartment separated by an anion permeable membrane, and each bounded by a face of a cation permeable membrane;
• (b) introducing into said concentrate compartment an aqueous solution of hydrogen fluoride and fluosilicic acid wherein the concentration of fluosilicic acid in said solution is greater than about 5 weight percent, and wherein the total acids concentration in said solution is such that hydrogen fluoride cannot be obtained therefrom by distillation;
• (c) passing direct current through said electrodialysis assembly for a time sufficient to increase the concentration of hydrogen fluoride in said concentrate compartment forming a product solution in which the total acids concentration is such that anhydrous hydrogen fluoride can be recovered therefrom by distillation;
• (d) withdrawing said product solution from said concentrate compartment;
• and
• (e) recovering anhydrous hydrogen fluoride from said solution of step (d) by distillation.