Process for treating sodium silico fluoride

In the process of reacting sodium silicofluoride having a particle size of less than 250 microns and ammonia in an aqueous medium to produce silica, sodium fluoride and ammonium fluoride, the improvement comprising:

• a. adding ammonia, in the form of a member selected from the group consisting of free ammonia gas and aqueous ammonium hydroxide having a concentration, calculated as free ammonia, of at least about 7.5 weight percent, at a rate such that the concentration of ammonium hydroxide in the reaction medium is less than about 5 weight percent over a period of at least about 60 minutes, until substantially all the sodium silicofluoride is reacted:
• b. agitating to a degree greater than that required to disperse sodium fluoride crystals and silica gel of said reaction medium produced in the course of said addition of ammonia and for a period of at least about 20 minutes after completion of said addition of ammonia:
• c. after completion of the reaction, adding a stoichiometric excess of said ammonia in an amount such that the total ammonium ion concentration consisting of ammonium fluoride and ammonium hydroxide in the reaction medium is more than 7.5 weight percent, calculated as free ammonia;
• and
• d. separating sodium fluoride crystals having a particle size of more than 100 microns and silica gel having a particle size of less than 5 microns thus prepared separately from ammonium fluoride solution.