oi water and subsequent cooling, 25.85 gins. of Water will contain at room temperature 6.07% of NaF, which corresponds to a yield of 63.15% sodium fluoride. UNITED STATES PATENTS Bishop .Q -QJune 21, 192i Stevenson June 28, 1927 Horn Dec. 23, I958 FOREIGN PATENTS France a July l6, 19l8 France t. July 22, 1924 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. This agent may also inhibit acid production by commensal oral bacteria. Even so, I can obtain substantially quantitative recovery of fluorine. The fluoride enhances the strength of teeth by the formation of fluorapatite, a naturally occurring component of tooth enamel. In the acidulation of phosphate rock to produce superphosphate fertilizer, the fluorine contained in the rock (of the order of 3%) is liberated as SiF If released to the atmosphere, this SiF constitutes a hazard to persons in the immediate area and a nuisance to persons living considerable distances from the site of operations. Formerly, this acid was neutralized and discarded. The method of obtaining utilizable values from gases containing SiF comprising the steps of absorbing the SiF in water to form a 20-30% H SiF solution, filtering said solution to remove insoluble constituents, adding to the filtered solution suflicient concentrated KC] solution to precipitate the fluorine as K SiF thereby forming a slurry of said K SiF in water, heating the resulting slurry for suflicient time to drive ofi the HCl formed by the reaction of H SiF with KCl, adding to the remaining slurry suificient concentrated NaOH solution to adjust the pH to a value of 8-9, thereby converting the K SiF to NaF solids in a solution of potassium silicate, filtering to remove the solid NaF and washing said solid free of adhering mother liquor, heating the filtrate to concentrate it to the point of incipient saturation with NaF, cooling said solution to crystallize additional NaF, filtering to remove said NaF and washing it free of mother liquor, and repeating the evaporation and filtration steps until a final desired silicate concentration is obtained. At the endof this step, the recovery of sodium fluoride would amount to 88.88% and the composition of the remaining mother liquor approximately 84.6% potassium silicate and 15.3 sodium fluoride. Such a slurry would be a semi-solid mass and could hardly be washed free of impurities. For these reasons, a single concentrating step cannot produce the results obtained by the present step-wise sodium fluoride removal. Adjusting the concentration of this mother liquor by the addition of water reduces the viscosity of the solution but, at the same time, it reduces the size of the crystals already formed and makes filtration even more diificult. The method of obtaining substantially pure NaF and concentrated potassium silicate and hydrochloric acid solutions from fluosilicic acid solution comprising the steps of adding to a concentrated solution of fluosilicic acid suflicient KCl in concentrated form to yield a slurry of K SiF in strong HCl solution, heating the slurry to drive otf HCl, recovering the HCl by condensing the vapors, adding to the remaining K SiF slurry suflicient NaOH to precipitate the fluorine as NaF in potassium silicate solution, separating the solids from the liquid, washing said solids free of adhering liquid, heating said liquid to evaporate water and concentrate said solution to the point of saturation with NaF, cooling said solution to crystallize NaF, separating the solid and washing it free of adhering mother liquid, and repeating the concentrating, cooling and filtering steps until a desired final solution concentration is obtained. Even if the evaporation could be carried out to this concentration, the solution would not be readily filtered and the sodium fluoride crystals would be extremely difiicult to wash. By the series of step-wise evaporation and filtration steps, impure crystals from the most concentrated silicate solution are given a chance to recrystallize in a less concentrated solution. This slurry is evaporated to dryness by removal of 184.8 parts of water and 40.09 parts of HCl. evaporate 1440 gms. Such dilute acid is not only uneconomical to recover but is practically worthless for industrial uses. The method of forming NaF and K SiO solution in high yield and purity comprising providing a system including a plurality of mixing, concentrating, cooling and filtering steps in sequence, introducing seriatim increments of K SiF and NaOH reactants into the system and moving said increments therethrough, said method comprising the steps of: (1) mixing K SiF with a stoichiometric quantity of NaOH in aqueous medium to form a slurry of NaF in K SiO solution; (2) admixing therewith the solid NaF recovered from step 8 below; (3) filtering to remove the solid NaF; (4) washing said solid free of mother liquor, thereby obtaining highly pure NaF; (5) adding to the mother liquor the washings from step 4 above and the solid NaF from step 12 below; (6) evaporating therefrom at least a portion of the water; (7) cooling the solution to crystallize additional NaF; (8) filtering to recover the solid NaF and adding said solid NaF to the slurry formed in step 1 above; (9) mixing with the filtrate from step 8 the solid NaF from step 16 below; (10) evaporating at least a portion of the water from the filtrate; (11) cooling the concentrated filtrate to crystallize additional NaF; (12) filtering the cooled concentrate to remove solid NaF and adding it back to the mixed washings and filtrate in step 5 above; (13) mixi-ng w'tih the filtrate from step 12 the solid from step subsequent to 16 wherein said solid is separated from the evaporated solution of K Si0 (14) evaporating more Water from the slurry to further concentrate it with respect to potassium silicate; (15) cooling the concentrate to crystallize additional NaF; (16) filtering the cooled concentrate and returning the solid NaF to the system at step 9 above; and repeating the sequence defined by steps 9-l6 inclusive until the solution reaches the desired concentration with respect to K SiO and finally removing the concentrated potassium silicate from the system as product. 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