Process for treating sodium silico fluoride
This is a continuation, of application Ser. No. 39,965, filed May 25, 1970, now abandoned. 
    
    
     The invention resides in the decomposition of sodium silico fluoride by means of a relatively high concentration of an aqueous ammonium solution or an ammonia gas. 
     The object of this invention resides in manufacturing pure sodium fluoride and silica gels as solid materials and an aqueous ammonium fluoride solution in high yield. 
     The other object of this invention resides in manufacturing pure sodium fluoride in a large crystalline form and a fine powder of silica gels, and separating them from an aqueous slurry solution of ammonium fluoride, respectively, in high efficiency by means of physical methods. 
     The another object of this invention resides in manufacturing a large crystal of pure sodium fluoride not bonding silica and silica gels having good dispersability in high yields and at same time. 
     The further object of this invention will be understood from the following descriptions. 
     Generally, sodium silico fluoride has been manufactured by treating silico fluoric acid produced in phosporic acid manufacturing industry, especially, in the manufacture of calcium superphosphate or phosphoric acid with an aqueous solution of sodium hydroxide. However, most of silico fluoric acid produced as by-product in the phosphoric acid manufacturing industry has been discarded in the form of waste gas or waste sludge of said industry, since an economical utilization of silico fluoric acid has not been found. Otherwise, the phosphoric acid fertilizer has been manufactured from the phosphoric acid containing silico fluoric acid without recovering it. 
     It is well known the fact that sodium silico fluoride, silica gels and ammonium fluoride are produced when sodium silico fluoride reacts with ammonia as depicted by Equation (I) 
     
         na.sub.2 SiF.sub.6 +4NH.sub.4 OH = 2NaF + SiO.sub.2 + 4NH.sub.4 F + 2H.sub.2 O 
    
     however, in ordinary process, said reaction has been accomplished by using a dilute aqueous ammonium solution in order to easily separate silica gels produced from the mixed solution of sodium fluoride and ammonium fluoride. It has been only studied to use the aqueous solution of sodium fluoride and ammonium fluoride for the manufacture of cryolite. 
     In order to find the economical use of sodium silico fluoride, many attempts have already been investigated and the most economical process has been found. This invention relates to a process characterized by decomposing sodium silico fluoride with a relatively high concentration of an aqueous ammonium solution or an ammonia gas while agitating, whereby forming the mixed aqueous slurry solution of ammonium fluoride containing sodium fluoride and silica gels as solid materials and then separating the decomposed products to each component thereof by means of physical methods. 
     The process mentioned above has been further developed to the improved process in which an aqueous solution of sodium hydroxide or sodium salt, namely, sodium sulfate or sodium chloride is added to said mixed slurry solution of ammonium fluoride containing sodium fluoride and silica gels while violently agitating to decompose ammonium fluoride to either sodium fluoride and ammonia or sodium fluoride and ammonium salt and then the mixed slurry solution obtained is treated to separate sodium fluoride, silica gels and ammonia or ammonium salt, respectively by means of physical methods. 
     The invention will be explained by the following examples and attached drawings. 
     The decomposition of sodium silico fluoride by an ammonium solution is shown as follows: 
     
         Na.sub.2 SiF.sub.6 (solid) + 4NH.sub.4 OH(liquid) → 2NaF (solid) + SiO.sub.2 (solid) + 4NH.sub.4 F (liquid) + 2H.sub.2 O (liquid) 
    
     Since the solubility of sodium fluoride to the aqueous solution is relatively high, the concentrated aqueous ammonia solution or ammonia gas is preferable to be used in said reaction to recover sodium fluoride from the reaction mixture as crystal in good yield. However, it makes difficult to separate sodium fluoride from silica gels owing to forming a very fine crystal of sodium fluoride or a sodium fluoride crystal bonding silica on its surface. 
     In order to overcome the disadvantages, the interrelationship between the concentration of ammonium ions, namely total ammonium ions concentration coming from ammonium fluoride and free ammonia in the reaction medium and the solubility of sodium fluoride to the reaction medium has been examined. As shown in Table 1, when the total ammonium ions concentration in the reaction medium is high, namely the amount of ammonia used is that more than the stoichiometrical amount of ammonia necessary to decompose sodium silico fluoride and the concentration of an aqueous ammonium solution used is also relatively high, it has been found that the solubility of sodium fluoride in the reaction medium becomes small and also that to enhance the yield of sodium fluoride, the total ammonia concentration of the reaction medium is necessary to be more than 7.5% calculated on the weight basis of NH 3 . Therefore, it is preferable to use the relative high concentration of an aqueous ammonium solution or an ammonia gas in the amount more than the stoichiometrical one of ammonia necessary to decompose sodium silico fluoride used. When an ammonium solution is used, the concentration of the aqueous ammonia solution is 18 ˜ 50% as calculated on the weight basis of NH 4  OH. 
     In this invention it is preferable to use sodium silico fluoride having the particle size of less than 250 μ in order to carry out the decomposition smoothly. 
     
                       Table 1                                                     
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Concentration                                                             
          Concentration                                                   
                       Concentration                                      
                                   Solubility                             
(wt%) of ammo-                                                            
          (wt%) of free                                                   
                       (wt%) of total                                     
                                   (25°C) of                       
nium fluoride                                                             
          ammonia      ammonia     sodium                                 
in the reaction                                                           
          (NH.sub.4 OH) in the                                            
                       (NH.sub.3) in the                                  
                                   fluoride                               
mixture   reaction mix-                                                   
                       reaction mix-                                      
                                   g/100g                                 
          ture         ture                                               
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0         0            0           0                                      
10        0            4.6         1.4                                    
          5            7.0         1.0                                    
          10           9.5         0.8                                    
15        0            6.9         0.9                                    
          5            9.5         0.7                                    
          10           11.8        0.4                                    
30        5            16.2        0.4                                    
          10           18.7        0.2                                    
          20           23.5        1.1                                    
50        0            22.9        0.2                                    
          10           27.8        0.1                                    
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     The process of this invention may be accomplished by means of a batch process in which sodium silico fluoride is added to a relatively high concentration of an aqueous ammonium solution, or a relatively high concentration of an aqueous ammonium solution or an ammonia gas is introduced into the aqueous slurry solution of sodium silico fluoride. Said process may also be accomplished by means of the continuous process in which the relatively high concentration of the aqueous ammonium solution or the ammonia gas and the aqueous slurry solution of sodium silico fluoride, or the relatively high concentration of the aqueous ammonium solution and sodium silico fluoride powder are simultaneously added. 
     According to the reaction condition, a sodium fluoride crystal bonding silica on its surface, a very fine crystal of sodium fluoride or hydrated silica gels coagulated may be formed and it may result in difficult to separate sodium fluoride and hydrated silica gels in high yield without any depression of purity thereof by the physical method. In this case, the separated silica gels includes sodium fluoride solution, and said sodium fluoride can be separated from silica gels by washing it with water, and the separated sodium fluoride includes silica gels and said silica gels can be eliminated with sodium hydroxide to rise the purity of silica gels or sodium fluoride, but, said process are not only troublesome but also uneconomical one. 
     Thus, it has been found by us to manufacture large crystals of sodium fluoride and fine powder of silica gels in high yield by decomposing sodium silico fluoride with the relatively high concentration of the aqueous ammonium solution or the ammonia gas while violently agitating in such reaction conditions that the total concentration of ammonium ions in the reaction medium reaches to more than 7.5% on the weight basis of NH 3  after the decomposition. 
     To obtain sodium fluoride and silica gels having the highest purity many attempts have been effected and led to the following results. 
     1. An aqueous slurry solution of sodium silico fluoride is decomposed with a high concentrated aqueous ammonium solution or an ammonia gas. 
     2. An aqueous ammonium solution or an ammonia gas is added slowly to the aqueous slurry solution containing sodium silico fluoride. 
     3. An agitating force to be used is quite enough to disperse the solid materials contained in the aqueous slurry solution. 
     The experimental results are explained as follows: 
     1. The influence of the order of adding the reactants on the purity of the products obtained. 
     a. Experiment No. 1 
     Three kg of sodium silico fluoride powders having a particle size of 100 μ were introduced in predetermined portions over 90 minutes into 10 l of a 25 wt. % aqueous ammonium solution (about 12.5 wt. % calculated on the basis of NH 3 ) filled into the cylindrical type reaction vessel having the capacity of 20 l and heated to 60°C, while agitating at the speed of 400 r.p.m. by means of a propeller type agitator. After charging, the reaction was continued for 30 minutes to complete the decomposition while agitating. Total concentration of ammonium ions in the solution mixture reached to 12.5% calculated on the weight basis of NH 3 . 
     b. Experiment No. 2 
     Six l. of 42% aqueous ammonium solution were introduced into the aqueous slurry solution comprising 3.0 kg of sodium silico fluoride having the particle size of 100 and l of water filled in said reactor and heated to 60°C, in the predetermined portions over 90 minutes while violently agitating. After charging, the reaction was continued for 30 minutes to complete the decomposition while agitating. The total concentration of ammonium ions in the reaction mixture reached to 12.5% calculated on the weight basis of NH 3 . In two experiments, resulted sodium fluoride and silica gels were separated from the reaction mixture by means of a levigation apparatus at the same condition. Sodium fluoride and silica gels obtained were analysed to determine the silica content in sodium fluoride and the sodium fluoride content in silica gels respectively. 
     The results obtained are shown as follows: 
     
                       Table 2                                                     
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          SiO.sub.2 % in NaF                                              
                      NaF% in SiO.sub.2                                   
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Exp. No. 1  15.6          10.6                                            
Exp. No. 2  0.9           1.5                                             
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     It has been considered from the result of the experiment No. 1 that the concentration of free ammonia in the reaction medium has influence on the formation of the products especially silica gels and also that silica gels produced are not only insufficient to disperse one and one through the reaction medium, but also bonds to sodium fluoride produced with resultant of falling the purity of sodium fluoride since a high concentration of free ammonia in the medium at the beginning of the reaction gradually reduces to a low concentration in the lapse of the reaction. While it has been considered from the result of the experiment No. 2 that since the concentration of free ammonia in the reaction medium always maintains low while the reaction is proceeding and silica gels disperse one and one through the reaction medium, silica gels produced does not bond to the sodium fluoride crystals at once resulting in the formation of silica gels. 
     2. The influence of the rate of adding ammonia on the purity of the products obtained. 
     Seven kg of a 45% aqueous ammonium solution were added continuously to an aqueous slurry solution containing 3.0 kg of sodium fluoride and 3 l of water at 45°C while violently agitating at the rate as shown in the table 3. The agitating was continued until the decomposition reaction was completed. The reaction mixture was separated to the aqueous slurry solution containing sodium fluoride and ammonium fluoride and the aqueous slurry solution containing silica gels and ammonium fluoride by means of the levigation apparatus. The reaction solid products, namely sodium fluoride and silica gels were filtered from each slurry solution and dried to obtain the final products, respectively. The purity of the products is shown in Table 3. 
     
                       Table 3                                                     
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The time required                                                         
            The retaining time                                            
                          SiO.sub.2 %                                     
                                   NaF%                                   
to adding an                                                              
            after adding an                                               
aqueous ammonium                                                          
            aqueous ammonium                                              
                          in NaF   in SiO.sub.2                           
solution    solution                                                      
(minutes)   (minutes)                                                     
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15          60            12.6     15.4                                   
30          60            10.8     16.1                                   
60          30            1.3      5.5                                    
120         30            0.5      1.5                                    
180         20            0.2      0.9                                    
240         20            0.4      2.0                                    
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     From the experimental results, it has been found that if the concentration of aqueous ammonium solution used is about 45% by weight and the rate of adding said solution to the aqueous slurry is more than 60 minutes, the satisfactory results may be obtained. In the process of this invention, as the time required to adding the aqueous ammonium solution into the slurry solution depends on the concentration of ammonia, it is to be noted that if the concentration of the aqueous ammonium solution is higher than 45% weight, the time required to adding into the aqueous ammonium solution takes long. When the ammonia gas instead of the aqueous solution thereof is used, it is necessary to pay their attention so as to maintain the concentration of free ammonia contained in the reaction medium to the low range. The concentration of free ammonia in the reaction medium is preferable to be less than 5 wt.%. 
     3. The influence of the agitating power on the purity of the products obtained. Six point three kg of a 40% aqueous ammonium solution were continuously added to the aqueous slurry solution containing 2.5 kg of sodium silico fluoride and 3.5 kg of water filled in the cylindrical reactor at 43°C while agitating by means of the propeller type agitator over 120 minutes periods and the reaction was continued until the reaction was completed. After the reaction, the reaction mixture was separated to the aqueous slurry solution of sodium fluoride and ammonium fluoride and the aqueous slurry solution of silica gels and ammonium fluoride. The reaction solid products, namely sodium fluoride and silica gels were filtered from each of said slurry solution and dried to the final products, respectively. The purity of the products is shown in the table 4. 
     
                       TABLE 4                                                     
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Revolution of                                                             
            SiO.sub.2 wt% NaF wt                                          
the agitator                                                              
            in NaF        in SiO.sub.2                                    
(r.p.m.)                                                                  
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 50 - 100   5.4           10.6                                            
100 - 200   2.1           9.8                                             
300 - 400   0.3           1.5                                             
600 - 700   0.2           0.5                                             
1000 - 1500 0.2           0.7                                             
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     From the experimental results as shown in table 4, it has been found that if revolution of the agitator is more than 300 r.p.m., the purity of sodium fluoride and silica gels may be considerably improved. As the agitation effect may vary by virture of the scale of the reactor used, the shape thereof, the type of agitating, the shape of propeller and the size thereof, the effect of agitation is not able to be estimated on the basis of the revolution of the agitator. At any way, the agitating power must be more than enough to disperse the solid materials contained in the aqueous slurry through the reaction medium. 
     The slurry solution of sodium silico fluoride used in the process of this invention may be not only aqueous slurry solution but also aqueous slurry solution of ammonium fluoride. 
     The water content of the aqueous slurry solution may be determined according to the concentration of an aqueous ammonia solution or an ammonia gas to be used, but it is preferable to be about 40 ˜ 90 wt % on the basis of the aqueous slurry solution. 
     The concentration of the aqueous ammonium solution to be used is suitable to be more than 15% calculated on the weight basis of NH 3  and the total concentration of ammonium ions in the reaction mixture is more than 7.5% calculated on the weight basis of NH 3  when the reaction was completed. 
     When sodium silico fluoride is directly treated with the aqueous ammonia solution, the concentration of the ammonium solution to be used is suitable to be more than 7.5% calculated on the weight basis of NH 3 . 
     It is preferable that the mole ratio of ammonia to sodium silico fluoride is more than 4:1. It is not preferable that said mole ratio is more than 8:1. In this case, the solubility of sodium fluoride in the reaction medium increases as the volume of the reaction medium increases in quantity and the yield of sodium fluoride formed decreases. 
     The reaction temperature is not necessary to define, but if the reaction temperature is too high, the solubility of sodium fluoride as well as silica to the reaction medium will increase, resulting the depression of the purity of ammonium fluoride solution and the yield of sodium fluoride and silica gels. Therefore, the reaction temperature is suitable to the less than 70°C. The agitating power is necessary to be more than the agitating power capable of dispersing the solid materials contained in the aqueous slurry through the aqueous slurry solution. 
     When ammonium fluoride in the aqueous slurry solution produced by the decomposition of sodium silico fluoride with ammonia is decomposed with sodium hydroxide or sodium salts to produce a sodium fluoride and ammonium solution or a ammonium salt solution. Sodium hydroxide or sodium salts to be used amounts to the stoichiometrical amount of ammonium fluoride thereof or a slight less amount than that of sodium hydroxide, and sodium chloride and sodium sulfate are used at the concentration of more than 10, 10 and 20 wt.%, respectively, or their solid states. The reaction conditions, namely agitating, the rate of adding etc. are same one as those of ammonia. 
     According to this invention, the products comprising sodium fluoride having large mono crystal more than 100 μ , silica gels having good dispersability as well as the particle size less than 5 μ and the aqueous ammonium solution are not only manufactured, but also the products comprising sodium fluoride not bonding silica and silica gels less containing sodium fluoride are produced by selecting the reaction conditions. The products manufactured according to the process of this invention are able to easily separated by means of the levigation or sedimentation apparatus. 
     When the decomposition reaction of ammonium fluoride with sodium hydroxide or sodium salts is carried out in the aqueous slurry solution of ammonium fluoride comprising silica and sodium fluoride crystals, more larger crystals of sodium fluoride may be produced by the aid of the crystal kern of sodium fluoride already existing in the said slurry solution. 
    
    
     The process of this invention will be explained by means of the attached drawings. 
     FIGS. 1, 2 and 3 show flow sheets of the process of this invention. The following examples and attached drawings will not limit the scope of this invention. 
    
    
     In FIG. 1, a sodium silico fluoride or its aqueous slurry solution and an aqueous ammonium solution or ammonia gas are continuously charged into the decomposition tank and decomposed while agitating, in other way, aqueous ammonium solution may be charged into the decomposition tank filled with an aqueous slurry solution of sodium silico fluoride while agitating, and sodium silico fluoride is decomposed to sodium fluoride, silica gels and ammonium fluoride. The aqueous slurry solution of ammonium fluoride produced comprising silica gels and sodium fluoride as solid materials is led to the middle portion of the levigation apparatus. From the neighborhood of the bottom of said apparatus, the aqueous ammonium fluoride solution obtained by the following procedure is introduced to form the upstream flow within said apparatus whereby silica gels contained in the aqueous slurry solution transfer upwardly and sodium fluoride crystals descend downwardly, thus, silica gels are discharged out from the top of the levigation apparatus in the form of the aqueous ammonium solution by means of the filter apparatus. Silica gels are washed with water and dried to make the final product. A part of the filtrate obtained is recycled to the levigation apparatus and the residual part thereof is used for various purposes. 
     In FIG. 2, a sodium silico fluoride or its aqueous slurry solution is decomposed with an aqueous ammonium solution or an ammonia gas in the decomposition tank as shown in FIG. 1. The aqueous slurry solution of ammonium fluoride produced comprising silica gels and sodium fluoride as solid materials is led to the middle portion of the levigation apparatus and silica gels are discharged out from the top of the levigation apparatus in the form of the aqueous slurry solution, filtered, washed with water and dried to make the final product as shown in FIG. 1. While sodium fluoride crystals precipitated in the bottom of said apparatus are drawn out in the form of the aqueous slurry solution, and separated from the aqueous ammonium fluoride solution by another filter apparatus, are washed with water and dried to make the final product. The filtrate obtained from the slurry solutions drawn out of the top and the bottom is gathered together, a part of the filtrate is recycled to the levigation apparatus and the residual part of the filtrate is used for various purposes. 
     In the FIG. 3, a sodium silico fluoride or its aqueous slurry solution is decomposed with an aqueous ammonium solution or an ammonia gas in the decomposition tank as shown in FIG. 1. The aqueous slurry solution of ammonium fluoride comprising silica gels and sodium fluoride produced is filtered by the filter apparatus and the mixture of silica gels and sodium fluoride is separated. After said mixture is washed with water and dried, said mixture is separated to sodium fluoride and silica gels by means of the cyclone type separator. 
     Since sodium fluoride obtained according to the process of this invention is high purity, sodium fluoride may be used in the industrial starting material, preservations for wood, fungicides and medical agents, while as silica gels obtained are high purity and fine particle size, they are used as a fuller of gum and synthetic resins and a drying agent. The aqueous ammonium fluoride solution may be used as the starting material for producing sodium fluoride, cryolite and the like, or may be treated to crystalline out and recover ammonium fluoride to use as an industrial chemical reagent. 
     In conclusion, sodium fluoride, silica gels and ammonium fluoride having wide utilization are produced by means of a simple process in high yield and high purity according to the process of this invention. The process of this invention has very important values in the industrial point of view. 
     EXAMPLE 1. 
     Four kg/hour of a 20% aqueous ammonium solution (calculated on the weight basis of NH 4  OH) were continuously charged to the decomposition tank together with 1.0 kg/hour of sodium silico fluoride in an aqueous slurry solution form of the concentration of 50 wt %. The decomposition reaction was carried out at 60°C while agitating at the revolution of about 60 r.p.m. by means of the propeller type agitator. The mean retention time in the reactor was about 3 hours. The aqueous slurry solution discharged from the reaction was introduced in the middle portion of the levigation tower. The aqueous ammonium fluoride solution produced from the following procedure was introduced into the levigation apparatus from the bottom thereof to form the upstream flow of about 20 cm per minute. The aqueous slurry solution of ammonium fluoride containing silica gels was drawn out from the top of said apparatus, while the aqueous slurry solution containing sodium fluoride was drawn out from the bottom of said apparatus. Sodium fluoride and silica gels contained in each aqueous slurry solution were filtered, washed with water and dried to recover sodium fluoride having a purity of better than 97% at the rate of 430 g/hour and silica gels having a purity of 99% at the rate of 320 g/hour respectively. 5.1 kg/hour of the total aqueous solution of amonium fluoride were recovered in the form of filtrate. The filtrate recovered comprised 14.8 wt % of ammonium fluoride and 0.4 wt % of sodium fluoride. 
     EXAMPLE 2 
     Four kg/hour of a 50% aqueous ammonium solution (calculated on the weight basis of NH 4  OH) were continuously charged to the decomposition tank together with 2.0 kg/hour of sodium silico fluoride. The decomposition reaction was carried out at 90°C while agitating at the revolution of about 120 r.p.m. by means of the propeller type agitator. The mean retention time in the reactor was about an hour. The aqueous slurry solution discharged from the reactor was allowed to settle for 12 hours while cooling to 20°C and then separated to the aqueous slurry solution of ammonium fluoride containing silica gels and the aqueous slurry solution of ammonium fluoride containing sodium fluoride as shown in the example 1. The upstream flow rate in the levigation apparatus was 15 cm per minute. Sodium fluoride and silica gels were filtered from the slurry solution and washed with water, dried to recover silica gels having a purity of better than 99% at the rate of 640 g/hour and sodium fluoride having a purity of 98% at the rate of 890 g/hour respectively. The total filtrate of ammonium fluoride amounted to 4.4 kgs. said filtrate comprised 35.0 wt % of ammonium fluoride and 0.1 wt % of sodium fluoride. 
     EXAMPLE 3 
     One hundred kg of sodium silico fluoride having a purity of 98% and 100 l of water were charged in the cylindrical type reactor having the capacity of 600 l to form an aqueous slurry solution thereof. After the aqueous slurry solution was heated to 60°C while agitating at the revolution of 400 r.p.m. by means of the agitator of the propeller type, 200 l of a 45 wt % aqueous ammonium solution (about 22.5% calculated on the weight basis of NH 3 ) were added to the slurry solution in the predetermined portion over 90  minutes. After the reaction, the mixed slurry solution obtained containing sodium fluoride and silica gels was cooled to 30°C. Sodium fluoride and silica gels contained in the slurry solution were separated by means of the sedimentation. Sodium fluoride crystals contained in the aqueous ammonium fluoride slurry solution were separated, filtered, washed with water and dried to recover 41 kg of sodium fluoride having a purity of better than 99 wt % and the crystalline size of 100 ˜ 500 μ and 31 kg of silica gels (containing 5% H 2  O) having an purity of 100% and the particle size of less than 3 μ respectively. The volume of aqueous ammonium fluoride solution obtained was 820 1 and its concentration was 22%. 
     EXAMPLE 4 
     One hundred kg of sodium silico fluoride having a purity of 98% were charged to the reactor of the cylindrical type. After 300 l of water were added to said reactor, the slurry solution was heated to 55°C while violently agitating at the revolution of 500 r.p.m. by means of the agitator of the propeller type, and 45 kg of an ammonia gas were introduced into said solution in the predetermined portions over 90 minutes to carry out the reaction. After the reaction, 175 kg of a 48% sodium hydroxide solution were added to said slurry solution in the predetermined portions to decompose ammonium fluoride. After the reaction, the aqueous slurry solution was cooled to 30°C and introduced into the levigation apparatus to separate the aqueous ammonium solution containing sodium fluoride crystals and the aqueous ammonium solution containing silica gels. Each of said aqueous slurry solutions was filtered, and sodium fluoride crystals and silica gels obtained were washed with water and dried to recover 120 kg of sodium fluoride crystals having the crystalline size of 100 ˜ 500 and a purity of better than 99% and 33 kg of silica gels (containing 7% H 2  O) having a purity of 100% and the particle size of less than 3 μ respectively. The volume of the aqueous ammonium solution obtained was 450 1 and its concentration was 16 wt % (calculated on the basis of NH 4  OH). 
     EXAMPLE 5 
     The aqueous slurry solution of ammonium fluoride containing sodium fluoride and silica gels produced at Example 1 was introduced in the filter apparatus. The mixture of sodium fluoride and silica gels was separated, washed with water and dried. The obtained mixture was then introduced in the cyclon type separator. Sodium fluoride having a purity of better than 98% at the rate of 430 g/hour and silica gels having a purity of 99% at the rate of 320 g/hour were obtained and 5.1 kg/hour of the total aqueous solution of ammonium fluoride was recovered. 
     EXAMPLE 6 
     One hundred kg of sodium silico fluoride having a purity of 98% were charged to the reactor of the cylindrical type. After 200 l of water were added to the reactor, the slurry solution was heated to 55°C while violently agitating at the revolution of 500 r.p.m. by means of the agitator of the propeller type, and 45 kg of an ammonia gas were introduced into said solution in the predetermined portions over 90 minutes to carry out the reaction. After the reaction, 175 kg of a 30% sodium chloride solution were added to said slurry solution in the predetermined portions to decompose ammonium fluoride. After the reaction, the aqueous slurry solution was cooled to 30°C and introduced into the levigation apparatus to separate the aqueous slurry solution containing sodium fluoride crystals and the aqueous ammonium chloride solution containing silica gels. Each of said aqueous slurry solutions was filtered and sodium fluoride crystals and silica gels obtained were washed with water and dried to recover 115 kg of sodium fluoride crystals having the crystalline size of 100 ˜ 150 μ and a purity of better than 99% and 33 kg of silica gels (containing 7% H 2  O) having a purity of 100% and the particle size of less than 3 μ , respectively. The volume of the aqueous ammonium chloride solution was 610 l and its concentration was 16 wt %.