Method of manufacturing longer fibers of potassium titanate
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of manufacturing potassium titanate fibers which are used as a composite filler material for plastics and metals, a filter material, as a membrane or in electronic appliances. More particularly, it relates to a method for manufacturing longer fibers of potassium titanate or derivatives of potassium titanate. 
     PRIOR DEVELOPMENT 
     Various methods for manufacturing potassium titanate fibers have been developed including the fusion process of reacting a mixture of raw materials by heating and fusing; the flux process of adding a flux to the mixture during heating and fusing; the calcination process of reacting the raw material at a temperature lower than the fusing point; and the method of calcination in which the temperature is controlled. These processes, however, are all defective in the fibers having only a short length of 100 μm or less result. 
     Because of the short length, potassium titanate fibers do not fully display their excellent features and the fiber length and this has limited their application. 
     Indeed longer lengths are advantageous, for example, when potassium titanate fibers are added as reinforcing materials to composite plastics since interlocking of these longer fibers is accelerated to achieve a higher reinforcing effect with a smaller amount of fiber addition. 
     In the conventional calcination method of manufacturing potassium titanate fibers, the mixing ratio of the raw material titanium-containing compound and the potassium-containing compound is set such that the potassium-containing compound is slightly in excess than in the titanium and potassium ratio in the resultant product. In such a condition, a portion of K 2  O evaporates the reaction of the above process and accelerates the fiber growth. The evaporation speed of K 2  O should not be too fast or too slow in order to obtain the longer fibers. 
     Since open state calcination for manufacturing potassium titanate fibers may cause excessive evaporation of K 2  O, there is a process using a vessel with a comparatively small opening. A sintering vessel of this type made of a porous material with adequate permeation to promote adequate evaporation and diffusion of K 2  O, thereby producing a comparatively longer fiber length product. 
     Conventional vessels are made primarily of alumina-silica materials. However, such calcination vessels react with its component, K 2  O, at about 700° C. and forms K 2  O. 4SiO (potassium tetrasilicate) or K 2  O. Al 2  O 3 . 6SiO (potash feldspar) to coat the inside surface of the alumina-silica vessel to thereby prevent gas permeation. This increases the K 2  O vapor pressure inside the container also serves to restrain fiber growth. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventors of the present invention have found that potassium titanate fiber length can be extended using a calcination vessel such as a saggar or crucible in the calcination method, the vessel made of a material containing 25 wt % or more MgO. The calcination method is for industrial production among those described above, and completed the present invention. 
     Since the MgO component in the material forming the vessel is not only non-reactant to K 2  O but also reduces or eliminates any reaction of the material with K 2  O, plugging of the micro pores is prevented. During the generation of potassium titanate, K 2  O is diffused and discharged outside the container through open pores. As a result, the K 2  O vapor pressure inside the vessel does not increase, nor is K 2  O evaporation restrained and as a result fiber growth is promoted to produce longer fibers. 
     As described in the above, the present invention is characterized in that the calcination vessel material contains 25 wt % or more of MgO present, for example, as magnesia dolomite or chrome-magnesite added at the time of the vessel is made. With less than 25 wt % of MgO, the reaction with other compounds, particularly of SiO 2  and K 2  O, cannot be restrained and plugging of the micro pores occurs, so that the desired effect to the MgO addition is not demonstrable. 
     If a basic material such as magnesia, dolomite or chrome-magnesite is used mainly for manufacturing the calcination vessel, a particularly preferable result is achieved since the MgO content will exceed 50 wt % and SiO 2  content, which is an impurity, will be reduced. If the MgO content is at least 25 wt %, if not as much as 50 wt %, MgO will react with SiO 2  to form compounds such as 2 MgO SiO 2  (Forsterite) and decrease any possible reaction with K 2  O. Thus, the present inventors confirmed that K 2  O diffusion through micro pores of the container is not particularly hampered. Although there is no particular restriction on the composition of the calcination vessel other than the MgO component, several experiments indicate that the SiO 2  content should preferably be at most 25 wt %. 
     The method of manufacturing potassium titanate fibers according to the calcination method using the basic calcination vessel of the present invention is now explained. 
     One or more materials selected from the group consisting of potassium carbonate, potassium hydrogen carbonate, potassium hydroxide, potassium sulfate, potassium nitrate, potassium chloride, potassium bromide, potassium and of fluoride, titanium dioxide and titanium hydroxide are weighed and mixed together to achieve the molecular ratio of 1 : 1 to 8, preferably 1 : 1 to 5, for K 2  O and TiO 2 , to the resultant mixture water is added and is kneaded to prepare a paste or is compression molded, and then heated at 900° C.-1250° C. for 30 minutes to 1000 hours, preferably for 20 hours to 50 hours. 
     Fibers of potassium titanate thus prepared are dipped into water, disentangled and then removed. 
     By further treating these fibers with an acid solution, potassium titanate or oxide fibers with a different K 2  O/TiO 2  ratio may be obtained, or by treating them in an alkaline aqueous solution with an alkaline earth metal compound, fibers of various titanium compounds, such as titanate alkaline earth metals, may be obtained. 
     The present invention enables the industrial scale manufacture of potassium titanate fibers with a length of 0.1-5 mm. 
     EXAMPLE 
     Table 1 shows the chemical composition and physical properties of the calcination vessel of the present invention. 
     To potassium carbonate and titanium dioxide in the amounts as shown in Table 2 were added 13 wt. parts water to 100 parts dry mixture and the resultant mixture was compression molded into a shape of 230×115×65 mm, which was placed in a calcination vessel and then heated for 50 hours at the temperatures shown in Table 2. After cooling, the product was dipped into water. The size of fibers taken out are reported in Table 2. 
     As is apparent from the results in Table 2, potassium titanate fibers of the Examples which were heated in the calcination vessel containing 25 wt % or more of MgO had much a longer fiber length compared to those obtained in Comparative Examples, thereby proving the superiority of the present invention method. 
     
                                           TABLE 1                                 
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                                     Apparent      Compression            
Saggar       Chemical composition (wt %)                                  
                                     porosity                             
                                          Apparent                        
                                               Bulk                       
                                                   strength               
No.                                                                       
   Material  SiO.sub.2                                                    
                Al.sub.2 O.sub.3                                          
                    Fe.sub.2 O.sub.3                                      
                        CaO                                               
                           MgO                                            
                              Cr.sub.2 O.sub.3                            
                                  TiO.sub.2                               
                                     (%)  density                         
                                               density                    
                                                   (kg/cm.sup.2)          
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1  Magnesia  0.3                                                          
                0.2 0.1 0.8                                               
                           98.6                                           
                              --  -- 17.3 3.51 2.90                       
                                                   500                    
2  Dolomite  1.5                                                          
                0.4 0.8 20.0                                              
                           77.3                                           
                              --  -- 16.8 3.50 2.91                       
                                                   550                    
3  Chrome-magnesite                                                       
             0.8                                                          
                12.9                                                      
                    3.9 0.7                                               
                           71.1                                           
                              10.6                                        
                                  -- 16.3 3.67 3.07                       
                                                   600                    
4  Spinel    0.2                                                          
                71.0                                                      
                    0.2 0.4                                               
                           28.0                                           
                              --  -- 17.5 3.48 2.87                       
                                                   500                    
5  Magnesia- 24.8                                                         
                34.5                                                      
                    1.6 0.7                                               
                           37.2                                           
                              --  0.6                                     
                                     18.5 3.01 2.45                       
                                                   400                    
   alumina-silica                                                         
6  Alumina-silica                                                         
             31.2                                                         
                64.7                                                      
                    1.6 0.2                                               
                            0.3                                           
                              --  1.3                                     
                                     18.0 3.05 2.50                       
                                                   500                    
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                                           TABLE 2                                 
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               Examples                        Comparative Examples       
               1   2   3   4   5   6   7   8   1   2   3   4              
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Potassium carbonate (kg)                                                  
               253 146 146 146 146 146 146 102 253 146 102 146            
Titanium dioxide (kg)                                                     
               147 254 254 254 254 254 254 298 147 254 298 254            
K.sub.2 O/TiO.sub.2 (mol ratio)                                           
               1/1 1/3 1/3 1/3 1/3 1/3 1/3 1/5 1/1 1/3 1/5 1/3            
Annealing temperature (°C.)                                        
               1000                                                       
                   1000                                                   
                       1100                                               
                           1100                                           
                               1100                                       
                                   1100                                   
                                       1100                               
                                           1100                           
                                               1000                       
                                                   1000                   
                                                       1000               
                                                           1100           
Saggar material No (Table 1)                                              
               1   1   1   2   3   4   5   1   6   6   6   6              
Fiber length (μm)                                                      
               100-                                                       
                   100-                                                   
                       300-                                               
                           300-                                           
                               300-                                       
                                   100-                                   
                                       100-                               
                                           100-                           
                                               20- 20- 20- 20-            
               300 500 3000                                               
                           2500                                           
                               2000                                       
                                   500 300 300 30  50  25  80             
Fiber diameter (μm)                                                    
               3-10                                                       
                   3-20                                                   
                       10-40                                              
                           10-40                                          
                               10-35                                      
                                   3-20                                   
                                       3-15                               
                                           4-10                           
                                               0.8-1                      
                                                   0.8-1.5                
                                                       0.8-1              
                                                           1-2            
Fiber/length diameter                                                     
               30-33                                                      
                   25-33                                                  
                       30-75                                              
                           30-62                                          
                               30-57                                      
                                   25-33                                  
                                       15-33                              
                                           25-30                          
                                               25-30                      
                                                   25-33                  
                                                       25-28              
                                                           25-33          
Reaction product                                                          
               K4T K4T K4T K4T K4T K4T K4T K6T K4T K4T K6T K4T            
                   K6T K6T K6T K6T K6T K6T                                
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 Note:                                                                    
 K4T: K.sub.2 O.4TiO.sub.2                                                
 K6T: K.sub.2 O.6TiO.sub.2