Abstract:
A precipitated silica having the following physico-chemical characteristics: 
     
       
         
               
               
               
             
                   
               
                 pH (5% in water) (ISO 787-9) 
                   
                 3–8 
               
                 BET surface area (DIN 66131) 
                 (m 2 /g) 
                 400–600 
               
                 DBP absorption value (DIN 53601, 
                 (g/100 g) 
                 380–420 
               
                 in relation to dried substance) 
               
                 Tapped density (ISO 787-11) 
                 (g/l) 
                 100–200 
               
                 ALPINE sieve residue &gt; 63μ (ISO 8130-1) 
                 (%) 
                 0.1–40.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to precipitated silicas having a high structure, to a process for the manufacture of the silicas and to the use of these silicas. 
     2. Description of the Background 
     The structure of a silica is understood to be the extent to which its primary particles congregate into secondary particles or, respectively, tertiary particles. The unit used to express the size of the structure is the Brabender dibutyl phthalate (DBP) absorption coefficient. 
     Spray dried precipitated silicas are known and sold, for instance, under the brand name of Sipernat®. The silicas are usually manufactured by precipitating water glass with sulfuric acid, wherein the entire width of possible precipitation variants can be deployed, as described, for example in EP 0 078 909, U.S. Pat. No. 4,094,771 or U.S. Pat. No. 6,013,234. 
     After precipitation the solid matter is removed by filtration and the filter cake is then spray-dried. If necessary, it is re-dispersed by adding an acid. Spray drying allows the manufacture of almost spherical, solid particles with a narrow size distribution range. 
     Silicas with DBP absorption values of up to 380 g/100 g are known as described in EP 0 078 909. In Example 1 of EP 0 078 909, a silica with 380 g/100 g DBP absorption value is obtained by spray-drying a silica suspension containing 11% solids by weight. According to Example 5 of EP 0 078 909 a DBP absorption of 346 g/100 g is obtained by spray-drying a silica suspension containing 16% solid matter by weight. 
     A similar process is disclosed in U.S. Pat. No. 6,013,234. In this case a silica suspension with pH&gt;4 and a solids content of more than 18% by weight was spray-dried to form particles with an average particle diameter of more than 150 μm and a BET surface area of 100 to 350 m 2 /g. 
     The known spray dried precipitated silicas can be improved with respect to their DBP absorption values. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to provide silicas of improved physical characteristics. 
     Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by precipitated silicas having the following physico-chemical data: 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 pH (5% in water) (ISO 787-9) 
                   
                 3–8 
               
               
                 BET surface area (DIN 66131) 
                 (m 2 /g) 
                 400–600 
               
               
                 DBP absorption value (DIN 53601, 
                 (g/100 g) 
                 380–420 
               
               
                 in relation to dried substance) 
               
               
                 Tapped density (ISO 787-11) 
                 (g/l) 
                 100–200 
               
               
                 ALPINE sieve residue &gt; 63μ (ISO 8130-1) 
                 (%) 
                 0.1–40  
               
               
                   
               
             
          
         
       
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Surprisingly, it has now been discovered that high-structured spray-dried silicas with DBP absorption values of more than 380 g/100 g can be obtained by employing a specific precipitation method. 
     Precipitated silicas of variant I of the present invention have the following physico-chemical characteristics. 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                 Variant 
                 II 
                 III 
                 IV 
               
               
                   
               
             
             
               
                 pH (5% in water) (ISO 787-9) 
                 3–8 
                 3–8 
                 3–8 
               
               
                 BET surface area (DIN 66131) (m 2 /g) 
                 400–600 
                 400–600 
                 400–600 
               
               
                 DBP absorption value (DIN 53601, 
                 380–420 
                 380–420 
                 380–420 
               
               
                 in relation to dried substance) 
               
               
                 (g/100 g) 
               
               
                 Tapped density (ISO 787-11) (g/l) 
                 140–200 
                 120–180 
                 100–130 
               
               
                 ALPINE sieve residue &gt; 63μ 
                 10–40 
                  1–10 
                 0.1–1   
               
               
                 (ISO 8130-1) (%) 
               
               
                   
               
             
          
         
       
     
     Another variant of the precipitated silicas of the present invention have the following physico-chemical characteristics: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                 Variant 
                 I 
                 II 
                 III 
                 IV 
               
               
                   
               
             
             
               
                 pH (5% in water) 
                 3–8 
                 3–8 
                 3–8 
                 3–8 
               
               
                 (ISO 787-9) 
               
               
                 BET surface area 
                 400–600 
                 400–600 
                 400–600 
                 400–600 
               
               
                 (DIN 66131) (m 2 /g) 
               
               
                 DBP absorption value 
                 380–420 
                 380–420 
                 380–420 
                 380–420 
               
               
                 (DIN 53601, in relation 
               
               
                 to dried substance) 
               
               
                 (g/100 g) 
               
               
                 Tapped density 
                 100–200 
                 140–200 
                 120–180 
                 100–130 
               
               
                 (ISO 787-11) (g/l) 
               
               
                 ALPINE sieve residue &gt; 
                 0.1–40  
                 10–40 
                  1–10 
                 0.1–1   
               
               
                 63μ (ISO 8130-1) (%) 
               
               
                   
               
             
          
         
       
     
     The variants of silica of the present invention can be prepared by a process in which, while stirring water in a vessel with a force sufficient to subject the medium to shear while the water is heated to 35° C. to 45° C., water and sulfuric acid are added together to the medium within at least 100 minutes. The pH is maintained at 6–7. The introduction of substances into the vessel is then interrupted for 60–120 minutes and when the addition of the substances has been completed, the medium contains a solids content of 36–42 g/l. The solids are removed by filtration, the filter cake is washed and the solid material is subjected to a short retention drying process 
     A special variant of the process is such that while stirring a vessel containing water heated to 35° C. to 45° C., preferably 36° C. to 40° C. and maintaining a pH of 6–7 and waterglass and sulfuric acid together, while the mixture is subjected to shear during the entire precipitation time with a disk agitator installed in addition to the agitator, by interrupting the precipitation for 90 minutes from the 13 th  to the 103 rd  minute, after a total precipitation time of 137 minutes, a final silica concentration of 38 to 42 g/l is established in the precipitation suspension. The precipitation suspension is then filtered and washed, the filter cake is dried or liquefied by adding water and/or acid to form a suspension containing 8% to 16% solid matter. The suspension is then spray-dried. 
     The short retention time drying in process stage c) can be conducted by liquefying the filter cake to a solids content of less than 18% by weight and spray-drying the suspension. 
     In another variant of the process of the present invention the short retention time drying process can be conducted by means of a spin-flash drying process on the washed filter cake from process stage b). 
     The pH of the end product is of decisive importance for many applications of the silicas. For instance, if the silica is used as a carrier material for vitamins, a neutral or weak acidic pH is required. The pH can be modified by subsequently treating the spray-dried silica with a base such as ammonia gas or by adjusting the pH of the re-suspended filter cake. 
     Determination of the pH of the silica is conducted on a 5% suspension of spray-dried silica according to the procedure of ISO 787-9. 
     The process of the present invention can be conducted under the condition that either the pH of the precipitation suspension is not modified after precipitation or the pH is lowered to 2–5, preferably 3, by adding acid, e.g. sulfuric acid. 
     The solid matter is separated from the suspension by known filtration methods such as a filter press (diaphragm filter press). The filter cake obtained in this way can be dried, for example by using a spin flash dryer. It is also possible to liquefy the filter cake by adding water and/or acid. If acid is employed, e.g. diluted sulfuric acid, a pH of &lt;5, preferably 2–4, is adjusted in the suspension. 
     In a special embodiment of the process of the present invention, the product that has been subjected to a short retention drying process is subsequently treated with ammonia gas, or, alternatively, the drying process is conducted in the presence of ammonia gas. 
     Adding ammonia gas increases the pH of the silica and enables higher DBP absorption values. 
     A specified particle size distribution can be adjusted with the aid of a spray dryer or a jet dryer (jet tower). This can be adjusted by selecting the dryer type (single material jet, 2 material jet, gas/liquid jet, atomizing disk) and the spraying pressure. Typically a dryer with an atomizing disk is used. 
     Silicas of the present invention can be used as a carrier for converting liquids into powder form, e.g. in the crop protection, pesticide and animal feed industries, e.g. vitamin A and E, choline chloride, as a free-flow or anti-caking agent, for example for table salt or instant meals, as well as in elastomers such as tires. 
     The silicas according to the present invention can be used to manufacture catalyst carriers. 
     Having now generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. 
     EXAMPLES 
     Reference Example 1  
     Precipitation of silica material is conducted as described in Example 1 of EP 0 078 909. For this purpose 60 m 3  of water at a temperature of 40° C. was added to a precipitation vessel, fitted with an EKATO MIG agitator and an additional EKATO shearing turbine. Commercially available water glass (26.8% SiO 2 ; 8.0% Na 2 O; density 1.346) and sulfuric acid (96%) flow into this vessel simultaneously at a speed of 10 m 3 /h and 0.9 m 3 /h respectively. After the 13 th  minute of precipitation the addition of water glass and acid is interrupted for 90 minutes. During this time both stirring devices continue to operate. From the 103 rd  minute the water glass and acid are added once more, at the above-mentioned rates, until the 146 th  minute. The solids content of the precipitation suspension is 47 g/l. 
     The solid of the suspension is removed by filtration in filter presses, washed and the resulting filter cake is liquefied under the effects of shearing. The solids content is 11.0%, the pH is 5. The silica suspension is then spray-dried. A DBP absorption value of 355 g/100 g was determined for the product that was obtained in the above manner. 
     The characteristics of the non-comminuted product are shown in the table below. 
     Reference Example 2  
     A precipitated silica was manufactured by the procedure described in Example 5 of EP 0 078 909. The procedure only differs from that of Reference Example 1 in the manufacture of the silica suspension for spray-drying. The filter cake is adjusted to a solids content of 16% by weight and a pH of 4.5 by adding water and acid under shearing conditions. The DBP absorption value is 349 g/100 g. 
     The characteristics of the non-comminuted product are found in the table below. 
     Example 1 
     An amount of 60 m 3  of water at a temperature of 38° C. was added to a precipitation vessel, fitted with an EKATO MIG agitator and an additional EKATO disk agitator (350 mm diameter). While maintaining a pH of 6.5, commercially available water glass (27.1% SiO 2 ; 8.07% Na 2 O; density 1.355) and sulfuric acid (96%) flow into this vessel simultaneously at a speed of 10 m 3 /h and 0.9 m 3 /h respectively. The acid is made to flow over the disk agitator, which is started when precipitation begins. After 13 minutes precipitation time, the addition of water glass and acid is interrupted for 90 minutes. During this time both the agitator and the EKATO disk agitator continue to operate. After this time the water glass and acid are added once more, at the above-mentioned rates and maintaining the above-mentioned pH for a further 34 minutes. The suspension is then acidified to pH 3 by adding sulfuric acid. The solids content of the precipitation suspension is 40 g/l. The suspension is subsequently filtered and washed in filter presses. For the further processing stage the filter cake is liquefied under shear with the addition of water and small quantities of sulfuric acid. The pH is 5.0; the solids content is 12%. The silica suspension is then spray-dried. The free sulfuric acid in the material is neutralized downstream of the spray dryer with ammonia gas. 
     The characteristics of the non-comminuted product are found in the table below. 
     Example 2 
     A precipitated silica is manufactured as described above in Example 1, except that the filter cake obtained after processing in the diaphragm filter press is liquefied under shear with the addition of water and small quantities of sulfuric acid. The pH is 5.0 and the solids content is 11.0%. The silica suspension is then spray-dried. 
     Example 3 
     The precipitated silica is manufactured by the procedure of Example 1 above, except that the filter cake obtained after processing in the diaphragm filter press is liquefied upon the addition of water. The solids content is 8%. 
     The characteristics of the non-comminuted product are described in the table below. 
     Example 4 
     The precipitated silica is manufactured according to the procedure of Example 1 above, except that the filter cake obtained after processing in diaphragm filter presses with a solids content of 18% is dried for further processing in a spin flash dryer. (O. T. Kragh, Keramische Zeitschrift, Vol. 30, issue 7, pp 369–370, 1978; T. Hoepffner, Informations Chemie, Vol. 342, pp 141–145, 1992). 
     The characteristics of the non-comminuted product are shown in the table below. 
     Example 5 
     The precipitated silica is manufactured according to the procedure of Example 1 described above, except that the precipitation suspension is not acidified after precipitation is complete. The suspension is filtered in diaphragm filter presses and washed with highly diluted sulfuric acid with a pH of 1.6 to 1.8. For further processing the filter cake with a solids content of 18% is dried in a spin flash dryer. The free sulfuric acid contained in the material from the washing process is neutralized by adding ammonia, or, alternatively, the pH in the powder is raised to 7.7. 
     The characteristics of the non-comminuted product are shown in the table below. 
     Example 6 
     Manufacture of a precipitated silica and the subsequent processing of the suspension, including washing were conducted according to the procedure described in Example 4 above, except that for further processing the filter cake is liquefied by adding water under shear conditions. The pH is 3.2; the solids content is 11%. To neutralize the free sulfuric acid, the material is neutralized with ammonia gas downstream of the spray-dryer. 
     The characteristics of the non-comminuted product are shown in the table below. 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 
               
             
             
               
                   
                   
               
               
                   
                 Reference 
                   
               
               
                   
                 Examples 
                 Examples 
               
             
          
           
               
                   
                 1 
                 2 
                 1 
                 2 
                 3 
                 4 
                 5 
                   
                   
                   
                   
                 6 
               
               
                   
                   
               
             
          
           
               
                 pH 
                   
                 6.4 
                 6.2 
                 7.6 
                 6.7 
                 6.7 
                 6.4 
                 3.1 
                 4.7 
                 6.3 
                 7.0 
                 7.7 
                 6.3 
               
               
                 DBP absorption 
                 g/100 g 
                 355 
                 349 
                 389 
                 382 
                 400 
                 387 
                 383 
                 391 
                 398 
                 406 
                 412 
                 387 
               
               
                 value 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Tapped density 
                 g/l 
                 180 
                 182 
                 154 
                 154 
                 135 
                 120 
                 119 
                 115 
                 111 
                 108 
                 111 
                 164 
               
               
                 ALPINE SR &gt; 
                 % 
                 10 
                 23 
                 15 
                 1.4 
                 1.8 
                 0.3 
                 0.2 
                 0.3 
                 0.3 
                 0.2 
                 0.1 
                 18 
               
               
                 63 μm 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 BET surface 
                 m 2 /g 
                 429 
                 515 
                 454 
                 458 
                 485 
                 484 
                 501 
                   
                   
                   
                 476 
                 495 
               
               
                 area 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
             
          
           
               
                 Suspension 
                 pH 
                 ~6 
                 ~3 
                 ~7 
               
               
                 Washing 
                   
                 normal 
                 normal 
                 acidic 
               
               
                 process 
                   
                   
                   
                   
               
             
          
           
               
                 Suspension of 
                 pH 
                 5 
                 4.5 
                 5 
                 5 
                 5 
                 N/A 
                 N/A 
                 (Filter cake has pH~3) 
                 3.2 
               
               
                 filter cake 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
             
          
           
               
                 Solids 
                 % 
                 11 
                 16 
                 12 
                 11 
                 8 
                 18 
                 18 
                 11 
               
               
                 content 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
             
          
           
               
                 NH 3  addition 
                 — 
                 — 
                 + 
                 — 
                 — 
                 — 
                 — 
                 + 
                 + 
                 + 
                 + 
                 + 
                 + 
               
               
                 Dryer 
                 SP 
                 SP 
                 SP 
                 SP 
                 SP 
                 SF 
                 SF 
                 SF 
                 SF 
                 SF 
                 SF 
                 SF 
                 SP 
               
               
                   
               
               
                 *SF = Spin flash dryer 
               
               
                 SP = Spray dryer 
               
             
          
         
       
     
     The disclosure of German priority Application No. 10058616.3 dated Nov. 25, 2000 is hereby incorporated by reference into the present application. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.