Silica, pyrogenic, manufacture

Figure 5.6 shows that pyrogenic manufacturing gives excellent control over particle size distribution and median particle size. These grades of fumed silica differ in properties and require a different technological approaches to their dispersion since small particle size filler is more difficult to disperse. At the same time, smaller particle sizes give more transparent products and better reinforcement. 

Amorphous silica exists also in a variety of forms that are composed of small particles, possibly aggregated. Commonly encountered products include silica sols, silica gels, precipitated silica, and pyrogenic silica (9,73). These products differ in their modes of manufacture and the way in which the primary particles aggregate (Fig. 8). Amorphous silicas are characterized by small ultimate particle size and high specific surface area. Their surfaces may be substantially anhydrous or may contain silanol, —SiOH, groups. These silicas are frequendy viewed as condensation polymers of silicic acid, Si(OH)4. 

For commercial processes, formed supports are more useful. Compared with other supports, fumed oxide supports showed new catalytic effects [41]. Some intensively investigated applications for these supports are abstracted in the following. SiC>2 pellets have been successfully introduced in a new generation of precious metal supports in vinylacetate monomer production [42]. This resulted in better selcctivities and an up to 50% higher space-time yield compared with supports based on natural alumo-silicates. In alkene hydration fumed silica pellets serve as a support for phosphoric acid. In this case, an increased catalyst lifetime and a higher space-time yield were observed [43]. Pyrogenic TiC>2 powder can be used as a starting material for the manufacture of monolithic catalysts [44] for the selective reduction of NOv with ammonia. 

The most important process for the manufacture of pyrogenic silicas is flame hydrolysis in which tetrachlorosilane is decomposed in an oxyhydrogen flame ... 

Pyrogenic silicas (Aerosils ) are manufactured by three processes ... 

Figure 12.1 Schematic diagrain of (he process for the manufacture of pyrogenic silica. (After Michael and Feich, 1993.)...

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