Precipitated silica production

The commercial grade of flaked sodium hydroxide dissolves readily and is convenient to handle. Any silica present is not objectionable since it is removed by filtering the redissolved sodium salt before precipitating the product with acid. 

Although natural quartz, cristobalite and opal are used as fillers, only synthetic products (fumed and precipitated silicas) find use as fillers in rubber base adhesives. 

The filter cake is stirred with 3.1 liters of acetone. The volume of acetone used is about 1.5 times the weight of the cake resulting in about a 65% acetone concentration. The benzoic acid dissolves in the acetone and the urokinase flocculates out. Sodium benzoate, about 1% of the weight of the cake, or 21 grams, is added to speed up the formation of the precipitate. The suspension of crude urokinase in acetone Is filtered on a Buchner funnel using filter paper precoated with a diatomaceous silica product (Celite 505). The precipitate is washed with acetone until the filtrate is water clear. The precipitate is then washed with ether and air dried. The yield of powder so obtained is 2.3 grams. 

Metal hydroxides (e.g., Fe, Mn, Al) can also be a problem (Rauten-bach and Albrecht, Membrane Processes, Wiley, New York, 1989). A chemical analysis of the feed solution composition along with consideration of solubility products allows one to determine the significance of precipitation. Solubility products can be affected by temperature, pH, and ionic strength. Seasonal temperature variations must be considered. Concentrations of silica need to be < 120 mg/L in the feed. 

The pyrogenic flame hydrolyzed silica Aerosil 200, a commercial product from Degussa, was used as a dispersion in doubly distilled water (1). The precipitated silica was prepared by hydrolysis of orthosilicic acid tetraethylester in ammoniacal solution according to the method of Stober, Fink and Bohn (11). The prepared suspension was purified by repeated centrifugation, separation from solvent and redispersion of the sediment in fresh water. Finally, the water was evaporated and the wet silica dried at 150°C for about half an hour. 

Physical Form. Fine white powder with particle sizes generally below Ipm. This is not the same as the commercial products fumes silica, silica gel, precipitated silica, or fused silica. It is formed during the electric arc production of elemental silicon from... 

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. 

Figure 12 shows an optional step at the end of the process in which the silica is chemically reacted with a silicone oil, typically polydimethylsiloxane, to render the product hydrophobic. Other aftertreatments are also commonly employed, such as wax coatings that enhance the performance of precipitated silicas used as flatting agents in paints (78). 

Two more recent applications for amorphous silicas are expected to grow to large volumes. Precipitated silicas are used in the manufacture of separator sheets placed between cells in automotive batteries. Their function is to provide a controlled path for the migration of conductive ions as a result of the porosity of the silica particles. Additionally, both precipitated silicas and aerogels are being developed for use in low temperature insulation, where the low thermal conductivity of the dry silica powders makes them useful in consumer products such as refrigerators (83). 

There are at least five types of synthetic silicas that can be considered for use in polymers. These are generally known as fumed, arc, fused, gel and precipitated. A detailed review of their production and uses has been given by Watson [96]. The types most often encountered in thermoplastics are the gel and precipitated silicas which are frequently used as antiblocking agents in polymer films and as gloss reducing agents in polymer sheets. 

Sodium silicate solutions are neutralized using sulphuric acid to obtain the Si(OH)4. The sodium silicate solution characteristics are dependent on the Si02 Na20 ratio. For production of precipitated silicas, sols and gels, a sodium silicate ratio 10 3 is usually employed rather than lower ratios, since less acid is required for neutralisation. Furthermore this ratio is available at low cost, since large quantities are produced. 

Silicon Dioxide occurs as an amorphous substance that shows a noncrystalline pattern when examined by X-ray diffraction. It is produced synthetically, either by a vapor-phase hydrolysis process, yielding fumed silica, or by a wet process, yielding precipitated silica, silica gel, colloidal silica, or hydrous silica. Fumed silica is produced in an essentially anhydrous state, whereas the wet-process products are obtained as hydrates or contain surface-adsorbed water. 

These two DOH compounds have only been obtained by the special "dry-synthesis" method described above [24, 25]. However, the reasons for the use of this method are different in the two cases. The benzene cyclopentadienyl iron complex decomposes quickly in aqueous synthesis systems at higher temperatures. Colourless needles are produced then. Under the conditions of the dry synthesis the complex is reasonably stable, although the synthesis is not easily reproduced. It works best when a reactive silica source such as fumed or precipitated silica is used and when the autoclaves in which the reactions take place have a large free volume. Such a dependence of the product on the starting material or the reaction vessel used is typical of reactions carried out under kinetic control [25]. 

In addition to the natural products, synthetic materials are in common use. Two methods of production are used pyrogenic or thermal (commonly known as fumed silica grades) and wet process (commonly known as precipitated silica). 

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