Silica hydrogels

Immiscible wetting in mixers Sol-gel processes Pellet flocculation <0.3 Low Up to 10 ton/hr Wet processing based on flocculation properties of particulate feed Coal fines, soot and oil removal from water Metal dicarhide, silica hydrogels Waste sludges and slurries... 

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

FIGURE 2.20 The effects on foam active ( ) and HA ( ) protein of treating unstabilized beer with silica hydrogel. Reprinted with permission from Siebert and Lynn (1997b). Copyright 1997 American Society of Brewing Chemists. 

Hydrogels are used in the refining of edible oils to adsorb phospholipids, trace metals, and soaps (103). The adsorption capacity depends on the ease of hydration of the adsorbates, so best performance demands careful control of moisture content in the system (104). Silica hydrogel in combination with alumina has been found to be useful for purifying used cooking oils in order to extend their life and enhance the quality of the fried food (105). 

Porous silica is one of the different forms of amorphous silica. It can be prepared by acidification of basic aqueous silicate solutions, and when reaction conditions are properly adjusted, porous silica gels are obtained [150], If water is evaporated from the pores of silica hydrogels prepared in this fashion, porous xerogels are obtained [153],... 

Silica applied as a support for catalysts is an X-ray-amorphous form of silicon dioxide [37], It is manufactured in two steps. First, a silica hydrogel is formed b> means of a sol-gel process [38] (Section A.2.1.4) Secondly, the silica hydrogel is subjected to aftertreatment followed by dehydration to remove water. The product... 

Silica beads 2-3 mm in size can be formed in the course of the sol-gel process. The sol passes an orifice where liquid droplets are formed which then settle in a water-immiscible liquid. During settling the liquid droplets solidify to silica hydrogel beads. 

As with powdered activated carbon, amorphous silica hydrogels are used in conjunction with bleaching clays in the adsorptive purification of fats and oils. Silica hydrogels possessing average pore diameters greater than about 60 A exhibit high... 

Typical Properties Typical physical properties for commercial amorphous silica hydrogels are shown in Table 4. Chemical purity is high typically >99.5% Si02, <0.1% AI2O3, <0.01% Fc203, with only traces of alkali and alkaline earth oxides on a volatile free basis. By definition, amorphous sihca is noncrystalline it contains no crystalline silica forms (i.e., quartz, ciistobahte, tridymite). 

TABLE 4. Typical Properties of Commercial Amorphous Silica Hydrogels. 

Table 1 Effect of stirring on the texture of fresh and aged silica hydrogels...

Table 2 Effect of gelation pH on the texture of monolithic silica hydrogels.

Another interesting fact appears in table 3 textural modifications induced in silica hydrogel texture still exist in silica-alumina hydrogels. 

These results point out that ageing step offers a simple way to strongly modify the texture of silica hydrogels. A further precipitation of alumina in the aged silica gel stops the... 

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