Silicate process, aqueous

Aqueous silicate product forms with widely varying properties such as leach rate, hardness, and volume can be produced at various costs by using the three alternative process routes. Typical leach rates for the products in water range from g/cm -day. In general, the volume... 

The structures of silicate polymers formed from alkoxides by the sol-gel process vary from weakly branched molecular networks to highly condensed particles that are similar to commercial aqueous silicates described by Iler (2). The sol-gel process combines this control of microstructure on molecular length scales with the ability to form specialized shapes such as fibers, films, monosized powders, and monoliths... 

A more economical route to MQ resin uses low cost sodium silicate and trimethylchlorosilane as inputs (eq. 35) (395). The sodium silicate process is initiated by acidifying an aqueous sodium silicate solution to a pH of 2. The resulting hydrosol quickly builds molecular weight. The rate of this increase is moderated by the addition of an alcohol such as 2-propanol. The hydrosol is subsequently silylated by the addition of trimethylchlorosilane. This process, which is kinetically sensitive and limited to synthesizing M/Q ratios of 1 or less, is preferred when MQ resins having high (>1%) OH content are required... 

In the course of synthesis, the template molecules act as structure-directing agents. Size and shape of the template determines the geometry of the pore. During crystallization, the amphiphilic nature of the template (mainly organic amines) allows for molecular interaction between the aqueous silicate solution and the growing hydrophobic silica framework. Thermodynamic analyses suggest that the kinetics of nucleation are the most important factors in the crystallization process. 

In Figure 18-lc, the formation of cyclic trimeric silicic acid (0 3) is observed. Probably due to the high ionization of silicic acid species in the aqueous silicate solution, oligomerization of the d eric silicic acid proceeds via the cyclic trimeric isomer of trimeric silicic acid. In a later stage of the dissolution process, the formation of the linear isomer of trimeric silicic acid is encountered in the silicon-29 nmr spectrum (Figure 18-Id). Preferential formation of cyclic trimeric sUicic acid is attributed to charge stabilization of the trimeric species. Ab initio quantum mechanic calculations of cyclic structures of silicate units (Van Beest, Verbeek, and Van Santen... 

During the process, the aqueous silicate solution is introduced into the upper portion of the gas-fired spray dryer and passes through a spray nozzle or a disk atomizer (see Figure 22.5). The speed of the spray wheel may be about 11,000 rpm. The finely and evenly dispersed liquid comes into contact with upwardly directed hot air. Typical spray tower tanperatures are about 180°C [21] with inlet temperatures of about 260-300°C and outlet air temperatures of above lOO C. The resultant spray-dried droplets adopt the form of hollow microspheres. The silicate particles are collected at the spray dryer s bottom and are withdrawn by a screw conveyor. The amorphous sodium silicate may have a bulk density on the order of 250-500 g/L, an SiOjiNajO molar ratio of 2.04 1, and an ignition loss on the order of 19-20%. Its mean particle size can be on the order of 100-200 pm. The material may be subjected to further milling to modify the form and density of the powder [51,63]. 

Despite the relatively long time scales and large length scales involved in gelation of aqueous silicate precursors, several groups have used molecular dynamics simulations and atomistic potentials to study the sol-gel process and sol-gel derived silica materials. 

Earlier in this chapter we discussed the sol-gel processing of aqueous silicates and mentioned the preparation of Si02 gels from fine particles made by flame oxidation (2)-(4). The preparation of alumina sols from alkoxides has been characterized by Yoldas (45). Aluminum alkoxides such as aluminum sec-butoxide and aluminum isopropoxide are readily hydrolyzed by water to form hydroxides. Which hydroxide is formed depends on the conditions used in the hydrolysis. The initial hydrolysis reaction of aluminum alkoxides can be written... 

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