Polysilicate particles

A serious problem in the control of silica polymerization is that, in general, initial particle formation is a result of random bond formation between a range of polysilicate species in solution. Moreover, the aggregation process is uncontrolled under conventional preparation conditions. It is important to understand the basic mechanisms of particle formation and aggregation in aqueous solution, as the physical properties of such silicas are determined primarily by the coordination number and nature and strength of interactions between essentially spherical primary particles. 

Alkalinity also has an important influence on the crystallization rate of zeolites. A remarkable example is the crystallization of zeolite A (LTA) from the precursor gel with a batch composition of 5Na20 Al203 2Si02 (100-200) H20.[13] Figure 3.8 shows the effect of different alkalinity (H2O/Na2O=20, 30,40) on the crystallization rate (including induction period and growth rate) and particle size of the product. Clearly, with an increase of alkalinity the crystallization process is speeded up, the particle size is decreased, and the distribution of the particle size is narrowed due to an increased nucleation rate and an increased polymerization rate between polysilicate and aluminate anions. 

Many productive methods have been developed for the preparation of silica sol including acidification/121 electrolysation-electrodialysis,[13] ion-exchange,[14] peptization/111 and hydrolysis of silicon compounds/101 which can be grouped into two main types. One is called the aggregation method that contains two steps the polymerization of silicate ions and the aggregation of these polysilicate anions via condensation reaction between the hydroxy groups of the particles. The other one is called the peptization method, i.e., dispersal of a precipitate of Si02 to form colloid. The acidification method will be discussed in detail below. 

Healy (Chapter 7) and Dumont also prefer the first approach. Healy sets down a model based on the control of coagulation by surface steric barriers of polysilicate plus bound cations. Healy s electrosteric barrier model is designed to stimulate new experimental initiatives in the study of silica sol particles and their surface structure. Dumont believes that many particular aspects of the stability of silica hydrosols could be explained not only by the low value of the Hamaker constant but also by the relative importance of the static term of the Hamaker equation. 

Dynamic light scattering indicated that the.colloids were monodisperse, and that their size increased steadily with (jstorage time after about 30 days they appeared to be roughly BOA in diameter.0 After heating to 95 C the particle size increased to over 100 A. Both the size and the spherical nature of the particles were confirmed by electron microscopy. These unstable alkaline solutions thus contain a relatively constant proportion of small species in true solution, and a large colloidal fraction that increases in size at the expense of the intermediate polysilicate fraction. This seems to suggest that possibly the colloidal material should be considered as a separate phase, in the same way as a precipitate would be. 

Similarly, the high ratio lithium silicates of Her U.S. Pat. No. 2,668,149 or the potassium polysilicates of Wol-tersdorp, application Ser. No. 728,926, filed May 14,1968, now Defensive Publication 728,926, dated Jan. 7, 1969, can be employed as the binder provided the requirements as to molar ratio, particle size and amount of amorphous silica are followed. 

Under these conditions the polymer resin macromolecules and/or colloidal particles are expected to coalesce and form an effective adhesive bond between the sand grains and reinforce the sodium polysilicate binder. 

Activated silica, niiich may have a favorable or an unfavorable effect on filtration, is composed of ionized micella formed by polysilicic acid-sodium polysilicate. This become negatively charged colloidal micella. The behavior of activated silicas depmds on the conditions of neutralization and the grade of the silicate used in the preparation of the material. Activated silica is a coagulant aid that contributes to coalescence of the particles. Hence, it brings about an improvement in the quality of settled or filtrated water, depending on the point at vdiich it is introduced. 

Hydrous sodium polysilicates are available as powders with bulk densities from 80 to 650 g/L and compacted granules with bulk densities in the range 750-950 g/L. Particle sizes are between 80 and 700 pm [2,11,65]. Made from sodium silicate solutions, they may contain 16-22 wt% moisture [1,5,7,11]. Water uptake is generally limited by their tendency toward caking at higher moisture levels. They are x-ray amorphous and have no defined stoichiometry [19,65]. The composition is... 

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