Alkali metal silicate solutions

Fig. 2 29gj NMR spectra of 3.0 mol% SiC>2, R = 2.0 alkali metal silicate solutions.

Alkali metal silicate solutions were prepared from Baker analyzed Si02 gel, reagent grade alkali metal hydroxides, and deionized water. Stock solutions at about 3 molX SiC, 5102 20 = 3 (M = alkali metal) were aged In polypropylene bottles for periods of several weeks, to assure that all the SiC>2 had dissolved. Samples were then formulated with extra water and base to achieve a desired composition. 

Figures 1, 2, and 3 show Si NMR spectra of alkali metal silicate solutions with silicate ratios (R = [SiC>2]/[M20]) of R - 0.4, 1.5, and 2.0, respectively. The total Si02 concentration is 1.75 molX...

Figure 1. Si NMR spectra of 1.75 mol% SiO, R O.A alkali metal silicate solutions.

Electrodialysis is a preferred process to prepare silica aquasols in areas where electric power is not expensive. The process consists in the electrolysis of an alkali metal silicate solution to continuously remove alkali metal ions until a silica aquasol of the desired particle size is obtained. 

A solution pH of as high as 10.5 may be used when ion exchange resins are used for nucleation. However, the pH of the alkali metal silicate solution should not exceed about 9.5 when nucleation is effected by electrodialysis or by acid neutralization. At pH higher than 9.5, silicate ions are present which tend to migrate to and deposit on the membrane thus restricting the free flow of cations therethrough. Therefore, while a pH above 9.5 can be tolerated for a short time, inversely related to the pH... 

To prepare a binder having Si02/M20 ratio of 3.5 to 3.8 it is therefore not necessary to add any colloidal silica if an alkali metal silicate solution is used already in the ratio range. On the other hand, if an alkali metal silica solution with ratio lower than 3.5 is used, it is necessary to add at least some colloidal silica in the form of a sol to prepare out binder. 

So far only batch precipitation processes have attained economic importance [47,48], although continuous precipitations have also been reported [49]. In general, acids and alkali metal silicate solution are fed simultaneously into water in a stirred vessel building silica seeds. In the course of the precipitation, three-dimensional silica networks are formed, accompanied by an increase in viscosity. The networks are reinforced by further precipitation of oligomeric silica and grow further into discrete particles with a decrease in viscosity. The formation of a coherent system and thus a gel state is avoided by stirring and increasing the temperature. 

The studies with liquid Si NMR spectroscopy for investigation of the chemistry and kinetics of the hydrolysis and condensation reactions either in alkali metal silicate solutions or in alkoxide sol-gel precursors are abundant and have been very... 

Table 2.1. Composition of Typical American Commercial Alkali Metal Silicate Solutions...

Since the concentration of hydroxyl ions and pH are reduced when concentrated alkali metal silicate solutions are greatly diluted, it is not surprising that hydrolysis... 

Colloidal silica has been made by various procedures involving electrodialysis whereby sodium ions are removed from a solution of sodium silicate to produce sol. These have been reviewed by Her (8), but in no case were stable products made. Sanchez (87) and Her (88) patented processes of electrolyzing alkali metal silicate solution to continuously remove alkali metal ions until a sol is obtained. 

---