Sodium and Potassium Silicates

It has been known since the seventeenth century that sand and sodium or potassium carbonate react at red heat to form a water-soluble glass called water glass. As noted by Vail (1), Johann Nepomuk von Fuchs was the first to investigate alkali silicates systematically and even before 1850 proposed their uses as adhesives, cements, and fireproof paints. By 1855 water glass was being made commercially, both in Europe and America. 

Manufacture has generally been carried out in large open-hearth furnaces above 1300 C by the following reactions  

Except under unusual circumstances, the latter reaction using soda ash is always -employed. 

The use of sodium salts other than carbonate is still of limited interest. In 1941, Her and Tauch (13) studied the equilibrium 

Calculations showed the impossibility of producing HCl at a practical concentration while obtaining a reasonable conversion based on NaCl. The equilibrium is more favorable under 1000 C, but the reaction rate is very low. In 1972, Hanf and Sole(14) reexamined the reaction at lower temperatures down to 700 C, and concluded that the equilibrium constant was favorable to using the reaction for processing certain copper ores containing silica. 

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Sometimes sodium and potassium silicates were used instead of sodium chloride. Thus the consumption of TNT did not need to be increased while the consistency... 

The most important property of sodium and potassium silicate glasses and hydrated amorphous powders is their solubility in water. The dissolution of vitreous alkali is a two-stage process. In an ion-exchange process between the alkali-metal ions in the glass and the hydrogen ions in the aqueous phase, the aqueous phase becomes alkaline, due to the excess of hydroxyl ions produced while a protective layer of silanol groups is formed in the surface of the glass. In the second phase, a nucleophilic depolymerization similar to the base-catalyzed depolymerization of silicate micelles in water takes place. 

Silicates. Both sodium and potassium silicate solids or solutions have valued functionality including emulsification, buffering, deflocculation, and antiredeposition ability. Silicates also provide corrosion protection to metal parts in washing machines, as well as to the surfaces of china patterns and metal utensils in automatic dishwashers. Silicates are manufactured in liquid, crystalline, or powdered forms and with different degrees of alkalinity. The alkalinity of the silicate provides buffering capacity in the presence of acidic soils and enhances the sequestration ability of the builder system in the formulation. The sili-cate/alkali ratios of the silicates are selected by the formulator to meet specific product requirements. Silicate ratios of 1/1 are commonly used in dry blending applications with silicate ratios of 2/1 and higher commonly used in laundry and autodish applications. 

Silicates. The addition of sodium and potassium silicates to synthetic detergent has proved very beneficial. They have in solid or solution form important characteristics such as emulsification, buffering, deflocculation, and antiredeposition ability. 

Industrially interesting alkali silicates (sodium and potassium silicates) are characterized either by the SiOi alkali oxide weight ratio or by the Si02 alkali oxide molar ratio. The latter is obtained by multiplication of the weight ratio by 1.032 for sodium silicates and 1.568 for potassium silicates. 

Table 12.1 Experimental and simulation details for mixed sodium and potassium silicates (p° is given in units of 10 2 atoms/A2).

A comprehensive review of sodium and potassium silicate was conducted by the Select Committee on GRAS Substances of the Life Sciences Research Office, Federation of American Societies for Experimental Biology (FASEB) for the Food and Drug Administration. It was concluded that... 

Sodium and potassium silicate are the soluble silicates of commercial importance. For potassium silicate, not nearly as extensive data from the laboratory or from human experience are available. The assumption of its similarity to sodium silicate in health and environmental effects appears to be valid, for an equivalent mole ratio of Si02 to alkali metal oxide. 

Because of this unpublished GRAS status, sodium and potassium silicates were included in the GRAS Review process. 

Sodium and potassium silicate and sodium metasilicate monographs... 

The Select Committee, in a 1977 tentative report (26), did not pass on its safety, feeling it had insufficient information. FDA then commissioned a literature review (28), and, in a 1981 final report, the Select Committee recommended Class I status for sodium metasilicate to the FDA. This was expressed in analogy to the statement on sodium and potassium silicates, quoted on page 2 (29). Codification, the final step in the GRAS affirmation process, could be another two years away. 

While alkali metal sihcates in general can be used in the process of the invention, sodium and potassium silicates are preferred. Because of its greater availability and economy, sodium silicate is particularly preferred and normally be used. 

The identity of the alkali oxide present has a relatively small effect on the viscosity of the melt. Although the isothermal viscosity does decrease in the order Cs > Rb > K > Na > Li, the differences among the alkali oxides are small as compared to the effect of alkali oxide concentration. In fact, if low viscosity data (< 10 Pa s) are plotted against the concentration of alkali per cm of melt instead of against mol% R2O, one finds that the data for lithium, sodium, and potassium silicate melts essentially lie on the same line for any given temperature between 1100 and 1400 °C. 

Formerly there was an opinion that the reaction of aggregate would be accompanied by the increase of volume, for example as in the case of CaO hydration. Nowadays, a more significant importance is attributed to the properties of reaction products. The hypothesis of the mechanism of concrete destruction assumes that the sodium silicate gel absorbing water swells [78]. Another type of expansion occurs when the sodium and potassium silicate gel transforms to sol. There is an opinion that this liquid of high viscosity exerts pressure after filling the pores in concrete [78, 80]. According to many authors, the osmotic pressure is causing expansion [80]. 

Pumice p3-m3s [ME pomis, fr. ME, fr. L pumic-, pumex] (15c) n. A highly vesicular (frothy), glassy, volcanic lava, usually rhyolitic (granitic) in composition composed of complex aluminum, calcium, magnesium, iron, sodium, and potassium silicates. Pumacite is the name for volcanic ash found in Kansas and Nebraska. Pumice is used as an abrasive, filler for plastics, polishing compounds and non-slip compounds. Density, 2.2g/cm (18.5lb/gal). Syn pumacite, pumice stone. 

Because lithium silicates are so different in behavior from sodium and potassium silicates, lithium polysilicate is discussed separately. 

The sodium and potassium silicates are available as two-component systems filler and binder with the setting agent in the filler. Sodium and potassium silicates are referred to as soluble silicates because of their solubility in water. This prevents their use in many dilute acid services while they are not affected by strong concentrated acids. This disadvantage becomes an advantage for formulating single component powder systems. All that is required is the addition of water at the time of use. The fillers of these materials are pure silica. 

Sorption reactions of silica in soils were postulated many years ago (Sreenivasan [1935]). In the early work, however, somewhat high concentrations of sodium and potassium silicates were used, and such systems would be subject to hydrolysis and polymerization reactions and also to pH changes. Thus, in recent studies on the sorption of soluble silica by soils (Eliassaf [1962] Beckwith and Reeve [1963] McKeague and Cline [1963]), dilute solutions (100 to 135 ppm) of monomeric silicic acid have been employed, and results indicate that the residual concentration of monosilicic acid is controlled by an adsorption equilibrium which is pH dependent. Sesquioxides make a considerable contribution to the capacity of soils to sorb soluble silica (Nejegebauer [1958] Beckwith and Reeve [1963]), and the apparent increase in solubility of silica in soil suspensions with increased acidity has been discussed in terms of... 

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