The Uses of Soluble Silica - American Chemical Society

30 T h e Uses of Soluble Silica James S. Falcone, J r .

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 19, 2015 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch030

West Chester University, Department of Chemistry, West Chester, PA 19383

The uses of soluble silica are reviewed in a concise manner. Ideas originally summarized by Ralph K. Iler are expanded and discussed with up-to-date references. Key technological factors associated with the application of soluble silica in the broad categories are described. The categories are: Adhesives, binders, and deflocculants; Cleaners and detergents; and Raw materials. These technological factors are related to current understanding of the chemical properties of these complex inorganic polymer solutions.

-IVALPH K. ILER (1), in The Chemistry of Silica, classified the uses of the soluble silicates into three categories: • Adhesives, binders, and deflocculants* function is dependent primarily on the presence of polysilicate ions. The soluble silicate used has a ratio range of 2.5 to 4.0, where the ratio is defined as one-half of the ratio of moles of Si to moles of cation (e.g., Na+ or tetramethylammonium ion, T M A ) . +

• Cleaners and detergents' function is primarily due to controlled alkalinity using silicates with ratios generally lower than 2.5. • Raw materials for the production of precipitated forms of silica, sols and gels from solutions with ratios equal to 3.3 or greater. A n analysis of the trends in U.S. production statistics for sodium silicates from the early 20th century to date is shown i n Figure 1. It suggests a macro growth pattern in volume of roughly 10 million kg/year 0065-2393/94/0234-0595$08.00/0 © 1994 American Chemical Society

In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


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T H E C O L L O I D CHEMISTRY O F SILICA

Figure 1. Sodium silicate production since 1900 as 40° Bé 3.3 ratio solution (data smoothed to show trends). CGR since 1925 is 2.6%. when the silicate production is calculated as 40° Bé 3.2 ratio equivalent (Bé refers to the Baume density scale). Growth plateaus as seen in the 1930s and the 1960s reflect, in a way, the three categories in the order described. In the first third of the century, silicate uses were predominantly in the first class. In mid-century there was a rise in the use of silicates in synthetic detergents. The overall volume of silicates grew after W o r l d War II in spite of great volume losses in applications of adhesives to both natural and synthetic organic polymers. New growth in production the final third of the century is due to increased use of soluble silicates as intermediates in making many classes of silica-based performance materials. The diversity of uses for the alkali silicates is a result of both their structural complexity (2, 3) and their complex reactivity. One might view them (4) as silica dissolved and/or dispersed in an hydroxide ion-rich aqueous system. In the chemical processing industry (CPI), they are valued as a reactive source of (Si02)n structural units. Soluble silica as an intermediate can be reacted with acids and bases to form a wide range of final products ranging from seemingly simple condensed forms of relatively pure noncrystalline silica, precipitates, gels, and sols, to highly complex crystalline metallosilicates like those found in the broad class of aluminosilicates, zeolites. The key properties of an intermediate are likely to be silica concentration, ratio, supporting cation, type and level of impurities, and consistency in these factors. These factors will be particularly important in the manufacture of catalysts, highly selective sorbents, and



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other high-performance materials where trace metals could effect final product performance. When alkali silicates are used as components, rather than reactants, in systems where partial contributions to overall performance are the dominant role, the utility factors are generally not as easy to identify. This is because these systems usually depend on the surface and solution chemical properties of the wide range of highly hydrophilic polymeric silicate ions deliverable from soluble silicate products or their proprietary modifications. In most cases, however, one or two of the many possible influences of these complex anions clearly express themselves in final product performance at a level sufficient to justify their use. These major values may be seen as resulting from the following broad functions: hydrogen ion buffering, metal ion complexation, and specific adsorption. The sodium silicates are salts of a weak acid, silicic acid (pK ~ 9.8), and a strong base, sodium hydroxide. Their chemistry is complicated by the fact that silicic acid is by no means a well-defined substance. In fact, it appears that silicic acid might be best viewed as a complex hydrous polymer which varies in Si, O, and H composition and connectivity between S i - O - S i or number of shared S1O4 tetrahedra corners. These variations influence silanol acidity and number of bonded and non-bonded oxygen atoms. The possible structures are difficult to study directly and model quantitatively; however, they are indirectly observable from the patterns of behavior in systems containing them. Evidence from studies of the interactions of soluble silica, metal ions, and oxide surfaces strongly suggests the presence of a wide range of metastable soluble silica species influencing the balance of other interaction in the system. For example: a

• Silica adsorbs on gamma-AI2O3 in a broad p H range (5, 6), which might be explained by assuming that the silica species in solution have a similarly broad range of pK values (between 6 and 10). The silica adsorption does not exhibit the sharp maximum value normally seen for weak acids at the p H value equal to the acid's pfC value. Silica adsorption was reduced by the presence of divalent metal ions, possibly as a result of reduced silicate species activity. a

fl

• The addition of solutions of soluble silica to oxide mineral suspensions increases the magnitude of the negative surface charge on the mineral particles with higher ratio silicates (increased oligomers and higher polymers) being more active. Soluble silica species also attenuate the influence of multivalent cations on the surface charge (7, 8). • Highly polymerized silicate anions appear to interact with metal ions in solution in a manner analogous to silica gel, and



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the interaction decreases as the degree of silicate polymeriza tion decreases (9). This behavior is consistent with Iler's generalization (10) that silica suspended in solutions of polyvalent metal ions begins to adsorb these ions when the p H value is raised to within 1 to 2 p H units of the hydroxide activity at which the corresponding metal hydroxide would precipitate. It is likely that much of the behavior of silica hydrogel and the larger soluble silicate polymers towards metal ions can be attributed to metal ion adsorption in the interfacial region of the larger silica oligomers/polymers. This results in localized concentration near the siliceous surface in excess of the metal ion hydroxide solubility product (11). • It also is well known that the addition of acids and bases shifts the polymer equilibrium in solutions of silicates as a result of the following generalized scheme:

\

/

—Si—OH

+

"Ο—Si^—

\

/

- ^ S i — O — S i —

+

OrT

W i t h these generalizations in mind, one can interpret the performance enhancements that are seen when soluble silicates are added to many industrial processes. Controlled Buffer. By the appropriate choice of ratio of silica to N a O H , one can effectively buffer hydrogen ion activity in the important industrial range of 9 to 11. W h e n systems performance is dependent on anionic surfactants and séquestrants, this property is valued greatly. Corrosion Inhibition. The presence of soluble silica in water exposed to various metals leads to the formation of a surface less susceptible to corrosion. A likely explanation is the formation of 'metallosilicate complexes' at the metal water interface after an initial disruption of the metal oxide layer and formation of an active site. This modified surface is expected to be more resistant to subsequent corrosive action via lowered surface activity and/or reduced diffusion. Red Water Control. Dissolved metal ions like iron and manganese play havoc with the aesthetics of ceramics systems that come into contact with waters containing them (e.g., bathtubs, wash basins, and stucco sprayed by lawn sprinklers). The addition of high-ratio silicates will effectively eliminate this problem through the formation of 'metallosilicate complexes,' which remain suspended i n the water (12).


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Bleach Stabilization. W h e n added to hydrogen peroxide based bleaching systems, soluble silicates are known to significantly enhance bleach performance. Many hypotheses have been put forth to explain this process, including buffering, peroxysilicate complex formation, and modification of peroxide equilibrium. However, the most recent and plausible explanation is that the silicate inactivates iron and manganese species which catalyze peroxide decomposition (13). Controlled Gelation. Solutions of soluble silica can be added to permeable matrices and jelled in place either by post-addition of acid, the action of acids in place, or the controlled reaction of a gelation agent added directly with the silicate. For example, CO2 is used in foundry applications where silicate jelled by CO2 gas binds sand to make metal molds. Hydrolyzable esters, like diacetin or triacetin, can be mixed with silicate solutions, yielding time-controlled setting systems. Recently, mixtures of Portland cement and silicates have been used to gel liquid waste systems. Various proprietary systems involving soluble silica are used to accomplish objectives which involve stabilizing soils and blocking fluid flows. The strengths and set times of these systems are generally a function of the concentration of the silica and the p H value of the solution. Coagulation-Dispersion. Soluble silica, particularly in the form called activated silica, is used in water treatment as a coagulant aide, where it improves the settling properties of alum-induced floes, apparently via densification. In addition, silicate added by itself to a mineral suspension can cause beneficial conditioning, coagulation, or dispersion, depending on the treatment levels and system. Judicious use of silicate can lead to an enhancement of mineral flotation processes which employ hydrophobic collectors, generally by dispersion improvement and suppression of the flotation of unwanted oxides. This is done by maintaining negative surface charges and/or hydrophilic surfaces on unwanted components. However, silicates if overused can lead to total suppression of a mineral slurry due to the adsorption of silicate on all minerals, which causes all surfaces to become hydrophilic, and thus non-floating. Anti-redeposition-Sacrificial Agent. The universal suppression described, while it is a negative in mineral flotation, becomes a positive value in such applications as de-inking and detergency, where the maintainance of particulate surfaces in a negative charge state aids i n the intended separation action (e.g., soil from cloth and ink from pulp fiber). In a related manner, the presence of anionic polysilicates will often improve a system whose performance is dependent on the effectiveness of anionic surfactant or poly electrolytes. This improvement is due to a sacrificial effect whereby the silicate preferentially " s o r b s " on active sites i n the system and helps to maintain high activity for the costly active ingredients which in the absence of silicate would be adsorbed.


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D e f l o c c u l a t i o n . Soluble silicates suppress the formation of ordered structures within clay slurries, thus increasing the solids which can be incorporated into a clay water system. This interesting surface phenome non finds practical expression i n the manufacture of bricks and cement. These examples attest to the diversity of values to be extracted from these important industrial materials. This brief review only highlights the applied chemistry i n this system. The interested reader is directed to the general references that follow for further insight.

References 1. Iler, R. K. The Chemistry of Silica; Wiley: N e w York, 1979;p667.

2. Dent Glasser, L . S.; Lachowski, Ε. E. J. Chem. Soc. Dalton Trans. 1980, 393, 390.

3. Falcone,Jr.,J.S.In Cements Research Progress—1988; Brown, P. W., E d . ; American Ceramic Society: Westerville, O H , 1989; 277. 4. McLaughlin, J. R. "The Properties, Applications and Markets for Alkaline Solutions of Soluble Silicate", paper presented to CMRA i n N e w York, May, 1976. 5. Hingston, F. J . ; Atkinson, R. J.; Posner, A. M.; Quirk, J. P. Nature (London) 1967, 215, 1459.

6. Huang, C . P. Earth Planet. Sci. Lett. 1975, 27, 265. 7. Hazel, F. J. J. Phys. Chem. 1945, 49, 520.

J.

8. Tsai, F.; Falcone, J . S. paper presented at ACS/CSJ Chem. Congress, Honolulu, H I , A p r . 6, 1979. 9. Falcone, Jr., J . S. In Soluble Silicates,ACSSymposium Series 194; Falcone, Jr., S., E d . ; American Chemical Society: Washington, D C , 1982;p133. 10. Iler, R. K. The Chemistry of Silica; Wiley: N e w York; p 667.

11. Ananthapadman, K. P.; Sumasundaran, P. Colloid and Surf. 1985, 13, 151. 12. Browman, M. G.; Robinson, R. B.; Reed, G . D . Environ. Sci. Technol. 1989, 23, 566. 13. Colodette, J . L.; Rothenberg, S.; Dence, C . W .J.Pulp and Paper Sci. 1989, 15, J3.

Additional

Reading

Falcone, Jr., J. S.; Boyce, S. D . In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Kroschwitz, J. I., E d . ; Wiley: N e w York, 1989; V o l . 15, pp 178-204. Liebau, F . Structural Chemistry of Silicates; Springer-Verlag: Berlin, Germany, 1985.

Dent Glasser, L . S. Chemistry in Britain 1982, Jan, 33. Ingri, N . In Biochemistry of Silica and Related Compounds; Bendtz, G.; Lindquist, I., Eds.; Plenum: N e w York, 1978; p 3.

Barby, D . ; et al. In The Modern Inorganic Chemical Industry; Thomson, R., E d . ; Chemical Society: London, 1977;p320.

Wills, J. H . In Encyclopedia of Chemical Technology, 2nd ed.; Wiley: N e w York, 1969; V o l 18, pp 134-166. Vail, J. G., Soluble Silicates;ACSMonograph 116; Van Nostrand Reinhold Co., Inc.: New York, 1952; V o l . 1 and V o l . 2. RECEIVED

1991.

for review March 4, 1991. ACCEPTED revised manuscript December 19,


The Uses of Soluble Silica - American Chemical Society

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