Silica Precipitated from Sodium Silicate Solution

Silica Precipitated from Sodium Silicate Solution 

There is a close relation between formation of silica gel and of predpitate. In precipitation, the silica concentration is lower and the particles are brought together into aggregates by forces of coagulation. In the absence of a coagulant, silica is not precipitated from solution at any pH. 

Barby (400) has defined precipitated silica as a dry silica with no long or short distance characteristic structure. This is based on the fact that in a gel each particle bears a certain average geometric relationship to the surrounding particles that is, all parts of a gel are similar in structure. This is probably not true of a predpitate. However, the silicas now to be considered are categorized on the basis of the process 

When silica particles are present in hot suspension at pH 9-10, they are coagulated when the concentration of sodium ion exceeds about 0.3 iV. 

In a solution. of sodium silicate with the commonly used ratio 3.3 SiOjr Naj it happens that the sodium ion normality is O.IC, where C is the silica concentration in grams per 100 ml..  

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Silica Precipitated from Sodium Silicate Solution... 

Surface-hydroxylated amorphous silica obtained by precipitation from sodium silicate solution in Rhone-Poulenc and Aubervilliers Laboratory (France). It is a nonporous adsorbent of high purity except that it contains some traces of sodium as impurities [49-52], This sample was used without further treatment. 

It should be pointed out that the coprecipitation or cogelation of silica-alumina from sodium silicate and aluminum salts (sulfate, chloride, or sodium aluminate) results in the formation of a product having strong zeolitic properties. It is necessary to remove the sodium ion by exchanging with another ion such as H+, NH4+ or A10+, and this is usually done by treatment of the precipitate or hydrogel with a dilute solution of ammonium chloride (or sulfate) or of aluminum sulfate. After the sodium is exchanged out, the material is washed free of electrolytes, dried, and calcined (700°C.). 

Precipitated silicas produced the highest melt elasticity that we have observed from simple Cr/silica catalysts [521]. Silica gels are usually "set (gelled) under acidic or neutral conditions, as the pH from mineral acid is adjusted upward by the addition of sodium silicate. Primary silica particles form a network of chains that occupies the entire volume of the solution as illustrated in Scheme 23. In contrast, precipitated silicas are usually made under basic conditions as the pH of sodium silicate solution is adjusted downward by the addition of acid. Primary silica particles coagulate into strong secondary aggregates that precipitate out of solution as a fine flocculent, but they do not gel. The precipitated silica does not occupy the entire reaction volume as a gel does. The process is illustrated in Scheme 29. After precipitation, the secondary aggregate structure can sometimes be "reinforced" by deposition of a further silica layer. 

Figure 6. The course of a DS coating procedure (at 90 °C) related to the solubility domains for amorphous silica. S marks the concentration of sodium silicate solution (at pH ca. 13.5) added to the titania slurry. The vertical arrow represents the change in aqueous silica concentration during a coating procedure (at pH 10). Point A represents coating procedures that stop short of the precipitation edge, and point B represents those during which the edge is traversed. (Reproduced with permission from reference 23. Copyright 1979.)...

In the example above, the adhesion forces are small, so the paint gel is soft. Paint gel is much softer than a silica gel made by drying the particulate precipitate made from acidified sodium silicate solution. In this case the silica particles are extremely adherent and can resist large stresses. However, even such a hard, glassy silica gel can be moulded by the application of large pressures, of the order of 1000 atm (100 MPa). 

In the rubber industry, silica is widely used as a non-black reinforcing filler to improve the mechanical properties of NR composites due to its high specific surface area, particularly tensile strength, tear resistance, abrasion resistance and hardness. Silica can be obtained by various methods, such as precipitated silica by precipitation of an aqueous sodium silicate solution, fumed silica by pyrogenic process, silica from natural resources e.g. rice husk ash and fly ash), and in situ silica or silica sol by the sol-gel process. 

A more practical precipitated silica of lower surface area that can be dried from water, yet retains an open structure with easy dispersibility, is described in detail in another patent to Alexander et al. (271). The main difference from the above is that the initial sol was made in a more concentrated solution 123 part of sodium silicate solution containing 3.71 grams SiOj per 100 cm was heated to 90 C, and 10.1 parts 13% H2SO4 solution was added in 30 min. Then added simultaneously over a 2 hr period were 119 parts of sodium silicate solution containing 8 grams SiOj per 100 cm and 111 parts 3.11% H3SO4 solution. At the end of the reaction more acid was added to reduce the pH to 7. To the product slurry, 3 parts of 1% cetyltrimethylammonium bromide solution was added to aid filtration. The recovered wet silica was washed to remove salt, the pH adjusted to 8, and the silica dried at 110 C. This... 

Precipitated silicas are normally made from solutions of sodium silicate (water glass). The sodium silicate solution is usually made by digesting a massive form of silica, such as sand, with sodium hydroxide solution. Precipitation is carried out by adjusting the pH with acid (sulfuric is usually used). Like all precipitations, the process is complex and governed by many variables, including temperature, pH profile, electrolyte concentration, agitation and time. Useful information can be found in Her [38], in Watson [39] and in manufacturer s literature [40]. 

Silicagel R (Reanal, Hungary, analytical grade), silica powder (precipitated from commercial sodium silicate solution by alcohol and then dried and sieved. The Na content at the surface of the precipitated silica was 3.07 mmol per 100 g solid powder, determined poten-tiometrically.), silicates Sepiolite (Spain), Zeolite 13X (Linde, FRG), kaolinite (Zettlitz, FRG), illite (Fiizerrad-vany, Hungary), montmorillonite (Mad, Hungary), and vermiculite (South Africa). The specific surface areas of the adsorbents are summarized in Table 1. Their detailed description has been published [23—26]. 

Silica gel. This is a hard, granular, very porous product made from the gel precipitated by acid treatment of sodium silicate solution. Its moisture content before use varies from roughly 4 to 7 percent, and it is used principally for dehydration of air and other gases, in gas masks, and for fractionation of hydrocarbons. It is revivified for reuse by evapoiatitm of the adsorbed matt. ... 

Silicon Dioxide occurs in nature as agate, amethyst, chalcedony, flint, quartz, sand, and tridymite. Silicon dioxide is obtained by acid precipitation from a sodium silicate solution to yield veiy fine particles. If silicon dioxide is obtained by the addition of sodium silicate to a mineral acid, the product is termed a silica gel. The pharmaceutical industry uses colloidal silicon dioxide, which is a submicroscopic fumed silica prepared by the vapor phase hydrolysis of a silica compound, such as silicon tetrachloride. Other names for colloidal silicon dioxide include light anhydrous silicic acid, silicic anhydride, and silicon dioxide fumed. Colloidal Silicon Dioxide NF must meet the following standards ... 

The product of the fusion of silica with sodium carbonate, sodium silicate (strictly called sodium poly trioxosilicate but usually metasilicate), dissolves in water to give a clear, viscous solution known as waterglass . It hydrolyses slowly and silica is precipitated. Besides the metasilicate, other silicates of sodium are known, e.g. the poly-tetroxosilicate (orthosilicate), Na4Si04. Only the silicates of the alkali metals are soluble in water. Other silicates, many of which occur naturally, are insoluble, and in these substances the polysilicate anions can have highly complicated structures, all of which are constructed from a unit of one silicon and four oxygen atoms arranged tetrahedrally (cf. the structure of silica). Some of these contain aluminium (the aluminatesilicates) and some have import ant properties and uses. 

Preparation. Precipitated silica is formed from an alkaline metal silicate solution, such as sodium silicate in a ratio of approximately 2.5—3.3... 

In the process to produce alumina (Fig. 1), bauxite is crushed and wet ground to 100-mesh, dissolved under pressure and heated in digesters with concentrated spent caustic soda solution from a previous cycle and sufficient lime and soda ash. Sodium aluminate is formed, and the dissolved silica is precipitated as sodium aluminum silicate. The undissolved residue (red mud) is separated from the alumina solution by filtration and washing and sent to recovery. Thickeners and Kelly or drum filters are used. The filtered solution of sodium aluminate is hydrolyzed to precipitate aluminum hydroxide by cooling. The precipitate is filtered from the liquor, washed, and heated to 980°C in a rotary kiln to calcine the aluminum hydroxide. 

Metal-support interaction was observed in two nickel-silica catalysts, but they were prepared either by precipitating a complex carbonate from nickel nitrate solution containing Si02 as a slurry or co-precipitating the carbonate from a solution of nickel nitrate and sodium silicate. The similar spectra from the unreduced catalysts resembled in shape and binding energy a NiSi03 standard and were quite distinct from NiO. The interaction was... 

Strongly Alkaline, Confined Environments, Because Si is most soluble in the form of silicate anions, it is not surprising that smectites form most rapidly in dilute alkaline solutions of sodium silicate and magnesium chloride. The layer silicate structure evolves by the precipitation of a planar Mg(OH)2 (brucite) sheet, on which monomeric silicate ions condense. Silica polymers are unable to reorganize into layered structures. It seems that the metal hydroxide sheet must be layered to begin with, a fact that necessitates 6-coordination of the metal ion that is to form the octahedral sheet of the 2 1 layer. Smectites can be formed from hydroxide sheets of the following cations (listed in order of increasing radius) ... 

Amorphous silica-aluminas are high-surface-area materials that can be prepared by co-precipitation or sol-gel techniques from solutions containing sodium silicate and sodium aluminate. Because of their high surface acidities they are potent hydrocarbon cracking catalysts, but their activity is strongly dependent on the silica/alumina ratio and on the method of preparation. Solid-state NMR studies have addressed the structural origins of these variations and have served to characterize the surface acidities. 

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