Neutralizing Soluble Silicates With Acids

In another patent, Alexander and Her (80) describe the isolation of particles formed in the above process by coagulating them with a metal ion such as calcium, washing the precipitate free from sodium salt, and then peptizing the product to a more concentrated silica sol by removing the calcium ions by ion exchange, for example. 

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Transportation and Disposal. Only highly alkaline forms of soluble silicates are regulated by the U.S. Department of Transportation (DOT) as hazardous materials for transportation. When discarded, these are classified as hazardous waste under the Resource Conservation and Recovery Act (RCRA). Typical members of this class are sodium silicate solutions having silica-to-alkali ratios of less than 1.6 and sodium silicate powders with ratios of less than 1.0. In the recommended treatment and disposal method, the soluble silicates are neutralized with aqueous acid (6 M H+ or equivalent), and file resulting silica gel is disposed of according to local, state, and federal regulations. The neutral liquid, a salt solution, can be flushed into sewer systems (86). 

From pH 9 to 10.7, there is an apparent increase in the solubility of amorphous silica, owing to the formation of silicate ion in addition to the monomer which is in equilibrium with the solid phase. Since the silicate ion is instantly converted to monomer in acid solution, both monomer and silicate ion are included in the determination as soluble silica by the molybdate reagent. In this range, amorphous silica is in solubility equilibrium with neutral monomer as well as silicate ions ... 

These simple facts are the basis of many precipitation processes for silica. They all involve neutralizing sodium silicate solution with an acid so that colloidal particles will grow in weakly alkaline solution and be flocculated by the sodium ions of the resulting soluble sodium salt. However, there is the further factor of reinforcement or strengthening of the aggregate structure that must also be taken into account. 

The ether extract containing the catalyst and neutral products was fractionally distilled (130°-160°C at 0.01 mm Hg). The soluble catalyst was concentrated in the pot residues. The distillation fractions were then chromatographed through a silicic acid column. Monoesters and cyclic ketones were eluted successively with 5 95 and 10 90 diethyl ether petroleum ether, and more polar material was eluted with 15 85 diethyl ether. -petroleum ether followed by pure diethyl ether. 

The most favourable pH range lies between 9 and 11. Other oxides containing the main part of Ge co-precipitate with the MFI-type zeolites in neutral or acidic medium. In a very alkaline medium, the formation of soluble germanates restricts the incorporation of germanium in the zeolite. Moreover if the pH is adjusted with alkali or ammonium hydroxide, the corresponding germanates admix with an almost purely siliceous MFI-type zeolite. These difficulties can be... 

Arsenate has chemical behavior similar to that of phosphate in soils it is chemisorbed by Fe and A1 oxides, noncrystalline aluminosilicates, and, to a smaller extent, layer silicate clays. Being the anion of the strong acid, H3ASO4, with pKa values (2.24, 6.94, and 11.5) similar to those of phosphoric acid, arsenate adsorbs most effectively at low pH. Consequently, its mobility is fairly low in acid soils with high clay or oxide content. In neutral to alkaline soils, especially those that are sodic, As may be mobile in the soluble Na arsenate form. Soil microbes and Mn oxides are able to promote the oxidation of arsenite to arsenate under aerobic conditions. 

The adsorption of organic cations is also dependent upon solution pH. At a given pH, the concentration of cations in solution relative to the concentration of uncharged molecules is dependent on the pK value of the base. If the pH is adjusted to be equal to the pK value, then the ratio of cation to free base is equal to unity, and cation exchange may be accompanied by adsorption of neutral molecules of the same organic species. For cations to be the dominant species in solution, the pH should be at least one or two units lower than the pK. If too acidic, the adsorption may be hindered due to competition with H+ ions or with metal cations released from the silicate lattice by acid attack. Adsorption will depend, too, on the solubility of the base in water that, in turn, may be pH dependent (22). 

Adsorption by Clays. — Owing to the possibility of chemical reactions between the clay and the adsorbed substances, the phenomena here are much more complicated than is ordinarily the case with many colloidal systems. According to Sullivan changes between the radicals are often involved. For instance when acid or neutral salts are adsorbed, sodium, potassium, and magnesium from the clay may be released or dissolved, while an equivalent amoimt of the adsorbed basic radical remains with the clay. The addition of alkaline solution is still more complicated. Not only may there be free alkali but basic solutions may be formed because of the hydrolysis of salts of a strong base and a weak acid, e.g., carbonates and phosphates. Three different reactions are now possible. First, the free alkali may react with the colloidal silica. Second, the silicate radical from the clay may form insoluble salts with the adsorbed base. Third, the sodium, potassium, or magnesium displaced from the clay may form soluble carbonates and phosphates, and these salts in turn be adsorbed by the clay constituents. These reactions are of great importance in the study of the fertilization of the soil. It has been claimed that the addition of lime not only neutralizes the undesirable acids, but also renders the potassium of the clay available for the plant. 

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