Fluorosilic acid

Liquid Dispersion Spray columns are used with slurries or when the reaction product is a solid. The absorption of SO9 by a hme slurry is an example. In the treatment of phosphate rock with sulfuric acid, offgases contain HF and SiF4. In a spray column with water, solid particles of fluorosilic acid are formed but do not harm the spray operation. The coefficient /cl in spray columns is about the same as in packed columns, but the spray interfacial area is much lower. Considerable backmixing of the gas also takes place, which helps to make the spray volumetri-caUy inefficient. Deentrainment at the outlet usually is needed. 

Anhydrous hydrogen fluoride and hydrofluoric acid react with substances containing silica and silicon oxide to form silicon tetrafluoridc and fluorosilic acid. SiF, a colorless gas at ambient temperature, is liighly toxic. An equilibrium mixture of SiF in the presence of moisture also contains hydrogen fluoride and hydrofluoric acid. 

The fluorosilicate acid can be used for fluoridation of drinking water, and fluorosilicate salts find use in ceramics, pesticides, wood preservatives, and concrete hardeners. The phosphoric acid can be purified by a solvent extraction process (Fig. 2). 

About half of the world s mined fluorspar comes from China. The second largest producer of fluorspar is Mexico, with about one-third of China s output. Other producers of fluorspar include France, Kenya, Mongolia, Morocco, Namibia, Russia, South Africa, and Spain. In addition, another, more limited source of fluorine is obtained from fluorosilic acid that is recovered from phosphoric acid plants processing phosphate rock. 

Discussion. The determination of this anion is of little practical importance. The methods available for its determination will, however, be outlined. Alkali fluorosilicates are decomposed by heating with sodium carbonate solution into a fluoride and silicic acid ... 

Insoluble fluorosilicates are brought into solution by fusion with four times the bulk of fusion mixture, and extracting the melt with water. In either case, the solution is treated with a considerable excess of ammonium carbonate, warmed to 40 °C, and, after standing for 12 hours, the precipitated silicic acid is filtered off, and washed with 2 per cent ammonium carbonate solution. The filtrate contains a little silicic acid, which may be removed by shaking with a little freshly precipitated cadmium oxide. The fluoride in the filtrate is determined as described in Section 11.59. 

If an acid solution of a fluorosilicate is rendered faintly alkaline with aqueous sodium hydroxide and then shaken with freshly precipitated cadmium oxide, all the silicic acid is adsorbed by the suspension. The alkali fluoride is then determined in the filtrate. 

Essentially all allylsilanes (M = SiR3, Section D.l.3.3.3.5.) with the exception of fluorosil-iconates11 and most of the trialkyl(allyl)stannancs (Section D.l. 3.3.3.6.), which have only very weak Lewis acidic properties, require a strong Lewis acid to trigger the reaction with a carbonyl compound by the preceding formation of an x-oxycarbenium ion, which attacks the allylic compound in an ionic open-chain pathway. These Lewis acid catalyzed carbonyl additions offer new possibilities for the control of the simple and induced diastereoselectivity12. 

Glass is resistant to attack by most chemicals. However, the silica in glass reacts with the strong Lewis base F from hydrofluoric acid to form fluorosilicate ions ... 

In most metallurgical operations, either aqueous or molten salt electrolytes are used only very rarely may an organic electrolyte be selected. The selection of the most suitable electrolyte is based on a variety of considerations. The choice of an electrolyte for lead, for example, is guided by the facts that lead forms an insoluble sulfate if sulfuric acid is used, and that a peroxide of lead is formed in solutions of other mineral acids. An electrolyte of lead fluorosilicate in hydrofluorosilicic acid (H2SiF6) is used in order to circumvent these problems. 

Earlier corrosion inhibitors limited the maximum strength of the acid to 15% by weight. Improved corrosion inhibitors (see below) have made the use of higher acid concentrations, such as 28% HCl more common. More dilute solutions may initially be injected in sandstone acidizing to reduce the formation of insoluble sodium and potassium fluorosilicates by displacing saline formation water before injection of hydrochloric acid. 

The 1980s saw major developments in secondary synthesis and modification chemistry of zeolites. SUicon-enriched frameworks of over a dozen zeolites were described using methods of (i) thermochemical modification (prolonged steaming) with or without subsequent acid extraction, (ii) mild aqueous ammonium fluorosilicate chemistry, (iii) high-temperature treatment with silicon tetrachloride and (iv) low-temperature treatment with fluorine gas. Similarly, framework metal substitution using mild aqueous ammonium fluorometaUate chemistry was reported to incorporate iron, titanium, chromium and tin into zeolite frameworks by secondary synthesis techniques. 

There are two useful side products. The H2Sip6 is shipped as a 20-25 % aqueous solution for fluoridation of drinking water. Fluorosilicate salts find use in ceramics, pesticides, wood preservatives, and concrete hardeners. Uranium, which occurs in many phosphate rocks in the range of 0.005-0.03% of UsOg, can be extracted from the dilute phosphoric acid after the filtration step, but this is not a primary source of the radioactive substance. The extraction plants are expensive and can only be justified when uranium prices are high. 

Americium may be separated from other elements, particularly from the lanthanides or other actinide elements, by techniques involving oxidation, ion exchange and solvent extraction. One oxidation method involves precipitation of the metal in its trivalent state as oxalate (controlled precipitation). Alternatively, it may be separated by precipitating out lanthanide elements as fluorosilicates leaving americium in the solution. Americium may also he oxidized from trivalent to pentavalent state by hypochlorite in potassium carbonate solution. The product potassium americium (V) carbonate precipitates out. Curium and rare earth metals remain in the solution. An alternative approach is to oxidize Am3+ to Am022+ in dilute acid using peroxydisulfate. Am02 is soluble in fluoride solution, while trivalent curium and lanthanides are insoluble. 

Natural barium sulfate or barite is widely distributed in nature. It also contains silica, ferric oxide and fluoride impurities. Silica is the prime impurity which may be removed as sodium fluorosilicate by treatment with hydrofluoric acid followed by caustic soda. 

Both gas and liquid are very sol in water. Can be prepd by distUlation from the product of reaction of Ca fluoride (fluorspar) and sulfuric acid. Used as fluorinating agent in organic and inorganic reactions production of fluorine and A1 fluoride prepn of hydrofluoric acid production of fluoroborates, fluorosilicates, etc as an additive in liquid rocket propellants and for refining of uranium (Ref 3, p 588-R)... 

Common aqueous acids do not attack silica, except for hydrofluoric acid, which forms fluorosilicate anions, eg, SiF2-. The rate at which the various... 

Fluorosilicates. Salts of FluosiLicic Acid, H2SiFe. See Aluminum-, Ammonium-, Barium-, Calcium -, Copper-, Lead-, Magnesium-, Potassium-, Sodium-, and Zinc Fluosilicates in this Section... 

At room temperature, hafnium dioxide is slowly dissolved by hydrofluoric acid. At elevated temperatures, hafnium dioxide reacts with concentrated sulfuric acid or alkali bisulfates to form various sulfates, with carbon tetrachloride or with chlorine in the presence of carbon to form hafnium tetrachloride, with alkaline fluorosilicates to form alkali fluorohafnates, with alkalies to form alkaline hafnates, and with carbon above 1500°C to form hafnium carbide. 

During maintenance work on casings of fans used to extract perchloric acid fumes, seven violent explosions occurred when flanges sealed with lead oxide—glycerol cement were disturbed. The explosions, attributed to formation of explosive compounds by interaction of the cement with perchloric acid, may have involved perchlorate esters and/or lead salts. Use of an alternative inorganic silicate—hexa-fluorosilicate cement is recommended. 

Fluoromethylbenzoic acids, metallation, 9, 26-27 Fluoro(phenyl) complexes, with platinum(II), 8, 482 Fluorosilanes, elimination in fluorinated alkene activation, 1, 732 in fluorinated aromatic activation, 1, 731 and hydrodefluorination, 1, 748 Fluorosilicate anions, hypercoordinated anions, 3, 484 Fluorotoluenes, metallation, 9, 21 Fluorous alkylstannanes, preparation, 3, 820 Fluorous biphasic system, as green solvent, 12, 844 Fluorous ligands, with supercritical carbon dioxide, 1, 82 Fluorous media... 

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