Fluorides removal with water

The removal of fluoride from water using defluoridation techniques is a common practice worldwide, both domestically and industrially. Current methods of fluoride removal from water include adsorption onto activated alumina, bone char and clay,1 precipitation with lime, dolomite and aluminium sulphate, the Nalgonda technique [4], ion exchange [5] and membrane processes such as reverse osmosis (RO), electrodialysis and very recently NF [6-9],... 

Uses Refractories (steel furnace linings) polycrystalline ceramic for aircraft windshields elec, insulations inorg. rubber accelerator paper mfg. white color standard reflector in optical instruments filler/ex-tender for paints, rubbers thickener for polyester resins fluoride removal in water treatment acid neutralization heavy metals removal water desilication alkali, anticaking agent, lubricant, nutrient in foods pharmaceuticals (antacid, mild laxative, buffer, mineral nutrient) absorbent, opacifier, buffer in cosmetics colorant in food-contact polymers, paper/paperboard in contact with aq./fatty foods in perfluoro-carbon cured elastomers for food contact activator in food-contact rubber articles for repeated use... 

Natural occurrence of magnesium in raw water enables fluoride removal due to formation of MgCOH) floes in the presence of lime. In this case, the water must be treated to a caustic alkalinity of 30 mg F-/L, a pH of >10.5 with option of recarbonation. Magnesia and calcined magnesite have also been tried for fluoride removal from water and their separation capacity was reported to improve with temperature. 

Another common method for alcohol protection is reaction with RjSiCl to give a silyl ether. Reaction conditions usually involve RjSiCl, with 4-dimethylaminopyridine (DMAP) as the base. Both the ease of preparation of the silyl ether and the stability of the protected species depend on the nature of the R groups. Trimethylsilyl ethers (ROTMS) are very labile and readily removed with water and dilute acid. The triethylsilyl group (ROTES) is a little more robust but may be removed with fluoride ion (the use of fluoride to cleave silyl groups reflects the strength of the Si-F bond). r-BuMejSiCl (TBDMSCl) reacts selectively with... 

Use of excess sodium drives the reaction, usually done under an argon or helium blanket, to completion. After cooling, the excess sodium is leached with alcohol and the sodium and potassium fluorides are extracted with water, leaving a mass of metal powder. The metal powder is leached with hydrochloric acid to remove iron contamination from the cmcible. 

The purified acid is recovered from the loaded organic stream by contacting with water in another countercurrent extraction step. In place of water, an aqueous alkafl can be used to recover a purified phosphate salt solution. A small portion of the purified acid is typically used in a backwashing operation to contact the loaded organic phase and to improve the purity of the extract phase prior to recovery of the purified acid. Depending on the miscibility of the solvent with the acid, the purified acid and the raffinate may be stripped of residual solvent which is recycled to the extraction loop. The purified acid can be treated for removal of residual organic impurities, stripped of fluoride to low (10 ppm) levels, and concentrated to the desired P2 s Many variations of this basic scheme have been developed to improve the extraction of phosphate and rejection of impurities to the raffinate stream, and numerous patents have been granted on solvent extraction processes. 

Samples for either Method 13A or Method 13B are obtained by the procedures outlined in Method 5 for particulates. As the gas stream passes through the samphng apparatus, the gaseous fluorides are removed by a chemical reaction with water, the particulate fluorides... 

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. 

Thus removal of water from classical rather inactive fluoride reagents such as tetrabutylammonium fluoride di- or trihydrate by silylation, e.g. in THF, is a prerequisite to the generation of such reactive benzyl, allyl, or trimethylsilyl anions. The complete or partial dehydration of tetrabutylammonium fluoride di- or trihydrate is especially simple in silylation-amination, silylation-cyanation, or analogous reactions in the presence of HMDS 2 or trimethylsilyl cyanide 18, which effect the simultaneous dehydration and activation of the employed hydrated fluoride reagent (cf, also, discussion of the dehydration of such fluoride salts in Section 13.1). For discussion and preparative applications of these and other anhydrous fluoride reagents, for example tetrabutylammonium triphenyldifluorosilicate or Zn(Bp4)2, see Section 12.4. Finally, the volatile trimethylsilyl fluoride 71 (b.p. 17 °C) will react with nucleophiles such as aqueous alkali to give trimethylsilanol 4, HMDSO 7, and alkali fluoride or with alkaline methanol to afford methoxytri-methylsilane 13 a and alkali fluoride. 

Aluminum fluoride is produced when partially dehydrated alumina hydrate reacts with hydrofluoric acids gas. The solid aluminum fluoride produced is cooled with noncontact cooling water prior to further processing, while the gases from the reactor are scrubbed with water to remove unreacted hydrofluoric acid from the gas stream. Aluminum fluoride is mainly used in the production of... 

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