Products and processes based on fluorine and their uses

1. Potassium perchlorate: Fluorine is a powerful oxidizing agent. When it is bubbled through a solution of potassium chlorate (KClO3), the latter is oxidized to potassium

perchlorate (KClO4) according to the reaction

F2 + 2KClO3 +H2O → 2HF + KClO4

2. Anhydrous hydrogen fluoride (HF): Fluorine reacts explosively with hydrogen even at low temperature. So much so that even water is dissociated by it as per the reaction

2F2 + 2H2O → 4HF + O2

The oxygen liberated oxygen is highly charged with ozone. The HF-gas is a powerful chemical.

3. Sodium silico-fluoride: Also called sodium fluoro-silicate or sodium hexafluorosilicate (F6Na2Si), it intensifies and accelerates glass-melting.

4. Uranium hexa-fluoride (UF6): It is used for refining and enrichment of uranium.

5. Fluoro-silicates of ammonium, magnesium and zinc: These are used as preservatives for hide, wood, etc.

6. Fluoro-carbon: These are used as filter membranes in semiconductor fabrication industry.

7. Fluoro-polymer filter: These are essentially fluoro-carbons and are inert. One such compound poly-tetra-fluoro-ethylene or PTFE is an excellent material for filter membranes which are made into cartridges for operation at nano-size level. The pores of (-) 0.1 micron size are suitable for filtering high-purity aqueous chemicals which are required for etching and cleaning of ultra-high-purity semiconductor chips.

8. Sodium fluoride (NaF), Stannous fluoride (SnF2) and Sodium monofluorophosphate

(Na2PO3F): These are added as components of toothpastes for preventing tooth decay by

virtue of the poisonous action of fluorine on bacteria.

9. Uranium metallurgy: In the process, both HF gas and fluorine are used. The uranium ore is first concentrated and the concentrate containing 50–90% U3O8 is refined for extraction

of uranium metal. For this purpose, the concentrate is first dissolved in n i t r i c a c i d , a n d t h e n a n aqueous solution of high purity uranium compounds is obtained by solvent extraction process using tributyl phosphate dissolved in kerosene or hexane as the solvent. The product is pure uranyl nitrate solution. This uranyl nitrate is thermally decomposed to obtain UO3, which is then reduced to UO2 by hydrogen. UO2 is heated in

presence of anhydrous HF gas to yield UF4 which is reduced to metallic uranium by reacting

it with either calcium metal or (more widely) magnesium metal at 6000C according to the reaction :

UF4 + 2Mg → U + 2 Mg F2

Sometimes it may be necessary to enrich the concentration of U235 in the metallic

uranium or its compounds. For this purpose UF4 is first fluorinated further to yield UF6.

Fluorite (Fluorspar) 185 lighter U2 3 5-containing UF6 (molecular weight 349) and the heavier U238-

containing UF6 (molecular weight 352) are partially separated. In this way the concentra-

tion of U235 may increase from about 0.7% to 3 per cent. The enriched UF6 so obtained

is reduced again to UF4 by means of hydrogen, and enriched uranium metal obtained by

metallic reduction as above. D. Uses based on a mix

1. Glass: Physically, common standard glass is an amorphous, hard, brittle and transparent substance and chemically, it is a mixture of the silicates of sodium, calcium and magnesium. The composition of typical ordinary glass is [18Na2O.2MgO.8CaO.72SiO2].

Essentially, the process of manufacture of glass consists in melting a mixture of 47% silica sand,14% soda (Na2CO3), 12% dolomite (CaCO3.MgCO3), 3.5% limestone (CaCO3), 2.5%

carbon (coke) and the balance broken waste glass (called cullet) at 1400-15000C, and then cooling it first to a thick viscous liquid at 800-9000C and then rapidly quenching it to a solid glass product. The low-melting soda and broken glass become liquid first and silica (quartz) being soluble in boiling soda, dissolves in this liquid to form low-melting sodium silicate which then reacts with the CaO and MgO to form glass. So far as glass is concerned, the main uses of fluorite are as follows.

(i) The compound sodium hexafluorosilicate (F6Na2Si) is added to serve double

purposes to supply the sodium to supplement the soda for fluxing, and to increase fluidity of the melt.

(ii) In the manufacture of float glass and fiber glass, fluorite is directly added. Float glass is a type of sheet glass (above 0.5 mm thickness) in large sizes that is exceptionally smooth on both surfaces and free from internal stress. For its manufacture, molten glass at 11000C is discharged from the furnace onto a bed of molten tin, on which it floats and spreads out freely in the form of a long strip. It is then cooled either slowly (annealed float glass) or suddenly (heat-strengthened float glass). Molten tin by virtue of its low shrinkage on solidification, higher-than-glass specific gravity, non- adherence to glass surface, and low coefficient of friction allows the molten glass to spread freely without any resistance and then solidify without any wrinkle. Fiber glass, on the other hand, is glass formed into fine fibers (thinner than human hair) by drawing molten glass through tiny holes. In both these processes, liquidity and free- flowing ability of the molten glass are very crucial, and role of fluorite assumes importance because of its ability to improve both these parameters. The Indian user industries generally use fluorite containing 90 to 98% CaF2 with size 75 to 150

microns.

(iii) In the manufacture of opalescent glass, fluorite, by virtue of its lower refractive index (1.433) than glass (1.53) serves to opacify glass (covering power of opacifier depends on the difference between the refractive indices of it and that of the substance opacified, and also on the particle size of the opacifier).

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1. Low-grade fluorite powder:- Nowadays low-grade fluorite powders are moulded and pressed into the shape of briquettes. Such fluorite briquettes have become popular in some European countries. For easy loading, olivine and bauxite are also mixed with the fluorite while briquetting.

2. Topaz: Chemically, topaz is aluminium fluorine hydroxyl silicate [Al2(SiO4)(OHF)2].

It is regarded as a future source of fluorine, particularly the non-gem type which have no other industrial use.