Chemical Compounds - Processes
Fluorspar is the most important source of fluorine. In the manufacture of hydrogen fluoride, powdered fluorspar is distilled with concentrated sulfuric acid in a lead or cast-iron apparatus. The hydrogen fluoride is obtained in a fairly anhydrous state by fractional distillation in copper or steel
vessels and stored in steel cylinders. The usual impurities in commercial hydrogen fluoride are sulfurous and sulfuric acids as well as fluorosilicic acid, arising from the presence of silica in
the fluorspar. Traces of moisture may be removed by electrolysis with platinum electrodes.
The preparation of the free element is carried out by electrolytic procedures in the absence of water. Generally these take the form of electrolysis of a melt of potassium fluoride-hydrogen
fluoride (in a ratio of 1 to 2.5-5) at temperatures between 30 and 70° C, 80 and 120° C, or at 250° C. During the process the hydrogen fluoride content of the electrolyte is decreased and
the melting point rises; it is therefore necessary to add hydrogen fluoride. In the high-temperature cell the electrolyte is replaced when the melting point reaches over 300 °C. Fluorine can be safely stored under pressure in cylinders of stainless steel if the valves of the cylinders are free from traces of organic matter.
The element is used for the preparation of various fluorides, such as chlorine trifluoride, sulfur(VI) fluoride, or cobalt(III) fluoride. The cobalt compound and certain other metal fluorides are important fluorinating agents for organic compounds. With appropriate precautions, the element itself may be used for the fluorination of organic compounds. Fluorine derivatives of hydrocarbons (compounds of carbon and hydrogen) are useful as refrigerants and as lubricants. The element is also used for the preparation of uranium(VI) fluoride, which is important in the process of separation of uranium-235 from natural uranium. Its outstanding oxidizing properties make elemental fluorine of possible interest as an oxidizer in rocket propulsion. Hydrogen fluoride and boron(III) fluoride are produced commercially because they are good catalysts for the alkylation reactions used to prepare organic compounds of many kinds. The addition of sodium fluoride to drinking water to reduce the incidence of dental caries in children is a matter of some controversy.
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