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Lecture 24 Fluorine.Pdf Module:5 Dr. N. K. Patel Lecture:24 Fluorine Module: 5 Lecture: 24 FLUORINE INTRODUCTION Fluorine is denoted by F, is the lightest halogen atom among others which having atomic number 9 and mostly found in compounds. As it is more electronegative element having extreme reactivity, it is handled with great care. Fluorine is a pale yellow gas composed of diatomic molecules which having single stable isotope, fluorine-19. Fluorine is the 13thmost abundant element comprising 600 – 700ppm by mass in Earth's crust while found rarely in stars as compared to other light elements. The industrial production of fluorine is carried out from following three important minerals: Fluorite Fluorapatite Cryolite In 1530, fluorite was first described, in the context of smelting. The name derived from the Latin verb “fluo”, it means “flow”, as fluorite was added to metal ores to lower their melting points. Henri Moissan was carried out the isolation of fluorine by electrolysis in 1886. The main use of elemental fluorine was uranium enrichment. Hydrofluoric acid is the key intermediate for the fluorochemical industry as the production of elemental fluorine is difficulty and rarely converted to free fluorine. The fluorides of low charged metals are ionic salts and high charged metals are volatile molecular compounds. Generally inorganic fluorides are used in steel making and aluminium refining while organic fluorine compounds are used in refrigerant gases due to its high chemical and thermal stability. Traditional chlorofluorocarbons are widely banned. NPTEL 1 Module:5 Dr. N. K. Patel Lecture:24 Fluorine MANUFACTURE Raw materials The main raw material for fluorine production is high purity anhydrous hydrofluoric acid. Typical specifications for hydrofluoric acid are Assay Wt % HF2, min 99.95 SO2, max 0.005 H2SiF6, max 0.001 H2O2, max 0.02 Non-volatile acid (as H2SO4), 0.01 max Potassium bifluoride, KF, HF is used as a raw material to charge the cells initially and for makeup when cells are rebuilt. A newly charged cell required about 1400kg KF.HF. Overall consumption of KF.HF per kg of fluorine generated is small. Specifications of commercial-grade flake potassium bifluoride are Assay Wt % KF.HF2, min 99.3 Cl, max 0.01 K2SiF6, max 0.50 SO4, max 0.01 Fe, max 0.02 Pb, max 0.005 H2O, max 0.1 Manufacture process The generation of fluorine on an industrial scale is a very complex operation. The anhydrous hydrogen fluoride was first stored in bulk and charged to a holding tank from which it was continuously fed to the cells followed by vapourizer where it was heated. Electrolyte for the cells was prepared by mixing KF.HF with HF to form KF.2HF. NPTEL 2 Module:5 Dr. N. K. Patel Lecture:24 Fluorine Cooling r Vent e n Makeup b o media b i HF t Air water u r Vaporizer a k r c To n e S a g k t atmosphere i n r f a e t Cooling R e F Flue media Burner g H a gases r Propane o t s Flue Stack Water gases H2 header Vent to Compressor compressor Water F2 header Electric heat k n Coolers s e a r t n e i e NaF r s Surge o g n Cooler F towers u a e tank l H r Electrolytic d Rectifiers F o n t cell o s Water c Distillation Water Fluorine r 2 e r H d 2 e a F o d e T a o h e T h Air Recovered preheater HF storage Main Air feeder To HF storage Power transformer Light transformers Figure: Manufacture of Fluorine by electrolysis of KHF2 Block diagram of manufacturing process Diagram with process equipment Animation The newly charged cells were started up at a low current, which was gradually increased at a conditioning station separate from the cell operating position until full current was obtained at normal voltages. After conditioning cell was connected in series using ca 12V provided for each cell by a low voltage, 6000A d-c rectifier. Hydrogen fluoride content was maintained between 40 – 42% by continuous additions. The electorate level must be set and the seal between the fluorine and hydrogen compartments were maintained by controlling the certain electorate level below the cell head. The cells were operated at 95 – 1050C and cooled with water at 750C via surge tank. The gases leaving from the cell contain hydrogen, fluorine and 10% HF. Hydrogen and fluorine stream were separately joined through header of each cell for further processing in the plant. Entrained electrolyte in the product gas streams were removed by demisters and filters. Then, the gas streams were cooled to -1100C in cooler to reduce the HF concentration to approximately 3mole%. The condensed HF was recycled, and stored while hydrogen stream was scrubbed with caustic and vented or burned. NPTEL 3 Module:5 Dr. N. K. Patel Lecture:24 Fluorine Sodium towers or further cooling were used to freeze out the HF, if less than 3% HF levels were required. The concentration of HF was reduced to less than 0.2mol%. Compression or exhausters were normally required in both fluorine and hydrogen streams at some point in the system beyond the HF condensers. Material of construction and corrosion Corrosion occurs due to the chemical reaction with environment which results into degradation of the properties of a material. Corrosion rates of some of metal at different temperature is tabulated in the following table Table - Corrosion of metals at various temperatures Temperature, 0C Material 200 300 400 500 600 700 Nickel 0.018 0.129 0.74 0.86 Monel 0.013 0.051 15 3.8 Inconel 0.96 1.6 4.3 13 Copper 4.1 3.0 25 74 Aluminium 0 0.33 0.46 Magnesium 0 0 Iron 0 0.23 0.61 295 Steel 0.27% carbon 0.051 0.23 0.38 503 Stainless, 310 0 0.79 14.2 Due to the stability of the nickel fluoride film, nickel or monel should be used for critical applications. Polytetrafluoroethylene (PTFE) is resistant to the gas under nominally static conditions. But the PTFE must be free of impurities, foreign materials that could initiate ignition with fluorine. As fluoropolymers are more susceptible to ignition than the metals so it should be used as little as possible in fluorine handling. The equipment lines and fittings used for fluorine handling must be tight, dry and thoroughly cleaned of all foreign matter before use. Foreign matter such as organics, which are not removed could burn with fluorine and initiates the metal equipment. After cleaning, the dry nitrogen should be NPTEL 4 Module:5 Dr. N. K. Patel Lecture:24 Fluorine filled. The corrosion resistance of the materials used with fluorine depends on the passivation of the system which is a pickling operation used to remove the traces of foreign matter. The dry nitrogen in the system is slowly replaced with gaseous fluorine. Equipment The design of compressors and blowers for gaseous fluorine service may vary from multistage centrifugal compressors to diaphragm and piston types. Using bourdon-type gauges or pressure transducers, pressure is measured. If parts are contact with fluorine then stainless steel or monel construction is recommended. For all fluorine temperature-measuring equipment standard thermocouples are used. While for high temperature service, nickel-shielded thermocouples can be used. Dilute mixtures such as 10 or 20% F2 in N2 are less hazardous than pure fluorine, but still the precautions and procedures should be taken. Health and safety Fluorine is the most reactive element and dangerous material but may be handled safely using proper precautions. Safety glasses, neoprene coat, boots and clean neoprene gloves should be worn to afford overall body protection. This is effective against both fluorine as well as the hydrofluoric acid. Hydrofluoric acid is produced from the reaction "moisture in the air. Ace shields made of conventional materials or highly fluorinated polymers should be worn during the handling of equipment containing fluorine under pressure. A mask having a self-contained air supply should always be available. Shields should be provided for valves as it is a susceptible area for fluorine fires. High pressure binders and valves should be located with proper protection. Toxicity Fluorine has a sharp, penetrating odour detectable at levels well below the TLV. Fluorine is corrosive in nature and also irritating to the skin. Even at low concentrations, irritates the respiratory tract while at high concentrations fluorine inhalation may result in severe lung congestion. Due to the over exposure of fluorine leads to irritation or burns of the eyes, skins and NPTEL 5 Module:5 Dr. N. K. Patel Lecture:24 Fluorine respiratory tract. The owing emergency exposure limits (EEL) for humans have been suggested: 15.0ppm for 10min; for 30min; and 7.5 ppm for 60min. An eye is the most sensitive area to fluorine. Chronic toxicity not occurs due to the corrosive effects and discomfort associated with inhalation of fluorine. As the metabolic fate of fluorine is not clear, it does not seem that much is converted to fluoride ion in the body. Burns When pure fluorine gas is contacted with skin the skin burn occurs which is comparable to thermal burns and differs considerably from those produced by hydrogen fluoride. Fluorine burns heal much more rapidly than hydrofluoric acid burns. Disposal For disposal of fluorine, it is converted to gaseous perfluorocarbons or fluoride salts. As the long atmospheric lifetimes of gaseous perfluorocarbons, disposal by conversion to fluoride salt is more preferred. For that the following methods are recommended: Scrubbing with caustic solution Reaction with solid disposal agents such as an alumina, limestone, lime, and soda lime Reaction with superheated steam In a caustic scrubbing system caustic potash is preferred because of the higher solubility of the resulting potassium fluoride.
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