Electrolysis fluorine

Chemical Production. Electrolytic production of chemicals is conducted either by solution (water) electrolysis or fused-salt electrolysis. Fluorine, chlorine, chlorate, and manganese dioxide are Hberated from water solutions magnesium and sodium are generated from molten salt solutions. 

All the free halogens are produced commercially by oxidation of their anions. Fluorine and chlorine are both produced by electrolysis fluorine from a molten 1 2 mixture of KF and HF, and chlorine from molten NaCl. 

Graphite reacts with alkali metals, for example potassium, to form compounds which are non-stoichiometric but which all have limiting compositions (for example K C) in these, the alkaU metal atoms are intercalated between the layers of carbon atoms. In the preparation of fluorine by electrolysis of a molten fluoride with graphite electrodes the solid compound (CF) polycarbon fluoride is formed, with fluorine on each carbon atom, causing puckering of the rings. 

Fluorine cannot be prepared directly by chemical methods. It is prepared in the laboratory and on an industrial scale by electrolysis. Two methods are employed (a) using fused potassium hydrogen-fluoride, KHFj, ill a cell heated electrically to 520-570 K or (b) using fused electrolyte, of composition KF HF = 1 2, in a cell at 340-370 K which can be electrically or steam heated. Moissan, who first isolated fluorine in 1886, used a method very similar to (b) and it is this process which is commonly used in the laboratory and on an industrial scale today. There have been many cell designs but the cell is usually made from steel, or a copper-nickel alloy ( Monel metal). Steel or copper cathodes and specially made amorphous carbon anodes (to minimise attack by fluorine) are used. Hydrogen is formed at the cathode and fluorine at the anode, and the hydrogen fluoride content of the fused electrolyte is maintained by passing in... 

Fluorine is produced by the electrolysis of anhydrous potassium biduoride [7789-29-9] KHF2 or KF HF, which contains various concentrations of free HF. The duoride ion is oxidized at the anode to Hberate duorine gas, and the hydrogen ion is reduced at the cathode to Hberate hydrogen. Anhydrous HF caimot be used alone because of its low electrical conductivity (see Electrochemical processing, inorganic). 

Significant amounts of cryoHte are also recovered from waste material ia the manufacture of aluminum. The carbon lining of the electrolysis ceUs, which may contain 10—30% by weight of cryoHte, is extracted with sodium hydroxide or sodium carbonate solution and the cryoHte precipitated with carbon dioxide (28). Gases from operating ceUs containing HF, CO2, and fluorine-containing dusts may be used for the carbonation (29). 

Manufacture and Economics. Nitrogen tritiuoride can be formed from a wide variety of chemical reactions. Only two processes have been technically and economically feasible for large-scale production the electrolysis of molten ammonium acid fluoride and the direct fluorination of the ammonia in the presence of molten ammonium fluoride. In the electrolytic process, NF is produced at the anode and H2 is produced at the cathode. In a divided cell of 4 kA having nickel anodes, extensive dilution of the gas streams with N2 was used to prevent explosive reactions between NF and H2 (17). 

Zirconium trifluoride [13814-22-7], ZrP, was first prepared by the fluorination of ZrH2 using a mixture of H2 and anhydrous HP at 750°C (2). It can also be prepared by the electrolysis of Zr metal in KF—NaF melts (3). Zirconium trifluoride is stable at ambient temperatures but decomposes at 300°C. It is slightly soluble in hot water and readily soluble in inorganic acids. This compound is of academic interest rather than of any industrial importance. 

The perchloryl fluoride [7616-94-6] FCIO, the acyl fluoride of perchloric acid, is a stable compound. Normally a gas having a melting poiat of —147.7° C and a boiling poiat of —46.7°C, it can be prepared by electrolysis of a saturated solution of sodium perchlorate ia anhydrous hydrofluoric acid. Some of its uses are as an effective fluorinating agent, as an oxidant ia rocket fuels, and as a gaseous dielectric for transformers (69). 

Diacyl peroxides (20, R = R = alkyl or aryl) have been obtained from the oxidation of carboxyhc acid potassium salts by Kolbe electrolysis or by elemental fluorine (187). 

Silver Fluoride. Silver fluoride, AgF, is prepared by treating a basic silver salt such as silver oxide or silver carbonate, with hydrogen fluoride. Silver fluoride can exist as the anhydrous salt, a dihydrate [72214-21-2] (<42° C), and a tetrahydrate [22424-42-6] (<18° C). The anhydrous salt is colorless, but the dihydrate and tetrahydrate are yellow. Ultraviolet light or electrolysis decomposes silver fluoride to silver subfluoride [1302-01 -8] Ag2p, and fluorine. 

The electrolyte used in fluorine cells is KF—HF in a ratio that minimizes melting point, HF vapor pressure, and corrosion of materials. Various ratios have been used. The manufacture of fluorine in the early 1990s was based on the electrolysis of KF 2HF, which allows cell operating temperatures of 100-105°C. 

NF3 was first prepared by Otto Ruffs group in Germany by the electrolysis of molten NH4F/HF and this process is still used commercially. An alternative is the controlled fluorination of NH3 over a Cu metal catalyst. 

The final route to fluorine compounds is electrofluorination (anodic fluorination) usually in anhydrous or aqueous HF. The preparation of NF tFl3 j (x = 1, 2, 3) has already been described (p. 818). Likewise a reliable route to OF2 is the electrolysis of 80% FIF in the presence of dissolved MF (p. 638). Perchloryl fluoride has been made by electrolysing NaC104 in FIF but a simpler route (p. 879) is the direct reaction of a perchlorate with fluorosulfuric acid ... 

The fluoride and chloride ions are very difficult to oxidize (Elx F = —2.889 V E°x Cl- = —1.360 V). Hence the elements fluorine and chlorine are ordinarily prepared by electrolytic oxidation, using a high voltage. As pointed out in Chapter 18, chlorine is prepared by the electrolysis of aqueous sodium chloride ... 

Oxidation A half-reaction in which there is an increase in oxidation number, 88 chromium, 548 electrolysis and, 498 fluorine, 557 halogens, 557-558 oxoacids, 568-570 oxoanions, 568-570 species strength, 506-507q transition metals, 546t zinc, 86-87... 

Oxidation through electrolysis is used to make fluorine and chlorine. Chlorine, for example, is... 

Gaseous fluorine is also prepared by electrolysis of molten fluoride salts but simpler methods are available for the preparation of bromine and iodine. Chemical oxidation, usually with chlorine as the oxidizing agent, provides Br2 and I2 economically because chlorine is a relatively inexpensive chemical. The reactions are... 

Figure 7. Section through cell after electrolysis starts (7 V) with operating carbon-based anode showing lenticular bubbles of fluorine on anode surface.

We have already described the refining of copper and the electrolytic extraction of aluminum, magnesium, and fluorine. Another important industrial application of electrolysis is the production of sodium metal by the Downs process, the electrolysis of molten rock salt (Fig. 12.15) ... 

Fluorine comes from the minerals fluorspar, CaF, cryolite, Na3AlF6 and the fluorapatites, Ca,F(P04)3. The free element is prepared from HF and KF by electrolysis, but the HF and KF needed for the electrolysis are prepared in the laboratory. Chlorine primarily comes from the mineral rock salt, NaCl. The pure element is obtained by electrolysis of liquid NaCl. Bromine is found in seawater and brine wells as the Br ion it ts also found as a component of saline deposits the pure element is obtained by oxidation of Br (aq) by Cl,(g). Iodine is found in seawater, seaweed, and brine wells as the I" ion the pure element is obtained by oxidation of I (aq) by Cl,(g). 

C19-0034. Fluorine is manufactured by the electrolysis of hydrogen fluoride dissolved in molten KF ... 

Some industrial uses of fluorine require molecular fluorine, F2, which is produced by electrolysis of HF. As Figure 21-16 shows, the cell uses liquid HF to which KF is added as an electrolyte. The redox chemistry is Anode 2 F" F2 + 2 e ... 

The electrochemical properties of fluorographite are also interesting in connection with the electrolysis of melted KF-2HF, which is used for industrial production of fluorine. Fluorine is here evolved at the carbon anode, which is spontaneously covered with a passivating layer of fluorographite hence it causes an undesired energy loss during the electrolysis. 

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