Fluorides, molten

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. 

Metalliding. MetaUiding, a General Electric Company process (9), is a high temperature electrolytic technique in which an anode and a cathode are suspended in a molten fluoride salt bath. As a direct current is passed from the anode to the cathode, the anode material diffuses into the surface of the cathode, which produces a uniform, pore-free alloy rather than the typical plate usually associated with electrolytic processes. The process is called metalliding because it encompasses the interaction, mostly in the soHd state, of many metals and metalloids ranging from beryUium to uranium. It is operated at 500—1200°C in an inert atmosphere and a metal vessel the coulombic yields are usually quantitative, and processing times are short controUed... 

Refractories in the Aluminum Industry. Carbon materials are used in the HaH-Heroult primary aluminum cell as anodes, cathodes, and sidewalls because of the need to withstand the corrosive action of the molten fluorides used in the process (see Aluminumand aluminum alloys). 

In the United States the alloy Inor 8 or Hastelloy N (Ni-16Mo-7Cr-5Fe) has been developed as a container material for molten fluorides containing uranium. The nickel-chromium-iron alloy originally considered as a suit-... 

For a long period of time, molten salts containing niobium and tantalum were widely used for the production by electrolysis of metals and alloys. This situation initiated intensive investigations into the electrochemical processes that take place in molten fluorides containing dissolved tantalum and niobium in the form of complex fluoride compounds. Well-developed sodium reduction processes currently used are also based on molten salt media. In addition, molten salts are a suitable reagent media for the synthesis of various compounds, in the form of both single crystals and powdered material. The mechanisms of the chemical interactions and the compositions of the compounds depend on the structure of the melt. 

Information exists about the use of measuring cells made entirely of diamond or graphite with or without embedded diamond windows. Diamond cells were used, for instance, by Toth and Gilpatrick [333] in the investigation of the Nb(IV) spectrum in a LiF - BeF2 molten system at 550°C. Windowless graphite cells for the IR spectroscopy of melts were developed by Veneraky, Khlebnikov and Deshko [334]. Diamond, and in some cases windowless sapphire or graphite micro-cells, were also applied for Raman spectroscopy measurements of molten fluorides. 

Figs. 70, 71 and 72 show schematic descriptions of the three latter above-listed methods. The main advantages and disadvantages of the methods were discussed in general by Volkov and Yazimirsky [294], for all types of molten salts. Regarding the application of the above-mentioned techniques in the investigation of molten fluorides, the following points should be mentioned. 

Opposite behavior was displayed by molten fluoride systems. For instance, no bands were observed in the thin layer emission spectrum of a KF - K2SiF6 melt, whereas increasing the melt layer to 10-20 mm led to the appearance of two intensive bands at 730 and 476 cm 1. These bands correspond to v3 and v4 vibrations of the complex ion SiF62 Solid K2SiF6 is characterized by IR absorption bands at 741 and 483 cm 1 [343]. 

Table 54 contains general information on molten fluoride systems reviewed here. 

The sodium reduction process was first implemented in Germany by Siemens and Halske A.G. and it remained a principally European method, while the American tantalum industry was founded exclusively on the electrolysis of molten fluorides [28, 576]. The production of tantalum by sodium reduction has gained worldwide acceptance and currently, nearly 90% of the world s tantalum is produced by the sodium reduction method [538]. 

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... 

Cl. Decroly, A. Mukhtar and R. Winand, Comparative Study of Electrocrystallization of Tantalum and Niobium from Molten Fluoride Mixtures, J. Electrochem Soc. Electrochemical Sciences, p. 905, Sept. 1968. 

Anhydrous lanthanum fluoride also may be made by passing dry hydrogen fluoride over lanthanum oxide. This process, however, produces trace amounts of lanthanum oxyfluoride, LaOF. Highly purified material may be obtained by passing dry purified HF over molten fluoride in a platinum crucible. 

Electrolysis in Molten Fluoride Salts or Fluoride Ion Solutions. . 199... 

Primary sources of the element are bastnasite and monazite, which contain from 4 to 8% praseodymium. Plant capacity involving liquid-liquid or solid-liquid organic ion-exchange processes for recovering the element is in excess of 100.000 pounds PreOn annually. Metallic praseodymium is obtained by electrolysis of Pr O] in a molten fluoride electrolyte, or by a calcium reduction of PrFj or PrCls in a sealed-bomb reaction. 

The high stability of the aluminate ion allows the production of concentrated solutions of aluminum with the virtual exclusion of the main metallic impurity, viz. iron as an oxide residue. The resultant impure aluminate solution is clarified and its temperature reduced when the reverse of the above reaction occurs with the formation of A1203,3H20 by a slow crystallization procedure. The high-purity alumina trihydrate product is calcined and then reduced electrochemically in a molten fluoride bath by the well-known Hall-Heroult process. The major problems in the Bayer process have their origin in the coordination chemistry of aluminum in alkaline solutions. The... 

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