Rare metal fluorides

Fig. 6.3. Free energy of formation of rare metal fluorides.

It is frequently possible to use a relatively inexpensive inert chloride salt mixture to which the rare metal fluoride is added, when for example the latter is less hygroscopic or more easily produced than the corresponding chloride. Uranium tetrafluoride in a bath of potassium chloride/lithium chloride, or... 

CoF is used for the replacement of hydrogen with fluorine in halocarbons (5) for fluorination of xylylalkanes, used in vapor-phase soldering fluxes (6) formation of dibutyl decalins (7) fluorination of alkynes (8) synthesis of unsaturated or partially fluorinated compounds (9—11) and conversion of aromatic compounds to perfluorocycHc compounds (see Fluorine compounds, organic). CoF rarely causes polymerization of hydrocarbons. CoF is also used for the conversion of metal oxides to higher valency metal fluorides, eg, in the assay of uranium ore (12). It is also used in the manufacture of nitrogen fluoride, NF, from ammonia (13). 

Re OPe . The final step in the chemical processing of rare earths depends on the intended use of the product. Rare-earth chlorides, usually electrolytically reduced to the metallic form for use in metallurgy, are obtained by crystallisation of aqueous chloride solutions. Rare-earth fluorides, used for electrolytic or metaHothermic reduction, are obtained by precipitation with hydrofluoric acid. Rare-earth oxides are obtained by firing hydroxides, carbonates or oxalates, first precipitated from the aqueous solution, at 900°C. 

The first experiments on the plasma chemical decomposition of fluoride solutions containing tantalum or niobium to obtain tantalum and niobium oxides were reported about fifteen years ago [524]. Subsequent publications were devoted to further development and expansion of the method for other refractory rare metals such as titanium and zirconium [525 - 532]. 

The double fluoride, ammonium hexafluorovanadate ((NH4)3VF6), forms from the oxide at 210 to 250 °C, but decomposes at 600 to 700 °C to yield pure vanadium fluoride. Examples of metal fluorides obtainable through the double fluoride route include uranium tetrafluoride, beryllium difluoride and the rare earth fluorides ... 

Fluoridizing roasting or fluorination is similar to chlorination, and is widely used in the treatment of several rare metal ores. Beryl, the most important ore of beryllium, can be opened by fusing with sodium silicofluoride at 850 °C ... 

All the rare earth metals except samarium, europium, and ytterbium can be prepared in a pure form by reducing their trifluorides with calcium. Magnesium fluoride is less stable than the rare earth fluorides and so magnesium does not figure as a reductant. Lithium forms a fluoride which is stabler than some of the rare earth fluorides and thus finds some use as a reductant. 

All the early work on plutonium was done with unweighable amounts on a tracer scale. When it became apparent that large amounts would be needed for the atomic bomb, it was necessary to have a more detailed knowledge of the chemical properties of this element. Intensive bombardment of hundreds of pounds of uranium was therefore begun in the cyclotrons at Berkeley and at Washington University in St. Louis. Sepa-ration of plutonium from neptunium was based on the fact that neptunium is oxidized by bromate while plutonium is not, and that reduced fluorides of the two metals are carried down by precipitation of rare earth fluorides, while the fluorides of the oxidized states of the two elements are not. Therefore a separation results by repeated bromate oxidations and precipitations with rare earth fluorides. 

The syntheses cited in this section have all involved attainment of high oxidation states in transition metal fluoro-anions, and thence in binary fluorides and in cationic species, by oxidation with F2 or other fluoro-oxidants in HF of metals or of compounds in lower oxidation states. A couple of examples are offered of syntheses of new fluoro-oxo-compounds involving fluorination of oxides already in high oxidation states with rare gas fluorides in HF. The ratio of F 0 in the ligands of these new compounds is greater than in the related compounds already reported. 

Alkali metal fluorides and ammonium bifluorides, fluoroborates, fluorophosphates, fluoroarsenates, fluoroantimonates, fluorotitanates, fluorozirconates, etc. require either anhydrous HF or aqueous HF for their preparations. Similarly, alkaline earth metals, transition metals, rare earth metal fluorides or their fluorosalts require... 

HYDRoaEN fluoride rarely occurs free in nature but its presence has been detected in the effluvia from vents in volcanic districts for example, R. V. Matteucci found it in the gaseous products of the fumeroles of Vesuvius. Hydrogen fluoride is formed by the direct union of the elements and by the action of fluorine on water, ammonia, hydrocarbons, and many organic compounds. It is also formed by the action of steam on some of the metal fluorides—-lead fluoride, silver fluoride, etc.—and by the action of some fluorides on water—e.g. iodine pentafluoride. The fluorides and fluosilicates are decomposed by sulphuric acid, with the evolution of hydrogen fluoride—the reaction is incomplete with hydrochloric acid in place of sulphuric acid. G. Gore used chromic fluoride and sulphuric acid R. Luboldt decomposed cryolite with the same acid. 

Other fluorofullerenes are obtained by reaction with metal fluorides. These may be less reactive than elemental fluorine, but consequently enable a much better selectivity in product generation. Depending on the metal chosen (either transition or rare earth metals might be considered), the composition of the fluorine compound ranges from C )F2 to about C60F36. Not only binary, but also ternary metal fluorides can be employed (Table 2.10). 

In reductions of rare earth metal fluorides and chlorides either with an active metal (Ca, Mg, Li) or by electrochemical means.7... 

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