Fluoride melt

The molten cryolite-based electrolyte (see above) is dissociated into Na cations and AIF , A1F4, F and various oxyfluoride anions, like Al204F4 . For aluminium electrolysis there are two possible cell reactions  

Molten Salts Chemistry and Technology, First Edition. Edited by Marcelle Gaune-Escard and Geir Martin Haarberg. 2014 John Wiley Sons, Ltd. Published 2014 by John Wiley Sons, Ltd. 

The reversible of these reactions at 1000 C are -1.187 and -1.065 V, respectively [2]. The difference in favour of Equation 1.1.2 reflects the fact that the so-called Boudouard reaction  

The question whether CO2 or CO is the primary anode product has been studied extensively [2, 3] and it has been shown that, except at very low current densities, the primary product is CO2 [2]. This conclusion has been based on carbon consumption studies (Equation 1.1.2) requires twice as much carbon per Faraday as Equation 1.1.1) and careful gas analysis, by avoiding disturbing side reactions, for example by using a diaphragm to separate the anode and cathode compartments [4], Side reactions are reactions between CO2 and carbon, either within pores in the interior of the anode, with carbon particles dispersed in the electrolyte, or with metal dissolved in the melt. In all cases the reaction product of these side reactions is normally CO. Dissolved aluminium can even reduce CO2 all the way to carbon [5]. By bubbling CO2 underneath a graphite anode it was shown [5] that, while Equation 1.1.3 did occur at zero current, the reaction ceased when the electrode was anodically polarised, even at quite low current densities (0.05-0.1A cm ). 

Most of the studies cited above were conducted in the time period between 1950 and 1980. However, as early as in 1936 Drossbach [6] demonstrated in a very elegant way how thermodynamic arguments could be applied to clarify the problem of identification of the primary anode product. If we consider the equilibrium  

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Investigation of fluoride melts containing tantalum or niobium is a very difficult procedure that involves high temperatures, corrosion activity and the... 

Three conceptual steps can be discerned in the definition of the ionic structure of fluoride melts containing tantalum or niobium. Based on the very first thermodynamic calculations and melting diagram analysis, it was initially believed that the coordination numbers of tantalum and niobium, in a molten system containing alkali metal fluorides, increase up to 8. 

Physicochemical properties of fluoride melts containing niobium... 

Analysis of the physicochemical properties of fluoride and oxyfluoride melts reveals that the complex ions are characterized by coordination numbers that do not exceed seven. Fluoride melts consist of the complex ions MeF72 and MeFe. Molten chloride-fluoride systems initiate the formation of heteroligand complexes of the form MeFgCl2 . Oxyfluoride and oxyfluoride-chloride melts can contain oxyfluoride complexes MeOF63 at relatively low concentrations. The behavior of the more concentrated melts can be attributed to the formation of oxyfluorometalate polyanions. 

Complexes in fluoride melts - vibration spectroscopy of fluoride melts... 

Vibration spectroscopy of fluoride melts 5.4.1. Experimental techniques... 

High refractory properties, extremely strong sensitivity to moisture and exceptionally high chemical activity of fluoride melts, especially of those containing ions of polyvalent metals, make spectral measurements of such melts extremely complicated. In order to obtain reliable results, the measurement cell must comply with three main requirements ... 

Due to the above requirements, typical optically-transparent materials, such as oxides (glass, quartz, alumina, zirconium oxide etc.) and halides (sodium chloride, lithium fluoride, calcium fluoride, potassium bromide, cesium bromide etc.) are usually unsuitable for use with fluoride melts. Therefore, no standard procedure exists at present for the spectral investigation of fluoride melts, and an original apparatus must be created especially for each particular case. 

The method based on a net melt-holder is used successfully for materials with relatively low melting points and is less promising for the investigation of fluoride melts containing tantalum and niobium. 

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