Cryolite melt

Successful electrolysis of aluminum requires a liquid medium other than water that can conduct electricity. The key to the Hall-Heroult process is the use of molten cryolite, Na AlFg, as a solvent. Cryolite melts at an accessible temperature, it dissolves AI2 O3, and it is available in good purity. A second important feature is the choice of graphite to serve as the anode. Graphite provides an easy oxidation process, the oxidation of carbon to CO2. ... 

Thermochemical data on inorganic fluorides are applied practically to energy balancing and equilibria in the fluorochemical and allied industries. In addition, they can be used to rationalize some of the peculiarities of fluorine chemistry. Thus, the almost monopolistic position of cryolite melts in the manufacture of aluminum—and possible e,x-... 

Sum and Skyllas-Kazacos [44] studied the deposition and dissolution of aluminum in an acidic cryolite melt. The graphite electrode was preconditioned (immersed in cryolite melt) to saturate the surface of the electrode in sodium before aluminum deposition could be observed. Current reversal chronoamperometry was used to measure the rate of aluminum dissolution in the acidic melt. Dissolution rate was mass transport controlled [45] and in the order of 0.8 10 7 and 1.8 10 7 molcm 2s 1 at 1030 °C and 980 °C respectively [44]. 

Sterten [33] used activity data and calculated the concentration of complex ions in cryolitic melts saturated with alumina and the distribution of anions as a function of the molar cryolite ratio (NaF/AlF3), as shown in Figure 5. Julsrud [34] and Kvande [35] suggested the existence of some ions of Al2OF84 for electrolytes with cryolitic ratio = 3, while the A1202F42 ions were in majority in... 

Figure 17 Influence of additives on the solubility of alumina [153,154] in cryolite melts at 1010°C.

Figure 18 Isothermal densities [141] of binary cryolite melts at 1000°C. 

Figure 21 Interfacial tension of aluminum in cryolite melts at 1273 K 1, Utigard and Toguri [183] 2, Gherasimov and Belyaev [184] 3, Zemchuzina and Belyaev [187] 4, Dewing and Desclaux [182].

The ionic species present in the electrolyte appear to be Na+, A1F63 , A1F4 , F and certain A1—O—F complexes (see above). The fact that sodium is present as free ions, whereas aluminum is bound in complexes, and the fact that the sodium ion is the carrier of current, have led many authors to the assumption that sodium is the primary discharge product at the cathode. However, the thermodynamic data favor primary aluminum deposition on aluminum in cryolite melts. 

In aluminium reduction cells the carbon anode is placed in the upper part of the bath parallel to the liquid aluminium layer at the bottom, which acts as the cathode. The electrolyte between these electrodes consists of cryolite melt Na3AlF6 and alumina A1203 dissolved in it it may also contain such admixtures as A1F3, CaF2 and others. Dissolving of alumina in the bulk of the cell may be described as follows ... 

Kvande (1980, 1986) suggested that Al20F is the most abundant species in cryolite melts with low alumina contents. He further claimed that the solubility of alumina in NaF-AlFs melts attains a maximum at the composition of cryolite. Based on this, he argued that it is reasonable to assume that alumina, when dissolving, reacts predominantly with A1F anions according to the following reaction scheme... 

Silny (1987) has carried out very detailed contact angle measurements of cryolite melts on graphite and the measurement of surface tension using the sessile drop method. He used the Leitz microscope for photographing the sessile drop and used a sophisticated computerized approach to calculate the contact angle and the surface tension from the shape of the drop. However, the results showed a dispersion of approximately 20%. 

Currently, one of the best-known examples is the interfacial tension between aluminum and cryolite melts. However, the results of Zhemchuzhina and Belyaev (1960) and Gerasimov and Belyaev (1958) have been found sufficiently discrepant, so that more work was desirable. Attempts to deduce the interfacial tension from the shape of frozen drops of metals were completely unsuccessful, due to distortion introduced by freezing. It was apparent that direct measurements on the liquid system were necessary. 

There are several methods for interfacial tension measurement. However, at high temperatures, the choice of the measurement technique is limited. Since most high temperature liquids are corrosive and often non-transparent to visible light, the sessile drop technique can rarely be used. However, by the use of the X-ray beam, the shape of sessile drops immersed in another liquid may be determined. This technique was used by Utigard and Toguri (1985) in the measurement of interfacial tension of aluminum in cryolite melts. On the basis of the curvature of the drop and the density difference between the metal and the salt. X-rays lead to a fuzzy outline of the drop shape and together with the sensitivity of the drop outline on the interfacial tension, this technique is limited to an accuracy of about 5-10%. 

Kvande (1986) - the structure of alumina dissolved in cryolite melts,... 

The frequency dependence of the measured resistance of the CVCC conductivity cell was tested using molten KCl and three different compositions of cryolite melts. The statistical analysis of the results indicated that the electrical conductivity of each electrolyte is independent of the applied frequency. Figure 8.11 shows the conductivity results as a function of the applied frequency. No variation of the conductivity values was observed within dispersion of 1%. This verifies the principle on which the technique is based, i.e. that the slope of resistance versus the distance L in the tube-type conductivity cell is independent of the applied frequency. Conventional methods, on the other hand, have to take into account the applied frequency and many conductivity values were derived or extrapolated to the infinite frequency of the measuring current. 

In another article, Wang et al. (1993) used the CVCC conductivity cell for measurement of a variety of molten cryolite melts with additives of aluminum fluoride, aluminum oxide, calcium fluoride, magnesium fluoride, and lithium fluoride. On the basis of the measured results, a multiple regression equation for the electrical conductivity of cryolite melts was derived. Influence of the bath composition on the electrical conductivity at different bath temperatures was discussed. A comparison of the measured results with the published electrical conductivity values for cryolite melts was made. The new regression equation can be used to calculate electrical conductivity of cryolite melts in modern industrial bath chemistry. 

For various carbon electrodes in cryolite melts saturated with alumina at 1010°C, values of 0.0048 [Pg.505]

Pure cryolite melts at 1000°, and the mixture of maximum fusibility (915°) consists of 95 % cryolite with 5 % alumina. 

Grjotheim [23] assumed that the most probable dissociation schemes of cryolite melts are... 

The aluminum oxide and the ions in the cryolite melt form oxy-fluoride complexes, e.g.,... 

The principal metal refined in a molten salt medium is aluminium. Something approaching 2% of the total aluminium produced is refined by a process based on the principle illustrated in Fig. 4.7. The density of the impure aluminium is increased by the addition of copper (25—30%) and that of a cryolite melt by the addition of barium fluoride so that three distinct layers, pure aluminium, melt and aluminium/copper, are formed in the cell. On electrolysis the aluminium is transferred from the anode of impure aluminium to the top layer while the major impurities (i) Na, Mg, Ca and Sr are oxidized from the anode pool to the melt but do not reduce at the cathode and therefore accumulate in the melt, and (ii) Fe, Si, Mn, Zn (and Cu) are oxidized less readily than aluminium and hence remain in the anode pool. The aluminium obtained is very pure, being in the range 99.99—99.999%. 

Fig. 1.2 Oscillations of weight in some non-stationary chemical film systems (1) Alumina sample in cryolite melt with Si02 additives (2) germanium metal in the molten mixture KF-NaF-K2GeF6-Ge02 and (3) fresh cathode deposit of germanium in the melt as in the case 2...

As follows from the examples given in this chapter, general reaction mechanism (1.9), though simplified, remains very good approximation for many real processes, from high-temperature molten salts to, as we can see in Chap. 6, low temperature ionic melts (ionic liquids). We believe that, to some extent, this approach is also applicable to the processes in aqueous media. However, there are obvious restrictions due to rather narrow electrochemical window of aqueous systems and solvatation effects, which would necessitate the consideratitMi of reactions other than intervalence ones only. Sometimes, the coupled chemical reactions do not confine exclusively to the IVR even in molten salt systems. One of such examples, the electroreduction of Si(lV) species in cryolite melts, is considered below. 

Joubert et al used DFT to determine the structure and relative stability of Al-containing species involved in cryolitic melts. A concomitant topological analysis of ELF proved to be a powerful tool to obtain insight into the bonding properties. 

Unfortunately, owing to experimental difficulties, interfacial measurements in molten salts often need to be made at solid metal substrates, and these give rather less reliable results.For example, the agreement between the reported values of the minimum capacitance for both solid and liquid metals tends to be quite poor. This may be ascribed to melt impurities, especially water and, at the higher temperatures, spurious parallel components arising from materials instabilities. However, progress has been achieved in a number of experimentally difficult situations, notably in the alumina-cryolite melt system which is so important in aluminum production. 

Cryolite melts (NasAlFe, mp 1273 K), which are of considerable importance in aluminum refining studies, need to be purified from refractory oxides (silica, alumina, etc.) and iron, among others. Preelectrolysis at 1.6 V using a molten silver cathode in a graphite or molybdenum crucible is recom-mended. " "" ... 

Cryolite melts with dissolved aluminum metal Advanced ceramics Alumina (AljOj)( ) ( )Only in contact with alumina saturated melts (12 wt.% of dissolved AljOj). Inert or oxidizing atmospheres until 1000°C. 

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