Cryolite-based melts

Among the systems containing a trivalent cation, technologically most important are the cryolite-based melts used as electrolytes for aluminum production. 

More information on cryolite-based melts can be found in specialized books by Grjotheim et al. (1982) and Thonstad et al. (2001) devoted to the fundamentals of aluminum electrolysis. 

For cryolite-containing systems the model of Utigard is, however, not suitable, since the activity of an ionic component cannot be assumed to be proportional to the surface concentration of this component. Fernandez and 0stvold (1989) have modified the Utigard s model and for the surface tension of the cryolite-based melts, they used the equation... 

The boiling point method using the apparatus described was applied to a number of cryolite-based melts, in the following studies... 

Gilbert et al. (1995) - the acid-base properties of cryolite-based melts with CaF2, MgF2, and AI2O3 additives,... 

Fig. 3.8 Transition times analysis for the process of Si(TV) rednction in cryolite-based melt at 900 °C with 0.75 wt% Si02- Dashed lines show hypothetical plots fin inactivation (deposition) of Si(ll) (4) and pre-dissociation (B) in the schcane (3.14)...

It is obvious that the gaseous products in the oxide-containing cryolite-based melts and in the oxide-containing chloride melts are very different. In fluoride melts the primary anode product is CO2 at normal current densities, and the CO contained in the off-gas from aluminium cells has mainly been formed through secondary reactions, as outlined above. Only at very low current densities CO may be a primary product. 

Aluminum is produced by the electrolysis of alumina dissolved in cryolite-based melts. The electrolyte also contains some impurities, that is, iron, silicon, phosphorus, sulfur, and so on [1], The impurities are introduced into the electrolyte with the alumina or fluoride salts or they originate from the carbon anodes. In the Hall-Heroult process we know that sulfur originates as sulfur in the anode carbon (1-5 wt% S) plus some sulfur contained in the alumina and in the aluminum fluoride. 

In this paper, the influence of alumina LOI (loss on ignition) on the dissolution behavior of alumina in cryolite-based melts was investigated. Besides bath temperature and bath composition, the dissolution rate of alumina depends on its physicochemical properties such as particle size distribution, density, specific surface area, crystal structure, and LOI. 

This strongly supports the notion that electrical charge in cryolite-based melts is transported mainly by cations. 

Julsrud (1979) proposed a thermodynamic model for cryolite-alumina melts based on cryoscopic and calorimetric measurements and considered the Al20Fg , Al20F , Al20FjQ, Al202F4, and Al202Fg species to be present in alumina saturated melts. 

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... 

The principal data on the transport phenomena in cryolite melts was discussed in the monograph Aluminium Electrolysis [2], Transference (transport) numbers are discussed also in the third edition of Aluminium Electrolysis [3]. The treatment is based on results published by Frank and Foster [4], Tual and Rolin [5, 6], and Dewing [7]. Frank and Foster investigated transport phenomena in cryolite-alumina melts by means of a radioactive tracer method. It was found that = 0.99. Tual and Rolin applied the classical Hittorf method. These authors also came to the conclusion that in neutral or basic electrolytes the transference number of the sodium cation is close to unity. With increasing acidity of the bath, the transport number of Na" " decreases. This is often explained by participation of the F ions in the conduction [2, 3]. Even in electrolytes with an excess of 7 mass% AIF3, the transference number of the sodium cation did not drop below Na L13A1F5 melt at 1030 K, Dewing [7] found that the transport number t+ = 0.957 0.08. 

Silicate-Based Melts. Silicate or Si02 melts have been studied by several investigators in an effort to develop a commercial process for electrowinning silicon. The molten solutions most often studied contained Si02 in cryolite. 

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. 

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%. 

However, there have been suggestions in the literature for reference electrodes based on the aluminalaluminium couple. An electrode comprising a molten salt and aluminium was first described by Drossbach [81]. Common types of reference electrodes involve a molten aluminium pool covered by molten cryolite contained within a thin walled tube of sintered alumina or boron nitride. This housing must be electrically insulating but able to transfer ions (i.e. porous and ionically conductive). Also it must be resistive to corrosion. Electrolytes must be in contact and a tortuous path is required such that the different melts do not mix but establish a stable liquid junction. 

Skybakmoen E, Gudbransen H, Stoen LI. Chemical resistance of sidelining materials based on SiC and carbon in cryolitic melts - a laboratory study. Light Met. 1999 128 215-22. 

The main problem in using the Hall process for silicon electrodeposition is that silicon melts at a much higher temperature than aluminum (1412°C compared to 660°C). Cryolite can not be used at this temperature due to volatization problems, so a binary or ternary melt containing SiOg had to be developed that would be stable above this temperature. Johnson (29) indicated that calcium and magnesium based silicate melts looked favorable, while other alkaline earth and alkali metal silicates were less desirable. 

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