Halides, molten

A signiflcairt property of the alkali metal halides is the solubility of the metals in their molten halides. Typical values of the consolute temperatures of metal-chloride melts are 1180°C in Na-NaF, 1080°C in Na-NaCl, 790°C... 

Molten halides are liquid electrolytes in many instances, and their decomposition may be canned out in principle to produce the metal and halogen, usually in the gaseous state. The theoretical decomposition voltage, E°, is calculated from the Gibbs energy of formation tlrrough the equation... 

In contrast to Li and Na, K cannot be produced by the electrolysis of molten halide mixtures. While the melting point of KCl (776°C) is lower than that of NaCl,... 

The vapor above mixtures of Cd and CdCl2 contains Cd(I) as CdCl. Solutions of Cd in anhyd Cd(II) salts arc deep red-black and diamagnetic, which implies an absence of Cd ions. At 600°C the solubility of Cd in the molten halides drops from 18 and 14 mol% for CdCl2 and CdBr2, respectively, to 1.5 mol% for Cdl2. The equilibrium ... 

Both liquid and solid electrolytes can be used, ranging from molten halides, such as a eutectic mixture of LiCl and KCl, to very sophisticated solid-state electrolytes such as calcia or yttria stabilised zirconia, CSZ, YSZ, which are conductors of oxygen ions. 

Many reactions thut cannol lake place in aqueous solutions because of the reactivity of water may be performed readily in molten sails. Both chlorine and fluorine react with water (the latter vigorously), and so the use of these oxidizing agents in aqueous solution produces hydrogen halides, etc., in addition lo the desired oxidation products. The use of the appropriate molten halide obviates this difficulty. Even more important is the use of molten halides in the preparation of these halogens ... 

A dissolution mechanism for zinc, cadmium and mercury in their molten halides has been proposed on the basis of experimental and literature data.949 Dissolution occurs at the metal-salt phase boundary. Adsorbed M24- cations are reduced to M+ ions which then migrate into the salt phase where Mi4" dimers form. The stability of the M2+ ions was found to increase in the order Zn < Cd Hg (the latter is so stable that Hg+ is undetectable in solution). 

The above work concentrated most of its attention on the use of zinc chloride as the molten halide and on the use of bituminous coal extract as feed to the process. Hydrocracking of the extract (1) and regeneration by a fluidized-bed combustion technique of the spent catalyst melt (2) from the process were both demonstrated in continuous bench-scale units. 

The addition of a hydrogen-donor solvent to the sub-bituminous coal at these mild conditions, appears to assist in melting the coal to permit access of the molten halide catalyst. 

Redkin [51] calculated the specific conductivity of molten halides using the equation... 

There are numerous data in the literature [59] which demonstrate that for molten halides the equivalent conductivity calculated by means of the Nernst-Einstein relation is significantly higher than the directly measured conductivity value. This is due to the fact that the structural entities of molten salts make unequal contributions to diffusion and electrical conductivity. 

The mercury electrode has been used by researches in melts up to about 250°C, especially for nitrate and chloroaluminate melts [82,83], Liquid lead (m.p. 327.5°C) is another liquid metal [84] used for molten halides. Liquid electrodes of bismuth (m.p. 271°C), indium (m.p. 157°C) and thallium (m.p. 303°C) have also been, used but to a limited extent [85],... 

Halogen electrodes in molten salts are being used especially for the study of molten halides. In 1930 Hildebrand and Salstrom [98-100] were the first who applied a chlorine reference electrode in molten salts. Up to the present, other authors used and improved this electrode [101-105], The reversible bromine/ bromide electrode was set up for the first time by Hildebrand and Salstrom [98] and improved by Murgulescu et al. [106,107], In the following an example is shown of how to obtain and use a reversible gas electrode in molten salts for the determination of thermodynamic activity. 

In Figure 4.10, values of ffLV for molten halides at TF are plotted versus the quantity C3.Le.vm 2/3 (Tanaka et al. 1996) (it should be noted that the constant C3 = 1.18 x 10 8 is similar but not equal to the constant Q of equation (4.1)). Fused halides have comparatively low surface energies (lying between 0.05 and 0.4 J/m2) for two main reasons (i) their heat of evaporation is low compared to that of metals (typical values being 30 as compared to 300 kJ/mole) and (ii) the coefficient of proportionality between (TLv and Le.vm 2/3 for halides is (as for oxides) only half of that for metals. 

Figure 4.10. Correlation between surface energy of molten halides and C3.Le.vm 2/3 (with C3 = 1.18 x 10" ). Reprinted from (Tanaka et al. 1996) with kind permission of the authors.

Figure 9.6. Contact angles of pure molten halides on graphite measured in a dry inert gas at 1000 C as a function of surface energies of halides (Morel 1970).

Electrolysis is often used to reduce the most active metals. In Chapter 11 we considered the electrolytic production of aluminum metal. The alkali metals are also produced by electrolysis, usually of their molten halide salts. 

A very large number of complex halides containing alkali-metal ions have been investigated. Many of these are covered in the section on Molten Halides. The majority of the remainder are presented in Table 5.180-192... 

The influence of absorption by the quartz cell was examined by measurements of blank cells and some molten salt samples. In the blank test, the displacement of the absorption baseline was not observed. The lowest energy, which could be used in this system, was estimated to be lOkeV. Thereby, the XAFS spectra of some molten halides could be successfully obtained. The results showed that a small difference was observed among their results. It was concluded that the developed device is also suitable for the XAFS measurement of hygroscopic molten salts. 

Capacitance measurements of carbon electrodes have also been made in molten halides, particularly chlorides [30-32], molten nitrates [33, 34], and in cryolite—alumina melts (graphite and glassy carbons). In cryolite—alumina melts, the double-layer capacitance of the basal plane of graphite, in the range 0.7—1.0 V (vs aluminum reference electrode) is about 20[xF/cm at 0.9 V, i.e., in a potential range where no appreciable flow of current has been observed. Data indicate that the capacitance is influenced by adsorbed species from the melt, possibly yielding intercalation compounds, and uncertainty in the true area of the electrode [34]. 

---