Alkaline earth chlorides, molten

Figure 6 Specific conductance K vs. ionic radii [52] for molten alkali chlorides MCI (M = Li, Na, K, Rb, Cs) at 1080 K and for molten alkaline earth chlorides MC12 (M = Be, Mg, Ca, Sr, Ba) at 1200 K.

The presence of oxides and the formation of oxofluoro-complexes in molten electrolytes may be sometimes unwanted, but in many cases they are the fundamental features of the system. For instance, the formation of oxide complexes in alkali-alkaline earth chloride melts may be mentioned. The formation of oxofluoride complexes in molten cryolite-alumina melts, used as electrolytes for aluminum production, is typical as well. On the other hand, the presence of oxofluoride complexes in electrolytes used for niobium production was initially regarded as unwanted. Recently, however, it has been proven that their presence in niobium electrolytes plays an important role in the niobium electrodeposition. In the following, some technologically important examples of systems containing halides and oxides will be described. 

The problem of solubility of the reactive vapors TiCl4, ZrCl4, and HfCl4 in molten alkali and alkaline earth chlorides is directly related to the phase diagrams of the binary systems MCI-XCI4 and any desired solubility may be obtained, providing that the pressure of the XCI4 vapor is maintained at the value required by composition and temperature. 

The general method for preparing the alkaline earth elements is to convert the mineral to a chloride or a fluoride by treatment with HC1 or HF. Then the molten salt is electrolyzed or, as in the case of BeF2, reduced with a chemical reducing agent such as Mg. 

Electrowinning Generally this method is limited to La, Ce, Pr and Nd because of their low-melting points. The rare earth salt (fluoride, chloride, etc.) mixed with an alkali or alkaline-earth salt is heated to 700-1100°C and then an electric dc current passed through the cell. If the bath temperature is above the melting point of the R, drops of the molten metal drip off of the cathode and are collected at the bottom of the cell. Generally, the electrowon metal is not as pure as that obtained by metallothermic reduction. 

The electrolysis Of fused alkali salts.—Many attempts have been made to prepare sodium directly by the electrolysis of the fused chloride, since that salt is by far the most abundant and the cheapest source of the metal. The high fusion temp. the strongly corrosive action of the molten chloride and the difficulty of separating the anodic and cathodic products, are the main difficulties which have been encountered in the production of sodium by the electrolysis of fused sodium chloride. Attention has been previously directed to C. E. Acker s process for the preparation of sodium, or rather a sodium-lead alloy, by the electrolysis of fused sodium chloride whereby sodium is produced at one electrode, and chlorine at the other but the process does not appear to have been commercially successful. In E. A. Ashcroft s abandoned process the fused chloride is electrolyzed in a double cell with a carbon anode, and a molten lead cathode. The molten lead-sodium alloy was transported to a second chamber, where it was made the anode in a bath of molten sodium hydroxide whereby sodium was deposited at the cathode. A. Matthiessen 12 electrolyzed a mixture of sodium chloride with half its weight of calcium chloride the addition of the chloride of the alkaline earth, said L. Grabau, hinders the formation of a subchloride. J. Stoerck recommended the addition of... 

Like the alkali metals, the pure alkaline earth elements are produced commercially by reduction of their salts, either chemically or through electrolysis. Beryllium is prepared by reduction of BeF2 with magnesium, and magnesium is prepared by electrolysis of its molten chloride. 

Alkaline-earth metals. Of these metals, Ca, Sr, and Ba, only calcium is produced commercially in appreciable quantities. These three metals are more difficult to produce than the alkali metals since the chlorides of the alkaline-earth metals melt at relatively high temperatures. Furthermore, when these metals are liberated at the cathode, they tend to become colloidally dispersed throughout the molten electrolyte. Accordingly, it is necessary to design the electrolytic cells in such manner to permit the immediate collection of the elemental metal. The type of cell used in the production of calcium serves as a suitable illustration. Molten calcium chloride is placed in a cylindrical vessel around... 

The relationship is valid for molten chlorides of the alkali metals, the alkaline earth metals and La, Fe2+, Mn2+. The author [51] proposed the following classification of molten chlorides ... 

Using that classification, Redkin modified Biltz and Klemm s table (see Table 3). That classification was confirmed by Nakamura and Itoh [52], who found that the specific conductivity of molten alkali chlorides increases monoton-ically with decreasing cation radius, as shown in Figure 6. In the case of chlorides of alkaline earth metals there is a break at CaCl2, and the specific conductivity decreases dramatically when going to Mg2+ and Be2+. This break may be attributed to complex formation among the component ions in those molten salts [53], This means that in the molten alkali halides there are free ions and no complex formation, a fact confirmed by Raman spectroscopy [52],... 

The alkaline earth metals show a wider range of chemical properties than the alkali metals. The IIA metals are not as reactive as the lA metals, but they are much too reactive to occur free in nature. They are obtained by electrolysis of their molten chlorides. Calcium and magnesium are abundant in the earth s crust, especially as carbonates and sulfates. Beryllium, strontium, and barium are less abundant. All known radium isotopes are radioactive and are extremely rare. 

Novozhilov et al. studied the solubility of HC1 in molten alkali- and alkaline earth metal chlorides [268, 269] and found the solubility of HC1 in these melts to obey Henry s law. The thermal dependences of the solubility (the Henry coefficient values) of HC1 in molten alkaline earth metal chlorides, which are exposed to the pyrohydrolysis, are presented in Fig. 2.5.3. The dependences are close to linear anOd their treatment by the least-squares method allows the calculation of the thermodynamic parameters of HC1 dissolution in the chloride melts. These values are presented in Table 2.5.2. 

Fig. 2.5.3. The dependence of the solubility of gaseous hydrogen chloride in molten alkaline earth metal chlorides against the inverse temperature 1, BaCl2 2, SrCl2 3, CaCl2 from the...

Recently, we have studied the solubility of some metal-oxides in chloride melts based on alkali-metal salts with the addition of approximately 25 mol% of alkaline-earth metal chlorides (CaCl2, SrCl2 or BaCl2). The solubility of MgO and NiO in the molten CaCl2-KCl (0.235 0.765) mixture at 700 °C was determined in Ref. [191]. The range of oxides available for potentiometric... 

As is well known, alkaline-earth metal-oxides possess a limited solubility in molten chlorides at 1000 K. In the saturated solutions of these oxides equilibrium (3.6.1) takes place. Therefore, there are two simultaneous reactions in the carbonate solutions, namely, equations (3.6.1) and (3.7.66), which may result in the precipitation of a solid phase. For equation (3.6.1) all we have said above for the carbonate remains true, i.e. the plateau (Fig. 3.7.23, sections 1-3) may also be a result of alkaline-earth metal-oxide precipitation. 

A.L. Novozhilov, E.I. Gribova and V.N. Devyatkin, Investigation of the State of HC1 in Molten Chlorides of Alkali and Alkaline Earth by IR Spectroscopy Method, Zh. Neorg. Khim. 17 (1972) 2078-2080. 

Rinck (6), from comprehensive investigations, described the equilibria between molten salts and metals in the alkali and alkaline earth groups. Potassium chloride was selected for the thermochemical reduction on the basis of availability and price per unit of contained potassium. 

On a laboratory scale, beryllium is prepared by electrolysis of a mixture of molten beryllium fluoride and alkali or alkaline earth fluorides. The product obtained is 99.7% pure the commercial material obtained by the same method is < 99%. Industrial electrolysis of beryllium chloride-alkali chloride melts yields beryllium with a purity usually greater than 99.8%. 

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