9th Intl. Symp. on Sustainable Molten Salt, Ionic & Glass-forming Liquids & Powdered Materials

For the first time the electrical conductivity of 13 compositions of (3LiCl-2KCl) - PbCl₂ molten mixtures has been measured. The measurements were carried out at the temperatures of 631-994 K in the concentration range of 0-100 mol. % with the increment of ~ 10 mol. % PbCl₂ using capillary quartz cells with platinum electrodes [1] and the AC-bridge method at the input frequency of 75 kHz.
The electrical conductivity of all melts increased smoothly with temperature and decreased as the concentration of PbCl₂ increased. Its deviations from the additive course did not exceed 3.5% at 750 K and 8% at 950 K. The maximum deviations are located near the PbCl₂ concentrations of 30–40 mol. %. The specific electrical conductivity (κ, S/cm) of several molten mixtures is exemplified below:
κ = -5.1439 + 1.2565*10^-2T - 4.5503*10^-6T² , (632-988 K) 4 mol.% PbCl₂;
κ = -4.3114 + 1.0750*10^-2T - 3.8926*10^-6T² , (662-976 K) 30 mol.% PbCl₂;
κ = -4.9041 + 1.1759*10^-2T - 4.4114*10^-6T² , (684-920 K) 70 mol.% PbCl₂.
The density, molar volumes and molar conductivity of all studied molten mixtures were calculated. Relative deviations of molar volumes from additive values decreased with increasing temperature. The maximum deviations were observed at the PbCl₂ concentration of about 20 mol. % (3.8% at 773 K and 2.9% at 923 K).
When PbCl₂ is added to the LiCl-KCl melt, the divalent lead cation forms complex groups with chlorine anions that are somewhat stronger than the lithium cation. However, the difference between the ionic moments of Li⁺ and Pb²⁺ is small. Relatively free and mobile K⁺ ions are located in the second coordination sphere. The molar electrical conductivity polytherms of molten mixtures (3LiCl-2KCl) - PbCl₂ are almost rectilinear over the entire concentration range, which also indicates a rather weak interaction in the system.
The data on electrical conductivity and molar volumes of the (3LiCl-2KCl) - PbCl₂ melts are compared with those earlier obtained in our studies on the (3LiCl-2KCl) - CdCl₂ [2] and (3LiCl-2KCl) - SrCl₂ molten mixtures [3]. The results are discussed in terms of the structure of these melts.

To perfect the technological processes of electrodeposition and electrorefining of zirconium, information on the electrical conductivity of ZrCl₄ solutions in molten alkali metal chlorides is needed. Most of ZrCl₄-containing melts have a high vapor pressure and, hence, are not suitable for industrial applications. Only two concentration windows in each MCl-ZrCl₄ system (M is an alkali metal) are suitable. These are the high and low temperature ranges with 0-30 or 55-70 mol. %; ZrCl₄, respectively, where the vapor pressure above the melt is less than 1 atm. Previously, we carried out a series of experiments on the study of the electrical conductivity of such melts [1–4].
In this work, for the first time the electrical conductivity of molten mixtures (LiCl-KCl)_eut.-ZrCl₄ have been measured in the ZrCl₄ concentration range of 0-30 mol. %; and in a wide temperature range (652–1075 K). The measurements were carried out in a capillary-type quartz cell of a special design [5]. The use of a low-melting solvent (LiCl-KCl eutectic) made it possible to significantly (by hundreds of degrees) lower the melting temperature and, accordingly, the saturation vapor pressure of molten mixtures.
It was found that the electrical conductivity of molten mixtures (LiCl-KCl)_eut.-ZrCl₄ (0.9-2.8 S/cm) is close to the electrical conductivity of high-temperature ZrCl₄ melts with the same concentration of chlorides of various alkali metals (0.6-3.5 S/cm). We studied these melts earlier [3, 4]. The obtained values of electrical conductivity are much higher than the electrical conductivity of low-melting, more concentrated (55–75 mol.%; ZrCl₄) molten mixtures (0.1–0.5 S/cm) [1, 2].
The experimental results are discussed taking into account the available spectroscopic data on the structure of ZrCl₄-containing molten mixtures. In particular, it was noted that with an increase in the concentration of ZrCl₄, the concentration of its relatively low-mobile complex groups ZrCl₆^2- increases proportionally. In this complex, all 6 chloride ions are strongly bound to the tetravalent zirconium cation. This leads to a decrease in the concentration of the main current carriers: K⁺, Li⁺, Cl^– and to a decrease in the electrical conductivity of the melts.
The liquidus line in the (LiCl-KCl)_eut.-ZrCl₄ quasi-binary system has been constructed for the first time.

Carbamide melts have found applications as electrolytes for electrochemical treatment of metals [1, 2]. The possibility of electrochemical synthesis of refractory compounds from carbamide melts at 135°С has been examined for Ti-B as an example. In the binary system (NH2)2CO–(NH4)2TiF6 a new process, which is more electropositive than the decomposition of carbamide has been observed. The scan rate independence of potential characterizes the observed processes as reversible charge transfer. This process corresponds to the one-electron irreversible charge exchange: Ti(IV)/Ti(III). In the binary system (NH2)2CO–NH4BF4 only the decomposition of carbamide has been observed. In the ternary system (NH2)2CO–(NH4)2TiF6–NH4BF4 a new process, which is more electropositive than the decomposition of carbamide and more electronegative than the one-electron charge exchange: Ti(IV)/Ti(III) has been observed. This process corresponds to the electrochemical synthesis of Ti-B compound. Micron coatings of titanium boride have been obtained on nickel and stainless steel by the electrolysis from a (NH2)2CO–(NH4)2TiF6–NH4BF4 melt at 135°C at current densities of 10-20 mA/cm2

The volatility of the components of molten mixtures of uranium tetrachloride with alkali metal chlorides UCl₄-MCl (MCl = Cs, Rb, K, Na, Li and Na-K (1:1)), containing from 2 to 50 mol. % UCl₄ were measured in the temperature range of 800-1200 K using the transpiration technique [1-3]. In vapors over molten UCl₄–MCl mixtures, monomeric (MCl and UCl₄) and dimeric (M₂Cl₂) molecules of alkali metal and uranium chlorides, as well as complex molecules of the MUCl₅ type predominate.
The dissolution of UCl₄ in molten alkali metal chlorides is accompanied by the complex formation, which manifests itself in a sharp decrease in the volatility of uranium tetrachloride and its content in saturated vapors. The strength of the complex chloride anions, formed in the melts (U₃Cl₁₄^2-, U₂Cl₁₀^2-, UCl₆^2-, UCl₇^3-), increases significantly as the concentration of UCl₄ decreases; in a series of solvent salts from LiCl to CsCl and as temperature decrease. As a result, the volatility of UCl4 in the composition of all gaseous compounds over its solutions in ionic melts varies over a very wide range.
It has been established that at 973-1173 K the volatility of UCl₄ in UCl₄-LiCl melts decreases by about 330-260 times as the concentration of UCl₄ in melts decreases from 50 to 2 mol. %. Under the same conditions, the volatility of UCl₄ in UCl₄–CsCl melts decreases by a factor of approximately 180,000–20,000. At lower temperatures, volatility decreases faster. An increase in the radius of cations in the series from Li₊ to Cs₊ reduces their counterpolarizing effect on chlorine anions, which are a part of the U(IV) chlorocomplex groups, which are strengthened in this case. Other things being equal (temperature and concentration), the complexes strengthening leads to a decrease in the volatility of UCl₄ by 3700–340 from its dilute (2 mol.%) solutions and by a factor of 7–5 from concentrated (50 mol.%) ones in molten alkali metal chlorides at 973-1173 K.
It has been established that the dependence of the logarithm of the UCl₄ volatility on the reciprocals of the effective ionic radii of alkali cations is almost linear. This makes it possible to estimate the volatility of uranium tetrachloride from its not yet studied solutions in molten mixtures of alkali metal chlorides, using the weighted average radius of monovalent cations as the effective ionic radius.