Molten Alkali Metal Chlorides Research Articles

In the LiCl-KCl melt at 500<sup>о</sup>С depending on the content of Li<sub>2</sub>О и LiOH

Molten alkali metal chlorides used in pyrotechnologies are aggressive corrosive agents. The high operating temperature of the process, the heterogeneity of the environment, and the significant corrosion activity of the molten salt necessitate both the search for stable structural materials and the development of methods for protecting the structural elements of high-temperature technological devices. Corrosion loss reduction techniques traditionally used in low temperature environments are not applicable at high temperatures. The article examines the influence of oxygen-containing impurities (lithium oxide and hydroxide) on the corrosion behavior of metallic nickel (grade NP1) – the main component of candidate structural alloys, a thermodynamically and structurally stable material in the melt for the process of electrolytic refining of spent nuclear fuel. A method for preparing the LiCl–KCl salt electrolyte and obtaining lithium oxide by thermal decomposition of anhydrous lithium hydroxide under vacuum is described, and the concentrations of impurities in the electrolyte and the synthesized lithium oxide are determined. An installation for conducting corrosion tests in an inert atmosphere of a glove box is presented. To assess the corrosion resistance of the material, the following were used: gravimetric analysis, X-ray diffraction analysis of the surface and cross-sectional sections, and X-ray diffraction analysis of the surface of the samples. The dependences of the corrosion rate of the material on the concentration of oxygen-containing additives Li2O and LiOH were obtained. Based on a combination of gravimetric, X-ray microspectral and X-ray phase analysis data, it was established that metallic nickel samples demonstrate high corrosion resistance in the studied melts with the introduction of Li2O and LiOH additives.

Corrosion electrochemical behavior of metal matrix composites “Al-nano-Al<sub>2</sub>O<sub>3</sub>” IN 0.5M NaCl aqueous solution

The corrosion-electrochemical behavior of nanocomposites of the “aluminum-nano-aluminum oxide” system, formed by direct chemical interaction of molten aluminum with titanium nanooxide in an environment of molten alkali metal chlorides at temperatures above 700оC, has been studied. Nanoalumina crystals in the α-Al2O3 modification, uniformly distributed throughout the volume of the metal matrix, were detected by means of electron microscopy and X-ray diffraction. The corrosion rate in 0.5M NaCl, determined by the gravimetric method, decreases by 3–4 times when moving from initial aluminum to Al-Al2O3 composites, while the nature of corrosion changes from pitting to uniform and the corrosion resistance class from 3 (resistant) to 2 (very persistent). This is due to the formation of a denser single-phase hydroxide coating on the surface of the composite compared to a two-phase loose coating on aluminum. The corrosion potential is not affected by the incorporation of aluminum oxide nanoparticles into the aluminum matrix.

Application of liquid Ga-Pb cathode for selective separation of samarium fission products from molten alkali metal chlorides

The efficient separation of the rare earth element Sm from spent nuclear fuel is of strategic importance for the nuclear industries. This study explored the feasibility of a novel liquid Ga-Pb hybrid electrode for electrochemical extraction of samarium compounds, and the electrochemical reduction mechanism and dynamic properties of Sm(III) ions were provided in NaCl-2CsCl molten salt. It was established that the reduction mechanisms of Sm(III) to Sm(II) and Pb(II) to Pb(0) on the Mo cathode were a one-step reaction processes, respectively, while Ga(III) to Ga(0) was a two-step reaction process. The co-deposition mechanisms of Sm(III), Ga(III) and Pb(II) ions on the Mo cathode were identified, and the types of intermetallic compounds, formation potentials and reaction processes were analyzed. The deposition potential of Sm on Ga-Pb mixed cathode is determined, and the value of its depolarization was greater than on the liquid Pb electrode. Simultaneously, Sm ions have higher exchange current densities and lower activation energies for electrode reactions on the liquid Ga-Pb mixed electrodes than on the liquid Pb electrodes. Furthermore, the electro-separation of NaCl-2CsCl-SmCl3 molten salt was executed via potentiostatic electrolysis technique, and the separation efficiency and the effect of cathode product type were analyzed and compared in Ga-Pb hybrid electrode and Pb electrode. It was obtained that the maximum extraction efficiency on the Ga-Pb hybrid electrode and the separation efficiency of Sm could reach 91.3 % after 8 h of electrolysis.

Ionic Conductivities of Molten Alkali Metal Chloride Binary Systems by Equilibrium Molecular Dynamics Simulation: Composition and Temperature Dependence

Conductivity of molten alkali metal chlorides is the foundation of molten-salt electrolysis process. In the study, equilibrium molecular dynamics was performed to investigate the ionic conductivities of the molten alkali metal chloride binary systems. The calculated values were in good agreement with the experimental results. Simple databases for conductivity have been built based on the verified algorithm. Mathematic expressions of conductivity vs. temperature & composition have also been obtained according to calculated data. Ionic conductivities of these melts exhibited positive dependences on temperature. Adding a smaller alkali metal cation into molten alkali chlorides could improve the conductive performance. With the gradual addition of LiCl, ionic conductivities of molten LiCl-NaCl, LiCl-KCl, LiCl-RbCl, and LiCl-CsCl all increased monotonously. The conductivities of molten NaCl-KCl, NaCl-RbCl, and NaCl-CsCl also increased with the continuous addition of NaCl. The improvement effects of LiCl on ionic conductivities of molten KCl, RbCl, and CsCl are more significant than those of NaCl.

Reaction of Oxygen with Uranium Trichloride in Molten Alkali Metal Chlorides

Reaction of Oxygen with Uranium Trichloride in Molten Alkali Metal Chlorides

The Electrochemical and Thermodynamic Properties of Dysprosium in Molten Alkali Metal Chlorides

This work presents the electrochemical study of Dy(III) ions in molten alkali metal chlorides at the temperature range 723–1073 K. Transient electrochemical techniques such as cyclic and square wave voltammetry, and open-circuit potentiometry have been used in order to investigate the reduction mechanism, transport parameters and thermodynamics properties of the reaction Dy + 3/2Cl2 → DyCl3. The obtained results show that the cathodic reaction of Dy(III) + 3ē → Dy is a single stage irreversible process being controlled by the charge transfer rate. The diffusion coefficient of [DyCl6]3− complex ions was determined at different temperatures in the fused 0.58LiCl-0.42KCl, 0.36NaCl-0.64CsCl eutectics and in an individual CsCl. The apparent standard potential of Dy3+/Dy couple was determined by the open-circuit potentiometry. The influence of the nature of the solvent on the electrochemical properties of [DyCl6]3− complex ions and the principal thermodynamic characteristics of the formation DyCl3 compound were discussed.

Wetting of a Charged Surface of Glassy Carbon by Molten Alkali-Metal Chlorides

Values of the contact angle of wetting of a surface of glassy carbon by molten chlorides of lithium, sodium, potassium, and cesium are measured by the meniscus weight method to determine the common factors of wettability of solid surfaces by ionic melts upon a change in the salt phase composition and a jump in electric potential. It is found that with a potential shift in the positive direction the shape of the curve of the contact angle’s dependence on the potential varies upon substitution of one salt by another: the angle of wetting shrinks monotonously in lithium chloride but remains constant in molten cesium chloride. This phenomenon is explained by the hypothesis that the nature of the halide anion adsorption on the positively charged surface of an electrode is chemical and not electrostatic. It is shown that the adsorption process is accompanied by charge transfer through the interface, with covalent bonding between the adsorbent and adsorbate.

Estance of gold in molten LiCl and the effect of the nature of cation on the estance zero potentials of Au in molten alkali metal chlorides

The dependences estance vs. potential in molten LiCl are obtained. They have three zeros of estance near the melting temperature of the salt. When the temperature increases by 400 K, only one cathode zero is retained for equilibrium estance. A comparison of the potentials of this zero of estance in a row of alkali metal chlorides at the same temperature indicates their nonmonotonic dependence on the cation radius, in contrast to the potentials of the ECC maxima of liquid Pb, Bi, and In and the PZC of gold in this row.

Electrochemical properties of molybdenum in individual molten alkali metal chlorides and their mixtures

The behavior of molybdenum in chloride melts is studied, and the standard potentials of Mo are determined in the melts based on eutectic mixtures LiCl-KCl-CsCl (at 633–1173 K) and NaCl-CsCl (at 793–1023 K), an equimolar NaCl-KCl mixture at 973–1123 K, and individual LiCl, NaCl, KCl, RbCl, and CsCl at 1123 K. The change in the conventional standard Gibbs energy of molybdenum trichloride formation in the NaCl-KCl, NaCl-CsCl, and LiCl-KCl-CsCl melts is calculated. The effect of the cation composition of the salt solvent on the conventional standard electrode potential of molybdenum in the chloride melts is considered. The diffusion coefficients of Mo(III) ions in the LiCl-KCl-CsCl melt are determined.

Stability of complex molybdenum(III) ions in molten alkali metal chlorides

High-temperature absorption spectroscopy is used to study the effect of temperature in the range 550–750°C and the cation composition of a solvent salt on the spectroscopic characteristics and stability of MoCl63− complex ions in melts based on alkali metal chlorides (LiCl; CsCl; LiCl-KCl, NaCl-CsCl, KCl-CsCl, and NaCl-KCl-CsCl eutectic mixtures; and equimolar NaCl-KCl mixture).

On some regularities of metal oxide solubility in molten CsI at T = 973 K

Solubility products of CdO ( pK s ,CdO = 6.80 ± 0.2), ZnO ( pK s ,ZnO = 10.0 ± 0.5), NiO ( pK s ,NiO = 11.2 ± 0.2) and EuO ( pK s ,EuO = 13.1 ± 0.2) in molten CsI at T = 973 K are determined by potentiometric titration of (0.02 to 0.03) mol · kg −1 solutions of the corresponding metal chlorides by strong base (KOH) using a membrane oxygen electrode Pt(O 2)|ZrO 2(Y 2O 3) as an indicator. On the basis of pK s ,MeO values, all the oxides studied are referred to practically insoluble in molten CsI. The values of the oxide solubility in CsI melt are lower than the corresponding values in molten alkali metal chlorides. This can be explained by ‘softer’ basic properties of I – as compared with Cl – in the frames of the Pearson ‘hard’ and ‘soft’ acid–base concept. In the oxide samples studied, the values of the solubility fall with the decreasing cation radius. The correlation between pK s ,MeO and the polarizing action by Goldshmidt ( Zr Me 2 + - 2 ) of the cation is practically linear and may be proposed for estimation of the solubility of s- and d- element oxides in molten CsI on the basis of their cation radii.

Study of reaction of curium oxy-compound formation in molten chlorides

Abstract The method of potentiometric titration using oxygen sensors with solid electrolyte membrane was applied to study the interaction of curium cations with oxygen anions in the molten alkali metal chlorides in the temperature range of 450–850 °C depending on oxy-acidity of melt. Assumptions were made concerning the ion and phase composition of the obtained compounds and the chemical reactions that take place in the melts at high temperature. Basic thermodynamic values were calculated for the resultant curium oxy-compounds.

Electrochemistry of ytterbium (III) in molten alkali metal chlorides

This work presents the electrochemical study of Yb(III) ions in molten alkali metal chlorides in the temperature range 723–1073 K. Transient electrochemical techniques such as linear sweep, cyclic and square wave voltammetry, and potentiometry at zero current have been used to investigate the reduction mechanism, transport parameters and thermodynamic properties of the reaction YbCl 2 + 1/2Cl 2 = YbCl 3 The results obtained show that the reduction reaction Yb(III) + e − ⇔ Yb(II) is reversible being controlled by the rate of the mass transfer. The diffusion coefficient of [YbCl 6] 3− complex ions has been determined at different temperatures in the fused eutectic LiCl–KCl, the equimolar NaCl–KCl and the CsCl media . The apparent standard potential of the soluble–soluble redox system Yb(III)/Yb(II) has been obtained by cyclic voltammetry. The influence of the nature of the solvent on the electrochemical and thermodynamic properties of ytterbium compounds is discussed.

Oxidative Dehydrogenation of Light Alkanes on Supported Molten Alkali Metal Chloride Catalysts

Potential and limitations of molten alkali metal (Li, Na, and K) chlorides supported on Dy2O3/MgO were explored for the oxidative dehydrogenation of lower alkanes, such as ethane and propane. The catalysts have high activity and selectivity to olefins compared to conventional catalysts. Optimum performance is obtained with catalysts on which the alkali metal chloride phase is molten under reaction conditions. Lower chloride melting point correlates with higher selectivity. The high selectivity to ethene or propene is attributed to the high mobility of cations and anions, which facilitates desorption of alkene (limiting further oxidation) and the generation of spatially isolated hypochloride anions acting as the active sites for the primary C–H bond activation.

Electronic absorption spectra of solutions of molecular chlorine in molten alkali metal chlorides

Electronic reflection absorption spectroscopy has been used for measuring the electronic spectra of chlorine solutions in molten alkali metal chlorides 2CsCl-NaCl, KCl-NaCl, and CsCl in the range 240–400 nm. Absorption bands of chloride melts are interpreted as electronic transitions in the molecular group Cl2 and in the triatomic linear group Cl 3 − of symmetry D ∞h and C ∞V .

Electronic Absorption Spectra of GaCl3 and Ga2O3 Solutions in Chloride—Fluoride Melts

Electronic absorption spectra of GaCl3 and Ga2O3 solutions in molten alkali metal chlorides and in NaCl-KCl-NaF and LiF-NaF melts were measured by reflection-absorption spectroscopy. The spectral data showed that the main structural units in the solutions are complexes GaHlg4− (Hlg = Cl, F) with Td symmetry and GaOHlg32− with C3v symmetry, respectively. Molecular oxygen occurring in molten systems as an admixture causes exchange decomposition with formation in all the melts of molecular complexes of chlorine, Cl2Cl−, with C∝v symmetry.

Behavior of molybdenum in pyrochemical reprocessing: A spectroscopic study of the chlorination of molybdenum and its oxides in chloride melts

The high temperature reactions of molybdenum and its oxides with chlorine and hydrogen chloride in molten alkali metal chlorides were investigated between 400 and 700 °C. The melts studied were LiCl–KCl, NaCl–CsCl and NaCl–KCl and the reactions were followed by in situ electronic absorption spectroscopy measurements. In these melts Mo reacts with Cl 2 and initially produces MoCl 6 2− and then a mixture of Mo(III) and Mo(V) chlorocomplexes, the final proportion depending on the reaction conditions. The Mo(V) content can be removed as MoCl 5 from the melt under vacuum or be reduced to Mo(III) by Mo metal. The reaction of Mo when HCl gas is bubbled into alkali chloride melts yields only MoCl 6 3−. MoO 2 reacts in these melts with chlorine to form soluble MoOCl 5 2− and volatile MoO 2Cl 2. MoO 3 is soluble in chloride melts and then decomposes into the oxychloride MoO 2Cl 2, which sublimes or can be sparged from the melt, and molybdate. Pyrochemical reprocessing can thus be employed for molybdenum since, after various intermediates, the end-products are chloride melts containing chloro and oxychloro anions of molybdenum plus molybdate, and volatile chlorides and oxychlorides that can be readily separated off. The reactions were fastest in the NaCl–KCl melt. The X-ray diffraction pattern of MoO 2Cl 2 is reported for the first time.

Thermodynamic Characteristics of Samarium and Europium Chlorides in Molten Alkali Metal Chlorides

Abstract The formal standard redox potentials of samarium (III)/(II) (E*Sm 3+/ Sm2+) and europium (III)/(II) (E*Eu3+ /Eu2+) in molten alkali metal chlorides were measured potentiometrically against a reference chlorine electrode. The Gibbs energy changes and equilibrium constants for the reaction LnCl2(I) + ½ Cl2(g) ⇔ LnCl3(I) were calculated for the salt systems studied. The effect of the cationic composition of the melt-solvent on the above thermodynamic characteristics is also reported.

Interaction of Hafnium Chlorides with Magnesium, Aluminum, and Lead in Molten Alkali Metal Chlorides

The thermodynamic characteristics of the interaction of hafnium(II) and hafnium(IV) chlorides with magnesium, aluminum, and lead in molten alkali metal chlorides were calculated.

Co-ordination of thorium(IV) in molten alkali-metal chlorides and the structure of liquid and glassy thorium(IV) chloride

Raman spectra of molten ThCl4–ACl (A = Li, Na, K or Cs) mixtures have been measured. The complete composition range has been studied for all systems at temperatures up to 960 °C. The spectral changes upon melting the binary compounds A2ThCl6, A3ThCl7 and the pure crystalline ThCl4 were also measured. The data indicate that in molten mixtures rich in alkali-metal chloride the predominant species are the ThCl62– octahedra [ν1(A1g) 297 and ν5(F2g) 125 cm–1] in equilibrium with the ThCl73– pentagonal bipyramid. With increasing ThCl4 mole fraction above 0.3 the frequency of the ν1(A1g) band increases continuously reaching its maximum (≈340 cm–1) in pure molten ThCl4. It appears that in mixtures rich in ThCl4 edge bridging of thorium(IV) octahedra occurs yielding chain species of the type [ThnCl4n + 2]2– and [ThnCl4n – 2]2+ where the end Th atoms of the chain are six- and four-fold co-ordinated for the anion and cation respectively. The known ionic character of these melts suggests that the chain length is rather small and that most probably the species have low n values (e.g. Th2Cl102–, Th3Cl142–, Th3Cl102+, . . .). Glassy ThCl4 was formed by slow cooling of the melt. The vibrational modes and the inferred structure of the glass are similar to those of the melt.