Chloride Melts Research Articles

Observation of the Same New Sheet Topology in Both the Layered Uranyl Oxide-Phosphate Cs11[(UO2)12(PO4)3O13] and the Layered Uranyl Oxyfluoride-Phosphate Rb11[(UO2)12(PO4)3O12F2] Prepared by Flux Crystal Growth.

Single crystals of four new layered uranyl phosphates, including three oxyfluoride-phosphates, were synthesized by molten flux methods using alkali chloride melts, and their structures were determined by single-crystal X-ray diffraction. Cs11[(UO2)12(PO4)3O13] (1) and Rb11[UO2)12(PO4)3O12F2] (2) contain uranyl phosphate layers exhibiting a new sheet topology that can be related to that of β-U3O8, while Cs4.4K0.6[(UO2)6O4F(PO4)4(UO2)] (3) and Rb4.4K0.6[(UO2)6O4F(PO4)4(UO2)] (4) contain layers of a known isomer of the prominent phosphuranylite topology. The location of the fluorine in structures 2-4 is discussed using bond valence sums. First principles calculations were used to explore why a pure oxide structure is obtained for the Cs containing phase (1) and in contrast an oxyfluoride phase for the Rb containing phase (2). Ion exchange experiments were performed on 1 and 2 and demonstrate the ability of these structures to exchange approximately half of the parent alkali cation with a target alkali cation in an aqueous concentrated salt solution. Optical measurements were performed on 1 and 2 and the UV-vis and fluorescence spectra show features characteristic of the uranyl group.

Liquid immiscibility and problems of ore genesis (according to experimental data)

The results of an experimental study of phase relations and distribution of elements in silicate melt–salt systems (carbonate, phosphate, fluoride, chloride) melt, silicate melt I–silicate melt II, and also in fluid – magmatic systems in the presence of alkali metal fluorides are presented. Salt extraction of a number of ore elements (Y, REE, Sr, Ba, Ti, Nb, Zr, Ta, W, Mo, Pb) was studied in liquid immiscibility processes in a wide temperature range of 800–1250°С and pressure of 1–5.5 kbar. It is shown that the partition coefficients are sufficient for the concentration of ore elements in the quantity necessary for the genesis of ore deposits. In the fluid-saturated melt of trachyrhyolite, the separation into two silicate liquids has been determined. The partition coefficients of a number of elements (Sr, La, Nb, Fe, Cr, Mo, K, Rb, Cs) between phases L1 and L2 has been obtained. The interaction processes of a heterophase fluid in the granite (quartz)–ore mineral–heterophase fluid (Li, Na, K-fluoride) system were studied at 650–850°C and P = 1 kbar. The formation of the phase of a highly alkaline fluid–saturated silicate melt – Ta and Nb concentrator is shown as a result of the reaction of the fluid with the rock and ore minerals.

A study of Eu2O3 solubilization in K2SrCl4 melt at 973 K under the action of CCl4 vapor

A comparative study of carbochlorination processes (treatment with CCl4 vapor in argon flow) of K2SrCl4 melt and K2SrCl4+Eu2O3,solid mixture at 973 K was performed by the potentiometric method with the use of Pt(O2)|ZrO2(Y2O3) membrane oxygen electrode as reversible to oxide ion.Solid Eu2O3 interacts with K2SrCl4 melt that results in partial transformation of the oxide into oxochloride, EuOCl (the degree of Eu2O3 transformation is 0.54) and the process is completely suppressed at oxide ion (SrO) concentration ca. 3∙10−3 mol kg-1; here the melt becomes saturated with respect to both Eu2O3 and EuOCl. The course of interaction of Eu2O3 with K2SrCl4 melt at 973 K obeys the Shchukarev equation.The analysis of van’t Hoff diagrams for these processes showed that the stage of Eu2O3 solubilization under the action of CCl4 is clearly detected. The data corresponding to the finishing of Eu2O3 solubilization in K2SrCl4 melt allowed to estimate the solubility product of Eu2O3 as 1.2·10−22 mol5·kg-5 in molality scale and 3.2·10-26 in mole fractions. The corresponding values for EuOCl are 1.47·10-7 mol3·kg-3 (mCl-= 13.1 mol kg-1) or 1.05·10-9 (mole fractions).An application of the Shreder-Le Chatelier equation for the recalculation of some literature data on the solubilization of rare-earth metal oxides and oxochlorides at different temperatures allowed us to compare them with the results obtained in this work. It was confirmed that the running of the carbochlorination process at low oxide ion concentrations was determined by the rare earth oxide (oxochloride) dissolution similarly to the situation observed in other chloride melts where HCl was used as a chlorinating agent. The literature data and data of this work expressed in units of oxoacidity function Ω and reduced to the same temperature (973 K) are in a good agreement.

РАВНОВЕСНЫЕ ПОТЕНЦИАЛЫ ОКСИДА СВИНЦА (II) В ЭКВИМОЛЬНОМ РАСПЛАВЕ KCl-PbCl2

The electromotive forces (EMF) of the electrochemical system (GC)Pb|(1-N)·KCl-PbCl2+N·PbO|ZrO2(Y2O3)|O2(Pt) in the concentration range of PbO 0.16-7.32 % mol. were measured at temperatures 776, 821 and 874 K. From the experimental values of EMF activities, coefficients of the activity and basic thermodynamic functions of lead oxide for dilute solutions were calculated. It is shown that PbO activities in the KCl-PbCl2 melt have moderate negative deviations from Raoul's law for ideal solutions. The activity coefficients of PbO at each temperature within the calculation errors are constant, that is, the diluted solutions of PbO in KCl-PbCl2 obey Henry's law. In the studied area of dilute solutions of PbO solvent activity coefficients are constant within the error, and their value is close to 1, as a consequence of the solvent activity within the calculation error will be equal to its molar fraction. This confirms the rule of Kubashevskii-Alcock about the obedience behavior of the solvent KCl-PbCl2 to Raoult's law for ideal solutions. Partial thermodynamic functions of the solvent - melt KCl-PbCl2 and integral thermodynamic functions of the KCl-PbCl2-PbO system are calculated using the standard equations. The interaction of lead oxide with chloride melts is caused by the formation of oxychloride compounds. According to the literature, there is a compound {[Pb2OCl]++Cl-} in the ion melt. The results of x-ray phase analysis show the presence of Pb2OCl2 compound in the frozen electrolyte melt. Using the device LECO ONH836 the concentration of oxygen in the metal lead was determined. It is shown that the oxygen concentration was 0.031 wt. % after the long-term exposure, which also exceeds the values in the Pb-O diagram almost twice.

Corrosion of Carbon Steel and Cast Iron in a Gas Phase over Salt Melts Used in the Magnesium Industry

Corrosion tests of SCh15 cast iron, St.3, and St.3 with an aluminized coating are performed in certain compositions of salt melts of 10% MgCl2–KCl–NaCl and 10% MgCl2–KCl–NaCl–CaCl2 systems with concentrations of 10, 25, and 40% CaCl2, as well as in the 10% MgCl2–45% KCl–20% NaCl–25% NaBr melt and in a gas phase over these melts at 700°C. Corrosion rates of metallic samples are found by a gravimetric method. Concentrations of impurities of halogenides and hydrogen halogenides in air blown through a reactor with melts and samples are determined by the chemical analysis of absorption solutions. It is shown that the aluminizing of carbon steel can decrease the corrosion rate in a gas phase over a salt melt by a factor of 5–70. The mechanism of formation of gases aggressive with respect to carbon steel and cast iron in atmospheric air contacting with the salt chloride melt is considered. The acceleration of the formation of hydrogen chrloride and chlorine during the interaction of the salt melt with atmospheric air under the effect of corrosion products of iron is revealed.

Corrosion of carbon steel and cast iron in gas phase above salt melts used in magnesium industry

Corrosion tests of SCh15 cast iron, Steel 3 and Steel 3 with aluminized coating in some compositions of 10%MgCl2-KCl-NaCl and 10%MgCl2-KCl-Naa-Caa2 salt melts with 10 %, 25 % and 40 % CaCl2 concentrations, and also in 10MgCl2-45%KCl-20%NaCl-25%NaBr melt, and in the gas phase above these melts at 700 °C. A gravimetric method was used to determine corrosion rates of metal samples. Chemical analysis of absorption solutions was used to determine concentrations of halide and hydrogen halide impurities in air blown through the reactor with melts and samples. It was shown that carbon steel aluminizing can reduce the corrosion rate in the gas phase over the salt melt by a factor of 5 to 70. The formation mechanism of gases aggressive in relation to carbon steel and cast iron in atmospheric air in contact with salt chloride melt was considered. Accelerated hydrogen chloride and chlorine formation during the salt melt interaction with atmospheric air under the influence of iron corrosion products was found.

Structural and Morphological Peculiarities of the Lithium Niobate and Lithium Tantalate Powders Synthesized in Chloride Melts

A new method for synthesizing lithium niobate and lithium tantalate is proposed and tested. It is based on the interaction between lithium, niobium, and tantalum oxides, which are formed during in situ reactions of atmospheric oxygen with homogeneous mixtures of molten lithium, niobium(V), and tantalum(V) chlorides. This method allows us to form single-phase lithium niobate and lithium tantalate powders (LiNbO3, LiTaO3) of a homogeneous morphological composition with an average particle size from 200 to 300 nm. The replacement of salt precursors (NbCl5, TaCl5) by oxide ones (Nb2O5, Ta2O5) under the same experimental conditions is shown to increase the average particle size of the synthesized powders and the heterogeneity of their morphological composition and to promote the appearance of by-products.

Wetting Behaviour of Gold Electrode and Molten Alkali Chlorides

Abstract The potential dependence of the contact angle between a gold electrode and lithium, sodium, potassium, rubidium, and caesium chloride melts was studied using the meniscus weight method to establish the patterns of wettability of solid surfaces by ionic melts when changing the composition of the salt phase and the jump of the electric potential. It is found that the forms of the contact angle versus the potential curve of Au change from a convex to a camel-like shape with two maxima upon replacing the lithium chloride with the caesium chloride melt. This phenomenon is explained by the assumption that the adsorption of the halide anions at the positively charged electrode surface has a chemical rather than electrostatic character. The adsorption process is accompanied by a charge transfer through the interface and the formation of covalent bonds between the adsorbent and adsorbate.

Polarization and adsorption effects on the wettability of a gold electrode by lithium, sodium, potassium, and cesium chloride melts

To establish the patterns of the wettability of solid surfaces by ionic melts, the wetting energy of a gold electrode by lithium, sodium, potassium, and cesium chloride melts was studied as a function of the electrical potential applied to the electrode. It was established that when the potential was shifted in the positive direction relative to the zero-charge potential of gold in the corresponding melt, the shape of the curve that defined the dependence of the wetting energy on the electrical potential varied according to the salt composition: for lithium and sodium chlorides, the wetting energy increased monotonically, while in potassium and cesium chloride melts there was a maxima. This phenomenon was explained from the standpoint of the mutual polarization of gold and chlorine ions in the place of their direct contact. At a certain electric field strength in the double electric layer and at a certain binding energy of the melt particles, the resultant mutual ion polarization led to the formation of ordered layers of ionic associations (presumably AuCln(n − 1)-) on the anode surface, which shielded the electrode charge.

Research on Installation Technologies of Corrosion Protection for Steel Structures

Rusted S275 steel profiles were used in the experimentation. According to standard ISO 8501-1 the steel rust grade - B. The steel was cut into nine samples with dimensions 150x150 mm. and brought into three groups with three samples in each group. In each group the surface for anticorrosive coating of specimen was prepared in three different methods. The steel surface was sandblasted using quartz sand with fraction 0-0,8 mm. Using this method the preparation level of surface Sa 3 according to standard 8503-1 was achieved. The specimen in second group were prepared by mechanical method using the wiry brush. The level of surface St 2 was achieved according to standard ISO 8503-1. In third group the specimen have not been prepared in any method. According to the standard ISO 12944-5:2007 three different anticorrosive coating systems were selected: one component alkyd paint system, two component epoxy and polyurethane paint system and two component zinc, epoxy and polyurethane paint system. The paint systems were selected considering the category of corrosion C3 and C4. The coating of paint systems were performed using the airless painting equipment. The thickness of each layer, wet and dry, were controlled. The thickness results of dry coating were measured with digital device “Positector”. To accelerate the process of corrosion, the aggressive artificial atmosphere was created according to the standard ISO 9227:2017 Corrosion tests in artificial atmospheres – Salt spray tests. Neutral salt spray test is the method in which 5 % neutral sodium chloride melting is automated in controlled environment. The sufficient amount of sodium chloride is melted in distilled water with specific conduction not exceeding the 20 µS/cm at 25 0C ± 2 0C to get the concentration of 50 g/l ± 5 g/l. The steel samples were kept in artificial atmosphere for 500 hours. Anticorrosive coating characteristics for differently prepared surfaces were analysed by testing the adhesion according the standard ISO 4624:2016 using digital adhesion gauge “Elcometer 506”. DOI: http://dx.doi.org/10.5755/j01.sace.22.1.20902

Electrode Potentials of Bismuth in a Mixture of Potassium and Lead Chlorides

The equilibrium potentials of Bi in a KCl–PbCl2–BiCl3 melt were measured by the EMF method at different temperatures and bismuth chloride contents. Empirical equations of isotherms and polytherms of the equilibrium potentials of bismuth were obtained. The conventional standard potentials of Bi in a molten mixture of potassium and lead chlorides were calculated. The changes in the Gibbs energy during the formation of bismuth trichloride from the elements in the melt were calculated. The coefficients of metal separation from alloys were estimated. The results indicate that it is promising to develop a technology for electrolytic processing of secondary raw materials for separating lead and bismuth in chloride melts.

Electrochemical Processes at the Bi0–Bi3+ Interface in Chloride Melts

For rational design of technological schemes for production of high-grade bismuth and to improve electrochemical bismuth refining, knowledge of physicochemical properties of low-valent bismuth compounds and the mechanisms underlying processes occurring at the Bi0–Bi3+ interface in molten salts is necessary. For this purpose, the kinetics and mechanism of formation of low-valent bismuth compounds in the Bi0–BiCl3–ZnCl2–NH4Cl system by UV–Vis spectroscopy was studied. Chemical interactions between metallic bismuth and Bi3+ species in the melt give rise to Bi+ intermediates and [Bi5]3+ clusters, which are registered by their characteristic absorption bands at approximately 18000 and 14000 cm–1. The kinetic parameters for the Bi+ and [Bi5]3+ formations are estimated from the dependence of intensities of these absorption bands on the time of contact between metallic bismuth and Bi3+ species in the melt. The rate constants characterizing the Bi+ and [Bi5]3+ formations are estimated to be 3.33 × 10–3 and 2.31 × 10–3 s–1, respectively. The mechanism of anodic bismuth dissolution is proposed, and the cathodic and anodic current efficiencies for bismuth are determined.

Selection of the Optimum Electrolysis Bath Composition for the Synthesis of Holmium–Iron Triad Metal Intermetallics

The results of high-temperature electrochemical synthesis of holmium–iron triad metal intermetallics in chloride melts are presented. The influence of the current density, the composition of an electrolysis bath, and the synthesis time on the electrolysis processes and the composition of the end product is studied. The electrolysis of the molten KCl–NaCl mixture containing 0.5–2.5 mol % holmium trichloride and 0.1–2.5 mol % nickel (cobalt) dichloride at a current density of 0.5–2.0 A/cm2, a temperature of 973–1073 K, and an electrolysis time of 30–90 min is shown to cause the formation of a cathode deposit in the form of a “metal–salt pear” on a tungsten electrode. This pear consists of a mixture of metallic nickel (cobalt) and HoNi, HoNi5, and HoNi3 (HoCo2, HoCo3, HoCo5, Ho2Co17) intermetallics. The intermetallic compound content in the cathode deposit is found to increase at a constant current density (1.2 A/cm2) and when the holmium chloride content in a melt or the ratio of the holmium chloride concentration to the nickel (cobalt) chloride concentration increases. Only a mixture of holmium–nickel (cobalt) intermetallics can exist in the cathode deposit if the electrolysis bath composition and the electrolysis parameters are controlled. The electrochemical synthesis of holmium–iron intermetallics was performed under galvanostatic conditions in molten KCl–NaCl–HoCl3. Iron ions are introduced in a melt via the anodic dissolution of metallic iron. The results of X-ray diffraction analysis of the electrolysis products demonstrate the fundamental possibility of synthesizing holmium–iron intermetallics. The optimum conditions of electrosynthesis of holmium–iron triad metal intermetallics are determined.

Formation of an Electrode Deposit under Galvanostatic Conditions

The laws of formation of a continuous deposit layer at a given direct current are considered. Equations are analyzed to calculate the time dependences of overpotential for instantaneous nucleation with kinetic or diffusion control of new-phase growth. The calculated dependences are compared with the experimental ones obtained for silicon electrodeposition from a fluoride–chloride melt.

Electrochemistry of SmCl3 and SmF3 in Molten NaCl-KCl

Lanthanide elements, which are present in spent fuel from fast nuclear reactors, can be converted into molten salts by anodic dissolution. Electrolysis of molten salts is promising for separation of lanthanides and actinides. To solve this problem, it is necessary to have experimental data on the electrochemical behavior of various lanthanides in molten salts. The aim of this study is determine the influence of the first coordination sphere composition of samarium complexes on its electrochemistry in molten equimolar mixture NaCl-KCl. Electrochemical investigations were carried out in a temperature range of 973-1173 K by cyclic voltammetry using an AUTOLAB PGSTAT 20 potentiostat with a package of application programs GPES (version 4.4.). The melt container was a glassy carbon crucible of SU-2000 brand, which also served as an auxiliary electrode. Tungsten wire was used as working electrode and Pt wire was utilized as a quasi-reference electrode. In the NaCl-KCl-SmCl3 melt it was found that peak current of the recharge process of Sm(III) to Sm(II) is directly proportional to the square root of the polarization rate, while the peak potential does not depend on the polarization rate up to v=0.1 V s-1. According to the theory of linear sweep voltammetry, up to the polarization rate 0.1 V s-1, the electrode process is reversible. However, in the NaCl-KCl-SmF3 molten system the recharge process was reversible up to the scan rate 1.0 V s-1. Diffusion coefficients of Sm(III) in chloride and chloride-fluoride melts were determined by cyclic voltammetry using the Randels–Sevchik equation. The quasi-reversible process for the Sm(III)/Sm(II) redox couple was determined in NaCl-KCl-SmCl3 melt at a sweep rate 0.1<v<0.3 V s-1. This mechanism is clearly evidenced by the deviation of the experimental points from linearity in the IC p vs. v 1/2 plot, by the dependence EC p on v and by the magnitude of the difference between EA p and EC p, which is larger than is required for a reversible process. A transition from a reversible to a quasi–reversible process was found at v > 1.0 V s-1 in the NaCl-KCl-SmF3 melt. The standard rate constants of charge transfer (k s) of the Sm(III)/Sm(II) redox couple were determined by cyclic voltammetry both in NaCl-KCl-SmCl3 and NaCl-KCl-SmF3 molten salts using the Nicholson’s equation, which is valid for quasi-reversible processes. The higher values of k s were obtained in chloride melt than in chloride-fluoride molten system.

Interaction of Neodymium Containing Chloride Melts with Oxygen Species

Alkali chloride based melts can be employed as working media in spent nuclear fuels pyrochemical reprocessing. Oxygen is on of common impurities in technology and oxygen species can seriously influence speciation of metals in molten electrolytes. From the other hand, one of the possible ways of separating uranium from rare earth fission products by selective precipitation of uranium oxide. The present work was aimed at studying reactions taking place in chloride melts containing neodymium (III) and uranium (III and IV) ions upon addition of oxygen species. In particular, reactions with oxygen and oxide ions (introduced in form of lithium oxide) were considered. Thermodynamic modeling of possible reactions was first performed to estimate the possibility of separating uranium from rare earths. The experiments were then conducted in the melts based on LiCl–KCl eutectic, NaCl–KCl equimolar mixture and individual LiCl at 550 oC (for LiCl–KCl) and 750 oC for all the melts). Reaction of the melts containing NdCl3, UCl3 or UCl4 with Li2O (at various O2– to Nd3+ (U3+, U4+) mole ratios) and with O2 was investigated. High temperature electronic absorption spectroscopy was employed to determine the kinetics of the reactions taking place. Phase composition of the solid phases was characterized by X-ray powder diffraction analysis. Size of the particles in the precipitates formed at various conditions was also determined to assess the feasibility of subsequent separation of the solids from the technological melts.

Metal Nitrides for Electrochemical Ammonia Synthesis in Molten Salts

Ammonia is one of the most important base chemicals in the world being a quintessential ingredient in most fertilizers. It is produced on the mega ton per year scale by the Haber-Bosch process from hydrogen and nitrogen at temperatures of around 400-500°C and pressures of up to 200 atmospheres contributing approximately 2% to global CO2 emissions.1 A promising alternative for small scale applications is electrochemical ammonia production utilizing molten salts as electrolytes.2 Such systems work at ambient pressure, moderate temperature and use renewable hydrogen sources like water enabling small local production of ammonia for energy storage and as building block chemical for fertilizer synthesis.2 Few materials have been evaluated as electrocatalysts for ammonia synthesis in molten salts, mainly transition metals.2Metal nitrides, especially Co3Mo3N, are highly active in classical ammonia synthesis with a Mars-van-Krevelen-type mechanism being suggested as the mechanism at play.3 Recently, computational studies have suggested using metal nitrides as electrocatalysts for ammonia formation assuming a similar mechanism.4 In this work, we explore the possibilities of using more complex gas electrode materials in alkali chloride melt to increase the rate and current efficiency of electrochemical ammonia synthesis. Furthermore, we hope to gain a deeper understanding of the mechanism of nitrogen reduction and more generally ammonia formation in molten salts. We synthesized several ternary metal nitrides using temperature-programmed nitridation of oxide precursor and characterized them by powder XRD, BET, XPS, UV-Vis, SEM, TEM and SQUID. Furthermore, the stability of the nitride phase in the chloride melt was tested and the necessary conditions to inhibit decomposition were optimized. A range of electrochemical tests (CV, amperometry, etc.) were carried out and XPS, as well as SEM were used to monitor compositional and structural changes on the surface. Finally, metal nitride electrodes were used for nitrogen reduction. Lan, R.; Irvine, J. T. S.; Tao, S.Sci. Rep. 2013, 3.Giddey, S.; Badwal, S. P. S.; Kulkarni, A. Int. J. Hydrogen Energy 2013, 38 (34), 14576.Abghoui, Y.; Skúlasson, E. Procedia Computer Science 2015, 51, 1897.Howalt, J. G.; Vegge, T. Phys. Chem. Chem. Phys. 2013, 15 (48), 20957.

Electronic Absorption Spectra of Uranium(V) Species in Fused Alkali Chlorides

Alkali chloride based melts can be used in pyrochemical reprocessing various types of spent nuclear fuels, especially those of fast neutron reactors. There is a possibility of refabricating uranium oxide or mixed uranium-plutonium oxide fuel by electrodeposition of UO2 or (U,Pu)O2 from chloride melts containing uranyl (or a mixture of uranyl and plutonyl) ions. At high temperatures uranyl chloro species undergo decomposition yielding UO2 + ions and finally UO2. Increasing temperature and lowering partial pressure of chlorine in the atmosphere above the melt shifts the equilibrium towards the decomposition products. In fact, uranium(V) ions are often present in uranyl-containing melts and, if unaccounted for, result in increased current efficiency of UO2 electrodeposition process. Electronic absorption spectroscopy allows a convenient method for detecting and measuring concentration of uranium(V) in uranyl-containing electrolytes. Absorption spectra of uranium(V) species in fused salts were reported over fifty years ago but still remain the least studied especially from the quantitative side concerning molar absorption (extinction) coefficient values. Uranium(V), f 1-electron configuration, absorbs in the visible region of the spectrum and the spectra have there two broad bands with the maxima around 620 and 775 nm not overlapping with the absorption of uranium(VI) ions, Fig. These bands correspond to two f–f transitions in UO2Cl4 3– complex ion, i.e., φ5u → 3π1u and φ5u → 3π3u. The aim of the present work was studying the effect of temperature and cationic melt composition on the absorption spectra of uranium(V) ions and determining molar absorption coefficients. The measurements were performed in individual alkali chlorides and their mixtures of various compositions including LiCl, CsCl, LiCl–KCl, LiCl–KCl–CsCl, NaCl–KCl, NaCl–CsCl. The temperature range varied from 350 to 850 oC depending on the melt. Uranium(V) ions were obtained in situ by reducing uranium(VI) ions with hydrogen. Uranium dioxide was also produced but no soluble uranium(IV) species in the melt were detected. After recording the spectrum a sample of the melt was taken and analyzed for the total uranium concentration and the mean uranium oxidation state which always was between five and six. From that, concentration of U(V) and V(VI) ions in the melt were calculated and U(V) extinction coefficients determined. The results obtained allow determination of U(V) concentration in technological electrolytes. Acknowledgement. This work was supported by the Ministry of Education and Science of the Russian Federation (project No. 4.5062.2017/8.9). Fig. Electronic absorption spectra of LiCl–KCl based melts containing U(VI) ions (line 1) and a mixture of U(VI) and U(V) ions at a 3 : 2 ratio (line 2). Total uranium concentration 0.038 mol/l, T = 550 oC. Figure 1

An Electrochemical Study of Divalent Ytterbium Species in NaCl–KCl and NaCl–KCl–Cscl Based Melts

A usual oxidation state of rare earth metals in chloride melts is +3. A number of lanthanides (particularly europium, ytterbium, samarium, and thulium) can also form thermodynamically stable ions in the oxidation state +2. Ln(II)/Ln(III) redox processes are normally investigated employing transient electrochemical techniques where the formation of Ln(II) ions is limited to the near electrode surface layer. The present work was aimed at studying the reduction of Yb(III) ions in the bulk of the melt and assessing the stability of Yb(II) chloro species in molten chloride media. The experiments were carried out in the melts based on NaCl–KCl–CsCl eutectic and NaCl–KCl equimolar mixtures at 550–850 and 700–850 oC, respectively, employing stationary and transient electrochemistry and high temperature spectroscopy methods. Concentration of ytterbium in the melt was varied from 1 to 10 wt. %. Ytterbium(II)/(III) formal standard redox potentials were obtained from the results of electrochemistry measurements (cyclic voltammetry and potentiometry). Electronic absorption spectra of Yb(II) species were recorded both in molten and quenched electrolytes. Reduction of Yb(III) to Yb(II) can be achieved by shifting the equilibrium of the reaction YbCl6 3– + Cl– ↔ YbCl6 4– + ½ Cl2 to the right by lowering partial pressure of chlorine in the atmosphere above the melt. Here, zirconium, as a metal with high affinity for chlorine, was used as a getter. The reduction process was followed by measuring the absorption spectra. As the reaction proceeded, the color of the melt changed from colorless of YbCl6 3– to brown. Analysis of the resulting melts showed that the mean oxidation state of ytterbium was below three, the value depended on temperature, duration of the reduction and melt composition. Increasing temperature or reduction time expectedly resulted in a greater degree of reduction. Ytterbium(III) can also be reduced with a suitable reductant and hydrogen was chosen here as an example. Sparging the melt containing Yb(III) ions with hydrogen resulted in decreasing mean oxidation state of ytterbium: YbCl6 3– + Cl– + ½ H2 ↔ YbCl6 4- + HCl. The course of the reduction process was followed by measuring the redox potential. The efficiency of hydrogen as a reducing agent was comparable to the thermal decomposition in the presence of zirconium getter. Hydrogen reduction was also a very convenient way of preparing the melts with a low Yb(II) content for spectroscopy measurements. Another way of reducing Yb(III) to Yb(II) is electrolysis: YbCl6 3– + e– ↔ YbCl6 4-. In a series of preliminary experiments Yb(II)/Yb(III) redox potentials were determined from the results of cyclic voltammetry measurements. Reduction of Yb(III) to Yb(II) was carried out by potentiostatic electrolysis under an inert (argon) or a reducing (hydrogen) atmosphere with and without agitating the melt. The lowest mean oxidation state of ytterbium in the melt thus obtained was 2.33 showing that two thirds of Yb(III) could be reduced to Yb(II). Acknowledgement. This work was supported by the Ministry of Education and Science of the Russian Federation (project No. 4.5062.2017/8.9).

Interaction of Neodymium Containing Chloride Melts with Oxygen Species

Reaction of lithium oxide with neodymium chloride was studied in LiCl, LiCl–KCl and NaCl–KCl based melts at various O2– : Nd3+ mole ratios. The effect of the experimental conditions on the degree and rate of neodymium precipitation, phase composition of the solid reaction products was investigated. Size of the particles in the precipitates formed at various conditions was determined to assess the feasibility of subsequent separation of the solids from the technological melts.