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Phase diagrams of the systems Na3AlF6 – NdF3 and (Na3AlF6 – NdF3)eut – Nd2O3 were experimentally determined by thermal analysis up to 60 mol % of NdF3, and 45 mol % of Nd2O3, respectively. The Na3AlF6 – NdF3 system was found to be a simple eutectic system with the eutectic point with the following approximate coordinates: 49 mol % NdF3, and 905°C. This eutectic composition was then used for the thermal analysis of the (Na3AlF6 – NdF3)eut – Nd2O3 system. The coordinates of the eutectic point of the (Na3AlF6 – NdF3)eut – Nd2O3 system were found to be at approximately 46 mol % Nd2O3 and 733°C. After the thermal analysis, the X – ray diffraction analysis of the solidified samples of both systems was performed. The XRD analysis of the Na3AlF6 – NdF3 system has shown the formation of two new compounds; NaNdF4 and NdOF. The formation of NdOF is probably the product of the high temperature hydrolysis between the moisture in the atmosphere and NdF3. The XRD analysis of the solidified samples of the (Na3AlF6 – NdF3)eut – Nd2O3 system has shown the formation of the following new compounds; NaNdF4, NdOF, NdAlO3, and NaF.

The physicochemical properties of the (LiF (1) + MgF2 (2))eut + LaF3 (3) molten system have been investigated. The phase equilibria, density and volume properties, electrical conductivity, and surf...

The electrical conductivity of molten K3AlF6, Rb3AlF6, Cs3AlF6, and CsF was measured at various temperatures using a pyrolytic boron nitride tube, platinum–rhodium alloy electrode, and alternating ...

Phase diagrams (solubility), density, electrical conductivity and viscosity of molten system (MgF2 – CaF2)eut – MgO have been investigated. The phase diagram of (MgF2 – CaF2 – LiF)eut – MgO and (MgF2 – BaF2)eut– MgO and the density of (MgF2 – CaF2 – LiF)eut – MgO have been also investigated. The solubility of MgO was measured by means of thermal analysis, the density by means of a computerized Archimedean method, electrical conductivity by means of a tube–cell (pyrolytic boron nitride) with stationary electrodes and the viscosity of the melt by computerized torsion pendulum method. It was found that the all investigated properties varied linearly with temperature in all investigated mixtures. On the basis of density values, the molar volume of the melts and partial molar volume have been calculated. The coordinates of the eutectic systems has been established as follows: (CaF2 – MgF2)eut – MgO as 0.30 mol% at 972 °C; (CaF2 – MgF2 – LiF)eut – MgO as 0.20 mol% at 941 °C; and (MgF2 – BaF2)eut. – MgO as 0.25 mol% at 883 °C. The density of the molten system of (CaF2 – MgF2)eut – MgO was found to be at 1000 °C: 2.687 g·cm−3 for the system with 0 mol% of MgO; 2.700 g·cm−3 for the system with 0.30 mol% of MgO and 2.728 g·cm−3 for the system with 0.50 mol% of MgO. The density of the molten system of (CaF2 – MgF2 – LiF)eut – MgO was found to be at 1000 °C: 2.875 g·cm−3 for the system with 0 mol% of MgO; 2.690 g·cm−3 for the system with 0.20 mol% of MgO and 2.650 g·cm−3 for the system with 0.30 mol% of MgO. The viscosity of basic eutectic mixture of (CaF2 – MgF2)eut at 1000 °C was found to be 7.806 mPa·s.

Molten system Na3AlF6 − SiO2 was examined by high temperature NMR. Further characterizations of the samples prepared by Rapid Solidification Processing (RSP) technique and by spontaneous solidification were performed by X–ray diffraction and solid state NMR. The high solubility of SiO2 in molten cryolite and the immiscibility phenomenon were observed. Using thermogravimetric analysis, weight losses were detected, which could be attributed to the formation of the volatile products. The results of the present study are the evidence that SiO2 reacts with molten Na3AlF6 under the formation of the volatile SiF4, non − volatile NaF, and alumino−silicate AlSi3O8− species. NMR spectroscopy coupled with diffraction−based methods was proved as a promising approach to obtain information about the structure and chemistry of this rather complex system.

International audience; A crystallographic approach incorporating multinuclear high field solid state NMR (SSNMR), X-ray structure determinations, TEM observation, and density functional theory (DFT) was used to characterize two polymorphs of rubid-ium cryolite, Rb3AlF6. The room temperature phase was found to be ordered and crystallizes in the Fddd (n°70) space group with a = 37.26491(1) Å, b = 12.45405(4) Å, c = 17.68341(6) Å. Comparison of NMR measurements and computational results revealed the dynamic rotations of the AlF6 octahedra. Using in-situ variable temperature MAS NMR measurements, the chemical exchange between rubidium sites was observed. The β-phase, i.e. high temperature polymorph, adopts the ideal cubic double-perovskite structure, space group 3 ̅ , with a = 8.9930(2) Å at 600 °C. Additionally, a series of polymorphs of K3AlF6 has been further characterized by high field high temperature SSNMR and DFT computation.

A crystallographic approach incorporating multinuclear high field solid state NMR (SSNMR), X-ray structure determinations, TEM observation, and density functional theory (DFT) was used to characterize two polymorphs of rubidium cryolite, Rb

Precise research on the RbF-Al2O3 system was carried out by means of combining X-ray powder diffraction, high-field solid-state NMR spectroscopy, and thermal analysis methods. α-Rb3AlF6, RbAlO2, Rb2Al22O34, and new phase, Rb2Al2O3F2, were identified in the system. The structure of this new rubidium oxofluoroaluminate was determined. It is built up from single layers of oxygen-connected AlO3F tetrahedra, those layers beeing separated by fluorine atoms. This type of structure exhibits a decent ionic conductivity at ambient temperature, 1.74 × 10-6 S cm-1. The similar structural arrangement of O3Al-O-AlO3 and FO2Al-O-AlO2F tetrahedra of the conduction planes in Rb2Al22O34 and Rb2Al2O3F2 were confirmed by 27Al NMR measurements. A thermal analysis of the RbF-Al2O3 system revealed that it can be defined as a pseudobinary subsystem of the more general quaternary RbF-AlF3-Al2O3-Rb2O phase diagram. From a phase analysis of individual phase fields, the mutual metastable behavior of all founded phases can be considered. It was observed that fluoro- and oxoaluminates exist together. Rb2Al2O3F2 is more stable under high temperature. Rubidium fluoro- and oxoaluminates are metastable precursors of the thermodynamically more stable structure of rubidium oxofluoroaluminate.

We present a techno-economic analysis of a 17,000–18,000 metric tons per year electrolytic process for producing Mg from MgO with and without out a concentrated solar thermal input. The solar thermal input is delivered via power tower technology and the evaporation and condensation of sodium. Energy requirements for the process at scale were based on thermodynamics and an extrapolation of laboratory measurements of the electrochemical kinetic and mass transport parameters via a finite-element numerical model. While technically possible, integrating a solar thermal input does not make economic sense without crediting avoided CO 2 emissions. A solar thermal input reduces energy operational costs from $0.654/kg to as low as $ 0.481/kg, but it also lowers the Mg production rate of the electrolytic cells such that more cells are required to achieve production capacity, which, in turn, increases capital and maintenance costs. The net operational savings are negligible. The estimated operational costs to produce Mg are ∼$2.46/kg. At this cost, the process without a solar thermal input is economically tantalizing vis a vis the current commercial processes for producing Mg, and its CO 2 emission level is 46% lower than that of the Pidgeon process, currently the predominant method for producing Mg.

Coal fly ash from coal power plants is a potential raw material for the production of alumina. An objective aluminothermic reduction method for the preparation of Al-Si master alloy and alumina from coal fly ash was investigated. The kinetic analysis using non-isothermal differential scanning calorimetry indicated that the reduction of Al6Si2O13, Fe2O3, and TiO2 by aluminum in coal fly ash occurs at 1618 K, 1681 K, and 1754 K, respectively. Moreover, the influence of reaction temperature on product composition was studied. The phases and morphologies of the products obtained by the aluminothermic reduction of coal fly ash at 1373-1773 K were analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy, respectively. The results from X-ray diffraction show that no oxide reduction has taken place at 1373 K and 1473K, the compositions of the product obtained by aluminothermic reduction of fly ash at 1573K- 1673 K are Al2O3, mullite, Al and Si, while the compositions of the product at 1773 K are Al2O3, Al, and Si. In addition, the chemical compositions of Al-Si alloy obtained at 1773 K are 86.81 wt% Al and 13.19 wt% Si.

We examined the kinetic and transport processes involved in Mg production from MgO via electrolysis at ca 1250 K with in a eutectic mixture of MgF 2 –CaF 2 , using a Mo cathode, and carbon anode. Exchange current densities, transfer coefficients, and diffusion coefficients of the electroactive species were established using a combination of cyclic and linear sweep voltammetry, chronoamperometry and electrochemical impedance spectroscopy. The cathode kinetics are described by a concentration dependent Butler–Volmer equation. The exchange current density and cathodic transfer coefficient are 11±4 A cm −2 and 0.5±0.12 respectively. The kinetics of the anode are described by two Tafel equations: at an overvoltage below 0.4 V, the exchange current density is 0.81±0.2 mA cm −2 with an anodic transfer coefficient of 0.5±0.1; above 0.4 V overvoltage the values are 0.14±0.05 mA cm −2 and 0.7±0.2 respectively. The diffusion coefficients of the electroactive species are D (Mg 2+ )=5.2±0.6E−5 cm 2 s −1 and D ( Mg 2 OF 4 2 - )=7.2±0.2E−6 cm 2 s −1 . The ionic conductivity of the electrolyte is ca 2.6 S cm −1 . A 3D finite element model of a simple cell geometry incorporating these kinetic and transport parameters suggest that up to 27% of the energy required to drive the electrolysis reaction can be supplied thermally for a current density of 0.5 A cm −2 , enabling a reduction in operating cost if the thermal energy is substituted for valuable electric work.

The physicochemical behavior of the molten system (LiF (1) + CaF2 (2))eut + LaF3 (3) has been studied and the phase equilibria, density and volume properties, electrical conductivity, and surface tension have been selected for investigation. Well-established methods for determination of these physicochemical properties of molten salts have been used, such as thermal analysis, the Archimedean method of hydrostatic weighing, and the phase shift and maximum bubble pressure methods. A significant change in all investigated properties was detected in the region of x3 = 0.04–0.06. In regard to phase equilibria, this region is close to and may contain the eutectic point; in volumetric properties, the initial volume contraction (on increasing mol %) starts to reverse. A sharp change of electrical conductivity reflects the changes in ionic composition, and surface tension also shows different behavior below and above this region.

TiB2 samples were exposed to molten KF−AlF3−Al2O3: 54.8-42.1-3.1 mol% salt, at 680 °C for 50, 100 and 200 h. The corroded samples of TiB2 were investigated by SEM-EDX, EBSD, XRD, FT-IR and MAS NMR analysis. Corrosion was noted to occur predominantly as pitting attacks on the surface of the investigated materials. An inter-crystal and trans-crystal corrosion were identified on the cross-sections of the samples. A perturbation of Ti B bonds was detected (SEM-EDX and NMR analysis), at which a formation of orthorhombic TiO2 was also identified (EBSD analysis). The subsequent NMR, XRD and FT-IR analysis of the behaviour of TiB2 powder in molten KF−AlF3−Al2O3 supports the statement about the formation of orthorhombic TiO2 and mullite type of aluminium borates.

Precise investigation of part of the phase diagram of KF–Al2O3 system was performed in an experiment combining different techniques. Solidified mixtures of KF–Al2O3 were studied by X-ray powder diffraction and high-field solid-state NMR spectroscopy over a wide range of compositions. To help with the interpretation of the NMR spectra of the solidified samples found as complex admixtures, we synthesized the following pure compounds: KAlO2, K2Al22O34, α-K3AlF6, KAlF4, and K2Al2O3F2. These compounds were then characterized using various solid-state NMR techniques, including MQ-MAS and D-HMQC. NMR parameters of the pure compounds were finally determined using first-principles density functional theory calculations. The phase diagram of KF–Al2O3 with the alumina content up to 30 mol % was determined by means of thermal analysis. Thermal analysis was also used for the description of the thermal stability of one synthesized compound, K2Al2O3F2.

Precise investigation of part of the phase diagram of KF-Al

Electrical conductivity of molten binary and ternary mixtures based on the system NaF–AlF 3 –SiO 2 was investigated by means of a tube–cell (made of pyrolytic boron nitride) with stationary electrodes. Viscosity of the binary system Na 3 AlF 6 –SiO 2 was measured by computerized torsion pendulum method. It was found that conductivity and viscosity varied linearly with temperature in all investigated mixtures. Obtained content dependence of electrical conductivity (isotherms) was divided into two parts. First, one represented the content region up to 10% (mole fraction) of SiO 2 ; second, the region was with a higher content of SiO 2 (from 10% up to 40%). While the conductivity considerably decreased with content of SiO 2 in the second part; it surprisingly rose in the low content range. A small addition of SiO 2 to the molten cryolite (up to 10%) could slightly increase viscosity, but had no influence on the slope of this dependence since it is responsible for a glassy-networks formation in the melt. Further addition of SiO 2 to the molten cryolite had a huge effect on the viscosity.

Density, viscosity and electrical conductivity of the molten system Na 3AlF 6–SiO 2 have been investigated in the concentration range up to 50 mole % of SiO 2. Density was measured by means of a computerized Archimedean method, viscosity of the melt by computerized torsion pendulum method and the electrical conductivity by means of the tube–cell (pyrolytic boron nitride) with stationary electrodes. It was found that the density, viscosity and conductivity vary linearly with the temperature in all investigated mixtures. On the basis of the density data, the molar volume of the melts and the partial molar volume have been calculated.

The X-ray diffraction (XRD) phase analysis of different solidified uranium-based fluoride systems ((LiF–NaF)eut–UF4; (KF–LiF–NaF)eut–UF4; (LiF–NaF)eut–UF4–ZrF4 and (KF–LiF–NaF)eut–UF4–ZrF4) were examined in order to provide the basis for pyro-electrochemical extraction of uranium in molten fluorides. Several uranium-based species (Na2UF6, Na3UF7, K2UF6, K3UF7, UO2, K3UO2F5) were identified in the solidified melts. The role of oxygen in argon atmosphere was found to be critical in the formation of uranium species during the melting and solidification. In order to reduce the accumulated level of free oxygen traces in our experiments, zirconium (in the form of ZrF4) was used inside the melt as an oxygen buffer. It was found that ZrF4 can really stabilize the uranium species by complexation and protects them against the oxygenation. The results of this work highlight the importance of oxygen removal for obtaining pure deposit in the electrorefinning of uranium.

This article is focused on the electrochemical investigation (cyclic voltammetry and related studies) of the redox couple Sm(III)/Sm(II) in an eutectic LiF–CaF2 melt containing SmF3. The first step of reduction for Sm(III) ions involving one electron exchange in soluble/soluble Sm(III)/Sm(II) system was found on a tungsten electrode. The study of the Sm(II)/Sm(0) electrode reaction was not feasible, due to insufficient electrochemical stability of LiF–CaF2. The first step was found reversible at temperatures 1,075 and 1,125 K up to polarization rate 1 V/s and at temperature 1,175 K the process was reversible at all sweep rates applied in this study. The diffusion coefficients (D) of Sm(II) and Sm(III) ions were determined by cyclic voltammetry, showing that D decreases when oxidation state increase, while the activation energy of diffusion (E a) increases. The standard rate constants of charge transfer (k s) were calculated for the redox couple Sm(III)/Sm(II) at 1,075 and 1,125 K based on the data of cyclic voltammetry.

Electrochemistry of gadolinium and uranium in LiF–CaF2 (79–21 mol%) melt was studied using reactive Ni electrode and alloying reactions were observed. Deposits of gadolinium and uranium in the form of Gd–Ni and U–Ni intermetallic alloys were obtained after electrolysis by modulated current. Electrolysis of the same parameters was used also in the complex system of LiF–CaF2–UF4–GdF3 to demonstrate feasibility of selective deposition of uranium and therefore its separation from the system. Compact deposit of U–Ni alloy containing only traces of gadolinium was obtained.

The electrochemical behaviour of samarium was investigated in LiF-BeF2 system on an inert (Mo) electrode by cyclic voltammetry and chronopotentiometry at 804, 833, 847 and 872 K. Redox process Sm3++e−→Sm2+ was recognized and analysed. Cyclic voltammetry data suggested that at potential sweep rates lower than 0.25 V/s, the reduction was limited by the diffusion of Sm3+ ions. It was not possible to observe reduction process of Sm2++2e−→Sm0 due to insufficient electrochemical stability of LiF-BeF2. Diffusion coefficients of Sm3+ ions in LiF-BeF2 were calculated from voltammetric and chronopotentiometric data in the temperature range 804–872 K. Diffusion coefficient values obeyed Arrhenius law. Activation energy was calculated to be 102.5 kJ/mol.

Sulphur doped nanoparticles of titanium dioxide were prepared by the reaction of titanium butoxide and thiourea in methanol. X-ray powder diffraction and High resolution transmission electron microscopy were used to investigate the crystallinity of the prepared nanoparticles. Based on the X-ray photoelectron spectroscopy measurements sulphur is present in the form of S 6+ which substitutes Ti 4+ in the titania lattice. The band gap energies of synthesised photocatalyst were calculated from the fitting of experimental dependences by double Boltzmann symmetrical functions. The photocatalytic activity of the samples was determinated by the measurement of the 4-chlorophenol (4-CP) degradation in the aqueous suspensions in the ultraviolet and visible region. Electron paramagnetic resonance spectroscopy confirmed the generation of OH radicals from S doped TiO 2 under vis and UV irradiation.

The present paper is an attempt to critically review the relevant techniques for the laboratory measurement of interfacial tension of high-temperature liquid-liquid systems, mostly linked with metallurgy and pyrochemistry. Even if the present paper is preferably devoted to interfacial high-temperature measurements, the same information concerning surface tension will be implicitly provided, as well. There will be no report of the techniques related to dynamic interfacial tension, measurements of ultralow interfacial tension, and the techniques used in microtensiometry and nanotensiometry. These methods (often call nonclassical techniques) are very hard to employ at high temperature, even if they could be in general used at these conditions.

Rapid solidification processing (with a cooling rate in the interval 105–106 K s−1) was used to prepare deeply undercooled cryolite–alumina melts. These samples were analyzed by XRD, infrared, and Raman spectroscopy. Besides cryolite, the amorphous phase and a low amount of ι-Al2O3 were detected. Annealing of the quenched sample revealed the transformation of metastable amorphous phases into different products depending on the annealing conditions. The results obtained showed that all of the elements (Na, Al, O, and F) are probably present in the amorphous parts of the quenched samples.

d ) Institute of Physics, Slovak Academy of Sciences, Dubravska cesta 9, SK-845 36 Bratislava e ) Instituto de Ciencia de Materiales de Aragon, CSIC-University of Zaragoza, ES-50009 Zaragoza The rapid-solidification processing (by a cooling rate of 105 -1 06 K/s) was used for the preparation of deeply undercooled cryolite/alumina (Na3AlF 6/Al2O3) melts. We found a mass of nanotubes on the surface of these undercooled melts. The nanotubes were preferentially located on the defect places of the surface with the following approximate dimensions: base � 100 � 100 nm, length � 1000 nm. The solidified samples with the nanotubes on the surface were analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and infrared spectroscopy (IR).

The interfacial tension (IFT) between aluminum and cryolite melts containing different salt additions (AlF3, NaF, Na2SO4, NaVO3) has been measured by the capillary depression method under the condi...

The interfacial tension (IFT) between aluminium and cryolite melts containing different salt additions (AlF3, NaF,Na2SO4) has been measured during electrolysis by the capillary depression method. The technique is based on the measurement of the capillary depression occurring when a capillary, which is moved vertically down through the molten salt layer, passes through the metal/salt interface. The depression is measured by simultaneous video recording of the immersion height of the alumina capillary. The interfacial tension is strongly dependent on the n(NaF)/n(AlF3) ratio. The addition of Na2SO4 decreases the IFT of the aluminium/electrolyte interface. We also found the different influence of the conditions of electrolysis on the IFT in systems with and without Na2SO4. In systems without Na2SO4 the IFT decreases with increasing current density, and in systems with Na2SO4 it increases.

Al-Si alloy, a precursor of solar grade silicon, was prepared by direct electrolysis in cryolite molten salt at 950 °C using high purity silica as material, liquid aluminum as the cathode and high purity graphite as the anode. The electrochemical behavior of Si(IV) ion was investigated using cyclic voltammetry method. The electrolysis products were characterized by XRD, SEM/EDS and ICP. The results indicate that the reduction process of Si(IV) on tungsten electrode is a two-step process and there is about 0.6 V gap between the two steps. The contents of boron and phosphorus in the aluminum-silicon alloy are 3 ppmw and 8 ppmw, which will make the directional solidification purification effectively and reduce the cost of preparation solar grade silicon from metallurgical grade silicon.

We are exploring the commercial viability for producing Mg from MgO for which thermal energy is supplied to the cell as a substitute for some electric energy. The thermal input source may be concentrated sunlight or natural gas. Laboratory-scale electrochemical studies near 1250 K for two cell concepts show that we reached current densities above 0.5 A-cm−2at an overvoltage of 1.0 V. Current efficiency values exceeded 80%. The discussion of the relationship between these bench-top experimental results and the industrial potential of the process has been initiated.

The cathodic concentration overvoltage in Na 3 AlF 6 –AlF 3 –Al 2 O 3 (sat.) melts was investigated at the temperature of 1000°C. The melts contained 10 and 20 wt.% of AlF 3 . Experimental data on the cathodic overvoltage agreed well with values calculated from the content of sodium in aluminium. The dependence of the cathodic overvoltage on temperature was studied as well. It was shown that the addition of 5 wt.% CaF 2 or 5 wt.% MgF 2 does not influence the overvoltage significantly, while the addition of 2 wt.% LiF decreases the overvoltage by 20 mV at a cathodic current density of 0.25 A cm −2 and by 50 mV at 0.75 A cm −2 . These results are consistent with the influence of these additions on the content of sodium in aluminium.

The electrical conductivity of molten binary and ternary mixtures based on the NaF-AlF3-SiO2 system was investigated by means of a tube-cell (composed of pyrolytic boron nitride) with stationary electrodes. An impedance/gain-phase analyser (National Instruments; a high-performance modular chassis controlled by Labview™ software) was used for the cell impedance measurement. The conductivity was found to vary linearly with temperature in all the mixtures investigated. The concentration dependence of electrical conductivity (isotherms) thus obtained was divided into two parts. The first represents the concentration region of up to 10 mole % of SiO2, the second the region with a higher concentration of SiO2 (from 10 mole % to 40 mole %). While the conductivity decreased considerably with the concentration of SiO2 in the second part, it increased surprisingly in the low concentration range. From these results, the influence of electrolyte composition and temperature on the electrical conductivity was examined.

Rapid solidification processing (cooling rate from the interval 105–106 K s−1) was used to prepare deeply undercooled cryolite-alumina melts. Such prepared samples were analyzed by the XRD method. Besides cryolite, XRD patterns belonging to ι-Al2O3 were recorded. The influence of annealing on the XRD patterns of deeply undercooled melts was also investigated.