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The chloro and oxy-chloro complex ions of vanadium, containing the metal in different oxidation states (II, III, IV and V), were investigated in the melts NaClCsCl (at 550–700°C) and NaClKCl (at 680–980°C) under a variety of conditions. Melts were examined using electronic absorption spectroscopy between 4,000 and 33,000 cm−1. Anodic dissolution of vanadium was studied in NaClKCl melts at various anodic current densities. Vanadium dissolves, forming V(II) ions, at current densities of 100–150 mA cm−2; increasing the current density above ca. 340 mA cm−2 leads to the formation of predominantly V(III) species. Reactions of vanadium metal and vanadium oxides (V2O3 and V2O4) with hydrogen chloride or chlorine were followed by in situ spectroscopy measurements. Depending on the experimental conditions, a range of vanadium chloro- and oxychloro-complexes was formed. Reaction of the V(III) chloro-complex with oxygen leads to the formation of VO(II) complex ions. Preliminary results indicate that the V(IV) chloro-complex, VCl6 2−, can be formed by oxidising VCl6 3− with chlorine. Vanadium(V) oxide reacts with hydrogen chloride in NaClKCl melts to form an oxygen-containing complex of V(III). Direct dissolution of V2O5 in NaClKCl melts yields sodium polyvanadate, NaV6O15.
Hafnium metal is widely used in nuclear industry for fabrication of neutron absorbing elements (control rods) and in metallurgy as a component of superalloys. The magnesium metallothermic reduction of hafnium tetrachloride is the best method for the metal production in terms of process conditions, purity of the product, and economic characteristics. However, synthesis of HfCl4 is a separate task. Direct reaction of hafnium oxide with chlorine gas is thermodynamically impossible. For this reason, chlorination is performed in the presence of a reducing agent such as carbon. In the present work thermodynamic analysis of hafnium oxide chlorination by gaseous chlorine in the presence of carbon was performed at temperatures between 450 and 1000°C. It was found that the main reaction products are gaseous hafnium tetrachloride, carbon monoxide and carbon dioxide. Rising temperature leads to increasing carbon monoxide partial pressure, while that of CO2 decreases. In the above temperature range, the variations of enthalpy, entropy, and Gibbs energy change of the chlorination reaction were calculated and the composition of the gas phase under the conditions of thermodynamic equilibrium was determined. However, the content of the components in real gas–vapor mixtures may significantly differ from the calculated equilibrium values due to kinetic limitation. A series of thermogravimetic experiments was performed under controlled atmosphere using a custom-built set-up based on a Mettler AT20 balances connected to a computer, and intended for the continuous measurement of the sample weight in gases (including aggressive) or in liquids. The kinetics of chlorination of hafnium oxide in the presence of carbon was investigated at temperatures ranging from 600 to 950°C. It was found that below 700°C the process is limited by the chemical reaction on the surface of nonporous solid spherical particles resulting in the formation of a volatile product. Above 700°C, the rate of the process is determined by the mass transfer of the gaseous substances. From the dependences of the logarithm of the reaction rate on the reciprocal temperature the values of the apparent activation energy (Ea) of the chlorination process were calculated. The value of Ea at 600–700°C equals to 65±5 kJ/mol, while at higher temperatures, 700–950°C, it is several times lower and equals to 14±7 kJ/mol. This confirms that at 600-700°C the chlorination proceeds in the kinetic mode, and the process speed is limited by the chemical reaction rate. At 700–950°C the rate of the chemical reaction is significantly higher and the slowest stage is the diffusion of Cl2 and CO to the reaction interface or removal of gaseous HfCl4.
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Neptunium behaviour in an LiCl-KCl eutectic melt at 723 K was studied using spectroelectrochemistry. Cathodic reduction of neptunium(IV)-containing melts led to the formation of Np(III) ions and then Np metal. Electronic absorption spectra of Np(IV) and Np(III) chloro species in LiCl-KCl melt were recorded and resolved into inidividual Gaussian bands. The nature of neptunium complex ions in the melt is discussed.
A comprehensive study of processes taking place during synthesis and subsequent thermal treatment of hafnium oxide was performed. Synthesis of HfO2 involves following stages: stepwise dehydration of hydrated hafnium hydroxide, crystallization of metastable tetragonal phase of HfO2 and subsequent irreversible transition to the stable monoclinic modification. Preliminary heat treatment lowered temperature of crystallization of HfO2 and resulted in changing process mechanism.
Series of alkali metal fluoroscandiates were prepared by reacting corresponding individual alkali fluorides MeF and ScF3 (were Me was Li, Na, K, Rb, Cs) at 400–600 °C for 100 h under argon atmosphere. The Me : Sc ratio varied from 1 : 1 to 3 : 1. X-ray powder diffraction analysis showed that the following phases were formed: Li3ScF6, Na3ScF6, NaScF4, K3ScF6, KSc2F7, Rb3ScF6, Cs3ScF6, CsScF4.
Some of the reports presented at the Sixteenth International Conference on High Temperature Materials Chemistry (Yekaterinburg, July 2–6, 2018) and r
