204 publications from this institution
Abstract—The magnesiothermic reduction of zirconium from its tetrachloride is a promising method for producing a nuclear-purity zirconium sponge. Hig
Melts based on mixtures of alkali and/or alkaline earth halides are considered as prospective media for electrowinning rare earth metals as well as for pyrochemical reprocessing spent nuclear fuels. Fluoride or mixed fluoride-chloride baths can be operated at high temperatures yielding molten rare earth metals (REMs) thus simplifying separation of the metal and salt. One of the problems in using fluoride melts is possible formation of fluorine at the anode. This can be avoided by adding a rare earth oxide to the melt both as the source of REM and oxide ions. The latter will be oxidized to oxygen producing carbon mono- or dioxide at the anode. The limiting factor for feeding the electrolysis bath with REM oxides is their solubility in the halide melt. The aim of the present study was determining the effect of temperature and melt composition on solubility of REM oxides in fused halides. The experiments were performed in CaCl 2 –CaF 2 mixtures containing 20 or 75 mol. % CaF 2 ; BaCl 2 –BaF 2 mixtures containing 15 or 73 mol. % BaF 2 ; equimolar CaF 2 –BaF 2 mixture and NaCl–NaF eutectic mixture (34 mol. % NaF). Solubility of REM oxides was determined by the method of isothermal saturation. Time required for reaching the equilibrium between solid REM oxides and fused salts was determined in a preliminary set of experiments. To compare the behavior of 4 f - and 5 f -elements, solubility of uranium dioxide was also measured. The measurements were performed at the temperatures up to 1400 o C under argon atmosphere. The lower limit of the temperature range varied from 700 to 1100 o C depending on the melting temperatures of the salt mixtures used. Oxides of yttrium, lanthanum, cerium, praseodymium, neodymium and samarium were selected for the study. To assess a possible mutual influence of rare earth elements on solubility of their oxides in fused salts the solubility of a mixture of REM oxides was determined in a separate series of experiments and concentrations of individual REMs in the melt was determined. Solubility of REM oxides increased with increasing temperature and an example of effect of temperature on solubility of neodymium oxide in various melts is shown in Fig. Fig. Concentration of neodymium in alkali and alkaline halide based melts saturated with Nd 2 O 3 . Melt: CaCl 2 –CaF 2 20 mol. % (1); CaCl 2 –CaF 2 75 mol. % (2); BaCl 2 –BaF 2 73 mol. % (3); BaCl 2 –BaF 2 15 mol. % (4); CaF 2 –BaF 2 50 mol. % (5; and NaCl–NaF 34 mol. % (6). Figure 1
No abstract is provided for this article.
Сидоров В.Е., Дубинин Н.Э., Половов И.Б., Никитина Е.В. Расплавы, 2019
Molten alkali chloroaluminates can be used as working media for molten salt nuclear fast reactors (MSNFR). One of the most important and difficult to solve problems that hinder practical realization of MSNFR is finding suitable durable construction materials. These can be selected from the existing ones or, in some cases, developing entirely new materials might be required. In the present work the corrosion behavior of a number of nickel alloys was studied at 450–650 o C in fused KCl–AlCl 3 mixtures with the initial AlCl 3 -to-KCl ratio of 1.1. The alloys were selected from those manufactured by Haynes International, Inc., VDM Metals, and Special Metals Co. and included high-temperature alloys Haynes 230, Hastelloy types S and X, VDM Alloy 600 (previously known as Nicrofer 7216); corrosion-resistant alloys Hastelloy types N, B-3, G-35, VDM Alloy C-4 (Nicrofer 6616), VDM Alloy 825 (Nicrofer 6020); and corrosion and heat resistant alloys Inconel 600 and 625. Samples of the alloys were kept in the melt for 6 to 1000 h. Alloys were tested in «as received» state, as well as after various typical technological operations (bending, welding, heat treatment, etc. ). The results of the tests showed that high-temperature alloys could not be used in contact with molten electrolytes at relatively high temperatures due to intergranular corrosion. The corrosion rates of the corrosion resistant nickel-based alloys were determined by red-ox reactions resulting in dissolution of the most electronegative alloy, i.e. Cr, Fe, and Mn. The corrosion of nickel-based superalloys in molten chloroaluminates has therefore electrochemical nature. Increasing temperature led to a noticeable increase of determined by red-ox reactions resulting in dissolution of the most electronegative alloy and also changed the corrosion processes nature. Transmission electron microscopy showed that a prolonged high-temperature exposure could result in the formation of intermetallic phases, i.e. sigma-phase in Hastelloy G-35 or Ni 2 (Cr,Mo) secondary phase in VDM Alloy C-4. Such phenomena can accelerate intergranular corrosion and stress corrosion cracking of the materials studied under industrial conditions. The results obtained agree well with the thermodynamics analysis, mechanical and thermophysical properties of the alloys, and "time-temperature-precipitation" diagrams constructed. Excessive sigma phases were also formed in the alloys’ surface layer along the grain boundaries after prolonged exposure at 650 ºC. Formation of the secondary phases in the surface layer was caused by selective chromium leaching and degradation of the nickel-based solid solution. Tests performed on welded and bent samples showed that their corrosion rates were higher than could be explained by changes in the structure of the alloy caused by high temperatures (in case of welds) and increased number of defects. Heat treatment of the alloys after welding and/or bending increased their corrosion resistance. The studies performed showed that physical and mechanical properties of the alloys studied were influenced by changes of their structure and composition. The results obtained here allowed determining maximum working temperatures and exposure time of the alloys thus providing the information on the limitation of their application in MSNFR technologies.
No abstract is provided for this article.
Abstract—The influence of the composition of nickel alloys on the mechanism of their corrosion in NaCl–KCl-based chloride melts has been analyz
Solubility of several transition metal chlorides (NiCl2, CrCl2, MoCl3, FeCl2) was measured in KCl-AlCl3 based melts. It was found that the solubility of studied metal chlorides depends on K : Al mole ratio. MoCl3 solubility decreases with increasing AlCl3 content. Solubility of CrCl2 and FeCl2 reaches maximum at K : Al ratio of 1 and decreases when this ratio either de-creases or increases. The dependence of NiCl2 solubility on K : Al mole ratio is V-shaped with the maximum near 0.9-0.95. The effect of temperature on solubility of transition metal chlorides in KCl-AlCl3 melts was also investigated. Increasing temperature does not alter the character of «solubility – K : Al mole ratio» dependences.
Low melting temperatures of KCl-AlCl 3 binary mixtures make chloroaluminate melts attractive as working media for transition metals’ electrowinning and electrorefining. These electrolytes can also be effectively used as heat transfer agents in the second loop of molten salt nuclear reactors. However the implementation of new electrochemical and nuclear technologies is limited by the lack of some essential information about physical and chemical properties of these ionic liquids; in particular there is no data on solubility of transition metal halides in chloroaluminate melts under different conditions. In the present study solubility of nickel, chromium, molybdenum and iron chlorides was determined in КCl-AlCl 3 based melts with varying K : Al mole ratio and working temperature. Potassium-to aluminum mole ratio and the transition metal content in the melt was determined by analyzing samples of chloroaluminate melts saturated with the required transition metal chloride at desired temperature and then rapidly quenched. Melt samples were analyzed by atomic emission spectroscopy combined with ICP. The relation between the solubility of nickel chloride and melt composition (different K : Al mole ratios) is presented in Fig. 1a. Solubility curve of NiCl 2 is V-shaped. The solubility minimum (<0.02 wt.% NiCl 2 ) is reached at the mole ratios of K : Al of 0.92 at 350 о С and 0.97 at 500 о С. Addition of potassium chloride added to the КCl-AlCl 3 equimolar mixture resulted in increase of nickel chloride solubility presumably due to formation of nickel-containing chloride complexes, NiCl 4 2- . Increasing nickel chloride solubility upon lowering K : Al mole ratio (from unity down) can be explained by the formation of six-coordinated complexes of nickel, NiCl 6 4- , existing in the form of Ni(Al 2 Cl 7 ) 2 compound. Increasing temperature does not alter the profile and behavior of the solubility curve of nickel chloride in КCl-AlCl 3 based melts. NiCl 2 solubility increases with temperature at potassium-to-aluminum mole ratios below unity and noticeable decreases in the melts with potassium chloride content exceeding the equimolar КCl-AlCl 3 composition. The results of molybdenum(III) chloride solubility determination at different melt compositions are presented in Fig. 1b. Molybdenum chloride solubility at K : Al molar ratios below 0,95 is small (equals to 0,015–0,03 wt.% MoCl 3 ) and does not depend on melt composition and temperature. When KCl content in the melt increases the molybdenum(III) chloride solubility becomes higher and at molar ratios of K : Al around 1.1–1.4 reaches 2.1 wt.% at 350 о С and 1.7 wt.% at 500 о С. The shape of the solubility curve indicates that molybdenum complex ions, MoCl 6 3- , are formed during dissolution of molybdenum chloride in the presence of excess of potassium chloride. Solubility of iron(II) chloride in КCl-AlCl 3 based melts depends on solubility of double chlorides KFeCl 3 and K 2 FeCl 4 , that have relatively high melting temperatures (399-406 and 374-380 о С, respectively). The obtained data (Fig. 1c) show that iron chloride solubility is relatively high and its dependence on the melt composition has a maximum at K : Al mole ratios between 1.05–1.15. Solubility behavior of chromium(II) chloride in chloroaluminate melts is more complex (Fig. 1d). At the molar ratio of K : Al<0.9 the CrCl 2 solubility is small (less than 0.01 wt.% CrCl 2 ) and does not depend on the melt composition. The increase of KCl concentration in the melt results in increasing chromium chloride solubility. It reaches maximum when K : Al mole ratio is close to 1.0. At further increasing potassium chloride content solubility of CrCl 2 shows a tendency to decreasing. This tendency is more pronounced at higher temperatures (500 о С). The maximum solubility of chromium(II) chloride in the melt is however not that high and does not exceed 0.15 wt.% CrCl 2 . The performed experiments thus indicate that the mole ratio of potassium and aluminum chlorides in the melt is the major factor controlling solubility of transition metal chlorides in the chloroaluminate melts. In a special series of experiments solubility of transition metal chlorides was investigated in chloroaluminate melts contacted with Hastelloy C-276 corrosion-resistant alloy. It was found that the presence of nickel-chromium-molybdenum-iron alloy in the contact with studied electrolyte does not lead to noticeable changes of solubility of transition metal chlorides in КCl-AlCl 3 based melts.
