Vanadium speciation in a variety of alkali chloride based melts was studied in situ using high temperature electronic absorption spectroscopy. The experimental spectra were resolved into constituent Gaussian bands. Octahedral VCl 6 3– complex ions is the only soluble species of V(III). Oxidation of VCl 6 3– complexes or chlorination of V 2 O 5 , V 2 O 4 or V 2 O 3 by hydrogen chloride produces soluble vanadyl-based complexes. The results obtained indicate that the coordination environment of vanadyl-species in NaCl–2CsCl melts is different from the melts containing smaller cations. Anodic dissolution of metallic vanadium in chloride melts produces vanadium(II) ions. Increasing temperature and the mean radius of the salt-solvent cation resulted in changes of the electronic spectra as a result of shifting VCl 6 4– ↔VCl 4 2– equilibrium towards the tetrahedral complexes.
Abstract—Molybdenum and molybdenum-containing alloys are used as structural materials in various high-temperature processes, including the operation
Link for citation: Nikitin D.I., Polovov I.B., Rebrin O.I. Possibility of uranium extraction from spent nuclear fuel in fused electrolytes containing rare elements . Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2023, vol. 334, no. 10, рр. 210-218. In Rus. The relevance of the research is caused by the plans of using electrolytic separation of metalized spent nuclear fuel as a stage in pyrochemical reprocessing of mixed nitride uranium-plutonium fuel. The main aim is to determine the parameters of uranium electrolytic separation from an alloy with simulators of fission products (precious metals and rare earth elements) simulating spent nuclear fuel in salt mixtures based on 3LiCl–2KCl with additives of rare earth element chlorides. Objects: model spent nuclear fuel – uranium alloy with simulators of fission product (precious metals and rare earth elements) with a mass compound of Pd:Ru:Ag:Rh=25:1:3:3, Nd:Ce:La:Pr:Sm:Y=15:10:5:5:5:1). Methods: electrorefining, X-ray fluorescence and X-ray diffraction method of analysis, inductively coupled plasma mass spectrometry, assessment of distribution of components in the system. Results. The data obtained showed that uranium deposits have dendrite formations of alpha-uranium at 550 °C in orthorhombic crystal system toward needle cathode current density. The resulting cathode deposits are free from impurities of ruthenium, rhodium, molybdenum, praseodymium and yttrium. The purification coefficient for palladium reaches 3000, and for silver 1700. Noble metals accumulate in the anode sludge, and even with the complete depletion of the anode material. The concentration of noble metals in the cathode deposit does not exceed 0,0015 wt %. Despite the high concentration of rare-earth chlorides in the electrolyte, which simulates the accumulation of rare-earth ions during repeated reprocessing of spent nuclear fuel, the concentration of rare-earth metals in the cathode product did not exceed 0,007 wt %. During uranium electrowinning from a model spent nuclear fuel in the 3LiCl–2KCl–UCl3 electrolyte (10,1 wt %) with REE chlorides, which imitate their accumulation in the electrolyte during repeated processing, at 550 °C, as well as the initial cathode current density of 0,2 A/cm2, the specific amount of electricity is 1,0 A∙h/cm2. A cathode uranium deposit is released with a current efficiency exceeding 90 % with all anode mass depletion, and purification coefficient 1800 for the sum of precious metals and for the sum of rare earth metals.
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 neptunium metal. Electronic absorption spectra of Np(IV) and Np(III) chloro species in LiCl-KCl melt were recorded and resolved into individual Gaussian bands. The nature of neptunium complex ions in the melt is discussed.
Corrosion behavior of stainless steel types AISI 316L, 316Ti and 321 and their components (iron, chromium, nickel and molybdenum) was studied at 750 0C in NaCl-KCl-VCl2 melts. It was found that iron, chromium and manganese species constitute the major corrosion products of austenitic steels and Fe2+, Cr2+, Ni2+, and Mo3+ ions are formed as a result of interaction of corresponded metal and vanadium-containing chloride melts. It was found that austenitic stainless steels are subjected to high temperature sensitization and this phenomenon determines the intergranular character of steel corrosion. Presence of vanadium ions in electrolyte leads to intensification of corrosion processes due to increased oxidation of electronegative steel components and alloy formation. Metallic molybdenum has highest corrosion resistance among studied construction materials.
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Molten salt mixtures can be effectively employed for electrowining and electrorefining of relatively chemically active rare refractory metals having negative reduction potential. One of such metals is vanadium. However quality of the metal produced (impurities content and size of crystals in the deposit) and current efficiency in a standard vanadium electrorefining process does not fully correspond to the requirements. The current work was carried out aiming to determine the optimal conditions of operating industrial vanadium electrolysis baths. The experiments on vanadium electrorefining were carried out in a semi-industrial electrolyser made of stainless steel. Crude vanadium metal was loaded in an anodic basket made of molybdenum. The experiments were performed in dry purified argon atmosphere, and glassy-carbon or nickel crucibles were used to contain the melt. Vanadium-containing melts were prepared by dry chlorination of vanadium metal with subsequent absorption of chlorination products by fused salt mixtures. To stabilize vanadium(II) ions, the molten electrolyte was kept in contact with vanadium for 12–18 hours. Mean oxidation state of vanadium in the melt determined oxidimetrically was always close to two and didn’t depend on vanadium concentration. The method of mathematical planning of experiments was used to determine optimal conditions of electrolysis. Current efficiently was chosen as a response factor. Variable parameters and their intervals were chosen on the basis of the preliminary experiments and the results of kinetic studies. Current density, vanadium concentration and specific quantity of electricity were selected as variables. Temperature had no effect on the current efficiency and increasing temperature only resulted in rising vapor pressure of the melt. Therefore, temperature was not included into variable parameters. Anodic current density was kept essentially constant between 0.1 and 0.15 A/cm 2 . Such anodic current density allowed maintaining average oxidation state of vanadium ions close to two in the course of the experiments. The 2 3 full factor experiment was performed and the regression equation was obtained using the experimental results. All variable parameters were significant and the equation obtained was adequate. The influence of the quadratic term was insignificant. The response function was then obtained in the form of a dependence of the cathodic current efficiency on current density, vanadium concentration and specific quantity of electricity passed. The most suitable parameters of vanadium electrorefining were determined using this equation and the experiment under the optimal conditions was carried out, Figure. The total amount of impurities in the refined product was below 0.09 wt. % and the current efficiency equaled to 0.96 g/A∙h. Figure. High-purity electrolytic vanadium Figure 1
The process of vanadium electrolytic refining in NaCl-KCl melt was performed in the specially designed semi-industrial scale electrolyser. The method of experiments mathematic planning was used to determine the electrolysis optimal conditions. In a special series of industrial experiments distribution of metallic and non-metallic impurities during vanadium electrorefining was investigated. The optimal parameters of the electrolysis for producing 99.5% pure vanadium were determined and the requirements for purity of crude materials formulated.
High-temperature absorption spectroscopy is used to study the effect of temperature in the range 550–750°C and the cation composition of a solve
Abstract—The technology of pyrochemical processing of mixed nitride uranium–plutonium spent fuel that is applied at the experimental and demons
Physico-chemical behavior of vanadium in chloride melts was studied using high temperature electronic absorption spectroscopy and spectroelectrochemistry. Dissolution of VCl3 in molten chloride leads to the formation V(III) species, VCl63-. Vanadium(II) complex ions VCl64- are formed during electroreduction of vanadium(III)-containing melts and vanadium metal anodic dissolution. V(II) chloro-species slowly react with silica to form an oxygen-containing insoluble phase. Due to high volatility of VCl4 oxygen-containing vanadyl species, VOCl42-, is the only stable form of V(IV) in fused chlorides. It is formed when V2O5, V2O4 or V2O3 react with HCl. Electroreduction of vanadyl complexes leads to the formation of insoluble oxygen-containing vanadium(III)species, whereas electrochemical oxidation has no effect on vanadium speciation, VO3+/VO2+ red-ox potential is more positive than Cl2/Cl-. Further oxidation VO2+ species gives vanadate anions. The electronic absorption spectra of soluble vanadium species were measured in 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl mixtures at 450-750 0C.
Molten salts have a wide range of unique properties that make them particularly useful in many areas of modern technology. From the fundamental point of view, analysis of speciation of vanadium in chloride melts leads to deeper understanding the nature of complex ion formation in ionic media. In the present work formation of vanadium complex ions in 3LiCl-2KCl, NaCl-KCl and NaCl-2CsCl mixtures at 450-750 o C was studied employing high temperature electronic absorption spectroscopy. The electronic absorption spectra (EAS) were recorded in the course of dissolution of vanadium(II) and vanadium(III) chlorides, anodic dissolution of metallic vanadium, and chlorination of various vanadium oxides (V 2 O 3 , V 2 O 4 , V 2 O 5 ). Dissolution of VCl 3 in fused alkali chlorides leads to the formation of V(III) species. Average oxidation state of vanadium in the quenched melt samples was 2.9-3.1. Spectral curves were resolved into two individual bands attributed to the 3 T 1g → 3 T 1g (P) and 3 T 1g → 3 T 2g electronic transitions in the octahedral VCl 6 3- complex. The positions of the band maxima shift towards longer wavelengths with increasing average radius of alkali metal cation, in agreement with the expected decrease of the polarizing effect of the second coordination sphere. Vanadium dichloride added to the alkali chloride melts partly disproportionated forming V(III) species and vanadium metal. The resulting spectrum represented a superposition of the absorption bands of V 2+ and V 3+ ions. The average oxidation state of vanadium in the quenched melts was higher than two and the concentration of vanadium was noticeably lower than expected. The best method for preparing vanadium(II) containing melts is the anodic dissolution of the metal. Position of the maximum in the EAS of V(II) in 3LiCl-2KCl at 450 o C (18770 cm -1 ) is very close to that of V(III), being shifted to higher energies only by about 900 cm -1 (Figure). This band corresponds to 4 A 2g → 4 T 1g (P) electronic transition in the octahedral VCl 6 4- complex (d 3 -configuration). Second band, with the maximum at 11890 cm -1 is attributed to 4 A 2g → 4 T 1g electron transition in the same ion. Increasing temperature and the mean radius of the alkali metal cation on the solvent melt resulted in certain changes of EAS of vanadium(II)-containing melts, i.e. , the intensity of the second peak decreased and a new, third, band appeared around 13300 cm -1 (Figure). The position of this third peak did not change with temperature and was the same in different melts. This band is attributed to 4 T 1 → 4 T 1 (P) electronic transition in the tetrahedral VCl 4 2- ion. Therefore, increasing temperature and the mean radius of the salt-solvent cation shifts the VCl 6 4- –VCl 4 2- equilibrium towards the tetrahedral species. The results of in situ spectroscopy measurements conducted during the reactions of various vanadium oxides with hydrogen chloride in alkali chloride melts showed that the nature of the products formed does not depend on the type oxide and the flow rate of HCl. Oxidation state of vanadium in all the resulting melts was close to four. The EAS corresponded to the absorption of vanadyl-ions. Oxygen-containing vanadium(IV) species exhibited somewhat anomalous behavior in NaCl-2CsCl melt as the temperature was varied. Increasing temperature resulted in decreasing intensity of the band at 13000-14000 сm -1 . This is explained by the coordination environment of vanadyl species in NaCl-2CsCl melts, different from the melts containing smaller cations. This conclusion was confirmed by XRD and Raman spectroscopy analysis of the quenched melts. Figure. Resolution into Gaussian bands of EAS recorded after anodic dissolution of vanadium metal in 3LiCl-2KCl (a) and NaCl-2CsCl (b) eutectic melts at 450 and 750 o C, respectively. Figure 1
Experimental set up for spectroelectrochemical measurements was designed. Uranium(III) ions, UCl6 3-, are formed during anodic dissolution of metallic uranium and prolonged contact of U(IV)- containing melts with uranium metal. UCl4 dissolution in 3LiCl- 2KCl melt leads to the formation of UCl6 2- ions. They undergo a two-step electrochemical reduction: U4+→U3+ recharge followed by U deposition. Neptunium(IV) chloride dissolves in 3LiCl-2KCl melt producing NpCl6 2- ions. Electrochemical reduction of Np(IV) on a glassy carbon electrode results in the formation of NpCl6 3- ions and then leads to the deposition of neptunium metal. Electronic absorption spectra of uranium (III, IV, V and VI) and neptunium (III and IV) species in 3LiCl-2KCl eutectic were measured at 450 oC.
