Fluoride Melts Research Articles

Molten Salt Reactor for 99Mo Production

99Mo production in a molten salt reactor with fuel salt based on metal fl uorides (LiF–BeF2–UF4) is examined. The proposed method of production is based on a particularity of the behavior of gaseous fission products in salt melts. Molybdenum, together with some other metals, does not form stable compounds in fluoride melt. At least 50% of fragment molybdenum in a molten salt nuclear-physical facility will be in a gas-aerosol phase above the surface of the molten salt. Neutron-physical and thermohydraulic calculations were performed taking into account the specific features of a molten salt reactor. The optimal configuration of the reactor, the materials used, the fuel salt composition, and the fuel cycle are described.

Surface tension of a titanium-containing oxide–fluoride melt calculated by the polymer theory

The surface tension of an aluminum–calcium oxide–fluoride melt is calculated using the polymer theory. It is shown that, at 15 mol % titanium oxide in the melt, aluminum–fluorine–oxygen complexes mainly form. When the titanium oxide content increases further, these complexes disappear and are partly replaced by titanium–oxygen TiO 4 4- and Ti2O 7 6- anions and titanium–fluorine–oxygen groups.

The amphiboles of the REE-rich A-type peralkaline Strange Lake pluton – fingerprints of magma evolution

Major and trace element compositions of amphibole in igneous environments commonly reflect evolving magma compositions. In this study, we use the amphibole-group minerals from the Strange Lake, REE-enriched peralkaline granitic pluton to gain insights into the evolution of the magma. This 1240Ma old pluton consists of two main intrusive facies, an early hypersolvus granite, which occurs as separate northern and southern intrusions, and a more evolved transsolvus granite. In the hypersolvus granite the amphibole is a late interstitial phase, whereas in the transsolvus granite, it is present as phenocrysts. The amphibole compositions vary from calcic–sodic (ferro–ferri-katophorite) in the southern hypersolvus granite to sodic (arfvedsonite, ferro–ferri-leakeite) in the other, more evolved granitic units. High Na, Si, Li, and low Al and Ca concentrations in the amphibole phenocrysts of the transsolvus granite indicate formation from a more evolved magma compared to the hypersolvus granite, despite the fact that these crystals formed early. We interpret the increasing Fe3+/Fe2+ ratios in the amphibole of the hypersolvus granite to reflect crystal chemical effects (Na/Ca-ratio) and increasingly oxidizing conditions in the magma, whereas in the phenocrysts of the transsolvus granite, the increasing ratio was the product of increasing proportions of F− and OH− in the melt. The amphiboles of all the granite units have elevated Nb, Zr, Hf and REE concentrations compared to the bulk rock, suggesting that these elements are compatible in amphibole. By contrast the much lower Ti concentration was due to saturation of the magma in sodium-titanosilicates. The amphibole REE concentrations vary greatly among the granite units. Amphibole of the southern and northern hypersolvus granite contains 0.16 and 0.07wt.% ∑REE+Y, on average, respectively, and in the transsolvus granite, the average ∑REE content is only 0.01wt.%, despite the more evolved nature of its host transsolvus granite. We intrepret this compositional difference to be due to the fact that the latter represents phenocrysts, which crystallized early, whereas the hypersolvus arfvedsonite is a late interstitial phase. Chondrite-normalized REE profiles emphasise the wide range in LREE-, and the narrow range in HREE-concentrations of the amphiboles. The variations in the LREE-profiles reflect the variable crystallization of primary LREE-bearing phases, including monazite-(Ce), pyrochlore group minerals and gagarinite-(Ce), prior to or contemporaneous with the amphibole, as well as the exsolution of a LREE-rich fluoride melt. The LREE are incompatible in the amphibole structure (apparent D<0.01) and are preferably accommodated by the octahedral C-site, whereas the HREE occupy the B-site. The chondrite-normalized HREE profiles are steep and display an increasing relative enrichment that culminates in compatible behavior for Yb and Lu (apparent D>1). Owing to their small ionic radius and their compatibility with the amphibole structure, HREE concentrations were more controlled by partitioning (crystal chemical effects) than by the concentrations in the corresponding magma. Large proportions of the bulk HREE content (up to 70%) reside in the amphibole, and their later release through hydrothermal replacement helps to explain the extreme and unusual HREE enrichment of the Strange Lake pluton.

Influence of alkaline earth metal cations on the charge transfer kinetics for the redox couple Ti(IV)/Ti(III) in a chloride–fluoride melt

The influence of strongly polarizing cations Mg2+, Ca2+, Sr2+, and Ba2+ on the charge transfer kinetics of the redox couple Ti(IV)/Ti(III) is studied. The standard rate constants of charge transfer (k s) for the Ti(IV)/Ti(III) redox couple in the (NaCl–KCl)eq–NaF (10 wt %)–K2TiF6 melt upon the addition of the strongly polarizing cations into the starting melt are determined. The dependence of the charge transfer rate constants on the ionic potential of the cations is established. The activation energies of the charge transfer process are calculated for the fluoride complexes of the Ti(IV)/Ti(III) redox couple with the outer-sphere cations Mg2+, Ca2+, Sr2+, and Ba2+.

Electrochemical behavior and electrowinning of uranium(IV) from FLiNaK molten salt

The electrochemical behaviors and electrowinning of uranium from LiF–NaF–KF (46.5–11.5–42.0 mol%, FLiNaK) molten salt were investigated. Cyclic voltammetric and chronopotentiometric studies revealed the presence of two steps during the electro-reduction of UF4 on W electode. U(IV) undergoes a one-electron reduction to form U(III) followed by a three-electron reduction to form U(0). The diffusion coefficient (D) of U(IV) ion was determined to be 1.33 × 10−6 cm2 s−1 at 550 °C. Based on the electrochemical behaviors of U(IV) in FLiNaK melt, pulse current electrolysis was carried out using Ni electrode. The black bulks obtained as electrolysis products were characterized to contain metallic U. Consecutive electrolysis experiments demonstrated up to 74.7% U can be separated from the fluoride melt.

Production of Nd-Fe Foundry Alloy by Electrolysis in Molten Salts

Nd-Fe-B-based permanent magnets have the highest magnetic characteristics. To improve their quality, one generally applies the technique of solid-phase alloying by materials with high content of neodymium, dysprosium, terbium, i.e. rare-earth metal (REM)-Fe(Co) foundry alloys. The paper presents the results of the study of the manufacturing process of Nd-Fe foundry alloys, with composition being close to eutectic, by electrolysis of neodymium oxides and fluorides from chloride and fluoride melts. It is shown that in the oxide electrolysis a sufficiently high current efficiency (over 98 %) is obtained. This is a promising method of obtaining of rare earth-containing materials.

ANODE GAS DYNAMICS IN HIGH-TEMPERATURE CRYOLITE MELT-ALUMINA SLURRY

The paper shows the results of simulating physical behavior of bubbles formed by oxygen electrowinning on an inert anode during hightemperature alumina slurry electrolysis in a fluoride melt. As part of the study, similarity criteria were calculated with experiments conducted on a water-based model of a cell with vertical electrodes, and the data on bubble behavior in slurry was obtained by video recording. The 20 % aqueous sulfuric acid solution with 30 vol.% alumina content was used as electrolyte for the model. Experiments were conducted in the electric current density range from 0,05 to 0,25 A/cm2. The video was recorded using the Nikon D3100 camera with 30 frames per second rate. The motion pattern of bubbles was obtained along with the quantitative data describing coalescence and bubble rise velocity. 125 bubbles with a thickness of 0,8 to 2,3 mm were analyzed to determine the average bubble rise velocity. Bubbles rose in a slug regime at 1,0–2,3 cm/s. The thickness of a bubble layer was about 5 mm. Further studies will be conducted to obtain new data on the bubble behavior at different solid contents, current densities, electrodes inclination angles, and particle size distributions.

CSLM study on the interaction of Nd2O3 with CaCl2 and CaF2–LiF molten melts

Neodymium is produced commercially by the fused salt electrolysis of Nd2O3–NdF3–CaF2–LiF melt. The efficiency of the process is low due to the limited solubility of Nd2O3 in the fluoride melt. The suitability of making Nd directly from Nd2O3 by the FFC Cambridge process seems very attractive. The basic requirements for FFC Cambridge process are that (a) the oxide should not dissolve in the molten salt (usually CaCl2 or CaF2–LiF) and (b) the decomposition potential of oxide should be below the decomposition potential of salt. The present study was to understand the first requirement. It reports for the first time the in situ chemical interactions of Nd2O3 with molten CaCl2 and CaF2–LiF using confocal scanning laser microscope. In molten CaCl2, Nd2O3 reacts vigorously to form NdOCl. The product detaches from the parent Nd2O3 surface, exposing the oxide surface to further attack from molten CaCl2. The reaction rate becomes faster as the temperature increases. In a molten CaF2–LiF melt, Nd2O3 dissolves. Owing to the limited solubility of the oxide in CaF2–LiF melt, the direct electroreduction may still proceed in an oxide-saturated CaF2–LiF melt.

Different types of liquid immiscibility in carbonatite magmas: A case study of the Oldoinyo Lengai 1993 lava and melt inclusions

Oldoinyo Lengai is situated within the Gregory Rift Valley (northern Tanzania) and is the only active volcano erupting natrocarbonatite lava. This study investigates the texture and mineralogy of the June 1993 lava at Oldoinyo Lengai, and presents petrographic evidence of liquid immiscibility between silicate, carbonate, chloride, and fluoride melt phases. The 1993 lava is a porphyritic natrocarbonatite consisting of abundant phenocrysts of alkali carbonates, nyerereite and gregoryite, set in a quenched groundmass, composed of sodium carbonate, khanneshite, Na-sylvite and K-halite, and a calcium fluoride phase. Dispersed in the lava are silicate spheroids (<2mm) with a cryptocrystalline silicate mineral assemblage wrapped around a core mineral. We have identified several textural features preserved in the silicate spheroids, melt inclusions, and carbonatite groundmass that exhibit evidence of silicate-carbonate, carbonate-carbonate and carbonate-halide immiscibility. Rapid quenching of the lava facilitated the preservation of the end products of these liquid immiscibility processes within the groundmass. Textural evidence (at both macro- and micro-scales) indicates that the silicate, carbonate, chloride and fluoride phases of the lava unmixed at different stages of evolution in the magmatic system.

Hydrogen Diffusion and Trapping in Fluoride Salt Melts and Graphite Electrodes

Molten salt processes were developed decades ago as a promising technology for material processing and industrial process heat applications. This technology is still considered applicable for thermal energy storage and power production. Liquid salts can indeed be considered legitimate candidates for several heat transfer applications, thanks to favorable physical properties such as high boiling point, low vapor pressures, large specific heat, and large thermal conductivity. In particular, fluoride-based molten salts are considered as a coolant for several nuclear reactor advanced design owing to favorable neutronic cross sections and reduced corrosion of structural materials. FHR, Fluoride-salt cooled High-temperature Reactors, use flibe salt (2LiF-BeF2) to cool a graphite pebble-bed-fueled core. A detailed knowledge of the chemistry of fluoride melts is important for an efficient corrosion control and reliable FHR reactor design. The corrosion mechanisms in a molten salt system are different from aqueous systems; molten salt chemistry prevents the formation of a thermodynamically stable passivation layer on structural metals, therefore the corrosion control has to be achieved through the control of the redox potential of the fluoride salt. Solubilities and diffusivities of metals, rare earths, fission products and tritium in the fluoride melt and within graphite framework, are of direct interest for the FHR design community. Ionic coordination in the liquid and gas phase also needs to be fundamentally understood in order to address the above mentioned problems. This paper will provide a review of the electrochemical techniques and spectroscopy methods used in the past to study fluoride salt systems. Furthermore, state of the art of techniques used in other high temperature molten salt environments and low temperature ionic liquids will be reviewed. Tritium management is the main design challenge for the FHR reactors. Tritium is produced through transmutation of Lithium-6 in a neutron environment and might be released into the atmosphere through the heat exchanger walls. However, in FHR reactors, the graphite pebble bed and graphite reflector may be the ultimate sink for tritium. Goal of the ongoing research at UW Madison is to define tritium diffusion and absorption into graphite, using hydrogen as a surrogate. This paper will address theoretical electrochemical techniques used in aqueous system to study absorption of hydrogen into metals (Potentiostatic Double-Step [1]) and also experimental techniques (Electrochemical Impedance Spectroscopy [2]) applicable to the same issue. These techniques will be applied initially to graphite/water systems and later to the graphite/flibe system. The paper will present preliminary results and report the challenges pertaining to experimental setup with fluoride salt systems, due to the high melting temperatures, the highly corrosive nature of the salt, in addition tothe toxicity of Beryllium, main component of flibe salt. REFERENCES: [1] B. Pound, G. Wright, et al., ‘A potentiostatic double-step method for measuring hydrogen atom diffusion and trapping in metal electrodes—II. Experimental’, Acta Metall., vol. 35, no. I, pp. 263–270, (1987). [2] D. D. Macdonald and M.C.H. McKubre. “Impedance Measurements in Electrochemical Systems”. Chapter 2 in Modern Aspects of Electrochemistry. 14, 61(1982).

Electrodeposition of Titanium in Water−Soluble KF−KCl Molten Salt

１． Introduction Metallic titanium has superior properties such as corrosion resistance, heat resistance, specific strength, etc. However, its high production cost and poor workability hinder the wide spread use of titanium. Coating technology is a fascinating method to utilize the characteristics of high corrosion resistance and high strength of titanium metal or compounds. Currently, PVD methods are commonly used for a coating of some titanium compounds, however, the deposition onto a substrate with complicated shape is difficult. Electroplating in molten salts has been expected as a coating method of titanium even for any shapes. Until now, there are some reports on the titanium electrodeposition from molten salts [1-7]. In previous studies, fluoride [3,6], chloride [1,2,4], and fluoride-chloride [4,5,7] molten salts have been used as electrolytes. Generally, compact and smooth Ti films were obtained from fluoride melts such as LiF–NaF–KF [3,6] or fluoride-chloride melts such as NaCl–KCl–NaF [5,7]. For the fluoride-chloride mixtures, when F/Cl atomic ratio became higher, better deposits (grain size and purity) were obtained and higher current efficiency were achieved [5,7]. However, one of the problems in the previous studies employing fluoride-based molten salts was the difficulty in removing the salt adhered to the deposited Ti, because the solubilities of LiF and NaF in water are very low. Recently, we have proposed a new electrodepositon process of Si using a KF–KCl binary system [8]. Since both KF and KCl are highly soluble in water, the adhered salt can be removed easily by washing with water. In the present study, we investigated the electrodeposition of Ti in molten KF–KCl after the addition of K2TiF6and sponge Ti. ２． Experimental The experiments were conducted in eutectic KF–KCl in a dry Ar atmosphere at 923 K. K2TiF6 (0.1-2.0 mol%) and Ti sponge (10 g) were added to the bath as Ti sources, and Ti (IV) ions were converted to Ti (III) ions by proportionation reaction. A Ni plate electrode was used as a working electrode. The counter and quasi-reference electrodes were a Ti rod and a Pt wire, respectively. The potential of the quasi-reference electrode was calibrated by the deposition potential of K metal on a Mo wire. Samples produced by the galvanostatic electrolysis were analyzed using SEM/EDX and XRD after washed with water to remove the adhered salts. ３． Result and discussion The samples preparation was carried out by galvanostatic electrolysis under the conditions of the added amount of K2TiF6 of 0.1-2.0 mol% and the cathodic current density of 25-250 mA cm-2. As a typical example, Fig. 1 shows an XRD pattern of the sample prepared at 100 mA cm-2 for 100 min in the melt after addition of K2TiF60.1 mol% and sponge Ti. Electrodeposition of crystalline Ti metal was confirmed. As shown in the surface SEM image in Fig. 2, the obtained granules have a diameter about 20 μm. No element was detected except for Ti by EDX analysis. In the presentation, the effects of concentration of Ti (IV) ions and current density on the morphology and crystalline orientation will be discussed. Acknowledgement A part of this study was conducted as a collaborative research with Sumitomo Electric Industries, Ltd. R eferences [1] J. O’M. Bockris, G.J. Hills, I.A. Menzies, L. Young, Nature, 178, 654 (1956). [2] D. Inman, S.H. White, J. Appl. Electrochem., 8, 375 (1978). [3] J. De Lepinay, J. Bouteillon, S. Traore, D. Renaud, M.J. Barbier, J. Appl. Electrochem., 17, 294 (1987). [4] D. Wei, M. Okido, T. Oki, J. Appl. Electrochem., 24, 923 (1994). [5] N. Ene, S. Zuca, J. Appl. Electrochem., 25, 671 (1995). [6] A. Robin, R.B. Ribeiro, J. Appl. Electrochem., 30, 239 (2000). [7] J. Song, Q. Wang, X. Zhu, J. Hou, S. Jiao, H. Zhu, Mater. Trans., 55, 1299 (2014). [8] K. Maeda, K. Yasuda, T. Nohira, R. Hagiwara, T. Homma, J. Electrochem. Soc., 162, D444 (2015). Figure 1

Electrolytic Aluminizing of Low Carbon Steel as Protection Treatment for Current Leads against High Temperature Oxidation

Application of protective aluminum-based coatings is one of the ways to increase the oxidation resistance of low-carbon steel. The electrolytic deposition of aluminum in the NaF33-KF15.2-(AlF3)51.8 wt.% melt at 920°C and the current density of 0.8-1.0 A·cm-2 (0.25 A·h·cm-2) have provided a continuous aluminide coating based on Fe2Al5 with a good adhesion to the steel substrate due to dissolution of oxide film from the surface of the treated products in fluoride melt. The resistance of steel samples to sulfide corrosion was investigated in a lab-scale three-electrode cell at 900°C in contact with a “Soderberg” carbon anode, which was obtained by carbonization of the coal tar pitch containing 2.0-2.5 wt.% of sulphur. The IRC (ohmic voltage drop in contact layer) growth rate was 3.3 times higher for uncoated steel due to formation of the oxide-sulfide layer based on FeS. The electrolytically aluminized steel with preliminarily formed α-Al2O3 layer possessed more stable value of the IRC in comparison with the uncoated steel because of the higher chemical resistance.

Critical evaluation of partial molar volumes and excess partial molar volumes of molten fluoride melts

Different procedures of evaluation of partial molar volume and excess partial molar volume have been applied for three binary or quasi-binary molten fluoride systems: KF–K2TaF7, K2ZrF6–K2TaF7 and (LiF–NaF–KF)eut–K2ZrF6. In the first procedure partial molar volume is calculated based on mathematical fitting of molar volume, while in the second procedure partial molar volume is calculated based on mathematical fitting of excess molar volume. It was shown that both procedures are in principle consistent. However, some inconsistencies on values of partial molar volume can occur when local extremes are observed on concentration dependence of molar volume. In such situation, proper selection of mathematical functions is essential which requires relatively high density of experimental data. Moreover, when data on only restricted concentration range are available, only procedure of molar volume fitting is applicable for calculation of partial molar volume.

Evaluating Composition Dependence in Surface Tension of Si–Ca–Na–O–F Reciprocal Oxide–fluoride Melts

Evaluating Composition Dependence in Surface Tension of Si–Ca–Na–O–F Reciprocal Oxide–fluoride Melts

Cathodic Behavior of Samarium(III) and Sm-Al alloys Preparation in Fluorides Melts

The electrochemical behavior of samarium ions in LiF-CaF2 melts at tungsten electrodes is investigated by cyclic voltammetry and square wave voltammetry. The electrochemical reduction of Sm(III) into Sm(II) according to Eq. (1) is reversible within 900mV/s.Sm3++e−→Sm2+(1)The reduction process for Sm(II) to Sm cannot be observed due to a more negative potential than that of Li(I) ions. Cyclic voltammograms results have proved that Sm and Al ions can be co-reduced from LiF-CaF2-AlF3-SmF3 melts. The cathodic product from LiF-CaF2-Sm2O3 (0.15mol/L)-AlF3 (8.89mol/L) melts at −731.4mA/cm2 is a thin layer metallic plate mainly composing Al, Al2Sm and Al3Sm. EDS mapping manifests that Sm-Al alloy exists in the cross-section of the deposit with the molar ratio of Sm/Al of 0.020, smaller than the original value of 0.034. By galvanostatic electrolysis in LiF-CaF2-Sm2O3 (0.225mol/L)-AlF3 (0.296mol/L) with Al (6mol/L) at –800mA/cm2 for 3h, only small metal particles dispersed in the molten salts are found, giving a lower Sm/Al molar ratio from 0.1 to 0.2 less than the original value of 1.52 in the melts. The results indicate that it is feasible to extract small amounts of Sm with larger amount of Al by codeposition from fluoride melts.

Electronuclear Subcritical Blanket Based on a Modular Core Design with Molten-Salt Solutions of Fissile Fluorides in Fluoride Melt

The concept of a research, electronuclear, molten-salt, subcritical blanket implementing a structural advantage of the requisite number of radiation-safety barriers is presented. The modular structure of the core is validated by research satisfying the initial requirements k eff ≤ 0.98 and neutron power in the blanket ~1 MW. The proposed technical solutions can serve to confirm the feasibility of the design.

The Effect of the Second Coordination Sphere on Electrochemistry of Niobium Complexes in Alkali Halide Melts

A study of the effect of the second coordination sphere on the standard rate constants of charge transfer (ks) for the Nb(V)/Nb(IV) redox couple in chloride, chloride-fluoride and fluoride melts has been presented. By using cyclic voltammetry, the standard rate constants of charge transfer were determined for chloride melts: ks (CsCl) < ks (KCl). The redox process in the NaCl–KCl (equimolar mixture) – NbCl5 melt was reversible. The standard rate constants of charge transfer were experimentally determined for chloride-fluoride melts: ks (KCl) < ks (CsCl) < ks (NaCl–KCl). The dependence of the standard rate constants was obtained for pure fluoride melts: ks (CsF) < ks (KF) < ks (NaF–KF). The influence of the first coordination sphere composition on the charge transfer standard rate constants was found experimentally: ks (fluoride melts) < ks (chloride-fluoride melts) < ks (chloride melts), both at glassy carbon and platinum electrodes. It was found that the ks increases with the temperature increase and substitution of a glassy-carbon electrode for a platinum one.

The evolution of immiscible silicate and fluoride melts: Implications for REE ore-genesis

The Mid-Proterozoic peralkaline Strange Lake pluton (Québec-Labrador, Canada) exhibits extreme enrichment in high field strength elements (HFSE), including the rare earth elements (REE), particularly in pegmatites. On the basis of a study of melt inclusions, we proposed recently that fluoride–silicate melt immiscibility played an important and perhaps dominant role in concentrating the REE within the pluton. Here we present further evidence for silicate–fluoride immiscibility at Strange Lake from a sample of the hypersolvus granite, which contains an inclusion composed largely of REE and HFSE minerals.The inclusion (∼5cm in diameter) comprises a narrow rim containing chevkinite-(Ce) and zircon in a fluorite matrix, a core of fluorbritholite-(Ce) and bastnäsite-(Ce) and a transition zone between the rim and the core consisting of a fine-grained intergrowth of bastnäsite-(Ce), gagarinite-(Y) and fluorite. We propose that the inclusion formed as a result of silicate–fluoride immiscibility, which occurred early in the emplacement history of the Strange Lake pluton, and that it represents the fluoride melt. After separation of the two melts, the boundary between them acted as a locus of crystallisation, where crystals formed repeatedly due to heterogeneous (surface catalysed) nucleation. Zircon crystallised shortly after melt phase separation, and was followed by the growth of perthite together with arfvedsonite and quartz. As a result, the silicate melt surrounding the fluoride inclusion became enriched in volatiles that facilitated crystallisation of progressively larger crystals in the inclusion; large crystals of arfvedsonite and perthite were succeeded by even larger crystals of quartz. Massive crystallisation of chevkinite-(Ce) followed, forming the rim of the inclusion. The fluoride melt, which constituted the matrix to the silicate minerals and chevkinite-(Ce), crystallised after chevkinite-(Ce), forming fluorbritholite-(Ce) and fluorite.Aqueous fluid exsolved from the silicate melt and altered the inclusion, replacing fluorbritholite-(Ce) with fluocerite-(Ce) and then bastnäsite-(Ce). This was followed by the formation of a fine-grained intergrowth of bastnäsite-(Ce), gagarinite-(Y) and fluorite at the expense of the earlier bastnäsite-(Ce). Chevkinite-(Ce) was not affected. Zircon, however, was replaced by anhydrous zirconosilicates and, in turn, by hydrous zirconosilicates.The inclusion represents the first macroscopic example of silicate–fluoride immiscibility in nature. We propose that globules of the fluoride melt were initially dispersed within the silicate melt and preserved only rarely in unaltered hypersolvus granite. They accumulated in the residual melt (and therefore in pegmatites) scavenging REE, Ca and F from the silicate melt and, with rare exceptions, were later destroyed by fluids. The latter process contributed significantly to the enrichment of the host rocks in REE, Ca and F.

Anodic processes during recovery of tellurium from oxyhalogenide melts

The process kinetics on a graphite anode in oxyhalide melts based on the CsCl–KCl–NaCl eutectic with TeO2 and NaF additives is investigated at 600°C. Dependences of limiting recharge currents of tellurium atoms and residual discharge currents of oxygen-containing ions on the TeO2 and NaF contents are investigated. Exceeding these currents leads to chlorine liberation in oxychloride melts and liberation of freons in chloride–fluoride melts. The dissolution mechanism of TeO2 in chloride and chloride–fluoride melts through the formation and subsequent dissociation of tellurium oxychloride with the formation of O2–, TeCl62-, and [TeO2Cl]– ions is proposed. The presence of these ions in the electrolyte is confirmed by infrared spectroscopy and explains high discharge rates of oxygen-containing ions at the anode.

Oxygen species in a niobium-containing fluoride melt

Oxygen species in a niobium-containing fluoride melt