Equimolar NaCl-KCl Melt Research Articles

Electrochemistry of Neodymium in an Equimolar NaCl-KCl Melt without and with Addition of Fluoride Ions

Abstract The paper presents the results of neodymium electrochemical behavior in chloride and chloride-fluoride melts. It was shown that the process of neodymium electroreduction in the NaCl-KCl-NdCl3 melt proceeds in two stages. By diagnostic criteria of voltammetry it was established that the discharge process of Nd(III) to Nd(II) at a sweep rate in the range of 0.6 ≤  ≤ 1.0 V s-1 is not complicated by disproportionation reaction. In this study diffusion coefficients, activation energy of diffusion for Nd(III) chloride complexes and standard rate constants of charge transfer for the Nd(III)/Nd(II) redox couple in the NaCl-KCl melt were determined. The nature of the working electrode on the standard rate constants of charge transfer for the Nd(III)/Nd(II) redox couple has been studied by cyclic voltammetry and electrochemical impedance spectroscopy. The formal redox potentials E*Nd(III)/Nd(II) in the NaCl-KCl melt were obtained from the cyclic voltammetry data. It was shown that the addition of fluorine anions into the NaCl-KCl-NdCl3 melt leads to stabilization of the higher oxidation state of neodymium in chloride-fluoride melts and the intermediate oxidation state Nd(II) does not exist in these melts.

Electrochemistry of Neodymium in an Equimolar NaCl-KCl Melt without and with Addition of Fluoride Ions

An investigation of compounds of those few elements in the oxidation state (II), by opposition to the general trivalent lanthanides is of special interest. The present work follows our previous investigations on samarium, europium ytterbium chloride systems and, in particular, the electrochemical behavior of the Sm(III)/Sm(II), Eu(III)/Eu(II) Yb(III)/Yb(II) redox couples. It deals with neodimium compounds.The electrochemical behavior of NdCl3 in molten salts was studied in few works, but only melts with a low melting point LiCl-KCl and LiCl-KCl-CsCl were investigated.The electroreduction of NdCl3 in an equimolar NaCl-KCl mixture was studied in the temperature range 973-1123 K by different electrochemical methods. It was shown that the discharge of Nd(III) to Nd(II) is complicated by the disproportionation reaction (DPP):3Nd(II) ↔ 2Nd(III) + Nd (1)Ignoring of reaction DPP can lead to incorrect determination of values diffusion coefficients and formal standard redox potentials of the Nd(III)/Nd(II) redox couple. The relatively narrow interval of polarization rate was found when the recharge process was reversible and not accompanied by the DPP reaction. At these scan rates the diffusion coefficients and formal standard redox potentials of the Nd(III)/Nd(II) redox couple were determined by cyclic voltammetry.The standard rate constants for the redox reaction Nd(III) + e- ↔ Nd(II) were calculated on the basis of cyclic voltammetry at sweep rates corresponding to the quasy-reversible process. The nature of working electrode on the rate of charge transfer for the Nd(III)/Nd(II) redox couple was studied. It was found that the values of k s determined at a molybdenum electrode were higher than those at a glassy carbon electrode.The introduction of fluorine anions into a chloride melt at a molar ratio of F-/Nd(III) = 5 leads to the disappearance of the recharge wave and only an ascending section associated with joint discharge of neodymium complexes and sodium and potassium cations was observed. Thus, the introduction of fluorine anions leads to stabilization of the highest oxidation state of neodymium in chloride-fluoride melt and neodymium in the oxidation state +2 does not exist in the melt.Introducing of fluoride ions to the melt NaCl-KCl containing Nd(II) accompanied by reaction of disproportionation:3NdCl4 2- + 12F- ↔ 2NdF6 3- + Nd + 12Cl- (2)The standard rate constants of charge transfer for the Nd(III)/Nd(II) redox couple were compared with the values obtained for other redox couples of lanthanides and these results were discussed in connection with the strength and stability of complexes.

Kinetic and Thermodynamic Properties of Ytterbium Chloride and Fluoride Complexes in Chloride Melts

An important problem concerning rare earth metals and molten salts is the reprocessing of nuclear fuel, since lanthanides are always present in spent nuclear fuel. According to the technology being developed for pyroelectrochemical processing of nuclear waste spent fuel is converted into molten salts by anodic dissolution, followed by electrochemical extraction of actinides from lanthanides.Thus a knowledge of the electrochemistry and thermodynamics of lanthanide compounds in molten salt systems is very useful for the understanding the recycling of spent nuclear fuel.The electroreduction of YbCl3 in alkali chloride melts (NaCl-KCl, KCl, CsCl) was studied in the temperature range 973-1173 K by different electrochemical methods. The diffusion coefficients (D) for Yb(III) and Yb(II) were determined by linear sweep voltammetry, chronopotentiometry and chronoamperometry methods. Decreasing values of D were obtained when the cation in the second coordination sphere changed from Na to Cs. It was shown that such changes are due to the decreasing of counter polarizing effect of cations with increasing cationic radius.The standard rate constants of charge transfer (k s) for the Yb(III)/Yb(II) redox couple were calculated on the basis of cyclic voltammetry data using Nicholson’s approach. The following row of the standard rate constants of charge transfer has been experimentally determined: k s (KCl) < k s (CsCl) < k s (NaCl-KCl).The formal redox potentials E* Yb(III)/Yb(II were obtained in alkali chlorides melts from the linear sweep voltammetry data. From the values of the formal redox potentials were calculated the Gibbs energies and equilibrium constants for the reaction:YbCl2(sol.) +1/2 Cl2(g.) « YbCl3(sol.) (1)It was determined the increasing of the formal entropy for the reaction (1) in transition from NaCl-KCl to CsCl melt that associated with a greater degree of ordering of the reaction products due to the complex formation.Influence of fluoride ions (NaF) on the electrochemical behavior of the Yb(III)/Yb(II) redox couple in an equimolar NaCl-KCl melt was studied. It was determined а decrease of diffusion coefficients and standard rate constants, as well as a shift of the formal redox potentials to the negative region.

Electrochemistry of Neodymium in an Equimolar NaCl-KCl Melt without and with Addition of Fluoride Ions

The paper presents results of neodymium electrochemical behavior in chloride and chloride-fluoride melts. It was shown that the process of neodymium electroreduction in the NaCl-KCl-NdCl3 melt proceeds in two stages. It was established by diagnostic criteria of voltammetry that the discharge process of Nd(III) to Nd(II) at a sweep rate in the range of 0.6 ≤ n ≤ 1.0 V s-1 is not complicated by reaction disproportionation. In this study diffusion coefficients, activation energy of diffusion for Nd(III) complexes and standard rate constants of charge transfer for the Nd(III)/Nd(II) redox couple in the NaCl-KCl melt were determined. It was shown that the addition of fluorine anions into the NaCl-KCl-NdCl3 melt leads to stabilization of the higher oxidation state of neodymium in chloride-fluoride melts and neodymium in the intermediate oxidation state +2 in these melts does not exist.

Peculiarities of Holmium and Iron Triad Ions Co-Reduction: Formation of HoxNiy (HoxCoy, HoxFey) Intermetallic Compounds in Chloride Melts

The present paper is focused on the analysis of Ni2+, Co2+, Fe3+, and Ho3+ ion co-reduction in the background equimolar NaCl-KCl melt at 973 K using the method of cyclic voltammetry. It was found that the co-reduction potentials of Ho3+ and iron triad ions differ greatly. The depolarization of metallic holmium electrodeposition on one of the iron triad metals preliminary deposited on the tungsten electrode was determined. This process resulted in the formation of the HoxNiy, HoxCoy, and HoxFey intermetallic compounds. It was observed that the HoxNiy, HoxCoy, and HoxFey intermetallic compounds may be synthesized in a kinetic regime. The influence of the current density, electrolytic bath composition, and electrolysis time on the composition of the obtained intermetallic compounds was studied. The possibility of synthesizing cathode deposits composed solely of intermetallic compounds is verified. It is demonstrated that the intermetallic compounds may be formed by the addition of the iron triad metals (in particular, metallic iron) via anode dissolution. Synthesized HoxNiy, HoxCoy, and HoxFey samples were characterized by X-ray diffraction analysis, scanning electron microscopy, and photon correlation spectroscopy.

Fabrication of an Ni–Pr Coating by the Application of EMF in an Equimolar NaCl–KCl Melt

Fabrication of an Ni–Pr Coating by the Application of EMF in an Equimolar NaCl–KCl Melt

Mechanism of Dy3+ and Nd3+ Ions Electrochemical Coreduction with Ni2+, Co2+, and Fe3+ Ions in Chloride Melts

The present paper is devoted to the study of the processes of the mechanism of electrochemical coreduction of Dy3+ and Nd3+ ions with Ni2+, Co2+, and Fe3+ ions in the equimolar NaCl-KCl melt at 973 K and characterization of the synthesized samples. The performed voltammetry analysis of the electrochemical coreduction processes elucidated a significant difference in the values of the extraction potentials of the studied metals. This melt testifies that intermetallic compounds of Dy and Nd with Ni, Co, and Fe may be synthesized in the kinetic regime. The intermetallic phases of Dy and Nd with Ni, Co, and Fe are found to be formed along with the phases of metallic Ni, Co, and Fe either during electrolysis at the cathode current densities exceeding the limiting diffusion current of Ni2+, Co2+, and Fe3+ ions or in the potentiostatic regime at the potentials of the corresponding voltammetry curves. Therefore, the following interrelated key parameters affecting the electrochemical synthesis of Dy and Nd intermetallic compounds with Ni, Co, and Fe were determined: (i) composition of the electrolyte, i.e., concentrations of FeCl3, CoCl2, NiCl2, DyCl3, and NdCl3; (ii) cathode current density or electrolysis potential and (iii) electrolysis time. The obtained samples were characterized by micro-X-ray diffraction analysis, cyclic voltammetry, and scanning electron microscopy methods.

Electroreduction of Holmium Chloride in the Equimolar NaCl–KCl Melt

Rare-earth metals and their alloys are widely used in various engineering fields due to their unique physical and chemical properties. To prepare rare-earth-containing coatings, data on the kinetics of electrode processes with the participation of the rare-earth metals in salt molten media should be available. These problems are poorly understood for rare-earth metals. In the present study, the kinetic parameters of the cathode reduction (transfer coefficient, heterogeneous charge transfer rate constants) of holmium ions in the equimolar NaCl–KCl melt are determined in a temperature range of 1073–1173 K in a holmium chloride concentration range of 1–10 wt % using chronopotentiometry and chronovoltammetry. The dependences of the kinetic parameters on the temperature and concentration of rare-earth metal chloride are determined. The following reduction mechanism of complex ion in the aforementioned melts is suggested: $${\text{HoCl}}_{6}^{{3 - }}$$ = $${\text{HoCl}}_{4}^{ - }$$ + 2Cl–, $${\text{HoCl}}_{4}^{ - }$$ + e → $${\text{HoCl}}_{4}^{{2 - }}.$$

REDUCTION KINETICS OF TERBIUM IONS IN EQUIMOLAR MELTING SODIUM AND POTASSIUM CHLORIDE

In this study, by means of chronopotentiometry and chronovoltamperometry we evaluated the kinetic parameters (transfer coefficients, heterogeneous charge transfer rates) of electroreduction of terbium ions. We also evaluated the dependence of the kinetic parameters on the concentrations of terbium chloride mass (%): 1, 3, 5, 7, 10 and temperatures from 1073 to 1173 K. The values of the kinetic parameters increase with the increasing temperature and decrease with the increasing concentration of terbium chloride. The experiment was carried out in a three-electrode cell in an atmosphere of purified argon. Based on the results obtained by chronopotentiometric and chronovoltamperometric methods, it was shown that the process of electroreduction of TbCl63- chloride complexes in the equimolar NaCl-KCl melt is irreversible in the whole studied temperature range and terbium chloride concentrations at potential scan rates from 0.2 to 20 V/s (chronovoltamperometry) and current densities from 0.01 to 1.2 A/cm2 and transitional time from 0.01 to 1.5 s (chronopotentiometry). In this study, it was shown that in an equimolar melt of NaCl-KCl, terbium is in the divalent and trivalent states, forming the TbCl63- and TbCl42- complexes. Based on the experimental data, in the studied temperature range and terbium chloride concentrations, we assumed that the reduction of terbium ions occurred in two stages. The electrode electroreduction is complicated by the kinetic current of the preceding chemical reaction. We determined the preceding chemical reaction's rate constants. Based on the data obtained, we could model the TbCl63- complex's reduction in the previously mentioned melt. The reduction process included the previous stage of the complex dissociation. The results obtained by different methods are consistent with each other.

ELECTROCHEMICAL REDUCING OF TERBIUM AND HOLMIUM IONS IN THE SODIUM AND POTASSIUM CHLORIDES MELT WITH EQUIMOLAR COMPOSITION

Interest to rare-earth metals (REM) and their alloys is due to the possibility of using them for the creation of new materials need for modern technology. For instance, REM as alloying components allows for preparation of material with special magnetic properties. A promising method for forming such coating is the surface treatment of metals. This process has an electrochemical character as such for the organization of technology the knowledge of kinetics and mechanism of these processes is important. Despite significant interest in rare-earth metals, these issues are not well described in the literature. In order to choose an adequate mathematical model for calculation of kinetic primers, preliminary experiments that allow evaluating the reversibility of the electrode process have been conducted. Based on that, it was concluded that cathodic reduction of terbium and holmium ions in equimolar NaCl-KCl melt is irreversible. By means of voltammetric analysis, kinetic parameters (transfer coefficients, heterogeneous constants of charge transfer rate) of terbium and holmium electroreduction in equimolar NaCl-KCl melt were determined. The experiment was conducted in a three-electrode cell under a purified argon atmosphere. A dependency of kinetic parameters on the concentration of terbium and holmium chlorides wt (%): 1, 3, 5, 7, 10, was determined. The experiment was conducted in 1073–1173K temperature range. Values of kinetic parameters increase with temperature but decrease with the increase of REM chloride. Based on obtained data, it was found that electroreduction of chloride complexes LnCl63– (Ln–Tb, Ho) in equimolar NaCl-KCl melt is irreversible in the studied range of temperatures and REM concentrations. In summary of experimental data, in range of temperature and rare-earth chloride concentration, and assumption was made that reduction of terbium and holmium ions occurs in two stages. The process includes the preceding stage of complex dissociation. A mechanism of LnCl63– complex reduction in the mentioned melt is proposed. The obtained results are in agreement with literate data for analogues systems.

The effect of surface treatment on the corrosion behavior of a pure Ta sheet in an equimolar NaCl–KCl melt at 850°C in air. Part 1: Laser surface melting

AbstractThe corrosion behavior of pure Ta and laser surface melting–treated Ta sheets was investigated in an equimolar NaCl–KCl melt at 850°C in air. The results show that the predominant factors involved in the consumption of materials were reactions that form sodium‐containing oxides and Ta2O5. High corrosion stress led to slip of the untreated Ta surface grains, further resulting in preferential corrosion along slip bands. The slip of surface grains substantially deteriorated the bonding between the corrosion scale and matrix, thus leading to rapid degradation of the corrosion scale. The melt layer consisted of a thin Ta2O5 film, and the inner dendrite layer demonstrated self‐healing characteristics during corrosion and presented a better hot corrosion resistance than the untreated Ta owing to the excellent bonding at the scale and melt layer interface.

The effect of surface treatment on the corrosion behavior of pure Ta sheet in an equimolar NaCl-KCl melt at 850℃ in air, part 2: Diamond film, TaC film, Ta-Si coating and Ta-Si-Al coating

The diamond film, TaC film, Ta-Si coating and Ta-Si-Al coating were developed on Ta sheet surface and their corrosion behavior were investigated in an equimolar NaCl-KCl melt at 850℃ in air, additionally, their corrosion behavior were concluded. The diamond film could prevent the corrosion mediums at beginning, while had no benefit in later period due to the corrosion product CO2 having no protective ability. The TaC film has good corrosion resistance mainly because of its low activity. The corrosion of Ta-Si and Ta-Si-Al coating could form continuous SiO2 amorphous film on surface to protect the un-corroded material.

Electroreduction of the tantalum oxofluoride complexes in an equimolar mixture of sodium and potassium chlorides

The electrochemical behavior of the tantalum mono-, di-, and trioxofluoride complexes in the equimolar NaCl–KCl melt is studied. The trioxofluoride TaO3F2– complexes are shown to discharge at the potentials that are more negative than those of alkali metal cations; that is, they are electrochemically inactive against the background of the equimolar mixture of sodium and potassium chlorides.

Electrochemical sulfur removal from chalcopyrite in molten NaCl-KCl

Electrolysis of solid chalcopyrite (CuFeS2) against a graphite inert anode has been studied in equimolar NaCl-KCl melt at 700°C. During electrolysis, S2− ions are released from the solid CuFeS2 cathode, transfer to the graphite anode and discharge to S2 gas. The reduction mechanism of CuFeS2 was investigated by cyclic voltammetry, potentiostatic and constant voltage electrolysis together with spectroscopic and scanning electron microscopic analyses. The reduction contains mainly three stages: the insertion of Na+ or K+ into CuFeS2, forming LxCuFeS2 (L=Na or K, x≤1); the partial reduction of LxCuFeS2 to Lx-wCuFe1-yS2-z and Fe; the complete reduction to a mixture of Cu and Fe, which can be magnetically separated. After the separation, pure Cu can be obtained by leaching out the residual Fe with acid. Electrolysis at a cell voltage of 2.4V has led to a rapid reduction of CuFeS2. The current efficiency and energy consumption were 85% and 1.68 kWh/kg-CuFeS2, respectively.

Reduction kinetics of neodymium chloride in the equimolar NaCl–KCl melt

The kinetic parameters (transfer coefficients, heterogeneous constants of the charge transfer rate) of cathode reduction of the neodymium ion in the equimolar NaCl–KCl melt are determined by chronopotentiometry and chronovoltamperometry. The dependences of the kinetic parameters on temperature and the rare-earth metal chloride concentration are presented. A mechanism is proposed for the reduction of the NdCl63– complex in the equimolar NaCl–KCl melt.

Electroreduction of yttrium ions in an equimolar NaCl–KCl molten mixture

The behavior of yttrium ions in an equimolar NaCl–KCl melt is studied in a temperature range of 973–1123 K using cyclic voltammetry at an inert electrode containing 5 wt % YCl3 in the range of potential sweep rates from 0.02 to 0.2 V/s. A cathodic reaction proceeds via the scheme Y(III) + 3e = Y. The diffusion coefficients of yttrium ions are determined, and the activation energy of the diffusion of yttrium ions is estimated.

Electrochemical reduction of dysprosium ions in a chloride melt at various electrodes

The mechanism of electroreduction of dysprosium chloride complexes on silver and tungsten electrodes is studied by linear and cyclic voltammetry in an equimolar NaCl-KCl melt at 973 K, and the kinetic parameters of the process are calculated. The diffusion coefficient of dysprosium ions is calculated. In contrast to a tungsten electrode, the electroreduction on a silver electrode is complicated by alloy formation with the cathode material.

The Electroreduction of Gadolinium and Dysprosium Ions in Equimolar NaCl-KCl Melt

The mechanism of rare earth metals (Gd and Dy) chloride complexes electroreduction on the tungsten electrode in equimolar NaCl-KCl melt at 973 K has been studied by linear and cyclic voltammetry. Some kinetic parameters of processes were calculated. It was shown that the tungsten electrode was indifferent to gadolinium and dysprosium which were reduced on the surface. We found that the discharge mechanism of gadolinium and dysprosium chloride complexes was described by three-electron step when the steady-state conditions of polarization were limited by the mass transfer stage. The conditions of nonstationary polarization made the slowness of the charge transfer stage. The diffusion coefficient of gadolinium and dysprosium ions was calculated, the diffusion coefficient of GdCl3-6 ions was (0.9 ± 0.2) × 10-5 cm2.s-1, and for DYCI3-6 ions, it was (1.60 ± 0.2) × 10-5 сm2.s-1.

New Method for Low-Cost Titanium Production

The present work deals with the investigation of an electrolytic method for titanium production that uses TiO2 enriched titania slag as raw material. The process involves two steps: i) carbothermal reduction of the slag to form titanium oxycarbide powder; and ii) electrolysis in a molten chloride-based electrolyte using a titanium oxycarbide consumable anode. Electrochemical studies show the stability of the different Ti species in the equimolar NaCl-KCl melt at 850oC. These results, together with previous work about the anodic oxidation mechanism of a consumable titanium oxycarbide anode in molten chlorides, allow us to optimize the anode and cathode voltages in the electrolysis experiments. The results show that best quality titanium deposits are obtained when the reduction occurs in a single electrochemical step, i.e. directly from di-valent titanium species to Ti metal. Then, the complete conversion of the Ti(III) ions released from the consumable oxycarbide anode to Ti(II) species by adding Ti sponge to the electrolyte, must be fulfilled.

Anomalous Diffusion Coefficients for W(IV) Ion Diffusion in NaCl-KCl Melt at 700-750°C

The electrodepostion of bi-valent iron, zinc and tungsten (IV) on tungsten electrodes in equimolar NaCl-KCl melt at 700-750oC was studied by Cyclic Voltammetry and Chronoamperometry. While iron (II) and zinc (II) ions demonstrate regular values of diffusion coefficients, which are all in the range of 10-6-10-5 cm2/sec, tungsten (IV) ions diffuse considerably slower. Plausible process mechanisms were proposed, according to which the tungsten (IV) ions form polynuclear ions and these massive species diffuse at considerably more moderate rates.