Molten NaCl-KCl Research Articles

Influence of Al2O3 overlay on corrosion resistance of plasma sprayed yttria-stabilized zirconia coating in NaCl-KCl molten salt

The pyrochemical reprocessing faces challenges in the selection of suitable materials for the fabrication of the container in electro-refining process. Yttria stabilized zirconia (YSZ) coating was considered as a promising material in this application, but the performance of YSZ coating in NaCl-KCl molten salt must be improved. In the present work, metal-organic decomposition (MOD) method was put forward to deposit an Al2O3 overlay on the top surface of sprayed YSZ coating. The corrosion behavior of the YSZ coatings with and without Al2O3 overlay in molten NaCl-KCl eutectic salt was investigated by immersion test. The results showed that the sprayed YSZ coating suffered severe corrosion and a NiO corrosion product formed on the coating surface. Meanwhile, a transformation of ZrO2 from a non-transformable tetragonal (t′) phase to a monoclinic (m) phase emerged. In contrast, the surface morphology of the YSZ coating with Al2O3 overlay remained almost unchanged. No corrosion product or new phase was formed. These results proved that the Al2O3 overlay prepared by the MOD method can significantly improve the corrosion resistance of YSZ coating.

Thermodynamic Properties of Hafnium Chlorides Dissolved in the NaCl-KCl Melt Obtained by Electrochemical Transient Techniques

The electroreduction of HfCl4 in the NaCl-KCl melt was studied in the temperature range 973–1123 K by cyclic voltammetry and impedance spectroscopy methods. It was determined that electrochemical reduction of Hf(IV) in the NaCl-KCl melt occurs via two successive stages involving transfer of two electrons at each stage. The formal standard potentials of E*Hf(II)/Hf, E*Hf(IV)/Hf and the formal standard redox potentials of E*Hf(IV)/Hf(II) were determined from the cyclic voltammetry and the values of the standard rate constants of charge transfer obtained by impedance spectroscopy method. The thermodynamic properties of the dilute solutions of hafnium di- and tetrachloride formation from the elements in an equimolar mixture of molten NaCl-KCl were determined. The free Gibbs energy changes for the reaction HfCl4(sol.)↔ HfCl2(sol.) + Cl2(g.) and the equilibrium constants of the metal-salt reaction Hf(IV) + Hf ↔ 2Hf(II) were calculated.

Direct preparation of V-Al alloy by molten salt electrolysis of soluble NaVO3 on a liquid Al cathode

Vanadium-aluminum (V-Al) alloy, the master alloy for the well-known Ti-6Al-4V alloy, is mainly produced via aluminothermic reduction process of V2O5 over the past decades. However, the reaction rate is too fast and it is difficult to control the product quality. Moreover, the utilization of aluminum is very low due to the production of Al2O3. In this work, a new method is proposed to prepare V-Al alloys by molten salt electrolysis of soluble NaVO3 on a liquid aluminum (Al) cathode in molten NaCl-KCl melt. The results show that VO3− ions can be reduced to metallic V-Al alloys on the liquid Al cathode during electrolysis process. Furthermore, thermodynamic and electrochemical analysis indicate that the reduction of VO3− ions on the liquid Al cathode results from the combined effect of electroreduction and aluminothermic reduction. In this process, the liquid Al can not only reduce VO3− ions to V, but also to combine with vanadium to form V-Al alloys. However, unlike the traditional aluminothermic reduction process, alumina produced due to the aluminothermic reduction will be in-situ electro-reduced to aluminum, which will be helpful to increase the utilization of aluminum. As a result, when the electrolysis voltage is enough large, VO3− ions will be completely reduced. Al3V alloy without alumina can be obtained.

Fundamental Studies on Induction Heating and Stirring of Non-Metallic Molten Fluid

Induction heating of non-metallic molten fluid has been investigated, mainly performed on melt glass stirring. Because silicate glass has relatively high viscosity and low electric conductivity for induction stirring, their experiments require very high temperature above 1500°C, and are difficult for observation and measurements. In this report, observation and simulation of heat generation and flow in molten salt are presented. And flow states of molten vanadium oxide glass is observed, both of them have low melting points. It is intended to investigate them from both experimental and theoretical directions, as a fundamental research. It is demonstrated that molten NaCl-KCl possessed the low impedance value in frequency range of hundred kHz to 1MHz, preferable for giving rise to induction current generation. Circulating flow of the fluids from the container wall to the center was observed. Temperature measurement in the melt was also conducted at various positions with respect to the coil configuration. The flow velocity and temperature distribution were analyzed by numerical simulation, and compared with the measurements, taking account of the transparent nature of the molten salts. They are shown to explain the experimental results, mostly. Molten vanadium oxide glass having the similar physical properties exhibited the similar flow characteristics.

Selection of the Optimum Electrolysis Bath Composition for the Synthesis of Holmium–Iron Triad Metal Intermetallics

The results of high-temperature electrochemical synthesis of holmium–iron triad metal intermetallics in chloride melts are presented. The influence of the current density, the composition of an electrolysis bath, and the synthesis time on the electrolysis processes and the composition of the end product is studied. The electrolysis of the molten KCl–NaCl mixture containing 0.5–2.5 mol % holmium trichloride and 0.1–2.5 mol % nickel (cobalt) dichloride at a current density of 0.5–2.0 A/cm2, a temperature of 973–1073 K, and an electrolysis time of 30–90 min is shown to cause the formation of a cathode deposit in the form of a “metal–salt pear” on a tungsten electrode. This pear consists of a mixture of metallic nickel (cobalt) and HoNi, HoNi5, and HoNi3 (HoCo2, HoCo3, HoCo5, Ho2Co17) intermetallics. The intermetallic compound content in the cathode deposit is found to increase at a constant current density (1.2 A/cm2) and when the holmium chloride content in a melt or the ratio of the holmium chloride concentration to the nickel (cobalt) chloride concentration increases. Only a mixture of holmium–nickel (cobalt) intermetallics can exist in the cathode deposit if the electrolysis bath composition and the electrolysis parameters are controlled. The electrochemical synthesis of holmium–iron intermetallics was performed under galvanostatic conditions in molten KCl–NaCl–HoCl3. Iron ions are introduced in a melt via the anodic dissolution of metallic iron. The results of X-ray diffraction analysis of the electrolysis products demonstrate the fundamental possibility of synthesizing holmium–iron intermetallics. The optimum conditions of electrosynthesis of holmium–iron triad metal intermetallics are determined.

Thermodynamic Properties of Hafnium Chlorides Dissolved in the NaCl-KCl Melt Obtained by Electrochemical Transient Techniques

The electroreduction of HfCl4 in the NaCl-KCl melt was studied in the temperature range 973-1123 K by cyclic voltammetry and impedance spectroscopy methods. It was determined that electrochemical reduction of Hf(IV) in the NaCl-KCl melt occurs via two successive stages involving transfer of two electrons at each stage. The formal standard potentials of E * Hf(II)/Hf, E * Hf(IV)/Hf and the formal standard redox potentials of E * Hf(IV)/Hf(II) were determined from the cyclic voltammetry and the values of the standard rate constants of charge transfer obtained by impedance spectroscopy method. The thermodynamic properties of the dilute solutions of hafnium di - and tetrachloride formation from the elements in an equimolar mixture of molten NaCl-KCl were determined. The free Gibbs energy changes for the reaction HfCl4(sol.) « HfCl2(sol.) + Cl2(g.) and the equilibrium constants of the metal-salt reaction Hf(IV) + Hf ↔ 2Hf(II) were calculated.

Electrochemical Co-Desulfurization-Deoxidation of Low-Grade Nickel-Copper Matte in Molten Salts

An electrochemical approach is able to convert low-grade nickel-copper matte (LNCM) to copper-, nickel-, and iron-alloys in both molten NaCl-KCl and MgCl2-NaCl-KCl without sulfur dioxide emissions. Since the LNCM contains the mixture of sulfides and oxides, the electrochemical deoxidation and desulfurization are taking place together, producing alloys in a one-step process. Thermodynamic properties of several sulfides/oxides were analyzed, and the electrochemical reduction behaviors of LNCM were investigated in by cyclic voltammetry (CV) using a powder modified working electrode. The morphologies of the electrolytic products could be tailored by adjusting both operating temperature and cell voltage of the electrolysis. Overall, this paper provides an environment-friendly pathway to comprehensively utilize minerals in molten salts.

In Situ Monitoring of O2− Concentration in Molten NaCl-KCl at 750°C

A very promising method for extraction of high pure titanium is electro-refining it from molten salt. Thus, a critical work for researchers is finding a molten salt which is impurity free. In order to clarify the behavior of the impurity element (oxide) in molten salt, the electrochemical behavior of oxide ions has been investigated in an equimolar NaCl-KCl by square wave voltammetry (SWV) and electrochemical titration technique (ETT) at 750°C. The purpose is to determine the residual content of oxide in the molten NaCl-KCl by using above in-situ methods. A distinct oxidation peak with a height proportional to the oxide content was observed from square wave voltammograms. The titrations of O2− were applied by sodium oxide additions. The results obtained through electrochemical titration technique show that the oxide content in the melt and the free oxide ions (O2−) detected content have a good linear relationship. The residual contents of oxide in the melt are around 700 ppm and the relationship between current density and concentration of oxide was determined by SWV method which is according to that obtained by using ETT method.

Electrochemical behaviors of Ti(III) in molten NaCl-KCl under various contents of fluoride

In order to investigate an optimal concentration of fluoride in alkali chloride melt which the Ti(III) can be reduced into metallic titanium in one step, a systematic study of the electrochemical properties of Ti(III) was carried out in molten NaCl-KCl. KF was used as a source of fluoride ions, and the molar ratio of fluoride ions and titanium ([F−]/[Tin+] ratio) was employed as a parameter to illustrate the influence of fluoride anions on the electrochemical behaviors of Ti(III). The experiments were carried out with various concentrations of fluoride ions using a glassy carbon electrode at a temperature of 1023K. Transient electrochemical techniques, such as cyclic voltammetry and square wave voltammetry, were performed to reveal the reduction steps and reduction potentials of the electroactive species. Results suggest that there are two steps for Ti(III) ion reduction in molten NaCl-KCl: Ti(III)→Ti(II) and Ti(II)→Ti. Ti(III) can be reduced directly to Ti in one step when KF was added in the melt in increasing amount.

Electrorefining of High Carbon Ferromanganese in Molten Salts to Produce Pure Ferromanganese

AbstractHigh carbon ferromanganese is used as a starting material to prepare pure ferromanganese by electrorefining in molten salts. High carbon ferromanganese was applied as the anode, molybdenum was the cathode and Ag/AgCl was the reference electrode. The anodic dissolution was investigated by linear polarization in molten NaCl-KCl system. Then potentiostatic electrolysis was carried out to produce pure ferromanganese from high carbon ferromanganese. The cathodic product was determined to be a mixture of manganese and iron by x-ray diffraction (XRD). The content of carbon in the product was analyzed by carbon and sulfur analyzer. The post-electrolysis anode was characterized by scanning electron microscope (SEM). The mechanism of the anode dissolution and the distribution of the main impurity of carbon and silicon after electrolysis were discussed.

Effect of oxygen vacancy and surface plasmon resonance: a photocatalytic activity study on Ag/Bi4Ti3O12 nanocomposites

Exposed {001} facets Bi4Ti3O12 (BTO-N2) nanosheets with oxygen vacancy were synthesized by heating a stoichiometric composition of α-Bi2O3 and TiO2 in molten NaCl–KCl at 800 °C for 2 h under the condition of nitrogen flow. Ag/Bi4Ti3O12 (Ag/BTO-N2) composites were further prepared by impregnation method from an aqueous solution of AgNO3. The typical cationic (Rhodamine B, RhB) and anionic (Congo red, CR) dyes were chosen as the model reaction to verify the influence of oxygen vacancy and surface plasmon resonance on the photocatalytic activity of Ag/BTO-N2 under artificial visible light irradiation. From the degradation kinetics of RhB and CR degradation, an important synergy effect between oxygen vacancy and surface plasmon resonance was detected with a remarkable increase up to a factor of 6 and 5 times (for RhB and CR, respectively) higher in the photocatalytic activity on Ag/BTO-N2 than that on BTO alone. A photocatalytic enhanced mechanism for the Ag/BTO-N2 composites under visible light irradiation was also discussed.

Molten salt synthesis of LaCoO3 perovskite

LaCoO3 perovskite was successfully synthesized in molten NaCl–KCl eutectic mixture. Heating the La-nitrate, Co-nitrate and NaCl–KCl mixture to 800 °C yielded large (up to 5 microns) LaCoO3 cubic particles. Conversely, when La- and Co-nitrates were fed into the NaCl–KCl mixture, molten and stabilized at 800 °C, much smaller (sub-micron) particles were obtained. These results expound the mechanistic aspects of the perovskites formation in molten salts involving nucleation and crystal growth phenomena.

Corrosion of Austenitic Steels and Their Components in Molten Nacl–Kcl–UCl3

The fused halides can be used for nuclear fuel reprocessing and as working media for molten salt nuclear reactors. However practical implementation of such technologies is limited by the problem of finding suitable corrosion resistant materials capable of long-term working in contact with molten salts. From the economical point of view the application of stainless steels as construction materials for the molten salt media is one of the most prospective ways. In the present study the corrosion behaviour of metallic iron, nickel, chromium, molybdenum, AISI 316L and 12Kh18N10T (analogue of AISI 321) austenitic steels was studied in NaCl–KCl–UCl3(1 wt.% U) melts at 750 ºC. Gravimetric measurements served as a basis for estimating corrosion resistance of studied materials. Quenched melts were analyzed using ICP-AES method. The surface of the alloy samples after corrosion tests was characterised using XRD, SEM and Х-ray microanalysis. The foils of corroded materials were examined by TEM. The corrosion rates of studied metals in NaCl–KCl–UCl3melt decrease in the following order: Cr>Fe>Ni>Mo, which correlates well with the formal standard potentials of metals in chloride melts indicating electrochemical nature of the corrosion processes. Molybdenum is the only corrosion resistant metal in the uranium-containing chloride melts and it can be used as a construction material for molten salt reactors. The major products of stainless steel corrosion in chloride melts are iron, chromium and manganese species. The surface of the corroded samples of austenitic stainless steels was depleted in manganese and chromium and enriched in nickel and molybdenum. Intergranular corrosion was observed for all types of austenitic steels. In terms of intensity and depth of the corrosion layer in NaCl–KCl–UCl3 the studied steels can be ranged in the order AISI 321>AISI 316L that correlates with the amount of excessive phases formed (figure). Figure 1

Electrochemical synthesis of an iridium powder with a large specific surface area

The synthesis of iridium powder in a molten NaCl–KCl medium at 700°C is carried out for the first time. The influence of the ratio of the cathode to the anode current density (i c/i a) on the structure and the morphology of the iridium powder is investigated. Single-phase and polycrystalline iridium powders with a specific surface of 16.8 m2/g are produced. The phase composition and the surface texture of the deposits are studied. The specific surface and the particle size of iridium powders as functions of the ratio i c/i a are analyzed.

Correction: Preparation of Mo nanopowders through electroreduction of solid MoS2 in molten KCl-NaCl.

Correction for 'Preparation of Mo nanopowders through electroreduction of solid MoS2 in molten KCl-NaCl' by Haiping Gao et al., Phys. Chem. Chem. Phys., 2014, 16, 19514-19521.

Growth of idiomorphic LiMnPO4crystals in molten NaCl–KCl and LiF–NaCl–KCl fluxes

Olivine-type idiomorphic polyhedral LiMnPO4 crystals grown from a molten LiF–NaCl–KCl ternary flux at 700 °C are found to be surrounded with well-defined facets of major {100}, {101}, and {210} faces with minor {011} faces. The average size of these crystals of ∼20.7 μm was around ten times larger than those grown from a molten NaCl–KCl binary flux. In situ XRD and TG-DTA measurements revealed that LiMnPO4 crystals grew from the melt during heating and holding at 700 °C with little assistance from the supersaturation produced by flux evaporation. Time-dependent scanning electron microscopy observation found that Ostwald ripening caused by repeated partial dissolution and reprecipitation of the LiMnPO4 crystals is the dominant factor in both the development of crystal facets and the mean diameter of individual crystals. The addition of LiF is therefore considered to promote the development of well-defined polyhedral crystals by increasing the solubility of LiMnPO4 in the molten flux.

Study on the high-temperature corrosion behavior of superheater steels of biomass-fired boiler in molten alkali salts’ mixtures

Alkali sulfates and chlorides may result in rapid corrosion attack of the materials during the process of biomass-fired power generation. The 9-mm-thick 12Cr1MoV superheater tube in biomass-fired boiler was cut into samples by wire cutting. Hot corrosion behaviors of the materials in molten NaCl–50% KCl, KCl–50% K2SO4, NaCl–50% Na2SO4, and Na2SO4–50% K2SO4 at 923 K were investigated by the weight change kinetics, scanning electron microscopy, as well as energy-dispersive spectroscopy. The weight gain curve was studied by electronic balance after each experiment cycle at 12, 24, 48, and 60 h. Compared to the microstructures of the corrosion surface and cross section of each material, concentrated pores and cracks were observed on the samples in molten NaCl–KCl, indicating that chlorides induce severest corrosion by active oxidation of chlorine in all cases. There is an extensive decrease in corrosion rates when mixing chlorides and sulfates together, where active oxidation was inhibited by the sulfates. It is interesting to note that sodium (Na) is not as active and detrimental as contents of potassium (K) to the corrosion of 12Cr1MoV steels, which results in the slightest corrosion effects by NaCl–Na2SO4 both on the surface and cross section of the specimens. Therefore, appropriate adjustment on the mixtures of chlorides and sulfates helps to improve the corrosion resistance of 12Cr1MoV.

Cathodic process of manganese (II) in NaCl-KCl melt

In order to achieve the preparation of pure ferromanganese by electrorefining of high-carbon ferromanganese in molten salt, the electrochemical behavior of MnCl2 was investigated. Molten NaCl-KCl system was selected as the electrolyte, tungsten was used as the working electrode, carbon as the counter electrode, and Ag/AgCl as the reference electrode. The cathodic process was studied by cyclic voltammetry, square wave voltammetry, and chronopotentiometry. The results show that it is a one-step process with 2 electrons transferred. The electrochemical reduction process of Mn2+ to Mn was determined as quasi-reversible and diffusion-controlled. Then, galvanostatic electrolysis was carried out to produce manganese metal from NaCl-KCl-MnCl2 melt, which was characterized by x-ray diffraction (XRD).

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.

Formation of Ti or TiC nanopowder from TiO2 and carbon powders by electrolysis in molten NaCl–KCl

A new route to produce pure Ti powder or TiC nanopowder with diameters of ∼50 nm by electrolysis in molten KCl–NaCl using TiO2 and carbon powder was reported in this paper.