KCl MgCl2 Research Articles

Chemical Analysis and Electrochemical Monitoring of Extremely Low-Concentration Corrosive Impurity MgOHCl in Molten MgCl2–KCl–NaCl

MgCl2–KCl–NaCl is a promising thermal energy storage (TES) material and heat transfer fluid (HTF) with high operating temperatures of >700°C for next-generation concentrating solar power (CSP) plants. One major challenge for future implementation of the molten chloride TES/HTF technology arises from the presence of some corrosive impurities, especially MgOHCl, a hydrolysis product of hydrated MgCl2. Even extremely low-concentration MgOHCl (tens of ppm O in weight) can cause unneglectable corrosion of commercial Fe–Cr–Ni alloys, which limits their service time as the structural materials in the molten chloride TES/HTF system. Thus, the chemical analysis and monitoring techniques of MgOHCl at the tens of ppm O level are vital for corrosion control. In this work, a chemical analysis technique based on direct titration and a high-precision automatic titrator was developed for an exact measurement of MgOHCl at the tens of ppm O level. It shows a standard deviation below 5 ppm O and an average error below 7 ppm O when the concentration of MgOHCl is 36 ppm O. Moreover, compared to other methods available in some literature reports, it can exclude the influence of co-existing MgO on the MgOHCl concentration measurement. This chemical analysis technique was used to calibrate the previously developed electrochemical method based on cyclic voltammetry (CV) to achieve reliable in situ monitoring of MgOHCl in the MgCl2–KCl–NaCl molten salt at a concentration as low as the tens of ppm O level. The in situ monitoring technique shows a monitoring limitation of <39 ppm O. The two techniques for MgOHCl measurement developed in this work could be used to develop an in situ corrosion control system to ensure the long service time of the molten chloride TES/HTF system in next-generation CSP plants.

Radiation-Induced Long-Lived Transients and Metal Particle Formation in Solid KCl–MgCl2 Mixtures

Radiation-Induced Long-Lived Transients and Metal Particle Formation in Solid KCl–MgCl₂ Mixtures

Simulation-Assisted Determination of the Minimum Melting Temperature Composition of MgCl2–KCl–NaCl Salt Mixture for Next-Generation Molten Salt Thermal Energy Storage

Molten chloride mixtures such as MgCl2–KCl–NaCl are potential thermal energy storage (TES) materials and heat transfer fluids (HTFs) for next-generation concentrating solar power (CSP) systems due to their high operation temperatures (>700°C) and low costs. This ternary MgCl2–KCl–NaCl salt mixture with a low material cost (<0.35 $/kg) is a promising salt for high-temperature TES/HTF applications, for example, it could enable next-generation CSP plants to have significant power cycle efficiency and lower levelized cost of electricity (LCOE). In this study, the minimum melting temperature composition for the MgCl2–KCl–NaCl mixture was estimated by FactSage simulation, while the binary and ternary phase diagrams of the MgCl2–KCl–NaCl salt system, including their phase transitions, were studied. Five different eutectic compositions of the MgCl2–KCl–NaCl mixture from literature and simulations were selected to determine the minimum melting temperature composition by the differential scanning calorimetry technique. Results show agreement between experiments and simulations of the melting temperature and its composition. Hence, values of the exact and reliable minimum melting temperature and salt composition in mol% and wt% are given. Based on the determined melting temperature, the minimum operating temperature of MgCl2–KCl–NaCl is recommended according to the safety margin for CSP applications.

Optimum design and key thermal property of NaCl–KCl–CaCl2 eutectic salt for ultra-high-temperature thermal energy storage

NaCl–KCl–CaCl 2 eutectic salt was developed using the thermodynamic calculation and experimental validation for the ultra-high-temperature thermal storage. Substitutional solution model (SSM) was used to describe the liquid phase and solid solution phase, and stoichiometric compound was applied to depict the intermediate phase. The predicted eutectic temperature (481.3 °C) of NaCl–KCl–CaCl 2 eutectic salt agrees well with experimental value (496.3 °C). The actual eutectic point of NaCl–KCl–CaCl 2 system is placed at T = 496.3 °C, X NaCl = 0.408 mol, X KCl = 0.075 mol and X CaCl2 = 0.517 mol. The fusion enthalpy is 171.5 J/g. The weight loss is only 1.7 wt% even at 900.0 °C and 939.0 °C is determined to be the upper limit of working temperature. The average liquid specific heat capacity is 0.80 ± 0.15 (J/g⋅ o C). The cost of NaCl–KCl–CaCl 2 eutectic salt is only 51.6% of solar salt and 54.3% of NaCl–KCl–MgCl 2 salt. SSM of thermodynamic calculation can be used as a reasonable model to optimize the chlorides salts in future. The developed NaCl–KCl–CaCl 2 eutectic salts have relatively better thermo-physical properties particularly excellent ultra-high-temperature thermal stability and lower cost, and can be considered as the ultra-high-temperature thermal energy storage materials. ∙NaCl–KCl–CaCl 2 salts were developed by thermodynamic calculation and experiment. ∙The calculated eutectic point agrees well with the experimental result. ∙The decomposition temperature of NaCl–KCl–CaCl 2 salt is 939.0 °C. ∙NaCl–KCl–CaCl 2 salts have better thermo-physical properties and lower cost.

Study of thermophysical properties of chloride salts doped with CuO nanoparticles for solar thermal energy storage

As heat storage and heat transfer medium, molten salt plays a vital role in achieving high heat transfer efficiency in concentrating solar power (CSP) plants. In this study, ternary chloride salts (NaCl–KCl–MgCl 2 ) is considered as the base salt and the nanocomposites doped with different mass fractions of CuO are synthesized using the solution evaporation method. The thermophysical properties of the nanocomposites, including melting temperature, phase change enthalpy, specific heat, thermal diffusivity, and thermal conductivity are investigated. The experimental results indicated the melting temperature of the nanocomposites is very close to that of the base salt. The specific heat of the nanocomposite in the liquid phase region is 10.51% higher than that of the base salt when the mass fraction of CuO was 0.7%. The addition of CuO nanoparticles significantly increase the thermal diffusivity and thermal conductivity of the base salt. The thermal diffusivity and thermal conductivity of the nanocomposites are increased by 10.95–37.23% and 15.85–39.55% compared to the base salt, respectively. The nanocomposites doped with CuO nanoparticles exhibit good application prospects and excellent thermal energy storage performance for high-temperature CSP systems. To improve the heat transfer characteristics of chloride salts, nanocomposites doped with 0.2, 0.7, and 2.0 wt% CuO nanoparticles were synthesized. The nanocomposites showed excellent TES performance for high-temperature CSP systems. • Nanocomposites doped with CuO exhibited excellent thermal energy storage performance. • Specific heat in liquid phase region had an enhancement of 10.51% by doping 0.7 wt% CuO nanoparticles. • Thermal diffusivity of nanocomposites increased with increasing temperature. • Thermal diffusivity and thermal conductivity of base salt can be increased by adding CuO nanoparticles. • The mechanism of the enhancement of thermophysical properties of nanocomposites was investigated.

Engineering molten MgCl2–KCl–NaCl salt for high-temperature thermal energy storage: Review on salt properties and corrosion control strategies

Conventional thermal energy storage (TES) media and heat transfer fluids (HTFs) currently used in commercial concentrated solar power (CSP) plants are nitrate-based molten salts with working temperature up to about 565 °C. Current interest in raising the working temperature in next-generation CSP for higher energy conversion efficiency leads to chloride-based molten salt technology (>700 °C). MgCl2–KCl–NaCl salt is selected to be such a TES/HTF material due to its excellent salt properties, abundance and low costs. In this review, survey and evaluation of its exact eutectic composition by evaluation of the phase diagram as well as most relevant salt properties for design, construction and operation of a test loop and projection of a real TES/HTF system are presented. They include minimum melting temperature, vapor pressure, heat capacity, density, thermal conductivity and dynamic viscosity. The working temperature range 420–800 °C is suggested for the TES/HTF system using this salt. Moreover, the recommended values of the heat capacity, density, thermal conductivity and dynamic viscosity in the suggested working temperature range are given. In addition, corrosion control strategies for this salt including corrosion monitoring and mitigation techniques are briefly reviewed, since controlling the salt corrosivity is one main concern of the engineering aspects using the molten chlorides at high temperatures. Overall, this review is expected to supply reliable values of the most relevant salt properties and recommend the most promising corrosion control strategies for further material studies, as well as modeling, simulation, detailed design and construction of molten MgCl2–KCl–NaCl salt TES/HTF systems.

Thermal properties of KCl–MgCl2 eutectic salt for high-temperature heat transfer and thermal storage system

Molten chloride salt is recognized as a promising heat transfer and storage medium in concentrating solar power (CSP) in recent years, but there is a serious lack for thermal property data of molten chloride salts under high-temperature condition. In this work, temperature and component dependence of local structure and physical properties for binary KCl–MgCl2 mixture were computed by molecular dynamics simulations (MD) method based on Born–Mayer–Huggins (BMH) potential. As the temperature rises, the distances between ions change and the structures turn into loose state, thus the density, thermal conductivity and the shear viscosity decrease, which also agree well with the experimental results. The calculation errors of thermal properties are mainly within 8%. The results show that density and thermal conductivity have negative linear correlation with temperature. Specific heat capacity is invariant with temperature at a constant pressure. When K+ increases in the salt, thermal conductivity is significantly improved, and shear viscosity and specific heat capacity decrease. The formulas of temperature-thermal properties-component correlations are obtained, which provide the fundamental property database for the precise design of high-temperature thermal storage system.

Electrochemical preparation and properties of a Mg-Li-Y alloy via co-reduction of Mg(ii) and Y(iii) in chloride melts.

Mg–Li based alloys have been widely used in various fields. However, the widespread use of Mg–Li based alloys were restricted by their poor properties. The addition of rare earth element in Mg–Li can significantly improve the properties of alloys. In the present work, different electrochemical methods were used to investigate the electrochemical behavior of Y(iii) on the W electrode in LiCl–KCl melts and LiCl–KCl–MgCl2 melts. In LiCl–KCl melts, typical cyclic voltammetry was used to study the electrochemical mechanism and thermodynamic parameters for the reduction of Y(iii) to metallic Y. In LiCl–KCl–MgCl2 melts, the formation mechanism of Mg–Y intermetallic compounds was investigated, and the results showed that only one kind of Mg–Y intermetallic compound was formed under our experimental conditions. Mg–Li–Y alloys were prepared via galvanostatic electrolysis, and XRD and SEM equipped with EDS analysis were used to analyze the samples. Because of the restrictions of EDS analysis, ICP-AES was used to analyze the Li content in Mg–Li–Y alloys. The microhardness and Young's modulus of the Mg–Li–Y alloys were then evaluated.

X-ray scattering reveals ion clustering of dilute chromium species in molten chloride medium.

Enhancing the solar energy storage and power delivery afforded by emerging molten salt-based technologies requires a fundamental understanding of the complex interplay between structure and dynamics of the ions in the high-temperature media. Here we report results from a comprehensive study integrating synchrotron X-ray scattering experiments, ab initio molecular dynamics simulations and rate theory concepts to investigate the behavior of dilute Cr3+ metal ions in a molten KCl–MgCl2 salt. Our analysis of experimental results assisted by a hybrid transition state-Marcus theory model reveals unexpected clustering of chromium species leading to the formation of persistent octahedral Cr–Cr dimers in the high-temperature low Cr3+ concentration melt. Furthermore, our integrated approach shows that dynamical processes in the molten salt system are primarily governed by the charge density of the constituent ions, with Cr3+ exhibiting the slowest short-time dynamics. These findings challenge several assumptions regarding specific ionic interactions and transport in molten salts, where aggregation of dilute species is not statistically expected, particularly at high temperature.

Volumetric Properties of Aqueous NaCl−MgCl2−H2O and KCl−MgCl2−H2O Solutions and Their Correlation with the Pitzer Model

The densities of aqueous solutions NaCl–MgCl2–H2O and KCl–MgCl2–H2O have been measured by vibrating-tube densimetry at 5 K intervals over the temperature range of 288 ≤ T/K ≤ 323 at atmospheric pressure. An experimental study was carried out for four total ionic strengths, for which the ionic strength fraction of MgCl2 was 0.2, 0.4, 0.6 and 0.8. The ion interaction approach developed by Pitzer was used to correlate the volumetric properties of aqueous mixtures. The mixing parameters of the Pitzer model were obtained by the least squares method. The calculated results are in good agreement with the experimental and literature data. The volume of mixing at constant ionic strength and at 298.15 K was calculated.

Design and performance evaluation of an innovative solar-nuclear complementarity power system using the S–CO2 Brayton cycle

In this paper, in order to deepen the grid penetration of solar energy, an innovative hybrid solar-nuclear complementarity power (SNCP) system using the supercritical CO2 Brayton cycle is proposed. A solar tower thermal system and a small modular lead-cooled fast reactor (LFR) are coupled in this system. The KCl–MgCl2 salt is chosen as both the heat transfer and energy storage materials for the solar energy block. The simulation model of the SNCP system is established by using the Ebsilon Professional code. Design point performance of the SNCP system is evaluated. The results demonstrate that the SNCP system has an incremental electric power of 77.6 MW compared with the standalone small LFR. The ratio of the incremental electric power to the total net electric power can be 31.1%. Moreover, the performance investigation of the SNCP system under the varying solar irradiance condition is conducted. The results reveal that with the solar irradiance increased, the net electric power and the ratio of the incremental electric power to the net electric power both increase. The SNCP system can operate stably under the pre-set modes and the operation behavior simulation results are in agreement with the pre-set operation strategy.

Redox Potential Measurements of Cr(II)/Cr Ni(II)/Ni and Mg(II)/Mg in Molten MgCl2–KCl–NaCl Mixture

The redox potential of Ni(II)/Ni, Cr(II)/Cr, and Mg(II)/Mg in MgCl2–KCl–NaCl (Magnak chloride) were studied utilizing chronopotentiometry (CP) method in a three-electrode electrochemical cell. Four different CrCl2 concentrations from 873 K to 1073 K, three NiCl2 concentrations from 773 K to 1073 K, and two MgCl2 concentrations from 823 K to 1073 K have been measured. The activity coefficients were calculated referring to a supercooled potential accordingly. The empirical equation of formal potential will be of great significance for the fundamental thermodynamic prediction in high-temperature Magnak chloride systems where corrosion product impurities are present.

Studies on Metastable Phase Equilibria in the Ternary Systems KCl–MgCl<sub>2</sub>–H<sub>2</sub>O and MgB<sub>4</sub>O<sub>7</sub>–MgCl<sub>2</sub>–H<sub>2</sub>O at 273 K

The present study investigates the metastable phase equilibria of ternary systems KCl–MgCl2–H2O and MgB4O7–MgCl2–H2O at 273 K by an isothermal evaporation method. The solubility data of the two ternary systems were determined. According to the solubility data, the corresponding metastable phase diagrams were plotted. The equilibrium solid phases of invariant points were identified by X-ray powder diffractometry. The results show that the ternary system KCl–MgCl2–H2O has a double salt KCl·MgCl2·6H2O. Its metastable phase diagram contains three univariate curves, two invariant points and three crystallization areas, which correspond to MgCl2·6H2O, KCl and KCl·MgCl2·6H2O. Compared with the stable phase diagram at 273 K, the unsaturated phase region increases slightly in the metastable phase diagram. The ternary system MgB4O7–MgCl2–H2O is of simple co-saturated type without double salt or solid solution. The metastable phase diagram contains two univariate curves, one invariant point and two crystallization fields, and the corresponding solid phases are MgCl2·6H2O and MgB4O7·9H2O. Comparing the metastable diagram against the stable phase diagram at the same temperature, it can be seen that the solubility of MgB4O7 in the metastable state obviously increased, which means that magnesium borate was significantly supersaturated under the metastable state.

Study on Phase Equilibrium of the K+, Mg2+//Cl–, SO42– + H2O Quaternary System at 353.15 K and Its Application

The stable phase equilibrium data of the K+, Mg2+//Cl–, SO42– + H2O quaternary interaction system and its four ternary subsystems KCl–K2SO4–H2O, KCl–MgCl2–H2O, K2SO4–MgSO4–H2O, and MgCl2–MgSO4–H2O ...

Molecular Oxygen Activation on Suspended Doped Cerium(IV) Oxide Particles in Molten Chloride Salts

Suspensions of cerium(IV) oxide (CeO2) particles in molten chloride salts were found to be active for molecular oxygen activation when doped with the larger Lanthanides, gadolinium (Gd), praseodymium, (Pr), and samarium, (Sm). The activity for O2 oxidation of gas phase CO on suspended doped ceria in molten mixtures of LiCl–LiBr–KBr and NaCl–KCl is significantly higher than undoped ceria or the salt mixtures alone in the temperature ranges of 350–600 °C, and 700–800 °C, respectively. Surface tension measurements show that the solid doped ceria particles suspended in the melt are contacting the gas–liquid interface of the bubbles. The activity for O2 oxidation of liquid phase MgCl2 in molten MgCl2–KCl to produce Cl2 and solid MgO was also observed to be significantly increased on suspended CeO2 doped with 5 wt% or 10 wt% Gd. The rates of oxidation were observed to increase with catalyst loading.

Effect of Mg dissolution on cyclic voltammetry and open circuit potentiometry of molten MgCl2–KCl–NaCl candidate heat transfer fluid for concentrating solar power

Molten chloride salts have recently gained attention for application to heat transfer and thermal energy storage in concentrating solar power (CSP) plants. In this study, we studied the electrochemical effect of addition of hydroxide ions and/or dissolved metals added to MgCl2–KCl–NaCl salt mixture. Open circuit potentiometry indicated that the salt's redox potential increased with addition of hydroxide ions and decreased with the addition of soluble magnesium metal. Cyclic voltammetry indicated that a reduction peak associated with hydroxides vanished with addition of Mg0. Hydrogen gas was evolved when a mixture of Mg metal and MgCl2–KCl–NaCl containing NaOH was heated to 650 °C. These results demonstrate the benefit and mechanism for redox potential control in this salt using dissolved Mg metal.

Amorphous and partially-amorphous metal coatings for corrosion resistance in molten chloride salt

Amorphous metal coatings were developed to improve the corrosion resistance of structural alloys in molten salts for use as salt-facing materials in concentrating solar power (CSP) and molten salt reactor (MSR) applications. Three coatings were tested. One Fe-based, fully amorphous coating, and two Ni-based partially amorphous coatings. The candidate coatings were applied to samples made from Ni-based alloy 230, and exposed to KCl–MgCl2 at 750 °C for 300 h in sealed Mo capsules. After exposure, uncoated alloy 230 samples were depleted in chromium near the surface, as measured by SEM-EDS. The attack had an average depth of 33 μm. All three coatings performed well, showing no observable degradation during exposure, despite chromium contents between 20% and 40%. XRD and TEM measurements showed the coatings were fully crystallized after exposure, so the excellent corrosion resistance can likely be attributed to the recrystallized microstructure of the coatings.

Invariant Equilibria in Multicomponent Systems

The translation method for predicting and constructing equilibrium phase diagrams of multicomponent systems is developed based on the compatibility of structural elements of partial constituent (n-component) systems and the general (n + 1)-component system with account for the requirements of the Gibbs phase rule. The invariant equilibrium states in the five-component system NaCl–KCl–MgCl2–CaCl2–H2O at 25°C are studied. This multicomponent system features the options of invariant equilibria where one and the same composition of equilibrium solid phases can be in equilibrium with saturated solutions of differing concentrations. Therefore, this invariant equilibrium is to be reflected in the diagram as a geometric (invariant) image that has a dimension (it may be conventionally referred to as a quasi-point) and not by a point. The dimension of this image is determined by the concentration bounds of components in the equilibrium saturated solution.

Interaction of Salt Melts of Magnesium Production with Atmospheric Air

A review of publications on the interaction of salt melts of magnesium production with atmospheric air is presented. The measurement procedure of the weight of interaction products of salt melts with air is described. The results of investigating the emission intensity of hydrogen chloride and chlorine by salt melts of MgCl2–KCl–NaCl, MgCl2–NaCl–BaCl2, and MgCl2–KCl–NaCl–CaCl2 systems, as well as the emission intensity of HCl + HBr and Cl2 + Br2 gases by salt melts of the MgCl2–KCl–NaCl–NaBr system, are presented. The thermodynamic analysis of interaction reactions of magnesium salt solutions with atmospheric air is performed. It is revealed that magnesium chloride in the salt melt most intensely interacts with atmospheric air with the emission of chlorine and hydrogen chloride. The specific rates of formation of halogen-containing gases from the unit surface area of the MgCl2–KCl–NaCl, MgCl2–KCl–NaCl–BaCl2, MgCl2–KCl–NaCl–CaCl2, and MgCl2–KCl–NaCl–NaBr systems are measured. The influence of calcium chloride, sodium bromide, and magnesium fluoride on the emission intensity of halogen-containing gases by the surface of salt melts is measured. It is determined that the addition of magnesium fluoride into the composition of chloride melts reduces the emission intensity of chlorine and hydrogen chloride.

Corrosion of Carbon Steel and Cast Iron in a Gas Phase over Salt Melts Used in the Magnesium Industry

Corrosion tests of SCh15 cast iron, St.3, and St.3 with an aluminized coating are performed in certain compositions of salt melts of 10% MgCl2–KCl–NaCl and 10% MgCl2–KCl–NaCl–CaCl2 systems with concentrations of 10, 25, and 40% CaCl2, as well as in the 10% MgCl2–45% KCl–20% NaCl–25% NaBr melt and in a gas phase over these melts at 700°C. Corrosion rates of metallic samples are found by a gravimetric method. Concentrations of impurities of halogenides and hydrogen halogenides in air blown through a reactor with melts and samples are determined by the chemical analysis of absorption solutions. It is shown that the aluminizing of carbon steel can decrease the corrosion rate in a gas phase over a salt melt by a factor of 5–70. The mechanism of formation of gases aggressive with respect to carbon steel and cast iron in atmospheric air contacting with the salt chloride melt is considered. The acceleration of the formation of hydrogen chrloride and chlorine during the interaction of the salt melt with atmospheric air under the effect of corrosion products of iron is revealed.