KCl MgCl2 Research Articles

Production of Low-Oxygen Ti Powder by Magnesiothermic Reduction of TiO2 in MgCl2–KCl–CeCl3 Molten Salt

Production of Low-Oxygen Ti Powder by Magnesiothermic Reduction of TiO2 in MgCl2–KCl–CeCl3 Molten Salt

Experimental analysis on improving heat storage efficiency of high-temperature packed bed system using spherical capsules filled with MgCl2–KCl–NaCl/Al2O3 nanoparticles composite phase change material

While thermal energy storage (TES) systems are becoming crucial to the stable operation of solar power plants, molten salt packed bed thermal energy storage (PBTES) systems attracted widespread attention due to their wide operating temperature range and low cost. However, achieving high-efficiency thermal storage systems is still a datum challenge to meet the higher energy demand, as well as sustainable development and utilization of renewable energy systems. This study developed spherical capsules employing nanocomposite salts (MgCl2–KCl–NaCl/Al2O3 nanoparticles) as composite phase change materials to experimentally study the heat storage process of a high-temperature PBTES prototype experimental system. Also, the heat storage performance of the PBTES system under different operating conditions is further investigated. The results showed that the nanocomposite salt gradually melted along the flow direction of the heat transfer fluid by exhibiting three-stage characteristics including solid phase sensible heat, phase change latent heat, and liquid phase sensible heat. Also, the temperature difference inside and outside the capsules showed the characteristics of peak-trough-peak. The PBTES system reached a heat storage density of 1290.51 MJ m−3 at a thermal efficiency of 86.95 %. The average heat storage rate increased by 49.55 % and the heat storage time was shortened by 29.67 % when the blower speed was increased from 800 r·min−1 to 1200 r·min−1. The experimental results implied that the PBTES system encapsulating nanocomposite salts has excellent thermal performance. This study demonstrated solving the problems of insufficient thermal energy supply in time or space and improving energy utilization could result in a high-efficiency thermal storage system.

Corrosion behaviour of FeCoNiCrAl high-entropy alloy in molten NaCl–KCl–MgCl2 chloride salt

Molten NaCl–KCl–MgCl2 is becoming a potential heat transfer fluid (HTF) and thermal energy storage (TES) material in the third generation of concentrated solar energy power (CSP) stations due to its excellent heat storage-capacity and thermal stability. However, the strong corrosion of the molten chloride salt at high temperature limits its industrial application. In this study, the corrosion behaviour of FeCoNiCrAl HEA with BCC + FCC phase structure in molten NaCl–KCl–MgCl2 (24.5–20.55–54.95 wt-%) at 650 °C under argon was investigated by a combination of weight loss and electrochemical methods. The results show that, the corrosion of FeCoNiCrAl HEA in molten chloride salt mainly manifests as the selective dissolution of Fe and Cr. With the further increase of immersion time, oxides gradually crack and slightly flake, resulting in large weight loss of FeCoNiCrAl HEA.

Study of the electrochemical behavior of Ni(II) impurity in MgCl2–KCl–NaCl melt

Study of the electrochemical behavior of Ni(II) impurity in MgCl2–KCl–NaCl melt

Corrosion behavior of iron-based and Ni-based alloys melted in NaCl–MgCl2–KCl mixed molten salt under vacuum atmosphere

In this work, static immersion and gravimetric methods were used to study the corrosion behavior of three common stainless steels and four Ni-based alloys in a NaCl–MgCl2–KCl mixed molten salt under vacuum atmosphere. The corrosion test was conducted at 700 °C, with a maximum exposure time of 100 h. Corrosion behavior was determined by measuring the mass loss of the sample at different time intervals and the corrosion rates at 100 h. The microstructure of the alloy was analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic force microscope (AFM), and X-ray diffraction (XRD). The results showed that stainless steels were generally more severely corroded than Ni-based alloys. 304 stainless steel and In800H exhibited the worst corrosion resistance, whereas Ni-based alloy In625 had the best corrosion resistance. Fe and Cr were dissolved in the molten chloride salts and reacted with the molten chloride salts. Mo played a positive role in suppressing the corrosion of the alloys in molten chloride salts by inhibiting the outward diffusion of the elements.

Study on the corrosion behavior of 316H stainless steel in molten NaCl–KCl–MgCl2 salts with and without purification

Study on the corrosion behavior of 316H stainless steel in molten NaCl–KCl–MgCl2 salts with and without purification

Core design study of the Wielenga Innovation Static Salt Reactor (WISSR)

This paper presents the design features and preliminary design analysis results of the Wielenga Innovation Static Salt Reactor (WISSR). The WISSR incorporates features that make it both flexible and inherently safe. It is based on innovative technology that controls a nuclear reactor by moving molten salt fuel into or out of the core. The reactor is a low-pressure, fast spectrum transuranic (TRU) burner reactor. Inherent shutdown is achieved by a large negative reactivity feedback of the liquid fuel and by the expansion of fuel out of the core. The core is made of concentric, thin annular fuel chambers containing molten fuel salt. A molten salt coolant passes between the concentric fuel chambers to cool the core. The core has both fixed and variable volume fuel chambers. Pressure, applied by helium gas to fuel reservoirs below the core, pushes fuel out of a reservoir and up into a set of variable volume chambers. A control system monitors the density and temperature of the fuel throughout the core. Using NaCl-(TRU,U)Cl3 fuel and NaCl–KCl–MgCl2 coolant, a road-transportable compact WISSR core design was developed at a power level of 1250 MWt. Preliminary neutronics and thermal-hydraulics analyses demonstrate the technical feasibility of WISSR.

High-temperature corrosion model of Incoloy 800H alloy connected with Ni-201 in MgCl2–KCl heat transfer fluid

Abstract Long-term dipping corrosion experiments on Incoloy 800H (800H) in molten eutectic MgCl2–KCl salt at 1,073 K with and without connection with Ni-201 alloy were carried out. While connecting with Ni-201 alloy, the corrosion rate of 800H was 4.47 ± 0.26 mg·cm−2·day−1 (10−3.55±0.02 A·cm−2). However, the corrosion rate for the 800H that was not connected to the Ni-201 alloy was 3.00 ± 0.27 mg·cm−2·day−1 (10−3.71±0.04 A·cm−2). Therefore, Ni-201 alloy accelerates the corrosion rate of 800H in the molten MgCl2–KCl salt while connecting to each other. In addition, based on the calculation of Gibbs energy of Ni–Fe–Cr alloy and exchange current density of 800H in MgCl2–KCl salt, a new Tafel model was developed to predict the corrosion rate of 800H alloys. The corrosion rate calculated by the new model is 10−3.74 A·cm−2, which agrees with the experimental data.

Molecular dynamics simulation on thermophysical properties and local structure of ternary chloride salt for thermal energy storage and transfer system

Ternary chloride salt (NaCl–KCl–MgCl2) is considered as a potential high temperature thermal storage material owing to its low melting point, excellent thermal properties and low cost. However, the thermophysical properties of ternary chloride salts are difficult to measure under high-temperature conditions. In this study, the microscopic model of ternary chloride salt is constructed by Materials Studio software. The ratio of Na ions, K ions, Mg ions and Cl ions in the simulation system is determined according to the molar ratio of NaCl–KCl–MgCl2 eutectic ternary chloride salt. The total number of ions is identified based on computational resources and the accuracy of simulation results, and the number of the four ions in the system is finally determined. LAMMPS software is used to simulate the microstructure and calculate the thermal properties. Simulation results indicate that as the temperature increases, the distance between cluster ions increases, while the coordination number decreases, resulting in the ion association weakens. Simulation results of thermophysical properties are in good agreement with the experimental measurements. The decrease in density and viscosity is attributed to the weakening of ion association with increasing temperature. All the results are expected to provide reliable guidance for the design and preparation of next-generation concentrating solar thermal energy storage materials.

Thermal conductivity measurement using modulated photothermal radiometry for nitrate and chloride molten salts

Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corrosive and conducting nature of these fluids also leads to time consuming sample preparation processes of many contact-based measurements. Here, we report the measurement of thermal conductivity of molten salts using a modulated photothermal radiometry (MPR) technique, which is a laser-based, non-contact, frequency-domain method adopted for molten salts for the first time. By unitizing the advantages of front side sensing of frequency-domain measurements and the vertical holder orientation, the technique can minimize the natural convection and salt creeping effects, thus yielding accurate molten salt thermal conductivity . The MPR technique is first calibrated using standard molten materials including paraffin wax and sulfur. It is then applied on measuring pure nitrate salts (NaNO3 and KNO3), solar salt (NaNO3–KNO3 mixture), and chloride salt (NaCl–KCl–MgCl2). The measurement results are compared with data from literature, especially those obtained from laser flash analysis (LFA). Our results demonstrate that the MPR is a convenient and reliable technique of measuring thermal conductivity of molten salts. Accurate thermal conductivity data of molten salts will be valuable in developing the next-generation high-temperature thermal energy storage and conversion systems.

Optimization of Dimensions of Smooth and Twisted-Tape-Inserted Tubes for Heat Transfer with NaCl/KCl/MgCl2 Molten Salts by Principle of Entropy Generation Minimization

Abstract The entropy generation minimization principle is used as the criterion to optimize the flow and heat transfer of solar collectors and heat exchangers that use molten salts NaCl–KCl–MgCl2 and KCl–MgCl2. The Gnielinski correlation for the Nusselt number versus Reynolds number, as well as the Moody friction factor given by Petukhov, was used for the calculation of the convective heat transfer coefficient and pressure loss due to friction in smooth tubes. For twisted-tap-inserted tube, equations of Nu and friction factor provided by Manglik and Bergles were used. The objective function, the entropy generation rate of the heat transfer system, was expressed as the function of Reynolds number, Prandtl number, heating flux, tube diameter, etc. As a result of the analysis, the optimum Reynolds number was determined and thereby to determine the optimum Nusselt number, convective heat transfer coefficient, friction factor, and tube diameter, which also allows the calculation of optimum flow velocity. The analysis was conducted in the fluid temperature range of 500–700 °C, which covers the operation temperature for supercritical CO2 power cycles in concentrated solar power (CSP) system. Optimized results from the smooth tube and twisted-tap-inserted tube are compared, which is important to the design of solar receivers for CSP systems.

Corrosion behavior of GH3535 alloy in KCl–MgCl2 eutectic salts purified with magnesium

Abstract The corrosion behavior of GH3535 alloys in KCl–MgCl2 eutectic salts with or without magnesium (Mg) was investigated using the static immersion tests. The effect of purification treatment on the corrosion of GH3535 specimens was also evaluated by the different analysis. The KCl–MgCl2 salts purified with Mg and reducing service temperature can slow down the corrosion rate of GH3535 specimens. Infrared spectra results showed that purification treatment reduced the content of crystal water in the KCl–MgCl2 salts. The difference of hydrate content in KCl–MgCl2 salts and the diffusion rate of Cr in alloy are the main reasons for the change of GH3535 specimens corrosion rate.

Preparation of low-oxygen titanium powder by magnesiothermic reduction of TiO2 in KCl–MgCl2–YCl3 molten salt

The Kroll process is currently the only effective method in industry to produce sponge titanium with a low-oxygen concentration (500 ppm). However, it has several limitations, such as a long processing time, low efficiency, and high energy consumption, resulting in the high production cost of titanium. To reduce the production cost, a method to prepare low-oxygen titanium powder by magnesiothermic reduction of TiO2 in KCl–MgCl2–YCl3 molten salt was designed. The thermodynamic calculation results showed that it was feasible to prepare titanium powder by the magnesiothermic reduction of TiO2 at 1073, 1173, and 1273 K. In addition, the deoxidation limits of Mg under Mg/MgCl2/YCl3/YOCl equilibrium were 3, 12, and 38 ppm at 1073, 1173, and 1273 K, respectively. The experimental results showed that titanium powder with a high oxygen concentration of approximately 10,000 ppm was obtained when the reduction was conducted in KCl–MgCl2 molten salt (i.e., the activity of YCl3 (aYCl3) was 0). Low-oxygen titanium powder was prepared with the addition of YCl3 in the molten salt and formed yttrium oxychloride (YOCl) (TiO2 (s) + 2Mg (l) + 2YCl3 (l) = Ti (s) + 2MgCl2 (l) + 2YOCl (s)). Moreover, at 1173 K, when the activity of YCl3 (aYCl3) was 1, titanium powder with an oxygen concentration as low as 150 ppm was obtained. Based on these results, a novel process for preparing low-oxygen titanium powder was designed. The proposed process is fast and highly efficient, and its future application in industry is anticipated.

Development of Deep Potentials of Molten MgCl2-NaCl and MgCl2-KCl Salts Driven by Machine Learning.

Molten MgCl2-based chlorides have emerged as potential thermal storage and heat transfer materials due to high thermal stabilities and lower costs. In this work, deep potential molecular dynamics (DPMD) simulations by a method combination of the first principle, classical molecular dynamics, and machine learning are performed to systemically study the relationships of structures and thermophysical properties of molten MgCl2-NaCl (MN) and MgCl2-KCl (MK) eutectic salts at the temperature range of 800-1000 K. The densities, radial distribution functions, coordination numbers, potential mean forces, specific heat capacities, viscosities, and thermal conductivities of these two chlorides are successfully reproduced under extended temperatures by DPMD with a larger size (5.2 nm) and longer timescale (5 ns). It is concluded that the higher specific heat capacity of molten MK is originated from the strong potential mean force of Mg-Cl bonds, whereas the molten MN performs better in heat transfer due to the larger thermal conductivity and lower viscosity, attributed to the weak interaction between Mg and Cl ions. Innovatively, the plausibility and reliability of microscopic structures and macroscopic properties for molten MN and MK verify the extensibilities of these two deep potentials in temperatures, and these DPMD results also provide detailed technical parameters to the simulations of other formulated MN and MK salts.

Insights into the local structure evolution and thermophysical properties of NaCl–KCl–MgCl2–LaCl3 melt driven by machine learning

The local structure evolution and thermophysical properties of the NaCl–KCl–MgCl2–LaCl3 melt were thoroughly understood, which facilitates the advancement and innovation of molten salt electrolytic production for Mg–La alloys.

Evolution of micro-pores in Ni–Cr alloys via molten salt dealloying

Porous materials with high specific surface area, high porosity, and high electrical conductivity are promising materials for functional applications, including catalysis, sensing, and energy storage. Molten salt dealloying was recently demonstrated in microwires as an alternative method to fabricate porous structures. The method takes advantage of the selective dissolution process introduced by impurities often observed in molten salt corrosion. This work further investigates molten salt dealloying in bulk Ni–20Cr alloy in both KCl–MgCl2 and KCl–NaCl salts at 700 ℃, using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction (XRD), as well as synchrotron X-ray nano-tomography. Micro-sized pores with irregular shapes and sizes ranging from sub-micron to several microns and ligaments formed during the process, while the molten salt dealloying was found to progress several microns into the bulk materials within 1–16 h, a relatively short reaction time, enhancing the practicality of using the method for synthesis. The ligament size increased from ~ 0.7 μm to ~ 1.3 μm in KCl–MgCl2 from 1 to 16 h due to coarsening, while remaining ~ 0.4 μm in KCl–NaCl during 16 h of exposure. The XRD analysis shows that the corrosion occurred primarily near the surface of the bulk sample, and Cr2O3 was identified as a corrosion product when the reaction was conducted in an air environment (controlled amount sealed in capillaries); thus surface oxides are likely to slow the morphological coarsening rate by hindering the surface diffusion in the dealloyed structure. 3D-connected pores and grain boundary corrosion were visualized by synchrotron X-ray nano-tomography. This study provides insights into the morphological and chemical evolution of molten salt dealloying in bulk materials, with a connection to molten salt corrosion concerns in the design of next-generation nuclear and solar energy power plants.

Preheating of solar power tower receiver tubes for a high-temperature chloride molten salt

Enhancement of solar-to-electric efficiency in future solar power tower (SPT) plants will require an improvement of the power cycle performance. An attractive option is the use of a supercritical CO2 Brayton cycle, which needs a heat transfer fluid (HTF) temperature of around 700 °C to be competitive. However, the current HTF employed in SPT plants, i.e. nitrate salt, cannot reach this temperature target. For this reason, new HTFs with a large temperature operation range have been investigated recently, but usually their freezing temperature is comparatively high, which complicates the preheating of the tubes during the receiver start-up. In the preheating operation, the heliostat field concentrates the solar irradiation onto the receiver tubes to elevate their temperature above a minimum value, while they are still empty, to avoid the risk of freezing of the incoming flow of HTF. This is a very critical operation, as too much incident heat flux in the preheating leads to extremely high thermal-stresses, causing the creep-fatigue damage of the receiver tubes. The present work numerically characterizes the temporal evolution of the temperature and thermal-stresses during the preheating of a typical configuration of a receiver tube made of Haynes 230 alloy. The aim is to check whether it is possible to preheat the receiver tubes above the freezing temperature of a chloride salt (i.e. NaCl–KCl–MgCl2) as an alternative high-temperature HTF to the nitrate salt. The results indicate that, although the original Vant-Hull algorithm of tube preheating is suitable for the nitrate salt, it is not adequate for the chloride salt. Therefore, a series of modifications of the Vant-Hull algorithm are proposed to meet the requirements of the high-temperature HTF. In the modified Vant-Hull algorithm, the resulting peak values of the temperature gradient and the von Mises stress are reached within the first 1.2 min of preheating. Besides, in the cases analyzed, the estimated creep-fatigue damage of the tube produced with the modified algorithm is much lower than an allowable limit set to protect the structural integrity of the receiver. However, the demanding operating conditions of the chloride salt receiver lead to a lower lifespan than that of the conventional nitrate salt receiver for the same heliostat field and receiver layout. Furthermore, the modified Vant-Hull algorithm requires longer preheating times, which delays the start-up of the SPT plant and reduces its available hours of operation per day. Finally, this work also demonstrates that the modified Vant-Hull algorithm is suitable to preheat receiver tubes that operate with other high-temperature molten salts and liquid metals.

High-temperature corrosion rate and diffusion modelling of Ni-coated incoloy 800H alloy in MgCl2–KCl heat transfer fluid

The polarization and long-term dipping corrosion behaviors of pristine and Ni-coated Incoloy 800H (800H) alloys were investigated in the eutectic MgCl 2 –KCl molten salt at different temperatures and compared their performance with expensive Haynes 230 (H230) alloy. The corrosion rates of all the studied alloys increased with temperature. The corrosion rates obtained from the electrochemical study showed Ni-coating increases the corrosion resistance of 800H alloy. Further, the linear polarization measurements were also performed at different temperatures to investigate the effect of Ni-coating on the exchange-current density. A long-term corrosion study showed that 800H has a higher corrosion rate than H230. However, Ni-coating significantly decreased the corrosion rate of 800H (even lower than H230) in the molten MgCl 2 –KCl salt. A diffusion model for Ni-coated 800H was presented to describe the diffusion behavior of Cr, Fe, and Ni elements in the Ni-coated 800H alloy in a long-term corrosion study. The diffusion model was simulated based on the partial differential equations and known initial boundary conditions. The simulated result is compared with experimental data by altering the diffusion coefficient and reasonable values of diffusion coefficients of Cr, Fe, and Ni in 800H alloy and Ni-coating. The estimated diffusion coefficient (D) values in the Ni-coating from 800H alloy are in the range of 2–4 × 10 −11 cm 2 s ˗1 , 4–6 × 10 −11 cm 2 s ˗1 , and 5–8 × 10 −11 cm 2 s ˗1 for Cr, Fe, and Ni, respectively. The overall results indicated that Ni-coated 800H could be employed as a suitable alternative construction material for solar thermal energy storage containers. • We report polarization and long-term dipping corrosion of pure and Ni-coated Incoloy 800H alloy in MgCl 2 -KCl molten salt. • Measured corrosion rates and weight loss showed Ni-coating on Incoloy 800H substantially increased the corrosion resistance. • Exchange current densities of Incoloy 800H with and without Ni-coating were investigated at different temperatures. • A diffusion model of Fe and Cr elements in Ni-coated 800H alloy was validated experimentally.

Corrosion behavior of pressureless-sintered SiC in molten NaCl–KCl–MgCl2 salt at 700 °C

The corrosion behavior of pressureless-sintered SiC in a molten NaCl–KCl–MgCl2 salt at 700 °C was investigated. Results indicate that it is prior for SiC and sintering aid B around grain boundaries to be corroded. MgCl2⋅xH2O(x=4,6) in the salt plays an important role in the corrosion of SiC. MgCl2⋅xH2O is hydrolyzed to HCl, H2O, and MgO, which react with SiC and B to form C, SiO2, SiCl4, B2O3, and Mg2SiO4. The corrosion depth of SiC and the concentrations of elements Si and B in the salt increase with the corrosion time, which are fitted well with Fick's law of diffusion.

Cesium ion sorption on hybrid pectin-Prussian blue beads: Batch and column studies to remove radioactive cesium from contaminated wastewater

Hybrid sorbents composed of various ratios of Prussian blue (PB) and pectin were tested for their ability to remove cesium ions in batch and column mode. The maximum sorption capacity calculated from Langmuir isotherms was obtained for a sorbent with 50% PB additive (63.8 ± 1.7 mg g−1). The sorption kinetics studies performed at 12°C, 22°C, and 32°C and the equilibrium studies indicate physical sorption. The good sorption kinetics and the bead form of the proposed sorbents allowed column tests to be performed. Using 3 g of sorbent (50% PB), 870 mL of solution with an initial cesium content of 120 mg L−1 was purified to <2.4 mg L−1 (most of the eluate contained cesium at a level of 0.1 mg L−1). The effect of NaCl, KCl MgCl2, and CaCl2 at different concentrations on the sorption of cesium ions was also examined. The results indicated that the sorbent was also effective in a strongly saline environment, as well as in substitute ocean water and real Baltic seawater. The proposed sorbent enables safe, long-term storage of radioactive item in an air-dry, dust free state and in the smallest volume of material of high sorption capacity. This type of treatment is crucial for the disposal of radioactive waste whose activity can only be reduced by decay. Therefore, our hybrid sorbent is a promising material for the removal of (radioactive) cesium ions using either continuous or batch processes from waste aqueous solutions from a variety of sources, even those containing significant amounts of other ions.