Physicochemical properties of the molten NaCl–CaCl₂–MgCl₂ salt containing magnesium

Abstract

A low-melting-point molten salt composed of NaCl, CaCl₂, and MgCl₂ was developed via thermodynamic calculations and experimental validation. A set of thermodynamic database for the NaCl–CaCl₂–MgCl₂ molten salt was built. The molten salt composition was optimised based on the phase diagram calculations, and the melting point measured in the experiment was 418.5°C. The molten salt was purified by adding excess Mg, resulting in a final Mg content of 738.0 µg/g. Compared with the NaCl–CaCl₂–MgCl₂ salt, the Mg-containing salt exhibited enhanced thermal stability, with a reduced weight loss of only 5.0wt% at 728.0°C. The average specific heat capacities of the solid and liquid phases were measured at 0.89 and 1.44 J/(g °C), respectively, indicating improved heat retention. Notably, the differences in the melting temperature and fusion enthalpy remained negligible. For the first time, the ultrafast dynamics of this ternary molten salt were characterized using polarization-selective mid-infrared pump-probe spectroscopy. The Mg²⁺ vibrational dynamics (4.5 ps) were slower than those of the water peak (2.3 ps), whereas the behavior of the water peak closely resembled that in pure water. Collectively, this study enriches the physicochemical database of molten salt and offers theoretical and technical guidance for salt purification, with significant implications for the comprehensive utilization of salt-lake resources with abundant reserves in China.

Keywords

molten salt / phase diagram calculation / thermophysical property / thermal energy storage / CaCl₂

References

[1] Kuravi S, Trahan J, Goswami DY, Rahman MM, Stefanakos EK. Thermal energy storage technologies and systems for concentrating solar power plants. Prog. Energy Combust. Sci., 2013, 39(4):285.

[2] Islam MT, Huda N, Abdullah AB, Saidur R. A comprehensive review of state-of-the-art concentrating solar power (CSP) technologies: Current status and research trends. Renewable Sustainable Energy Rev., 2018, 91: 987.

[3] Jelley N, Smith T. Concentrated solar power: Recent developments and future challenges. Proc. Inst. Mech. Eng. Part A, 2015, 229(7):693.

[4] Palacios A, Barreneche C, Navarro ME, Ding Y. Thermal energy storage technologies for concentrated solar power–A review from a materials perspective. Renewable Energy, 2020, 156: 1244.

[5] Vignarooban K, Xu XH, Arvay A, Hsu K, Kannan AM. Heat transfer fluids for concentrating solar power systems-A review. Appl. Energy, 2015, 146: 383.

[6] Nunes VMB, Queirós CS, Lourenço MJV, Santos FJV, Castro CAND. Molten salts as engineering fluids–A review. Appl. Energy, 2016, 183: 603.

[7] Mohan G, Venkataraman M, Vidal JG, Coventry J. Assessment of a novel ternary eutectic chloride salt for next generation high-temperature sensible heat storage. Energy Convers. Manage., 2018, 167: 156.

[8] Guo SQ, Zhang JS, Wu W, Zhou WT. Corrosion in the molten fluoride and chloride salts and materials development for nuclear applications. Prog. Mater. Sci., 2018, 97: 448.

[9] Tian Y, Zhao CY. A review of solar collectors and thermal energy storage in solar thermal applications. Appl. Energy, 2013, 104: 538.

[10] X.K. Xu, X.X. Wang, P.W. Li, et al., Experimental test of properties of KCl–MgCl₂ eutectic molten salt for heat transfer and thermal storage fluid in concentrated solar power systems, J. Sol. Energy Eng., 140(2018), No. 5, art. No. 051011.

[11] Ding WJ, Vidal JG, Bonk A, Bauer T. Molten chloride salts for next generation CSP plants: Electrolytical salt purification for reducing corrosive impurity level. Sol. Energy Mater. Sol. Cells, 2019, 199: 8.

[12] Li YY, Xu XK, Wang XX, Li PW, Hao Q, Xiao B. Survey and evaluation of equations for thermophysical properties of binary/ternary eutectic salts from NaCl, KCl, MgCl₂, CaCl₂, ZnCl₂ for heat transfer and thermal storage fluids in CSP. Sol. Energy, 2017, 152: 57.

[13] P.W. Li, E. Molina, K. Wang, et al., Thermal and transport properties of NaCl–KCl–ZnCl₂ eutectic salts for new generation high-temperature heat-transfer fluids, J. Sol. Energy Eng., 138(2016), No. 5. art. No. 054501.

[14] Xu XK, Dehghani G, Ning JX, Li PW. Basic properties of eutectic chloride salts NaCl–KCl–ZnCl₂ and NaCl–KCl–MgCl₂ as HTFs and thermal storage media measured using simultaneous DSC–TGA. Sol. Energy, 2018, 162: 431.

[15] Kubíková B, Šimurda M, Robert E, Shi ZN, Boča M. Surface tension, electrical conductivity and viscosity of the LiCl–NaCl–ZnCl₂ molten system. J. Mol. Liq., 2016, 224: 672.

[16] Wei XL, Song M, Wang WL, Ding J, Yang JP. Design and thermal properties of a novel ternary chloride eutectics for high-temperature solar energy storage. Appl. Energy, 2015, 156: 306.

[17] Wei XL, Chen DD, Zhang XC, et al.Experimental and simulation study on enhancing thermal energy storage and heat transfer performance of ternary chloride eutectic salt by doping MgO nanoparticles. J. Therm. Sci., 2024, 33(6):2221.

[18] Z.Z. Rong, J. Ding, W.L. Wang, G. Pan, and S.L. Liu, Ab-initio molecular dynamics calculation on microstructures and thermophysical properties of NaCl–CaCl₂–MgCl₂ for concentrating solar power, Sol. Energy Mater. Sol. Cells, 216(2020), art. No. 110696.

[19] X.X. Wang, J.D. Rincon, P.W. Li, Y.Y. Zhao, and J. Vidal, Thernophysical properties experimentally tested for NaCl–KCl–MgCl₂ eutectic molten salt as a next-generation high-temperature heat transfer fluids in concentrated solar power systems, J. Sol. Energy Eng., 143(2021), No. 4, art. No. 041005.

[20] Mohan G, Venkataraman M, Vidal JG, Coventry J. Thermo-economic analysis of high-temperature sensible thermal storage with different ternary eutectic alkali and alkaline earth metal chlorides. Sol. Energy, 2018, 176: 350.

[21] Lv XY, Dong AP, Dai YB, Wang J, Shu D, Sun BD. Temperature and concentration dependence of the physical properties and local structures of molten NaCl–KCl–LiCl mixtures. J. Mol. Liq., 2017, 229: 330.

[22] Li CJ, Li PW, Wang K, Molina EE. Survey of properties of key single and mixture halide salts for potential application as high temperature heat transfer fluids for concentrated solar thermal power systems. AIMS Energy, 2014, 2(2):133.

[23] Z.G. Yang, H.Q. Yin, F.Y. Guo, et al., Design optimization and key thermo-physical properties of the NaCl–KCl–FeCl₃ molten salts for low-temperature thermal energy storage, J. Energy Storage, 72(2023). art. No. 108255.

[24] Ambrosek JWMolten Chloride Salts for Heat Transfer in Nuclear Systems, 2011, Madison, Wisconsin: University of Wisconsin-Madison

[25] Zhao YF, Li ZY, Li SJ, Song WL, Jiao SQ. A review of in situ high-temperature characterizations for understanding the processes in metallurgical engineering. Int. J. Miner. Metall. Mater.2024, 31(11):2327.

[26] Song X, Li SL, Liu SS, Fan Y, He JL, Song JX. Coordination states of metal ions in molten salts and their characterization methods. Int. J. Miner. Metall. Mater., 2023, 30(7):1261.

[27] Li HX, Xu WJ, Zhang YF, et al.Prediction of the thermal conductivity of Mg–Al–La alloys by CALPHAD method. Int. J. Miner. Metall. Mater, 2024, 31(1):129.

[28] H.Q. Yin, Z.R. Wang, X. Lai, Y. Wang, and Z.F. Tang, Optimum design and key thermal property of NaCl–KCl–CaCl₂ eutectic salt for ultra-high-temperature thermal energy storage, Sol. Energy Mater. Sol. Cells, 236(2022), art. No. 111541.

[29] H.Q. Yin, J.L. Song, W.G. Liu, and B. Hu, Thermodynamic reoptimization of the ACl_x–MgCl₂ (A = Na, K, Rb, Cs and Ca, x = 1 or 2) systems, Calphad, 79(2022), art. No. 102476.

[30] X. Lai, H.Q. Yin, P. Li, B.X. Liu, L. Gao, and Z.F. Tang, Design optimization and thermal storage characteristics of NaNO₃–NaCl–NaF molten salts with high latent heat and low cost for the thermal energy storage, J. Energy Storage, 52(2022), art. No. 104805.

[31] Pelay U, Luo LG, Fan YL, Stitou D, Rood M. Thermal energy storage systems for concentrated solar power plants. Renewable Sustainable Energy Rev., 2017, 79: 82.

[32] M. Vaka, R. Walvekar, P. Jagadish, M. Khalid, N.M. Mubarak, and H. Panchal, High-temperature molten salts optimisation usingmixture design for energy storage application, J. Energy Storage, 32(2020), art. No. 101981.

[33] Du LC, Ding J, Tian HQ, Wang WL, Wei XL, Song M. Thermal properties and thermal stability of the ternary eutectic salt NaCl–CaCl₂–MgCl₂ used in high-temperature thermal energy storage process. Appl. Energy, 2017, 204: 1225.

[34] Yuan RF, Yan C, Fayer M. Ion-molecule complex dissociation and formation dynamics in LiCl aqueous solutions from 2D IR spectroscopy. J. Phys. Chem. B, 2018, 122(46):10582.

[35] Yuan RF, Fayer MD. Dynamics of water molecules and ions in concentrated lithium chloride solutions probed with ultrafast 2D IR spectroscopy. J. Phys. Chem. B, 2019, 123(35):7628.

[36] Hung ST, Roget SA, Zheng WZ, Fayer MD. Concentration dependence of dynamics and structure among hydrated magnesium ions: An ultrafast infrared study. J. Phys. Chem. B., 2023, 127(14):3278.

Rights & permissions

University of Science and Technology Beijing