Interactions of molten salts with cathode products in the FFC Cambridge Process

Abstract

Molten salts play multiple important roles in the electrolysis of solid metal compounds, particularly oxides and sulfides, for the extraction of metals or alloys. Some of these roles are positive in assisting the extraction of metals, such as dissolving the oxide or sulfide anions, and transporting them to the anode for discharging, and offering the high temperature to lower the kinetic barrier to break the metal-oxygen or metal-sulfur bond. However, molten salts also have unfavorable effects, including electronic conductivity and significant capability of dissolving oxygen and carbon dioxide gases. In addition, although molten salts are relatively simple in terms of composition, physical properties, and decomposition reactions at inert electrodes, in comparison with aqueous electrolytes, the high temperatures of molten salts may promote unwanted electrode-electrolyte interactions. This article reviews briefly and selectively the research and development of the Fray-Farthing-Chen (FFC) Cambridge Process in the past two decades, focusing on observations, understanding, and solutions of various interactions between molten salts and cathodes at different reduction states, including perovskitization, non-wetting of molten salts on pure metals, carbon contamination of products, formation of oxychlorides and calcium intermetallic compounds, and oxygen transfer from the air to the cathode product mediated by oxide anions in the molten salt.

Keywords

FFC Cambridge Process / molten salts / electrolysis / extraction / oxides / sulfides / metals / alloys / reaction mechanisms

References

[1] D.J. Fray, T.W. Farthing, and Z. Chen, Removal of Oxygen from Metal Oxides and Solid Solutions by Electrolysis in a Fused Salt, International Patent, Appl. WO9964638, 1999.

[2] Chen GZ, Fray DJ, Farthing TW. Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride. Nature, 2000, 407(6802): 361.

[3] Flower HM. Materials Science: A moving oxygen story. Nature, 2000, 407(6802): 305.

[4] Science and Technology, Dr. Chen and the philosopher’s stone, The Economist, 21st September 2000. [2020-05-4] https://www.economist.com/science-and-technology/2000/09/21/dr-chen-and-the-philosophers-stone

[5] Faller K, Froes FHS. The use of titanium in family automobiles: Current trends. JOM, 2001, 53(4): 27.

[6] Fray DJ, Chen GZ. Reduction of titanium and other metal oxides using electrodeoxidation. Mater. Sci. Technol., 2004, 20(3): 295.

[7] G.Z. Chen and D.J. Fray, Understanding the electro-reduction of metal oxides in molten salts, [in] A.T. Tabereaux ed., Light Metals 2004, Wiley-TMS, 2004, p. 881.

[8] Wang DH, Jin XB, Chen GZ. Solid state reactions: An electrochemical approach in molten salts. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem., 2008, 104, 189.

[9] Xiao W, Wang DH. The electrochemical reduction processes of solid compounds in high temperature molten salts. Chem. Soc. Rev., 2014, 43(10): 3215.

[10] Fray DJ, Schwandt C. Aspects of the application of electrochemistry to the extraction of titanium and its applications. Mater. Trans., 2017, 58(3): 306.

[11] Hu D, Chen GZ. Breitkopf C, Swider-Lyons K. Advanced extractive electrometallurgy. Springer Handbook of Electrochemical Energy, 2017, Berlin, Springer, 801.

[12] Chen GZ, Fray DJ. Fang ZZ, Froes HS, Zhang Y. Invention and fundamentals of the FFC Cambridge Process. Extractive Metallurgy of Titanium: Conventional and Recent Advances in Extraction and Production of Titanium Metal, 2020, Oxford, Elsevier, 227.

[13] Metalysis, Technology, 2019 [2020-11-17]. http://www.metalysis.com/technology/.

[14] GLABAT, Development of Negative Electrode Materials, 2015 [2020-11-17]. http://www.glabat.com/article/content/view?id=23.

[15] Hu D, Xiao W, Chen GZ. Near-net-shape production of hollow titanium alloy components via electrochemical reduction of metal oxide precursors in molten salts. Metall. Mater.Trans. B, 2013, 44(2): 272.

[16] Schwandt C, Hamilton JA, Fray DJ, Crawford IA. The production of oxygen and metal from lunar regolith. Planetary Space Sci., 2012, 74(1): 49.

[17] B.A. Lomax, M. Conti, N. Khan, N.S. Bennett, A.Y. Ganin, and M.S. Symes, Proving the viability of an electrochemical process for the simultaneous extraction of oxygen and production of metal alloys from lunar regolith, Planetary Space Sci., 180(2020), art. No. 104748.

[18] Siambun NJ, Mohamed H, Hu D, Jewell D, Beng YK, Chen GZ. Utilisation of carbon dioxide for electro-carburisation of mild steel in molten carbonate salts. J. Electrochem. Soc., 2011, 158(11): H1117.

[19] Tang DY, Yin HY, Mao XH, Xiao W, Wang DH. Effects of applied voltage and temperature on the electrochemical production of carbon powders from CO₂ in molten salt with an inert anode. Electrochim. Acta, 2013, 114, 567.

[20] W. Wang, B.M. Jiang, Z. Wang, and W. Xiao, In situ electrochemical conversion of CO₂ in molten salts to advanced energy materials with reduced carbon emissions, Sci. Adv., 6(2020), No. 9, art. No. 9278.

[21] O. Al-Juboori, F. Sher, A. Hazafa, M.K. Khan, and G.Z. Chen, The effect of variable operating parameters for hydrocarbon fuel formation from CO₂ by molten salts electrolysis, J. CO ₂ Util., 40(2020), art. No. 101193.

[22] Peng C, Guan CZ, Lin J, Zhang SY, Bao HL, Wang Y, Xiao GP, Chen GZ, Wang JQ. A rechargeable high-temperature molten salt iron-oxygen battery. ChemSusChm, 2018, 11(11): 1880.

[23] Chen HL, Jin XB, Yu LP, Chen GZ. Influences of graphite anode area on electrolysis of solid metal oxides in molten salts. J. Solid State Electrochem., 2014, 18(12): 3317.

[24] Chen GZ, Fray DJ. Cathodic refining in molten salts: Removal of oxygen, sulfur and selenium from static and flowing molten copper. J. Appl. Electrochem., 2001, 31(2): 155.

[25] Mohamedi M, Børresen B, Haarberg G, Tunold R. Anodic behavior of carbon electrodes in CaO-CaCl₂ melts at 1123 K. J. Electrochem. Soc., 1999, 146(4): 1472.

[26] Chen GZ, Fray DJ. Voltammetric studies of the oxygen-titanium binary system in molten calcium chloride. J. Electrochem. Soc., 2002, 149(11): E455.

[27] Barnett R, Kilby KT, Fray DJ. Reduction of tantalum pentoxide using graphite and tin-oxide-based anodes via the FFC-Cambridge Process. Metall. Mater. Trans. B, 2009, 40(2): 150.

[28] Hu LW, Song Y, Ge JB, Jiao SQ, Cheng J. Electrochemical metallurgy in CaCl₂-CaO melts on the basis of TiO₂RuO₂ inert anode. J. Electrochem. Soc., 2016, 163(3): E33.

[29] Ramanarayanan TA, Rapp RA. The diffusivity and solubility of oxygen in liquid tin and solid silver and the diffusivity. Metall. Trans., 1972, 3(12): 3239.

[30] Zou XL, Lu XG, Zhou ZF, Li CH. Direct electrosynthesis of Ti₅Si₃/TiC composites from their oxides/C precursors in molten calcium chloride. Electrochem. Commun., 2012, 21, 9.

[31] Hu HB, Gao YM, Lao YG, Qin QW, Li GQ, Chen GZ. Yttria stabilized zirconia aided electrochemical investigation on ferric ions in mixed molten calcium and sodium chlorides. Mater. Metall. Trans. B, 2018, 49(5): 2794.

[32] Wang T, Gao HP, Jin XB, Chen HL, Peng JJ, Chen GZ. Electrolysis of solid metal sulfide to metal and sulfur in molten NaCl-KCl. Electrochem. Commum., 2011, 13(12): 1492.

[33] Wu CH, Tan MS, Ye GZ, Fray DJ, Jin XB. High-efficiency preparation of titanium through electrolysis of carbosulfurized titanium dioxide. ACS Sustainable Chem. Eng., 2019, 7(9): 8340.

[34] Li W, Yuan YT, Jin XB, Chen HL, Chen GZ. Environmental and energy gains from using molten magnesium-sodium-potassium chlorides for electro-metallisation of refractory metal oxides. Prog. Nat. Sci., 2015, 25(6): 650.

[35] Yuan YT, Li W, Chen HL, Wang ZY, Jin XB, Chen GZ. Electrolysis of metal oxides in MgCl₂ based molten salts with an inert graphite anode. Faraday Discuss., 2016, 190, 85.

[36] Sadoway D. Inert anodes for the Hall-Heroult cell: The ultimate materials challenge. JOM, 2001, 53(3): 34.

[37] Yin HY, Gao LL, Zhu H, Mao XH, Gan FX, Wang DH. On the development of metallic inert anode for molten CaCl₂-CaO system. Electrochim. Acta, 2011, 56(9): 3296.

[38] Yin HY, Tang DY, Zhu H, Zhang Y, Wang DH. Production of iron and oxygen in molten K₂CO₃-Na₂CO₃ by electrochemically splitting Fe₂O₃ using a cost affordable inert anode. Electrochem. Commun., 2011, 13(12): 1521.

[39] Wang DH, Xiao W. Lantelme F, Groult H. Inert anode development for high-temperature molten salts. Molten Salts Chemistry-From Lab to Applications, 2013, Oxford, Elsevier, 171.

[40] Schwandt C, Fray DJ. Use of molten salt fluxes and cathodic protection for preventing the oxidation of titanium at elevated temperatures. Metall. Mater. Trans. B, 2014, 45(6): 2145.

[41] Schwandt C, Fray DJ. Determination of the kinetic pathway in the electrochemical reduction of titanium dioxide in molten calcium chloride. Electrochim. Acta, 2005, 51(1): 66.

[42] Jiang K, Hu XH, Ma M, Wang DH, Qiu GH, Jin XB, Chen GZ. “Perovskitization”-assisted electrochemical reduction of solid TiO₂ in molten CaCl₂. Angew. Chem. Int. Ed., 2006, 45(3): 428.

[43] Gordo E, Chen GZ, Fray DJ. Toward optimisation of electrolytic reduction of solid chromium oxide to chromium powder in molten chloride salts. Electrochim. Acta, 2004, 49(13): 2195.

[44] Wu T, Jin XB, Xiao W, Liu C, Wang DH, Chen GZ. Computer-aided control of electrolysis of solid Nb₂O₅ in molten CaCl₂. Phys. Chem. Chem. Phys., 2008, 10(13): 1809.

[45] Chen GZ, Gordo E, Fray DJ. Direct electrolytic preparation of chromium powder. Metall. Mater. Trans. B, 2004, 35(2): 223.

[46] Deng Y, Wang DH, Xiao W, Jin XB, Hu XH, Chen GZ. Electrochemistry at conductor/insulator/electrolyte three-phase interlines: A thin layer model. J. Phys. Chem. B, 2005, 109(29): 14043.

[47] Xiao W, Jin XB, Deng Y, Wang DH, Chen GZ. Three-phase interlines electrochemically driven into insulator compounds: A penetration model and its verification by electroreduction of solid AgCl. Chem. Eur. J., 2007, 13(2): 604.

[48] Li W, Jin XB, Huang FL, Chen GZ. Metal-to-oxide molar volume ratio: The overlooked barrier to solid-state electro-reduction and a green bypass through recyclable NH₄HCO₃. Angew. Chem. Int. Ed., 2010, 49(18): 3203.

[49] W. Li, H.L. Chen, F.L. Huang, X.B. Jin, F.M. Xiao, and G.Z. Chen, Fast electro-reduction of TiO₂ precursors with macro-micro-bimodal porosity in molten CaCl₂, [in] The 3rd Asia Conference on Molten Salts and Ionic Liquids, Harbin, 2011.

[50] Chen GZ, Fray DJ. A morphological study of the FFC chromium and titanium powders. Miner. Process. Extr. Metall., 2006, 115(1): 49.

[51] TWI Ltd., What is friction welding? [2020-07-04] https://www.twi-global.com/technical-knowledge/faqs/faq-what-is-friction-welding

[52] Muir Wood AJ, Copcutt RC, Chen GZ, Fray DJ. Electrochemical fabrication of nickel manganese gallium alloy powder. Adv. Eng. Mater., 2003, 5(9): 650.

[53] Wang DH, Qiu GH, Jin XB, Hu XH, Chen GZ. Electrochemical metallisation of solid terbium oxide. Angew. Chem. Int. Ed., 2006, 45(15): 2384.

[54] Qiu GH, Wang DH, Ma M, Jin XB, Chen GZ. Electrolytic synthesis of TbFe₂ from Tb₄O₇ and Fe₂O₃ powders in molten CaCl₂. J. Electroanal. Chem., 2006, 589(1): 139.

[55] Pistorius PC, Fray DJ. Formation of silicon by electrodeoxidation, and implications for titanium metal production. J. South Afr. Inst. Min. Metall., 2006, 106(1): 31.

[56] Delimarskii YK, Gorodiskii OV, Grishchenko VF. Cathode liberation of carbon from molten carbonates. Dokl. Akad. Nauk SSSR, 1964, 159(3): 650.

[57] Ingram MD, Baron B, Janz GJ. The electrolytic deposition of carbon from fused carbonates. Electrochim. Acta, 1966, 11(11): 1629.

[58] Kawamura H, Ito Y. Electrodeposition of cohesive carbon films on aluminum in a LiCl-KCl-K₂CO₃ melt. J. Appl. Electrochem., 2000, 30(5): 571.

[59] Massot L, Chamelot P, Bouyer F, Taxil P. Studies of carbon nucleation phenomena in molten alkaline fluoride media. Electrochim. Acta, 2003, 48(5): 465.

[60] Ijije HV, Lawrence RC, Siambun NJ, Jeong S, Jewell DA, Hu D, Chen GZ. Electro-deposition and re-oxidation of carbon in carbonate containing molten salts. Faraday Discuss., 2014, 172, 105.

[61] K. Dring, Direct electrochemical production of titanium, [in] The 3rd Workshop on Reactive Metal Processing (RMW3), Cambridge, 2007 [2020-07-09] http://www.okabe.iis.u-tokyo.ac.jp/core-to-core/rmw/RMW3/slide/RMW3_06_Dring_F.pdf.

[62] G.Z. Chen and D.J. Fray, Prevention of Unwanted Reactions at Three Phase Boundaries, UK Patent, Appl. GB0329541.7, 2003.

[63] Stevenson A. Development of a Novel Electrochemical Pyro-processing Methodology for Spent Nuclear Fuels, 2016, Nottingham, University of Nottingham

[64] Hu ML, Qu ZF, Bai CG, Hu D, Chen GZ. Effect of the changed electrolytic cell on the current efficiency in FFC Cambridge Process. Mater. Trans., 2017, 58(3): 322.

[65] Xiao W, Jin XB, Deng Y, Wang DH, Chen GZ. Rationalisation and optimisation of solid state electro-reduction of SiO₂ to Si in molten CaCl₂ in accordance with dynamic three-phase interlines based voltammetry. J. Electroanal. Chem., 2010, 639(1–2): 130.

[66] Yang X, Yasuda K, Nohira T, Hagiwara R, Homma T. Cathodic potential dependence of electrochemical reduction of SiO₂ granules in molten CaCl₂. Metall. Mater. Trans. E, 2016, 3(3): 145.

[67] Juzeliūnas E, Fray DJ. Silicon electrochemistry in molten salts. Chem. Rev., 2020, 120(3): 1690.

[68] Gao P, Jin XB, Wang DH, Hu XH, Chen GZ. A quartz sealed Ag/AgCl reference electrode for CaCl₂ baeed molten salts. J. Electroanal. Chem., 2005, 579(2): 321.

[69] Wang H, Siambun NJ, Yu LP, Chen GZ. A robust alumina membrane reference electrode for high temperature molten salts. J. Electrochem. Soc., 2012, 159(9): H740.

[70] Al-Shara NK, Sher F, Yaqoob A, Chen GZ. Electrochemical investigation of novel reference electrode Ni/Ni(OH)₂ in comparision with silver and platinum inert quasi-reference electrodes for electrolysis in eutectic molten hydroxide. Int. J. Hydrogen Energy, 2020, 44(50): 27224.

[71] Xie HW, Zhang H, Zhai YC, Wang JX, Li CD. Al preparation from solid Al₂O₃ by direct electrochemical deoxidation in molten CaCl₂-NaCl at 550°C. J. Mater. Sci. Technol., 2009, 25(4): 459.

[72] Yan XY, Fray DJ. Direct electrolytic reduction of solid alumina using molten calcium chloride-alkali chloride electrolytes. J. App. Electrochem., 2009, 39(8): 1349.

[73] Kadowaki H, Katasho Y, Yasuda K, Nohira T. Electrolytic reduction of solid Al₂O₃ to liquid Al in molten CaCl₂. J. Electrochem. Soc., 2018, 165(2): D83.

[74] Clark RJH, Bradley DC, Thornton P. The Chemistry of Titanium, Zirconium and Hafnium, 1973, Oxford, Pergamon Press, 367.

[75] Zhu Y, Wang DH, Ma M, Hu XH, Jin XB, Chen GZ. More affordable electrolytic LaNi₅-type hydrogen storage powders. Chem. Commum., 2007, 24, 2515.

[76] Chen GZ, Fray DJ, Farthing TW. Cathodic deoxygenation of the alpha-case on titanium and alloys in molten calcium chloride. Metall. Mater. Trans. B, 2001, 32(6): 1041.

[77] Fray DJ. Novel methods for the production of titanium. Int. Mater. Rev., 2008, 53(6): 317.

[78] Alexander DTL, Schwandt C, Fray DJ. Microstructural kinetics of phase transformations during electrochemical reduction of titanium dioxide in molten calcium chloride. Acta Mater., 2006, 54(11): 2933.

[79] Sri Maha Vishnu D, Sanil N, Shakila L, Panneerselvam G, Sudha R, Mohandas KS, Nagarajan K. A study of the reaction pathways during electrochemical reduction of dense Nb₂O₅ pellets in molten CaCl₂ medium. Electrochim. Acta, 2013, 100, 51.

[80] Choi EY, Lee JW, Park JJ, Hur JM, Kim JK, Jung KY, Jeong SM. Electrochemical reduction behavior of a highly porous SIMFUEL particle in a LiCl molten salt. Chem. Eng. J., 2012, 207–208, 514.

[81] Stevenson A, Hu D, Chen GZ. Molten salt assisted electrochemical separation of spent fuel surrogates by partial direct reduction and selective anodic dissolution. ECS Trans., 2014, 64(4): 333.

[82] Mohandas KS. Direct electrochemical conversion of metal oxides to metal by molten salt electrolysis: A review. Miner. Process. Extra. Metall, 2013, 122(4): 195.

[83] Schwandt C, Doughty GR, Fray DJ. The FFC-cambridge process for titanium metal winning. Key Eng. Mater., 2010, 436, 13.

[84] Hu D, Dolganov A, Ma MC, Bhattacharya B, Bishop M, Chen GZ. Development of the fray-farthing-chen cambridge process: Towards the sustainable production of titanium and its alloys. JOM, 2018, 70(2): 129.

[85] Yu ZL, Wang N, Fang S, Qi XP, Gao ZF, Yang JY, Lu SG. Pilot-plant production of high-performance silicon nanowires by molten salt electrolysis of silica. Ind. Eng. Chem. Res., 2020, 59(1): 1.