George Z. Chen, Interactions of molten salts with cathode products in the FFC Cambridge Process, Int. J. Miner. Metall. Mater., 27(2020), No. 12, pp. 1572-1587.
Cite this article as:
George Z. Chen, Interactions of molten salts with cathode products in the FFC Cambridge Process, Int. J. Miner. Metall. Mater., 27(2020), No. 12, pp. 1572-1587.
Invited ReviewOpen Access

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

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    George Z. Chen    E-mail:

  • Received: 14 July 2020Revised: 22 September 2020Accepted: 30 September 2020Available online: 1 October 2020
  • 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.

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  • [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.
    G.Z. Chen, D.J. Fray, and T.W. Farthing, Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride, Nature, 407(2000), No. 6802, p. 361. doi: 10.1038/35030069
    H.M. Flower, Materials Science: A moving oxygen story, Nature, 407(2000), No. 6802, p. 305. doi: 10.1038/35030266
    Science and Technology, Dr. Chen and the philosopher’s stone, The Economist, 21st September 2000. [2020-05-4]
    K. Faller and F.H.S. Froes, The use of titanium in family automobiles: Current trends, JOM, 53(2001), No. 4, p. 27. doi: 10.1007/s11837-001-0143-3
    D.J. Fray and G.Z. Chen, Reduction of titanium and other metal oxides using electrodeoxidation, Mater. Sci. Technol., 20(2004), No. 3, p. 295. doi: 10.1179/026708304225012242
    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.
    D.H. Wang, X.B. Jin, and G.Z. Chen, Solid state reactions: An electrochemical approach in molten salts, Annu. Rep. Prog. Chem.,Sect. C:Phys. Chem., 104(2008), p. 189. doi: 10.1039/b703904m
    W. Xiao and D.H. Wang, The electrochemical reduction processes of solid compounds in high temperature molten salts, Chem. Soc. Rev., 43(2014), No. 10, p. 3215. doi: 10.1039/c3cs60327j
    D.J. Fray and C. Schwandt, Aspects of the application of electrochemistry to the extraction of titanium and its applications, Mater. Trans., 58(2017), No. 3, p. 306. doi: 10.2320/matertrans.MK201619
    D. Hu and G.Z. Chen, Advanced extractive electrometallurgy, [in] C. Breitkopf and K. Swider-Lyons, eds., Springer Handbook of Electrochemical Energy, Springer, Berlin, 2017, p. 801.
    G.Z. Chen and D.J. Fray, Invention and fundamentals of the FFC Cambridge Process, [in], Z.Z. Fang, H.S. Froes, and Y. Zhang, eds., Extractive Metallurgy of Titanium: Conventional and Recent Advances in Extraction and Production of Titanium Metal, Elsevier, Oxford, 2020, p. 227
    Metalysis, Technology, 2019 [2020-11-17].
    GLABAT, Development of Negative Electrode Materials, 2015 [2020-11-17].
    D. Hu, W. Xiao, and G.Z. Chen, Near-net-shape production of hollow titanium alloy components via electrochemical reduction of metal oxide precursors in molten salts, Metall. Mater. Trans. B, 44(2013), No. 2, p. 272. doi: 10.1007/s11663-013-9800-5
    C. Schwandt, J.A. Hamilton, D.J. Fray, and I.A. Crawford, The production of oxygen and metal from lunar regolith, Planetary Space Sci., 74(2012), No. 1, p. 49. doi: 10.1016/j.pss.2012.06.011
    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. doi: 10.1016/j.pss.2019.104748
    N.J. Siambun, H. Mohamed, D. Hu, D. Jewell, Y.K. Beng, and G.Z. Chen, Utilisation of carbon dioxide for electro-carburisation of mild steel in molten carbonate salts, J. Electrochem. Soc., 158(2011), No. 11, p. H1117. doi: 10.1149/2.017111jes
    D.Y. Tang, H.Y. Yin, X.H. Mao, W. Xiao, and D.H. Wang, Effects of applied voltage and temperature on the electrochemical production of carbon powders from CO2 in molten salt with an inert anode, Electrochim. Acta, 114(2013), p. 567. doi: 10.1016/j.electacta.2013.10.109
    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. doi: 10.1126/sciadv.aay9278
    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. CO2 Util., 40(2020), art. No. 101193. doi: 10.1016/j.jcou.2020.101193
    C. Peng, C.Z. Guan, J. Lin, S.Y. Zhang, H.L. Bao, Y. Wang, G.P. Xiao, G.Z. Chen, and J.Q. Wang, A rechargeable high-temperature molten salt iron-oxygen battery, ChemSusChem, 11(2018), No. 11, p. 1880. doi: 10.1002/cssc.201800237
    H.L. Chen, X.B. Jin, L.P. Yu, and G.Z. Chen, Influences of graphite anode area on electrolysis of solid metal oxides in molten salts, J. Solid State Electrochem., 18(2014), No. 12, p. 3317. doi: 10.1007/s10008-014-2645-2
    G.Z. Chen and D.J. Fray, Cathodic refining in molten salts: Removal of oxygen, sulfur and selenium from static and flowing molten copper, J. Appl. Electrochem., 31(2001), No. 2, p. 155. doi: 10.1023/A:1004175605236
    M. Mohamedi, B. Børresen, G. Haarberg, and R. Tunold, Anodic behavior of carbon electrodes in CaO–CaCl2 melts at 1123 K, J. Electrochem. Soc., 146(1999), No. 4, p. 1472. doi: 10.1149/1.1391789
    G.Z. Chen and D.J. Fray, Voltammetric studies of the oxygen–titanium binary system in molten calcium chloride, J. Electrochem. Soc., 149(2002), No. 11, p. E455. doi: 10.1149/1.1513985
    R. Barnett, K.T. Kilby, and D.J. Fray, Reduction of tantalum pentoxide using graphite and tin-oxide-based anodes via the FFC-Cambridge Process, Metall. Mater. Trans. B, 40(2009), No. 2, p. 150. doi: 10.1007/s11663-008-9219-6
    L.W. Hu, Y. Song, J.B. Ge, S.Q. Jiao, and J. Cheng, Electrochemical metallurgy in CaCl2–CaO melts on the basis of TiO2·RuO2 inert anode, J. Electrochem. Soc., 163(2016), No. 3, p. E33. doi: 10.1149/2.0131603jes
    T.A. Ramanarayanan and R.A. Rapp, The diffusivity and solubility of oxygen in liquid tin and solid silver and the diffusivity, Metall. Trans., 3(1972), No. 12, p. 3239. doi: 10.1007/BF02661339
    X.L. Zou, X.G. Lu, Z.F. Zhou, and C.H. Li, Direct electrosynthesis of Ti5Si3/TiC composites from their oxides/C precursors in molten calcium chloride, Electrochem. Commun., 21(2012), p. 9. doi: 10.1016/j.elecom.2012.05.008
    H.B. Hu, Y.M. Gao, Y.G. Lao, Q.W. Qin, G.Q. Li, and G.Z. Chen, Yttria stabilized zirconia aided electrochemical investigation on ferric ions in mixed molten calcium and sodium chlorides, Mater. Metall. Trans. B, 49(2018), No. 5, p. 2794. doi: 10.1007/s11663-018-1371-z
    T. Wang, H.P. Gao, X.B. Jin, H.L. Chen, J.J. Peng, and G.Z. Chen, Electrolysis of solid metal sulfide to metal and sulfur in molten NaCl–KCl, Electrochem. Commum., 13(2011), No. 12, p. 1492. doi: 10.1016/j.elecom.2011.10.005
    C.H. Wu, M.S. Tan, G.Z. Ye, D.J. Fray, and X.B. Jin, High-efficiency preparation of titanium through electrolysis of carbo-sulfurized titanium dioxide, ACS Sustainable Chem. Eng., 7(2019), No. 9, p. 8340. doi: 10.1021/acssuschemeng.8b06801
    W. Li, Y.T. Yuan, X.B. Jin, H.L. Chen, and G.Z. Chen, Environmental and energy gains from using molten magnesium–sodium–potassium chlorides for electro-metallisation of refractory metal oxides, Prog. Nat. Sci., 25(2015), No. 6, p. 650. doi: 10.1016/j.pnsc.2015.11.002
    Y.T. Yuan, W. Li, H.L. Chen, Z.Y. Wang, X.B. Jin, and G.Z. Chen, Electrolysis of metal oxides in MgCl2 based molten salts with an inert graphite anode, Faraday Discuss., 190(2016), p. 85. doi: 10.1039/C5FD00231A
    D. Sadoway, Inert anodes for the Hall-Heroult cell: The ultimate materials challenge, JOM, 53(2001), No. 3, p. 34.
    H.Y. Yin, L.L. Gao, H. Zhu, X.H. Mao, F.X. Gan, and D.H. Wang, On the development of metallic inert anode for molten CaCl2–CaO system, Electrochim. Acta, 56(2011), No. 9, p. 3296. doi: 10.1016/j.electacta.2011.01.026
    H.Y. Yin, D.Y. Tang, H. Zhu, Y. Zhang, and D.H. Wang, Production of iron and oxygen in molten K2CO3–Na2CO3 by electrochemically splitting Fe2O3 using a cost affordable inert anode, Electrochem. Commun., 13(2011), No. 12, p. 1521. doi: 10.1016/j.elecom.2011.10.009
    D.H. Wang and W. Xiao, Inert anode development for high-temperature molten salts, [in] F. Lantelme and H. Groult, eds., Molten Salts Chemistry–From Lab to Applications, Elsevier, Oxford, 2013, p. 171.
    C. Schwandt and D.J. Fray, Use of molten salt fluxes and cathodic protection for preventing the oxidation of titanium at elevated temperatures, Metall. Mater. Trans. B, 45(2014), No. 6, p. 2145. doi: 10.1007/s11663-014-0134-8
    C. Schwandt and D.J. Fray, Determination of the kinetic pathway in the electrochemical reduction of titanium dioxide in molten calcium chloride, Electrochim. Acta, 51(2005), No. 1, p. 66. doi: 10.1016/j.electacta.2005.03.048
    K. Jiang, X.H. Hu, M. Ma, D.H. Wang, G.H. Qiu, X.B. Jin, and G.Z. Chen, “Perovskitization”-assisted electrochemical reduction of solid TiO2 in molten CaCl2, Angew. Chem. Int. Ed., 45(2006), No. 3, p. 428. doi: 10.1002/anie.200502318
    E. Gordo, G.Z. Chen, and D.J. Fray, Toward optimisation of electrolytic reduction of solid chromium oxide to chromium powder in molten chloride salts, Electrochim. Acta, 49(2004), No. 13, p. 2195. doi: 10.1016/j.electacta.2003.12.045
    T. Wu, X.B. Jin, W. Xiao, C. Liu, D.H. Wang, and G.Z. Chen, Computer-aided control of electrolysis of solid Nb2O5 in molten CaCl2, Phys. Chem. Chem. Phys., 10(2008), No. 13, p. 1809. doi: 10.1039/b719369f
    G.Z. Chen, E. Gordo, and D.J. Fray, Direct electrolytic preparation of chromium powder, Metall. Mater. Trans. B, 35(2004), No. 2, p. 223. doi: 10.1007/s11663-004-0024-6
    Y. Deng, D.H. Wang, W. Xiao, X.B. Jin, X.H. Hu, and G.Z. Chen, Electrochemistry at conductor/insulator/electrolyte three-phase interlines: A thin layer model, J. Phys. Chem. B, 109(2005), No. 29, p. 14043. doi: 10.1021/jp044604r
    W. Xiao, X.B. Jin, Y. Deng, D.H. Wang, and G.Z. Chen, Three-phase interlines electrochemically driven into insulator compounds: A penetration model and its verification by electroreduction of solid AgCl, Chem. Eur. J., 13(2007), No. 2, p. 604. doi: 10.1002/chem.200600172
    W. Li, X.B. Jin, F.L. Huang, and G.Z. Chen, 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., 49(2010), No. 18, p. 3203. doi: 10.1002/anie.200906833
    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.
    G.Z. Chen and D.J. Fray, A morphological study of the FFC chromium and titanium powders, Miner. Process. Extr. Metall., 115(2006), No. 1, p. 49. doi: 10.1179/174328506X91365
    A.J. Muir Wood, R.C. Copcutt, G.Z. Chen, and D.J. Fray, Electrochemical fabrication of nickel manganese gallium alloy powder, Adv. Eng. Mater., 5(2003), No. 9, p. 650. doi: 10.1002/adem.200300369
    D.H. Wang, G.H. Qiu, X.B. Jin, X.H. Hu, and G.Z. Chen, Electrochemical metallisation of solid terbium oxide, Angew. Chem. Int. Ed., 45(2006), No. 15, p. 2384. doi: 10.1002/anie.200503571
    G.H. Qiu, D.H. Wang, M. Ma, X.B. Jin, and G.Z. Chen, Electrolytic synthesis of TbFe2 from Tb4O7 and Fe2O3 powders in molten CaCl2, J. Electroanal. Chem., 589(2006), No. 1, p. 139. doi: 10.1016/j.jelechem.2006.02.002
    P.C. Pistorius and D.J. Fray, Formation of silicon by electrodeoxidation, and implications for titanium metal production, J. South Afr. Inst. Min. Metall., 106(2006), No. 1, p. 31.
    Y.K. Delimarskii, O.V. Gorodiskii, and V.F. Grishchenko, Cathode liberation of carbon from molten carbonates, Dokl. Akad. Nauk SSSR, 159(1964), No. 3, p. 650.
    M.D. Ingram, B. Baron, and G.J. Janz, The electrolytic deposition of carbon from fused carbonates, Electrochim. Acta, 11(1966), No. 11, p. 1629. doi: 10.1016/0013-4686(66)80076-2
    H. Kawamura and Y. Ito, Electrodeposition of cohesive carbon films on aluminum in a LiCl–KCl–K2CO3 melt, J. Appl. Electrochem., 30(2000), No. 5, p. 571. doi: 10.1023/A:1003927100308
    L. Massot, P. Chamelot, F. Bouyer, and P. Taxil, Studies of carbon nucleation phenomena in molten alkaline fluoride media, Electrochim. Acta, 48(2003), No. 5, p. 465. doi: 10.1016/S0013-4686(02)00646-1
    H.V. Ijije, R.C. Lawrence, N.J. Siambun, S. Jeong, D.A. Jewell, D. Hu, and G.Z. Chen, Electro-deposition and re-oxidation of carbon in carbonate containing molten salts, Faraday Discuss., 172(2014), p. 105. doi: 10.1039/C4FD00046C
    W. Li, Studies of New Mode and Mechanism for Electrolysis of Solid Oxides in Molten Salts [Dissertation], Wuhan University, 2010.
    K. Dring, Direct electrochemical production of titanium, [in] The 3rd Workshop on Reactive Metal Processing (RMW3), Cambridge, 2007 [2020-07-09]
    G.Z. Chen and D.J. Fray, Prevention of Unwanted Reactions at Three Phase Boundaries, UK Patent, Appl. GB0329541.7, 2003.
    A. Stevenson, Development of a Novel Electrochemical Pyroprocessing Methodology for Spent Nuclear Fuels [Dissertation], University of Nottingham, Nottingham, 2016.
    M.L. Hu, Z.F. Qu, C.G. Bai, D. Hu, and G.Z. Chen, Effect of the changed electrolytic cell on the current efficiency in FFC Cambridge Process, Mater. Trans., 58(2017), No. 3, p. 322. doi: 10.2320/matertrans.MK201623
    W. Xiao, X.B. Jin, Y. Deng, D.H. Wang, and G.Z. Chen, 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., 639(2010), No. 1-2, p. 130. doi: 10.1016/j.jelechem.2009.12.001
    X. Yang, K. Yasuda, T. Nohira, R. Hagiwara, and T. Homma, Cathodic potential dependence of electrochemical reduction of SiO2 granules in molten CaCl2, Metall. Mater. Trans. E, 3(2016), No. 3, p. 145. doi: 10.1007/s40553-016-0081-1
    E. Juzeliūnas and D.J. Fray, Silicon electrochemistry in molten salts, Chem. Rev., 120(2020), No. 3, p. 1690. doi: 10.1021/acs.chemrev.9b00428
    P. Gao, X.B. Jin, D.H. Wang, X.H. Hu, and G.Z. Chen, A quartz sealed Ag/AgCl reference electrode for CaCl2 based molten salts, J. Electroanal. Chem., 579(2005), No. 2, p. 321. doi: 10.1016/j.jelechem.2005.03.004
    H. Wang, N.J. Siambun, L.P. Yu, and G.Z. Chen, A robust alumina membrane reference electrode for high temperature molten salts, J. Electrochem. Soc., 159(2012), No. 9, p. H740. doi: 10.1149/2.033209jes
    N.K. Al-Shara, F. Sher, A. Yaqoob, and G.Z. Chen, 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, 44(2020), No. 50, p. 27224. doi: 10.1016/j.ijhydene.2019.08.248
    H.W. Xie, H. Zhang, Y.C. Zhai, J.X. Wang, and C.D. Li, Al preparation from solid Al2O3 by direct electrochemical deoxidation in molten CaCl2–NaCl at 550°C, J. Mater. Sci. Technol., 25(2009), No. 4, p. 459.
    X.Y. Yan and D.J. Fray, Direct electrolytic reduction of solid alumina using molten calcium chloride-alkali chloride electrolytes, J. App. Electrochem., 39(2009), No. 8, p. 1349. doi: 10.1007/s10800-009-9808-3
    H. Kadowaki, Y. Katasho, K. Yasuda, and T. Nohira, Electrolytic reduction of solid Al2O3 to liquid Al in molten CaCl2, J. Electrochem. Soc., 165(2018), No. 2, p. D83. doi: 10.1149/2.1191802jes
    R.J.H. Clark, D.C. Bradley, and P. Thornton, The Chemistry of Titanium, Zirconium and Hafnium, Pergamon Press, Oxford, 1973, p. 367.
    Y. Zhu, D.H. Wang, M. Ma, X.H. Hu, X.B. Jin, and G.Z. Chen, More affordable electrolytic LaNi5-type hydrogen storage powders, Chem. Commum., 2007, No. 24, p. 2515. doi: 10.1039/b701770g
    G.Z. Chen, D.J. Fray, and T.W. Farthing, Cathodic deoxygenation of the alpha-case on titanium and alloys in molten calcium chloride, Metall. Mater. Trans. B, 32(2001), No. 6, p. 1041. doi: 10.1007/s11663-001-0093-8
    D.J. Fray, Novel methods for the production of titanium, Int. Mater. Rev., 53(2008), No. 6, p. 317. doi: 10.1179/174328008X324594
    D.T.L. Alexander, C. Schwandt, and D.J. Fray, Microstructural kinetics of phase transformations during electrochemical reduction of titanium dioxide in molten calcium chloride, Acta Mater., 54(2006), No. 11, p. 2933. doi: 10.1016/j.actamat.2006.02.049
    D. Sri Maha Vishnu, N. Sanil, L. Shakila, G. Panneerselvam, R. Sudha, K.S. Mohandas, and K. Nagarajan, A study of the reaction pathways during electrochemical reduction of dense Nb2O5 pellets in molten CaCl2 medium, Electrochim. Acta, 100(2013), p. 51. doi: 10.1016/j.electacta.2013.03.135
    E.Y. Choi, J.W. Lee, J.J. Park, J.M. Hur, J.K. Kim, K.Y. Jung, and S.M. Jeong, Electrochemical reduction behavior of a highly porous SIMFUEL particle in a LiCl molten salt, Chem. Eng. J., 207-208(2012), p. 514. doi: 10.1016/j.cej.2012.06.161
    A. Stevenson, D. Hu, and G.Z. Chen, Molten salt assisted electrochemical separation of spent fuel surrogates by partial direct reduction and selective anodic dissolution, ECS Trans., 64(2014), No. 4, p. 333. doi: 10.1149/06404.0333ecst
    K.S. Mohandas, Direct electrochemical conversion of metal oxides to metal by molten salt electrolysis: A review, Miner. Process. Extra. Metall, 122(2013), No. 4, p. 195. doi: 10.1179/0371955313Z.00000000069
    C. Schwandt, G.R. Doughty, and D.J. Fray, The FFC-cambridge process for titanium metal winning, Key Eng. Mater., 436(2010), p. 13. doi: 10.4028/
    D. Hu, A. Dolganov, M.C. Ma, B. Bhattacharya, M. Bishop, and G.Z. Chen, Development of the fray-farthing-chen cambridge process: Towards the sustainable production of titanium and its alloys, JOM, 70(2018), No. 2, p. 129. doi: 10.1007/s11837-017-2664-4
    Z.L. Yu, N. Wang, S. Fang, X.P. Qi, Z.F. Gao, J.Y. Yang, and S.G. Lu, Pilot-plant production of high-performance silicon nanowires by molten salt electrolysis of silica, Ind. Eng. Chem. Res., 59(2020), No. 1, p. 1. doi: 10.1021/acs.iecr.9b04430
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