Towards a sustainable technology for production of extra-pure Ti metal: Electrolysis of sulfurized Ti(C,N) in molten CaCl₂

Abstract

A new concept for producing highly pure Ti metal powder from ilmenite (FeTiO₃) is proposed in this article. Titanium nitride (TiN) or titanium oxycarbonitride (TiO _xC _yN _z) could be synthesized in the first step via the simultaneous carbothermal reduction and nitridation (CTRN) of FeTiO₃ to remove oxygen roughly. To separate oxygen completely, high-quality TiS₂ samples were then synthesized from TiN and TiC using S₂ gas, and the clean sulfides were finally reduced to α-Ti powders with spherical morphology using electrolysis in molten CaCl₂. X-ray diffraction (XRD), scanning electron microscopy (SEM) in conjunction with energy-dispersive X-ray spectroscopy (EDS), and elemental LECO analysis were used to study the phases and microstructures of the sulfides and the electrochemically reduced powders. The Ti powder showed no carbon contamination and consisted of high-purity foil-like Ti sheets with very low oxygen, carbon, and nitrogen contents of less than 0.15wt% O, 0.02wt% C, and 0.003wt% N, respectively. The quality of the Ti powder was much higher than that of the powder obtained using the conventional OS process (proposed by K. Ono and R.O. Suzuki) starting directly from the oxides.

Keywords

titanium / molten salt electrolysis / sulfidation / titanium oxycarbonitride / ilmenite

References

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

[2] Ono K, Suzuki RO. A new concept for producing Ti sponge: Calciothermic reduction. JOM, 2002, 54(2): 59.

[3] Suzuki RO. Calciothermic reduction of TiO₂ and in situ electrolysis of CaO in the molten CaCl₂. J. Phys. Chem. Solids, 2005, 66(2–4): 461.

[4] Suzuki RO. Direct reduction processes for titanium oxide in molten salt. JOM, 2007, 59(1): 68.

[5] Suzuki RO, Inoue S. Calciothermic reduction of titanium oxide in molten CaCl₂. Metall. Mater. Trans. B, 2003, 34(3): 277.

[6] Xie HW, Zhao HJ, Qu JK, Song QS, Ning ZQ, Yin HY. Thermodynamic considerations of screening halide molten-salt electrolytes for electrochemical reduction of solid oxides/sulfides. J Solid State Electrochem., 2019, 23(3): 903.

[7] R.O. Suzuki and K. Ono, A new concept of sponge titanium production by calciothermic reduction of titanium oxide in the molten CaCl₂, [in] C. Delong, R. W. Bradshaw, M. Matsunaga, G. R. Stafford, and P. C. Trulove, eds., Proceedings 13th International Symposium on Molten Salt, Philadelphia, 2002, p. 810.

[8] Natsui S, Sudo T, Shibuya R, Nogami H, Kikuchi T, Suzuki RO. Visualization of TiO₂ reduction behavior in molten salt electrolysis. Metall. Mater. Trans. B, 2020, 51(1): 11.

[9] Suzuki N, Tanaka M, Noguchi H, Natsui S, Kikuchi T, Suzuki RO. Reduction of TiS₂ by OS process in CaCl₂ melt. ECS Trans., 2016, 75(15): 507.

[10] Suzuki N, Tanaka M, Noguchi H, Natsui S, Kikuchi T, Suzuki RO. Calcium reduction of TiS₂ in CaCl₂ melt. Mater. Trans., 2017, 58(3): 367.

[11] Ahmadi E, Fauzi A, Hussin H, Baharun N, Ariffin KS, Rezan SA. Synthesis of titanium oxycarbonitride by carbothermal reduction and nitridation of ilmenite with recycling of polyethylene terephthalate (PET). Int. J. Miner. Metall. Mater., 2017, 24(4): 444.

[12] Ahmadi E, Yashima Y, Suzuki RO, Rezan SA. Formation of titanium sulfide from titanium oxycarbonitride by CS₂ gas. Metall. Mater. Trans. B, 2018, 49(4): 1808.

[13] Ahmadi E, Suzuki RO. An innovative process for production of Ti metal powder via TiS_x from TiN. Metall. Mater. Trans. B, 2020, 51(1): 140.

[14] Ahmadi E, Rezan SA, Baharun N, Ramakrishnan S, Fauzi A, Zhang GQ. Chlorination kinetics of titanium nitride for production of titanium tetrachloride from nitrided ilmenite. Metall. Mater. Trans. B, 2017, 48(5): 2354.

[15] Ahmadi E, Shoparwe NI, Ibrahim N, Sheikh Abdul Hamid SAR, Baharun N, Ariffin KS, Hussin H, Ahmad Fauzi MN., et al Davis BR, Moats MS, Wang SJ., et al The effects of experimental variables on iron removal from nitrided malaysian ilmenite by becher process. Extraction 2018, The Minerals, Metals & Materials Series, 2018, Cham, Springer, 1383.

[16] Suzuki RO, Ono K, Teranuma K. Calciothermic reduction of titanium oxide and in-situ electrolysis in molten CaCl₂. Metall. Mater. Trans. B, 2003, 34(3): 287.

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

[18] Sadrnezhaad SK, Ahmadi E, Malekzadeh M. Mechanism of reaction of molten NiTi with EBM graphite crucible. Mater. Sci. Technol., 2009, 25(6): 699.

[19] R.O. Suzuki, H. Noguchi, and M. Tanaka, Method for Producing Metallic Titanium, Japanese Patent, Appl. 6495142B2, 2019.

[20] Suzuki RO, Suzuki N, Yashima Y, Natsui S, Kikuchi T., et al Davis BR, Moats MS, Wang SJ., et al Calciothermic reduction and electrolysis of sulfides in CaCl₂ melt. Extraction 2018, The Minerals, Metals & Materials Series, 2018, Cham, Springer, 763.

[21] Basu SK, Taniguchi M. Thermal analysis and kinetics of oxidation of “TiS₂” and “Ti₂S₃”. Thermochim. Acta, 1986, 109(1): 253.

[22] T. Kimura, Molten salt synthesis of ceramic powders [in] C. Sikalidis, ed., Advances in Ceramics — Synthesis and Characterization, Processing and Specific Applications, Intech, 2011, p.75.

[23] Fang ZZ, Paramore JD, Sun P, Ravi Chandran KS, Zhang Y, Xia Y, Cao F, Koopman M, Free M. Powder metallurgy of titanium — past, present, and future. Int. Mater. Rev., 2018, 63(7): 407.

[24] Kanou O, Fukada N, Takenaka S. The use of HDH titanium alloy powder for additive manufacturing application. J. Jpn. Soc. Powder Powder Metall., 2017, 64(6): 295.