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

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  • Corresponding author:

    Eltefat Ahmadi    E-mail: eltahmadi@eng.hokudai.ac.jp

  • Received: 30 July 2020Revised: 4 August 2020Accepted: 6 August 2020Available online: 10 August 2020
  • A new concept for producing highly pure Ti metal powder from ilmenite (FeTiO3) is proposed in this article. Titanium nitride (TiN) or titanium oxycarbonitride (TiOxCyNz) 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.
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Towards a sustainable technology for production of extra-pure Ti metal: electrolysis of sulfurized Ti(C,N) in molten CaCl2

  • Corresponding author:

    Eltefat Ahmadi    E-mail: eltahmadi@eng.hokudai.ac.jp

  • 1. Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan
  • 2. Japan Society for the Promotion of Science (JSPS), Tokyo 102-0083, Japan

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 (TiOxCyNz) 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.

1.   Introduction
  • The electrochemical reduction of metal oxides in molten salts has opened a pathway for direct extraction of metals, particularly Ti metal through direct electrolysis of TiO2 in molten calcium chloride (CaCl2) [12]. Reduction in molten salts has been expanded as an effective way to alleviate the high energy consumption in high-temperature synthesis of titanium tetrachloride (TiCl4) and its conversion to sponge Ti by Mg in the Kroll process [35]. This approach also leads to replacing the energy intensive recycling of liquid Mg and gaseous Cl2 in the electrolysis of MgCl2. The CaCl2-based molten salts, including CaCl2–NaCl, CaCl2–KCl, CaCl2–LiCl, and CaCl2–MgCl2–NaCl, have been used as alternative electrolytes for reducing metal oxides or sulfides to metals or alloys [6] because of their high solubility for CaO [68], in which the dissolved Ca acts as a reductant of the oxides or sulfides. Nevertheless, the high solubility of oxygen of approximately 31.9at% O in α-Ti has limited the emerging and newly developed methods for obtaining high-quality Ti metal in molten salts [910]. It should also be noted that Mg and Ca, having strong affinities for oxygen and chlorine, have been mainly used for extracting reactive metals that cannot be readily produced via carbothermal reduction. The effects of operational factors and the choice of CaCl2-based molten salts on the efficiency of the process have been widely investigated to produce Ti from TiO2, but the electrochemical reduction of TiO2 by liquid Li in molten LiCl–KCl, which is operable at lower temperatures [8], did not work with sufficient efficiency to produce highly pure Ti metal from Ti ore, ilmenite (FeTiO3).

    On the other hand, TiN or TiOxCyNz can be sustainably synthesized via carbon reduction in N2 gas (carbothermal reduction and nitridation, CTRN) of FeTiO3 [1112]. The previous works on CTRN of FeTiO3 were conducted using a mixture of waste polyethylene terephthalate (PET) and coal as a reducing agent in an H2–N2 gas atmosphere, for example, at 1250°C for 3 h. The possible reactions to form TiN from FeTiO3 in N2 and H2–N2 atmospheres can be described as reactions (1)–(2) [11]:

    TiN is formed through a three–step mechanism in CTRN (${\rm{FeTi}}{{\rm{O}}_3}\xrightarrow{1}{\rm{Fe}} + {\rm{Ti}}{{\rm{O}}_2}\xrightarrow{2}{\rm{Fe}} + {\rm{T}}{{\rm{i}}_3}{{\rm{O}}_5}\xrightarrow{3}{\rm{Fe}} + {\rm{TiN}}$). The product Fe is oxidized in the presence of NH4Cl at 70°C and leached out in the form of Fe2O3·H2O from the nitrided sample by the Becher process. The TiOxCyNz synthesized via CTRN from FeTiO3 (characteristically called TiN) could be then applied for production of Ti metal via sulfidation and electrochemical reduction in molten salt [1213]. The reduction of iron oxide in FeTiO3 and separation of Fe by oxidation and leaching consume energy, but they are useful for preparing high-quality TiN. This process may result in a better separation of Fe particles from the matrix (i.e., TiN) in comparison with the current practice in industry for production of synthetic rutile (TiO2) [11,1415] used in the Kroll process. In the synthesis of TiO2, FeTiO3 is also carbothermally reduced to a mixture of Fe and TiO2 in the initial step. Then, the Fe content of the reduced product is removed by the oxidative leaching via the Becher process. The proposed concept enables us to remove Fe almost completely from TiN, which is impossible to achieve using the traditional methods. The iron oxide generated in this method may be used in the steel industries to avoid environmental contamination.

    Notably, TiN and TiC were also easily converted to TiCl4 at low temperatures of approximately 400°C [14], which is another attribute of the proposed process in the synthesis of TiOxCyNz from FeTiO3. In terms of energy consumption, the low-temperature chlorination of TiN may be compared with the carbo-chlorination of TiO2 at high temperatures of approximately 1000°C. In this process, TiS2 was prepared and reduced to Ti metal in only approximately 2–3 h in molten salt, whereas the Kroll process via TiCl4 may take 2–5 d for sponge Ti metal production and recycling of MgCl2 [14,16]. The proposed process consumes less energy even when the sulfidation step is compared to the Kroll process consisting of Mg reduction of TiCl4, MgCl2 electrolysis, and the difficulties in handling the hazardous chlorine gas. More importantly, the electro-calciothermic reduction in molten salt has the potential for continuous processing in contrast to the batch-wise Kroll process. Hence, the energy consumption in the proposed method for reducing sulfides in molten salts can be remarkably reduced when TiN is used for low-temperature chlorination to synthesize TiCl4 or for sulfidation.

    TiN is easily converted to TiSx (1 ≤ x ≤ 2) because of its strong affinity for S [23], and the exhaust gas from this reaction is mainly N2 gas, a non-greenhouse gas, demonstrating a merit of the present work. Needless to say, formation of a small amount of TiC in solid solution with TiN is inevitable because of the excess carbon used in CTRN. Thus, TiC should also be considered in sulfidation using S2 gas, but this approach has not been addressed in the literature. Previously, we reported the synthesis of TiN or TiOxCyNz from ilmenite (FeTiO3) [1112,14] and sulfidation of individual TiO, TiC, TiN, and TiO0.02C0.13N0.85 powders using CS2 gas [12]. Hence, in the present work, the preparation of TiN or TiOxCyNz from FeTiO3 was not examined. The sulfidation of pure TiN to TiS2 and its conversion to Ti metal has also been recently disclosed [13]. TiS2 can be also prepared from TiO2 via CS2 sulfidation [10] and was converted to Ti metal with a trace amount of sulfur because of the very slight solubility of sulfur (0.02at% S) in α-Ti [45]. Fig. 1 shows the various approaches for producing Ti metal from TiO2, TiS2, and TiN or FeTiO3 via TiS2.

    Figure 1.  Schematics of the approaches for producing Ti metal by molten salt electrolysis via the OS process (a), modified OS process (b), and this work (c) using TiN or FeTiO3.

    The formation of the intermediate phases, such as calcium titanite (CaTiO3), is one of the most important limitations of TiO2 reduction, acting as kinetic barriers and reducing the diffusion of ionized oxygen from the sample to molten salt. It also increases the carbon contamination of the product because of a slow reaction rate. The use of TiS2 can suppress the carbon contamination of the product, and S2 can be recycled from the electrolysis [13,17]. Nevertheless, sulfidation of TiO2 using CS2 may increase the carbon contamination of the product, and carbon has a detrimental effect on the mechanical properties of the Ti product [13,18]. TiN can be taken as an oxygen-free starting material, followed by sulfidation in a stream of S2 gas to prepare a suitable material for Ti metal production in molten salt, as shown in Fig. 1. In the recently patented electrochemical reduction of TiS2 to Ti metal, calcium sulfide or calcium oxide is added to the molten electrolyte to provide the calcium metal required for the reduction reaction [19]. This approach could be a great interest to the Ti community, especially when the feedstock is synthesized from TiN or FeTiO3 because of the lack of TiS2 natural resources. Suzuki et al. [910] reduced commercially available TiS2 to Ti metal using the calciothermic reduction and electrolysis in CaCl2–CaS melts. They showed that the sulfur concentration could be decreased to approximately 0.03wt% at the molar ratio of Ca/TiS2 = 2.0. In addition, the sulfur content of Ti powder was reported to be as low as approximately 0.01wt% when the electric charge was four times larger than the stoichiometric amount for reduction of TiS2 [10]. The Ti metal produced from the commercial grade TiS2 was, however, contaminated by approximately 1.0wt% O because of the high oxygen contamination of the commercial TiS2 powder.

    The authors also reported the synthesis of TiSx from FeTiO3 using the CS2 sulfidation method and electrochemical reduction of the sulfurized TiN samples in molten CaCl2 [1213,20]. However, the effect of the supplied charge in the electrochemical reduction of the sulfide prepared from TiN has not been discussed. In addition, the synthesis of TiS2 from TiC using S2 gas has not been investigated, although it has a great potential for production of clean titanium sulfides. The total reaction for the sulfidation of Ti(O,C,N) using S2 gas at 1200°C could then be expressed by the following reactions:

    The excess carbon formed from reaction (4) may act as a reductant in the sulfidation of TiO as written in reaction (5), or it can be simultaneously removed in the form of CS2 gas by the excess sulfur through reaction (6).

    Hence, the sulfidation reaction of TiO in the presence of TiC can be written as reaction (7):

    In this work, we have initially sulfurized TiC and TiN powders to clean TiSx (1 ≤ x ≤ 2) using S2 gas, and the intermediate sulfides were reduced to extra-pure Ti metal at the cathode. The electrochemical reduction of TiSx by the metallic Ca dissolved in the CaCl2 melt is expressed as follows [10,13]:

    The reduction of TiSx synthesized from Ti(C,N) could be a promising scenario for establishing a new sustainable process for Ti metal production from FeTiO3 via TiOxCyNz and TiS2; however, the carbon contamination of the product has not been well clarified in the literature. On the other hand, electrolysis of the sulfides is more favorable than that of the oxides, and this approach significantly reduces the energy consumption. Furthermore, sulfur is also cheap, nontoxic, readily available globally, and it can be applied to the synthesis of intermediate clean sulfides, while S2 gas plays an important role in reducing the oxygen content and particularly the carbon content of titanium metal obtained via the OS process (proposed by K. Ono and R.O. Suzuki). The purpose of this work is to prepare clean TiS2 from Ti(C,N) via CS2 and S2 sulfidations and investigate the effects of the supplied charge and molar ratio of CaS in molten CaCl2 on the production of industrial grades of Ti metal powders from TiN and TiC via TiS2. This goal can be considered a noteworthy achievement toward sustainably processing Ti metal from FeTiO3 via TiN and successive TiS2 electrolysis in molten salt.

2.   Experimental
  • Commercial TiN and TiC powders (99.5%, 50 μm, Wako Chemical Co., Japan) were sulfurized in a dual-zone furnace. The sulfur powder was evaporated at 300°C and used for the sulfidation of Ti (C,N) at 1200°C for 3 h. A high-purity Ar gas with a flow rate of 10 mL/min was used to transport S2 gas to the reaction site. In comparison, the sulfide samples were also prepared using CS2 sulfidation at 1200°C for 3 h. Details on the preparation of the sulfides and the experimental setup have been provided elsewhere [13]. In the synthesis of sulfides from TiN and TiC, CS2 or S2 gas was used for synthesis of TiSx. However, the samples prepared via CS2 may be slightly contaminated by free carbon, which must be clarified. Hence, the two sulfur sources were used to evaluate the possibility of clean sulfide production.

    The sulfides were electrochemically reduced at 900°C and 3.0 V via the OS process in 600 g of molten CaCl2 with the addition of 0.5mol% CaS and various supplied charges (Q/Q0) ranging from 200% to 600%, where Q is the total supplied electricity (Coulomb) and Q0 is the stoichiometric electric charge applied for the reduction of the samples. The experimental conditions were previously reported by the authors [13], and some outlines are briefly reported here. In addition to the supplied charge (Q), voltage, and current, the vacuum pressure and temperature of the electrolysis cell were precisely monitored using a data acquisition unit (Agilent 34972A Data Acquisition Switch Unit). The sulfide sample prepared from TiC was also reduced to Ti metal powder using this method at Q/Q0 = 400%. The molten salt was dehydrated at 900°C for 12 h in a vacuum (≤ 10 Pa). Then, the purification of the salt was conducted through pre-electrolysis at 2.0 V under an Ar atmosphere with a flow rate of 50 mL/min for 1 h using a supplementary carbon electrode prior to the electrolysis.

    The Ti powders obtained from electrolysis of the sulfides were carefully separated from the Ti net used as the cathode and washed three times with distilled water and dilute acetic acid. Eventually, the products were washed with water, ethanol, and acetone and dried in vacuum for a day. The chemical analyses and morphologies of the final products were investigated using X-ray diffraction (XRD, Philips, X'Pert Pro, Cu Kα) and scanning electron microscopy (SEM, JEOL JSM-6500F) in conjunction with energy-dispersive X-ray spectroscopy (EDS). The residual O, C, N, and S concentrations in the sulfides and the reduced samples were determined using LECO analyzers (TC600 and CS600).

3.   Results and discussion
  • The previous results showed that the oxygen content of TiOxCyNz samples prepared from FeTiO3 is negligible [11]. Additionally, the small amount of oxygen in the samples can be reduced in the molten salt. Hence, the oxycarbonitride obtained from CTRN could be considered TiCN. As a case in point, the TiO0.02C0.13N0.85 intermediate was synthesized from FeTiO3 and its sulfidation was also experimentally confirmed [12]. In this article, the sulfides were prepared through the sulfidation of TiN and TiC samples, and they were served for the electrochemical reduction to produce Ti metal. The S2 gas in the present work was generated through the evaporation of sulfur at 300°C for 3 h in the low temperature region of the designed furnace. The details on the experimental set up have been presented elsewhere [1213]. Notably, the sulfidation system may be modified on an industrial scale, and sulfur can be melted and purified in separate tanks to be continuously fed into the sulfidation furnace. The samples A, B, and C in Fig. 2(a) show the XRD patterns of the commercial TiN and the sulfides synthesized from TiN under the CS2 and S2 gases, respectively. TiN was transformed to the single phase of Ti2.45S4 (ICSD # 01-072-0820) at 1200°C for 3 h in a stream of CS2 gas with a flow rate of 20 mL/min. Meanwhile, the samples synthesized in a S2 gas atmosphere mainly contained the TiS2 (ICSD # 01-088-2479) phase with a small amount of nonstoichiometric Ti2.45S4 phase. The synthesis of the low carbon samples is required for the present work; hence, samples with high sulfur contents (i.e., TiS2) are more favorable for high-purity Ti metal production. The impurities concentration of these samples was determined using the LECO method, and the results are provided in Fig. 2(b). As shown, the sample prepared via CS2 sulfidation was slightly contaminated with carbon, and the residual nitrogen content of the sample was as high as approximately 0.1wt%. However, a very clean TiS2 sample was obtained using S2 gas instead of CS2. The carbon content of this sulfide prepared from TiN decreased to 0.01wt%, indicating a remarkable potential for preparation of clean sulfide from TiN.

    Figure 2.  XRD patterns (a) and LECO analyses (b) of pure TiN (A) and the sulfides prepared from TiN using CS2 (B) or S2 (C) gas.

    Carbon removal is a crucial issue in the synthesis of clean sulfides, particularly from Ti(C,N) as a starting material. Therefore, the sulfidation of TiC samples was also investigated separately, and the XRD and LECO analyses are shown in Fig. 3, where A, B, and C represent pure TiC, and the sulfides are prepared from TiC using CS2 or S2 gas, respectively. We found that the single phase of Ti2.45S4 could also be formed through sulfidation of TiC samples using CS2 gas at 1200°C for 3 h. However, the carbon content of the sulfide sample was remarkably high compared to that of the sample obtained from TiN in CS2 gas. The carbon impurity is derived from CS2 decomposition, or the product of the reaction between TiC and CS2 increased the carbon contamination of the sulfurized sample. The use of S2 instead of CS2 suppressed the carbon content of the sulfurized samples, which leads to a new approach for the synthesis of the sulfide. The XRD spectra also indicated the formation of mainly TiS2 containing a tiny amount of Ti2.45S4. Interestingly, the residual carbon in the sulfide was entirely removed as CS2 gas by the excess S2 gas, which was detected via the CS2 colorimetric tubes. In addition, TiS3, TiC, and free carbon were not observed in the XRD patterns.

    Figure 3.  XRD patterns (a) and LECO analyses (b) of pure TiC (A) and the sulfides prepared from TiC using CS2 (B) or S2 (C) gas.

    The excess amount of carbon was reduced to a very low concentration when TiC samples were sulfurized using S2 gas. As a result, the carbon contamination decreased to approximately 0.05wt% for the sample prepared at 1200°C for 3 h. It can be further decreased to approximately 0.024wt% by increasing the sulfidation time of the carbide sample to 5 h. It has been demonstrated that carbon removal in CS2 sulfidation of TiC is barely possible. In practical processes, TiCN or TiOxCyNz should be converted to clean TiS2 samples through sulfidation in a stream of S2 gas. The stoichiometry of the sulfide products synthesized from TiN or TiC using S2 gas is consistent with that of TiS2.

    The excess carbon in CS2 sulfidation of TiC samples is an important issue. In previous studies, we reported the formation of nanosized carbon fibers in the sulfide product obtained in a CS2 gas stream. However, we have shown that the sulfides prepared in the present work from TiN and TiC samples via S2 gas were extremely clean and without carbon contamination. The free carbon generated from the sulfidation reaction of the TiC sample was in situ reacted with excess sulfur and was removed as CS2 gas. Nevertheless, the off-gas in this approach mainly consists of N2 gas, which is a good feature of the designed process. In the case of TiC sulfidation or carbides recycling, the off-gas may contain CS2, which can be used for sulfidation of FeTiO3 to remove Fe as FeS [13,20]. Therefore, all the sulfide products in our designed process can be recycled in several steps without generation of sulfur waste. Apparently, CO gas is the main waste generated in CTRN, and Fe is recycled as a Fe2O3·H2O by-product [15]. The sulfide obtained from TiN is converted to extra-pure Ti metal, and S2 gas is recycled and used for synthesis of the sulfides. The total reaction can be summarized as

    where the reductant required for reaction (12) could be mainly supplied by recycling the waste PET plastic of the CTRN process.

    The SEM images of the sulfurized TiN and TiC are shown in Fig. 4. The well-crystallized hexagonal plates of titanium sulfides are obviously apparent in the products obtained from TiN or TiC powder using S2 gas. The sulfide products were entirely homogeneous plates with a mean size of approximately 5 μm. The EDS analysis showed that the S to Ti molar ratios were approximately 1.84 and 1.78 for the samples synthesized from TiN and TiC, respectively. These S/Ti ratios are inconsistent with the reported molar ratio of S/Ti = 1.97 for the TiS2 phase [21]. The slightly lower analytical values are probably due to the coexistence of Ti2.45S4 and TiS2 phases, although they could not be classified in the SEM images. The LECO analyses indicated that the sulfur contents of the samples synthesized using S2 gas were higher than those prepared using CS2 gas, which is consistent with the EDS analysis. For instance, the hexagonal-shaped particles obtained from TiN via CS2 gas at 1200°C for 3 h had an S to Ti molar ratio of approximately 1.71, which is lower than those determined for the samples synthesized using S2 gas.

    Figure 4.  SEM images of the sulfides synthesized from (a) TiN and (b) TiC using S2 gas.

  • The thermodynamic data (from HSC Ver. 8.08) predict that the calciothermic reduction of TiN and TiC in molten CaCl2 is barely possible. Hence, the high-purity titanium sulfides were initially prepared from TiN as an alternative to TiOxCyNz using S2 sulfidation at 1200°C for 3 h. The sulfide samples consisting of ≤ 0.003wt% N, 0.48wt% O, and ≤ 0.01wt% C were served as starting materials in the electrolysis. Fig. 5 shows the current per 1 g of the sulfide sample (TiS2) with respect to the supplied charge (Q/Q0). The current at the constant voltage of 3.0 V was substantial (3.5 A/g) in the initial stage of the electrolysis. Notably, the surface area of the anode immersed in the melt is estimated to be approximately 1020 mm2. The currents decreased moderately with some fluctuations up to approximately Q/Q0 = 100%, and approached constant values of approximately 1.0 A/g. Thereafter, the currents were constant during the electrolysis up to Q/Q0 = 600% in the second step. The applied voltages in the experiments were much higher than the theoretical decomposition voltage of CaS (2.01 V), where liquid Ca can be formed from CaS [10]. Hence, the calcium is dissolved in the molten CaCl2 and acts as a reducing agent. Notably, to avoid chlorine gas generation, the electrolysis voltage should not exceed 3.21 V, where the theoretical decomposition of CaCl2 occurs at 900°C.

    Figure 5.  Cathodic current per 1g of the TiS2 vs. supplied charge during electrolysis of the sulfurized TiN samples.

    The influence of the CaS molar fraction in the molten CaCl2 salt on the current efficiency of the reduction was examined. Slightly increasing the molar fraction of CaS in CaCl2 from 0.1mol% to 0.5mol% substantially improved the sulfur removal because of the increased uptake of Ca. The higher CaS concentration in CaCl2 melt as solvent results in a faster reduction. The oscillation of current can also be explained in terms of an excessive release of gaseous S2 or CS2 as bubbles on the carbon anode. The S2 gas is formed from decomposition of CaS at the carbon anode, and it can be used in the sulfidation of Ti(O,C,N) or FeTiO3.

    The samples reduced at 2Q0 (Run 1) showed α-Ti, Ti3O, and Ti6O. However, the products prepared at 4Q0 (Run 2) and 6Q0 (Run 3) were composed entirely of the single phase of α-Ti, and neither TiC nor Ti6O was identified in the products. Fig. 6 presents the XRD patterns of α-Ti metal obtained from TiS2 (Run 2). No oxides or carbides of titanium were observed in the products. A photograph (insert) of the α-Ti product consisting of high-purity Ti sheets is also shown in Fig. 6. The oxygen, carbon, nitrogen and sulfur contents of the Ti sheets, formed from sulfurized TiN, were respectively less than 0.15wt% O, 0.02wt% C, 0.003wt% N, and 0.005wt% S as determined by the LECO measurements.

    Figure 6.  XRD pattern and the product photograph (insert) of the α-Ti powder and Ti sheets obtained from the sulfurized TiN by S2 gas and electrochemical reduction in CaCl2−0.5mol%CaS.

    The residual concentrations of O, C, N, and S in the α-Ti products plotted against the supplied charge (Q/Q0) during electrolysis are presented in Fig. 7. The results demonstrate that the sulfide is mainly reduced by calcium in the first stage, whereas the residual oxygen is removed from the sample in the second step, particularly by increasing the supplied charge from 200% to 600%. This result is due to the relatively high concentration of Ca in the cathode vicinity, producing a rapid reduction of TiS2 at the beginning of electrolysis. The sample reduced for 200% of the supplied charged contained approximately 2.2wt% of unreacted sulfur and a large amount of the oxygen impurity. No carbon impurity was detected in the Ti metals, which could be due to the low contamination with oxygen of the sulfide samples and the carbon anode acting as a non-consumable electrode in the electrolysis. In addition, α-Ti metal consisting of 0.32 ± 0.3wt% O, ≤ 0.07wt% C, and ≤ 0.05wt% S was successfully produced from the sulfurized TiC in conditions similar to those applied for TiN. As a result, TiS2 was fully transformed to the single phase of α-Ti metal at Q ≥ 4Q0 without contamination in molten CaCl2–0.5mol%CaS, as confirmed via the XRD spectra and LECO analysis. In this process, half of the deposited Ca may be dissolved into molten CaCl2 because of the use of a basket-type cathode. The presence of a small amount of water impurity in CaCl2 is also inevitable and results in formation of CaO and finally decomposition to calcium in the molten bath. The generated calcium from decomposition of CaO in CaCl2 may also partially reduce the sulfide near the cathode. The electrolysis was mainly completed in two stages, as shown in Fig. 5. The initial stage (Q/Q0 < 100%) was quite efficient for reduction of the sulfides. However, the later stage of deoxidation from the residual oxygen in TiSx was slower, and an excess supplied charge of 3Q0 was applied to remove oxygen completely. Therefore, the current efficiency of the electrolysis can be estimated to be roughly 25% by considering a complete oxygen removal from the sulfurized sample.

    Figure 7.  Residual concentration of O, C, N, and S in the α-Ti powders vs. the supplied charge (Q/Q0) during electrolysis of the sulfurized TiN and TiC samples prepared using S2 gas.

    Fig. 7 also indicates that the powder samples prepared from the sulfides prepared from TiN or TiC by S2 sulfidation showed less oxygen and particularly carbon contamination (0.07wt% C) than the powder samples prepared from TiO2 by CS2 sulfurization or reduction of TiO2 in molten CaCl2 [910]. This result strongly indicates that the current efficiency is highly enhanced and CO/CO2 does not form in this improved OS process. Furthermore, the oxygen and nitrogen contents of the sample reduced at the supplied charge of 6Q0 were much lower than those of the sample reduced at 4Q0. They were analytically determined to be as low as 0.44wt% O and 0.003wt% N in the reduction of the TiSx prepared from TiN using S2 gas.

    The SEM images of the α-Ti powders obtained from sulfurized TiN and TiC are shown in Fig. 8. The α-Ti powders consisting of relatively spherical or interconnected, round particles with diameters ranging from 1 to 5 μm were formed through the electro-calciothermic reaction between TiS2 and dissolved calcium in molten salt. This result might be due to the higher diffusion rate of ions, strong dissolving capability of CaS in the molten salt, and the effect of the interfacial tension of the molten salt [22]. Powders obtained at Q ≥ 4Q0 also contained high-purity Ti foils (0.125wt% O). The low oxygen concentration in the product is well compatible with the defined international standards for high-purity Ti powder.

    Figure 8.  SEM image of the α-Ti powders obtained from electro-calciothermic reduction of (a) the sulfurized TiN at Q/Qo = 600% and (b) the sulfurized TiC at Q/Qo = 400% in molten CaCl2–0.5mol%CaS salt.

    The spherically shaped Ti powders from this work could have promising applications in additive manufacturing or 3D printing technology for manufacturing complicated, near-net-shape products [2324]. Therefore, TiN or TiOxCyNz has a great potential as a starting material for the production of commercial grade Ti metal powders using TiS2 via sustainable sulfidation and calciothermic reduction in molten CaCl2 salt.

4.   Conclusion
  • In summary, a single phase of Ti2.45S4 was formed from TiC and TiN using CS2 gas, whereas TiS2 consisting of small amounts of Ti2.45S4 was obtained under a S2 atmosphere at 1200°C for 3 h. The present process enables the synthesis of clean TiS2 from Ti(C,N) samples without carbon contamination of the products. We found that production of α-Ti powders with trace amounts of oxygen and especially carbon is attainable not only from TiN but also from TiC via TiS2 instead of TiO2. The electro-calciothermic reduction of the sulfides for 400% of the supplied charge in molten CaCl2–0.5mol%CaS indicated the formation of spherical α-Ti powders containing the foil-like Ti sheets from the sulfurized TiN and TiC samples. The extra-pure Ti sheets obtained from the sulfurized TiN and TiC samples contained less than 0.15wt% O, 0.02wt% C, 0.002wt% S, and 0.003wt% N and less than 0.125wt% O, 0.02wt% C, and 0.005wt% S, respectively. This process has, therefore, a great potential for commercialization and sustainable production of low-cost Ti metal powders from TiSx via TiN and TiC, possibly synthesized from FeTiO3.

Acknowledgements
  • This work is the financially supported by The Japan Society for the Promotion of Science (JSPS) Postdoctoral Fellowship (No. P18054), Grants-in-Aid for Scientific Research (KAKENHI) (Nos. 18F18054 and 17H03434), the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Nanotechnology Platform Program (No. A-19-HK-0034), and the Japan Mining Industry Association. The kind support from JSPS and the International affairs office of the Faculty of Engineering, Hokkaido University, are also gratefully acknowledged.

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