2023 Volume 64 Issue 12 Pages 2792-2800
Recently, manufacturing of metallic titanium ingots based on a thermal decomposition process using titanium disulfide TiS2 as the intermediate product with a significantly lower decomposition temperature of approximately 4000 K was reported. However, this process involves a carbothermic reaction for preparing the intermediate product. Carbonic acid gas, a cause of global warming, was generated, and the process is inadequate from the viewpoint of environmental protection. In the present study, a reaction process without the generation of carbonic acid gas was used. Dilute hydrogen-mixed argon gas was used for the synthesis of the intermediate product. The phases of the synthesized intermediate products were investigated using X-ray diffraction and were matched to several titanium sulfide phases, such as TiS, and Ti2S3, which have a higher titanium ratio than the product, TiS2, synthesized by carbothermic reaction. The thermodynamic conditions of the synthesis process, such as the partial pressures of oxygen and sulfur, were also predicted from the product phases.
Recently, Suzuki et al.1) summarized the reported laboratory-scale manufacturing processes of metallic titanium. Electrochemical methods in the presence of molten salt2–5) and metallurgical processes in the presence of active reactants6–8) were used for the production of titanium particles in these studies. We also reported a thermolysis (thermal decomposition) method to produce titnaium ingots9,10) as part of a metallurgical process. Thermal decomposition processes in a plasma atmosphere11–15) and an electron-beam remelting method16) have also been reported.
In our previous manufacturing process, titanium nitride (TiN) and titanium disulfide (TiS2), with comparatively low decomposition temperatures, were easily produced from TiO2 via a carbothermic reaction17) and were used as intermediates materials. Subsequently, the intermediates were heated to 4000–5000 K using an arc flame of conventional arc-melting equipment and was immediately decomposed to a metallic the titanium ingot. This is a reasonable technique for high-speed mass production because decomposition is determined by the heat-transfer rate independent of the reaction area.
The carbothermic reaction used to manufacture TiN and TiS2 generates carbon dioxide (CO2), a primary contributor to global warming.18,19) The use of hydrogen as an effective substitute for carbon to decrease CO2 emission in metallurgy showed differences in the quality, thermodynamic stability, and kinetic properties of the products in the ironmaking field.20–23) Ardani et al. assessed the efficiency of hydrogen gas in reducing titanium tetrachloride (TiCl4) to manufacture titanium powder.24) The extensive use of hydrogen is more reasonable than the widely used carbothermic reactions to decrease CO2 emission in metallurgy. Therefore, in the present study, the efficiency of using hydrogen in the manufacturing of titanium-based intermediates and their reduction was thermodynamically and experimentally investigated for the previous research, which is the production of TiS2 as an intermediate in the titanium manufacturing using the carbothermic process.10)
The present study used a dilute hydrogen gas. Since the partial pressure of oxygen is not determined by the hydrogen content in the atmosphere, it has to be estimated thermodynamically as a function of temperature. The precipitation phases of titanium hydride and hydride sulfide and their influence should be considered.
It is assumed that the atmospheric gas flow inside a reaction chamber is in equilibrium with the temperature in a phase relation. The phase relationship can be determined from the equilibrium reaction of hydrogen, sulfur, and oxygen remaining in the reaction chamber without argon, a nonreactive gas, considering that the atmospheric gas flow consists of argon, hydrogen, and sulfur gases. The ratio of atmospheric gases depending on the partial pressure of oxygen is described by the reaction H2 + 1/2O2 = H2O and is determined as a function of the temperature based on the literature.25) Under the coexisting atmosphere of H2, H2O and S2 gases, genarations of some oxidized and sulfdized gases such as H2S, H2SO4 and SO2 are expected and is also described by the reactions, H2O + 1/2S2 = H2S + 1/2O2, H2 + 2O2 + 1/2S2 = H2SO4 and 1/2S2 + 2H2O = SO2 + 2H2 for example. Because the ratio of H2/H2O is determined by temperature and partial pressure of oxygen, ratio of the gases are also determined by these reactios under the conditions. Figure 1 shows the ratio of these gases for oxygen and sulfur gas pressures controlled by the relations. As shown in Fig. 1, the relationship of the gas ratio indicates that (vertical column) a partial pressure of oxygen of less than 10−12 atm results in a stable H2 gas at 1600 K and above 10−10 atm results in stable steam (H2O). H2S is generated in low partial pressure region of oxygen ($\text{P}_{\text{O}_{2}}$ < 10−12). On the other hand, although less H2SO4 is only generated in the high partial pressure region of oxygen ($\text{P}_{\text{O}_{2}}$ > 10−1) and sulfur ($\text{P}_{\text{S}_{2}}$ > 10−3), it seems to have no significant influence on the reduction of Ti and Ti-based intermediates. SO2 is also negligible for the wide region for the partial pressure oxygen and sulfur. A comparison of the hydrogen-stabilized area in Fig. 1 indicates that Ti, titanium oxides in low oxidation states, and titanium sulfides occupy the hydrogen gas-filled area, as shown in Fig. 2 obtained in later. It can be concluded that the precipitated phases of titanium oxides and sulfides depend on the hydrogen gas ratio, except for the generation of complex phases such as SO2 and H2SO4 depending on the phase relation.
Dependence of the partial pressure of oxygen and sulfur for gas ratios for the reactions (a) H2O + 1/2S2 = H2S + 1/2O2, (b) H2 + 2O2 + 1/2S2 = H2SO4 and (c) 1/2S2 + 2H2O = SO2 + 2H2 at 1600 K, respectively.
Phase relations of Ti–H–O–S system at 1600 K for partial pressures of oxygen and sulfur.
Figure 2 shows the Ti-based phase relations equilibrated in an H2–H2O–H2S–S2 gas atmosphere at 1600 K. The reduction of titanium oxides can be expressed as eq. (2-1) for investigating the phase relationship.
| \begin{equation} \text{4TiO$_{2}$} + \text{H$_{2}$} \rightarrow \text{Ti$_{4}$O$_{7}$} + \text{H$_{2}$O} \end{equation} | (2-1) |
Here, the equilibrium constant Keq(2-1) of the reaction can be determined from the literature.25) The ratio of the partial pressures of H2 and steam (H2O) is also determined as a function of temperature. Similarly, sulfurization and its equilibrium constant can be expressed using eqs. (2-2) and (2-2)*
| \begin{equation} \text{TiO$_{2}$} + \text{H$_{2}$} + \text{S$_{2}$} \rightarrow \text{H$_{2}$S} + \text{TiS} + \text{O$_{2}$} \end{equation} | (2-2) |
| \begin{equation} K_{\text{eq (2-2)}} = (p_{\text{H}_{2}\text{S}}\cdot a_{\text{TiS}}\cdot p_{\text{O}_{2}})/(a_{\text{TiO}_{2}}\cdot p_{\text{H}_{2}}\cdot p_{\text{S}_{2}}) \end{equation} | (2-2)* |
Here, the equilibrium constant Keq(2-2) is determined from the literature.25) The partial pressures of H2, O2, S2, and H2S in the atmospheric gas composition in the reaction chamber are thermodynamically determined to be the ratios of H2/H2O as a function of temperature depending on reactions H2 + 1/2O2 = H2O. Although the stable structure of sulfur is S8, the sulfur vapor is conventionally used as S2 here for thermodynamic investigations. The equilibrium constant Keq can be estimated using eq. (2-2)* from the determined gas ratios. The activities of Ti, TiO, Ti2O3, Ti3O5, Ti4O7, TiO2, TiS, and TiS2 are treated as solids for the thermodynamic investigation and are interpreted as aX = 1 for pure substances. The melting temperatures of these oxides are high, and the thermodynamic data of hydrides and sulfides are insufficient for this definition. Hence, the estimation effect is limited for the investigated phase relation. The stability of the products determined by the relations can be judged from the difference in the chemical potentials (Δμ) of the reactions which are expressed using eq. (2-3) as the difference between the equilibrium constants of the experimental and standard values.
| \begin{equation} \varDelta \mu = RT\cdot \ln(K_{\text{app}}/K_{\text{eq}}) \end{equation} | (2-3) |
Here, Kapp means actual gas ratio of ($p_{\text{H}_{2}\text{S}}$ · aTiS · $p_{\text{O}_{2}}$)/($a_{\text{TiO}_{2}}$ · $p_{\text{H}_{2}}$ · $p_{\text{S}_{2}}$) from (2-2)*, and is regarded to be ($p_{\text{H}_{2}\text{S}}$ · $p_{\text{O}_{2}}$)/($p_{\text{H}_{2}}$ · $p_{\text{S}_{2}}$) when the TiO2 and TiS are treated as pure substance. Namely, the chemical potential means degree of misalignment for the equilibrium state, and boundary of these phases, i.e. TiO2/TiS, can be determined. Thus, the phase relations equilibrated with these gases are shown in Fig. 2. Mills26) reported the thermodynamic investigation of Ti-based stable phases, and this relation is indicated by thin lines. The thermodynamic data of Mills referred to an original study performed under an H2 atmosphere.27,28) Hence, it was necessary to consider the precipitation of Ti2S3 under an H2 atmosphere which is represented by a thin line. As the result, the difference between the phase relations in the previous study10) obtained by the carbosulfurization reaction and those in this study considering the influence of the H2–H2O–H2S–S2 gases [Fig. 2] can be negligible.
Figure 3 illustrates the experimental setup used for the synthesis of titanium sulfides under an Ar + 4 vol% H2 gas (dilute H2 gas) flow atmosphere. Since the TiO2 samples settled in the reaction boat allowed only the surface to react, parts of the samples were preformed to a granular shape with a diameter of 2 mm and a length of 5–10 mm. A regent rutile (TiO2) powder (>99.0 mass%, Wako Pure Chemical Industries, Ltd.) was used. The rutile powder was kneaded with an equal weight of distilled water, formed using a syringe to shape of the granular, and dried. The preformed granular-shaped rutile sample was placed in an alumina reaction boat as shown in Fig. 4(a) and inserted into an outer quartz tube. Granular and powder TiO2 of 0.80–1.50 g was used for each experiment. The reaction boat was also connected to an inner quartz tube containing a reagent sulfur powder (>99.0%, Kishida Chemical Co., Ltd.) of about 3.00 g for sulfurization experiments.
Schematic of experimental equipment for the synthesis process under Ar + 4 vol% H2 gas flow.
Photographs show the surface textures of (a) original, (b) deoxidized, and (c) sulfurized samples after deoxidization.
For deoxidization, the experiments were performed for several conditions, and the samples were heated within 1473–1593 K for 7.2–108 ks (2–30 h) under Ar + 4 vol% H2 gas at a flow rate of 0.00–3.33 mL s−1 (0–200 mL min−1) under an atmospheric pressure of 1 atm [Table 1(a)]. A sample synthesized under an air atmosphere was also prepared for comparison.
The samples were also heated to attain the same temperature for the same time under the same gas flow with sulfur gas for simultaneous oxidization and sulfurization [Table 1(b)]. The synthesized conditions of continuous sulfurization after deoxidizations are also summarized in Table 1(c). The deoxidized samples at 1593 K for 86.4 ks (24 h) under the gas at a flow rate of 3.33 mL s−1 (200 mL min−1) were continuously kept at the same temperature for 14.4 ks (4 h) under a sulfur gas supply at a dilute H2 gas flow rate of 0.00–0.67 mL s−1 (0–40 mL min−1) as a carrier gas. The sulfur powder in the inner quartz tube was heated stepwise from 373 to 823 K during the supply of evaporated sulfur gas to the reaction chamber. The supplying rate of sulfur gas was appropriately controlled by the heating temperature and rate experimentally. After that, the samples were rapidly cooled to room temperature (300 K). The cooling rates were measured to be 8.3 K s−1 (500 K min−1) at 1473 K and 5.3 K s−1 (320 K min−1) at 723 K.
The synthesized products were analytically investigated using an X-ray diffractometer (XRD) (Ultima III, Rigaku Co.) with Cu Kα radiation, and the change in weight before and after synthesis was measured.
Figures 5(a)–(d) show the XRD patterns of the deoxidized products of TiO2 obtained under a dilute hydrogen gas flow. Figure 5(a) shows the product Sp88 prepared under an air atmosphere as the reference. Figures 5(b)–(c) show sp97, sp116, and sp109 prepared using a dilute hydrogen gas mixture at a flow rate of 3.33 mL s−1 (200 mL min−1) for 14.4–108 ks, respectively. The analytically investigated phases using XRD and the gas flow rate with the synthesis time are summarized in Table 1(a). Although two temperature ranges of 1473 K and 1593 K were used, different temperature ranges were corrected among the experiments from the phases of the synthesized products.
X-ray diffraction patterns of the synthesized product: (a) 14.4 ks under an air flow and (b) 14.4 ks, (c) 86.4 ks, and (d) 108 ks under Ar + H2 gas flow at a flow rate of 3.33 mL s−1.
The XRD pattern of the oxidized sp88 is similar to that of TiO2 (rutile, PDF #00-021-1276) and does not structurally differ before and after synthesis. Thus, rutile–TiO2 is structurally stable at the investigated temperatures in an atmosphere of high partial pressure of oxygen, similar to previous research.10) The XRD pattern of sp88 can be regarded as that of the original phase before synthesis. The XRD patterns of sp97, sp116, and sp109 reveal strongly deoxidized products under the dilute H2 gas at a flow rate of 3.33 mL s−1. sp97 shows Ti4O7 (PDF #00-050-0787) and Ti3O5 (PDF #00-040-0806 and PDF #00-009-0309). sp116 shows Ti3O5 (PDF #00-009-0309, PDF #00-023-0606, PDF #01-082-1138, and PDF #00-076-1066) and Ti3O5 (PDF #00-009-0309) similar to sp97. sp109 shows the XRD patterns of Ti3O5 (PDF #00-023-0606, PDF #01-082-1138, PDF #00-011-0217) and Ti3O5 (PDF #00-023-0606, PDF #01-082-1138) similar to sp116. Although symbols indicating peak positions as reference patterns are showed in the diffraction patterns, some symbols shift from the peak top of the reference patterns. Because the diffraction peaks are shifted by influencing impurity elements or non-stoichiometric state, the shift can be interpreted as difference of stability of the products. The peak shifts of these products of sp109 are smaller than that of sp97 and sp116, and this means that the products reach stoichiometric and stable state by progression of the deoxidation. Thus, the XRD patterns of the strongly deoxidized product Ti3O5 reveal it as a stable phase in the stationary state at a given temperature under the dilute hydrogen gas flow rate.
4.2 Sulfurized products after deoxidizationFigures 6(a)–(c) show the XRD patterns of the products sulfurized for 14.4 ks (4 h) under the dilute H2 mixture gas at the flow rates of (a) 0.00 mL s−1 (sp119), (b) 0.33 mL s−1 (sp122), and (c) 0.67 mL s−1 (sp123) after deoxidization under the dilute H2 mixture gas flow rates of 3.33 mL s−1. Although the XRD patterns of the products synthesized with simultaneous oxidization and sulfurization are not indicated in this paper, the results are summarized in Table 1(b) with the experimental conditions. The XRD patterns of the sulfurized products after deoxidation are also indicated in Table 1(c) with the experimental conditions.
X-ray diffraction patterns of the sulfurized products under Ar + H2 gas flow at a flow rate of (a) 0.00 mL s−1, (b) 0.33 mL s−1, and (c) 0.67 mL s−1 after deoxidization for 86.4 ks under the gas flow at a flow rate of 3.33 mL s−1.
From the XRD patterns, for the simultaneous oxidization and sulfurization, some titanium-based oxides still regardless of the conditions. For the continuous sulfurization after the deoxidization, although sp119 shows the XRD patterns of Ti1.2S2 and TiS2 (PDF #00-041-0930, PDF #03-065-3372), the XRD pattern of TiO2 (rutile, PDF #00-021-1276) is also significant. Similarly, sp122 shows the XRD pattern of Ti1.2S2 (PDF #00-041-0930) and smaller peaks of TiO2 (rutile, PDF #00-021-1276) than that of sp119. The XRD pattern of sp123 shows that TiO2 (rutile, PDF #00-021-1276) peaks disappear with the appearance of Ti1.2S2, Ti7S12, Ti2S3, and TiS (PDF #00-041-0930, PDF #03-065-2120, PDF #01-089-1500, PDF #03-065-2119) peaks.
The parts of starting material (TiO2 powder) was preformed to a granular shape in the present study. Conversely, carbothermic sulfurization performed in a previous study10) used a mixture of graphite and TiO2 powders. The TiO2 powder was expected to react with graphite via the carbothermic sulfurization process. Therefore, the space to pass through the reaction gas is increased depending on reaction as shown by the reaction model in Fig. 7(a). The supplied sulfur gas passed through the space formed by the reaction of TiO2 and graphite and reacted with TiO2. Therefore, carbothermic sulfurization can be performed rapidly and effectively.
Schematics models of reacting images of the synthesized intermediates: (a) carbothermic reaction of the powdered sample mixed with graphite powder, (b) hydrogen reduction of the powdered sample, and (c) the hydrogen reaction of the granular-shaped sample.
Versus, because the hydrogen reduction process is not requiring the graphite to the sample, the space to pass through the reaction gas is not formed depending on the reaction. Moreover, because the space between particles of the powdered sample is small, the reaction gas is carried by gas-laminer film on the sample surface. The reduction of the powdered TiO2 occurs mainly on the surface, as shown by the model in Fig. 7(b).
Thus, it anticipated that the space for hydrogen and sulfur gases would be necessary to pass through for the reduction to progress. The reactivity of powdered sample depends on the shape of sample,29) to increase the sample surface, the original powdered TiO2 was converted to granular shape to create space for the gas to pass as shown in Fig. 7(c). The preformed sample homogeneously reacts to form titanium sulfides, eliminating titanium oxide.
5.2 Dependence of the gas flow rate and synthesis timeFigure 8 shows the relationship between the analytically investigated phases (titanium oxides) using XRD and reaction time. The phases show depending on the equilibrating partial pressure of oxygen at 1600 K, and the figure shows an estimated partial pressure of oxygen from the investigated phases. TiO2, the starting material, is stable at temperatures under atmospheric conditions. The partial pressure of oxygen in the synthesis of sp88 is $\log P_{\text{O}_{2}}$ ≈ −1 ($\text{P}_{\text{O}_{2}}$ = 0.2 atm), as shown in Fig. 8(a). For sp98 listed in Table 1(a), the phases of TiO2 and Ti4O7 coexist under the dilute hydrogen gas at the flow rate of 0.67 mL s−1. The partial pressure of oxygen in the synthesis condition of that can be expected to be above the equilibrium partial pressure of oxygen of Ti4O7/Ti3O5 with a value of $\log P_{\text{O}_{2}}$ = −14.4 ($\text{P}_{\text{O}_{2}}$ = 10−15 atm). sp111 synthesized after 108 ks shows similar phases as that of sp98. Because the deoxidation is not progressed after that, it is regarded that the partial pressure of oxygen is limited by the gas flow rate.
Relationship of the synthesized time of the products and the analytically investigated phases of titanium oxides using XRD.
Under the dilute H2 gas at a flow rate of 3.33 mL s−1, it is confirmed that phases existed Ti4O7 and Ti3O5 in the product synthesized for 14.4 ks, as shown in Fig. 5(b). The precipitated phases of sp91 synthesized for 7.2 ks are not essentially different. Therefore, it can be expected that the partial pressure of oxygen is between $\log P_{\text{O}_{2}}$ = −13.4(TiO2/Ti4O7) to −16.9(Ti3O5/Ti2O3). The phases are confirmed to be only Ti3O5 for the products synthesized for a prolonged time of 86.4 ks and 108 ks [Fig. 5(c)–(d)]. Thus, the partial pressure of oxygen can be interpreted as between $\log P_{\text{O}_{2}}$ = −14.4(Ti4O7/Ti3O5) and −16.9(Ti3O5/Ti2O3) and is also regarded as the limited level in the present condition.
5.3 Sulfurization of deoxidized productsIn this study, two types of sulfurization methods are used: (i) sulfurization and deoxidation performed simultaneously and (ii) sulfurization performed after deoxidation. The precipitated phases of the synthesized products through simultaneous sulfurization and deoxidization are the complex phases of titanium sulfides and titanium oxides. Moreover, titanium sulfides cannot be separated from titanium oxides, as presented in Table 1(b). Although TiO2 is reduced to titanium oxides, such as Ti4O7 and Ti3O5, with an increase in the dilute hydrogen gas flow rate, the formation and separation of titanium sulfides are not sufficiently performed under this condition. This is because the supply of sulfur gas is also diluted depending on the increasing gas flow rate.
The reaction process of sulfurization after deoxidization is shown in Fig. 9. Although products with complex phases, including titanium sulfides and titanium oxides, are confirmed for some of the conditions, products having only titanium sulfides are also confirmed, as presented in Table 1(c). The phases of the products are titanium oxides with low oxidation states when the synthesis condition has a prolonged deoxidization time with a high gas flow rate. The phases of sp116 and sp109 synthesized over a prolonged time are experimentally determined to be Ti3O5 [Table 1(a)]. Moreover, the phases of sp123 as shown in Fig. 9(a) are converted to titanium sulfides by suppressing the precipitation of titanium oxides during continuous sulfurization under an appropriate gas flow rate and the partial pressure of oxygen [Table 1(c)]. Although the product suppressing the precipitation of titanium oxides, such as sp123, coexists with TiS2, Ti2S3, and TiS, it might be influenced by hydrogen in the atmosphere.26–28) For sp119 as shown in Fig. 9(b), TiO2 is re-precipitated due to reoxidization caused by increasing the partial pressure of oxygen by stopping the supply of dilute hydrogen gas into the reaction chamber because TiO2 is eliminated once during deoxidation by the dilute hydrogen gas flow such as sp116. On the other hand, in the present study, although sulfurization of sp89, sp84, and sp81 is performed under high partial pressure of oxygen [Table 1(c)], TiO2 is not eliminated because of the sulfurization of TiO2 is not easy performed and requires higher partial pressure of sulfur than that of the lower titanium oxides such as Ti3O5.
Expected reaction passes by the deoxidization and sulfurization processes of the equilibrating phases of titanium oxides and sulfides in the Ti–H–O–S system.
From the sulfurization behaviors, it is necessary to deoxidize sufficiently them for a prolonged time to eliminate TiO2 by converting it to titanium oxides of lower oxidation states. For that, during sulfurization, it is also necessary to sulfurize under a lower partial pressure of oxygen to suppress reoxidation under the appropriate the dilute hydrogen gas flow rate. Moreover, because the partial pressure of oxygen is equilibrated by the atmosphere controlled by the dilute H2 gas flow and the titanium oxides, it is necessary to reduce sufficiently to the lower titanium oxides before the sulfurization. From the experimental results, the partial pressure of sulfur in the present condition seems to be $\log P_{\text{S}_{2}}$ = −6.4 to −4.3 (Ti3O5/TiS2) from the expected sulfurization process as shown in Fig. 9. However, it might be influenced depending by the supply of sulfur gas. The supplying rate of sulfur is only controlled by the heating rate and temperature of sulfur powder and carrier gas flow rate of the dilute H2 gas. This is because it is difficult to control the partial pressure under the supplying gas compared with the carbothermic sulfurization under the stopping of the carrier gas.10) Extensive research on this issue is necessary and will be addressed in future studies.
5.4 Comparison of the sulfurization processes under hydrogen and carbothermic reductionsTiO2 is precipitated in sp119 in the sulfurization process after deoxidization, as shown in Fig. 9(b). The product is reoxidized when it is not under the low partial pressure of oxygen. sp123, under the appropriate partial pressure of oxygen, when equilibrated with Ti3O5 during sulfurization, is sulfurized to TiS, Ti2S3, or TiS2 [Fig. 9(a)]. It is necessary to supply sulfur gas to increase the partial pressure of sulfur accompanied by an appropriate dilute hydrogen gas flow to avoid increasing the partial pressure of oxygen and produce titanium sulfides under a hydrogen reduction atmosphere. Conversely, it is sufficient to supply only sulfur gas even when the carrier gas flow is stopped to produce titanium sulfides under a carbothermic sulfurization atmosphere.10) The partial pressure of oxygen is thermodynamically maintained by the Boudouard reaction (C + CO2 = 2CO, CO2 = 1/2O2 + CO) in the carbothermic sulfurization atmosphere. Therefore, it is unnecessary to increase the partial pressure of oxygen. In this study, the Boudouard reaction does not occur. Hence, it is necessary to supply the dilute H2 gas into the reaction chamber to decrease the partial pressure of oxygen. The partial pressure of sulfur is decreased by the supplying gas. The phase relation of Ti–H–O–S system and Ti–C–O–S system10) is basically matched and characteristic phases for the Ti–H–O–S system are not confirmed as shown in Fig. 9. Thus, titanium sulfides are produced under a dilute sulfur gas atmosphere, similar to the process of carbothermic sulfurization in the previous study.
One issue in the previous study10) was the purity of products owing to carbon used in the carbothermic sulfurization process. Since it is not used in the present study, carbon contamination can be ignored. The remaining oxides in the titanium sulfides are influenced by the purity of thermally decomposed Ti as precipitated titanium oxides.10) These oxide phases can be suppressed from the product in the present study, and the issue of thermally decomposed Ti is ignored. Actually, although the oxygen concentration in the thermal decomposed titanium can be assumed from content of remaining titanium oxides in the sulfurized products or oxygen content in the reaction chamber using the thermal decomposition, because it is difficult to estimate that value theoretically, it is desirable to investigate experimentally. Extensive experimental research is necessary to reduce the reaction time and purify the products using this approach, which will be addressed in future studies.
In this study, the manufacturing process of titanium sulfides as intermediate materials was found to be suitable for the manufacturing of metallic Ti via thermal decomposition. The starting material TiO2 can be stably reduced to Ti3O5 under a typical dilute hydrogen gas flow rate of less than the explosive limit (4 vol% H2–Ar) at 1600 K, although the reduced phases depend on the gas flow rate. The partial pressure of oxygen is thermodynamically regarded as 10−17 to 10−15 atm at 1600 K from the equilibrated phase during the processes. Moreover, sulfurization after deoxidization under a dilute hydrogen gas flow changed the phases in the products to TiS and Ti2S3, apart from TiS2. The precipitation of titanium sulfides via sulfurization provides the partial pressure of sulfur thermodynamically as 10−6 to 10−4 atm at 1600 K at the partial pressure of oxygen equilibrated with Ti3O5. The partial pressure of sulfur under carbothermic conditions is almost 10−5 atm at the same temperature. Hence, these intermediate products are produced in a wide condition by supplying a dilute sulfur gas atmosphere under a hydrosulfurization atmosphere than under a carbosulfurization atmosphere.
This study was supported financially by the Mukai Science and Technology Foundation.
