2019 Fiscal Year Annual Research Report
An innovative method of production of metallic Ti powder via titanium oxycarbonitride and titanium sulfide synthesized from ilmenite
| Project/Area Number |
18F18054
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| Research Institution | Hokkaido University |
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Principal Investigator |
鈴木 亮輔 北海道大学, 工学研究院, 教授 (80179275)
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| Co-Investigator(Kenkyū-buntansha) |
AHMADI ELTEFAT 北海道大学, 工学研究院, 外国人特別研究員
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| Project Period (FY) |
2018-10-12 – 2021-03-31
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| Keywords | Titanium metal powder / sulfides / oxycarbonitride / ilmenite / molten salt electrolysis |
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| Outline of Annual Research Achievements |
A new processing concept for production of Ti metal powder via TiSx (1<x<2) and TiN from ilmenite (FeTiO3) has been proposed. TiN or TiOxCyNz was synthesized by carbothermal reduction and nitridation (CTRN) of FeTiO3.
The preparation of clean TiS2 from TiN plays a key role in production of high-purity Ti powders. Either CS2 or S2 gas could be used for synthesis of titanium sulfides from TiOxCyNz or FeTiO3. The sulfidation results showed that CS2 gas stream leads to contamination of the samples. Hence, a dual-zone furnace was manufactured to gradually evaporate the sulfur and to control the partial pressure of S2 gas in the experiments. The carbon contamination was remarkably suppressed using S2 instead of CS2 gas.
TiN was perfectly transformed to the single phase of Ti2.45S4 and TiS2 at 1473 K for 3.6 and 10.8 ks, respectively. Nitrogen contents of the sulfides were also reduced to <0.005 mass% N when the samples were sulfurized at 1473 K for 10.8 ks. Then, the sulfides were reduced to α-Ti metal powders by the electrochemical OS process in molten CaCl2 with a small addition of CaS. The carbon impurity decreased to 0.01 mass% C for the sulfide and <0.1 mass% C for the Ti powder samples.
Sulfidation of TiC was also studied and pure TiS2 samples with carbon content of <0.025 mass% C were prepared from TiC at 1473 K for 18 ks, which is applicable for producing Ti powder. We successfully produced α-Ti powders with spherical morphology consisted of the foil-like Ti sheets with < 0.15 wt% O via TiS2 from TiN and TiC by the electrochemical reduction in molten CaCl2-0.5 mol% CaS.
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
The sulfidation of TiN and TiC powders to TiS2 powders has been completed. The process parameters were determined with the focus on removing the impurities such as C, S, O and N in the sulfides and Ti powders via sulfidation and electrochemical reduction in molten CaCl2 salt. Currently, TiC and TiO powders are being sulfurized using S2 gas to reduce the oxygen and carbon contents of the samples which enables us to synthesis the clean sulfides with only trace amounts of oxygen from Ti(O,C,N) prepared from natural Ti ore or FeTiO3. In other words, the path is being opened to treat the samples from FeTiO3. The sulfides from Ti(O,C,N) are reduced by varying the conditions such as CaS mol% and supplied charge (Q/Q0) in CaCl2-CaS melts for future industrialization.
We have also found that TaN or TaC can be promising starting materials for capacitor grade Ta powder production. Ta powder without residual C or N is obtained via sulfidation and reduction by OS process. Therefore, Ta powder production is also being studied via the electro-calciothermic reduction of the metal sulfide, TaS2 in molten CaCl2. The effect of sulfidation and reduction parameters are investigated to produce Ta powder for capacitor applications. The sulfidation thermodynamics of TaN, TaC and Ta2O5 to TaS2 are studied using S2 gas. The sulfides are also being reduced to Ta powders and the achievements have been submitted as a patent to the japan patent office (JPO).
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| Strategy for Future Research Activity |
The process will be expanded for refractory metals production via molten salt electrolysis of the sulfides. The sulfides could be prepared either from the sulfidation of the carbides and nitrides or carbo-sulfidation of the oxides.
In this work, TaS2 is prepared by the high-efficiency carbo-sulfidation of tantalum oxide (Ta2O5) by S2 gas in a dual-zone furnace. The sulfidation process is optimized and the sulfides synthesized from carbides, nitrides or oxides will be electrochemically reduced to Ta metal powder by in-situ generated Ca from the decomposition of CaS in the electrolyte.
For preparation of the sulfide, Ta2O5 is mixed with graphite at different molar ratios of C to Ta2O5 and carbo-sulfurized at various conditions using S2 gas. Alternatively, TaC and TaN powders are sulfurized by S2 gas, and the sulfides are finally reduced to high-purity Ta metal powder for capacitor applications. Thermodynamics of the reactions will be studied in formation of the sulfides. Conversion of Ta2O5 to TaN by carbothermal reduction and nitridation (CTRN) in an N2 atmosphere using carbon is theoretically favorable. Therefore, TaN is also synthesized by nitridation and will be used for high-purity TaS2 synthesis using S2 gas.
The conversion of niobium oxides (Nb2O5) to NbS2 can be also carried out by sulfidation of Nb2O5 or NbN under steam of CS2 and S2 gases. Therefore, in addition to the sulfidation of the oxides by CS2, we will propose a sequential research through CTRN and sulfidation using CS2, S2 gases and conversion of the sulfides to these value-affordable metals.
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