Oxygen removal from metallic Ti is extremely difficult and, currently, there is no commercial process for effectively deoxidizing Ti or its alloys. The oxygen concentration in Ti scraps is normally higher than that in virgin metals such as in Ti sponges produced by the Kroll process. When scraps are remelted with virgin metals for producing primary ingots of Ti or its alloys, the amount of scrap that can be used is limited owing to the accumulation of oxygen impurities. Future demands of an increase in Ti production and of mitigating environmental impacts require that the amount of scrap recycled as a feed material of Ti ingots should also increase. Therefore, it is important to develop methods for removing oxygen directly from Ti scraps. In this study, we evaluated the deoxidation limit for b-Ti using Y or light rare earth metals (La, Ce, Pr, or Nd) as a deoxidant. Thermodynamic considerations suggest that extra-low-oxygen Ti, with an oxygen concentration of 100 mass ppm or less can be obtained using a molten salt equilibrating with rare earth metals. The results presented herein also indicate that methods based on molten salt electrolysis for producing rare earth metals can be utilized for effectively and directly deoxidizing Ti scraps.

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A novel electrochemical deoxidation process for Ti in the magnesium chloride−holmium chloride (MgCl 2 −HoCl 3 ) system was developed. The reaction mechanism was proposed on the basis of the E-pO 2− diagram constructed in this study and the experimental results. Metallic Mg was electrodeposited on a Ti cathode to remove O in the form of O 2− from Ti via electrolysis conducted in molten MgCl 2 −HoCl 3 , with C and Ti as the anode and cathode, respectively. The activity of the generated O 2− (i.e., a O 2− ) in the system was effectively maintained at a very low value by the generation of holmium oxychloride (HoOCl) and/or the effusion of CO x gas from the anode. By applying a voltage ranging from 2.1 to 3.0 V between the cathode and anode at 1173 K, the O concentration in Ti was effectively deoxidized to below 1000 mass ppm O. The deoxidation capability of this process was maintained even after the addition of MgO into the system as an additional source of O 2− . This novel deoxidation method can be used to recycle large quantities of Ti scrap with high O concentration.

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