Thermodynamic Analysis of Deoxidation of Titanium Through the Formation of Rare-Earth Oxyfluorides

In this work, the possibility of the direct removal of oxygen species from metallic Ti through the formation of rare-earth oxyfluorides has been investigated from a thermodynamic viewpoint. The deoxidation limit of β-Ti using rare-earth metals (M: Y, La, Ce, and Nd) as deoxidants was evaluated. It was found that Ti metal with an oxygen concentration of 200 mass ppm or less could be theoretically obtained under the M/MOF/MF₃ equilibrium at 1300 K (1027 °C), which suggested a possibility of reducing the oxygen content in Ti below 500 mass ppm utilizing a fluoride-based molten salt. Furthermore, a new deoxidation process, in which oxygen was removed in the form of MOF compounds using Mg deoxidant, was discussed as well. The obtained results revealed that the oxygen content in β-Ti could be theoretically reduced to a level below 1000 mass ppm using a MF₃-containing molten salt equilibrated with Mg. Rare-earth metals and their alloys are usually produced by the electrolysis in a fluoride-based molten salt; hence, the modern industrial electrolysis techniques can be potentially utilized for deoxidizing Ti scrap.