Direct Deoxidation of Ti by Mg in MgCl₂–HoCl₃ Flux

Abstract

A new method for removing the dissolved oxygen (O) in titanium (Ti) is developed, wherein magnesium chlorideholmium chloride (MgCl 2 HoCl 3) and Mg are used as a flux and a reducing agent, respectively. Through the thermodynamic assessment using a p O2 p Cl2 diagram as well as the experimental results, the deoxidation of Ti to a level below 1000 mass ppm O (and even 500 mass ppm O) via the formation reaction of holmium oxychloride (HoOCl), O (in Ti) + Mg + HoCl 3 ¼ HoOCl + MgCl 2 , was confirmed. The deoxidation limit decreases with the increase of the activity of HoCl 3 in the MgCl 2 HoCl 3 flux. One advantage of this method is that the activity of the deoxidized product, a MgO , in the system can be effectively maintained at a low level by the formation of HoOCl. The EpO 2¹ diagram of the MOCl system (M = Ho, Mg) constructed in this study indicates that the electrochemical deoxidation of Ti scraps in MgCl 2 HoCl 3 system will be more effective because the a MgO can be further decreased via the formation of HoOCl, and/or the electrochemical oxidation of oxide ions on the carbon anode. This new deoxidation technique using rare-earth-containing MgCl 2 flux can be applied to the recycling of Ti scraps in the future.

Electrochemical Deoxidation of Titanium in Molten MgCl2-HoCl3

“…The O concentration in β-Ti cannot be reduced to less than 19300 mass ppm O by Mg at 1173 K (aMg = 1, aMgO (s) = 1) [14] . However, the deoxidation limit can be lowered if the activity of oxide ion, a O 2 (or aMgO), in the system can be reduced.…”

“…Each 0.1 g of Ti sample was melted with approximately 1 g of high-purity nickel basket (Ni flux 502-344, LECO Corporation) containing 4 ± 2 μg O in order to evenly extract the O. A standard Ti sample containing 980 ± 40 mass ppm O (Ti pin 501-657, LECO Corporation) was used for calibration in the O analysis [14,22] .…”

“…Deoxidation of Ti using Mg as a deoxidant is generally believed to be impossible because of its weak deoxidation ability. The equilibrium O concentration under the Mg/MgO equilibrium is approximately 1.9 mass% at 1173 K [14] . For reference, as shown in Fig.…”

“…For reference, as shown in Fig. 1, the equilibrium O concentration of Ti under the Ca/CaO equilibrium at 1173 K is plotted as a filled circle (approximately 0.03 mass% O) [14] . This shows that it is thermodynamically difficult to remove O directly from Ti to levels below 1 mass% O by Mg deoxidation when the activity of MgO (s) (aMgO (s)) is high.…”

“…Thus, a new method using Mg as a deoxidant in molten MgCl2-HoCl3 was developed, in which the aMgO was decreased and maintained at a low level via the formation of holmium oxychloride (HoOCl). The O content in Ti was effectively reduced to a level of approximately 1700 mass ppm O by Mg when deoxidation was conducted in MgCl2-20 mol% HoCl3 molten salt [14] . More recently, a new deoxidation method that uses a combination of the formation of HoOCl and the electrochemical deoxidation technique was developed to further decrease the O concentration of Ti [22] .…”