Electrochemical Deoxidation of Titanium Scrap in MgCl₂-HoCl₃ System

Abstract

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.

Electrochemical Deoxidation of Titanium in Molten MgCl2-HoCl3

“…Cl2 gas evolution occurs at the C anode according to Eq. (5) in parallel with COx effusion when the applied voltages higher than 2.4 V [22] .…”

“…When Ti undergoes cathodic polarization with a C electrode, metallic Mg precipitates on the Ti cathode according to Eq. (1) in the molten MgCl2HoCl3 during electrolysis and works as a deoxidant because the deposition potential of Mg is higher than that of Ho [22] .…”

“…The concentration of O 2 in the molten salt, or a O 2 in the system, can be further decreased by electrochemical oxidation on the C anode, which forms COx gas when the applied voltage at the electrodes is higher than 1.6 V according to Eq. (4) [22] .…”

“…Approximately 50 g of silver shot was placed in the chamber to absorb the Mg generated during the displacement reaction and during electrolysis. Approximately 20 g of Ti sponge was also placed in the chamber as a getter to absorb gaseous impurities [22] . The molten salt with a composition of MgCl220 mol% HoCl3 was produced by the following reaction: Ho (s) + 3/2…”

“…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] .…”