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To produce high-purity Mg metal directly from MgO, the scale-up of a novel electrolytic process using a metal cathode and vacuum distillation was investigated. The electrolysis of MgO was conducted in MgF2–LiF molten salt using Cu or Ag cathode and graphite anode at 1053 K. The influence of the cathode metal and the concentration of Mg in the Mg alloy on the current efficiency was investigated. In addition, the interaction of Al2O3 in contact with the electrolyte and cathode was studied. When electrolysis was carried out with an applied current of 8.81–9.59 A for 75–95 h, Mg alloys such as Mg2Cu and MgAg were produced with a current efficiency of 82.2–88.0%. In addition, owing to the reaction between the Mg alloy and Al2O3, a dense layer of magnesium aluminum oxide was produced. Afterward, when the vacuum distillation of the Mg alloys was conducted at 1300 K, 99.9997% Mg metal was obtained.

Steel containers and instruments are used to hold and transport Mg melts. Thus, quantitative analysis of the dissolution of metallic elements from the steel materials into the liquid Mg is important for controlling the impurities in processes such as Mg alloy production and Ti smelting. When austenitic stainless steels containing Cr and Ni come in contact with a Mg or Mg alloy melt, a large amount of Ni and limited amounts of Fe and Cr dissolve into the melt. In this study, the composition of a Mg-Ni-based melt in equilibrium with SUS316 was investigated to determine the maximum amounts of impurities dissolved from SUS316 into the liquid Mg. Mg-Ni melts of different compositions were held in a closed crucible of SUS316 at 1073 K (800 °C) to 1273 K (1000 °C), and the compositions of the inner wall of the crucible and the Mg-Ni alloy were evaluated after quenching. Subsequently, the relationships between the solubility limit of element i (i: Ni, Fe, and Cr) in liquid Mg with the coexistence of SUS316, $$ C_{{{\text{sol}},{i}}}^{*} $$ (mass pct), and the temperature, T (K), were determined: $$ \log \,(C_{{{\text{sol}},{\text{Ni}}}}^{*} ) = {{ - 1.40 \times 10^{3} }}/{T} + 2.57 \, ( \pm \,0.12), $$ $$ \log \,(C_{{{\text{sol}},{\text{Fe}}}}^{*} ) = - {{5.20 \times 10^{3} }}/{T} + 3.93 \, ( \pm \,0.16),\;{\text{and}} $$ $$ \log \,(C_{{{\text{sol}},{\text{Cr}}}}^{*} ) = - {{5.96 \times 10^{3} }}/{T} + 3.80 \, ( \pm \,0.05). $$ These solubility limits were compared with those estimated based on the available thermodynamic data for the SUS316 and Mg-i binary systems. The validity of the obtained data and the reliability of the previously reported thermodynamic data were also discussed.

Titanium (Ti) is produced by a multi-step process from Ti ore composed of Ti oxide including chlorination process. However, the smelting cost is a major factor that leads to an increase in the cost of Ti. To lower the cost of Ti, the development of a new technique for the direct production of metallic Ti from an oxide feed material is highly desired. However, owing to the high affinity between Ti and oxygen (O) and the high solubility of O in Ti, it is extremely difficult to produce metallic Ti with low O concentration which meets the level required by the industry. In this study, we developed a new method for producing Ti with a low O concentration directly from TiO2 by using Mg as a reductant, assisted by a rare earth oxychloride formation reaction. The production of Ti with a low O concentration was demonstrated using a magnesiothermic reduction of TiO2 in MgCl2–LaCl3 molten salt via a reduction reaction involving LaOCl formation: (TiO2 (s) + 2 Mg (l) + 2 LaCl3 (l) = Ti (s) + 2 MgCl2 (l) + 2 LaOCl (s)). An electrochemical method that produces metallic Ti from TiO2 with no consumption of rare earth elements and Mg was designed based on the experiment results. This process is expected to replace the current smelting processes of Ti as well as numerous other rare metals produced from raw oxide materials and contribute significantly to a decrease in the price of rare metals.

A novel electrolytic process that uses a liquid–metal cathode was investigated to produce high-purity magnesium (Mg) metal using magnesium oxide (MgO). Electrolysis of MgO in a magnesium fluoride (MgF2)-lithium fluoride (LiF) molten salt was carried out with applied voltages of 2.5 V to 3.0 V at 1053 K to 1083 K using tin (Sn), silver (Ag), or copper (Cu) as the cathode and graphite or platinum (Pt) as the anode. After electrolysis, Mg alloys with Mg2Sn, AgMg, and Cu2Mg phases were produced with current efficiencies of 77.2 to 83.8 pct when the concentration of Mg in the Mg alloys was 11.9 to 12.9 mass pct. For the production of high-purity Mg metal directly from the Mg alloys, vacuum distillation was performed. When vacuum distillation was conducted at 1200 K to 1300 K, the concentration of Mg in the Mg alloys feed decreased from 30.2 to 34.1 mass pct to 0.32 to 1.75 mass pct, and Mg metal with a purity of 99.975 to 99.999 pct was obtained. Therefore, this study demonstrates that the production of high-purity Mg metal through an efficient and environmentally sound method using the electrolysis of MgO is feasible.

This paper presents a brief review of the history of titanium smelting and the current trends in related research and development. Presently, both electrolytic and metallothermic reduction processes utilizing various feed materials such as titanium oxide are widely studied. However, many challenges remain to be addressed before realizing the practical application of smelting processes utilizing oxide feed. To make titanium a “common metal”, a new reduction process that is high speed, energy-efficient, low cost, and of low environmental impact is required. The current status of titanium recycling is likewise outlined, and the development of the recycling process is discussed. Low-grade titanium scraps heavily contaminated by oxygen and iron are currently used for producing additive alloys (ferro-titanium) in the steel industry. In the near future, if the demand for titanium metal increases dramatically, there could be an oversupply of low-grade titanium scraps in the market. Therefore, the development of anti-contamination and efficient removal processes for oxygen and iron is essential for the efficient utilization of titanium. The development of these technologies is vital for expanding the titanium industry through innovation in both titanium smelting and recycling technologies.

The development of a new titanium (Ti) smelting process is highly desired to decrease the cost of Ti metal. In this study, a new method to produce a low-oxygen-concentration Ti directly from TiO2 using La as the reductant was developed. First, deoxidation experiments of metallic Ti using La as the deoxidant were conducted, and the ability of La to deoxidize metallic Ti was demonstrated. The oxygen concentrations in Ti were approximately 3900 ± 400 mass ppm O and 120 ± 50 mass ppm O at La/La2O3 and La/LaOCl/LaCl3 equilibria, respectively, at 1200 K (927 °C). Then, TiO2 reduction experiments utilizing La as the reductant were conducted to produce low-oxygen-concentration Ti directly. TiO2 pellets and Ti pieces were held in LaCl3 molten salt with a sufficient amount of La at 1200 K (927 °C) for 86 ks. After the reaction, the TiO2 pellets were reduced to metallic Ti without reaction inhibition by the formation of intermediate products. The oxygen concentration of the Ti pieces, which were placed in the molten salt with the TiO2 pellets, was 100 ± 50 mass ppm O. These results indicate that low-oxygen-concentration Ti containing less than 200 mass ppm O could be produced directly from TiO2 by utilizing La as the reductant in LaCl3 molten salt at 1200 K (927 °C). This new method is expected to lead to the development of a new industrial process for the production of ultra-low-oxygen-content Ti from Ti ore without a chlorination process.

Deoxidation methods of titanium (Ti) scrap and Ti powder have become increasingly important in recent years. Some rare earth (RE) metals with strong deoxidizing capabilities, including Y, La, Ce, and Ho, are candidate agents for the development of a new deoxidation technology. In this study, a new method was developed to directly remove oxygen (O) from Ti using the Y/YOCl/YCl3 equilibrium. According to the calculation based on available thermodynamic data in the literature, the O concentration in β-Ti can be reduced to less than 10 ppm O at 1300 K (1027 °C) in the Y/YOCl/YCl3 equilibrium. To demonstrate the effectiveness of this method using the Y/YOCl/YCl3 equilibrium, the deoxidation limits of Ti samples using Y metal in YCl3 (l) or in YCl3-NaCl-KCl (l) at 1300 K (1027 °C) were experimentally investigated in this study. As a result, the O concentrations in the Ti samples were from 30 to 60 ppm O in YCl3 (l). This result revealed that Ti with extremely low O concentration can be reliably obtained using the RE/REOCl/RECl3 equilibrium for the first time. The establishment of this process will realize efficient recycling of Ti scrap and production of low O concentration Ti powder, which contribute to the large-scale use of Ti products.

A new electrochemical deoxidation process for Ti, where a mixture of magnesium chloride and yttrium chloride (MgCl2–YCl3) is used as flux, was developed. In the new process, Ti and carbon were used as the cathode and anode, respectively. By elucidating the system using an E-pO2− diagram and the experimental results, the reaction mechanism was proposed. Mg is deposited on the Ti cathode and reduces the oxygen in Ti to oxide ions (O2−). The activity of the generated O2−, $$a_{\text O^{2-}}$$ , in the system is effectively kept at a low level by the formation of yttrium oxychloride, and is further decreased by the electrochemical oxidation reaction on the anode. The process effectively deoxidizes Ti to the level of 100 mass ppm of oxygen concentration at 1200 K. In addition, the oxygen concentration in the Ti sample was maintained at the level of 500 mass ppm O even with the addition of O2− sources. Furthermore, a new concept of the industrial Ti-recycling process based on this new deoxidation process was depicted. The obtained results indicate that this new deoxidation technique can be applied in the recycling process of Ti scrap containing a large amount of oxygen.

To reduce the oxygen level in titanium (Ti) to less than 1000 mass ppm O, using magnesium as the deoxidant at 1300 K (1027 °C), the activity of the deoxidation product (MgO), i.e., aMgO, in the system must be reduced to less than 0.04, from a thermodynamic viewpoint. In this study, we developed a new deoxidation technique for Ti, by adding yttrium chloride (YCl3) to magnesium chloride (MgCl2) flux, which effectively decreases and maintains the aMgO in the system at a low level, via the formation of yttrium oxychloride (YOCl). Through thermodynamic assessment using a $$ p_{{{\text{O}}_{ 2} }} {\text{-}}p_{{{\text{Cl}}_{ 2} }} $$ diagram, as well as experiments, the deoxidation of Ti to an oxygen level below 1000 mass ppm O, via the reaction O (in Ti) + Mg + YCl3 → MgCl2 + YOCl, was confirmed. Furthermore, using the E-pO2− diagram of the M-O-Cl system (M = Y, Mg), the possibility of electrochemical deoxidation is discussed. In the MgCl2-YCl3 flux, Mg deposits on the Ti cathode and simultaneously deoxidizes it. The activity of the deoxidation product, MgO, decreases due to the formation of YOCl and/or the electrochemical oxidation of oxide ions on the carbon anode; thus, the deoxidation of Ti becomes feasible. This new deoxidation technique using rare-earth-containing MgCl2 flux can be applied to the recycling of Ti scraps, in the future.

The major resource for recycling Ti is currently in-house Ti scrap generated in smelting and fabrication processes instead of postconsumer Ti products, and the actual recycling rate including cascade recycling in the smelting and fabrication industry is high. The major impurities in Ti scrap are O and Fe. High-grade Ti scrap with low O and Fe concentrations is remelted to obtain Ti and its alloys. On the other hand, low-grade Ti scrap with high O and Fe concentrations is used as ferrotitanium for the steel industry. However, if demand for Ti drastically increases, the amount of low-grade Ti scrap generated would exceed the demand for ferrotitanium. Before this happens, technologies for anti-contamination or for efficient O and Fe removal must be developed for efficient utilization of Ti. Herein, the current status of Ti scrap generation and its recycling flow are reviewed. New developments in Ti recycling technology are also discussed.

In order to produce high-grade titanium dioxide (95% TiO2) directly from a titanium ore using titanium scrap and iron chloride waste, chlorine recovery and selective chlorination processes were investigated. The mixture of titanium scrap and ferrous chloride in carbon crucible and titanium ore in quartz crucible were placed inside gas-tight quartz tube. The experiments were conducted at 1193–1248 K. Titanium tetrachloride and metallic iron were produced by the chlorination of titanium scrap in the carbon crucible, and the chlorination ratio of titanium chip was 94.6%. In the quartz crucible, 96.2% TiO2 was obtained under certain conditions by the selective removal of iron from the ore because of the reaction with the generated TiCl4. The iron removal ratio of titanium ore was 98.2%. Therefore, the feasibility of the effective use of titanium scrap and iron chloride waste for upgrading titanium ore was demonstrated in this study.

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.

The dissolution of metallic elements from Fe and steel materials into pure liquid Mg was investigated with the aim of improving control of impurities in Mg during processes such as Mg-alloy production and Ti smelting. Pure Mg was melted between 1073 and 1323 K (800 and 1050 °C) for 24 to 96 hours in closed crucibles made of pure Fe, low-carbon steel, or austenitic stainless steel SUS316. From the experiments using the pure Fe crucible, the relationship between the solubility of Fe in liquid Mg [Csol,Fe (mass pct)] and temperature [T (K)] was determined to be log(Csol,Fe) = − 3.67 × 103/T + 2.48 (± 0.06), from which the standard Gibbs energy of Fe dissolution in liquid Mg was evaluated. The amount of Fe dissolved from low-carbon steel was the same as that from pure Fe. From SUS316, not only Fe but also Cr and Ni dissolved into liquid Mg; the Fe and Cr concentrations in liquid Mg did not change significantly over time, whereas the Ni concentration increased monotonically. Preferential Ni dissolution resulted in a Ni-poor layer on the SUS316 surface. Finally, using the experimental data of Fe dissolution from low-carbon steel, the Fe contamination route in the current Ti smelting process, where a large amount of Mg is used as a reducing agent, is discussed.

The recovery of platinum group metals (PGMs) from spent automobile catalysts is a difficult process because of their relatively low contents in the scrap. In this study, to improve the efficiency of the existing recycling techniques, a novel physical concentration method involving treatment with FeCl2 vapor has been examined. The reactions occurring between typical catalyst components and FeCl2 vapor are discussed from the thermodynamic point of view, and the validity of the proposed technique was experimentally verified. The obtained results indicate that the vapor treatment at around 1200 K (927 °C) can effectively alloy PGMs (Pt, Pd, and Rh) with Fe, resulting in the formation of a ferromagnetic alloy. It was also confirmed that cordierite and alumina (the major catalyst components) remained unreacted after the vapor treatment, while ceria species were converted into oxychlorides. The samples simulating the automobile catalyst were also subjected to magnetic separation after the treatment with FeCl2 vapor; as a result, PGMs were successfully extracted and concentrated in the form of a magnetic powder. Thus, the FeCl2 vapor treatment followed by magnetic separation can be utilized for recovering PGMs directly from spent catalysts as an effective pretreatment for the currently used recycling methods.

Fundamental experiments are conducted with the aim of developing an efficient recycling process for rare earth elements (REEs) from neodymium-iron-boron (Nd-Fe-B) permanent magnet scrap. Molten magnesium dichloride (MgCl2) was chosen as an extraction medium, which can selectively chlorinate and extract REEs in magnet alloys. Dysprosium-containing Nd-Fe-B magnet alloy was immersed in molten MgCl2 at 1273 K (1000 °C) for 3 to 12 hours. The results of the experiments clearly show that the REEs in the magnetic alloy were successfully extracted into the molten salt, while the Fe-B alloy remained in a solid form. The extraction ratios of Nd and Dy were at most 87 and 78 mass pct, respectively. After the extraction experiment, excess MgCl2 and Mg were removed by vacuum distillation and the rare earth chlorides were recovered. Thus, the feasibility of this method for efficient recovery of rare earths using molten MgCl2 is demonstrated.

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 β-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.

The phase diagram of the barium sulfide-copper(I) sulfide system was investigated above 873 K (600 °C) using a custom-built differential thermal analysis (DTA) apparatus. The melting point of barium sulfide was determined utilizing a floating zone furnace. Four new compounds, Ba[subscript 2]Cu[subscript 14]S[subscript 9], Ba[subscript 2]Cu[subscript 2]S[subscript 3], Ba[subscript 5]Cu[subscript 4]S[subscript 7], and Ba[subscript 9]Cu[subscript 2]S[subscript 10], were identified through quench experiments analyzed with wavelength dispersive X-ray spectroscopy (WDS) and energy dispersive X-ray analysis (EDS). A miscibility gap was observed between 72 and 92 mol pct BaS using both DTA experiments and in situ melts observation in a floating zone furnace. A monotectic was observed at 94.5 mol pct BaS and 1288 K (1015 °C)., United States. Office of Naval Research (N000141210521), Massachusetts Institute of Technology. Undergraduate Research Opportunities Program