희토류 산화물의 염소화 및 열환원 공정을 통한 희토류 금속 및 합금 제조

Abstract

This study presents an energy-efficient and carbon-free smelting process for converting rare earth metal oxides into metals and alloys. The process consists of calcination and chlorination of rare earth metal oxides, followed by metallization of the chlorides. In the calcination step, the process conditions were established to improve the purity of the metal oxides and produce single-phase sesquioxides (Nd2O3 and Tb2O3). Nd(OH)3 in the Nd oxide was converted into Nd2O3 during a 2 h calcination at 800 ℃. The Tb oxide consisting of multiphase, nonstoichiometric compounds, was converted into a mixture with an O/Tb elemental ratio of 1.5-1.6 after 24 h of calcination at 850 ℃. In the chlorination step, process optimization was carried out by adjusting the reaction ratio of metal oxides to NH4Cl, reaction time, and gas atmosphere to produce single-phase anhydrous rare earth chlorides (NdCl3, TbCl3, and DyCl3). The powdered form of NdCl3 produced at 400 ℃ and under argon gas flow was identified as NdCl3·6(H2O), while the bulk form of NdCl3 produced by melting at 760 ℃ after a chlorination process consisted of anhydrous NdCl3 and NdCl3·n(H2O). The powdered NdCl3 produced in an argon gas environment with a controlled level of oxygen (below 16.05 ppm) and moisture (below 1 ppm) content was identified as single-phase anhydrous NdCl3 and showed the highest chlorination conversion rate of 98.65%. The addition of overstoichiometric ratios of NH4Cl in the chlorination process decreased the total amount of impurities (N, H, and O) in the NdCl3 product and increased the conversion rate of NdCl3. TbCl3 and DyCl3 were produced using the same mechanism as NdCl3. Anhydrous TbCl3 was produced through a chlorination process at 450 ℃ for 8 h with an NH4Cl/Tb2O3 reaction ratio of 12.1281, while anhydrous DyCl3 was produced at 450 ℃ for 9 h with an NH4Cl/Dy2O3 reaction ratio of 14.0338. In the metallothermic reduction process, calciothermic reduction was studied to produce rare earth metals and alloys at a temperature range of 850-1050 ℃ using rare earth chlorides (NdCl3, TbCl3, and DyCl3) synthesized through chlorination. Our method decreased the process temperature while improving the recovery rate of Nd using the thermodynamic parameters of the CaCl2-KCl-NaCl and Nd-Fe liquid solutions. To reduce the activity of the product (CaCl2), the optimal composition of the CaCl2-KCl-NaCl molten salt was WCaCl2=0.4(XKCl:XNaCl=6:4). The molten metal bath (Nd or Nd-Fe) that formed at the bottom of the reaction zone during Nd and Nd-Fe alloy production absorbed metal particles generated in the molten salt during the reaction, thereby facilitating ingot formation. In Nd produced at 1050 ℃ using 1.2× the stoichiometric amount (by mass) of Ca, the Nd recovery rate was 97.0%. Moreover, in the Nd-Fe alloys produced at 1050 ℃ targeting eutectic compositions, the Nd recovery rate was 96.3%. Increased Fe contents in the Nd-Fe liquid solution reduced the Nd recovery rates, and the Nd-Fe alloy (Nd recovery rate: 89.8%) was produced at 850 ℃, suggesting the possibility of increasing the energy efficiency of the Nd production process. The Nd-Fe alloy produced through this proposed process could be used as a raw material in the NdFeB strip casting process. Dy and Tb were produced as Tb-Fe and Dy-Fe alloys using the same mechanism as Nd-Fe alloy production. Tb-Fe and Dy-Fe alloys, produced through a 3 h reaction at 1000 ℃ and designed to target eutectic compositions, showed recovery rates of 86.8% for Tb and 72.8% for Dy, respectively.