Experimental Investigation and Thermodynamic Modeling of the B 2 O 3 -FeO-Fe 2 O 3 -Nd 2 O 3 System for Recycling of NdF
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INTRODUCTION
THE transition from fossil fuels to renewable energy sources is creating an increasing demand for highly efficient electrical engines and electrical generators. One of the core components of these engines is NdFeB magnets, and hence the demand for neodymium is increasing rapidly. Neodymium like all other rare earth elements is mainly present in low-grade ores in a limited number of geographic locations. This has caused concerns over the supply of neodymium. The nature of these ores (low grade, often containing radioactive isotopes) makes mining and extraction of neodymium a high environmental impact operation. Recycling of neodymium from NdFeB magnet containing waste is an attractive alternative to reduce the supply concern and environmental impact of neodymium production. Neodymium, however, reacts with oxygen to form one of the most stable oxides that exist, which makes it very hard to recover neodymium in metallic form. Understanding the oxidation of NdFeB magnets would make it easier to make a process for recovery of neodymium. Thermochemical and phase diagram data are essential for this understanding, but presently such data are very limited for the iron-saturated FeO-B2O3-Nd2O3 system. LARS KLEMET JAKOBSSON, formerly Postdoctoral Fellow with NTNU Norwegian University of Science and Technology, Høgskoleringen 1, NO-7491 Trondheim, Norway, is now R&D Engineer with Elkem Technology, Fiskaaveien 100, NO-4675 Kristiansand, Norway. Contact e-mail: [email protected] GABRIELLA TRANELL, Professor, is with NTNU Norwegian University of Science and Technology, Høgskoleringen 1, 7491 Trondheim, Norway. IN-HO JUNG, Professor, is with McGill University, 3610 University, H3A 0C5, Montreal, QC, Canada. Manuscript submitted February 10, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
The present work was conducted to find a self-consistent dataset for the iron-saturated FeO-B2O3-Nd2O3 system. The modified quasichemical model (MQM)[1] was used for the liquid solution while all ternary compounds were assumed to be stoichiometric. All relevant literature was critically reviewed, and optimization of all relevant systems was carried out. In addition, key phase diagram experiments were conducted to prepare the final thermodynamic description of the iron-saturated FeO-B2O3-Nd2O3 system. Fe2O3 also had to be included in the model to account for varying oxidation state of iron. Therefore, thermodynamic modeling of the entire B2O3-FeO-Fe2O3-Nd2O3 system was conducted.
II.
THERMODYNAMIC MODELING
A. Stoichiometric Compounds The Gibbs energy of a stoichiometric compound (solid and liquid) or gas can be expressed by: GoT ¼ HoT TSoT ;
HoT
¼
DHo298K
ZT
þ
Cp dT;
½1
½2
298K
SoT
¼
So298K
ZT Cp þ T dT; 298K
½3
where DHo298K is the standard enthalpy of formation of a given compound at 298 K (25 C). DHo298K of elemental species stable at 298 K (25 C) and 1 atm is assumed to be 0 J mol1 as a reference. So298K is the standard entropy at 298 K (25 C) and Cp is the heat capacity of a compound. The G
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