Research On Preparation Of Mg-Nd Based Alloy By Electrolysis In Molten Chlorides

During the production and application of NdFe B permanent magnet materials, a large amount of NdFeB scraps is generated. And the inadequate processing of the waste Nd Fe B magnets not only leads to the environmental pollution, but also is a waste of rare earth resource. Currently, the waste NdFeB magnets are generally recovered via hydrometallurgical route. The hydrometallurgical recycling process has the advantages of high recovery rate, and it is easy to realize industrialization. However, the hydrometallurgical route also has some shortcomings, such as high acid consumption, high waste acid emission and tedious processing steps.Our group has present a new method for recycling of rare earths in the NdFeB scraps which includes selective chlorination of REs with molten MgCl₂, followed by direct electrolysis in the obtained molten chlorides, including MgCl₂ and RECl₃, to prepare RE-Mg based alloys. In this paper, the formation mechanisms of Mg-Nd based alloys during molten chlorides electrolysis are systematically investigated to provide theoretical support for the recovery and comprehensive utilization of NdFeB scraps.Firstly, the electrochemical behavior of Nd³⁺ on Mo electrode in molten NaCl-KCl and NaCl-KCl-MgCl₂ at 750 ?, as well as the formation mechanisms of Mg-Nd alloy, is investigated by using electrochemical techniques, such as cyclic voltammetry, square wave voltammetry and Open Circuit Chronopotentiometry, etc. Then, the electrochemical behaviors of Pr³⁺ and Dy³⁺ in NaCl-KCl and NaCl-KCl-MgCl₂ melts are further studied, and the deposition mechanisms of Mg-Pr as well as Mg-Dy alloys are also invested. Finally, Mg-Nd based alloys are prepared by molten slats electrolysis using pure rare earths chlorides and the molten extractant as the base electrolyte, respectively. Further, the phase composition, microstructure and element content of the alloys are analyzed by XRD, SEM and EDS.Through the systematical investigation, the research results in this thesis can be concluded as follows.?1? The electrochemical results show that the reduction of Nd³⁺ ions in NaCl-KCl molten salts is a one-step reaction with the reduction potential is approximately-2.10 V?vs Ag/AgCl?. In molten NaCl-KCl-MgCl₂-NdCl₃ system, Mg²⁺ ions are first reduced to Mg at-1.80 V, and Nd³⁺ ions are then reduced on Mg via underpotential deposition between-1.80 and-2.10 V, and alloyed with Mg forming three intermetallic compounds MgNd, Mg2 Nd and Mg3 Nd. Mg-Nd alloy is prepared by electrolysis by potentiostatic electrolysis at-2.40 V for 10 hours, and it composed of Mg41Nd5?Mg12Nd?Mg?Nd.?2? The reduction of Pr³⁺, as well as Dy³⁺ ions in NaCl-KCl molten salts at 750 ?, is also a one-step reaction. Their reduction potentials are approximately-2.10 V. In molten NaCl-KCl- RECl₃-MgCl₂ at 750 ?, Mg-RE alloy can be prepared via constant current and constant potential electrolysis. And the main content of the alloy is Mg and RE?Nd?Pr?Dy?.?3? Using the molten slats, which is obtained from the chlorination reaction between MgCl₂ and NdFeB scrap, as the base electrolyte, Mg-Nd based alloy has been prepared via electrolysis at 750 ?. The content of Mg in the alloy is above 90 wt%, while the content of rare earths is relatively low.The results of this thesis indicate that it is feasible to prepare Mg-Nd based alloy via electrolysis using molten salt extractant from the chlorination of waste NdFeB, and provide a new technology for the recycling of NdFe B scraps.