Liquid metal extraction of Nd from NdFeB magnet scrap
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This research involves using molten magnesium (Mg) to remove neodymium (Nd) from NdFeB magnet scrap by diffusion. Mg was melted over pieces of NdFeB scrap and held at temperatures in the range 675–705 °C for 2–8 h. The Mg was allowed to solidify, and the castings were then sectioned and characterized using scanning electron microscopy, x-ray diffraction, and chemical analysis. Nd was found to have diffused out of the solid scrap into the molten Mg. The thickness of the diffusion layer was measured, the diffusion of Nd through the NdFeB scrap into liquid Mg was described, and the diffusion coefficient of Nd in liquid Mg was estimated.
I. INTRODUCTION
In recent years NdFeB-based magnet materials have emerged as the leaders in high-field permanent magnet applications. They exhibit the highest energy product of all permanent magnets and possess excellent resistance to demagnetization at normal operating temperatures. NdFeB magnets have become widely used in automotive cranking motors, computers, audio-visual components, magnetic separators, military and aerospace systems, and other devices that require high-field magnets of reduced size and weight. During this last decade, the United States Bureau of Mines investigated several processing techniques for recovery of valuable rare-earth elements from NdFeB magnet scrap. The best separation of rare-earth from NdFeB magnet scrap obtained by Morrison1 and Lyman and Palmer2 was using sulfuric acid to dissolve the scrap, followed by precipitation of sodium and ammonium intermediate double salts, and then converting those rare-earth salts to useful neodymium product NdF3 by using HF. In 1993, Greenberg3 obtained neodymium trifluoride from NdFeB swarf (a mixture of oil, machining chips, and other solid residue) and slag by using hydrofluoric acid. As in Ref. 1, this method involved a number of chemical extraction and reduction steps, and both methods require the use of large amounts of dangerous solutions. Due to the complexity and expense of returning to an oxide or fluoride state these methodologies are impractical for recycling purposes.5 With the current expansion of the NdFeB magnet market, more and more scrap is being stockpiled due to the lack of a cost-efficient recycling method.1 It is clear that development of a viable method to recycle NdFeB scrap is desirable for economic and environmental reasons.5 2296
http://journals.cambridge.org
J. Mater. Res., Vol. 15, No. 11, Nov 2000 Downloaded: 17 Mar 2015
This study focused on liquid metal extraction processing to recycle Nd from NdFeB scrap. Researchers at Ames Laboratory are pioneers in this area, applying the general method to a number of systems.4,5 In this study liquid Mg was used as the extractant to remove Nd from magnet scrap. Nd is soluble in liquid Mg and forms a number of intermetallic compounds while Fe and B are essentially immiscible in Mg. Therefore, liquid Mg readily accepts Nd, forming a Mg–Nd solution, while leaving the Fe and B behind. The production of a Nd-rich alloy should find application in the
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