Electrochemical recovery of rare-earth elements from NdFeB magnets

Abstract

The use of the rare-earth elements (REEs) is steadily increasing and, on the other hand, possibilities for their substitution in modern advanced technologies, especially in green energy applications, are limited. Recently, recycling REEs from spent products has become an alternative to primary mining. NdFeB magnets are of particular interest, since they are present in various forms of waste, such as end-of-life wind turbines and electric vehicles. They contain considerable amounts of REEs, mostly Nd but also Ce, Pr, Gd and Tb. The main constituent in NdFeB magnets is iron (55−60 wt-%), which makes selective REE recycling challenging. Feasible and effective technology for selective recovery of REEs from NdFeB magnets is thus needed.

This dissertation focuses on electrochemical and chemical leaching as an essential step in hydrometallurgical recycling. The mechanisms in electrochemical leaching and in acid leaching were investigated. With proper adjustment of the leaching acid composition and operating conditions, effective separation of magnet constituents was achieved. Electrochemical leaching (ia=10−50 A/dm2) in a mixture of sulfuric (0.05−0.5 M) and oxalic (0.05−0.2 M) acids allowed to leach up to 99.7% of magnet with the maximum REE purity 97.2%. Applying electricity led to increase of the dissolution rate of magnets up to 10 times and removal of passive oxalate layer. In particular, a one-step electrochemical process was proposed for the recovery of REEs as cathodic REE-oxalate deposits with a yield of up to 31% and REE purity of 91−93%, while the rest of the oxalates were recovered as a powder. The unexpected result was explained by the electrostatic attraction of the positively charged REE-oxalate particles. Chemical leaching in 0.1 M H2SO4 of roasted NdFeB powder led to an REE leaching yield of 70% with a final ratio between Nd and Fe of 1/11. The selectivity in chemical leaching was explained by changes in the chemical NdFeB particle composition during roasting. In particular, NdFeO3 was observed in the porous outer layer and a higher Fe/Nd mole ratio and a lower oxygen content were observed in the inner core. These findings provide a new insight into electronic waste recycling by implementing principles of electrochemical/selective chemical leaching and helps in the search for new environmentally sustainable hydrometallurgical recycling methods.