Quantitative analysis of dysprosium in coated NdFeB used magnets by laser-induced breakdown spectroscopy for sorting prior to recycling

Permanent magnets containing strategic rare earth elements (REE) such as Nd, Pr or Dy are being used more and more around the world, and their recycling is becoming a major stake in a context of virtual monopoly of China on the production of REEs. This paper addresses the sorting of solid used NdFeB magnets upstream of a recycling process. Dysprosium was chosen as representative of heavy REEs, and the LIBS technique was selected to perform online quantitative analysis under a residual coating or oxidation layer when present.The studies were carried out using two different LIBS systems. The first one was a commercial laboratory equipment with a short-range working distance of 30 cm. Laser ablation conditions were optimized based on the LIBS signal intensity and stability, on the shot-to-shot signal evolution, and on the quality of the calibration curve. This led to 60 pre-ablation shots and 40 analysis shots. Under these conditions, we showed the feasibility of quantifying the dysprosium content under a depth of 35 to 60 μm with a relative uncertainty of less than 10 % at 95 % confidence.The second LIBS system was a homemade setup designed to make measurements at 1 m distance, a working range representative of a possible online implementation above a conveyor belt. The ablation efficiency of this system was found much too low, but it was considerably increased by using the laser in the free-running mode. In the standard Q-switched mode, the results also showed the feasibility of quantifying dysprosium with uncertainties of the same order than with the short-range equipment.