Oxidative roasting of Nd‒Fe‒B permanent magnets prior to leaching improves the selectivity in the recovery of rare-earth elements (REEs) over iron. However, the dissolution rate of oxidatively roasted Nd‒Fe‒B permanent magnets in acidic solutions is very slow, often longer than 24 hours. Upon roasting in air at temperatures above 500 °C, the neodymium metal is not converted to Nd2O3, but rather to the ternary NdFeO3 phase. NdFeO3 is much more difficult to dissolve than Nd2O3. In this work, the formation of NdFeO3 was avoided by roasting of Nd‒Fe‒B permanent magnet production scrap in an argon atmosphere, having an oxygen content of pO2 ≤ 10–20 atm, and the addition of 5 wt.% of carbon as iron reducing agent. For all the non-oxidizing iron roasting conditions investigated, the iron in the Nd‒Fe‒B scrap formed a cobalt-containing metallic phase, clearly distinct from the rare-earth phase at microscopic level. The thermal treatment was optimized to obtain a clear phase separation of metallic iron and rare-earth phase also at the macroscopic level, to enable easy mechanical removal of iron prior to the leaching step. The sample roasted at the optimum conditions (was leached in the water-containing ionic liquid betainium bis(trifluoromethylsulphonyl)imide, [Hbet][Tf2N]. A leaching time of only 20 min was sufficient to completely dissolve the REEs. The REEs/iron ratio in the leachate was about 50 times higher than the initial REEs/iron ratio in the Nd‒Fe‒B scrap and the full process (roasting leaching) more than tenfold faster than previously reported processes. Therefore, roasting in argon with addition of a small amount of carbon is an efficient process step to separate the REEs from the iron by avoiding the formation of NdFeO3, for ta faster and more selective recovery of REEs from Nd‒Fe‒B permanent magnets.
Full reference paper
Martina Orefice, Amy Van den Bulck, Bart Blanpain and Koen Binnemans, Selective roasting of Nd‒Fe‒B permanent magnets as a pre-treatment step for intensified leaching with an ionic liquid, J. Sustain. Metall.., 2019, 20, 1-12, DOI: 10.1007/s40831-019-00259-1
This work received funds by etn-DEMETER, European Training Network for the Design and Recycling of Rare-Earth Permanent Magnet Motors and Generators in Hybrid and Full Electric Vehicles, grant agreement 674973; SOLCRIMET, Solvometallurgy for critical metals an European Research Council (ERC) Advanced Grant under the European Union’s Horizon 2020 Research and Innovation Programme: Grant Agreement 694078.