Recycling neodymium-iron-boron magnet alloy by direct sulfation with SO₂ gas

Abstract

Neodymium-iron-boron magnets possess strong magnetic field strength which makes them popular in small electronics, such as hard disc drives and television. These magnets can also be used for larger applications where an even stronger magnetic field is required such as MRI machines or electric cars. Due to the supply risk associated with these magnet alloys, investigating a recycling method for neodymium from electronic waste has become critical. However, there are some challenging aspects associated with these rare earth magnets when producing an effective method to recycle this electronic waste. Selective sulfation has been used to separate the neodymium from the iron in permanent magnets. Sulfuric acid is used to convert the metals to sulfates, which are then roasted to produce a calcine of neodymium sulfate and iron oxide. The calcine could then be leached with water to create a pure neodymium solution. To further investigate the selective sulfation reaction, direct sulfation using SO2 would be attempted. Solid particle samples comprised of the pure magnet alloy were directly contacted with SO2 and O2 in a tube furnace. The variables investigated were temperature of the material bed and the ratio of SO2 to O2. The product from the tube furnace is then water leached to recover any of the converted neodymium sulfates. The leachate and solid residue were analyzed by ICP-MS and XRD respectively. The tube furnace results suggest that direct sulfation using the pure magnet alloy is not feasible. In all experiments the recovery of neodymium never exceeded 2% and the iron in the system either stayed as the metal phase or oxidized to Fe2O3.