Selective separation of rare earths from spent Nd-Fe-B magnets using two-stage ammonium sulfate roasting followed by water leaching

The research presents an effective approach to achieve the selective separation of rare earth elements (REEs) from waste Nd-Fe-B magnets. Investigations show that the use of a two-stage roasting process can significantly reduce the amount of ammonium sulfate required and improve the separation efficiencies of REEs. During the first low-temperature roasting stage, almost 80% of REEs can be transformed into RE₂(SO₄)₃ or NH₄RE(SO₄)₂ at 400 °C within 1 h, whilst simultaneously iron and other impurities are converted into insoluble metal ammonium sulfates. These intermediate products can then be subjected to a further roasting procedure at 750 °C for 2 h, leading to an extraction of REEs of up to 96%. In contrast, the extraction of the related impurities: Fe, Al, Cu and Co is only 0.008%, 0.27%, 1.64% and 3.48%, respectively. Through the analysis and characterization of the calcine and leach residue, it was found that the decomposition of NH₄RE(SO₄)₂ and the reaction of Fe₂(SO₄)₃ and RE₂O₃ together improve the extraction of REEs during the second roasting stage. After separation of REEs, the main phase present in the leach residue is hematite, which can be recycled as a feedstock material for iron or steelmaking processes. Moreover, waste gases from this process – like NH₃, SO₂, and SO₃ - can also be recovered and reused in the preparation of (NH₄)₂SO₄, which significantly reduces the costs of the recycling operations. Overall, this newly developed process has considerable environmental and economic advantages for the recovery of valuable metals from waste Nd-Fe-B magnets.