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

Abstract

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.