Impact of Ferric and Ferrous Iron on the Crystallization of Rare Earth Sulphate Hydrates

Rare earth elements (REEs) are important for permanent magnets used in for example wind turbines and motors. There is an imbalance in supply and demand of this commodity and the REE have been identified as critical raw materials by the European Union. This study focuses on recovery of REEs from sulfuric acid solutions using antisolvent crystallization in recycling of magnet waste. Ethanol is used as an antisolvent to crystallize Nd₂(SO₄)₃·8H₂O and (Nd/Dy)₂(SO₄)₃·8H₂O. The impact of the presence of Fe in ferrous and ferric states, and of different seeding strategies, on the quality of the crystal product in terms of purity, crystal size, morphology and agglomeration has been investigated. Higher purity (above 99%) is obtained for seeded experiments and the purity is higher for higher seed loading and lower antisolvent dosing rate. Furthermore, Fe(III) has a higher tendency to be incorporated into the pure Nd phase compared to the Nd phase containing 10% of Dy, while Fe(II) is not detected in any of the phases. By balancing the addition of antisolvent and seed loading the optimum conditions in terms of high purity and productivity can be found. The results provide insights to improve the recovery of REEs as a pure concentrate.