Preparation of high-purity iron oxide from end-of-life NdFeB magnet waste

The acid-soluble residue from end-of-life NdFeB magnet waste is often introduced into the ferrous metallurgical process to recover iron oxide as a value added product, but this approach has a low economic value. In this work, a ferrous chloride solution from NdFeB magnet waste containing rare earth elements, which was obtained after the leaching and impurity removal process from the acid-soluble residue, was used as the raw material. The crystallization mechanism and reaction kinetics in the oxidation precipitation process were studied to prepare high-purity iron oxide. The results showed that the growth of γ-FeOOH flakes with smooth surfaces and uniform morphologies could be explained by Ostwald's ripening theory. The apparent activation energy of the oxidative precipitation reaction of ferrous iron was 56.8 kJ/mol, indicating a chemically controlled reactions. Under the optimal oxidative precipitation conditions, the precipitation efficiency of ferrous ions and rare earth elements was 98.8 % and < 1 %, respectively. The precipitated product was well-crystallized γ-FeOOH with an aggregated flake morphology. It was roasted at 700 °C for 2 h to obtain an iron oxide product with a purity of 99.6 %. This study could provide technical support for the valuable utilization of acid-soluble residues from NdFeB magnet waste and to propose new methods for the utilization of other iron wastes and residues.