Oxidation of ferrous ions enhanced by CaMg(CO3)2 for their efficient separation from neodymium solution

The waste products of NdFeB magnets are an attractive resource for the recovery of rare earth elements (REEs), especially for neodymium [Nd]. Conventional methods employed to remove ferric (Fe[II]) from Nd solutions involve high energy consumption and the extensive use of chemicals, including oxidants and pH regulators, leading to substantial wastewater generation. In this work, an environmentally friendly approach utilizing CaMg(CO3)2 was presented to remove Fe(II) from Nd solution. Under optimized conditions, including a reaction temperature of 70°C, a CO32-/Fe2+ ratio of 1.5/1, a reaction time of 240 min, and an initial concentration of 200 mg/L, the heterogeneous precipitation reaction on the surface of CaMg(CO3)2 exhibited an exceptional Fe(II) removal efficiency of 98.6%, with a negligible loss of Nd (only 0.8%). It was observed that the CaMg(CO3)2 not only provide a suitable alkaline environment for the selective precipitation of Fe(II), but also significantly enrich the concentration of dissolved oxygen at a high temperature. In fact, at a CaMg(CO3)2 concentration of 2 g/L, the dissolved oxygen concentration in the solution increased significantly to 15.5 mg/L at 70°C. Consequently, the usage of CaMg(CO3)2 facilitated the rapid conversion of Fe(II) to Fe(III) without the need for additional oxidants, which was perfect for the removal of Fe(II) rather than Nd.