Hydrometallurgical recycling of NdFeB magnets: Complete leaching, iron removal and electrolysis

NdFeB magnets currently dominate the magnet market. Supply risks of certain rare earth metals (REM), e.g. Nd and Dy, impose efficient recycling schemes that are applicable to different types and compositions of these magnets with minimum use of chemicals and waste generation. In this study, a hydrometallurgical method was studied that could be adjusted to recover not only REM, but also other valuable metals (e.g. Co, Ni and Cu) that co-existed in the magnet. The magnet powders were completely dissolved in a dilute sulfuric acid solution giving more than 98% of dissolved iron in the ferrous state. Chemical oxidation of Fe2+ into Fe3+ by the addition of MnO2 required only 1 h at ambient temperature. It was then possible to precipitate more than 99% of this ferric iron by adjusting the pH of the solution above 3 with either Ca(OH)(2) or MnO additions. However, the addition of Ca(OH)(2) resulted in the formation of gypsum and up to ca. 23% REM losses, possibly via co-precipitation into the gypsum. MnO elevated the Mn2+ concentration in the solution. However, it was found to be problematic that subsequent direct electrolysis removed Mn and Co. Low anodic current efficiencies (ACE) resulted in high energy consumption (EC), while incomplete Mn and Co removals and undesired REM losses were reported. Pre-electrolysis removals of REM and/or Co by oxalate and/or sulfide precipitation were proven to be successful and selective, but this enlarged the flowsheet considerably with only minor improvement of the Mn removal, ACE and EC.