Recycling of NdFeB magnets: Model supported iron minimization via in-situ hydrolysis during leaching and hematite precipitation in an autoclave

The NdFeB magnets are a significant component of technological applications, which contain a remarkable quantity of rare earth elements (REEs). For this reason, the recycling of NdFeB magnets is essential for the reuse of these elements. This research represents the recycling strategy of NdFeB magnets, which consists of iron minimization through in-situ hydrolysis during oxidative leaching and hematite precipitation in an autoclave. The influence of the experimental parameters in the hydrolysis of Fe³⁺ in a nitric acid medium on the formation of a mixture of iron oxide hydroxide was examined in detail. Afterward, the effect of the temperature, time and, water addition on the iron and REEs precipitation in the autoclave was investigated by the ANOVA analysis based on the Box-Behnken design. According to the particle swarm optimization algorithm, maximum REEs extraction and minimum iron extraction were achieved in a solution of 1.82 mol/L nitric acid, at a solid/liquid ratio of 11/100, a temperature of 60 °C, and a stirring speed of 415 rpm. The mean leaching efficiencies of REEs and iron are calculated to be 76% and 7%, respectively. The optimal parameters for hematite precipitation in the autoclave were determined to be a process temperature of approximately 137 °C, a processing time of approximately 3 h and 55 min, and a water addition of approximately 20%. The optimum parameters are in harmony with the validation experiments.