Preparation of high-purity ferrous sulfate from NdFeB magnetic material secondary waste slag

A large amount of secondary waste residue is produced after rare earth recovery from NdFeB magnetic waste material. In view of the high iron content in waste residue component, the iron extraction from the waste residue was studied, and the iron was recycled in the form of ferrous sulfate products. In the stage of iron extraction by acid leaching, the effects of acid concentration, extraction temperature, leaching time, liquid-solid ratio (acid volume/waste residue mass), and leaching frequency on iron ion leaching were investigated in detail. The optimal acid leaching process parameters obtained by univariate experiments are acid concentration of 6 mol/L, leaching and extracting temperature of 80 ℃, leaching time of 120 min, liquid-solid ratio (mL/g) of 4∶1 and leaching frequency of 2, under which the leaching rate of iron ions is approximately 97.8%. In the reduction stage, the effects of temperature, reaction time and scrap iron excess coefficient on the conversion of Fe³⁺ to Fe²⁺ were investigated. The obtained optimal reduction process parameters are reduction temperature of 90 ℃, reaction time of 120 min, and scrap iron excess coefficient of 1.2, under which the conversion rate of Fe³⁺ to Fe²⁺ is 97.69%. Finally, the ferrous sulfate product is obtained by concentration, cooling crystallization and recrystallization, with the purity of 99.92%.