Recovering And Recycling Sintered NdFeB Wastes By Chemical Methods

Sintered NdFeB magnets with high coercive force,high remanence and high energy product are key materials for high efficiency and energy saving devices and have been widely used in many fields such as electric cars and hybrid cars,consumer electronics products and clean energy.Rare earth elements?REEs?is one of the main raw materials of sintered NdFeB magnets.Due to the increasing demand and Chinese policy adjustment,the global prices of REEs exhibit great fluctuation in recent years.To release the supply pressure of REEs,much attention has been focused on recovering and recycling Nd Fe B wastes.In this work,the waste formation during the production process of sintered NdFeB magnets is analyzed.The chemical composition,phase composition analysis,microstructure analysis and magnetic properties of various types of wastes have been investigated.Based on the understanding of the existing recovering and recycling approaches for NdFeB wastes at present,three improved recovering and recycling methods were proposed and their processes were optimized.The main work and conclusions are as follows:The wastes were classified into slurry wastes,flake wastes and bulk wastes according to their forms.The slurry wastes were badly oxidized and contaminated,and they did not show any typical hard magnetic phases,microstructures and magnetic properties as typical NdFeB magnets.The flake wastes and bulk wastes contained a small amount of impurities,and they showed the typical three-phase structures of sintered NdFeB.But there were some defects such as large grains,porosities and aggregated rich-Nd phases,which lead to slightly lower magnetic properties than qualified magnets.A selective precipitation approach using oxalic acid as precipitant was proposed to recover REEs from NdFeB wastes.Tartaric acid and HMTA were used as the leaching reagents.The optimized process of leaching,based on the factorial experiment,was determined as the hydrochloric acid concentration of 6 mol/L,the tartaric acid concentration of 50 g/L,and the temperature of 313K,by which the extraction yields of Fe and REEs up to67.99%and 99.27%,respectively,were obtained.For the precipitation process,the optimized oxalic acid dosage and pH value were determined to be 1.5:1 and 1.5,respectively.The produced REEs oxides have the purity and recovery yield up to 95.83%and 90.18%,respectively.A co-precipitation method using urea as precipitant was proposed to recovering REEs and Fe from Nd Fe B wastes.The optimized process of precipitation was determined as the pH value of 3 and the temperature of 65?,by which the recovery yields of Fe and REEs up to94.92%and 100.94%,the total recovery yield up to 98.81%,and the purity up to 98.86%,respectively,were obtained.REEs and Fe elements in NdFe B wastes were recovered by the microwave assisted combustion method,and the regenerated Nd₂Fe₁₄B/?-Fe nanocomposite magnetic powders were recovered by reduction process with hydrogenated calcium.The optimized oxidation and reduction process were determined as the nominal component of RE₁₅Fe₇₇B₈ and the CaH₂ dosage of 1.1:1,by which the produced magnetic powers displayed coercive force H_c,remanence B_r,saturation magnetization M_s up to 2253 Oe,0.37 T,and 99 emu/g,respectively.The average grain size of the regenerated magnetic powders was 200³00 nm.The special two-phase structure is beneficial for studying nanocomposite NdFe B-based magnets.Overall,this thesis work not only improved the recovery of NdFeB wastes by leaching-precipitation and co-precipitation chemical methods,but also proposed,for the first time,a new approach to synthesize the NdFeB powders by microwave assisted combustion and reduction.These results are important for developing new recovering and recycling approaches for sintered NdFeB magnet wastes.