Study On The Recycling Technology Of Waste Nd-Fe-B Sintered Magnets In Bulk Form

Nd-Fe-B sintered magnets,which exhibit high permanent magnetic properties,are the most important kind of permanent magnets,and widely used in different fields,such as electronic information,automobile,health care,domestic appliances and wind power.The recent development of electronic information industry,wind power and new energy automobile results in the increasing demand of Nd-Fe-B magnets.146 thousand tons of Nd-Fe-B magnets were produced in 2016,and the annual production keeps rising,which means the increasing quantity of Nd-Fe-B wastes,from no matter manufacturing or the out-of-service products.As the Nd-Fe-B wastes,with a high content of rare earth elements,are valuable,it is significant to recycle the Nd-Fe-B wastes.In this thesis,the study aims at the recycling of the bulk wastes of Nd-Fe-B sintered magnets.Based on the overall analysis of the bulk wastes,grain boundary diffusion?GBD?process was applied to promote the coercivity of the wastes,bonding technique was used to prepare the recycled anisotropy bonded magnets,and liquid meatal extraction?LME?method was applied to extract the rare earth elements of the wastes.The main works and the results in this thesis were as followed:As the first part of the work,overall testing and analysis were carried out on the bulk wastes from both manufacturing process and waste hard disk drives.The result showed that the bulk wastes had a low degree of oxidization and had complete microstructure as the normal magnets.The bulk wastes from manufacturing,with defects on their shape,had low magnetic properties within a range of distribution,but the magnetic properties at different position of the same waste showed no obvious difference.The coatings of the coating wastes adhered well but the thickness of them were not homogenious.Wastes from hard disk drives,with well-adhered coating,were not damaged,and their magnetic properties and microstructures had no obvious change.Based on the above work,grain boundary diffusion process was applied to optimize the coercivity of the wastes as the first recycling way in this thesis.Pr-Cu alloy,with a lower price than the heavy rare earth,was used as the diffusion medium in the GBD process for the bulk waste with low coercivity,to achieve recycling by coercivity optimization.The effects of processing parameters on optimization were investigated.As a result,after Pr₇₀Cu₃₀ grain boundary diffusion,the microstructure of the waste magnet was optimized,resulted in the improvement of the coercivity.The coercivity of the waste magnet increased by 51.9%,from7.88 kOe to 11.97 kOe,after 800°C/4 h diffusion and 500°C/3 h annealing.The temperature stability of the remanences decreased after diffusion,while that of the coercivity increased.This recycling process with the advantage of simplicity and low cost,is an effective way to recycle the Nd-Fe-B bulk wastes with low coercivity.Preparation of recycled Nd-Fe-B anisotropic bonded magnet was investigated as the second method for recycling in this study.The bulk wastes was mechanically pulverized and sieved,and were directly used for preparation.Anisotropic bonded magnets were obtained with remanence of 0.52 T,coercivity of 5.50 kOe and energy product of 3.4 MGOe.The result also indicated that,for better magnetic properties of the recycled magnet,the size of waste magnet powder should be larger,the range of solidification time was 1¹.5 h,and the solidification should be carried out in shielding gas or vacuum.The result also showed that the recycled bonded magnets prepared from waste magnet powder in medium size?80-200mesh?had the highest magnetic anisotropy field.This study about recycled bonded magnet provides a useful reference for the low-cost rapidly recycling of the Nd-Fe-B bulk wastes.Finally,liquid metal extraction method was also studied for rare earth extraction from the bulk wastes in this thesis.Copper was used as the extraction medium,and the relevant processing parameters were investigated.It was found that the extracting rate of rare earth of the samples treated at 1200°C was higher than that of samples treated at 1100°C,which is related to the eutectic melting of Fe and Fe₂B at higher temperature,forming the liquid phase that is immiscible with the liquid Cu,and enhancing the extraction of rare earth.Lower Cu/waste ratio resulted in better separation of rare earth element and Fe element.Smaller waste scraps made them easier surrounded by the liquid Cu,which was advantageous to the extraction process.When extracted in 1100°C,the holding time should not exceed 2 h in case of the raise of Fe content in the extracted product?Cu-rich region?.This work is meaningful for the research on the dry-recycling method for the Nd-Fe-B wastes.