| As key strategic resources, rare earth(RE) had been widely applied in national defense and high-tech civilian use. In recent year, the RE raw resources in China drops substantially due to the rapid development of the national economy and continuous global supply, so it is urgent to recycle the RE renewable resources. In present thesis, high-efficiency extraction and integrated utilization of RE and their associated resources in waste sintered Nd-Fe-B magnets, a kind of typical RE renewable resources, were studied. For bulk wastes, the thermodynamics and kinetics of hydrogenation process of the magnets were firstly investigated, and regenerated bonded Nd-Fe-B magnets with good magnetic properties were prepared. Large batch recycling of waste magnets up to several hundred kilograms was successfully carried out. Thermal stability, chemical stability, and mechanical properties of regenerated sintered Nd-Fe-B magnets were then evaluated. For sintered Nd-Fe-B sludge, a new recycling route based on co-precipitation and calcium reductive sintering was invented, and regenerated sintered Nd-Fe-B magnets with useable magnetic properties were prepared. The present researches provide beneficial exploration for recycling RE renewable resources. The thermodynamics and kinetics of hydrogen decrepitation(HD) process for sintered Nd-Fe-B waste bulks and strip casting flakes were systematically studied. Effects of hydrogen pressure, temperature, and size of the bulks on the HD process as well as the decrepitation ability were investigated. First,the hydrogenation amount of both the waste magnets and flakes increases with increasing pressure, but it decreases with the increment of HD temperature and size of the magnets. The hydrogenation amount of the waste magnets is lower than that of the flakes. Second, the HD process composes of four stages of magnets surface activation,slow hydrogenation of Nd-rich grain boundary phase, quick hydrogenation of Nd2Fe14 B main phase grains, and slow hydrogenation of inner part of the magnets.The surface activation process accelerates with increasing pressure, and almost disappears when the pressure reaches to 6 MPa. Under 15 MPa, the HD activation energy of the flakes and waste bulks are 6.64 and 6.71 k J/mol, respectively, indicating the former is easier for HD process. Moreover, blasting power of HD process of waste sintered Nd-Fe-B magnets decreases with the increase of the initial hydrogen pressure.Magnets with particle size smaller than 10 mm are desirable for HD process at 423 K.Regenerated Nd-Fe-B bonded and sintered magnets with high magnetic property were prepared from Nd-Fe-B waste bulks via HD and sintering method. For bonded magnets, the fracture manner of mechanical crushing(MC) and HD powders are intra-granular and inter-granular fracture, respectively. The(BH)max of bondedmagnets from MC and HD powders are 91.4 and 111.6 k J/m3. The coercivity of the bonded magnets from HD powders increases by 22.3% via 10 wt.% Nd Hx nanoparticles doping. Mass production experiments were carried out to prepare high performance regenerated Nd-Fe-B sintered magnets. Compared with their original magnets, the regenerated magnets with brand of 35 SH recover their Br, Hci, and(BH)max by 99.2%, 105.65%, and 98.65%, respectively, while the regenerated magnets with brand of 42 H recover their Br, Hci, and(BH)max by 99.27%, 96.76%, 99.29%,respectively, indicating that the regenerated magnets are qualified for commercial use.Thermal stability, chemical stability, and mechanical properties of regenerated Nd-Fe-B sintered magnets with brand of 35 SH and 42 H were investigated. For comparison, corresponding original magnets of same brand were also studies. It is found that the remanence temperature coefficient(α), coercivity temperature coefficient(β), and the operation temperature(TO) of the regenerated magnets are more or less same as the original magnets. The chemical stability of the regenerated magnets is slightly lower than that of the original magnets due to the higher rare earth content and more Nd-rich grain boundary phase in the former. Moreover, the tensile strength, compressive strength, and hardness of the regenerated magnets are slightly lower than those of the original magnets, while the fracture toughness of the former is better. Further observation indicates that the amount of grain boundary phase is responsible for the mechanical properties difference between the regenerated magnets and the original magnets. Finally, it is concluded that the thermal stability, chemical stability, and mechanical properties of regenerated Nd-Fe-B sintered magnets are comparable to those of the original magnets, which are qualified for commercial application.New recycling route from Nd-Fe-B sintered sludge to regenerated sintered magnets was set up. First, co-precipitation technique was applied to obtain oxide powders containing rare earth and main transition metal from the sludge. Second,calcium reduction diffusion technique was used to prepare the oxide powders into single phase Nd2Fe14 B alloy powders. Finally, Nd Hx nanoparticles were doped into the alloy powders to fabricate regenerated sintered magnets via conventional sintering technique. Based on analysis to specific mechanism of above method process, key preparation conditions were optimized, and Nd-Fe-B magnets with useable magnetic properties were obtained. The best magnetic properties are Br of 1.2 T, Hci of 517.6k A/m, and(BH)max of 258 k J/m3. |
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