Study On New Technology And Mechanism Of Regeneration Of Sintered Nd-fe-b Sludge Waste

In recent years,with the rapid development of new energy and electronic information industries,the market demand and output of sintered Nd-Fe-B have increased rapidly,resulting in an increasing amount of sludge waste generated during machining.Green and efficient recycling of sludge waste has become an urgent problem to be solved.Among them,the in-situ regeneration method of recovering sludge waste by calcium reduction diffusion method to prepare regenerated magnets has become a hot spot in the field of rare earth secondary resource recovery.At present,there are still some problems in the recovery of sludge waste by calcium reduction diffusion method,such as unclear reaction mechanism,high reduction temperature,uneven particle size of regenerated magnetic powder,small processing capacity and still incapable of mass production,and low performance of regenerated magnet.Therefore,this thesis firstly studies the reaction mechanism of the recovery of sludge waste by calcium reduction diffusion method through systematic experimental research and thermal-kinetic calculation analysis.On this basis,the regenerated magnetic powder with uniform particle size distribution and low impurity elements was prepared by adjusting and optimizing the reduction diffusion process and the washing and decalcification process.Finally,by introducing the rotary diffusion technology,a new process of small batch recovery of sludge waste by rotary-reduction diffusion was developed,and then high-performance regenerated magnets were prepared by doping rare-earth-rich Nd₄Fe₁₄B and high-remanence Nd₂Fe₁₄B alloy powders.Based on Nd-Fe-B sludge calcium reduction-diffusion reaction and thermal and kinetic calculations,the reaction mechanism was elucidated.The results show that the recovery of sludge waste by calcium reduction-diffusion method is a complex reaction of multi-step selective reduction and diffusion.Calcium preferentially reduces iron oxides in sludge.As the reaction temperature increases,Nd₂O₃is reduced,and the reduced Nd diffuses to the surface of the iron core to react to form Nd-Fe-B alloys.The formation and growth process of Nd-Fe-B alloy is mainly divided into nucleation stage,high-speed growth stage and completion stage.The thickness of the alloy layer increases rapidly at 1-3 h,and its growth law is more linear,and then slows down with the increase of reaction time.The reaction takes 4 h to complete at 900℃.The calcium reduction-diffusion reaction of sludge conforms to the unreacted shrinking core model,and its rate-controlling step is the internal diffusion process.The size and properties of the regenerated powders were regulated by optimizing the reduction diffusion process.The results show that:（1）CaH₂ reducing agent reduces the minimum dosage of reducing agent and reaction temperature,and obtains regenerated magnetic powder with finer particle size,better distribution and lower impurity content.The average particle size of the regenerated magnetic powder was reduced to 3.43μm,the grain size showed a normal distribution,and the residual Ca and O contents were reduced to about 0.08 wt.%and 0.32 wt.%,respectively.Compared with initial sludge,the magnetic properties of its M_3T are improved by about33%.（2）The calcium chloride diffusion medium helps to reduce the reaction temperature of calcium reduction and diffusion,and the reaction temperature for obtaining single-phase Nd₂Fe₁₄B is reduced to 800°C;the potassium chloride diffusion medium can effectively promote grain refinement and optimize the size distribution,and the average grain size is refinement to 1.83μm.The loose and porous structure is beneficial to the washing and decalcification process,and the Ca,O and H contents in the regenerated magnetic powder are reduced to 0.04 wt.%,0.26 wt.%and 0.27 wt.%,respectively.（3）The high-energy ball milling assisted calcium reduction diffusion process can effectively reduce the reaction temperature and reaction time,and obtain nano-scale Nd-Fe-B recycled powder.After 4 h of high-energy ball milling,the reduction-diffusion reaction temperature and time decreased by 100°C and 2 h,respectively,and the average particle size of the regenerated magnetic powder was 300-400 nm.（4）Compared with deionized water and dilute acetic acid solution,the washing and decalcification effect of ammonium chloride-methanol solution is better.The Ca,O and H in the regenerated magnetic powder were reduced to 0.07 wt.%,0.31 wt.%and 0.27 wt.%,respectively.Rotation-reduction diffusion process was used to realize the treatment of sludge waste in small batches of 100 grams.The effects of rotation rate,calcium content,and reduction species on the performance and microstructure of the regenerated magnetic powder were studied:when the rotation speed was 10 rpm,the amount of CaH₂ was 15wt.%,the comprehensive magnetic properties of the powder are the best,the particle size is more uniform,the average particle size is 3.59μm,the Ca,O and H contents in the regenerated magnetic powder are 0.12 wt.%,0.34 wt.%and 0.17 wt.%,respectively,M_3T is 143.94 emu/g.Regenerated sintered Nd-Fe-B magnets with different properties were prepared by doping rare earth-rich Nd₄Fe₁₄B or high remanence Nd-Fe-B alloy powder.Doping Nd₄Fe₁₄B alloy can significantly improve the coercivity of the magnet.When the doping amount is 40 wt.%,the remanent magnetization B_r=11.7 kG,the coercivityH_cj=16.5 kOe,and the maximum energy product（BH）_max=31.8 MGOe are obtained.The compound doping of a certain amount of rare earth-rich Nd₄Fe₁₄B and high remanence alloys can prepare multi-brand regenerated magnets.When the doping content of Nd₄Fe₁₄B alloy is 5 wt.%,and the doping content of high remanence alloy is25～45 wt.%,the magnetic properties of the regenerated magnets reach the commercial level of 38M～40M.When the high remanence alloy doping content is increased to 65wt.%,the magnetic properties of the regenerated magnet are B_r=13.3 kG,H_cj=14.7 kOe,（BH）_max=42.9 MGOe,reaching the commercial level of 42M.