Study On Classification Evaluation And Recovery Of Sintered Nd-Fe-B Sludge Waste

In recent years,as the application of NdFeB has been continuously expanded,the production of NdFeB has been increasing,and wastes such as scraps and sludge generated during the production and processing have also increased substantially year by year.NdFeB sludge waste is rich in rare earth elements,but with many kinds of impurities due to a variety of processing techniques.At present,manufacturers do not distinguish sludge,and deal with all of the sludge using a wet process which result in low recycling efficiency,long recycling period,high cost,and serious environmental pollution.Therefore,it is of great significance in terms of both economic and social benefits to classify and find a specific way to handle different sludge.In this paper,the sludge produced by the main processing methods in China is taken as the research object to be evaluated and classifiedbased on diverse tests,and the recovery of NdFeB sludge and the preparation of regenerated sintered magnets are explored.Eight kinds of commercial sludge are collected to test the phase structure,elemental composition,carbon,hydrogen and oxygen concentration,particle size,and magnetic properties.According to main phase structure and oxidation degree,the sludge is divided into the following three categories:slightly oxidized sludge,moderately oxidized sludge and severely oxidized sludge.The sludge with Nd₂Fe₁₄B as main phase,and low C/H/O content is defined as slightly oxidized sludge where that produced by double-sided grinding,trepanning processing,centerless grinding,vertical grinding,and slicing with organics as coolant are included.The sludge from drilling processing is seen as moderately oxidized one which composes of main phase Nd₂Fe₁₄Bsmall content of Nd₂O₃ and Fe phase,and higher sludge/H/O concentration.For the severely oxidized sludge cut by wire electrical discharge machining and slicing with water as coolant,Nd₂Fe₁₄B phase is often heavily oxidized,or completely decomposed into Nd₂O₃,Fe,Fe₂O₃ phases with ultra-high oxygen content.Moderately oxidized sludge produced by centerless grinding and double-sided grinding sludge is selected to explore the pretreatment process in order to effectively reduce the impurities and C/H/O content.It was found that the combination of acid and organic dissolution,along with ultrasonic stirring and magnetic separation is a suitable method.15vol.%acetic acid solution can be used to remove the oxides under continuous stirring,and then ultrasonic cleaning with anhydrous ethanol is used to remove the organic impurities in the sludge.After magnetic separation,the powder is dried.After this pretreatment,the oxygen content of the centerless grinding sludge is reduced from 13.53wt.%to 1.14wt.%,the carbon content is reduced from 0.30wt.%to0.13wt.%,the hydrogen content is reduced from 0.15wt.%to 0.10wt.%,and the main phase Nd₂Fe₁₄B phase is maintained.At the same time,the impurity content of the sludge decreases,and the saturation magnetization increases.The oxygen content of double-sided grinding sludge after pretreatment greatly decreases from 8.08wt.%to1.52wt.%,the carbon and hydrocarbon content also decrease from 0.62wt.%and0.56wt.%,respectively,to 0.30wt.%,0.42wt.%.The pretreated centerless grinding sludge is used as raw material for preparing regenerated sintered Nd-Fe-B magnets.Nd₄Fe₁₄B and NdHx nano-powders are added to the pretreated sludge powder,respectively.The effects on the magnetic properties and microstructure of the regenerated magnets are studied.When doped with NdHx nano-powder,the sintered magnet with 20wt.%addition has the best magnetic properties with the maximum energy product of 16.04MGOe,coercivity of 22.91kOe and remanence of 8.35kGs,respectively.When doped with Nd₄Fe₁₄B powder,the sintered magnet with 54wt.%addition has the best magnetic properties with the maximum magnetic energy product of 31.43MGOe,coercivity of 16.51kOe and remanence of 11.52kGs,respectively.