Development Of Novel Processes On The Recovery Of Rare Earth Elements From Ndfeb-based Permanent Magnet Wastes

China presently possesses about 40% of global rare earth reserves,and produces more than 90% of the rare earth elements(REEs)output of the world.The high demand and the expected supply shortages of REEs,additionally triggered by Chinese export restrictions,lead to a significant increase in rare earth price.In order to reduce the dependency on Chinese rare earth market,US,Japan and EU countries promoted to extract REEs from electronic wastes.Among the rare earth containing wastes,NdFeB permanent magnets became one of the most critical materials to be recycled.The NdFeB permanent magnets contain more than 30 wt.% of REEs.It is estimated that 20-30% NdFeB magnet scraps will be produced in the manufacturing process,e.g.,processing,sintering,cutting and polishing.Furthermore,the permanent magnet is easy to be oxidized at high working temperatures.The oxidized scraps can't be reused in the conventional NdFeB magnet production.It is essential to recycle and extract the REEs from the magnet scraps.This thesis focuses on the proposal of noval processes to extract REEs from permanent magnet.Based on the different affinity of REEs and Fe to oxygen,selective reduction method,FeO-B2O3 selective oxidation method were proposed.Moreover,on the basis of the hydrolysis of rare earth carbides,VIM-HMS method was also raised.The selective reduction process involves two steps,i.e.,oxidizing the magnet particles and selective reduction of the oxides.REEs in the NdFeB alloy particles were first oxidized to rare earth oxides.The experimental oxidation conditions were controlled at 700 oC for 1h with the thickness of the sample controlled under 2mm in the crucible.The oxidation products mainly contain Nd2O3,Fe2O3,Al2O3 and B2O3.Fe2O3 can be easily reduced to the metal phase by carbon at experimental conditions(reduction temperature: 1400-1550 oC,holding time: 1-5h).Almost all the REEs remained in oxide phase.The purity of the rare earth oxide reached upto 94.9wt.% at 1550 oC for 5h.Increasing the reduction temperature and extending the time of treatment helps in removing of B2O3 in the rare earth oxides.Al2O3 in the oxides phase can be hardly reduced into metal phase,and forms REAlO3 perovskite phase.Ferrous oxalate dehydrate(FeC2O4·2H2O)and boric acid(H3BO3)were used to synthetize FeO-B2O3 fluxes.In order to recover the REEs from permanent magnets,the Fe2O3,B2O3 and FeO-B2O3 fluxes were designed to extract the REEs from NdFe B magnet scraps,repectively.Through the observation of the products gotten at different temperature(1300-1550oC),it suggests the experimental temperature should ?1400oC.Increasing the reduction temperature and prolonging the time of treatment helps in reducing of B2O3 in the rare earth oxides(temperature: 1400-1550 oC,holding time: 1-9h).Flux 2 worked the best for the extraction of REEs from magnet scraps.The highest purity of the rare earth oxides obtained from the present study can reach 98.4wt.%.The optimal recovery rate for REEs was more than 99.0%.The VIM-HMS process involves three basic steps,i.e.,the vacuum induction melting(VIM)process,the hydrolysis process and the magnetic separation process(HMS).In VIM process,the NdFeB magnets were melted in graphite crucible under vacuum(< 1Pa),in which way the rare earth carbides formed by the reaction of REEs and carbon,and the carbon saturated NdFeBCsat alloy was gotten.Based on the hydrolysis of rare earth carbides,the REEs were separated from the NdFeBCsat alloy by the reaction of the rare earth carbides phase with water.In which way,the rare earth hydroxides and iron-based metal residues were produced.Magnetic separation was further used to remove the iron residues from the rare earth hydroxides.Through this method,the optimal recovery ratio reached 97.0%,and the purity of the rare earth hydroxides was as high as 99.7wt.%.In the rare earth hydroxides,it clearly showed that there are two individual grain shapes existing: nanorods with length of around 200 nm and width of around 20 nm and particles.Finally,the rare earth hydroxides were converted to rare earth oxides for industry application.By comparing the full oxidation and selective reduction method,the FeO-B2O3 selective oxidation method and VIM-HMS method,the VIM-HMS method showed great advantage on other two methods.Then,the VIM-HMS method was extended to be applied to the Al,Zr,Co,Ni,Cu and Nb containing NdFeB materials.In order to investigate the properties of rare earth carbides,the Nd-C binary system has been assessed based on the experimental investigation,thermodynamic calculations and the literatures survey.