Valuable Metal Recycling From NdFeB And Ni-metal Hydride Battery Wastes Using Combined Pyro-and Hydrometallurgical Techniques

Wastes from processing of NdFeB permanent magnet materials,Ni-metal hydride(NiMH)battery electrode materials,and end-of-life rare-earth materials are used as secondary sources of rare earths and other valuable metallic elements.Hydrometallurgical processes are extensively used to recover valuable elements from these wastes.The extent of recovery of rare earths is high,but excess acid used in the processes causes environmental pollution and there is low recovery of other valuable elements.To effectively recover rare earths and other valuable metals from NdFeB and NiMH battery wastes,a new combined recycling method using both hydro-and pyrometallurgical techniques is introduced.High-purity alloys and rare-earth oxide(REO)-SiO2-Al2O3 slags,which were obtained from these two wastes by selective hydrogen reduction then separation by melting,were processed using a hydrometallurgical approach(leaching-purification-sedimentation)to recover valuable metallic elements.The results showed that:(1)When the hydrogen concentration exceeded 76.14%and 4.95%,selective reduction of Fe and Co in the NdFeB waste and of Ni and Co in the NiMH battery wastes,respectively,was possible and other oxides of inert elements were reduced to simple substances.The reduction was controlled by diffusion limitation,diffusion and chemical reactions simultaneously.The best thermodynamic conditions for selective reduction by H2 were at 800 ? for 2 h.(2)When the ratio of Al2O3 to SiO2 in the La2O3-SiO2-Al2O3 ternary slag was 0.5-0.75,the slag exhibited a low melting temperature and low viscosity.FeO,MnO,and B2O3 decreased the slag melting temperature and viscosity.The final slag from the NdFeB waste contained 60%La2O3-19.30%SiO2-9.70%Al2O3-7%Fe0-4%B2O3,while that of NiMH battery waste contained 55%La2O3-25.30%SiO2-12.70Al2O3-7%MnO.(3)Rare earth precipitated phases,which formed La2O3-SiO2-Al2O3-FeO-B2O3 and La2O3-SiO2-Al2O3-MnO slag systems,gave similar apatite-type lanthanum silicates at various quenching temperatures.Rare-earth phases were in the shape of long hexagonal prisms,which grew as the quenching temperature was lowered.(4)In the melting separation,SiO2 and Al2O3 slag-making materials were added to the product obtained after hydrogen reduction of NdFeB waste to obtain Fe-Co alloys of 99.88%purity and a slag comprising 57.58%La2O3-16.10%SiO2-8.00%Al2O3-6.04%FeO-4.20%B2O.Similar treatment of the NiMH battery waste gave a Fe-Co alloy with a purity of 99.95%and a slag comprising 46.44%La2O3-26.05%SiO2-17.68%Al2O3-6.32%MnO.(5)These two slags were separately treated to recover rare earths by hydrochloric acid leaching both under low temperature and ordinary pressure and under high pressure and high temperature.The extents of REO leaching of the slags of the NdFeB and NiMH battery wastes were 98.13%and 96.70%,and their purities were 99.56%and 99.51%,respectively.