Study On The Extraction And Separation Of Nd(Ⅲ) And Dy(Ⅲ) With Neutral Phosphonte Extractants In Non-aqueous Solvent Extraction

Solvent extraction（SX）is the most commonly used technique for purification and separation of REEs.However,the efficient separation of REE from conventional aqueous solution by solvent extraction is facing huge challenge due to the high activity,high interfacial tension and the high dielectric constants of water that affect the coordination of REEs with the extractant.Non-aqueous solvent extraction（NASX）is a novel extraction system that replaces the aqueous phase with polar molecular organic solvents（PMOSs）.NASX presents several advantages,including high extraction and separation efficiency,low acid consumption for stripping,and is an efficient and environmentally friendly extraction system.This study focuses on the efficient separation and recovery of Nd（Ⅲ）and Dy（Ⅲ）from waste neodymium iron boron magnets（NdFeB）.The extraction and separation of Nd（Ⅲ）and Dy（Ⅲ）by NASX system consisting of two different types of neutral extractants Trioctylphosphine oxide（TOPO）and di（1-methylheptyl）methyl phosphonate（P350）with Polyethylene glycol 200（PEG 200）,Ethylene glycol（EG）and Dimethylsulfoxide（DMSO）,respectively,was investigated and the extraction mechanism was studied to provide a theoretical basis and technical support for further applications of the NASX system.the specific research work and main results of this paper are summarized below:（1）The extraction and separation of Nd（Ⅲ）and Dy（Ⅲ）by TOPO-PEG 200,TOPO-EG,TOPO-DMSO system and the conventional aqueous phase system was investigated.The result showed that the extraction performance of the four extraction systems was in the following order:PEG 200>EG>DMSO>>H₂O.The TOPO-PEG 200 extraction system exhibited the best extraction and separation efficiency and the influence factor such as PEG 200 content,salting-out agent and TOPO concentration on the extraction performance of TOPO-PEG 200were further investigated systematically.It was found that under the optimum extraction conditions,85.13%of Dy（Ⅲ）could be extracted with 41.83 of theβ_Dy/Nd from 70 vol%PEG200solution with 0.15 mol/L TOPO.The extraction capacities of 0.15 mol/L TOPO towards Dy（Ⅲ）and Nd（Ⅲ）are 11.37 g/L and 7.91 g/L.The stripping experiment showed that the acid consumption required for the stripping of loaded-TOPO was low,with 0.05 mol/L of HCl could strip Nd（Ⅲ）and Dy（Ⅲ）from the loaded phase sufficiently.The extraction mechanism was determined to be a neutral complexation mechanism and the extracted complex was identified as RECl₃·2TOPO by FT-IR and slope analysis.thermodynamic studies indicated that the extraction process was exothermic.（2）The separation of Nd（Ⅲ）and Dy（Ⅲ）by P350 in the NASX system was investigated using kerosene as the diluent.The results revealed that the extraction and separation performance of P350 for Nd（Ⅲ）and Dy（Ⅲ）in the NASX system remained unaffected by the presence of the salting-out agent Li Cl.Therefore,P350 exhibited great application prospects over TOPO as an extractant for rare earth elements in NASX system.The effects of the type and content of PMOSs,P350 concentration and temperature on the separation of Nd（Ⅲ）and Dy（Ⅲ）by P350 were systematically investigated.It was encouraging to find that the favorable separation factor of Nd（Ⅲ）and Dy（Ⅲ）elements（βNd/Dy）was determined to be 201 when extracted from 70 vol%PEG 200 with 0.2 mol/L P350,which was higher than that the TOPO-PEG 200 system.The saturation loadings of 0.2 mol/L P350 for Dy（Ⅲ）and Nd（Ⅲ）in the non-aqueous phase system were 12.51 g/L and 8.98 g/L,which were almost consistent with the conventional aqueous phase system.The extracted complex was determined to be RECl₃·2P350by slope analysis and the extraction mechanism was confirmed to be neutral complex mechanism by FT-IR analysis.Furthermore,the P350-PEG 200 extraction system was used to separate and recover Nd（Ⅲ）and Dy（Ⅲ）from simulation NdFeB immersion solution.It was found that 85.24%of Dy（Ⅲ）could be extracted from simulation immersion solution with aβ_Dy/Nd of 83 by a single counter-current extraction.According to the Mc Cabe-Thiele diagram,3-stages counter-current solvent extraction was needed to completely extract Dy（Ⅲ）.97.11%of Nd（Ⅲ）and 98.5%of Dy（Ⅲ）in the loaded organic phase could be extracted by 0.04 mol/L,respectively.Moreover,97.11%of Nd（Ⅲ）and 98.5%of Dy（Ⅲ）in the loaded organic phase could be sequentially stripped by 0.04 mol/L and 0.4 mol/L HCl,respectively.Eventually,a P350-PEG 200 extraction system-based separation process was proposed in the hope of providing a novel and low-cost alternative to the efficient recycle of Nd（Ⅲ）and Dy（Ⅲ） from end-of-life NdFeB.