Enhanced Separation Of Praseodymium From Neodymium And Its Mechanism By Kinetic "Push And Pull" System

Separation and purification of rare earths by solvent extraction have becoming one of the most important techniques in industrials.However,the practical industrial processes are very difficult to be operated due to the extreme similarity in physical-chemical properties of rare earths,especially for adjacent rare earths.To achieve a pure rare earth chemical,several hundreds or even thousands of stages of traditional mixer-settlers are required.How to improve the separation factor of rare earths gains great attention.Many progresses have been made in separation of rare earths at equilibrium extraction,which make some improvement to the rare earth separation process.However,kinetic separation of rare earths by solvent extraction can obtain a more satisfying separation than equilibrium extraction in some studies.So far,the researches on kinetic separation data and mechanism are few.Therefore,it is important to investigate the extraction separation of rare earth by kinetic methods.Pr(?)and Nd(?)are the most difficult separated elements in light rare earths.In our work,the extraction kinetics of Pr(?)and Nd(?)by[A336][NO3]was investigated.Kinetic"push and pull"system of[A336][NO3]-DTPA for Pr(?)and Nd(?)separation was investigated.The main research contents and results are shown as follows:Extraction kinetics of Pr(?)and Nd(?)by[A336][N03]in nitrate medium were investigated by single drop technique.The dependence of Pr(?)and Nd(?)extraction rates on column height,nozzle diameter,concentrations of Pr(?)and Nd(?)in the aqueous phase,concentration of salting-out agent,pH,concentration of[A336][NO3]were studied.The rate equations of[A336][NO3]extracting Pr(?)and Nd(?)were obtained.The apparent activation energy,activation enthalpy,activation entropy were calculated by temperature experiments and the extraction mechanism was deduced.The rate constant of[A336][N03]extracting Pr(?)was double than that of[A336][NO3]extracting Nd(?).The apparent activation energy of[A336][NO3]extracting Pr(?)and Nd(?)were 9.145 kJ/mol and 11.604 kJ/mol,respectively,which indicated that the extraction processes of Pr(?)and Nd(?)by[A336][NO3]were both diffusion controlled.The extraction rates of Pr(?)and Nd(?)by[A336][NO3]decreased with increasing the nozzle diameters,which suggested that the extraction reactions were occurred at the liquid-liquid interface.The activation enthalpy of[A336][NO3]extracting Pr(?)and Nd(?)were 6.573 kJ/mol and 9.055 kJ/mol,which indicated that the extraction reactions were fast reactions.The activation entropy of[A336][NO3]extracting Pr(?)and Nd(?)were-833.61 J/(mol·K)and-836.25 J/(mol.K),which indicated that the extraction reactions obeyed to SN2 mechanism and the structures of the extracted species were more ordered than the extractant molecules.As the rate of[A336][NO3]extracting Pr(?)was faster than that of Nd(?),and the rate of DTPA complexing with Nd(?)was faster than that of Pr(?),we presented a kinetic"push and pull”system to separate Pr(?)and Nd(?)in a column extractor.The effects of the adding way of DTPA,the pH of DTPA,the adding amount of DTPA,the concentration of LiNO3,the initial mole ratio of Pr(?)to Nd(?)in the aqueous phase,the pH of the aqueous phase,the concentration of[A336][NO3]on separation factor of Pr(?)to Nd(?)were investigated.As[A336][NO3]was continuously pumped into the column extractor in the form of dispersed oil droplets and at the same time DTPA was injected into the aqueous feed solution when the extraction was just started,the separation factor of Pr(?)to Nd(?)could achieve 21.7,nevertheless the separation factor of this extraction system in extraction equilibrium was 5.8.The extraction time corresponding to the maximum separation factor was 290 min,and the phase ratio(O/W)was 20:1.The different adding ways of DTPA had an obvious effect on separation factor of Pr(?)and Nd(?),which indicated that the difference in the rates of DTPA complexing with Pr(?)and Nd(?)played a crucial role in enhancing the kinetic separation of Pr(?)and Nd(?)by[A336][NO3].Varying the pH of DTPA,the adding amount of DTPA and the pH of the aqueous phase affected the rates of DTPA complexing with Pr(?)and Nd(?),which indirectly affected the rates of[A336][NO3]extracting Pr(?)and Nd(?).The kinetic "push and pull"effect was obvious as the rates of DTPA complexing with Pr(?)and Nd(?)matched with the rates of[A336][NO3]extracting Pr(?)and Nd(?).Organic phase added into the extraction column in the form of oil bubbles can improve the kinetic separation process.The effects of the pH of DTPA,the adding amount of DTPA,the concentration of LiNO3,the initial mole ratio of Pr(?)to Nd(?)in the aqueous phase,the pH of the aqueous phase,the concentration of[A336][NO3]on kinetic separation of Pr(?)and Nd(?)were investigated.As the initial mole ratio of Pr(?)to Nd(?)was 1:4,the pH of DTPA was 5.0,the concentration of DTPA was 0.0223 mot/L,the concentration of LiNO3 was 4 mol/L,the pH of the aqueous phase was 3.0,the concentration of[A336][NO3]was 0.4 mot/L,the maximum separation factor can reach 11.5.The extraction time corresponding to the maximum separation factor was 160 min,and the phase ratio(O/W)was 1:2.Contrasted to oil droplets,oil bubbles as the organic phase carrier greatly reduced the extraction time and the phase ratio of O/W.The mathematical model of[A336][NO3]-DTPA system was built.The concentration ratio of uncomplexed Pr(?)and Nd(?)in the aqueous phase was crucial to enhance the kinetic "push and pull"effect.As the concentration ratio of uncomplexed Pr(?)and Nd(?)was above 1:0.925,it had a disadvantage impact on the kinetic separation of Pr(?)and Nd(?).As the concentration ratio of uncomplexed Pr(?)and Nd(?)was in the range from 1:2.04 to 1:0.925,it was in favor of kinetic separation of Pr(?)and Nd(?).