| Rare earth is the most important strategic non-renewable resources, also known as the "industrial vitamin". In recent years, the exploitation and utilization of the rare earths got more and more attention. Our country put the project of "the basic research of utilization and green science of rare earth resource" on the list of national key basic research development program (973 Program) in 2012, this action would effectively promote the development of rare earth industry in the direction of green metallurgy, and our research would laid a good scientific basis for rare earths separation, purification and comprehensive utilization.Saponification P507-HC1 system is the main system for the extraction and separation of single rare earth at present, this system occupies a very important position in our country and even all the world. Summarizes the experiences of industrial production and studies of preliminary experimental found that the current saponification process caused higher production cost and polluted the environment, specific performance in large amount of ammonia nitrogen wastewater, emulsification and three-phase. Therefore, to develop a new green and non-saponification extraction process and replace the original process is imperative.In this paper, a new efficient process by adding the complexing agent to the aqueous phase to improve the extraction efficiency and separation selectivity was developed, and this process put an end to the generation of waste water. The extraction and synergistic extraction of La, Ce, Pr, Nd in P507 sysem and P507+P204 sysem in the presence of lactic acid and citric acid was systematically researched. The main results of our study are summarized as follows:(1) The study of extraction behaviors in P507-LA+H3cit-HCl system: ①The addition of LA and H3cit improved the ablilty of P507 in rare earths extraction and separation, the extract capacity(C), distribution ratio(D) and separation factor(β) increased with the concentration of complexing agent and the increase of pH value in the aqueous phase. ②The double complexing effect comes from the mixture of LA and H3cit was better than single LA, the C, D and β follow the rule as:C(LA+H3cit-Hcl)>C(LA-Hcl)>C(Hci)、D(LA+H3cit-HCl)>D(LA-HCl)>A(HCI)、 β(LA+H3cit-HCl)>β(LA-HCl)>β(HCl), the maximum of βCe/La,βPr/Ce、βNd/Pr in LA-HCl system and LA+H3cit-HCl system was 13.30,2.39,2.13 and 14.04,10.47,2.4, respectively, and these β were higher than 8.0,2.2,1.6 that in the existing saponification P507 system.(2) The study of extraction behaviors in P507+P204-LA+H3cit-HCl system: ①The extraction of La, Ce, Pr, Nd by P507 and P204 was belonged to synergistic extraction, and the maximum synergistic enhancement coefficient R for La, Ce, Pr, Nd was 2.17,2.22,2.67,2.58 when the Xp507 was 0.8. ②The addition of LA improved the ability of system’s extraction and separation in the P507+P204-LA-HC1 synergistic system, the D increased with Xp204 and the β increased with Xp507. ③The double enhanced effect on extraction in P507+P204-LA+ H3cit-HCl system was better than P507+P204-LA-HC1 system and P507+P204-HC1 system, the D{β) decreased(increased) with the Xp507.④The result of multistage extraction experiment shown that the extraction equilibrium time is short(only three stage) in the synergistic and complexing system(Xp507=0.8), and this system not only possess good separation ability but also increase the amount of organic phase saturated extraction, the maximum is 53.48g/L which is far more than 28g/L in saponification system.(3) Extraction mechanism and the behavior of La, Ce, Pr, Nd in HC1 medium, LA-HC1 medium, LA+H3cit-HC1 medium with P507 or P204 was researched by slope method and constant molar method, indicated that the both extraction mechanism of RE3+ by P507 and P204 was cation-exchange. In the process of extraction the Cl+, Lac+, H2cit- are not involved in complexing reaction, and the extraction reaction can be expressed as:one RE3+ with three P507(P204) dimmers and generates the complexes in the form of RE(HA2)3, RE(HB2)3, and three H+ was released. (H2A2, H2B2 is replace the P507, P204 dimmers molecules, respectively)(4) On the basis research of light rare earths extraction by P507 or P204, the synergistic extraction mechanism of La, Ce, Pr, Nd by P507 and P204 was also duplicated by slope method and constant molar method. In different medium, Cl-, Lac- and H2cit- would not participate in the synergistic-complexing reaction, the cation-exchange was the synergistic extraction mechanism all the same. In the synergistic extraction, each RE3+ combines with the mixture of P507 and P204 and three H+ was released into the aqueous phase.(5) The research of thermodynamic equilibrium constants in different systems shown: ①In the extraction system of P507, P204 and synergistic extraction system of P507 and P204, the equilibrium constant K was according to the laws of KLA+Hscit-HCl)>K(LA-HCl)>K(HCl) and this proved that the addition of complexing agents were conducive to the positive extraction of rare earths. ② The equilibrium K of La, Ce, Pr, Nd was according to the laws of KNd>Kpr>KCe>KLa and this proved that the synergistic extraction of light rare earlhs was positive sequence extraction. ③The influence of the temperature on the P507 system and P507+P204 synergistic system can be ignored. Study of the relationship between 1000/T and logD according to Van’t hoff equation found that the extraction of La, Ce, Pr, Nd by P507 was endothermic reaction, on the contrary, the synergistic extraction of La, Ce, Pr, Nd by P507 and P204 is exothermic reaction.(6) The FT-IR,1H NMR,13C NMR,31P NMR and MS spectrum were used to examine the combination mechanism of P507 and P204, the extraction mechanism of P507 system and P507+P204 synergistic system in the absence and presence of complexing agents. The spectrum results were listed as follows: ①Lac- and H2cit- would not enter into the organic phases, ② The changes of organic structure characterized by spectral data demonstrated the existence of cation-exchange extraction mechanism, in the process of REs extraction RE3+ combined with P507 and P204 and generated the rare earth chelate complex with the formation of P-O-RE and coordination polymers between RE and P=O ligands, and the coordination number is six. ③Synthetically the changes of dimmers hydrogen bond detected by IR spectrum, the shifts of H detected by 1H NMR and the analys of SEI-MS tested the formation of P507+P204 dimers. In the synergistic extraction the RE3+ get P507 and P204 together at the same time and generated mixed chelating ligand, the ligand compound has greater stability and more soluble in organic phase, this characteristic make synergistic system showed positive association estraction effect in the process of REs extraction. |
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