| Rare earths have been identified as one of the nation's most important strategic non-renewable resources, playing an important role in the high-technology applications. With the increasing demand for them in the high and new-tech areas, the production scale becomes larger and larger. Solvent extraction is presently one of the major techniques for separating rare earths on industrial scale, and P507 is a chief extractant used for separating rare earths. At the same time, this system suffers from some main disadvantages with respect to waste-water disposal and a higher consumption about acid/base, arousing the higher production costs and polluting environment seriously. Therefore, to develop an environment-friendly extraction and separation technology is essential.In this paper, a new technology of two synergistic complexing agents lactic acid (LA) and citric acid (H3cit) with unsaponified P204 to separate light rare earths has been developed, pointing to the above problems, the extraction behaviors and mechanisms of La, Ce, Pr and Nd have been studied in the P2O4-REC13-LA and P204-RECl3-LA-H3cit systems, as well as the recovery of the complexing agents, which would provide a theoretical basis for developing clean separation processes from rare earths.Firstly, the aqueous solution of RECl3-LA-H3cit has been studied by UV-Vis absorption spectra and FT-IR, the combination between rare earth and lactic acid is 1:1 and 1:2, namely RE(Lac)2+and RE(Lac); and RE(Lac)2+, RE(H2cit)2+and RE(Lac)(H2cit)+are the predominant species in the RECl3-LA-H3cit solution, at the same time, the structure of RE(Lac)(H2cit)+complex in the form of carboxylic oxygen bridge bidentate ligand is speculated.The extraction parameters of La, Ce, Pr and Nd are studied in the P204-REC13-LA and P204-RECl3-LA-H3cit systems, respectively, and the main results in the former system are listed below:the distribution ratios of each rare earths show an increasing trend with pH increasing and the concentrations of lactic acid; at pH=3.50 and lactic acid concentration of 0.60 mol/L,?Ce/La,?Pr/Ce and ?Nd/Pr are 3.42,2.04 and 1.60, respectively, and the maximum extraction capacity for P204 extractant is 28.85 g/L, close to those in the saponified P507 system. The distribution ratios increase with citric acid concentrations under the fixed of pH value in the latter system, and at pH=2.50, lactic acid concentration of 0.60 mol/L and [H3cit]:[LA]=1:10, the S. F. value ofPr(?)Ce(?) is as high as 5.78, meanwhile, the extraction capacity can be up to 27.08 g/L, higher than that in the P204-RECI3-LA system under the same conditions; synergistic coefficients (R) of La, Ce, Pr and Nd increase with the increase of aqueous pH value, and the maximum values are 1.44,1.79,1.98 and 2.09, respectively, showing the positive cooperativity, and the synergistic coefficients follow the order of Nd>Pr>Ce>La, which is beneficial to enhance effect of extraction and separation.The extraction processes are analyzed by slope analysis in these two systems, indicating that the hydrogen atom in P-OH is replaced by RE3+, and the complexing agents are retained in the aqueous phase in the extraction process; the relationship of equilibrium constants in the different systems is:the values in the P204-RECl3-LA-H3cit system>the values in the P204-RECI3-LA system>the values in the P204-HC1 system; the influence of temperature on the extraction of RE3+ with P204 in different systems shows that the extraction processes are endothermic reactions, and temperature has little effect on the extraction; separation of the La(?), Ce(?, Pr(?), Nd(?) mixture with P204 using the complexing method has been studied, and ? values of Ce/La, Nd/La and Pr/La are 3.92,4.56 and 3.41, respectively, larger that those in the P204-REC13-LA system, indicating that this complexing system using lactic acid and citric acid has a potential industrial application for La(?) separation.The extraction mechanism is studied by FT-IR and NMR, indicating that the extraction status of P=O in the dimeric P204 can be displayed at a higher lactic acid concentration, and the extraction process is that RE3+locates in the center of the extracted complex which is formed by the chemical bond and the dative bond between RE3+ and the oxygen of P-OH as well as the oxygen of P=O, respectively; the analysis of saturated extracted complexes reveals that there is no free and dimeric species of P204 in the organic phase; the reactivities of P-OH and P=O in P204 are affected by the lactic acid and citric acid in the extraction process; the studies of 1H NMR and 31P NMR further show that RE3+ replaces the hydrogen atom in P-OH, and has a strong coordination interaction with P=O.The mass transfer kinetics are investigated using a constant interfacial area cell with laminar flow, the effect of stirring speed, temperature, and specific interfacial area on the extraction rate demonstrates that the extraction process is a diffusion-controlled kinetics process with an interfacial reaction; the activation energies (Ea) of La, Ce, Pr and Nd are calculated to be 17.66 kJ·mol-1.13.57 kJ·mol-1.10.01 kJ·mol-1 and 9.61kJ·mol-1 from the slopes of logao against 1000/T, respectively; the rate equations are deduced from the studies on the effects of acidity in aqueous phase and extractant concentrations on the extraction rate; the rate-controlling step, extraction mechanism at the interfacial zone and interfacial reaction model are evaluated from analysis of the experimental results.Lastly, the experiments of recovery, stripping and concentration of complexing agents are studied, and the results are listed as follows:using a countercurrent extraction process at a phase ratio O/A=1:1, t=20 C, pH=0.60, and 75 vol.% TBP in kerosene, the recovery rates of lactic acid and citric acid reach more than 99% for 10 extraction stages, and the COD values after the 11 extraction stages can be up to 57.78 mg/L and 8.14 mg/L, respectively, and the total COD value of 65.92 mg/L can meet the emission standards of pollutants from rare earths industry (COD? 80mg/L) from the Ministry of Environmental Protection, People's Republic of China (2011); using the alkaline solution of pH=13 to strip lactic acid and citric acid,100% stripping is obtained using a cross-flow extraction process with ten stages at a phase ratio O/A=1.5:1 and t=50 ?, and the concentrations are enrichment to 1.40 mol/L and 0.54 mol/L, respectively, which will provide the probabilities for dissolving minerals in the circular process.Overall, the studies on P204-RECl3-LA-H3cit system could be regarded as a promising environment-friendly and effective separation technology. |
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