Study On The Solvent Extraction And Separation Of Light Rare Earth Elements By Using Membrane Dispersion Micro-extractor

The rare earth solvent extraction process based on the mixer settler owns many defects such as slow mass transfer rate, low stage efficiency, large equipment area occupation, and etc. The green high efficient rare earth separation process becomes a hot area of research. It has not been reported to separate the rare earth by using membrane dispersion technology. Membrane dispersion technology can effectively control the dispersed droplet size to increase the contact area between two phases and the mass transfer rate. It supplies one new method to solve these problems faced in rare earth separation processes. For the first time, this dissertation studied light rare earth elements extraction and separation process by using home-made membrane dispersion micro-extractor, in which saponified P507 was used as the extractant.The equilibrium extraction process of La and Ce by using saponified P507-kerosene system was studied. In this dissertation, the reaction mechanism of the saponified P507 and light rare earth elements ions was studied. And the extracted complex was determined as RE(HL)3L3 by using saturation capacity method. By evaluating the equilibrium aqueous pH and the initial aqueous pH, the reactions occurred under different situations were determined. According to the reaction mechanism, a mathematical model was established to predict the rare earth element distribution between two phases.The membrane dispersion process of the saponified P507-kerosene-hydrochloride acid system was studied. According to single hole dispersion theory, the droplet diameters of saponified P507-kerosene-hydrochloride acid system with different physic-chemical properties and operational parameters were calculated. A home-made membrane dispersion micro-extractor and high speed microscope video camera were used to determine the real diameter size distribution. The growth of both the saponified P507 concentration and initial aqueous pH can reduce the interfacial tension between the two phases which can cause a reduction of the droplets diameter. Increasing the continuous phase flow rate can increase the shear force, which can also reduce the droplet diameter greatly. By using equipment that our own setup and at the optimal conditions, the minimum sauter diameter of dispersed phase is 38μm.By using the membrane dispersion micro-extractor, the solvent extraction process of La, Ce, Pr, and Nd were studied. For the first time, by using the membrane dispersion micro-extractor, the effects of P507 concentration, initial aqueous pH, rare earth ions concentration, temperature, total flow rate, and etc. on the extraction efficiency and mass transfer rate were determined. When phase ratio is 1:1, and total flow rate is 160 m L/min, extraction efficiency of all four elements can reach 100% in less 2 seconds.Newman’s mass transfer coefficient empirical model for the dispersed phase was used to calculate the reaction resistance. By evaluating the effect of temperature on the reaction rate constant, the apparent activation energy of La, Ce, Pr, Nd reacting with the saponified P507 were obtained. They are 71.3kJ/mol, 47.4kJ/mol, 24.2kJ/mol and 21.1kJ/mol for La, Ce, Pr, and Nd reacting saponified P507, respectively. Increasing the initial aqueous pH can turn an extraction process controlled by interfacial reaction rate into an extraction process co-controlled by both the interfacial reation rate and the mass transfer in the organic phase rate. Increasing the total flow rate can turn an extraction process controlled by mass transfer rate in dispersed phase into an extraction process controlled by reaction rate.By using the membrane dispersion micro-extractor, the solvent extraction separation processes of Ce/La, Pr/Ce, and Nd/Pr were studied. A dynamic separation method was brought out.The effects of P507 concentration, initial aqueous pH, total flow rate and dispersed phase flow rate on the extraction efficiency and separation coefficient were evaluated. For Ce/La system with equilibrium separation coefficient of 1.50, by using membrane dispersion micro-extractor the separation coefficient can reach 16.2 with 91% Ce and 38% La extracted under the optimal conditions. Similar results can be found for Pr/Ce and Nd/Pr. The equilibrium separation coefficient of Pr/Ce is 1.21, and it can reach 2.28 with 81% Pr and 65% Ce extracted. The equilibrium separation coefficient of Nd/Pr is 1.1, and it can reach 1.57 with 54% Nd and 43% Pr extracted.