Separation of Praseodymium and Neodymium from Heavy Rare Earth Elements Using Extractant-Impregnated Surfaces Loaded with 2-Ethylhexyl Phosphonic Acid-mono-2-ethylhexyl Ester (PC88A)

In the rare earth industry, the next step after leachingand impurityremoval is separation. The most common technology for separation issolvent extraction. Although promising, it faces a few challengesincluding the large consumption of organic solvents, large volumesof waste generation, and the need for multiple stages to achieve thedesired separation factor. An alternative approach to solvent extractionis supported-liquid extraction (SLE) in which the extractant phaseis supported in place by a solid support media in which the liquidextraction takes place. The main advantages of SLE over solvent extractionare lower solvent consumption and less generation of hazardous waste.Here, an extractant-impregnated surface (EIS) made of a microtexturedsilicon substrate coated with octadecyltrichlorosilane for hydrophobicityand impregnated with 2-ethylhexyl phosphonic acid-mono-2-ethylhexylester (PC88A) is developed to separate praseodymium and neodymiumfrom a mixture of heavy rare earth elements. The possible contactmodes including impaled, impregnated, and encapsulated are investigated,and it is found that the impregnated mode can be achieved when 0 <& theta;(es(w)) < & theta;( c ).The feed contains 10 mg/L of all REEs at pH 2.5. The key separationperformance indicators including yield, purity, and separation factorare determined, and the results indicate that the final praseodymium+ neodymium purity is 92% with 96% yield, and a separation factorof 171 that is comparable with solvent extraction is achieved. Kineticstudies indicate that the pseudo-second-order kinetic model fits thekinetic data, which means that the adsorption is controlled by chemisorptionwith an activation energy of 69.3 kJ mol(-1). Thermodynamicstudies indicate that the adsorption process of the studied REEs onPC88A-EIS is endothermic (& UDelta;H (ads) =31.3 kJ/mol). The Gibbs free energy of praseodymium and neodymiumis positive, whereas that of heavy rare earth elements is negative(-8.56 kJ/mol), indicating that the heavy rare earth elements'adsorption on PC88A is spontaneous whereas that of praseodymium andneodymium is not spontaneous. Therefore, the system can selectivelyseparate heavy rare earths over praseodymium and neodymium. Isothermstudies indicate that the Langmuir model better fits the adsorptiondata, suggesting that the monolayer homogeneous adsorption mechanismis the controlling mechanism. The maximum heavy rare earths'adsorption amount was found to be 671.4 mg/cm(2), which iscomparable to those obtained using functionalized adsorbents. Theadsorbed heavy rare earths were eluted with 4.5 M H2SO4, and the EIS was regenerated and reused for several cycles,indicating a cost-effective potential material in real applications.