Mass transfer efficiency in rare earth extraction using a hollow fiber pertraction device

Abstract

Extraction of neodymium by N,N-dibutylacetamide (DBAc) has been investigated using a single hollow porous fiber pertraction device. It consists of a polypropylene hydrophobic fibre maintained in a cylindrical glass calender. The Nd loaded aqueous phase flows inside the porous fiber. The organic phase flows outside in the calender and fills the pores of the membrane. Neodymium is extracted by DBAc, transported through the membrane by molecular diffusion and collected in the solvent flow. DBAc was chosen taking into account its efficiency with regard to the extraction of rare earths, estimated initially by liquid–liquid extraction, and because of its low variation of viscosity with metal concentration. Accurate measurements of diffusion coefficient of neodymium in the solvent by Taylor Dispersion Analysis (TDA) confirmed the low influence of the neodymium concentration on its diffusivity. These results, combined with the determination of the distribution coefficient of Nd in DBAc, gave good agreement between experimental and simulated data of pertraction test. Nevertheless, the results showed a high resistance of mass transfer mainly due to the diffusion in membrane. A possible explanation was that the distribution coefficient of Nd was not high enough to ensure a sufficient gradient of concentration along the thickness of the membrane filled by the solvent. To prove it, a second pertraction experiment was performed using HDEHP at 1 mol L−1 in n-dodecane as extractant. In this case, the distribution coefficient of Nd is 3 times higher than for DBAc. Good fitting of experimental data with simulated extraction of neodymium has been obtained and confirm the aforementioned hypothesis which assumes that high mass transfer rate requires high distribution and diffusion coefficients.