Extraction of rare earth metals with a multistage mixer-settler extraction column

Extraction behavior of rare earth metals within a mixer-settler extraction column has been analyzed with the stage efficiency calculated from mass transfer coefficients and interfacial area. The mass transfer coefficient within the dispersed drops is determined from a rigid sphere model by taking into account the residence time distribution of drops, and the coefficient around the drops is calculated by Ranz-Marshall's correlation with the terminal settling velocity of a rigid sphere. The interfacial area of dispersed drops is calculated by the use of correlations for the drop diameter and the holdup of dispersed phase in the mixer-settler extraction column. The calculated results for the separation of samarium and gadolinium with a five-stage mixer-settler extraction column are compared with the experimental results at various agitation speeds and flow ratios between two phases. The extraction behavior in the multistage column is explained by a model based on the hydrodynamics and the mass transfer within the mixer. Effects of the pH value in aqueous phase, the extractant concentration in organic phase and the number of stages on the extraction behavior in the mixer-settler column are also shown.