Interfacial effects in a multistage mixer–settler operation

Abstract

A pilot‐scale mixer–settler with twenty‐one stages was used to investigate the interfacial tension change during extraction cycle for the complicated system: NdCl3–SmCl3–EuCl3–GdCl3–TbCl3–DyCl3–HCl– 1 M D2EHPA–kerosene. Interfacial tension, total rate earth (TRE) concentrations in both phases, aqueous acidities, and organic entrainment in the raffinate, etc., were measured for each stage. Murphree stage efficiencies based on organic phase were calculated and related to the interfacial tension profiles. In general, the lower the interfacial tension, the higher the stage efficiency observed. For the extraction section, the stage efficiency ranged from 80%–100%, but for stripping (including scrubbing) section, it varied from 100%–15%. For high acidic stripping agent, 5 M HCl, the relatively lower stage efficiency might be due to the protonation of the acidic extractant, therefore the interfacial resistance increased significantly. From the information of stage efficiency, mass transfer direction, and interfacial tension versus solute concentration etc., the Marangoni effect could be used to explain the interfacial phenomena of this complicated extraction system. The results of real stream tests in this investigation will be useful in future plant design.