Interfacial tensions play an important role on settling in rare earth element separation process. The interfacial tensions of binary liquid-liquid systems (P507-kerosene/water) and (P507-kerosene/hydrochloric acid which contain rare earths) were measured by the pendant drop method. Viscosity of P507-kerosene with different proportions was obtained by rotational viscometer. The influence of the volume of kerosene

the saponification rate

the concentration of rare earths

the kinds of rare earths and number of rare earths on the interfacial tension and viscosity have been discussed that increase the volume of kerosene would raise the interfacial tensions and lower the viscosity

and the increase of saponification rate brought the decrease of interfacial tension and increase of viscosity. The varied concentration of rare earths did not show great impact on the interfacial tension within a certain degree of saponification rate. Too low concentration and too high saponification rate would raise emulsification

which would hinder the measurements. Different kinds and number of rare earths presented little difference on interfacial tensions for their similar properties. CFD simulations in settler with different P507-kerosene systems have been conducted and the results showed that the phase separation was tougher with lower interfacial tension and higher viscosity.