A new design of mixer-settler is presented. The mixer has several mixing chambers in series and with water dispersed in oil in the mixer. The mixer with several mixing chambers in series can improve the residence time distribution of both phases in the mixer significantly. The dispersion of water in oil can increase the rate of mass transfer. The new design also has the advantages of low backmixing in the mixer and very little intrastage and interstage short circuiting. Both the new design and the existing mixer settler in which mixer is a single mixing chamber with oil dispersed in water in the mixer were used to separate rare earth elements totest their performance. Laboratory results indicated that the ratio of efficiency for the separation of light group and heavy group of rare earth elements is 1. 426 and the ratio of efficiency for the separation of two neighbouring rare earth elements is mostly greater than 1 when the flow capacity is between 1. 46 and 2. 92m3/(m2·h).