This work is aimed at optimizing a settler structure in a mixer-settler. Two different aspects have been considered. Firstly, the flowcharacteristics of a settler have been examined by computational fluid dynamics (CFD) simulation with various agitation speeds of the mixer, as well as organic phase volume fractions ranging from 0.075 to 0.6. The aqueous and organic phase turbulent flow fields were measured by particle image velocimetry (PIV) technique to verify the CFD simulation. Two organic phases with different physical properties were assessed in the CFD simulation to simulate the liquid-liquid systems related to rare earth element extraction, i.e., 0.072 mol.L-1 P507/kerosene and 1.8 mol.L-1 P507/kerosene. Secondly, the CFD simulation was carried out in a settler equipped with baffles. The effects of number and location of the baffle in the settler on flow features and entrainments of the aqueous and organic outlet were analyzed. Meanwhile, different settler/mixer volume ratios were also examined. By analyses and comparisons, an optimal design for settler was proposed. CFD can provide a significant guidance to better mixer-settler design. (C) 2020 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved