The present study concerns a new type of mixer-settler unit where coalescence is promoted by means of the phase inversion method (MSPI). Using two different systems (alkaline etching solution - LIX 54 in kerosene and acidic aqueous solution - LIX 984 in kerosene) the influence of the total flowrate, the A/O phase ratio and the hydrodynamic conditions in the nozzles of the scattering device on the extraction and separation efficiencies are studied. Two modes of operation are clearly outlined and a theoretical model predicting the separation efficiency is proposed and compared to experimental results. A comparison between mixer-settlers with classic decanters and the MSPI unit has demonstrated the advantages of the latter where throughputs in the range 20 -25 m3.m-2h-1 can be easily realised.