The aim of this work is to improve the separation efficiency in the case of liquid-liquid dispersions using a new type of mixer-settler design called MSPI (mixer-settler based on phase inversion) and a so-called solvent assisted method. The phase inversion is used to decrease the distance between the drops of the disperse phase and the interface in spherical decanters called carrier drops, while a solvent injection in the mixer allows for optimal drop diameters and, therefore, for better and easier separation. In this study, a new design method for the mixing zone in a mixer-settler is proposed. It is based on the capacity of the settler to provide high separation efficiency while throughput increases. This separation is of essential importance when residual water of good quality has to be released. A number of correlations are identified that allow reasonable prediction of the D-3,D-2 of carriers formed using a perforated plate, as well as the drop size distribution and the Sauter mean diameter ((d) over bar (min)) of the primary dispersion. They allow predicting the influence of the organic/aqueous phase ratio as well as the contact angle on these parameters. The method is applied to design operating conditions in MSPI devices used in two particular cases, a heavy metal extraction and a crude oil emulsion separation. The results show that it is possible to obtain very good extraction efficiency coupled with good separation efficiency. They make the MSPI a viable alternative in the treatment of residual water contaminated with crude oil as well as in hydrometallurgy. At the same time the study shows that the MSPI is a simple piece of equipment to work with and very easy to maintain. While the results obtained are for some specific cases, the design methodology presented here can allow the rapid evaluation of other separation processes in the MSPI unit.