Separation and purification of rare earth elements (REE) is of great importance due to the varietyof their technological applications. In this context, liquid membrane separation stand out as an alternative to traditional techniques in order to selectively separate and concentrate REE. Some of the most significant advantages of the liquid membrane technologies are the minimum liquid membrane inventory and the low power consumption (V. S. Kislik 2010). However, membrane instability can be a significant problem for industrial application and the mechanisms behind membrane instability are poorly understood.This study analyses the separation of rare earth elements by means of liquid membranes with the final objective of recovering REE from an apatite concentrate (M. Alemrajabi, 2015, M.Mohammadi, 2015). In this study, the feed phase will consist of an aqueous phase containing REE, the membrane phase will contain organic solvent and carrier (mainly D2EHPA and EHEHPA), and the stripping phase will be a hydrochloric acid solution. In order to analyse the suitability of liquid membranes in the extraction ofREE, a deeper insight into the mechanisms causing instability is needed. In this study, surface tension is considered as the key factor of Marangoni instabilities and spontaneous emulsification (A. M. Neplenbroek 1992. H-D. Zheng 2009, F. F. Zha 1995). The influence of capillary forces and the effect of mass transfer through the membrane will be examined. Furthermore, a comprehensive evaluation of mass fluxes, separation efficiency and membrane performance on a wide range of operating conditions will be conducted in order to optimise the separation process. This study is expected to provide valuable information for the design of more stable and efficient liquid membrane processes.