This work evaluated nanofiltration (NF) as a potential step in the recovery process of Rare Earth Elements (REEs) from leachates of coal combustion fly ashes. A pre-treatment step, by pH adjustment and microfiltration (MF), has been studied to separate REEs by major elements. The individual and combined effects of applied pressure (12-24 bar) and NF feed acidity (pH 1.5-3.5) on rejection of six critical REEs and permeate flux have been investigated via response surface methodology (RSM). The resulting model equations were used to develop a cost model for the recovery chain, in order to select the optimum NF operating conditions. The optimization of the REE recovery chain, including pre-treatment and NF, was done with respect to the objective of maximizing the difference between NF concentrate economic value and treatment cost. NF with an appropriate MF pre-treatment has been effective in concentrating REEs from fly ash leachate, reaching the maximum potential gain at the optimum operating conditions of 12 bar and pH 3.5.