Experimental and economic evaluation of nanofiltration as a pre-treatment for added-value elements recovery from seawater desalination brines

Abstract

Desalination brine mining emerges as a solution to supply raw materials to the European Union industry in a circular economy approach since valuable minerals and metals (e.g., B(III), Mg(II), Ca(II), Sc(III), V(V), In(III), Ga(III), Li(I), Mo(VI), Rb(I)) are present in seawater but in low concentrations. Brine pre-treatment is important to remove species that may impair the performance of other technologies involved and to increase recovery efficiencies. Hence, nanofiltration and calcium precipitation were proposed as pre-treatment stages. Nanofiltration was studied to separate monovalent from multivalent elements (Fonsalía desalination plant case study), while economically, it was evaluated whether it would be better to place it before (Scenario 1) or after (Scenario 2) a Ca(II) precipitation stage, considering that Ca(II) can produce scaling in membranes. Three commercial membranes were tested using synthetic brines at 30 bar. Experiments (65 % permeate recovery) showed that Fortilife XC-N and PRO-XS2 membranes presented higher Ca(II) and Mg(II) rejection than NF270. Heating the brine for Ca(II) precipitation jeopardizes the economic feasibility of the project. Scenario 1 was the best configuration since it presented lower total levelized cost (≈1.6 €/m3 inlet, without heating the brine for Ca(II) removal). In such scenario, PRO-XS2 reported the best selectivity between monovalent and multivalent elements.