Magnesium Recovery from Nanofiltration Brine by Membrane Distillation Crystallization

Membrane distillation crystallization (MDCr) is gaining recognition as a sustainable and cost-effective method for treating hypersaline brine. The current study explores magnesium sulfate (MgSO₄) crystallization by using MDCr from synthetic nanofiltration (NF) brine. The study evaluates three feed temperature conditions (41.8 °C, 54.9 °C, and 64.5 °C), along with the corresponding permeate temperatures (19.9 °C, 23.2 °C, and 26.2 °C) and flow rates (1.3 and 0.7 L/min). The tested conditions revealed that temperature impacts the MDCr performance and MgSO₄ crystallization more effectively than the flow rate. The presence of other ions (Na⁺, K⁺, and Cl^‑) decreases the solubility of MgSO₄ (compared with the theoretical solubility at the tested temperature) and increases the tendency of co-crystallization with NaCl, which poses a significant challenge in the final separation stage. The examined process conditions (feed temperature 64.5 ± 0.5 and flow rate 1.3 L/min) successfully delay the crystallization of MgSO₄, toward a higher water recovery factor (65.98 %), owing to the higher solubility of MgSO₄ at higher temperatures, which minimizes the extent of co-crystallization. The recovered crystals (a mixture of NaCl and MgSO₄) are then separated by selectively dissolving NaCl in a saturated solution of MgSO₄. No compromise with the permeate purity (<5 μm/cm) was observed under all tested conditions.