Membrane distillation crystallization’s parametric analysis for magnesium sulphate crystallization from simulated nanofiltration brine

Abstract

Magnesium (Mg) is a valuable cation in waste brine, abundant in oceanic reserves (∼1 million tons/km3), and essential for various industries. Conventional crystallization methods often produce inconsistent crystals due to limited control over supersaturation. Membrane distillation crystallization (MDCr), however, provides precise control of nucleation and crystallization, making it promising for mineral extraction. This study explores MgSO₄ crystallization using MDCr, initially with pure MgSO₄ solutions, then individually adding NaCl, KCl, and MgCl₂, and finally using a mixed solution. Key parameters studied included feed temperature, flow rate, and ion concentrations. PHREEQC simulations were compared against experimental outcomes. Tests were conducted at feed temperatures of approximately 41 °C, 51 °C, and 62 °C, with permeate temperatures of about 20 °C, 24 °C, and 30 °C, respectively, and flow rates of 13.3, 23.6, and 30.1 mL/sec. Results indicated temperature significantly affects MDCr performance (transmembrane flux, log mean temperature difference and permeate conductivity) and crystallization (mean crystal size, coefficient of variation, turbidity and induction time). NaCl and MgCl₂ triggered earlier crystallization at lower supersaturation levels and recovery factors, increasing induction time which results in increased mean crystal size and coefficient of variation (CV%), whereas KCl showed minimal impact on MDCr but have noticeable effect on crystallization. With all tested conditions MDCr maintained CV% below 50, highlighting its ability to ensure crystal uniformity. Experimental crystallization points aligned closely with PHREEQC predictions (deviation < 6 %). Permeate conductivity remained below 1 μS/cm, underscoring MDCr’s efficiency in producing high-purity water from concentrated brines. These findings provide essential insights into MgSO₄ recovery from NF brines.