Magnesium recovery from brines by a novel non-dispersive membrane-assisted reactive crystallization (ND-MARC) process...

Efficient recovery of magnesium from saline solutions is essential for resource valorization and sustainable brine management. Conventional precipitation processes typically require large excesses of alkaline reagents, leading to high operational costs and secondary waste. This study proposes a novel non-dispersive membrane-assisted reactive crystallization (ND-MARC) process that couples selective NH₃ transfer through a hydrophobic membrane with controlled in situ precipitation of Mg(OH)₂. The process was experimentally assessed by varying temperature, flow rate, and Mg and NH₃ concentrations, achieving Mg recoveries above 95% and identifying NH₃ concentration, Mg concentration, and ammonia solution temperature as the most statistically significant parameters, with the NH₃:Mg ratio as an influential condition on performance. The ND-MARC system achieved high magnesium recoveries with apparent NH₃:Mg molar ratios ranging from 1.17 to 2.07 under specific operating conditions. under specific operating conditions. These values are competitive with those reported for conventional stirred reactors, while also allowing for control of local supersaturation by regulating the NH₃ feed and hydrodynamic conditions, as demonstrated by SEM images, which revealed systematic changes in crystal morphology and size distribution. This behavior may contribute to enhanced reagent utilization efficiency and reduced effective ammonia consumption. A phenomenological model incorporating NH₃ transfer, chemical equilibria, and Mg(OH)₂ precipitation kinetics reproduced the experimental trends, with prediction errors below 12%. The analysis revealed that fouling resistance becomes dominant at high NH₃ concentrations and high ammonia solution temperature, accounting for up to approximately 80% of the total resistance. Overall, the ND-MARC configuration provides a controlled, efficient, and reagent-saving alternative for magnesium recovery, while enabling Mg(OH)₂ production, establishing a promising platform for sustainable reactive crystallization and selective resource recovery from industrial brines.