The Role of Operating Conditions in the Precipitation of Magnesium Hydroxide Hexagonal Platelets Using NaOH Solutions

Magnesium hydroxide, Mg(OH)₂, is an inorganic compound extensively employed in several industrial sectors. Nowadays, it is mostly produced from magnesium-rich minerals. Nevertheless, magnesium-rich solutions, such as natural and industrial brines, could prove to be a great treasure. In this work, synthetic magnesium chloride and sodium hydroxide (NaOH) solutions were used to recover Mg(OH)₂ by reactive crystallization. A detailed experimental campaign was conducted aiming at producing grown Mg(OH)₂ hexagonal platelets. Experiments were carried out in a stirred tank crystallizer operated in single- and double-feed configurations. In the single-feed configuration, globular and nanoflakes primary particles were obtained, as always reported in the literature when NaOH is used as a precipitant. However, these products are not complying with flame-retardant applications that require large hexagonal Mg(OH)₂ platelets. This work suggests an effective precipitation strategy to favor crystal growth while, at the same time, limiting the nucleation mechanism. The double-feed configuration allowed the synthesis of grown Mg(OH)₂ hexagonal platelets. The influence of reactant flow rates, reactant concentrations, and reaction temperature was analyzed. Scanning electron microscopy (SEM) pictures were also taken to investigate the morphology of Mg(OH)₂ crystals. The proposed precipitation strategy paves the road to satisfy flame-retardant market requirements.