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 nano-flakes 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 favour crystal growth while, at the same time, limiting nucleation mechanism. The double-feed configuration allowed the synthesis of grown Mg(OH)2 hexagonal platelets. The influence of reactants flow rates, reactants concentrations and reaction temperature was analyzed. 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.