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 ismostly produced from magnesium-rich minerals. Nevertheless, magnesium-richsolutions, such as natural and industrial brines, could prove to bea great treasure. In this work, synthetic magnesium chloride and sodiumhydroxide (NaOH) solutions were used to recover Mg(OH)(2) by reactive crystallization. A detailed experimental campaign wasconducted aiming at producing grown Mg(OH)(2) hexagonal platelets.Experiments were carried out in a stirred tank crystallizer operatedin single- and double-feed configurations. In the single-feed configuration,globular and nanoflakes primary particles were obtained, as alwaysreported in the literature when NaOH is used as a precipitant. However,these products are not complying with flame-retardant applicationsthat require large hexagonal Mg(OH)(2) platelets. This worksuggests an effective precipitation strategy to favor crystal growthwhile, at the same time, limiting the nucleation mechanism. The double-feedconfiguration allowed the synthesis of grown Mg(OH)(2) hexagonalplatelets. 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 roadto satisfy flame-retardant market requirements.Hexagonal magnesium hydroxide crystalswere successfullysynthesized using magnesium chloride and sodium hydroxide solutionswithout any post-treatment. This was achieved thanks to the accuratecontrol of the supersaturation level in a double-feed semi-batch reactor.The approach can be exploited to manage nucleation and crystal growthmechanisms in the extremely fast precipitation processes of metalhydroxides.