Shaping Mg(OH)2 crystals through hydrothermal treatment

Shaping Mg(OH)2 crystals through hydrothermal treatment Ferdinando Micilettaa,b,*, Giuseppe Battagliaa, Giuseppe Polilloa, Fabrizio Vicarib, Alessandro Tamburinia,b, Andrea Cipollinaa, Giorgio Micalea,*. aDipartimento di Ingegneria, Università degli Studi di Palermo, Palermo, Viale delle Scienze, 90128, Italy bResourSEAs SrL, Via Notarbartolo n. 38, 90141 Palermo, Italy E-mail: ferdinandodomenico.miciletta@unipa.it - giorgiod.maria.micale@unipa.it Magnesium hydroxide (MDH) is a compound widely employed in several industrial sectors, with increasing interest as halogen-free flame-retardant agent. Battaglia et al. [1] demonstrated a practical approach to recover MDH from waste brines via reactive crystallization, achieving high conversions and purities (about 100%). On the other hand, nanometric strongly aggregated crystals have been mainly obtained. These crystals do not comply with market requirements for flame-retardant applications: (i) a specific surface area (SSA) below 10 m2/g, (ii) hexagonal plate-like morphology and (iii) a narrow particle size distribution without aggregation [2]. To enter the flame-retardant market, as part of the European project LIFE23-ENV-IT-MareMag LIFE, this work focuses on the hydrothermal modification of magnesium hydroxide powders obtained from waste bittern solutions. The hydrothermal treatment relies on a dissolution–recrystallization mechanism, promoting the rearrangement of the solid phase to minimize its surface energy and increase crystal size. For the first time, a detailed comparative study was conducted exploring Mg(OH)2 suspensions and particles (after filtration and drying) with controlled particle sizes and morphologies. The influence of key operating parameters, namely temperature, reaction time, feed suspension concentration, and the presence of a mineralizing agent such as sodium hydroxide, was also analyzed. A specific surface area of 8 m²/g and a hexagonal morphology were achieved at 200 °C for reaction times exceeding 30 hours (using dried solids). Notably, similar performances were obtained under milder operating conditions (180°C and 5h) using suspensions with controlled initial nanoparticles distributions, leading to routes for industrial applications. Results will be adopted for scale-up and energy optimization studies. Keywords: Hydrothermal recrystallization, Magnesium hydroxide, Crystal morphology. References [1] [2] G. Battaglia, M. A. Domina, S. Romano, A. Tamburini, A. Cipollina, and G. Micale, “Magnesium Hydroxide Nanoparticles Production from Natural Bitterns,” Chem Eng Trans, vol. 96, pp. 43–48, 2022 M. Ren, M. Yang, S. Li, G. Chen, and Q. Yuan, “High throughput preparation of magnesium hydroxide flame retardant: Via microreaction technology,” RSC Adv, vol. 6, no. 95, pp. 92670–92681, 2016