Preparation of nano-hexagonal flake magnesium hydroxide from seawater brine and the crystallization-based separation of inorganic salt products from the mother liquor

This paper presents a novel technology for highly efficient and comprehensive utilization of seawater brine. First, utilizing simulated seawater brine as the raw material, the high-purity nano-hexagonal flake Mg(OH)₂ flame retardant could be synthesized through a two-step process including ammonia precipitation and hydrothermal synthesis. Optimal conditions and reaction mechanisms for both ammonia precipitation and hydrothermal synthesis are elucidated. The optimal conditions for ammonia precipitation are as follows: an addition of 1 wt% Mg(OH)₂ seed, a dropping time for NH₃·H₂O of 60 min, an aging time of 3 h, and a reaction temperature maintained at 30 °C. The optimal conditions for hydrothermal synthesis are as follows: a dosage of 1-tetradecyl-3-methylimidazolium bromide ([C₁₄mim]Br) at 4 wt%, a hydrothermal time of 6 h, a hydrothermal temperature set at 140 °C, and a solid-liquid ratio of 5:100. The nano-hexagonal flake Mg(OH)₂ prepared with real seawater brine exhibits almost the same properties as that synthesized from simulated seawater brine. The addition of solid NaOH to the mother liquor after ammonia precipitation could achieve the recycling of NH₃·H₂O and complete recovery of Mg²⁺. Subsequently, a cascade separation process for sodium, potassium, bromine, and other compounds from magnesium-removed simulated seawater brine through crystallization has been developed. The optimal process conditions were identified and experimentally validated. This work provides a simple, low-energy and high value-added pathway for comprehensive utilization of seawater brine.