Preparation of high-purity hexagonal flaky magnesium hydroxide and high-purity magnesium oxide from seawater bittern

Currently, comprehensive utilization technology of seawater bittern faces several challenges, including low resource utilization efficiency, limited value of magnesium salt products, and high energy consumption. This study proposes an innovative approach to prepare high-purity MgO. This process begins with the reaction of brine with a mixed alkaline solution of NaOH-Na 2 CO 3 to synthesize basic magnesium carbonate (4MgCO 3 ·Mg(OH) 2 ·4 H 2 O). Subsequently, 4MgCO 3 ·Mg(OH) 2 ·4 H 2 O is transformed into a high-purity Mg(OH) 2 structure through the hydrothermal conversion method. Finally, high-purity MgO is obtained by calcining magnesium hydroxide. The effects of mixed alkali precipitation and hydrothermal reaction conditions on the structure, morphology, and purity of intermediate products and MgO are systematically investigated. The optimal conditions for preparing high-purity MgO are determined as follows: the molar ratio of Mg 2 + :CO 3 2− :OH − in seawater bittern to the NaOH-Na 2 CO 3 mixed alkali is 5:4.7:2, at a reaction temperature of 60 °C, stirring speed of 200 rpm, stirring time of 10 min, and reaction time of 35 min. Under these conditions, spherical 4MgCO 3 ·Mg(OH) 2 ·4 H 2 O is initially obtained. Subsequently, the 4MgCO 3 ·Mg(OH) 2 ·4 H 2 O underwent hydrothermal treatment at a solid-to-liquid ratio of 8:100 and a temperature of 120 °C. Polyethylene glycol (PEG) is added as a surfactant at 3  wt % of the solid content of 4MgCO 3 ·Mg(OH) 2 ·4 H 2 O, along with 5 g of NaOH (accounting for 62.5  wt %), for 2 h. This process yielded high-purity Mg(OH) 2 with a thin flake morphology. Finally, the high-purity Mg(OH) 2 is calcined at 500 °C for 4 h, resulting in cubic crystal nano-MgO with a purity of 99.8 % and an average particle size of approximately 30 nm.