Synergetic Effects of Initial Na ⁺ and NH ₄ ⁺ Concentrations on Mg(OH) ₂ Precipitation and Kinetic Parameters Using Bischofite from Salt Lake

Alkali metal ions (e.g., Na⁺) are extremely common in abandoned bischofite from salt lakes. Clarifying the role of these ions in salt lake systems during the crystallization and growth of magnesium hydroxide (Mg(OH)₂) holds notable value for supporting the large-scale industrial production of Mg(OH)₂ using abandoned bischofite. This study aims to elucidate the effects of Na⁺ on the nucleation and growth patterns of Mg(OH)₂ in an ammonia-based preparation system derived from abandoned bischofite. The effects of Na⁺ on the physicochemical properties of Mg(OH)₂─including crystal structure, morphology, particle size, and size distribution─were also systematically investigated. The variation patterns of pH, conductivity, and other parameters during the reaction process under different NH₄⁺:Mg²⁺ molar ratios and Na⁺ concentrations were studied. The synthesized Mg(OH)₂ products were characterized using X-ray diffraction, scanning electron microscopy, laser particle size analysis, and thermogravimetric analysis. Results indicated that Na⁺ and the NH₄⁺:Mg²⁺ molar ratio exerted a strong synergistic effect on crystal face polarity, particle size, morphology, and the conversion efficiency of Mg(OH)₂. Mechanistic analysis revealed that the presence of Na⁺ accelerated nucleation, altered the growth system, regulated growth kinetics, and ultimately determined the physicochemical properties of the final product. This research provides a theoretical basis for the preparation of high-performance Mg(OH)₂ from abandoned bischofite in salt lakes.