Investigation of Phase Equilibria and Polythermal Solubility in the MgCl₂–NaCl–H₂O System for the Extraction of Magnesium-Bearing Raw Materials from Natural Brines

Abstract

Natural brines are potential sources of magnesium-bearing raw materials, but their effective utilisation requires an understanding of salt–water phase equilibria and crystallisation behaviour. This study investigated phase equilibria and polythermal solubility in the MgCl₂–NaCl–H₂O system and evaluated natural brine from the Kuyimazor area of Kogon district, Bukhara region, Uzbekistan, as a possible source of magnesium-containing products. Model solutions containing magnesium chloride, sodium chloride, and water were examined by the visual polythermal method over the temperature range from −35.6 to −3.0 °C. Polythermal solubility diagrams and internal sections were constructed to identify crystallisation regions and phase-transition behaviour. The natural brine and crystallised salt products were characterised by standard chemical analysis, X-ray fluorescence spectroscopy, scanning electron microscopy with energy-dispersive spectroscopy, elemental mapping, and Fourier-transform infrared spectroscopy. Magnesium recovery was also examined by alkaline precipitation using sodium hydroxide solution. The constructed diagram indicated crystallisation regions corresponding to ice, NaCl, NaCl·2H₂O, MgCl₂·12H₂O, MgCl₂·8H₂O, and MgCl₂·6H₂O. Characteristic phase-transition temperatures were observed at approximately −35.6, −34.6, −20.6, −17.0, −4.4, and −3.0 °C. XRF analysis showed that the investigated salt sample contained 16.5 mass% Mg. The results showed that increasing magnesium chloride concentration reduced sodium chloride solubility, indicating a salting-out effect that may support magnesium enrichment in the liquid phase. XRF and SEM–EDS analyses showed that sodium, magnesium, chlorine, and oxygen were the dominant components of the studied salt materials. FTIR spectra confirmed the hydrated nature of the crystallised phases and indicated the formation of hydroxyl-containing magnesium precipitate after alkaline treatment. The findings provide physicochemical information relevant to the selective recovery of magnesium-bearing materials from local natural brines.