<p>Magnesium (Mg) is one of the main impurities in the zinc hydrometallurgy process. The objective of this study was to separate and recover Mg from freezing crystallization from waste electrolyte. In this study, magnesium sulfate whiskers were successfully prepared through a combined process of neutralizing precipitation and hydrothermal synthesis. First, waste electrolyte was treated using freezing crystallization at 258.15&#xa0;K, producing co-crystallization products of ZnSO₄·7H₂O and MgSO₄·7H₂O. Second, these products were subjected to water dissolution and neutralizing precipitation to achieve Zn-Mg separation. Under these conditions, the Zn precipitation ratio exceeded 99.9%, and the recovered Zn was returned to the hydrometallurgical process, while Mg remained in the solution. Finally, magnesium sulfate whiskers were synthesized hydrothermally from the Mg-containing solution. Characterization through b-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy-energy-dispersive x-ray spectroscopy confirmed that the obtained whiskers were MgSO₄·5Mg(OH)₂·3H₂O, with an average size of 205.79&#xa0;μm. Furthermore, thermogravimetry and differential thermal analysis revealed that the decomposition temperature of MgSO₄ in the whiskers was 1001°C, indicating excellent thermal stability. These results demonstrated that the synthesized magnesium sulfate whisker is a promising flame-retardant filler with high decomposition temperature and no release of hazardous by-products.</p>