<p>Molten CaCl₂ is increasingly recognized for its potential in developing innovative mineral and material processing technologies as an effective solvent and electrolyte. However, controlling the accumulation of free O²⁻ remains a major challenge. This work explores the feasibility of eliminating free O²⁻ in molten CaCl₂ by adding MgCl₂. Thermodynamic analysis suggests that the introduction of Mg²⁺ favors the precipitation of O²⁻ as insoluble MgO (Mg²⁺ + O²⁻ → MgO↓). Experimental results confirm a rapid and vigorous reaction producing sub-micron MgO particles, which initially suspend, then coalesce, and eventually settle at the bottom. The process’s efficiency depends on the dosage of MgCl₂ added. A sub-stoichiometric dosage inadequately removes O²⁻, while a hyper-stoichiometric dosage disrupts the melt stability due to the widespread dispersion of soluble Mg(II) and MgO particles. A near-stoichiometric dosage, on the other hand, leads to optimal performance by facilitating the formation of Mg₂O²⁺, enabling localized MgO precipitation and thus preventing contamination of the bulk melt. These findings provide a foundation for designing more efficient and environmentally friendly molten CaCl₂-based processes for industrial applications.</p>