<p>The phase diagram is essential for optimizing slag compositions for smooth tapping and effective separation of matte and slag in the copper flash smelting process. Increased CaO, MgO, and Al₂O₃ impurities from the copper concentrates and flux require accurate and reliable phase diagrams to adjust the operating conditions accordingly. A series of pseudo-ternary phase diagrams were experimentally determined in the form of “FeO”−CaO−SiO₂ at the fixed MgO and Al₂O₃ concentrations under copper flash smelting conditions. The synthetic slags were equilibrated at 1300&#xa0;°C and Po₂ 10⁻⁹ to 10⁻⁷&#xa0;atm followed by quenching and electron probe microanalysis. Spinel, wüstite, and tridymite were identified to be the primary phases in the composition range of copper flash smelting slags. 1300&#xa0;°C isotherms were constructed as a function of impurity concentration and Po₂. Fully-liquid slag can be obtained by adjusting oxygen partial pressure and fluxing conditions at a fixed operating temperature using the multi-component phase diagrams. The accurate liquidus and solid solution compositions can also be used for optimization of the thermodynamic database in the future.</p>