<p>Saprolite laterite (SL) is primarily utilized in the production of ferronickel through the rotary kiln-electric furnace (RKEF) process. Nonetheless, the substantial SiO₂ and MgO content in SL leads to notable slag production with a high melting point and elevated energy consumption. This research explores the properties of the MgO-SiO₂-FeO-(CaO, Al₂O₃, Cr₂O₃) slag system with varying FeO levels to formulate an optimized slag system to diminish energy consumption in the RKEF process. The outcomes reveal that increasing the FeO content in the slag beyond 10% notably decreases its melting temperature and viscosity while enhancing its electrical conductivity. Moreover, a higher FeO content in the slag leads to decreased sulfur and phosphate capacities; nevertheless, it considerably enhances the kinetic circumstances for desulfurization and dephosphorization. Industrial trials demonstrate that elevating the FeO content in the slag to 12–16% results in a substantial reduction in energy consumption throughout the RKEF process, with negligible alterations in phosphorus, sulfur, or other impurity levels in the ferronickel alloy. These research findings offer noteworthy insights into optimization of both the slag system and production operations, thereby serving as theoretical guidance for effective and low-carbon ferronickel production via the RKEF process.</p>