Carbothermic Reduction and Sulfidation Behavior for Fe–Ni–S Matte Production from Synthetic Saprolitic Nickel Ore

This study investigates the production behavior of Fe–Ni–S matte from synthetic nickel ore designed to simulate low-grade saprolitic laterite. The synthetic feed was formulated based on XRF and XRD analyses of magnetically upgraded laterite concentrate. Thermodynamic modeling, including phase stability analysis, Ellingham evaluation, viscosity prediction, and sulfidation equilibria, was employed to define optimal smelting conditions. Carbothermic reduction at 1550 °C enabled selective reduction in NiO and FeO, leading to the formation of Fe–Ni alloy droplets, which subsequently reacted with FeS to produce Fe–Ni–S matte. The carbon ratio played a critical role in controlling FeO content in slag, thereby influencing slag basicity and viscosity. An optimal carbon ratio of 0.2–0.4 mol maintained slag viscosity within the industrially favorable range (2–5 poise) and minimized crucible dissolution. Thermodynamic analysis confirmed that FeS is the only stable sulfide phase at high temperature and dissolves into the Fe–Ni melt, promoting stable matte formation. Under optimized carbon and FeS addition conditions, a maximum nickel recovery of approximately 88% was achieved, attributed to improved slag composition, controlled viscosity, and enhanced matte–slag separation. These results demonstrate that simultaneous carbothermic reduction and sulfidation is an effective route for Fe–Ni–S matte production from saprolite-derived oxide feed. Control of carbon ratio, FeS addition, and Al2O3 flux is essential for achieving stable matte formation and efficient metal–slag separation.