Selective Ni/Fe separation from nickel laterite ore via two-stage reduction-sulfidation: Role of carbonaceous reductants

Ni in nickel laterite ore is mainly hosted in the crystal lattices of silicate minerals such as serpentine through isomorphous substitution, which makes conventional reduction–smelting processes suffer from poor Ni/Fe selectivity, substantial sulfur loss, and high carbon dependence. Herein, a two-stage carbon-assisted reduction–sulfidation strategy is proposed using CaSO4 as the sulfidizing agent. CaSO4 is first converted to active CaS during pre-reduction at 900 °C, and the reduction–sulfidation of Ni/Fe oxides and selective partitioning of Ni/Fe between low-nickel matte and slag are then achieved in a high-temperature molten stage at 1500 °C. By integrating HSC/FactSage thermodynamic calculations, TG-DSC, online gas analysis, XRD, SEM-EDS, and chemical titration, the mechanisms of anthracite, coke, and straw biochar were systematically compared. The results show that at C/CaSO4 = 2–3, CaSO₄ was almost completely converted to CaS, with a CaSO4‐to‐CaS conversion ratio of up to 98.94%. Notably, excessive carbon increased residual CaSO₄ and decreased the conversion ratio to 76.13%, which decoupled the sulfur retention ratio from the CaS conversion ratio; a high sulfur retention level cannot serve as direct evidence for efficient sulfidation conversion. Thermodynamic predominance diagrams combined with online gas monitoring defined a suitable window for selective Ni sulfidation: PO2 = 1×10-7–1×10-5 Pa and PSO2 = 1.00–17.38 Pa. The coke system maintained a persistently low oxygen potential and a moderate sulfur potential, promoting preferential reduction–sulfidation of Ni and its enrichment from the FeS matrix into Fe–Ni composite sulfides, thereby yielding the highest Ni grade of 19.32%. In the anthracite system, the relatively higher oxygen potential and lower sulfur potential led to Ni mainly dispersed in the FeS matrix, giving the highest Ni recovery of 89.34%. The straw biochar system was intermediate between these two systems, demonstrating the potential of biomass reductants to partially replace fossil carbon. Overall, the carbonaceous reductant governs nickel matte formation and selective Ni partitioning through its carbon supply capacity, gasification behavior, and regulation of oxygen and sulfur potentials. This work provides a thermodynamic basis and a synergistic reductant/atmosphere regulation strategy for low-carbon and selective Ni extraction from nickel laterite ore.