<p>Iron extraction from nickel-copper smelting slag has remained an unresolved problem up to now. In this study, original slag produced by the traditional nickel smelting process and modified slag produced by replacing silicon dioxide (SiO₂) with calcium oxide (CaO) as a flux were used as raw materials for iron-extraction experiments by carbothermal reduction. The influences of reduction time, temperature, and slag type on the iron reduction extent were investigated. The results show that the Fe-containing compounds in modified slag are more easily reduced by carbon. When CaO is added or supplemented to each nickel smelting slag to adjust the alkalinity to 0.8 uniformly, and the reduction time is 30 minutes at 1473 K, the iron reduction extent of the modified slag reaches 73.81 pct, which is 26 pct higher than that of the original slag. Further analysis reveals that in the reduction product of the modified slag, the precipitation amount of iron particles is significantly increased and the particle size is enlarged. The unreduced Fe-containing minerals are mainly hortonolite. The results of kinetic analysis indicate that the reduction of the original slag is controlled by the interfacial chemical reaction and diffusion. The activation energy of the modified slag reduction is 16.09 kJ/mol in the range of 0–15 minutes and 14.82 kJ/mol in the range of 15–30 minutes, and it is mainly controlled by diffusion with a lower activation energy, making the reduction reaction easier to proceed. From the perspective of the reduction mechanism, the reduction of the original slag mainly depends on the substitution effect of CaO on ferrous oxide in the Fe-containing minerals, while the reduction process of the modified slag mainly focuses on the reduction of iron oxide generated after the decomposition of ferrite. The main reason for the low reduction extent in the original slag is that the ferrous oxide in the hortonolite is difficult replaced by CaO.</p>