Recovering Magnesium from Ferronickel Slag by Vacuum Reduction: Thermodynamic Analysis and Experimental Verification

The feasibility of recovering magnesium from ferronickel slag by vacuum reduction was evaluated. The thermodynamic calculations indicated that the magnesia in slag can be reduced to gaseous magnesium by Si, FeSi, Al, and C, with the minimum reduction temperatures of 2324, 2530, 1678, and 2580 K at 100 000 Pa, respectively. As the system pressure decreases, the minimum reduction temperatures decline significantly. Si maintains the minimum reduction temperature of 1585-1673 K at the atmospheric pressure of 10-100 Pa, acting as a suitable reducing agent for recovering magnesium. To verify the findings, preliminary vacuum reduction experiments, in which CaO was added to eliminate the adverse impact of SiO₂ in slag, were carried out. By reducing slag with additions of 50 wt % Si and 30 wt % CaO at 1573 K for 3 h at 10 Pa, the recovery of magnesium reached 97.74%.