Nonisothermal Crystallization, Growth, and Shape Control of Magnetite Crystals in Molten Nickel Slag During Continuous Cooling

Efficient recovery of valuable metals from metallurgical slag has attracted an increasing amount of attention in recent years. In this paper, iron from nickel slag was recycled efficiently via a molten oxidation method. Nonisothermal crystallization of oxidized molten nickel slag and the growth of magnetite crystals were observed in situ by high-temperature confocal laser scanning microscopy (HT-CLSM). The growth and shape-control mechanism of magnetite crystals were also analyzed. The results show that the initial crystallization temperature of magnetite crystals in the melt was approximately 1450 °C, and the stable growth temperature ranged from 1400 °C to 1200 °C. The average growth rate of the crystals ranged from 0.013 to 0.141 μm/s at cooling rates of 5 to 50 °C/min. The magnetite crystals formed in the molten slag exhibit granular and dendritic morphologies. Stirring in the melt favors the formation of granular crystals with octahedral structures and depresses dendrite growth. The crystallization dynamics of magnetite crystals in molten slag is dominated by diffusion control, and the crystallization changed from three-dimensional growth to low-dimensional growth with the decrease of crystallization temperature. The apparent crystallization activation energy is in the range of − 581.98 ± 46.86 to − 339.36 ± 34.01 kJ/mol at a cooling rate of 5 to 50 °C/min.