There is a high iron content in nickel slag that mainly exists in the fayalite phase. Basic oxide can destroy the stable structure of fayalite which is beneficial to the treatment and comprehensive utilization of nickel slag. The research was based on the composition of the raw nickel slag, taking the CaO-SiO₂-FeO-MgO system as the object and CaO as a modifier. The effect of basicity on the melting characteristics, viscosity and structure of the CaO-SiO₂-FeO-MgO system was studied. The relationship between the viscosity and structure of the CaO-SiO₂-FeO-MgO system was also explored. The results show as follows: (1) When the basicity is lower than 0.90, the primary phase of the slag system is olivine phase. When the basicity is greater than 0.90, the primary phase of the slag system transforms into monoxide. When the basicity is 0.90, olivine and monoxide precipitate together as the temperature continues to decrease. At the same time, the liquidus temperature, softening temperature, hemispherical temperature, and flow temperature all reach the lowest value. (2) With the increase of basicity, the critical viscosity temperature of the CaO-SiO₂-FeO-MgO system decreases first and then increases. Critical viscosity temperature is the lowest at the basicity of 0.90, which is 1295 °C. (3) When the slag system is heterogeneous, the viscosity of the molten slag increases rapidly because of the quantity of solid phase precipitated from the CaO-SiO₂-FeO-MgO system. (4) When the slag system is in a homogeneous liquid phase, the molar fraction of O⁰ decreases with the increase of basicity and the mole fraction of O⁻, and O²⁻ increases continuously at the basicity of 0.38~1.50. The silicate network structure is gradually depolymerized into simple monomers, resulting in the degree of polymerization, and the viscosity, being reduced. The mole fraction of different kinds of oxygen atoms is converged to a constant value when the basicity is above 1.20.

The iron resources in nickel slag were recycled by oxidation and magnetic separation. The effects of holding time, temperature, air flow rate and basicity on the crystallization of magnetite were investigated systematically. Moreover, the influence of particle size and magnetic flux density on the recovery and grade of iron during the magnetic separation was also explored. Results showed that the magnetite particles were significantly influenced by holding time, and the average diameter size reached about 20 μm after holding for 20 min at 1623 K. The holding temperature obviously affected the microstructure of magnetite phases: with the increase in holding temperature, the shapes of the magnetite particles changed from polyhedral form to skeletal particles. As the air flow rate was increased to 170 mL/min, the magnetite developed into tiny spherical particles due to the strong stirring. It was also found that the crystallization of magnetite was slightly effected by basicity. The iron recovery reduced with the decrease of particle size, while the iron grade first increased to a maximal value of 38 μm, and then decreased. As the magnetic flux density increased, the iron recovery initially increased rapidly, reaching a maximal value at 120 mT, while the iron grade remained almost constant. The final iron recovery and grade were 75.99% and 54.08%, respectively, via multi-step magnetic separation instead of single magnetic separation. Iron in concentrate mainly exists in the form of magnetite and magnesium ferrite, and contents of siderophile elements (Ni, Co) in final concentrate were also higher than that of raw slags.

Nickel smelting produces a large amount of nickel slag which contains a lot of Fe in the form of FeO. The effect of FeO content on the melting properties and structure of nickel slag is investigated by thermodynamic calculations and experiments, using the nickel slag of water quenching furnace as raw material, which is simplified to CaO-SiO2-FeO-MgO slag system. The results are as follows: with increasing FeO content, the precipitated primary phase changes from melilite to olivine. The liquidus temperature, softening temperature, hemispherical temperature, flow temperature, viscosity, and critical viscosity temperature of the slag system tend to decrease gradually, but the solid phase temperature tends to decrease first and then increase. The centre of the symmetric stretching vibration band of the [SiO4]4 tetrahedron -in the slag system shifts to the low wavenumber region. The mole fraction of O0 in the slag continuously decreases, and the mole fraction of O- and O2- increases. The complex silicon-oxygen tetrahedron structure gradually disintegrates and the structural unit tends to be simple.