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.

High-percentage iron resources in nickel slags were recovered as magnetite via molten oxidation process, and the transfer behavior of Fe element was studied. The elemental distribution in oxidized slag samples, the influence of atmosphere, holding temperature and time on magnetite crystal growth, and Fe element distribution in magnetic materials were also investigated. It was found that magnetite could be produced from fayalite or hortonolite in nickel slags during molten oxidation with CaO as a modifier, air as an oxidizer, accompanying with the enrichment of Fe, Co, Ni and Cu. The select of atmosphere is very important during the precipitation and growth of the magnetite crystals. The magnetite crystals precipitated invisibly or slightly in argon atmosphere, while exhibited dendritic structures with crystallization content of ³18.5% in air atmosphere. Especially, after blowing air into molten slag for 30 min, magnetite crystals develop well-distributed and complete, resulting in its crystallization content increases up to 33.5%. The Fe content in the matrix of oxidized samples remained approximately constant after holding for 20 min. Mössbauer spectra analysis indicates that the 89.6% of Fe exists in magnetite phases, while only 10.4% of Fe in hedenbergite. It was also found that Ni and Co simultaneously concentrate in the magnetite phase, indicating that Fe, Ni, and Co can be recovered effectively from nickel slag.

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