Na₂S, Na₂CO₃, FeO, FeS and carbon were used to regulate the properties of slag or metal fractions, and their effects on metal growth and metal–slag separation behavior were investigated. The growth of ferronickel grains can be enhanced by adding these additives, and Na₂S was the most effective. Na₂S, Na₂CO₃ and FeO mainly affected the properties of slag, while carbon and FeS affected the metal fraction. The onset temperature of metal–slag separation was 1297 °C for the sample without additive, which was decreased to 1123 and 1101 °C after adding 3.30 wt.% Na₂S and 4.47 wt.% Na₂CO₃, respectively. The onset temperature of metal–slag separation was mainly controlled by the slag fraction. The average apparent activation energy of metal grain growth was 125.32 kJ/mol without additive, and it decreased obviously after adding different additives. Na₂S also had the most remarkable effect on the decrease in activation energy.