The present study aims to investigate possible factors contributing to valuable metalsMetals losses, particularly nickelNickel, in slagsSlag from the nickel smeltingNickel smelting process. It involves a high-temperature phase equilibriaEquilibria study in the slagSlag systems relevant to nickel smeltingNickel smelting and industrial slagSlag sample analyses. Phase equilibriaEquilibria study defines the thermodynamic limits of the system, providing the foundation for analyzing the industrial samples. Deviations from the equilibrium help identify the potential factors restricting equilibrium leading to insights into finding the causes of metalMetals losses and recovery limitations. Quenched slagSlag samples were collected from various locations across the flash smeltingFlash smelting furnace at the Kalgoorlie Nickel SmelterNickel smelter (KNS) to analyze compositional changes in the molten slagSlag throughout the furnace. High-temperature equilibration laboratory experiments were also conducted to investigate phase equilibriaEquilibria in the “FeO”-MgO-SiO2\“FeO\”-MgO-SiO slagSlag system in equilibrium with Fe–Ni alloys under controlled oxygen partial pressures. Additionally, thermodynamic prediction and process modelling were performed using a thermodynamic database from the PyrometallurgyPyrometallurgy Innovation Centre (PYROSEARCH). The results indicate that nickelNickel losses in industrial samples occur through both entrainment of high-Ni matteMatte particles and the dissolutionDissolution of Ni in liquid slagSlag and olivineOlivine solid solution. Experimental findings suggest that, at constant temperature, oxygen partial pressure significantly influences nickelNickel solubilitySolubility in slagSlag and olivineOlivine solid solution. Assessing the mechanisms of nickelNickel loss provides valuable insights for optimizing smeltingSmelting conditions to enhance metalMetals recovery and improve the efficiency of industrial smeltingSmelting operations.