The rotary kiln-electric furnace (RKEF) process has undergone a rapid development during the past decade, producing a significant quantity of ferronickel slag. At present, the ferronickel slag has become the fourth largest industrial solid waste in China with a utilization ratio of less than 10 wt%. It is urgent to seek an efficient method for utilization of ferronickel slag. In this study, the thermodynamic characteristics of ferronickel slag sintered in the presence of magnesia (additive) for preparing refractory materials were assessed by calculating relevant thermodynamic functions and phase diagrams. The thermodynamic results showed that by sintering ferronickel slag with the addition of magnesia at appropriate temperatures, it is possible to promote the formation of forsterite and spinel phases, which would contribute to high refractoriness of the refractory materials derived from the slag.