In electric arc furnace (EAF) steelmaking, the properties of slag play a decisive role in refining efficiency and steel quality. Among these properties, the melting point of slag is a key parameter.
The melting point of slag refers to the temperature at which solid slag completely transforms into a uniform liquid phase upon heating, or the temperature at which liquid slag begins to crystallize upon cooling. Since steelmaking slag is a multi-component system formed from various complex oxides, its melting process does not occur at a single fixed temperature, but within a certain temperature range.
In practice, the slag used in steelmaking is generally required to have a melting point 40–220°C lower than that of molten steel. Specifically, in EAF steelmaking, the slag melting point is usually 60–120°C lower than that of liquid steel. This ensures that slag can perform its essential metallurgical functions—such as covering the molten bath, absorbing impurities, and protecting the steel—during the refining process.
Typical Melting Point of Slag in Electric Arc Furnace Steelmaking
Based on industrial practice, the melting point of different types of EAF slag can be summarized as follows:
- Oxidizing slag: around 1230–1525°C
- Reducing slag: around 1430–1520°C
Interestingly, the melting point of individual oxides that form slag (CaO, MgO, SiO₂, FeO, etc.) is often much higher than the actual melting point of the slag. This difference is explained by chemical bonding: complex compounds weaken bond energies, resulting in a significantly lower melting point. This allows slag to form and flow properly in the steelmaking temperature range.
Examples of Common Slag Compounds and Their Melting Points (°C)
| Compound | Mineral Name | Melting Point (°C) |
| CaO·SiO₂ | Calcium Silicate | 1550 |
| MnO·SiO₂ | Manganese Silicate | 1285 |
| MgO·SiO₂ | Magnesium Silicate | 1557 |
| 2CaO·SiO₂ | Dicalcium Silicate | 2130 |
| 2FeO·SiO₂ | Fayalite | 1205 |
| 2MnO·SiO₂ | Tephroite | 1345 |
| 2MgO·SiO₂ | Forsterite | 1890 |
| CaO·MgO·SiO₂ | Monticellite | 1390 |
| CaO·FeO·SiO₂ | Kirschsteinite | 1205 |
| 2CaO·MgO·SiO₂ | Akermanite | 1450 |
| 3CaO·MgO·2SiO₂ | Merwinite | 1550 |
| 2CaO·P₂O₅ | Dicalcium Phosphate | 1320 |
| CaO·Fe₂O₃ | Calcium Ferrite | 1230 |
| 2CaO·Fe₂O₃ | Dicalcium Ferrite | 1420 |
Factors Affecting the Melting Point of EAF Slag
- Slag Basicity (CaO Content)
Slag basicity directly affects its phase composition. A higher basicity typically raises the melting point. Excess lime addition can also slow down slag formation during steelmaking. - Magnesia (MgO) Content
Moderate MgO promotes the formation of low-melting-point calcium–magnesium silicates, reducing slag formation temperature. However, excessive MgO increases the melting point, reducing slag fluidity. - Iron Oxide (FeO, Fe₂O₃) Content
Iron oxides can combine with lime to form low-melting-point compounds, lowering slag melting temperature and improving fluidity. In the EAF oxidizing stage, if slag is too viscous or lime remains unmelted, oxygen blowing can increase FeO content, reducing melting point and enhancing slag reactions.
Conclusion
The control of slag melting point is essential for efficient electric arc furnace steelmaking. Ideally, slag should remain fully liquid at steelmaking temperatures to ensure effective refining, impurity removal, and protection of molten steel. By optimizing slag basicity, MgO levels, and FeO content, steelmakers can achieve the desired melting characteristics and improve overall steel quality.
