In electric arc furnace (EAF) steelmaking and other metallurgical processes, foamy slag plays a vital role in saving energy, protecting the furnace lining, and improving refining efficiency. So, what are the essential characteristics of good foamy slag?
1. Proper Foaming Ability
The slag material should melt quickly and form a liquid film with suitable viscosity. This allows gas bubbles to remain stable for 4–8 seconds, known as the foaming index, which is a key indicator of slag quality.
2. Suitable Viscosity and Stabilizing Particles
Slag viscosity must remain within an appropriate range:
- If too high, it will hinder metallurgical reactions such as dephosphorization.
- If too low, it can accelerate furnace lining erosion.
Good foamy slag should also contain suspended particles such as 2CaO·SiO2 and MgO-containing compounds (e.g., 2CaO·SiO2·MgO, MgO·SiO2). These compounds stabilize the slag and improve foaming performance.
3. Optimal FeO Content
The iron oxide (FeO) content in good foamy slag should remain within 14–20%. This ensures proper slag dissolution and supports refining reactions like dephosphorization.
When carbon content in molten steel is low, injecting carbon powder reacts with FeO to generate gases, which are necessary for foaming. However, if FeO exceeds 20%, it degrades the suspended particles (such as dicalcium silicate), reducing foaming quality.
4. Controlled Metallic Iron Beads
Foamy slag should contain metallic iron beads at a reasonable level. Ideally, slag flowing from the furnace door should not show emulsification, and slag entering the pit should not contain excessive glowing iron droplets.
For best results:
- Slag with basicity above 2.2 should appear flocculent.
- Slag with basicity around 2.0 should appear scaly.
5. Effect of Temperature
Temperature is a fundamental condition for slag melting. Once melted, as long as the carbon–oxygen reaction occurs in the bath, slag foaming can be achieved.
In general, temperature has a minor impact, but when slag basicity is 2.0–3.0 and the temperature exceeds 1650 °C, slag foaming quality declines significantly. Operators often use this phenomenon as an indicator of molten bath temperature.
