In modern steelmaking, the stability and performance of foamy slag play a critical role in metallurgical reactions and overall production efficiency. One of the key mechanisms behind foamy slag behavior is the Marangoni Effect, which helps explain why slag foams remain stable or collapse during refining.
Slag Composition and the Marangoni Effect
Steelmaking slag is mainly composed of CaO, with additional surface-active components such as P₂O₅, SiO₂, CaF₂, and FeO. These substances tend to accumulate on the bubble film, increasing its elasticity.
Gas bubbles in slag are usually formed with high-melting compounds like 2CaO·SiO₂ and MgO·SiO₂ as nucleation points. Meanwhile, suspended solid particles such as CaO and MgO can attach to the bubble films, enhancing their strength but reducing elasticity. On the other hand, FeO and P₂O₅ improve elasticity but weaken film strength.
When surface-active agents are unevenly distributed, a surface tension gradient is created, driving liquid flow within the film. This phenomenon is known as the Marangoni Effect.
Impact of the Marangoni Effect on Steelmaking Quality
If bubble films are too weak or too elastic, they rupture quickly, causing rapid foam collapse. As a result, the foaming index decreases, the Marangoni Effect becomes less significant, and the metallurgical efficiency of steelmaking is negatively affected.
In practice, the foaming index is considered a reliable indicator of both the Marangoni Effect and foamy slag quality. Generally, a higher foaming index means a stronger Marangoni Effect and better metallurgical performance.
What is the Foaming Index?
The foaming index refers to the duration (measured in seconds) from the moment slag starts foaming until the bubbles break and collapse completely. It is one of the most important parameters to evaluate foamy slag quality and serves as a visual indicator for steelworkers to control metallurgical reactions.
In practical operations, the foaming index is measured as follows: under suitable slag basicity, when the molten pool carbon content is appropriate, a certain amount of carbon powder is blown into the furnace to create slag foaming. The slag height increases to its maximum, and after stopping carbon injection, the time taken for the foamy slag height to drop from maximum to minimum is defined as the foaming index.
Relationship Between MgO Content and the Foaming Index
Magnesium oxide (MgO) plays a vital role in foamy slag formation. MgO can react with CaO and SiO₂ to form low-melting calcium–magnesium olivine, accelerating slag formation. In addition, fine MgO particles and their compounds, with relatively high melting points, act as suspended nuclei that improve the foaming index.
In steelmaking practice, using light-burned dolomite, magnesia-lime, or other MgO-rich minerals can increase slag foaming ability while reducing refractory erosion. However, if the MgO content in electric furnace slag exceeds 10%, the quality of foamy slag significantly deteriorates.
