In steelmaking, sulfur significantly reduces the mechanical properties of steel, increasing brittleness and lowering ductility. Therefore, desulfurization in the electric arc furnace (EAF) is a critical process to ensure high-quality steel production. According to molecular theory, the desulfurization mechanism of basic reducing slag can be divided into the following steps:
- Diffusion of sulfur from molten steel to slag
[FeS] → (FeS) - Formation of stable compounds in slag
(FeS) + (CaO) → (CaS) + (FeO) - Overall reaction
[FeS] + (CaO) → (CaS) + (FeO)
This shows that the essence of desulfurization in EAF steelmaking is the diffusion of sulfur at the steel–slag interface and its reaction with CaO to form CaS.
Key Factors Affecting Desulfurization in the Electric Furnace
The efficiency of sulfur removal in basic reducing slag depends on several factors:
Slag Basicity
Calcium oxide (CaO) is the primary condition for desulfurization. In acidic slag, CaO is bound with SiO₂, leaving no free CaO for desulfurization. With increasing basicity, the free CaO content rises and improves desulfurization ability. However, excessive basicity increases slag viscosity and reduces interfacial kinetics. In practice, a basicity ratio of 2.0–3.0 offers optimal desulfurization conditions.
Iron Oxide Content in Slag
Lowering FeO content in the slag favors the equilibrium shift toward desulfurization, thereby improving operational efficiency.
Addition of Fluorspar (CaF₂)
A moderate amount of CaF₂ enhances slag fluidity, accelerates slag formation, and promotes sulfur diffusion toward the interface. CaF₂ can also directly react with sulfur to form volatile desulfurization products. However, long-term reliance on fluorspar has negative side effects, making it an unsustainable approach.
Temperature
Within the temperature range of EAF steelmaking, equilibrium constants do not change much. Yet, higher temperatures improve the fluidity of both molten steel and slag, increase sulfur diffusivity, and accelerate desulfurization.
Slag Volume
Increasing slag volume helps dilute desulfurization products, raising sulfur capacity and enhancing reactions. On the other hand, excessive slag thickness hinders diffusion and slows down desulfurization.
The Role of Kinetics
Even under the same slag composition, kinetic conditions have a decisive influence on desulfurization efficiency. Stirring the molten pool or intensifying steel–slag interface movement can:
- Increase contact area
- Accelerate sulfur diffusion
- Enhance overall desulfurization rate
This is why kinetic control is widely used in practical steelmaking operations.
Why Is Desulfurization Easier in High-Carbon Steel?
At the same steelmaking temperature, high-carbon steel desulfurizes more easily than low-carbon steel because:
- Lower oxygen content: Higher carbon content reduces the equilibrium oxygen level in molten steel, which is favorable for desulfurization.
- Better fluidity: High-carbon steel has greater molten fluidity, which enhances steel–slag reactions.
Therefore, in industrial practice, high-carbon steel typically achieves better desulfurization results than low-carbon steel.
