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Metallurgical Principles of P and S Removal During Steelmaking Molten Steel Refining
Aug 31,2020
(1) Metallurgical Reaction Principle of De-phosphorization
Phosphorus exists in molten steel in the form of Fe2P, which has high solubility in molten steel. It readily reacts with (FeO) that diffuses from the slag into the molten steel, releasing heat in the process. The reaction equation is:
2[P] + 5(FeO) → (P₂O₅) + 5[Fe]; ΔH = -260,000 J
Phosphorus oxides have very low solubility in molten steel but are readily soluble in slag and react with (FeO) present in the slag to form (3FeO·P₂O₅). The reaction is as follows:
(P2O5) + 3(FeO) → (3FeO·P2O5); ΔH = -127,900 J
Both (P2O5) and (3FeO·P2O5) are unstable oxides that decompose at even slightly elevated temperatures during smelting, causing phosphorus to re-enter the molten steel. Consequently, slag dominated by FeO has very poor dephosphorizing ability. To achieve an excellent dephosphorizing effect, it is essential to add a strongly basic oxide—CaO (lime)—to the slag, which combines with (P2O5) to form stable calcium phosphate. The reaction equation is as follows:
(P2O5) + 4(CaO) → [(CaO)4·P2O5]; ΔH = -689,700 J
The overall reaction equation for the dephosphorization process is as follows:
2[P] + 5(FeO) + 4(CaO) → [(CaO)₄·P₂O₅] + 5[Fe]; ΔH = -949,700 J
As shown in the reaction equation above, to effectively carry out dephosphorization, pay attention to the following aspects:
(1) High oxidizability of the molten steel and high slag basicity are essential conditions for P removal.
(2) Control the molten steel temperature: Since dephosphorization is an exothermic reaction, a lower molten steel temperature is conducive to P removal.
(3) Slag with good fluidity can enhance the activity of CaO in the slag, which is conducive to phosphorus removal.
(4) Strengthening the stirring of the molten steel and slag is conducive to P removal.
(2) Metallurgical Reaction Principles of Desulfurization
Sulfur exists in both the molten steel and the slag in the form of FeS. The [FeS] in the molten steel and the (FeS) in the slag can mutually transfer through diffusion. At a given temperature, the mass fraction ratio between the two remains constant. The desulfurization process exploits this principle. Under conditions where the molten steel has been sufficiently deoxidized and reduced, the (FeS) in the slag reacts with Ca²⁺ to form CaS, thereby reducing the (FeS) content in the slag. As a result, the [FeS] in the molten steel diffuses into the slag, achieving the goal of desulfurizing the molten steel. The reaction equation is as follows:
(FeS) + (CaO) = (CaS) + (FeO)
Therefore, when carrying out desulfurization work, pay attention to the following aspects:
(1) The more thoroughly the molten steel is deoxidized, the better. During the reduction stage, it is essential to carry out effective deoxidation of the molten steel—this is a prerequisite for desulfurization.
(2) Increase the basicity of the slag to ensure sufficient (CaO) is available for the reaction. It is generally believed that a slag basicity of 2.5 to 3.5 is optimal.
(3) An appropriately higher molten steel temperature. Raising the molten steel temperature can reduce the viscosity of the slag, facilitate the diffusion of FeS, and improve desulfurization efficiency.
(4) Strengthen the stirring action between the molten steel and the slag. The desulfurization process is a slow diffusion process, and enhancing stirring to increase the contact area between the molten steel and the furnace slag is the most effective measure for improving desulfurization efficiency. Based on this, the “simultaneous discharge of steel and slag” during tapping is an optimal and highly effective operational method.
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