Types and Processes of Secondary Refinement

Types and Processes of Secondary Refinement

Secondary refining encompasses critical processes performed after primary steelmaking (in a BOF or EAF) to achieve precise control over chemistry, temperature, and cleanliness. Key objectives include deep desulfurization and dephosphorization.

I. Dephosphorization of Molten Steel

While modern converters can achieve phosphorus levels of 40-100 ppm, this depends heavily on the initial silicon and phosphorus content of the hot metal. Effective dephosphorization requires forming P₂O₅ and fixing it into a basic slag. Current advanced practices focus on minimizing slag volume in the final converter stage.

   Hot Metal Pretreatment (Dephosphorization): A popular strategy, especially in Japan, involves desiliconizing and dephosphorizing the hot metal in a separate vessel or an initial converter operation before the main converter charge. This allows the main converter to operate with ultra-low silicon inputs, enabling efficient "less-slag" or "slag-free" blowing. A key trade-off is a lower scrap charge ratio.

   Two-Stage (Duplex) Converter Process: This method uses two converter stages:

    1.  The first converter performs desiliconization and dephosphorization.

    2.  The slag from the first stage is completely removed ("slag-off").

    3.  The semi-refined metal is then decarburized and finished in the second converter.

    By preventing phosphorus reversion from slag, this process can achieve end-of-blow phosphorus levels as low as 30 ppm.

   The Role of Secondary Refining: Final phosphorus content is influenced by tapping practices and downstream steps. Any slag carry-over during tapping can lead to phosphorus reversion as P₂O₅ in the slag is reduced. Furthermore, additions of phosphorus-containing alloys (e.g., some ferro-manganese grades) can increase phosphorus. Typically, the final product's phosphorus is about 10 ppm higher than at the converter endpoint.

   Advanced Ladle Refining for Ultra-Low Phosphorus: Utilizing a Ladle Furnace (LF) with specialized slag practices allows for even lower final phosphorus. A strategic approach is to tap the converter at a ~50°C lower temperature, which favors phosphorus retention in the slag. The necessary thermal energy is then supplied by arc reheating in the LF under controlled, reducing conditions to prevent reversion. Modeling shows that with optimized slag (e.g., ~18% FeO, 0.4% P₂O₅) and controlled tapping, steel with ~20 ppm phosphorus is achievable.

II. Desulfurization

Desulfurization in integrated steelmaking occurs in three stages: hot metal desulfurization, limited in-converter desulfurization, and ladle desulfurization during secondary refining.

   Hot Metal Desulfurization: Achieved by injecting reagents like calcium carbide, magnesium, or lime-magnesium mixtures into the iron ladle. This can reduce sulfur to very low levels (e.g., 20 ppm), depending on reagent consumption.

   Ladle Desulfurization (Key Stage): For efficient deep desulfurization in the ladle, three conditions are paramount:

    1.  Strong Reducing Conditions: Sufficient aluminum must be added to deoxidize the steel thoroughly, creating a low oxygen potential.

    2.  Optimal Slag Chemistry: The ladle slag must be lime-saturated (CaO-saturated). The "Lime Saturation Degree" is a key parameter:

           =1: Slag is CaO-saturated (optimal for high CaO activity).

           <1: Slag is unsaturated, liquid, but with lower CaO activity, reducing efficiency.

           >1: Slag is supersaturated, heterogeneous, and less reactive.

    3.  Intensive Stirring: The molten steel must be violently stirred (via argon bubbling) to ensure strong slag-metal contact and kinetic conditions for reaction.

Under optimal conditions (CaO-saturated slag + intense stirring), desulfurization rates can reach 95%. Efficiency drops significantly with weaker stirring or non-optimal slag.

   Complexities During Stirring: Intense ladle stirring triggers other simultaneous reactions:

       Reduction of SiO₂ in the slag by dissolved [Al], increasing the steel's silicon content.

       Re-oxidation of aluminum by air.

       Reduction of MnO from slag, slightly increasing manganese content.

    The increase in silicon content is particularly detrimental for producing low-silicon steels (e.g., for thin strips) and can itself hinder desulfurization.

   Integrated Desulfurization Strategy: Achieving ultra-low sulfur (e.g., <20 ppm) requires an integrated approach:

       Hot Metal Desulfurization should achieve an efficiency of ~75%, reducing sulfur to <30 ppm.

       Ladle Desulfurization must then operate at high efficiency (>90%). If ladle desulfurization efficiency is low (e.g., 35%), hot metal desulfurization must be even more aggressive to reach a starting point of ~30 ppm S to achieve a final target of ~50 ppm.

       For a final target of ~100 ppm S, hot metal pretreatment typically needs to reduce sulfur to ~150 ppm.

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