LF Refining Process Technology: Refining Practice and Operation System

Basic Operating Process of LF Ladle Furnace

 Ladle Preparation

Ⅰ. Check the permeability of the porous plug and clean the ladle to ensure safe operation.

Ⅱ. Preheat the ladle to 1200°C.

Ⅲ.Move the ladle to the tapping position and add synthetic slag materials.

Ⅳ.Based on the final sample from the primary furnace, add alloys and deoxidizers for initial alloying and deoxidation.

Ⅴ.Prepare for slag blocking or slag-free tapping.

Tapping from Primary Furnace

Ⅰ. The tapping temperature is determined based on the different steel grades, the amount of slag added, and the amount of alloying materials.

The tapping temperature should be based on the liquidus temperature, taking into account the temperature drop caused by the addition of slag and alloying materials, as well as the heating capacity of the LF (sulfuric acid furnace), and then appropriately increased according to the furnace volume to account for temperature drops during transportation.

Ⅱ. Be careful to block the slag as completely as possible;

Ⅲ. For steels requiring deep desulfurization, add synthetic slag into the steel stream during tapping.

Ⅳ. When 1/3 of steel is tapped, begin argon stirring (~200 L/min for ladles >50t) to ensure proper mixing.

Ⅴ. When 3/4 is tapped, reduce argon flow to ~100 L/min to prevent excessive cooling.

Slag Formation (Slagging Practice)

Proper slag formation during LF refining is critical because it:

Ⅰ. Enables desulfurization, deoxidation, dephosphorization, and even denitrification

Ⅱ. Absorbs non-metallic inclusions

Ⅲ. Controls inclusion morphology

Ⅳ. Forms foamy slag (submerged arc slag) to:

• Improve thermal efficiency
• Reduce refractory erosion

Rapid slag formation and early power-on heating are essential to enhance refining efficiency.

Foamy Slag (Submerged Arc Slag) in LF Refining

Functions of Foamy Slag

• Improves power factor and reduces electricity consumption per ton of steel
• Reduces heat loss and increases thermal efficiency (from ~30% to ~60%)
• Minimizes arc radiation damage to refractory lining
• Improves refining conditions and steel cleanliness

Two Methods of Submerged Arc Operation

• Increase slag quantity and thickness
• Add foaming agents to expand slag volume and thickness

Key Factors Affecting Foamy Slag Operation

Arc Length

Arc length is related to arc voltage，Formula：

L_arc=(U_arc-α)/β

α = voltage drop at cathode/anode (~10–20V)

β = potential gradient (~1.1 V/mm in LF refining)

Alternative formula (at 1600°C, basic slag):

L_arc=(U_arc-9)/8.4

Optimal condition:Slag thickness ≈ 2 × arc length for best thermal efficiency and submerged arc operation.

Foaming Index

The foaming index represents the average residence time of gas bubbles in slag — a key indicator of slag foaming performance.

Selection of Foaming Agents

To achieve effective slag foaming,Slag must have:

Ⅰ.  The refined base slag must have suitable physical properties, namely, high viscosity, low surface tension, and suitable basicity.

Ⅱ. Sufficient gas supply is required.

A. Argon gas stirring provides a portion of the gas supply during the reaction between the electrodes and the slag;

B. Gas can be generated by adding an external foaming agent.

Types of Foaming Agents

Ⅰ. Carbonates: Commonly used ones include limestone, dolomite, and industrial alkali. They mainly undergo the following reactions at high temperatures.

CaCO₃ → CaO + CO₂

MgCO₃ → MgO + CO₂

Na₂CO₃ → Na₂O + CO₂

Ⅱ.Carbon and carbon-containing compounds: Common examples include coke, silicon carbide, and calcium carbide. Because the oxygen content in the steel and the FeO content in the slag are both high in the initial stage of an LF furnace, these substances will react with the slag.

C + (FeO) → Fe + CO

SiC + 3(FeO) → 3Fe + SiO₂ + CO

CaC₂ + 3(FeO) → 3Fe + CaO + 2CO

Experimental results show that carbonates decompose rapidly at high temperatures, with short reaction times and relatively small gas volumes.

Fogging agents primarily composed of SiC and CaC2 exhibit good foaming effects, but CaC2 shows even better foaming performance.

A mixture of SiC and CaC2 foaming agents provides the best foaming effect.

Important Notes

• CaC₂ offers excellent foaming but is difficult to transport and store
• Foaming efficiency is higher during early LF stages (high FeO content)
• Foaming weakens during later stages after deoxidation
• For low-silicon steels, watch for silicon pickup due to SiC and slag reduction

Industrial Performance Data

• After implementing full submerged arc operation in LF:
• Electricity consumption ↓ 12.5% per ton
• Electrode consumption ↓ 26.6%
• Steel ladle service life ↑ 26.8%