What is Desulphurisation in Ladle Furnace?

Ladle furnace desulfurization is a key technology in the secondary refining process of steel. It utilizes the ladle refining furnaces to further treat molten steel after it flows out of the main smelting unit, removing sulfur from the molten steel.

Excessive sulfur can adversely affect the properties of steel, such as reducing its toughness and plasticity. By adding desulfurizing agents such as pre-melted slag, ferroaluminum alloys, aluminum slag, and lime to the LF , combined with processes such as argon stirring and heating, sulfur can be effectively transferred to the slag, thereby improving the quality of the molten steel and meeting the purity requirements of subsequent casting.

Ladle Furnace Desulfurization Process

Ladle refining furnaces desulfurization are core step in the production of high-quality steel, especially low-sulfur and ultra-low-sulfur steel, in modern steel plants. Its goal is to reduce the sulfur content in the molten steel to extremely low levels (e.g., <0.005%, or even <0.001%). The main process steps are as follows:

1. Loading & Slag Receiving: 

• The ladle is hoisted from the primary refining furnace (converter/electric furnace) into the LF station.

• Typically, oxidizing slag (high in FeO and MnO) from the primary refining furnace needs to be removed, or a reducing agent is added to modify it into reducing slag.

2. Forming Reducing Slag (White Slag): 

• During refining, lime (CaO, the main slag-forming agent), fluorite (CaF₂, a fluxing agent that lowers the slag's melting point and viscosity), and aluminum granules (deoxidizer) are added to the LRF.

• The slag is rapidly melted by electric arc heating, forming a liquid, highly basic reducing slag (its color changes from black to white, hence the name "white slag").

3. Deep Deoxidation & Preliminary Composition Adjustment: 

• Depending on the steel grade requirements, aluminum wire or other deoxidizers are fed into the LRF to reduce the oxygen activity of the molten steel to an extremely low level.

• Alloys are added for fine-tuning of the composition.

4. Core Desulfurization Operation: 

• Under conditions of strongly reducing white slag and deep deoxidation, efficient desulfurization is achieved in the ladle furnace through high-power electric arc heating (providing high temperatures, typically >1550℃) and strong argon stirring (increasing the steel-slag reaction interface and mass transfer rate).

• During this stage, sulfur transfers from the molten steel into the slag, and the sulfur content in the slag (existing as CaS) continuously increases.

• Additional slag material can be added as needed during operation.

5. Calcium Treatment (Optional but Commonly Used): 

• For aluminum-deoxidized steel, calcium treatment is often performed in the later stages of desulfurization using calcium-feeding wire (pure calcium or calcium silicate wire).

Purpose: 

• Modification treatment: To transform solid Al₂O₃ inclusions into low-melting-point calcium aluminates (such as 12CaO·7Al₂O₃), improving the fluidity of the molten steel and preventing nozzle blockage.

• Deep Desulfurization: [Ca] dissolved in molten steel can react directly with [S] to form fine CaS, achieving "precipitation desulfurization" within the molten steel, further reducing sulfur content.

6. Soft Blowing & Static Agitation: 

• After desulfurization and calcium treatment, switch to weak argon agitation (soft blowing).

Purpose: 

• To promote the flotation of fine inclusions (including CaS, Al₂O₃, etc.) in the molten steel for absorption by the slag, while preventing secondary oxidation and slag entrapment. This is a crucial step in purifying molten steel and stabilizing sulfur content.

7. Departure: 

• Temperature measurement and sampling are performed. After confirming that the sulfur content and all components meet the temperature standards, the molten steel is hoisted to the continuous casting process.

Main Factors Affecting LRF Desulfurization Efficiency

1. Initial Steel Conditions: [O] and [S] content and temperature upon arrival at the station.

2. Slag Properties: Basicity, fluidity (viscosity), and reducing properties are decisive factors.

3. Agitation Intensity: Directly affects the transport rate of reactants and products.

4. Operating Process: Slag formation timing, deoxidation degree, soft blowing effect, etc.

The LF ladle refining furnaces desulfurization process is a composite refining technology combining "high-basicity reducing slag + high temperature + strong stirring." By precisely controlling metallurgical thermodynamics and kinetic conditions, it efficiently removes sulfur from molten steel into the slag, making it an indispensable core process in modern clean steel production.

Successful implementation of the ladle furnace relies on a deep understanding of the three-phase reactions of slag, steel, and gas, and meticulous process control. For any questions or needs regarding the refining furnace, please feel free to contact us for further discussion.