Tapping Process and Slag Retention Operation of the Eccentric Bottom Tapping (EBT) Electric Arc Furnace

Tapping Process and Slag Retention Operation of the Eccentric Bottom Tapping (EBT) Electric Arc Furnace

I. EBT Tapping Process

Once molten steel temperature and composition meet the specified requirements, the tapping sequence for an Eccentric Bottom Tapping (EBT) furnace is initiated as follows:

1.  Ladle Positioning: Transport the pre-heated ladle into position directly beneath the EBT spout (tapping box) of the furnace.

2.  Pre-Tap Furnace Tilting: Prior to opening the taphole, tilt the furnace approximately 3°–5° towards the tapping side. This initial tilt creates sufficient static pressure head from the molten steel above the taphole, which is critical for initiating a strong, steady stream and helps prevent early slag entrainment.

3.  Tapping Execution:

       Remove the taphole filler material (support plate/plug) to open the taphole and begin the tap.

       As tapping proceeds, gradually increase the furnace tilt to a maximum of about 12°. This controlled tilting maintains a near-constant depth of molten steel above the taphole, ensuring a consistent flow rate.

       When approximately 95% of the steel has been tapped, initiate a rapid return tilt of the furnace body back to its horizontal position. This fast return (at a rate of about 3°/s) is crucial. It sharply reduces the steel head above the taphole, minimizing or eliminating the vortex that would otherwise draw slag into the stream, thereby achieving effective slag-free tapping.

Critical Considerations During Tapping:

   Tilt Rate Control: The tilting motion, especially during the initial and final phases, must not be too rapid. Excessive speed could cause molten steel in the EBT box to surge and contact the upper water-cooled panel, risking severe damage or a breakout.

   Maintain EBT Box Level: It is imperative to ensure a sufficient residual height of molten steel remains in the EBT box throughout the tap. This acts as a buffer to prevent the formation of a vortex at the taphole, which is the primary mechanism for slag carry-over into the ladle.

II. Steel and Slag Retention Operation

A key design feature of the EBT furnace is its ability to perform a "steel and slag retention" operation at the end of the tap. By concluding the tap with the rapid back-tilt, the furnace retains approximately 10–15% of the molten steel and over 95% of the slag inside the main vessel. This operational strategy delivers several significant benefits:

1.  Process Advantages: It enables a true slag-free tap to the ladle. The retained steel and slag provide excellent conditions for the subsequent heat, fundamentally changing the traditional furnace reset process.

2.  Improved Smelting Performance:

       Stable Arcing: The retained molten pool ensures a stable, conductive path for the arc from the very beginning of the next melt-down period, improving electrical efficiency and reducing electrode consumption.

       Faster Melting: The existing liquid pool accelerates the formation of a full molten bath, advancing melt-down completion by 10–15 minutes.

       Enhanced Refining: The early establishment of a liquid pool allows for earlier and more effective oxygen injection, significantly improving dephosphorization conditions from the start of the oxygen blow.

Operational Notes for Steel/Slag Retention:

1.  Grade Change Considerations: When switching to a significantly different steel grade, evaluate whether the chemistry of the retained steel will adversely affect the final composition of the next heat.

2.  Charging Sequence: Do not add lime onto the furnace bottom before charging the first scrap bucket. The retained liquid metal will cause the lime to sinter prematurely, reducing its efficiency.

3.  Charging Height: Control the drop height of the first scrap bucket to avoid "cannonballing," which could damage the refractory lining of the side walls or the slag line by plunging through the retained liquid.

4.  Process Timing: Initiate oxygen blowing and any melting aids (e.g., carbon injection) earlier in the process to capitalize on the head start provided by the liquid heel.

5.  Furnace Maintenance: Regularly inspect and clean the EBT spout area. Monitor the erosion state of the furnace bottom and taphole region to schedule timely maintenance and prevent failures.

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